NC0002879_Solids Holding Plan_20190909

SEP 09 2019
MN Cape Fear P f ty
Public Utility Authority �` ' � Sec
on
Stewardship.Sustainability.Service.
Solids Handling Plan for Sweeney Water Treatment Plant
NPDES Permit # NC0002879
Solids are generated at Sweeney Water Treatment Plant(WTP)by way of two different processes. The first
process that generates solids at the Sweeney WTP is filter backwashes. All solids produced as a result of filter
backwashes travel to the Washwater Equalization Basin.
The second process that generates solids at the Sweeney WTP is excess floc that is periodically wasted from our
Super-Pulsator units. These units are solids contact upflow clarifiers with agglomeration plates set at a 60
degree angle inside them. Excess floc,or solids,that accumulates in the Super-Pulsator units overflow into a
chamber where they are stored until a valve,which is set on a timer,opens allowing the excess solids to flow,or
"blow down",from the super-pulsator through a meter vault and into the Thickener Splitter Box. At the
Thickener Splitter Box,which functions to equally split flow to either of our Thickeners,anionic polymer is
added as a thickening agent before the solids overflow from a mixing chamber into one of two Thickeners. In
the Thickeners, solids settle further until they are dense enough to be removed. The supernatant from the
Thickeners flows by gravity through five(5) separate man holes before arriving at the Washwater Equalization
Basin.
From the Washwater Equalization Basin,fluidized solids that are produced as a result of the two processes
mentioned above are pumped to the Clarifier Splitter Box. At the Clarifier Splitter Box,an anionic polymer is
added as a thickening agent before the solids flow into one(1)of our two(2)Westech Upflow Clarifiers.
Supemate from the clarifiers flows through a manhole,where sodium hydroxide(50%)is added for pH
adjustment before flowing into the Wet Well where a majority of it is pumped back to the head of the plant and
into the raw influent of the treatment plant. What is not"recycled"back to the head works of the plant goes to
the river per NPDES permit#NC0002879.
Excess solids that build up in the Thickeners are pumped to the CFPUA's James A. Loughlin Waste Water
Treatment Plant(WWTP). Excess solids that build up in the Clarifiers are pumped to the Thickener Splitter
Box where they are thickened further before being pumped to the WWTP for dewatering and disposal. At the
WWTP,the solids are dewatered and combined with biosolids. These solids are listed as source materials in the
CFPUA Land Application permit number WQ0001271. The combined solids are currently taken to a landfill.
235 Government Center Drive, Wilmington, NC 28403
t: 910-799-6064 f: 910-799-6066 www.cfpua.org

Effluent Mixing Analysis for the
Cape Fear Public Utility Authority
Southside and Sweeney NPDES Outfalls
November 16, 2018
PREPARED FOR PREPARED BY
Cape Fear Public Utility Authority Tetra Tech, Inc.
235 Government Center Drive, One Park Drive, Suite 200
Wilmington, North Carolina 28403 PO Box 14409
Tel [phone] Research Triangle Park, North Carolina 27709
Tel [phone]

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
TABLE OF CONTENTS
1.0 INTRODUCTION 1
1.1 Defining Critical Flow 1
2.0 SOUTHSIDE/SWEENEY JP-EFDC MODEL 4
2.1 Model Grid Update 4
2.1.1 Model Bathymetry 4
2.1.2 Vertical Layers 5
2.2 Model Simulation Period 9
2.3 Model Boundary Conditions 9
2.3.1 Freshwater Flows 9
2.3.2 Water Temperatures 11
2.3.3 Open Boundary Salinity 14
2.3.4 Water Surface Elevation 16
2.3.5 Meteorological Inputs 18
2.3.6 Point Sources 20
2.3.6.1 Effluent Flows 20
2.3.6.2 Effluent Temperatures 20
2.3.6.3 JP-EFDC Representation 20
2.4 Model Calibration 21
2.4.1 Calibration Stations 21
2.4.2 Calibration Methodology 23
2.4.3 Water Surface Elevation 24
2.4.4 Water Temperature 28
2.4.5 Salinity 38
3.0 MODEL APPLICATION: EFFLUENT MIXING ANALYSES 47
3.1 Effluent Mixing Analyses 48
3.2 Effluent Mixing Analysis Results 48
4.0 SUMMARY AND CONCLUSIONS 56
5.0 BIBLIOGRAPHY 57
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Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
LIST OF TABLES
Table 1-1 Critical flow analysis for the 2018 JP-EFDC model 2
Table 2-1 2018 EFDC model hydrodynamic calibration and validation stations 21
Table 2-2 2018 EFDC model WSE calibration and validation statistics 27
Table 2-3 2018 EFDC model temperature calibration and validation statistics 37
Table 2-4 2018 EFDC model salinity calibration and validation statistics 46
Table 3-1 2018 JP-EFDC model predicted dilution ratios at Southside WWTP and Sweeney WTP for existing
conditions 54
Table 3-2 2018 JP-EFDC model predicted dilution ratios at Southside WWTP discharging at 12 MGD 54
Table 3-3 2018 JP-EFDC model predicted dilution ratios at Southside WWTP discharging at 16 MGD 54
Table 3-4 2018 JP-EFDC model predicted dilution ratios at Southside WWTP discharging at 20 MGD 55
Table 3-5 2018 JP-EFDC model predicted dilution ratios at Southside WWTP discharging at 24 MGD 55
LIST OF FIGURES
Figure 1-1 Location of the CFPUA Southside WWTP and Sweeney WTP discharges 3
Figure 2-1 2018 EFDC model grid for the Lower Cape Fear River Estuary 6
Figure 2-2 2018 EFDC model grid bottom elevations of the Lower Cape Fear River Estuary 7
Figure 2-3 2018 EFDC model grid bottom elevations at the Southside WWTP and Sweeney WTP 8
Figure 2-4 2018 EFDC model USGS flow stations used to develop the flow boundary time series 10
Figure 2-5 2018 EFDC model freshwater flow boundary time series 11
Figure 2-6 2018 EFDC model LCFRP temperature stations used to develop the temperature boundary time
series 12
Figure 2-7 2018 EFDC model freshwater temperature boundary time series 13
Figure 2-8 2018 EFDC model open boundary temperature boundary time series 13
Figure 2-9 2018 EFDC model LCFRP salinity station used to develop the salinity boundary time series 15
Figure 2-10 2018 EFDC model open boundary salinity boundary time series 16
Figure 2-11 2018 EFDC model NOAA WSE station used to develop the WSE open boundary time series 17
Figure 2-12 2018 EFDC model weather station used to develop the meteorological time series 19
Figure 2-13 Location of calibration and validation stations used in the 2018 EFDC model 22
Figure 2-14 Simulated and observed WSE at Wilmington, NC, NOAA 8658120, 01/2011 —12/2011 25
Figure 2-15 Simulated and observed WSE at Wilmington, NC, NOAA 8658120, June 2011 25
Figure 2-16 Simulated and observed WSE at Wilmington, NC, NOAA 8658120, 01/1992—06/2018 26
Figure 2-17 Simulated and observed temperature at Northeast Cape Fear River at US 421 at Wilmington, NC,
NCDEQ B974000, 01/2011 —12/2011 29
Figure 2-18 Simulated and observed temperature at Cape Fear River at Horseshoe Bend near Wilmington, NC,
LCFRP HB, 01/2011 —12/2011 29
Figure 2-19 Simulated and observed temperature at Wilmington, NC, NOAA 8658120, 01/2011 —12/2011 30
Figure 2-20 Simulated and observed temperature at Cape Fear River CM 61 at Wilmington, NC, NCDEQ
B980000, and LCFRP M61, 01/2011 — 12/2011 30
Figure 2-21 Simulated and observed temperature at Brunswick River dns NC 17 at park near Belville, NC,
LCFRP BRR, 01/2011 —12/2011 31
Figure 2-22 Simulated and observed temperature at Cape Fear River at CM 56 near Wilmington, NC, NCDEQ
B982000, 01/2011 —12/2011 31
Figure 2-23 Simulated and observed temperature at Cape Fear River at Channel Marker 23, LCFRP M23,
01/2011 —12/2011 32
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Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
Figure 2-24 Simulated and observed temperature at Cape Fear River at Channel Marker 18, LCFRP M18,
01/2011 —12/2011 32
Figure 2-25 Simulated and observed temperature at Northeast Cape Fear River at US 421 at Wilmington, NC,
NCDEQ B974000, 01/1992—06/2018 33
Figure 2-26 Simulated and observed temperature at Cape Fear River at Horseshoe Bend near Wilmington, NC,
LCFRP HB, 01/1992—06/2018 33
Figure 2-27 Simulated and observed temperature at Wilmington, NC, NOAA 8658120, 01/1992—06/2018 34
Figure 2-28 Simulated and observed temperature at Cape Fear River CM 61 at Wilmington, NC, NCDEQ
B980000, and LCFRP M61, 01/1992—06/2018 34
Figure 2-29 Simulated and observed temperature at Brunswick River dns NC 17 at park near Belville, NC,
LCFRP BRR, 01/1992—06/2018 35
Figure 2-30 Simulated and observed temperature at Cape Fear River at CM 56 near Wilmington, NC, NCDEQ
B982000, 01/1992—06/2018 35
Figure 2-31 Simulated and observed temperature at Cape Fear River at Channel Marker 23, LCFRP M23,
01/1992—06/2018 36
Figure 2-32 Simulated and observed temperature at Cape Fear River at Channel Marker 18, LCFRP M18,
01/1992—06/2018 36
Figure 2-33 Simulated and observed salinity at Northeast Cape Fear River near Wrightsboro, NC, LCFRP
NCF6, 01/2011 —12/2011 39
Figure 2-34 Simulated and observed salinity at Northeast Cape Fear River at US 421 at Wilmington, NC,
NCDEQ B974000, 01/2011 —12/2011 39
Figure 2-35 Simulated and observed salinity at Cape Fear River at Horseshoe Bend near Wilmington, NC,
LCFRP HB, 01/2011 —12/2011 40
Figure 2-36 Simulated and observed salinity at Cape Fear River CM 61 at Wilmington, NC, NCDEQ B980000,
and LCFRP M61, 01/2011 —12/2011 40
Figure 2-37 Simulated and observed salinity at Brunswick River dns NC 17 at park near Belville, NC, LCFRP
BRR, 01/2011 —12/2011 41
Figure 2-38 Simulated and observed salinity at Cape Fear River at CM 56 near Wilmington, NC, NCDEQ
B982000, 01/2011 —12/2011 41
Figure 2-39 Simulated and observed salinity at Cape Fear River at Channel Marker 23, LCFRP M23, 01/2011 —
12/2011 42
Figure 2-40 Simulated and observed salinity at Northeast Cape Fear River near Wrightsboro, NC, LCFRP
NCF6, 01/1992—06/2018 42
Figure 2-41 Simulated and observed salinity at Northeast Cape Fear River at US 421 at Wilmington, NC,
NCDEQ B974000, 01/1992—06/2018 43
Figure 2-42 Simulated and observed salinity at Cape Fear River at Horseshoe Bend near Wilmington, NC,
LCFRP HB, 01/1992—06/2018 43
Figure 2-43 Simulated and observed salinity at Cape Fear River CM 61 at Wilmington, NC, NCDEQ B980000,
and LCFRP M61, 01/1992—06/2018 44
Figure 2-44 Simulated and observed salinity at Brunswick River dns NC 17 at park near Belville, NC, LCFRP
BRR, 01/1992—06/2018 44
Figure 2-45 Simulated and observed salinity at Cape Fear River at CM 56 near Wilmington, NC, NCDEQ
B982000, 01/1992—06/2018 45
Figure 2-46 Simulated and observed salinity at Cape Fear River at Channel Marker 23, LCFRP M23, 01/1992—
06/2018 45
Figure 3-1 Dilution ratios for the Southside WWTP during existing conditions using four-day moving average
50
Figure 3-2 Dilution ratios for the Southside WWTP at 12 MGD using four-day moving average 51
Figure 3-3 Dilution ratios for the Southside WWTP at 16 MGD using four-day moving average 51
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Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
Figure 3-4 Dilution ratios for the Southside WWTP at 20 MGD using four-day moving average 52
Figure 3-5 Dilution ratios for the Southside WWTP at 24 MGD using four-day moving average 52
Figure 3-6 Dilution ratios for the Sweeney WTP during existing conditions using four-day moving average 53
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Cape Fear Public Utility Authority Southside& Sweeney Effluent Mixing Analysis
ACRONYMS/ABBREVIATIONS
Acronyms/Abbreviations Definition
CFPUA Cape Fear Public Utility Authority
cfs Cubic Feet per Second
CRM Coastal Relief Model
EFDC Environmental Fluid Dynamics Code
IA Index of Agreement
JP-EFDC Jet Plume Environmental Fluid Dynamics Code
LCFRP Lower Cape Fear River Program
MAE Mean Absolute Error
MGD Million Gallons per Day
MLLW Mean Lower Low Water
NCDC National Climate Data Center
NCDEQ North Carolina Department of Environmental Quality
NOAA National Oceanic and Atmospheric Administration
NPDES National Pollutant Discharge Elimination System
NRMSE Normalized Root Mean Squared Error
PSU Practical Salinity Units
RMSE Root Mean Squared Error
SA Surface Airways
SABSOON South Atlantic Bight Synoptic Offshore Observational Network
UM Updated Merged
UNCW University of North Carolina Wilmington
USACE U.S. Army Corps of Engineers
USEPA U.S. Environmental Protection Agency
USGS U.S. Geological Survey
WBAN Weather Bureau Army Navy
WRDB Water Resources Database
WSE Water Surface Elevation
WTP Water Treatment Plan
WWTP Wastewater Treatment Plant
NI]
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Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
1.0 INTRODUCTION
The Cape Fear Public Utility Authority(CFPUA) is required to conduct effluent mixing analyses for its Southside
and Sweeney facility National Pollutant Discharge Elimination System (NPDES)discharges:
1. Southside Wastewater Treatment Plant(WWTP; NPDES NC0023973)
2. Sweeney Water Treatment Plant(WTP; NPDES NC0002879)
The Southside WWTP—currently permitted at 12 million gallons per day(MGD)—discharges to the Lower Cape
Fear River Estuary, and the 0.6 MGD Sweeney WTP discharges to the Northeast Cape Fear River(Figure 1-1).
Both facilities discharge to tidally influenced waterbodies in New Hanover County, North Carolina.
The effluent mixing analyses will allow the North Carolina Department of Environmental Quality(NCDEQ)and
CFPUA to accurately determine the environmental impacts of the effluent on the receiving waterbodies by
calculating dilution ratios for each NPDES discharge. The dilution ratios can be used by NCDEQ to establish
more accurate instream waste concentrations needed for permitting decisions including establishing effluent toxic
limits and whole-effluent toxicity testing requirements and serving as the basis for pretreatment headworks
analysis for the Southside WWTP.
Two mechanistic models, the Environmental Fluid Dynamics Code (EFDC)hydrodynamic model and the Jet
Plume Environmental Fluid Dynamics Code(JP-EFDC) model, were used to simulate conditions in the tidally-
influenced Lower Cape Fear River Estuary. JP-EFDC is a module within EFDC. The three-dimensional
hydrodynamic model represents both far-field (EFDC)and near-field mixing (JP-EFDC)and was used to establish
the dilution ratios for the Southside WWTP and Sweeney WTP at selected distances from each outfall.
In 2001, EFDC and JP-EFDC models were developed for the Lower Cape Fear River Estuary system, and the
results of the model analysis were used to evaluate the near-field and far-field mixing of effluent from the
Wilmington Northside and Southside WWTPs(Tetra Tech 2001). The models simulated conditions in the estuary
system from 1993 through 1999. In 2008 the models, which are hereafter referred to as the 2008 EFDC model
and 2008 JP-EFDC model,were revised and used to evaluate effluent mixing for expansion of the Wilmington
Northside WWTP(Tetra Tech 2008).
Tetra Tech extended the 2008 EFDC model to simulate conditions in the Lower Cape Fear River Estuary system
through June 2018. The extended model, hereafter referred to as the 2018 EFDC model,was calibrated and
validated to measured data collected in the Lower Cape Fear River Estuary between 1992 and 2017. The model
calibration also focused on the 2011 low freshwater flow period, which was defined as the critical period based on
recent and historic flows. The 2008 JP-EFDC model was also updated, and results from this updated 2018 JP-
EFDC model were used to establish dilution ratios at selected distances from the Southside WWTP and the
Sweeney WTP outfalls respectively.
This report describes the 2018 EFDC and 2018 JP-EFDC model setup, development, and calibration. A summary
of model results for use by NCDEQ in establishing appropriate dilution ratios to apply for regulatory purposes for
both the NPDES permits is also provided.
1.1 DEFINING CRITICAL FLOW
A critical low-flow freshwater period was defined based on historical freshwater flows, and results from the critical
period were used to establish dilution ratios for the Southside WWTP and the Sweeney WTP. The previous
dilution ratio studies in the Lower Cape Fear River Estuary defined the critical period as the period with the 10th
percentile freshwater flows(Tetra Tech 2001, Tetra Tech 2008).
Freshwater daily average flows used in the 2018 EFDC model for the Cape Fear River, Black River, and North
East Cape Fear River were used to determine the critical flow period in the Lower Cape Fear River Estuary.
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Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
Continuous daily average flows from the U.S. Geological Survey(USGS)station 02105769 were used for Cape
Fear River, USGS station 02106500 for Black River, and USGS station 02108000 for Northeast Cape Fear River.
The 20-year record from 1998 through 2017 was used to identify the potential critical flow periods for the dilution
analysis. The lowest freshwater flows occurred during the summer months(June through September) in the years
2002, 2007, and 2011. The calculated average summer monthly 10th percentile freshwater flow at USGS
02105769 for the 20-year record was 1,071 cubic feet per second(cfs),which is similar to the flow observed in
July 2011, 997 cfs. The critical flow period was selected based on a combination of lowest average summer
monthly flows and the 10t'' percentile summer flows. Therefore, the timeframe for the dilution analysis was in July
2011.
Table 1-1 summarizes the average flows for July 2011 in each of the three primary drainages(Cape Fear River,
Black River, and Northeast Cape Fear River). The percent of summer flows shown in the third column was
calculated as the sum of the flow between the months of June through September for 2011 divided by the entire
flow for that year at each gaging station, respectively. The low percentages substantiate that 2011 summer flows
reflect a critical low flow period for the modeling.
Table 1-1 Critical flow analysis for the 2018 JP-EFDC model
10th
Percent of Average Percentile
Drainage Summer Flows for Summer
Station ID Area, sq. Flows, % July 2011, Flows, cfs
miles (2011) cfs (1998 -
2017)
02105769 5,255 1.9 997 1,071
(Cape Fear River)
02106500 676 1.5 87 168
(Black River)
02108000
(Northeast Cape 599 2.8 21 154
Fear River)
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Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
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Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
2.0 SOUTHSIDE/SWEENEY JP-EFDC MODEL
The 2008 EFDC model developed for the Lower Fear River Estuary wasused to investigate potential
p Cape ea s ary est gate pote t
discharge options for CFPUA's Northside WWTP(Tetra Tech 2001, Tetra Tech 2008). The 2008 EFDC model
simulates the estuary system from the 1-140 bridge approximately 3.5 miles north of Wilmington, North Carolina to
the Cape Fear River mouth at the Atlantic Ocean. The model domain also includes the lower segments of three
major Lower Cape Fear River Estuary tributaries: (1)Cape Fear River, (2) Northeast Cape Fear River, and (3)
Black River. The Black River discharges to the Cape Fear River above the 1-140 bridge.The Southside WWTP
and the Sweeney WTP discharge locations are within the model domain. The 2008 EFDC simulates model
conditions in the estuary from 1993 through 1999.
The 2018 EFDC model uses the same model domain as the 2008 EFDC model, but includes an updated and
refined grid and extends the model to simulate conditions in the Lower Cape Fear River Estuary through June 30,
2018(Section 2.1, Section 2.2). The bathymetric data were updated using the data collected within the last 10
years(Section 2.1). In addition, the model boundary conditions were updated with the best available data to
simulate several years that encompass the critical period (Section 2.2). Results from the 2018 EFDC model were
compared to measured data collected in the Lower Cape Fear River Estuary(Section 2.4).
2.1 MODEL GRID UPDATE
The 2018 EFDC model grid refined the 2008 EFDC model grid to better represent hydrodynamics near the
Sweeney WTP facility. Additional grid cells were added to reduce the overall cell size and improve the resolution
surrounding the discharge location. The model grid was also extended at the open boundary, located at the
mouth of the Cape Fear River, by three rows of horizontal grid cells. The open boundary was extended to provide
a smoother transition for the tide to enter the riverine portion of the Lower Cape Fear River Estuary from the open
ocean. The refined grid consisted of 827 horizontal grid cells that ranged in size from approximately 8.5 acres
(105 meters x 328 meters)in the riverine area to approximately 166 acres(1,025 meters x 656 meters)near the
offshore area(Figure 2-1).
2.1.1 Model Bathymetry
The 2018 EFDC model bathymetry was re-interpolated using data collected within the last 10 years to better
reflect the conditions during the 2018 EFDC model critical period. All bathymetry data used in the 2018 EFDC
model were referenced to the National Oceanic and Atmospheric Administration (NOAA) Mean Lower Low Water
(MLLW)datum'. Bathymetric data were available from the U.S. Coastal Relief Model(CRM)and the U.S. Army
Corps of Engineers(USACE). The CRM data were collected in 1999 and covered a large portion of the model
domain. The USACE data were collected in 2017 and 2018 and covered the navigational channel in the Lower
Cape Fear River.
The 2017/2018 USACE data were used to represent conditions in the navigational channel in the 2018 EFDC
model because it reflected the current conditions of the system prior to the proposed new dredging being funded
by the USACE(note: the increased circulation provided by the new dredging would be expected to further
improve tidal mixing, so assuming present conditions is conservative). The 1999 CRM data were used for all other
areas. The bathymetric data were interpolated by averaging the survey data points that fell within a model cell.
The USACE data had an average of 815 data points per cell, and the CRM data had an average of 16 data points
1 The MLLW is the average height of the lowest tide recorded at a tide station each day during a 19-year
recording period. The 19-year recording period is the nearest full year count to the 18.6-year cycle of the lunar
node regression, which influences tides.
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Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
per cell. The processed bathymetric data used in the model are shown in Figure 2-2. Figure 2-3 shows the bottom
elevation of the 2018 EFDC model grid at Southside WWTP and at Sweeney WTP.
Bathymetric data were not available in the Black River and in the Northeast Cape Fear River two miles upstream
of Sweeney WTP. For these portions of the river, a uniform slope was calculated using the bottom elevations of
USGS stations 02108000 (Northeast Cape Fear River near Chinquapin, North Carolina)and 02108566
(Northeast Cape Fear River near Burgaw, North Carolina). For each river, the slope was calculated by
interpolating between the last model cell where bathymetry data were available and the USGS station. The
calculated slopes were applied uniformlyto all cells in the rivers. The estimated slopes were duringthe
PPadjusted
model calibration phase to improve model representation of water depths and velocities in the 2018 EFDC model.
2.1.2 Vertical Layers
A sigma grid, which uses a constant number of vertical layers throughout the model domain, was used for the
2018 EFDC model. The cell thickness of each vertical layer changes depending on the water surface elevation
such that all layers at a grid cell location are of uniform depth. Based on the grid system, the vertically hydrostatic
equations of motion for turbulent flow are solved numerically to determine transport between cells, instream
velocity, momentum,free surface elevation, and cell water volume. The 2018 EFDC model has four vertical grid
cells, which is the same as the 2008 EFDC model. Near the open boundary, the cells averaged 21 feet thick,
while in the riverine areas the cells averaged 8 to 10 feet thick.
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Cape Fear Public Utility Authority Southside& Sweeney Effluent Mixing Analysis
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[It)TETRA TECH 6 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
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Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
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Model Bat hymetry 0 730 1.400 2.800 4200 r,ea0 l`tEt I'*L]TETRATECH
M-19N,;TL_tli\
1C 30 201E v. n Zraa,s-nr Fives
Figure 2-3 2018 EFDC model grid bottom elevations at the Southside WWTP and Sweeney WTP
UTETRA TECH 8 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
2.2 MODEL SIMULATION PERIOD
The simulation period for the 2018 EFDC model covers a 26.5-year period from January 1, 1992 through June 30,
2018. Boundary conditions from the 2008 EFDC model were extended to represent hydrodynamic and
meteorological conditions in the 2018 EFDC model for the simulation period.The 2018 EFDC model was
calibrated for water surface elevation(WSE), salinity, and temperature for the critical year 2011 and verified with
data collected from January 1, 1992 through June 30, 2018. Refer to Section 1.1 for the discussion on defining
the critical period.
2.3 MODEL BOUNDARY CONDITIONS
2.3.1 Freshwater Flows
Three major river systems contribute freshwater flows to the Lower Cape Fear River Estuary: (1)Cape Fear
River, (2) Black River, and (3) Northeast Cape Fear River(Figure 2-4).
Daily average flows from five USGS gages were used to estimate the watershed flows contributing to the model
domain(Figure 2-4):
• USGS 02105769(Cape Fear River at Lock#1 near Kelly, North Carolina)was used to represent the
flows from the Cape Fear River.
o The flows from the USGS 02105769 station were area weighted to capture the contributing
drainage area from Lock#1 to the confluence with Black River(Figure 2-5).
• USGS 02106500 (Black River near Tomahawk, North Carolina)was used to represent the flows from the
Black River(Figure 2-5).
o The flows from the USGS 02106500 station were area weighted based on the contributing
drainage area to represent the tributary flows at the confluence with Cape Fear River.
• USGS 02108000(Northeast Cape Fear River near Chinquapin, North Carolina)was used to represent
the flows from the Northeast Cape Fear River(Figure 2-5).
o The flows from the USGS 02108000 station were area weighted based on the contributing
drainage area to represent the tributary flows at the start of the model grid.
The Northeast Cape Fear River and Black River flows were not area-weighted in the 2008 EFDC model. There is
a significant amount of land that drains to these rivers below the gages: 5 percent below the Cape Fear River
gage, 57 percent below the Black River gage, and 66 percent below the Northeast Cape Fear River
gage.Therefore, area-weighting the flows provides a better approximation of freshwater flows reaching the model
domain. Of the area-weighed flows, 57 percent were from the Cape Fear River, 20 percent were from the Black
River, and 23 percent were from the Northeast Cape Fear River.
Flow data were available for the entire simulation period, January 1, 1992 through June 30, 2018, at all three
USGS stations. There were some data gaps of three days or less in the measured time series, and the gaps were
filled by averaging available data before and after the gaps.
OTETRA TECH 9 November 16, 2018

Cape Fear Public Utility Authority Southside & Sweeney Effluent Mixing Analysis
02108000
eYMwe-
Ines
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,,
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N.•
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a "'
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t:etienei Gecgrac4z..Esri ceiIvit? =,86.A.EF-:•Cr:+C,I,CS _-
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M.^.1981?U 76N 0 :a:5f.250 10.500 1!,750 21.000 (-_.___
XI SA AIMaNdmr AlttMs
ce
Figure 2-4 2018 EFDC model USGS flow stations used to develop the flow boundary time series
n TETRA TECH 10 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
90000 , , , , , , , , , , , , , , , , , , , , , , ,
80000 -
70000 -
,. 60000 = -
t 50000
40000
`- 30000
20000 = •
10000 li
0 ••Jr", _. _:a.►.?:..I. .V . Lam__._.. ..- :J,11..4.- _ .1: _.r x
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
Cape Fear River _ Black River NE Cape Fear River
Figure 2-5 2018 EFDC model freshwater flow boundary time series
2.3.2 Water Temperatures
The water temperatures of the freshwater and open boundaries impact the density of the receiving water and,
thus, the mixing of the effluent water. Monthly water temperature data were collected as part of the joint venture
between the Lower Cape Fear River Program (LCFRP)and University of North Carolina Wilmington (UNCW)
Aquatic Ecology Laboratory at stations throughout the Lower Cape Fear River Estuary. Data collected at NC11
(Cape Fear River), B210 (Black River), NCF117 (Northeast Cape Fear River), and M18(open boundary)were
used to develop the monthly water temperature time series for the rivers and the open boundary(Figure 2-6,
Figure 2-7, and Figure 2-8). The water temperature data were collected between 1996 and 2014. Gaps of three
months or less were filled by averaging the months before and after the gaps. Gaps greater than three months
were filled by using long-term monthly averages.
[ ]TETRA TECH 11 November 16, 2018

Cape Fear Public Utility Authority Southside& Sweeney Effluent Mixing Analysis
LCFRP ID: "'`
B210
LCFRP ID: tevi Wos (is ay
lvaH!} LCFRP ID:
NC11 +NA• NCF117
;ui to
t./L
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t
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ii Southside WWTP
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A LCFRP Station
• NPDES Location
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P Ccetec:r''ay re!retl�:t.at,crai Cecgraprics mover!rraF policy Sc.'
t.al.crel CeccraFrio.E-ri Celorre.I-EPE.I.NEP-..:CT."C.,SC'S _-
'- ES.=. f."ETi ':PCAt..CEECC.r.CFA.i^cerrer!F CcrF
CFPUA Diutton Analysis Study N o e 0332000 3:ooa 32 333 as,000
Temperature Staton Locaan FEE, TETRA TECH
M:19N„71;t5� 3 'S lO 0 7'DJ 11:S0 1'_.533 irt
X 3 :]15 1•M3t17.d'nt-
I.IEtsrs
Figure 2-6 2018 EFDC model LCFRP temperature stations used to develop the temperature
boundary time series
-,N TETRA TECH 12 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
100 ,
i 90 - -
CU
80 . A A g 4 I • k r $ _ S 4 A A I '
a) 70 i
60 =
•
L.
a 50 la C '-- -
# 11 (
bOH
1 1 �
i 1 ii--
i i 1 i i i i + i i i i i i
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Date
Cape Fear River Black River NE Cape Fear River
Figure 2-7 2018 EFDC model freshwater temperature boundary time series
100
i •
90 =
i 80 - f ^ 4 i r 1 t 4\ \. \ \: I{ " 1 h 1 ..
r
CU
.. 70
4
t
L.
E r
30 ! I F , t if 4 -I----I --- _ --- I i 1 1 , 1 11 1 1 1 I 1 1
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
l Open Boundary
Figure 2-8 2018 EFDC model open boundary temperature boundary time series
't'TETRA TECH 13 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
2.3.3 Open Boundary Salinity
Tidal circulation in the Lower Cape Fear River Estuary mixes denser saline water with fresh riverine water.
Realistic predictions of water density due to salinity intrusion is crucial for characterizing mixing processes in the
dilution study. The prediction of salinity is the best measure of a model's representation of advective transport of
dissolved and suspended materials by the velocity field (Tetra Tech 2008).
Salinity data collected offshore by the South Atlantic Bight Synoptic Offshore Observational Network(SABSOON)
were used to generate the open boundary condition for salinity for the 2008 EFDC model. The SABSOON is a
real-time coastal ocean observing system on the continental shelf off Georgia, which was developed at offshore
towers maintained by the U.S. Navy for a flight training range. The 2001 and 2008 EFDC models used a constant
salinity of 33 practical salinity units(PSU) due to data limitations at that time (Tetra Tech 2001, Tetra Tech 2008).
However, salinity concentrations vary daily and seasonally depending on tidal cycles and seasonal freshwater
flows.
For the 2018 EFDC model, salinity conditions at the open boundary were defined using data collected as part of
the joint venture between LCFRP and UNCW Aquatic Ecology Laboratory. Monthly salinity concentrations were
collected on an ebb tide throughout the estuary(LCFRP 2015). Annual reports that list monthly average salinity
by site were available for 2002 through 2015. Data from M18, located near the open boundary, were processed to
create a monthly salinity time series for the open boundary (Figure 2-10). Gaps of three months or less were filled
by averaging the months before and after the gaps. For the January 1992 through December 2002 and January
2016 through June 2018 periods, when no salinity data were available, the long-term M18 average salinity was
applied because salinity is independent of seasonality and dependent on instream flow conditions; therefore,
long-term monthly averages were not used to fill the gaps. Salinity concentrations at M18 ranged from 7.9 PSU to
35.5 PSU and averaged 27.5 PSU with a median of 29.3 PSU.
lit]TETRA TECH 14 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
�j
\-----\--\\., \ ''' '1...-//7--....-~" '
1 t t v n s �•, Gs aYr1e'
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4;�,/
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r.e',c i GectraGhic.E=ri Cely me.i-ERE.1.!.EF-:1;Ct:,C.LEGS.1.AEF
ESL. t:'ET 1.P CA'..GEE CC '.CLL 1- er^ertF Cot
CFPUA Ddutlon Analysts Study N 3 e.3332 330 24 330 3f.O00 S 330
Salinity Station Location I _ Fe
et
TETRA TECH
M3 1951„TA+.16\ N 0 1.873 !O FDO 11.250 1E 003
.y 1)2)18 V AtatatN48F F.lstes
Figure 2-9 2018 EFDC model LCFRP salinity station used to develop the salinity boundary time
series
OTETRA TECH 15 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
40 •
35 713.
3 30 - Av irpo y
I
25 -
a 15
10
5 i 1 F l l l l i l l l l l l l l i 1 i 1 t 1 1 1 1
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
Open Boundary
Figure 2-10 2018 EFDC model open boundary salinity boundary time series
Average salinity monitored between 2002-2015 was applied for periods lacking data(pre-2002 and post-2015).
2.3.4 Water Surface Elevation
Information about WSE is critical in determining the tidal dynamics throughout the Lower Cape Fear River
Estuary. The open boundary was forced with the hourly water levels(datum -MLLW)from the NOAA station
8658120 located in the Lower Cape Fear River Estuary at Wilmington, North Carolina (Figure 2-11). NOAA
station 8658120 is located approximately 27 miles north of the open boundary but is the closest station to the
open boundary that measured water levels. The timing of tidal crests and troughs can vary over 27 miles;
therefore, the amplitude and phase used at the open boundary were adjusted to improve the representation of the
tidal cycle throughout the system. Data gaps were filled with a reconstructed tide signal based on the seven main
harmonic tide constituents for the station. The phase and amplitude of the constituents were estimated based on
a least squared harmonic analysis of the available records.
l l TETRA TECH 16 November 16, 2018

Cape Fear Public Utility Authority Southside &Sweeney Effluent Mixing Analysis
a`O1�ND a„-N US qY�'
O
rMr
f{tt' taC.
Fn.J
jcland N �qr Sweeney WTP
IT '74 st , NC0002879
W IlnNrt Ytnyhtsvdlc B
gt 8658120
Southside WVJTP vacs
it
w ; NC0023973,
IOW
TA it
Witt
+till•
trans
tr+ur
tow./
td 1 tq Catdina Beach
•
alll/r11
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oak t,land. �.�.
• NOAA Station ^ t����*`
•
• �4:4:
NPDES Location �4
Com utattonalGrid �` °'
P Cmter: ay let reflect t.a!cral eecgrapr,cs C r;eet ep pcl: Sc,ram:
t,eticral Cecgrsphia.Esri CeLcarre.SERE.I EF-MCt:"C !'SCS.
------ ESA.t"ETI.JlA.0 . GEE CC.riCAA.i^ae,"eetF Cap
CF PU+;Gnut:on Ani:ys+s&tidy N 0 e 3 3_.030 .4.000 3e 300 <s 300
Pressure Station Location ^ case TETRA TECH
M't9d3Aa A'16F'. N 0 1.s i°3.i50 i.5a0 11,7.50 1'_,(b3
0533 Mi13t•. W3. M ��- r.let�,
Figure 2-11 2018 EFDC model NOAA WSE station used to develop the WSE open boundary time
series
ICI TETRA TECH 17 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
2.3.5 Meteorological Inputs
Meteorological data from the closest National Climate Data Center(NCDC)weather station, located
approximately 2.5 miles from the Sweeney WTP and 7.5 miles from the Southside WWTP,were used to develop
atmospheric conditions and wind time series files for the EFDC model (Figure 2-12). The data needed for the
2018 EFDC model development included:
• Precipitation,
• Pressure,
• Air temperature,
• Relative humidity,
• Wind speed and direction, and
• Cloud cover.
Data from NCDC-Surface Airways(SA),Weather Bureau Army Navy(WBAN) 13748,Wilmington International
Airport, North Carolina were used to develop the EFDC meteorological time series. Data for WBAN 13748 station
were downloaded from two sources and processed into the weather boundary time series:
• NOAA Local Climatological Data:
o Hourly records were available for the period of July 1996 through December 2013.
o The records from this period did not have significant gaps(e.g., not multi-day).
• NOAA Integrated Surface Database:
o Hourly records were available for the period January 1992 through June 1996 and from January
2014 through June 2018.
o Gaps in the precipitation, air temperature, dew point temperature,wind speed and direction, and
solar radiation data were less than three hours. Data gaps were filled using the average of the
hourly data before and after the data gaps.
o Longer gaps of three hours and less were present in the air pressure and cloud datasets.
• To fill the air pressure record gaps, altimeter setting(in millibars)was calculated from
station pressure data from NCDC site 13748 and the station elevation (9.14 meters)
using the appropriate conversion equations.
• Sky cover summation state records for NCDC site 13748 were used to fill multi-day gaps
in the cloud cover record.
QTETRA TECH 18 November 16, 2018

Cape Fear Public Utility Authority Southside &Sweeney Effluent Mixing Analysis
11/4
CAs Yn•-•
,048
11.1
'II 7
vi.tmn q100 cy.den.
IIIIMImington International AP
NCDC-SA: 13748
eland"la -
1.1 Sweeney WIP
NC0002879
•
wil
• ;
A
41/11
rill yitightsViRe Beach
ar- Southside WV
NC0023973
tu,6.0;
•-
VVrvsbc...44
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111/13
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soufisti ANA 7 f!,;!?1'1.1
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„.
5 nu
Legend ll7 •
tJPDES Location
Oil,I si Ana. t.
• Meteorological Station
Computational Grid
CFPUA Cilut,on Ana ySIS Stut N 3 e 0332 033 24 030 M 333 43 333
Meteorological Station Locabon
A Feet
TETRA TECH
3 18752 750 7.530 11 250 15.003 mit
1911.3 Flete's
Z:13 Xtan2....611e-
Figure 2-12 2018 EFDC model weather station used to develop the meteorological time series
lit)TETRA TECH 19 November 16, 2018

Cape Fear Public Utility Authority Southside& Sweeney Effluent Mixing Analysis
2.3.6 Point Sources
CFPUA's Sweeney WTP, Southside WWTP, and Northside WWTP were the only point sources represented in
the 2018 EFDC model (Figure 1-1). These point sources were included because the effluent mixing analysis is
focused on the Sweeney WTP and Southside WWTP, and the Northside WWTP is the only other point source
located near these facilities. CFPUA provided recent effluent flow and temperature records for Sweeney WTP and
Southside WWTP. The Northside WWTP historic effluent flow and temperature records were available in the 2008
EFDC model.
2.3.6.1 Effluent Flows
CFPUA provided sub-hourly flow data from the Sweeney WTP for the months of July through September for the
years 2015 through 2017. To maintain a conservative approach, the maximum observed daily average flow was
calculated from the facility records and applied as a steady-state discharge in the model. A constant discharge of
1.41 cfs was applied in the 2018 EFDC model.
CFPUA also provided monthly average flows from the Southside WWTP from January 1997 through June 2018.
A monthly time series was developed from the data and input into the 2018 EFDC model for the Southside
WWTP. Short-term gaps of three months or less were filled by averaging the before and after months. Data gaps
greater than three months were filled using long-term monthly average flows from 1997 through 2003 (limited to
these years because gaps were only present in the early years of the simulation).
Average effluent flow from the Northside WWTP was extracted from the 2008 EFDC model (Tetra Tech 2008)
and input into the 2018 EFDC model as a steady-state discharge for the full model period at a rate of 8.69 cfs.
2.3.6.2 Effluent Temperatures
CFPUA provided bi-monthly effluent temperature records for the Sweeney WTP from January 2015 through July
2018. To be consistent with the representation of flows from the facility, the average effluent temperature from
July through September was applied as a steady-state representative temperature. A constant temperature of
82.4°F (28°C)was applied in the 2018 EFDC model.
CFPUA provided monthly average effluent temperature records for the Southside WWTP, which were used to
generate a monthly average temperature series for the 2018 EFDC model. Short-term gaps of three months or
less were filled by averaging the before and after months. Any data gaps greater than three months were filled
using long-term monthly average temperatures from 1997 through 2003 (limited to these years because gaps
were only present in the early years of the simulation).
The 2008 EFDC model did not specify an effluent temperature for Northside WWTP, so the mean effluent
temperature from the Southside WWTP was applied (22°C).
2.3.6.3 JP-EFDC Representation
The point sources were input in the JP-EFDC model within the 2018 EFDC model to estimate the near-field
mixing within the plume from the Southside WWTP and Sweeney WTP. The Southside WWTP has two discharge
ports, each with a 24-inch diameter. However, the JP-EFDC model cannot represent more than one port.
Therefore, the Southside WWTP was represented as a single discharge port, which is a reasonable
representation due to the proximity and directionality of the ports. To produce the same exit velocity and the near-
field jet-plume behavior expected with multiple ports in the actual diffuser design, a single port with a 34-inch
diameter representative of to two 24-inch pipe diameters, was selected.
Field visits from Tetra Tech personnel and information provided by CFPUA indicated that the bottom of the
discharge pipe of Sweeney WTP was set at approximately five feet above the surface of the river. However, in
JP-EFDC, discharge pipes cannot be located above the WSE, therefore, the pipe was set to discharge at the
water surface in the model with the same vertical angle as the discharge pipe.
TETRA TECH 20 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
2.4 MODEL CALIBRATION
2.4.1 Calibration Stations
The 2018 EFDC model was calibrated to observations of hydrodynamic data collected during 2011, which is the
critical year for the EFDC model (refer to Section 1.1 for the discussion on defining critical period). The modeled
hydrodynamic outputs were compared to measured hydrodynamic data at NOAA, NCDEQ, and LCFRP
monitoring stations located throughout the Lower Cape Fear River Estuary. Model validation was conducted by
comparing the model to observed data for the modeling period of January 1, 1992 through June 30, 2018. Table
2-1 lists the calibration and validation stations used in the 2018 EFDC model. Figure 2-13 presents the location of
calibration and validation stations.
Table 2-1 2018 EFDC model hydrodynamic calibration and validation stations
Station ID Station Location Agency Parameter Purpose
NCF6 Northeast Cape Fear River LCFRP Salinity Calibration/Validation
near Wrightsboro, NC
Northeast Cape Fear River
B974000 at US 421 at Wilmington, NCDEQ Salinity, Calibration/Validation
NC Temperature
Cape Fear River at
Salinity,
HB Horseshoe Bend near LCFRP Calibration/Validation
Wilmington, NC Temperature
8658120 Wilmington, NC NOAA WSE, Calibration/Validation
Temperature
B980000 Cape Fear River CM 61 at NCDEQ Salinity, Calibration/Validation
Wilmington, NC Temperature
Cape Fear River at Channel Salinity,
M61 Marker 61 at Wilmington, LCFRP Temperature Calibration/Validation
NC
BRR Brunswick River dns NC 17 LCFRP Salinity, Calibration/Validation
at park near Belville, NC Temperature
B982000 Cape Fear River at CM 56 NCDEQ Salinity, Calibration/Validation
near Wilmington, NC Temperature
M23 Cape Fear River at Channel LCFRP Salinity, Calibration/Validation
Marker 23 Temperature
M18 Cape Fear River at Channel LCFRP Salinity, Calibration/Validation
Marker 18 _ Temperature
(lb)TETRA TECH 21 November 16, 2018

Cape Fear Public Utility Authority Southside &Sweeney Effluent Mixing Analysis
.1\\
"4"5 Cas
ciao*t as u.4
11-1 LCFRP ID:
NCF6
v.,.
, .. • ,
<, NCDECHD:, Og
89740000
•,. .land N I
LCFRP
.st Sweeney WTF
i7
.T4 HB • NC0002879
LCFRPlla: • N0X4 ID: virightsvtIle Beach
' •
BRR 8658120
NCDEQ/LCFRP ID.
B9800000IM61
Southside VATTP
kt. 0,
NCDED ID: ‘‘ NC002;
B9820000flII
nit
=
7,31.
4,104
EMI
'Ail •Cat ohna Beach
4 •
mvor
0 I
olft+419:gft t:
,0041,0117 ,t,41 star
LCFRP
M23 'Ai
Legend
A Calibration Stations
• tiPIDES Location 3
Computational Grid ect carertr-Ec rd.; Sc..•:s
Celame.1-ERE.Lt.EF-.•:CVC
:17EECC.1.CAA o^cre—e-'F Ccrr
CFPUA Diution Analysis Study, N3 e 303:330 .74•000 000 4.0 300
Fec
Calibration Stations Location
141 RA IFE-
3 18753 750 7 530 11 250 15 DOD
19 TV VS% k1etes
1:31 XIS A Xtasa.d
Figure 2-13 Location of calibration and validation stations used in the 2018 EFDC model
(11 TETRA TECH 22 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
2.4.2 Calibration Methodology
The 2018 EFDC model was calibrated using graphical and statistical comparisons between the model predictions
and the observations. The 2018 EFDC parameters were initially defined using values from the 2008 EFDC model
(Tetra Tech 2008). The model results were analyzed to determine how well the model captured the trends and
magnitudes of measured data, and the statistical results were reviewed to determine if there were biases
compared to measured data. Parameters were adjusted to improve the model performance.
The visual(temporal)analyses were performed using the postprocessor Water Resources Database(WRDB)
Graph(version 6.1.0.29),which creates comparison plots using the model results from the EFDC output file
(*.BMD2)and available field observations from a WRDB database. WRDB Graph was also used to generate the
following goodness-of-fit statistics based on the time series of model predictions(P)and the time series of
observations(0):
Coefficient of Determination: R —
z
2 — (n En 1(Pixoi))-(E 1PixE 1Oi)
J[nE 1(Pi2)-E;`_1(Pi)2]x[nEr 1(Oi2)-En 1(Oi)2]
Mean Absolute Error: MAE = En 11P`-oil
n
' (Pi-oi)2
Root Mean Squared Error: RMSE = I
n
Normalized Root Mean Squared Error: NRMSE = e osE*100
Index of Agreement: IA = 1.0 — E 11Pi-oi12 2
En 1[IPi-of+Ioi-ol]
The coefficient of determination (R2)is an estimate of the portion of the variance in the observations that is
explained by the simulations. It can range from 0 to 1. A value of 0 means that the simulations cannot explain the
variance of the observations. A value of 1 means that the simulations explains the variance of all the
observations.
The mean absolute error(MAE)and the root mean squared error(RMSE)are estimates of the average deviation
of the model predictions from the observations. The normalized root mean squared error(NRMSE) provides an
estimate of the relative importance of the errors with respect to the observations. The MAE, RMSE, and NRMSE
constitute indicators of model prediction accuracy(Stow et al. 2003), and the smaller their values, the higher the
agreement between the observations and the predictions. Finally, the index of agreement(IA)evaluates the
global agreement between the predictions and the observations. Values of the IA range between 0 and 1,with the
highest value indicating a perfect match between the two time series.A value of zero indicates that the model
predicts individual observations no better than the average of the observations.
n TETRA TECH 23 November 16,2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
2.4.3 Water Surface Elevation
The 2018 EFDC hydrodynamic model was calibrated to reproduce WSE using data collected by NOAA at station
8658120 in Wilmington, North Carolina, the only location where WSE data were available within the model
domain. During the calibration,the phase and amplitude of the forcing conditions imposed at the open boundary
were adjusted to match the phase and amplitude of the water level observations. The datum used for water
surface elevation and bathymetry was MLLW.
The open boundary conditions had to be adjusted because they were constructed using the hourly records of
WSE collected at the NOAA station in Wilmington, North Carolina, located 27 miles upstream of the open
boundary. A delay was introduced in the signal by adjusting the phase of the observations to account for the time
it takes a tidal wave to propagate from the open boundary to NOAA station 8658120. A summary of the calibration
and validation statistics at WSE stations are presented in Table 2-2. Figure 2-14 and Figure 2-15 show the WSE
comparison for the calibration period of January 1, 2011 through December 31, 2011. Figure 2-16 shows the
WSE comparison for the validation period of January 1, 1992 through June 30, 2018.
The calibration results for the period of January 1, 2011 through December 31, 2011 indicated that the 2018
EFDC model was capable of reproducing WSE with a high degree of agreement with the observations. Water
levels during high freshwater inflow periods, such as that observed during September 1996 and 1999, were
overestimated at the Wilmington station (Figure 2-16), but the model successfully reproduced the phase and
amplitude of the observations during spring and neap tide periods, and the statistics of model performance were
very good with correlation coefficients above 0.95, lAs above 0.98, and NRMSE below 0.15(Table 2-2). The
statistics of model performance and the visual comparison of the model predictions versus the observations of
WSE were very good.
OTETRA TECH 24 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
12
F01IIIIiIIII .III . .IIII,
d .
W
10
u 2 ``\\ 1
; `
id
3 -2
-4
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-14 Simulated and observed WSE at Wilmington, NC, NOAA 8658120, 01/2011 — 12/2011
12
3 10 -
,11
Z
8
v 6
c ••
a w
A 4
d . •~
W
u 2 .
to 0 - •« - •• •..
•
I-
d
a+
3• -2
-4
13 14 15 16 17 18 19
June 2011
Date
• Measured Simulated
Figure 2-15 Simulated and observed WSE at Wilmington, NC, NOAA 8658120, June 2011
NTETRA TECH 25 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
e2
I:: IiiIiIIpp. . IIIII
m 4
u 2
0 . . •
I i
{ ,
•
-2
a, -4
3 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
Measured Simulated
Figure 2-16 Simulated and observed WSE at Wilmington, NC, NOAA 8658120, 01/1992 —06/2018
n TETRA TECH 26 November 16, 2018

eta C.)rn Co 0
cn cn aiCD
m C — T
-.IN N.) O mA3 O O G1
D _
m a -O
n cr
Mean
Ni Ni C
Measured
Ai..,.) '
CO (MLLW-ft)
Median C
iv
J Measured
cn co (MLLW-ft) v
a-
5th Percentile o
N
o Measured ry
CO o (MLLW-ft)
95th Percentile N
0
J Measured CO
CS)' C`i (MLLW-ft) m
— c_ = T
= cu Mean 0
Ni m Ni < Simulated 3
w 0 Co (MLLW-ft) 0
— N CD
0
Median
co
Ni N) N , Simulated (n
N 1 cn co
v (MLLW-ft) m
— t — 0 n
m
3 5th Percentile iT
O c,, o CD Simulated
iv
N Ni ' (MLLW-ft)
— o v
o Ni 95th Percentile a
Simulated v
- (MLLW-ft) v
O o 0 0
ifl ifl D -
01 s
p1 v'
iz
(7).. lD
O O — m U'
'co w o
rn 01 o
— — -- - m
0
O o 1- 0 m
z N Ni m N
O o
CD Z
cr •
X
co O O
0 co
cn cn m D
rn _ — D
Ni v
O O O
5 `
CO CO CO N.

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
2.4.4 Water Temperature
The 2018 EFDC model was calibrated to represent heat distribution using temperature measurements collected
by NOAA, NCDEQ, and LCFRP at locations throughout the estuary. The fundamental parameters adjusted during
the model calibration were the solar radiation attenuation coefficient and the heat transfer coefficient between the
water column and the solid bed. These parameters control most of the vertical structure of heat in the model
(Hamrick 1992; Ji 2008). A summary of the calibration and validation statistics at temperature stations are
presented in Table 2-3. Figure 2-17 through Figure 2-24 show the comparison for water-column averaged
temperature for the calibration period of January 1, 2011 through December 31, 2011, except for stations LCFRP
HB, and NOAA 8658120 where surface temperatures were measured. Figure 2-25 through Figure 2-32 show the
comparison for the water-column averaged temperature for the validation period of January 1, 1992 through June
30, 2018.
The calibration results indicated that the model was capable of reproducing with high precision the temperature
variations observed in the evaluated stations. The calibration statistics were excellent at most stations, with R2
close to 1.0, lAs close to 1.0, and RMSEs around 1°F to 2°F. Also, the computed observed and simulated means
suggested that the predictions were unbiased with respect to the observations. These results were comparable to
those reported for other calibrated estuarine models(Blumberg et al. 1999; Kim 2013; Camacho et al. 2014a).
f lbl TETRA TECH 28 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
100
•
90 I80T
E 70 -
a
•
t 60 -
E
g 50 -
A
40 -
30
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-17 Simulated and observed temperature at Northeast Cape Fear River at US 421 at
Wilmington, NC, NCDEQ B974000, 01/2011 — 12/2011
100
90 •
-
u •
g 80
vp
E 70 - •
60 - •
dl •
a- 50 -
a
40
30 • •
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-18 Simulated and observed temperature at Cape Fear River at Horseshoe Bend near
Wilmington, NC, LCFRP HB, 01/2011 — 12/2011
QTETRA TECH 29 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
100
90
v_
I 80
E 70
y
a 60
E
50
A
40
30
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-19 Simulated and observed temperature at Wilmington, NC, NOAA 8658120, 01/2011 —12/2011
100
90
80
• •
vy
G 70
a 60
50
40
•
30
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ,
2011
Date
• Measured(M61) • Measured(B9800000) Simulated
Figure 2-20 Simulated and observed temperature at Cape Fear River CM 61 at Wilmington, NC,
NCDEQ B980000, and LCFRP M61, 01/2011 — 12/2011
OTETRA TECH 30 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
100
90 -
I80
E 70
Its - , '
E
I- 50
3 .
40
30 -
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-21 Simulated and observed temperature at Brunswick River dns NC 17 at park near Belville, NC,
LCFRP BRR, 01/2011 — 12/2011
100
90
mg 80
F
♦u 70
lD
a 60 '
1- 50
B
40
30
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-22 Simulated and observed temperature at Cape Fear River at CM 56 near Wilmington,
NC, NCDEQ B982000, 01/2011 — 12/2011
TETRA TECH31 November 16, 2018

Cape Fear Public Utility Authority Southside &Sweeney Effluent Mixing Analysis
100
•
90 -
LL
80 -
an
0
` 70 -
z
60 -
E
ai
y 50 -
to
3 '
40 -
30
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-23 Simulated and observed temperature at Cape Fear River at Channel Marker 23, LCFRP M23,
01/2011 —12/2011
100 — -- — — — ---
90
LL
aJ
80
a) 70
ro
f0
L.
a 60
50
ro
3 1 '
40
30 —Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-24 Simulated and observed temperature at Cape Fear River at Channel Marker 18, LCFRP
M18, 01/2011 — 12/2011
ICI TETRA TECH 32 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
s 100 , , . , r , 1 ' :
C.)
a) 90
a,
0 80 li,N,11 1\1 f\1\1111tit
ai
70 1\1\rt \)
•
a3 60 '
a
— 50
, t
CC) 40 - •
�a
3 30 I I I I I I + f + I + + + + + + I -I I + + + '
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
• Measured Simulated
Figure 2-25 Simulated and observed temperature at Northeast Cape Fear River at US 421 at
Wilmington, NC, NCDEQ B974000, 01/1992— 06/2018
u. 100 ,
•
a)
I) E
, • •
al � li iIifi'ililliii\il\ii\lii ,�� i
a 1
60 ii, i 11411 .111
50 0 1 111 ,1 iii
a) 40 ._ . -
•
3 30 ; I I I i i i I i I I -I 1 iI If I I I
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
• Measured Simulated ]
Figure 2-26 Simulated and observed temperature at Cape Fear River at Horseshoe Bend near
Wilmington, NC, LCFRP HB, 01/1992 —06/2018
I I TETRA TECH 33 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
r 100 ,
90
80 •
cti 607
f, 50 Y
W 40
3 30 i i i i i i i i i I
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
• Measured Simulated
Figure 2-27 Simulated and observed temperature at Wilmington, NC, NOAA 8658120, 01/1992—
06/2018
LL 100
I- 90
I
80 70 - %10111\11\1\1
!to
40
f0 30
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
Measured ( M61) • Measured (B9800000)
Simulated
Figure 2-28 Simulated and observed temperature at Cape Fear River CM 61 at Wilmington, NC,
NCDEQ B980000, and LCFRP M61, 01/1992 —06/2018
l )TETRA TECH 34 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
100 , . . , , , . . . , , , , . , , , , , , , , , , ,
c 90 :cri t)\(111\1
0 80 =
2, 70
to
ill\1\1\1\IPINNI
w 50 -
a�`� 40 - , \IIVV\I
30 : if I I I E I I I I f I Ii I I I I I I I I I I I
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
• Measured Simulated I
Figure 2-29 Simulated and observed temperature at Brunswick River dns NC 17 at park near
Belville, NC, LCFRP BRR, 01/1992 —06/2018
100 ,
•
L 90
al
0 80w tb\I 1\el\i\ii z ,t i i i\it, i 4 11J il I
1 1\1)
L 70
to tii /is i * 1 \t \r ,\1, \I :‘
a£i 50 -11\1 ( i
3 30 1 1 1 1 1 1 1 IIIIIIIII I I I I I I 1 I
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
• Measured Simulated
Figure 2-30 Simulated and observed temperature at Cape Fear River at CM 56 near Wilmington,
NC, NCDEQ B982000, 01/1992 —06/2018
TETRA TECH 35 November 16, 2018

Cape Fear Public Utility Authority Southside & Sweeney Effluent Mixing Analysis
100 ( 11111111 .
v :
II 90
i"):71 80 i\ V ' #II##N#
70
11:1 1 f
at 60II 11 It
v 50
16 40
30 i r i
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
• Measured Simulated
Figure 2-31 Simulated and observed temperature at Cape Fear River at Channel Marker 23, LCFRP
M23, 01/1992 —06/2018
100 < , , , ,
v
i 90 .
oso Akit
.\(1 \ ttlf •
A I\
t 60
a
E50
40
30 + III
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
L • Measured Simulated
Figure 2-32 Simulated and observed temperature at Cape Fear River at Channel Marker 18, LCFRP
M18, 01/1992 —06/2018
(mil TETRA TECH 36 November 16, 2018

Cape Fear Public Utility Authority Southside & Sweeney Effluent Mixing Analysis
Table 2-3 2018 EFDC model temperature calibration and validation statistics
I
_ d a, _ _ _ a, _ _a _
N LL C N C a1 G a) Ci 3 C a C a� . 0
gc ►- N a 2 a> d E d m 2 m c r m C a) m e w a> r 6 MAE RMSE
= d a N m - = a) O = L ca w m a m m - d d
Station ID 6 y y a) 3 R2 (Degree (Degree NRMSE IA
2 2 a0) i y QQi a y ?,, - y W 2 E aa, E d a E m a E GGi F)
EC EC 20 2 o o in o in 0 o F)
January 01, 2011 -December 31, 2011
B974000 65.60 68.61 43.08 86.43 65.61 67.11 44.90 85.72 1.00 0.96 1.17 0.02 1.00
HB 74.58 81.44 42.67 87.76 73.60 80.07 43.18 87.66 0.99 1.43 1.85 0.03 1.00
8658120 68.04 72.86 43.16 86.00 67.59 71.93 44.90 85.76 0.99 1.13 1.42 0.02 1.00
B980000 65.59 68.46 43.65 86.49 65.00 66.73 44.27 85.58 1.00 0.90 1.10 0.02 1.00
M61 72.25 77.99 44.85 86.00 71.64 77.47 45.73 85.69 0.99 1.00 1.29 0.02 1.00
BRR 68.30 72.88 44.01 86.50 69.28 72.72 44.88 89.65 0.98 2.04 2.49 0.04 0.99
B982000 65.64 68.31 43.78 86.52 65.38 67.67 44.67 85.84 1.00 0.91 1.02 0.02 1.00
M23 71.81 79.18 45.79 85.10 71.79 78.72 46.29 85.63 1.00 0.44 0.53 0.01 1.00
M18 71.48 78.60 45.79 85.14 71.69 78.73 46.09 85.51 1.00 0.32 0.40 0.01 1.00
January 01, 1992-June 30. 2018
B974000 67.15 70.83 45.06 84.71 67.95 70.65 47.28 85.48 0.97 1.60 2.29 0.03 0.99
HB 72.31 78.44 47.84 86.00 72.54 78.46 48.07 86.47 0.98 1.23 1.98 0.03 0.99
8658120 67.19 71.42 44.24 85.46 67.77 71.65 47.03 85.21 0.97 1.85 2.50 0.04 0.99
B980000 66.70 70.28 45.31 84.95 66.97 69.40 46.77 85.08 0.97 1.54 2.22 0.03 0.99
M61 69.73 75.56 47.07 85.46 69.82 75.10 48.25 86.05 0.99 1.17 1.54 0.02 1.00
BRR 66.88 70.34 44.78 85.64 68.28 71.36 46.26 90.31 0.97 2.25 2.95 0.04 0.99
B982000 66.65 69.58 45.82 84.56 67.14 69.49 47.56 85.07 0.98 1.51 2.11 0.03 0.99
M23 70.09 76.46 49.10 84.74 70.30 76.18 49.28 84.80 0.99 0.82 1.10 0.02 1.00
M18 70.14 76.21 49.39 84.56 70.31 76.34 49.67 84.75 1.00 0.59 0.82 0.01 1.00
it TETRA TECH 37 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
2.4.5 Salinity
Salinity is an indicator of mass transport in estuaries; therefore, a correct representation of its distribution is one of
the most important goals during the development of hydrodynamic models. The calibration of salinity is
particularly important to evaluate the capacity of the model to represent transport caused by density gradients
between the freshwater and the ocean water(i.e. baroclinic or density-driven currents), and to ensure the model
can represent vertical stratification in the system.
The 2018 EFDC model was calibrated to reproduce the temporal and spatial variability of salinity in the system.
The calibration was performed based on direct observations of salinity records available at NCDEQ and LCFRP
stations(Figure 2-13). During calibration, no adjustments were made to the parameters or coefficients for salinity
from the 2008 EFDC model because the comparisons between the measured and modeled salinity
concentrations were very good.
The calibration results are presented at the NCDEQ and LCFRP monitoring stations from Figure 2-33 through
Figure 2-39 and in Table 2-4. The validation results are presented at the NCDEQ and LCFRP monitoring stations
from Figure 2-40 through Figure 2-46 and in Table 2-4. Monthly average salinity records were available at the
LCFRP stations. Therefore, for visual comparisons, modeled salinity from the surface layer was compared to the
monthly average salinity values at the LCFRP stations. However, to calculate statistics, monthly averages from
the modeled salinity were used.
The calibrated model successfully reproduced the distribution and temporal dynamics of salinity in the system.
The level of intrusion was correctly represented by the model as can be inferred from the salinity comparisons
developed in the upstream portion of the estuary at LCFRP station NCF6(Figure 2-33 and Figure 2-40). The
small salinity values measured at LCFRP stations on the riverine portion of the rivers(less than 5 PSU)were
slightly overpredicted by the model but were overall well represented, suggesting the salinity intrusion was weak
in this area of the estuary. As expected, the salinity intrusion was higher at the mouth of the estuary with values
above 20 PSU.
In most of the evaluated stations, the model was able to match the timing and magnitude of the salinity
observations. The computed RMSEs were small with values below 2 PSU to 4 PSU,the R2 values and IA were in
general good, and the predictions were unbiased or had a small bias with respect to the observations(Table 2-4).
The difference between the measured and simulated means provided the approximate indication of the bias of the
model and indicated that the model slightly overpredicted salinity at stations NCF6, HB, BRR, and M61, with all
the other stations not presenting any significant bias.
OTETRA TECH 38 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
30
25
20 - -
Z 15
c
10
5
•
0 •
, Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-33 Simulated and observed salinity at Northeast Cape Fear River near Wrightsboro, NC,
LCFRP NCF6, 01/2011 — 12/2011
35
30 - -
25
204046010001\110110/1111
v
15
10
5
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-34 Simulated and observed salinity at Northeast Cape Fear River at US 421 at Wilmington,
NC, NCDEQ B97400O, 01/2011 — 12/2011
OTETRA TECH 39 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
30
25 -
20 -
16iii
Z 15 -
c
10
5
0 •
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ,
2011
Date
• Measured Simulated
Figure 2-35 Simulated and observed salinity at Cape Fear River at Horseshoe Bend near Wilmington, NC,
LCFRP HB, 01/2011 —12/2011
35
30
25
a 20 - • •
•
C 15
lit411°\11\til
10 -
5
•
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured(M61) • Measured(B9800000) Simulated
Figure 2-36 Simulated and observed salinity at Cape Fear River CM 61 at Wilmington, NC, NCDEQ
B980000, and LCFRP M61, 01/2011 — 12/2011
TETRA TECH 40 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
30
25
20
z. 15
`c iiiiiiillifil?"1#1111\111111)\ti)111.
11,0001
10 111111
5
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-37 Simulated and observed salinity at Brunswick River dns NC 17 at park near Belville, NC,
LCFRP BRR, 01/2011 — 12/2011
30
25 - -
20 2
z. 1510 -liti\iiiiiiii40/110101011y16v!
5
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-38 Simulated and observed salinity at Cape Fear River at CM 56 near Wilmington, NC,
NCDEQ B982000, 01/2011 — 12/2011
OTETRA TECH 41 November 16,2018

Cape Fear Public Utility Authority Southside& Sweeney Effluent Mixing Analysis
40
30 - \A"
z. 20 - •
•
10 -
0
, Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2011
Date
• Measured Simulated
Figure 2-39 Simulated and observed salinity at Cape Fear River at Channel Marker 23, LCFRP M23,
01/2011 — 12/2011
35
30
D 25 N 1 I
Et 20
• 15
iTo •
10
5 ( i
0
� I ► I �� , ill • iI . ,�
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
[ • Measured Simulated
Figure 2-40 Simulated and observed salinity at Northeast Cape Fear River near Wrightsboro, NC,
LCFRP NCF6, 01/1992— 06/2018
TETRA TECH 42 November 16, 2018

Cape Fear Public Utility Authority Southside& Sweeney Effluent Mixing Analysis
35 , , , , , , , , , , , , , , , , , , , , , , , , ,
30 -
25 - ( -
cn
20 -
I
1 •
•
c 5 • I
cn 10 i
, 1
5 ' ( I 1 i j
'A i i41111 ;,, . ii iiiii: ill il 1 1 Willii
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
• Measured Simulated
Figure 2-41 Simulated and observed salinity at Northeast Cape Fear River at US 421 at Wilmington,
NC, NCDEQ B974000, 01/1992 - 06/2018
35 , , : ! 1 1 ! I , 1 t .
.
30 a
25 - F---i--4--- ----F---4,----f---1- 1 f
N
11 20 _
'E 15 j
0 10
5 H. A ..:.'i. IL II i . i.klil d
, it L, 14' I t ,'
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
F. Measured Simulated
Figure 2-42 Simulated and observed salinityat Cape Fear River at Horseshoe Bend near
9 P
Wilmington, NC, LCFRP HB, 01/1992 -06/2018
lb TETRA TECH 43 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
35 _
•
30
S 25 = •
cn •
20 - •
r I •
= 15 r • • I I
(/) 10 I �!Irl I I � �1
ill
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
• Measured (M61) • Measured (B9800000)
Simulated
Figure 2-43 Simulated and observed salinity at Cape Fear River CM 61 at Wilmington, NC, NCDEQ
B980000, and LCFRP M61, 01/1992 —06/2018
35 , , , , , , , , , , , , , , , , , , , , ,
30
3 25
U
0 20
E 15 •
cn 10 ,
L. LfliILU I .1 �- ,l- i ►�
IiUj.1P
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
• Measured Simulated
Figure 2-44 Simulated and observed salinity at Brunswick River dns NC 17 at park near Belville, NC,
LCFRP BRR, 01/1992—06/2018
TETRA TECH 44 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
35 , , , , , , , , , , , , , , , , , , , . , , , ,
30
25 - •
v 20 - (11
i
15 I .
1 ., 1 i
._ .iliti. J 1
cA 10 �I 'i1 ]1 ,
I
1 I Ii
)i1
1111
I
• 1 •
0 It • - .I 1 1 1 I 14 1 1 1 41
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
L • Measured - Simulated j
Figure 2-45 Simulated and observed salinity at Cape Fear River at CM 56 near Wilmington, NC,
NCDEQ B982000, 01/1992 - 06/2018
40 1 1
i
30 _ I 1 1
I ,------- -
•
•
I :: i!J1T1 _. (! (t1
. r 1 •
0 14 1 1 1 1 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I
1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
Date
• Measured Simulated I
Figure 2-46 Simulated and observed salinity at Cape Fear River at Channel Marker 23, LCFRP M23,
01/1992-06/2018
I'1>41 TETRA TECH 45 November 16, 2018

If) O O CO ,-- (fl O �t N N r- (f) r- O) N
co O O O co O O co co O co co co co co co _�
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T N
(D
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Q W N Cr) CO N O N O I- co O CO O N 10 N-
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C 2 O O O O O O O O O O O O O O O O
ce
.f Z a)
c >
0
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Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
3.0 MODEL APPLICATION: EFFLUENT MIXING ANALYSES
The JP-EFDC model is based on the Lagrangian formulation used in the Updated Merged(UM)component of the
U.S. Environmental Protection Agency(USEPA)PLUMES model(Tetra Tech 2001).The JP-EFDC predicts the
average properties of the plume by conserving horizontal and vertical momentum,conservation of mass, and
conservation of tracer mass/heat(Lee and Cheung 1990).
The calibrated and validated 2018 EFDC model was used to evaluate the near-field and far-field mixing and
transport of the Southside WWTP and Sweeney WTP. The concept of dilution is used to quantify the degree of
mixing and the transport of the effluent discharge in the Lower Cape Fear River Estuary system. The dilution of
the effluent discharge was determined by the following equation:
D = Ce
C
where, D is the dilution ratio,
Ce is the concentration of a conservative constituent in the effluent, and
C is the in-stream concentration of a conservative constituent.
Discharges and receiving water conditions were evaluated for the individual facilities to consider critical flow
conditions for the dilution analysis study. Section 1.1 discusses the evaluation of critical period for the dilution
analysis study.
Average, 10th percentile, and 90th percentile dilution ratios using a four-day moving average calculated during the
critical period of July 2011 were summarized at select distances from the facility locations.A four-day average
dilution value was used for the scenario analysis based on recommendations of USEPA's Technical Support
Documents for mixing zone analysis (USEPA Technical Support Documents 1991). The dilution ratios were
evaluated for 12, 16, 20, and 24 MGD discharge flows from the Southside WWTP, along with the peak 24-hr
average flows permitted for the Sweeney WTP discharges. This will provide the dilution analysis ratios for the
expected highest discharges and the low critical ambient flows.
OTETRA TECH 47 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
3.1 EFFLUENT MIXING ANALYSES
The effluent mixing analyses were conducted using the JP-EFDC model which is nested in EFDC. The JP-EFDC
sub-model allows for the two-way interaction between near-field and far-field processes. For the 2018 JP-EFDC
model simulations for the Sweeney WTP and Southside WWTP, a conservative tracer with a concentration of Ce
was introduced into the effluent discharges. The hydrodynamic and transport model was then used to simulate
the distribution of the tracer in the system for the critical period of July 2011. June 2011 was also run as a spin-up
period.
The near-field dilution analysis was conducted to determine the mixing and dilution of the effluent discharge in the
immediate vicinities of the Sweeney WTP and Southside WWTP discharge locations. The 2018 JP-EFDC model
outputs were used for the near-field mixing, and the 2018 EFDC model outputs were used for the far-field mixing.
For continuous results between the near and far-field regions, outputs from the two models were interpolated
between the furthest near-field value and the EFDC model value of the grid cell where the dischargers are located
(Tetra Tech 2008).
Assuming a first order dispersion equation, the dispersion coefficient(oc)at the four layers is calculated as
follows:
log(C-)
a= —
(REFoc—R)2
where,
C is the far-field tracer concentration at a distance REFDc from the discharge,
Co is the tracer concentration from the effluent discharge at the end of the near-field plume,
RFDDC is the radius of the EFDC grid cell calculated using the length and width of the cell (dx, dy)as
REFDC = (dx.dy/n), and
R is the radial distance of the jet plume.
Using the dispersion coefficient of the jet plume, the concentrations at select points of interest in the four layers
were calculated based on the same first order dispersion equation rearranged to solve for the concentration of the
tracer:
C = Coe—«(R—Ro)2
where,
C is the far-field tracer concentration at a select distance R from the discharge,
Co is the tracer concentration from the effluent discharge at the end of the near-field plume,
oc is the dispersion coefficient as calculated above,
R is the select distance from the effluent discharge, and
Ro is the radial distance of the jet plume at the end of the near-filed plume
The tracer concentrations were averaged from the four layers and a four-day moving average was calculated for
July 2011.
3.2 EFFLUENT MIXING ANALYSIS RESULTS
The dilution ratios were calculated for potential discharge flows of 12, 16, 20, and 24 MGD from the Southside
WWTP, as well as the peak 24-hour average flow permitted discharge for the Sweeney WTP. For the duration of
QTETRA TECH 48 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
the critical period, four-day moving average dilution ratios were computed. The average, 10th percentile, and 90th
percentile dilution ratios were assessed from the four-day moving averages in July 2011. Dilution ratios were
calculated at near-field locations in close proximity to the outfalls, and at distances that were one-third and one-
half the width of the river to provide information about effluent dilution further away from the outfalls. At the
Southside WWTP, the width of river is 500 meters(1,640 feet),while at the Sweeney WTP the width of the river is
180 meters(590 feet). The Southside WWTP dilution ratios were calculated at 25 meters(82 feet), 75 meters
(246 feet), 167 meters(548 feet), and 250 meters(820 feet)from the discharge pipe. The Sweeney WTP dilution
ratios were calculated at distances of 10 meters(33 feet), 25 meters(82 feet), 60 meters(197 feet), and 90
meters(295 feet)from the discharge pipe.
Figure 3-1 and Figure 3-6 show dilution ratios at the Southside WWTP and Sweeney WTP during existing
discharge rates, respectively. Figure 3-2 through Figure 3-5 present the dilution ratios at the Southside WWTP
when the effluent discharge was set at 12, 16, 20, and 24 MGD. In the figures, dilution ratios are on the left Y-axis
and WSE on the right Y-axis. The WSE was included in the figures to show the dilution ratios during the spring-
neap tidal cycle during the critical flow period of July 2011.
Table 3-1 summarizes the average, 10th percentile, and 90th percentile dilution ratios for the selected distances
from the Southside WWTP and Sweeney WTP for the existing conditions. Table 3-2 through Table 3-5 summarize
the average, 10th percentile, and 90th percentile dilution for the selected distances from the Southside WWTP for
discharge flows of 12, 16, 20, and 24 MGD.
The dilution ratio for the Southside WWTP was the lowest at 25 meters(82 feet)from the facility with an average
dilution ratio of 2.2 and 10th percentile ratio of 2.1 (Figure 3-1, and Table 3-1 ). The low dilution ratios are caused
partly by the shallow depth of water at the discharge location. The depth of water where the Southside WWTP
discharges is on average 1.2 meters(4 feet). At this shallow water depth, salinity intrusion causes the plume to
become trapped near the surface and does not let the plume diffuse and dilute. The Southside WWTP dilution
ratio was highest at 250 meters(820 feet)from the facility, one-half the distance of the river width. Here,the
average was 702 and the 90th percentile was 874. The small dilution ratios at 25 meters and 75 meters indicate
that the discharge conditions(i.e. discharge volume and velocity) control the mixing extent at these locations, but
at 167 meters the effluent mixing is likely controlled by buoyant spreading and passive diffusion caused by
ambient turbulence(Figure 3-1). As the plume moves farther away from the discharge location, the dilution ratios
are influenced by the tides and salinity intrusion.
Increases in discharge flows from the Southside WWTP resulted in higher dilution ratios within 75 meters of the
discharge locations, and lower dilution ratios further out(Figure 3-2 through Figure 3-5, Table 3-2 through Table
3-5). The higher discharge flows resulted in higher velocities, which increased the mixing in the near-field. At
longer distances, this immediate velocity impact is negated, and the higher flows diminish the ability of ambient
conditions to diffuse the plume.
The average dilution ratios for the Sweeney WTP are 8.3 at 10 meters and 9.3 at 25 meters(Figure 3-6 and Table
3-1).With the water depth an average of 10.5 meters deeper at the Sweeney WTP discharge location than the
Southside WWTP and a smaller discharge rate, the mixing in the near-field is less effected by the characteristics
of the discharge jet. Further out at 60 meters and 90 meters, the average dilution ratios are 29.8 and 1,045,
respectively.
OTETRA TECH 49 November 16,2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
1,100 - 4A
1,000
600
7
6A
00
`o 600
2.5W
0 600 t ;
2.0
300 I
I
200 1.6
100
• • .• • • ♦._yam-_.___._.— = ♦---'--•--.-i
0 • .�- - • • _ • • - - • - • • - - • 10
7/1/2011 7/612011 7711/2011 71112011 7/2112011 7/26/2011 7131/2011
Date
- -25m from Discharge 75m from Discharge 167m from Discharge -.-260m from Discharge —WSE
Figure 3-1 Dilution ratios for the Southside WWTP during existing conditions using four-day moving
average
OTETRA TECH 50 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
1,100 4.0
1,000
900 3.6
800
3.0
700
co 600
8 26W
0 600
Opp _1 ._ <
20
300 •
\
200 1.6
100
711/2011 7/612011 7/11/2011 711612011 7121/2011 7r26/2011 7/31/2011
Date
-.-26m from Discharge 76m from Discharge 167m from Discharge - 260m from Discharge —WSE
Figure 3-2 Dilution ratios for the Southside WWTP at 12 MGD using four-day moving average
1,100 4.0
1,000
900 3.6
800
3.0
700
10 : •
400
2.0
300
•
100 _
0 •-+ i...._•. 4 Y' * f • ► • ♦ �-t.` r-4 • r 4 4 ♦ •-a 1.0
71112011 7/6/2011 7111/2011 7118 2611 712112011 7/26/2011 7/31/2011
Date
-.-25m from Discharge 76m from Discharge 167m from Discharge 260m from Discharge —WSE
Figure 3-3 Dilution ratios for the Southside WWTP at 16 MGD using four-day moving average
n TETRA TECH 51 November 16,2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
1,100 4.0
1,000
900 3.6
800
7
3.0
00
2.6 w
O 600
400
2.0
300
200 ` 1.6
•
100
O . = r i; i 4- 4 1 i i 1 i i ± T 2—f—f" ► i-t -i ♦ 1.0
711 r20t 1 7I6/2011 711112011 7116/2011 7/21/2011 7/26/2011 7131/2011
Date
-.-26m from Discharge . 76m from Discharge 167m from Discharge -.-260m from Discharge —WSE
Figure 3-4 Dilution ratios for the Southside WWTP at 20 MGD using four-day moving average
1,100 4.0
1,000
900 3 6
800
3.0
700
c 600
2.6 W
▪ 600 ai
3
400
2.0
300
200 1.6
100 . . •
O - e • a
71112011 7/612011 7/11/2011 7/16/2011 7121/2011 7/26/2011 7131/2011
Date
-.--26m from Discharge 76m from Discharge 167m from Discharge • 260m from Discharge —WSE
Figure 3-5 Dilution ratios for the Southside WWTP at 24 MGD using four-day moving average
OTETRA TECH 52 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
1,400 —4.0
1,200 -3.6
1,000 `y.
800
Z6
5
600
-2.0
400
- 1.6
200
71112011 71112011 7/11/2011 7116/2011 712112011 7/26/2011 7131/2011
Date
—.-10m from Discharge 25m from Discharge 60m from Discharge 90m from Discharge—WSE
Figure 3-6 Dilution ratios for the Sweeney WTP during existing conditions using four-day moving
average
OTETRA TECH 53 November 16, 2018

Cape Fear Public Utility Authority Southside& Sweeney Effluent Mixing Analysis
Table 3-1 2018 JP-EFDC model predicted dilution ratios at Southside WWTP and Sweeney WTP for existing conditions
Scenario Southside WWTP Sweeney WTP
Distance 25 m 75 m 167 m (548 ft); 250 m (820 ft); 10 m 25 m 60 m (197 ft); 90 m (295 ft);
from Outfall (82 ft) (246 ft) 1/3 river width 1/2 river width (33 ft) (82 ft) 1/3 river width 1/2 river width
Average* 2.2 3.6 33.6 701.8 8.2 9.3 29.8 1,045.0
1 0th
Percentile*
2.0 3.4 28.4 471.6 7.2 8.2 27.2 949.4
90'h
2.3 3.9 38.9 873.6 9.3 10.4 31.9 1,179.6
Percentile*
* Calculated using four-day moving averages
Table 3-2 2018 JP-EFDC model predicted dilution ratios at Southside WWTP discharging at 12 MGD
Scenario Southside WWTP
Distance 25 m 75 m 167 m (548 ft); 250 m (820 ft);
from Outfall (82 ft) (246 ft) 1/3 river width 1/2 river width
Average* 2.6 4.1 26.2 326.8
10'h
2.5 3.8 21.8 231.8
Percentile
90'h
2.8 4.3 30.5 426.1
Percentile*
*Calculated using four-day moving averages
Table 3-3 2018 JP-EFDC model predicted dilution ratios at Southside WWTP discharging at 16 MGD
Scenario Southside WWTP
Distance 25 m 75 m 167 m (548 ft); 250 m (820 ft);
from Outfall (82 ft) (246 ft) 1/3 river width 1/2 river width
Average' 3.0 4.4 22.1 200.2
10th
Percentile'
2.9 4.1 18.4 147.6
90'h
Percentile'
3.2 4.7 25.3 242.0
* Calculated using four-day moving averages
TETRA TECH 54 November 16, 2018 I,I

Cape Fear Public Utility Authority Southside &Sweeney Effluent Mixing Analysis
Table 3-4 2018 JP-EFDC model predicted dilution ratios at Southside WWTP discharging at 20 MGD
Scenario Southside WWTP
Distance 25 m 75 m 167 m (548 ft); 250 m (820 ft);
from Outfall (82 ft) (246 ft) 1/3 river width 1/2 river width
Average* 3.3 4.6 19.5 129.7
10th
Percentile"
3.1 4.3 16.4 104.1
90th
Percentile 3.6 5.0 22.6 152.1
* Calculated using four-day moving averages
Table 3-5 2018 JP-EFDC model predicted dilution ratios at Southside WWTP discharging at 24 MGD
Scenario Southside WWTP
Distance 25 m 75 m 167 m (548 ft); 250 m (820 ft);
from Outfall (82 ft) (246 ft) 1/3 river width 1/2 river width
Average" 3.6 4.8 17.6 93.7
lath
Percentile'
3.4 4.5 15.1 75.4
Bath
Percentile'
3.9 5.2 20.6 116.1
* Calculated using four-day moving averages
Th TETRA TECH 55 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
4.0 SUMMARY AND CONCLUSIONS
An EFDC model and nested JP-EFDC model were developed to represent conditions in the Lower Cape Fear
River Estuary from January 1, 1992 through June 30, 2018. The main purpose of the modeling efforts was to
evaluate effluent mixing at varying distances from the outfalls to help NCDEQ define the critical mixing zone to
establish dilution ratios to apply for regulatory compliance.
The 2018 EFDC model accounted for the dynamic processes and unique physical features affecting the Lower
Cape Fear River Estuary system.The 2018 JP-EFDC model allowed for a more robust evaluation of near-field
conditions affecting the Southside WWTP and Sweeney WTP. The nested models were able to represent the
effluent mixing zone of the facilities by accounting for the near-field jet-plume dynamics and the far-field transport
and mixing.
The 2018 EFDC model was calibrated to reproduce observed hydrodynamic conditions(water surface elevation
[WSE], temperature, salinity) in the system for the year 2011, which was defined as the critical year for the
effluent mixing analysis study. For the calibration, model parameters were adjusted to achieve a good graphical
and statistical match between model outputs and measured data throughout the Lower Cape Fear River Estuary.
Model calibration was conducted using observations available for the year 2011. Once calibrated, the
performance of the model was validated against hydrodynamic observations available from January 1992 through
June 2018. The 2018 EFDC model was well calibrated and validated and able to reproduce the trends and
magnitudes of WSE, temperature, and salinity. For WSE and temperature, the model achieved R2 values greater
than 0.95 and Index of Agreement values(lAs)greater than 0.98. For salinity, the model achieved average R2
values of approximately 0.7 and average lAs of 0.88.
The calibrated 2018 EFDC and JP-EFDC models were applied to evaluate effluent mixing during the July 2011
critical period. Mixing was assessed for discharges of 12, 16, 20, and 24 MGD for the Southside WWTP, and the
peak 24-hour average discharge was evaluated for the Sweeney WTP. Dilution ratios were calculated at multiple
distances from the outfall locations. For the existing conditions, the average of four-day moving average dilution
for Southside WWTP at 167 meters(548 feet)--a distance equal to 1/3 the width of the river—is estimated to be
33.6:1. For comparative purposes, the dilution at 250 meters(820 feet)—a distance equal to 1/2 the width of
river—is estimated to be 702:1. USEPA guidelines recommend that mixing zone definition not exceed 1/3 the
width of the receiving water. The significant difference in dilution ratios predicted by the JP-EFDC model at the 1/3
and 1/2 widths, respectively, indicate that the 1/3 width likely reflects the transition point from near-to far-field
mixing.When the Southside WWTP discharge is increased to 24 MGD, the average of four-day moving average
dilution at 167 meters(548 feet)is estimated to be 17.6:1, increasing to 93.7:1 at a distance equal to 1/2 of the
width of river. For the Sweeney WTP, the four-day moving average dilution at a distance equal to 1/3 of the width
of the river at the discharge location (60 meters—197 feet)is estimated to be 29.8:1. At a distance equal to 1/2
the width of river, the dilution ratio for the Sweeney WTP effluent is predicted to increase significantly to 1,045:1.
The model results support using a distance of 1/3 of the receiving stream width to define the near-field mixing
zone for the Southside WWTP and Sweeney WTP effluent discharges.
OTETRA TECH 56 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
5.0 BIBLIOGRAPHY
Blumberg,A. F., and G. L. Mellor. 1987. A description of a three-dimensional coastal ocean circulation model. In:
Three-Dimensional Coastal Ocean Models, Coastal and Estuarine Science, Vol. 4. (Heaps, N. S., ed.)
American Geophysical Union, pp. 1-19.
Blumberg, A. F., Khan, L. A., and St. John, J. P. 1999. Three-dimensional hydrodynamic model of New York
Harbor region. Journal of Hydraulic Engineering, 125(8), 799-816.
Camacho, R.A., Martin, J. L., Diaz-Ramirez, J., McAnally, W., Rodriguez, H., Suscy, P., and Zhang, S. (2014a).
Uncertainty analysis of estuarine hydrodynamic models: An evaluation of input data uncertainty in the
Weeks Bay estuary, Alabama. Applied Ocean Research, 47, 138-153.
CORMIX Frequently Asked Questions(FAQ). 2018. http://www.cormix.info/faq.php.Accessed on 03/30/2018.
Doneker L. Robert, Jirka, H. Gerhard. 2007. CORMIX User Manual. A Hydrodynamic Mixing Zone Model and
Decision Support System for Pollutant Discharges into Surface Waters. EPA-823-K-07-001.
FLOW Science 2018. https://www.flow3d.com/products/flow-3d/why-flow-3d/. Accessed on 03/30/2018.
Frick,W.E. 1984. Non-Empirical Closure of the Plume Equations. Atmospheric Environment, 18, 653662.
Galperin, B., L. H. Kantha, S. Hassid, and A. Rosati. 1988. A quasi-equilibrium turbulent energy model for
geophysical flows. J. Atmos. Sci., 45, 55-62.
Hamrick, J.M. 1992. Three-Dimensional Environmental Fluid Dynamics Computer Code: Theoretical and
Computational Aspects. The College of William and Mary, Virginia Institute of Marine Science. Special
Report 317.
Johnson, B.H., K.W. Kim, R.E. Heath, B.B. Hsieh, and H.L. Butler. 1993. Validation of three-dimensional
hydrodynamic model of Chesapeake Bay. J.Hyd. Eng., 119, 2-20.
Ji, Z.-G. (2008) Hydrodynamics, in Hydrodynamics and Water Quality: Modeling Rivers, Lakes, and Estuaries,
John Wiley and Sons, Inc., Hoboken, NJ, USA. doi: 10.1002/9780470241066.ch2.
Kim, S. C. (2013). Evaluation of a Three- Dimensional Hydrodynamic Model Applied to Chesapeake Bay Through
Long Term Simulation of Transport Processes. JAWRA Journal of the American Water Resources
Association, 49(5), 1078-1090.
LCFRP. 2015. Environmental Assessment of the Lower Cape Fear River System. CMS Report No. 16-02. Center
for Marine Science. University of North Caroline Wilmington.
Lee, J.H.W. and V. Cheung, 1990. Journal of Environmental Engineering, Volume 116, Issue 6, December 1990,
pp 1085-1106.
Mellor, G. L., and T. Yamada. 1982. Development of a turbulence closure model for geophysical fluid problems.
Rev. Geophysics. Space Phys., 20, 851-875.
NCSU 2018. Elevation Data Sources. https://www.lib.ncsu.edu/qis/elevation.Accessed 03/26/2018.
Stow, C., Roessler, C., Borsuk, M., Bowen,J., and Rechkhow, K. 2003. Comparison of Estuarine Water Quality
Models for Total Maximum Daily Load Development in the Neuse River Estuary. J.Water Resources
Planning and Management, 10.1061/(ASCE)0733-9496(2003) 129:4(307), 307-314.
Tetra Tech. 2001. 3-Dimensional EFDC Water Quality Model of the Lower Cape Fear River and Its Estuary.
Prepared for the City of Wilmington and New Hanover County. May 2001.
Tetra Tech. 2007. MetADAPT User's Manual, Meteorological Data Analysis and Preparation Tool. Prepared for
U.S. Environmental Protection Agency. November 2007.
Tetra Tech 2008. Technical Memorandum: Cape Fear Public Utility Authority Northside WWTP Effluent Dilution
Analysis. Prepared for Cape Fear Public Utility Authority, September 2008.
OTETRA TECH 57 November 16, 2018

Cape Fear Public Utility Authority Southside&Sweeney Effluent Mixing Analysis
USEPA. 1991. Technical Support Document for Water Quality-Based Toxics Control, Office of Water, March
1991.
n TETRA TECH 58 November 16, 2018

Addendum to Effluent Mixing Analysis for the Cape Fear Public Utility Authority Southside and Sweeney NPDES Outfalls
(Tetra Tech, March 29, 2019)
At the request of the NC Division of Water Resources, additional points at distances from the outfall are presented for dilution ratio (DR) and
instream waste concentration (IWC). The tables below incorporate this information as an addendum to the original report(Tetra Tech, Feb 2019).
Table 1-A. 2018 JP-EFDC model predicted dilution ratios (DR) and instream waste concentrations (IWC)at Sweeney WTP for existing conditions
Scenario Sweeney WTP- Existing Conditions
Distance 2 m 3 m 4 m
from Outfall (6.6 ft) (9.8 ft) (13.1 ft) 5 m (16.4 ft)
DR IWC DR IWC DR IWC DR IWC
Average* 4.4 22.7% 5.8 17.2% _ 7.0 14.3% 7.8 12.8%
10th
Percentile* 3.6 27.8% 4.9 20.4% 6.0 16.7% 6.5 15.4%
90th
Percentile* 5.0 20.0% 6.5 15.4% 8.1 12.3% 9.1 11.0%
* Calculated using four-day moving averages
Table 1-B. 2018 JP-EFDC model predicted dilution ratios (DR) and instream waste concentrations (IWC)at Sweeney WTP for existing conditions
(Table 3-7 in Tetra Tech, 2019)
Scenario Sweeney WTP-Existing Conditions
Distance 6 m 10 m 25 m 60 m (197 ft); 1/3 river 90 m (295 ft);
from Outfall (20 ft) (33 ft) (82 ft) width 1/2 river width
DR IWC DR IWC DR IWC DR IWC DR IWC
Average* 8.1 12.3% 8.2 12.2% 9.3 10.8% 29.8 3.4% 1,045.0 0.1%
10th
Percentile* 6.7 14.9% 7.2 13.9% 8.2 12.2% 27.2 3.7% 949.4 0.1
90th
Percentile* 9.4 10.6% 9.3 10.8% 10.4 9.6% 31.9 3.1% 1,179.6 0.1
* Calculated using four-day moving averages
Tetra Tech, February 2019. Effluent Mixing Analysis for the Cape Fear Public Utility Authority Southside and Sweeney NPDES Outfalls. Prepared
for Cape Fear Public Utility Authority by Tetra Tech, Research Triangle Park, North Carolina.

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