NC0063860_Comments_20181207

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December 3,2018
Ms.Teresa Rodriguez
Division of Water Resources REC LVED/DENRIDWR
Water Quality Permitting Section—NPDES
1617 Mail Service Center DEC 0 7 2018
Raleigh,NC 26699-1617
Water Resources
Re: Harbor Estates WWTP—NC 0063860 Permitting Section
Mecklenburg County
Cormix Model
Dear Ms.Rodriguez:
Enclosed for your review is the Harbor Estates Cormix Model final report.
If you have any questions or concerns regarding the data or outcome of this model, please do not hesitate
to contact me.
Best regards,
56\1AkWilafAdYA
Amanda Berger
Manager of Environmental Compliance
Aqua-North Carolina
202 MacKenan Court, Cary, NC 27511 • AquaAmerica.com

Dilution Modeling Report
for
Harbor Estates WWTP
Prepared by
Jerry K. Snyder, P.E., DEE, DWRE
for
Aqua North Carolina, Inc.
November 2018
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Pennsylvan-1 ',. se No. PE032686E

Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
TABLE OF CONTENTS
EXECUTIVE SUMMARY 1
1.0 INTRODUCTION 1
1.1 BACKGROUND 1
1.2 DESCRIPTION OF EXISTING OUTFALL 2
1.3 PURPOSE OF DILUTION MODELING STUDY 2
2.0 MODELING APPROACH 3
2.1 MODELING OBJECTIVES 3
2.2 MODELED CONDITIONS 3
2.3 INPUT DATA 6
3.0 MODEL RESULTS 11
4.0 SUMMARY AND RECOMMENDATIONS 13
5.0 REFERENCES 15
6.0 ATTACHMENTS 17
LIST OF TABLES
1 Discharge Limits for Harbor Estates WWTP NPDES Permit#NC0063860 1
2 Outfall Pipe Configuration 2
3 CORMIX Model Scenarios 6
4 Measured Data for Lake Wylie and Harbor Estates 7
5 Water Density Calculations 7
6 Zone of Initial Dilution(ZID) Calculation 9
7 Harbor Estates CORMIX Input Data 10
8 Water Quality Criteria 11
9 Input Concentrations and Water Quality Criteria 11
10 Dilution and Distance Required to Meet Water Quality Criteria 14
11 Dilution Achieved at the Zone of Initial Dilution Distance 14
12 Proposed Effluent Limits for Harbor Estates 16
LIST OF FIGURES
1 Facility Location Map 4
2 Outfall Pipe Construction Drawing 5
3 Plan View of Plume from Harbor Estates Outfall 9
4 Dilution vs. Distance from Outfall 12
5 In-Stream Waste Concentration along Plume Centerline—Harbor Estates 12
i

Modeling Plan for Harbor Estates WWTP—NPDES #NC0063860 November 2018
LIST OF ATTACHMENTS
1 CORMIX Input and Output Files
ii

Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
EXECUTIVE SUMMARY
The mixing behavior of the Harbor Estates WWTP discharge into Lake Wylie was simulated
using the CORMIX model. Three discharge constituents were modeled, chloride, copper, and
zinc. A generic simulation was also performed that could be applied to any constituent in the
discharge. The simulations indicated that the slotted diffuser at the end of the outfall pipe is
highly effective at creating rapid mixing and dilution within a short distance of the outfall. A
zone of initial dilution(ZID)was calculated as 4.92 meters based on the outfall configuration.
The ZID is an initial mixing area within a short distance from the outfall. Based on a dilution
factor of 29.1 achieved at the ZID and accounting for background concentrations where
applicable, discharge limits for chloride (6,516 mg/L), copper (0.066 mg/L), and zinc (0.479
mg/L) are proposed. The proposed discharge limits will allow the most stringent water quality
criteria to be met within the ZID distance.
1.0 INTRODUCTION
1.1 BACKGROUND
This Dilution Modeling Report presents the results of a dilution modeling study performed in
accordance with a Modeling Plan submitted in September 2018 and approved on October 4,2018.
Some information provided in the Modeling Plan is included in this Modeling Report for
completeness.Aqua North Carolina,Inc.operates the Harbor Estates Wastewater Treatment Plant
(WWTP)under NPDES Permit NC0063860.The WWTP treats 100%domestic wastewater from
the Harbor Estates. The WWTP is located on Pine Harbor Road (NCSR1113), Charlotte 28278,
Mecklenburg County,North Carolina.The WWTP is permitted to discharge up to 0.0416 million
gallons per day(MGD). The average discharge flow rate in 2017 was 0.0124 MGD. The current
discharge limits for Harbor Estates for chloride, copper, and zinc are listed in Table 1 below.
Table 1
Discharge Limits for Harbor Estates WWTP NPDES Permit#NC0063860
Parameter Monthly Average Units Daily Maximum Units
Flow 0.0416 MGD1
Total
Chloride 230 mg/L2 230 mg/LZ
Total
Copper 7.88 ug/L3 10.47 ug/L3
Total
Zinc 125.7 ug/L3 125.7 ug/L3
Notes:
1. MGD is million gallons per day
2. mg/L is milligrams per liter
3. ug/L is micrograms per liter -
1

Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
1.2 DESCRIPTION OF EXISTING OUTFALL
The WWTP discharges treated domestic wastewater to Lake Wylie in the Catawba River Basin
through a permitted outfall pipe. Lake Wylie is classified as WS-III&B (Water Supply III and
Full Contact Recreation). A location map showing the locations of the facility and the outfall
pipe is included as Figure 1 below. The outfall is a 3-inch diameter slotted polyvinyl chloride
(PVC)pipe that ends in Lake Wylie 210 feet from the shore. A construction drawing showing
the profile of the outfall pipe is included as Figure 2 below. The pipe ends with two 20-foot
sections of PVC pipe. The end section is capped and filled with concrete,presumably to anchor
the pipe on the bottom. The adjacent section of pipe has slotted perforations covering the length
of the 20-foot section. These were described as being approximately 1/8-inch wide and 4 inches
long at 1-inch spacing on the crown(top) of the pipe. The end sections of pipe were located
under 30-35 feet of water when they were inspected and cleaned out several years ago.
On occasion the concentrations of total chloride and total copper have exceeded the permit limit
of 230 mg/L and 7.88 ug/L respectively. The basic configuration of the outfall is described in
Table 2 below.
Table 2 Outfall Pipe Configuration
Harbor Estates
Location Lake Wylie
Orientation of pipe Horizontal on bottom
Distance from Shore,feet 210
Diameter, inches 3
Diffuser Perforated Pipe(slotted)
Material PVC
Easement Width,feet 24
1.3 PURPOSE OF DILUTION MODELING STUDY
In 2017 the Facility exceeded its monthly average limit for the concentration of total chloride and
total copper in September and December. The measurements of copper in September and
December were 38 ug/L and 21 ug/L respectively while the Monthly Average limit for total
copper is 7.88 ug/L. The total chloride concentrations measured in September and December
2017 were 570 mg/L and 405 mg/L respectively while the Monthly Average limit for total
chloride is 230 mg/L.
The study will establish effluent dilution credits for total chloride, total copper, and total zinc.
The objective will be to determine the in-stream waste concentration(IWC) at various distances
from the end of the discharge pipe. The distance where the permitted concentrations are met will
be determined as an objective of this dilution study. This will be the basis for a recommended
dilution credit.The dilution credit will be an allowance of end-of-pipe concentrations greater than
the permit value as long as the permitted concentrations are achieved within a short distance from
the end of the pipe. Dilution credits may be developed and applied to other constituents as well.
2

Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
2.0 MODELING APPROACH
2.1 MODELING OBJECTIVES
The objective of the modeling study is to simulate the behavior of the discharge plume from the
outfall. The simulation will allow evaluation of the IWC of total chloride,total copper, and total
zinc in the discharged water as the plume mixes with ambient water. Although total chloride,
total copper, and total zinc are the main constituent of interest in this study, the simulation
results will be general and could be applied to any constituent in the discharged water.
2.2 MODELED CONDITIONS
The simulations were accomplished using a well-established and accepted hydrodynamic
model, Cornell Mixing Zone Expert System(CORMIX). The CORMIX model was used to
simulate the plume behavior of the existing discharge under existing conditions. The model will
be used to develop a steady-state concentration profile along the discharge plume centerline.
The location and distance from the end of the outfall pipe where the permitted total chloride,
total copper, and total zinc concentrations are met will be determined. A total of four
simulations were performed. Since the pumps cycle on an off with the treated effluent level in
the pump tank, a single instantaneous pumping rate was used in the model to represent the
discharge flow rate. This reflects the actual mixing behavior at the outfall pipe while the
discharge is active. The instantaneous pumping rate is independent of the flow rate passing
through the treatment plant. If the average daily flow to the plant changes,the pumps cycle
more or less frequently,but the instantaneous pumping rate while the pumps are on will remain
the same. Specific model runs will be made for total chloride, total copper, and total zinc. The
parameter-specific simulations will account for the background concentration of total chloride,
total copper, and total zinc in the ambient environment. The generalized plume simulation will
assume 100% as the discharge concentration and 0%in the background concentration. The
results of the generalized simulation can be used to calculate the IWC of any constituent in the
discharged water by multiplying the model-predicted percentage concentration in the plume by
the concentration of that constituent at the end of the pipe. The four model scenarios are
summarized in Table 3 below.
3

Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
Figure 1 Facility Location Map
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4

Dilution Modeling Report for Harbor Estates WWTP-NPDES #NC0063860 November 2018
Figure 2 Outfall Pipe Construction Drawing
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5

Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
Table 3 CORMIX Model Scenarios
Discharge Modeled Discharge
Flow 1 _ Constituent Concentration Units
(gpm)
Total
83 Chloride 230 ug/L
Total
83 Copper 10.47 ug/L
83 Total Zinc 125.7 ug/L
Generalized
83 Parameter 100
1.The instantaneous pumping rate will be used for all
scenarios.
2.3 INPUT DATA
The CORMIX model is a hydrodynamic computer code that simulates the behavior of hydraulic
plume or"jets"in various ambient environments. It has been applied extensively to model or
simulate the behavior of water discharges into lakes and stream environments. The model
calculates utilizes data on the discharge configuration, ambient environment, and relative
densities between the ambient water and the discharged water. The CORMIX model has
separate modules to simulate submerged single-port discharges (CORMIX 1), submerged multi-
port diffusers (CORMIX2), and buoyant surface discharges (CORMIX3). The CORMIX2
module will be used for this study.
CORMIX models the concentrations of constituents in water,no matter which units of measure are used,
as the concentrations above background. Therefore,if there is a non-zero background(ambient)
concentration of any modeled constituent,that background concentration is subtracted from the
discharge concentration and the water quality criterion as well.
For interpretation of the calculated model output concentrations,the background concentrations are
added to the predicted plume concentrations.Because both the discharge concentration and water quality
criteria are adjusted for background levels, the dilution factors(S)calculated by the model is the ratio of
the adjusted discharge concentration to the adjusted water quality criterion.
In addition to the model scenarios listed in Table 3 above,two additional scenarios were run. These
represented the maximum chloride and copper concentrations reported in September 2017.Additional
water samples were collected on October 11,2018 and analyzed for use in this dilution modeling study.
These are listed in Table 4 below.
6

Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
Table 4 Measured Data for Lake Wylie and Harbor Estates October 11,2018
Measurement Location Parameter Value Units
Effluent Temperature 23 °C
Effluent Total Dissolved Solids 834 mg/L
Effluent Chloride 205 mg/L
Effluent Copper 0.037 mg/L
Effluent Zinc 0.079 mg/L
Upstream Temperature 24 °C
Upstream Total Dissolved Solids 34 mg/L
Upstream Chloride 6.3 mg/L
Upstream Copper <0.005 mg/L
Upstream Zinc 0.014 mg/L
Downstream Temperature 25 °C
Downstream Total Dissolved Solids 41 mg/L
Downstream Chloride 8.3 mg/L
Downstream Copper <0.005 mg/L
Downstream Zinc <0.010 mg/L
In Table 4 the measurements reported as less than,indicated by the"<"symbol were treated as zero.The
downstream samples were not used as their location was not provided.
Water Density Calculations
The behavior of the discharge plume modeled in CORMIX is affected by the energy(velocity)of the
discharged jet and the relative density of the discharged water and the receiving water.These densities
were calculated from the measured total dissolved solids (TDS)concentration and temperatures
measured on October 11,2018.These are summarized in Table 5 below.
Table 5 Harbor Estates Water Densities
Total Dissolved
Solids,TDS Temperature Density'
mg/L °C kg/m3
Discharge 834 23 998.174
Lake Wylie 34 24 997.325
1. Density was calculated from Sea Water Equation of State
online calculator, http://fermi.jhuapl.edu/denscalc.html.
7

t
Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
Discharge Rate
The discharge rate was calculated from the measured time required for the discharge pump to draw
down a holding tank between the upper and lower set points on the tank.The 46-inch diameter tank was
drawn down 23 inches in 2 minutes. This resulted from a pumping rate of 83 gallons per minute(gpm)
0.120 million gallons per day,mgd).
Downstream Velocity
The downstream velocity was calculated from the relative drainage areas between the discharge location
at Pine Run(2810 square miles, s.mi.)and the Catawba River near Rock Hill, SC(U.S.G.S.,gage
02146000 drainage area 3050 s.mi.)and the average annual flow at the gage station(3,895 cubic feet per
second, cfs)based on five years of daily discharge data. The calculated average annual flow along the
Catawba River at Pine Run is 3,589 cfs.The cross-sectional area of the Catawba River was measured
from a U.S.G.S. Quadrangle map(Lake Wylie, SC,7.5-minute quadrangle, 1993) as 44,500 square feet,
s.f. The calculated downstream velocity if 0.08 feet per second,fps.
Diffuser Configuration
The diffuser at the end of the discharge pipe consists of a 20-foot section of 3-inch diameter PVC pipe.
The diffuser was modeled as 10 groups of three openings on the top of the pipe. Each opening was 1/8-
inch wide and 4 inches long,which gave it an opening area of 0.5 square inches,int. And an equivalent
diameter of 0.798 inches (0.0665 feet for model input). The total opening area for the 30 openings is 15
int and the equivalent diameter is 4.37 inches.
Diffuser Orientation
The diffuser is perpendicular to the orientation of the Catawba River.The origin for the coordinate
system for model output is at the middle of the diffuser pipe on the bottom.The coordinates are oriented
as follows.The downstream direction of flow is the positive X direction.The opposite river bank is the
positive Y direction,and straight up from the bottom is the positive Z direction.
Figure 3 shows the dimensions of the plume originating from the diffuser pipe.The plume originates on
the left side and proceeds toward the right(southerly).
Zone of Initial Dilution(ZID)
The ZID is a limited area around a discharge pipe or outfall where the water quality criteria for toxic
constituents may be exceeded as long as the criteria are met at the edge of this limited zone and the
limited nature of the ZID allows for passage of aquatic life around the zone. The dimensions of the ZID
can be calculated as the minimum of 5 * ambient depth or 50* discharge length scale(DLS).The DLS
is defined as the square root of the area of the discharge.The ZID for the diffuser is 16.14 feet(4.92
meters).Table 6 below shows the calculation of the ZID distance for Harbor Estates.
Table 7 below lists the input values used in the generalized model scenario. Only the discharge
concentration and water quality criterion values,listed in Table 4,changed for the other scenarios.
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Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
Figure 3 Plan View of Plume from Harbor Estates Outfall
Horizontal Plume Trajectory and Width
Harbor Estates
4
3Left Edge of Plume
c 2
cuU
E 1
O
Plume Centerline
u LI 0
v
C
co
171. 2
v -2
Right Edge of Plume
E -3 i
v -4
w 0 1 2 3 4 5 6
Downstream Distance from Outfall,Meters
Table 6 Zone of Initial Dilution (ZID)for Harbor Estates Diffuser
Discharge
Length Scale, Avg. Depth Zone of Initial Zone of Initial
Area DLS 50 x DLS at Discharge 5 x Depth Dilution,ZID Dilution,ZID
(s.f.) (ft.) (ft.) (ft.) (ft.) (ft.) (m)
Harbor Estates 0.10 0.32 16.14 35.25 176.25 16.14 4.92
The model input values are listed in Table 7 below.
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Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
Table 7 Harbor Estates CORMIX Input Data
Module:CORMIX2 Submerged Mulit-Port Diffuser
TAB INPUT DATA UNITS OF MEASURE
Project Tab
Name: Harbor Estates-Lake Wylie
Scenario: Generalized
Ambient Tab
Average Depth: 35.25 feet
Discharge Depth: 35 feet
Ambient Velocity: 0.08 feet per second,fps
Bounded/Unbounded Bounded
Width: 600 feet
Manning's n: 0.025
Wind Speed: 0 meters/second,m/s
Ambient Density Data Fresh/Uniform
Water Density: 997.325 kilograms per meter squared,kg/m2
Effluent Tab
Flow Rate: 0.120 million gallons per day,MGD
Concentration: 100 percent,
Discharge Density 998.174 kilograms per meter squared,kg/m2
Pollutant Type: Conservative
Discharge Tab
Nearest Bank: Left
Diffuser Length: 20 feet
Distance to one endpoint: 200 feet
Distance to other endpoint: 220 feet
Port Height: 0.25 feet
Port Diameter: 0.0665 feet
Contraction Ratio: 1
Total No.of Openings: 30
Alignment Angle Gamma(y): 90 degrees,°
Tab-No.of Nozzles per Riser Several
No.of Nozzles per Riser: 3
Tab-Net Horizontal Momentum Flux Zero
Nozzles orientation: Same Direction
Mixing Zone Tab
Toxic/NonToxic NonToxic
WQ Standard No WQStandard (for generalized parameter scenario)
Specified Mixing Zone Tab Mixing Zone Specified
Distance: 4.92 meters
Region of Interest 2000 meters
Grid Intervals for Display 200 No.of Display Steps
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Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
Water Quality Criteria and Background Concentrations
Lake Wylie is classified as WS IIUB water uses.Class WS III is a water supply classification and B is
for primary contact recreation.In addition,there are chronic and acute limits that apply to freshwater
aquatic life. Table 8 below lists the water quality criteria for chloride copper and zinc for Lake Wylie.
Table 8 Water Quality Criteria
FW Aquatic
•
Parameter Units B WS III Chronic Acute Background
Chloride mg/L N/A 250 230 230 6.3
Coppers ug/L N/A N/A 2.26 2.95 0
Zinc2 ug/L N/A N/A 30.21 29.97 14
1.A hardness value of 20 mg/L as CaCO3 was used to calculate the criterion
for copper.
2.A hardness value of 20 mg/L as CaCO3 was used to calculate the criterion
for zinc.
Adjustments for Background Concentrations
The input concentrations for chloride,copper,and zinc adjusted for background concentrations are listed
in Table 9 below.
Table 9 Input Concentrations and Water Quality Criteria Adjusted for Background Concentration
Discharge Water Quality
Concentration above Criterion above
Parameter Units Background background Scenario Name
Chloride mg/L 223.7 223.7 Daily Max.Chloride
Copper mg/L 0.01047 0.002 Daily Max.Copper
Zinc mg/L 0.1117 0.016 Daily Max.Zinc
Max. Chloride mg/L 563.7 223.7 Highest Measured Chloride
Max.Copper mg/L 0.038 0.002 Highest Measured Copper
3.0 MODEL RESULTS
The slotted diffuser creates rapid mixing and significant dilution within short distances from the outfall
pipe.A dilution factor, S,is defined,for the purposes of CORMIX,to be the discharge concentration
divided by the in-stream concentration at a location along the plume centerline. For example,if the
dilution factor is 25,the concentration at the plume centerline is 4%of the discharge concentration. A
plot of the calculated dilution factor versus distance from the outfall pipe is shown in Figure 4.Note that
all output from CORMIX is in metric units even though it does allow input of English units for most
parameter values.Figure 5 is a corresponding plot of the Instream Waste Concentration(IWC)versus
distance from the outfall pipe.Figure 5 is based on the discharge concentration of 100%and can be
applied to any constituent in the discharged water.
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Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
Figure 4 Dilution vs.distance from Outfall—Harbor Estates
Dilution vs. Distance from Outfall
Harbor Estates
35 -- — --— — —
—
I
30 — _-- — — --- — —
c25 [ � R,
0
m20 -a°z:. _ ..
0 15 _
2 10 — -- --- —
0
5
0
0 1 2 3 4 5 6
Distance from Outfall,Meters
Figure 5 In-Stream Waste Concentration along Plume Centerline—Harbor Estates
Concentration along Plume Centerline
Harbor Estates
100 _ --- --
e
• 90
o 80
E 70
Y
c 60
c 50
o
u 40
CU• 30
c 10 _ I
U 0 -- 0
N
c) 0 1 2 3 4 5 6 l
a Distance from Outfall, Meters
Note that the concentration is greatest at the centerline of the plume and drops of with increased lateral
distance from the centerline. Therefore,the IWC illustrated in Figure 5 above is the maximum
concentration at the distance from the outfall.
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Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
Parameter-Specific Model Results
Three scenarios were run showing calculated in-stream concentrations of chloride,copper,and zinc.
Two additional scenarios were run using the maximum chloride and copper concentration measured in
the discharge. Since CORMIX evaluates excess concentration(i.e.,concentrations above background
levels,if present)the discharge concentrations and water quality criteria were reduced by the measured
background levels of chloride and zinc.There was no copper measured in the background water sample.
The results of the parameter-specific scenarios are summarized on Table 10.
The worst-case scenario is for a discharge concentration of copper at.038 mg/L (38 ug/L).For
reference,the current daily maximum copper concentration is 10.47 ug/L.Note that even though the
dilution factor required to meet the water quality criterion for copper is 19.0,the fresh water aquatic life
chronic criterion of 0.002 mg/L is met within 1.72 meters downstream from the diffuser.
4.0 SUMMARY AND RECOMMENDATIONS
The results of the dilution modeling indicate that the slotted diffuse is highly effective and it promotes
rapid mixing within a short distance from the outfall.A ZID distance of 4.92 meters was calculated in
Table 6 based on the minimum criterion of 50*DLS.This method for calculating a ZID is generally
recognized by the USEPA,Environmental Regulators, and is referenced in the CORMIX user's manual.
Table 11 below summarizes the predicted centerline plume concentration for the five scenarios noted I
Table 10 plus the generic or generalized parameter case,where the discharge concentration was set at
100%.Note that the centerline concentrations at 4.92 meters is significantly below the most stringent
water criterion,which is the chronic fresh water aquatic life criterion.
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Dilution Modeling Report for Harbor Estates WWTP-NPDES#NC0063860 November 2018
Table 10 Dilution and Distance Required to Meet Water Quality Criteria
Minimum Distance Distance Distance Dilution
Discharge Water Quality X Y Z Factor Centerline
Parameter Units Conc. Criterion Meters Meters Meters S Concentration
Chloride mg/L 223.7 223.7 0.00 0.00 0.00 1.0 224
Copper mg/L 0.010 0.002 0.10 0.00 0.56"-- 5.5 0.002
Zinc mg/L 0.112 0.016 0.14 0.00 0.63 7.0 0.016
Max. Chloride mg/L 563.7 223.7 0.020 0.00 0.34 2.50 224
Max. Copper mg/L 0.038 0.002 1.72 0.00 0.71 19.0 0.002
Table 11 Dilution Achieved at the Zone of Initial Dilution Distance of 4.92 meters-Harbor Estates
Maximum Minimum Distance Distance Distance Dilution Predicted
Discharge Water Quality X Y Z Factor Centerline
Parameter Units Conc. Criterion Meters Meters Meters S Concentration
Chloride mg/L 223.7 223.7 4.92 0.00 0.39 29.1 7.69
Copper mg/L 0.010 0.002 4.92 0.00 0.39 29.1 0.0004
Zinc mg/L _ 0.112 0.016 4.92 0.00 0.39 29.1 0.004
Max.Chloride mg/L 563.7 223.7 4.92 0.00 0.39 29.1 19.4
k Max. Copper mg/L 0.038 0.002 4.92 0.00 0.39 29.1 0.001
Generic % 100 N/A 4.92 0.00 0.39 29.1 3.44
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Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
Table 11 demonstrates that a dilution factor of 29.1 is achieved at a ZID distance of 4.92 meters
downstream from the outfall and is quite protective of all measured chloride,copper,and zinc
concentration measured at the outfall.The maximum daily discharge limits could be raised significantly
and still be protective of the most sensitive water quality criteria.The maximum discharge
concentrations of chloride copper and zinc were calculated as:
Cnew=S *WQ' +BG
Where:
Cnew=the daily maximum discharge concentration that will allow the most sensitive water quality
criterion to me met at the ZID distance from the outfall.
S=29.1,the calculated centerline dilution ratio calculated at the ZID distance from the outfall.
WQ' =the water quality criterion adjusted for background concentration,WQ -BG.
WQ=the water quality criterion.
BG=the background concentration of the parameter of interest.
Table 12 lists the maximum discharge concentrations of chloride, copper,and zinc that will allow the
most sensitive water quality criterion to me achieved within the ZID distance from the outfall.
Our recommendation is that the discharge limits for Harbor Estates be raised to reflect the limits
in Table 12. Note that these are based upon achievement of the water quality criteria at the ZID
of 4.92 meters.
5.0 REFERENCES
1. Mixing Zones in North Carolina July 23, 1999.
2. Technical Support Document for Water-quality Based Toxics Control,pp. 71-72(EPA/505/2-90-
001 March 1991).
3. User's Manual for CORMIX: A Hydrodynamic Mixing Zone Model and Decision Support
System for Pollutant Discharges into Surface Waters.G.H.Jirka,R.L.Doneker,and S. W.Hinton
September 1996.
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Dilution Modeling Report for Harbor Estates WWTP—NPDES#NC0063860 November 2018
Table 12 Proposed Effluent Limits for Harbor Estates
Proposed Minimum Distance Distance Distance Dilution
Discharge Water Quality Background X Y Z Factor
Parameter , Units Conc. Criterion Concentration Meters Meters Meters S
Chloride. mg/L 6,516 223.7 6.3 4.93 0.00 0.39 29.1
Copper mg/L 0.066 0.002 0.000 4.93 0.00 0.39 29.1
Zinc mg/L 0.479 0.016 0.014 4.93 0.00 0.39 29.1
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Dilution Modeling Report for Harbor Estates WWTP—NPDES #NC0063860 November 2018
6.0 ATTACHMENTS
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