NC0069841_Rocky River Model Application Scenarios Memo_20240607

TETRA TECH 4000 Sancar Way,Suite 200•PO Box 14409
Research Triangle Park, NC 27709
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MEMORANDUM
To: NC DEQ DWR Date: March 26, 2024
Cc: Black&Veatch, WSACC Revised June 7, 2024
Subject: Rocky River Model Scenario
From: Trevor Clements, Hillary Yonce, Application to Support Speculative
Will Hicks (Tetra Tech) Limits Determination
This memo details the model application performed for critical condition scenarios, including seasonal low
flows and warm temperatures, with existing and speculative permitted flow limits for modeled wastewater
treatment plants (WWTPs).
1.0 INTRODUCTION
This technical memorandum is intended to support the Water and Sewer Authority of Cabarrus County
(WSACC) in seeking approval for the expansion of discharge capacity associated with WWTP effluent
discharge from the Rocky River Regional WWTP and/or the Muddy Creek WWTP. Assimilative capacity
of the receiving water was assessed using a QUAL2K model developed based on water quality
monitoring conducted during low flow conditions along the Rocky River mainstem and its tributaries
spanning from May to November of 2022. The model underwent calibration and corroboration and
received review and approval from the North Carolina Division of Water Resources. The overall project
goal is to develop speculative limits for the Rocky River Regional WWTP and/or the Muddy Creek WWTP
that protects the designated uses of the Rocky River under the proposed net increase in wasteload
allocation (WLA).
North Carolina Water Quality Regulations (15A NCAC 02B .0206)specify that water quality standards
related to oxygen-consuming wastes be protected using the minimum average flow for a period of seven
consecutive days that has an average recurrence of once in ten years (7Q10 flow). NC regulations (15A
NCAC 02B .0404) also provide for seasonal variation for the discharge of oxygen-consuming wastes, with
the summer period defined as April through October and winter period as November through March.
Additionally, all existing WLAs must be accounted for to evaluate available assimilative capacity for a
speculative WLA for the proposed WSACC expansion.
2.0 CRITICAL CONDITIONS
2.1 LOW FLOWS
To evaluate assimilative capacity in the Rocky River, the calibrated QUAL2K model was modified to
simulate critical low flow conditions. Tetra Tech estimated 7Q10 flows for the Rocky River QUAL2K model
using a drainage area-based approach. The estimates incorporated 7Q10 data from National Pollutant
Discharge Elimination System (NPDES) permits and United States Geological Survey (USGS) literature
NTETRA TECH 1

Memorandum- Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
(Table 1). The NPDES fact sheet for Mallard Creek WWTP and Long Creek WWTP provided 7Q10
estimates for each respective waterway. Significant studies and statistical evaluations have been
conducted by USGS to characterize low flow conditions across the Rocky River watershed. Unique
differences in geology across the watershed reveal variable low flow rates systemwide, which were used
to modify boundary condition flows seasonally in the model. USGS estimated annual and winter 7Q10
flows for individual tributaries across the watershed using a network of partial-record and continuous-
record local USGS flow gages (Weaver 2016, Weaver and Fine 2003). Unit-area low flow statistics were
assigned to the tributaries they were developed for based on drainage area, and where necessary,
applied to adjacent modeled tributaries without records (Table 2).
Low flow estimates at the Rocky River Regional WWTP outfall and the most downstream gage (USGS
02126000 Rocky River near Norwood) were used as anchor points for the low flow water balance.
Residual flows unaccounted for by tributaries were calculated from interim drainage areas as added
baseflow. The Rocky River Regional outfall 7Q10 estimate from the NPDES fact sheet for summer and
winter are 20.7 cubic feet per second (cfs) and 31.9 cfs, respectively. The Norwood USGS gage
estimates a 7Q10 of 47 cfs during summer and 79 cfs during winter.
Table 1. 7Q10 low flow estimates for Rocky River tributary locations from USGS and NPDES facilities.
Location of Estimate Source ID Summer Winter Summer Winter
.. .e or Drainage 7e10 7Q10 7Q10
permit) Area(sq mi) Flow Flow Area Flow Area Flow
West Branch Rocky River 02123932 4.98 0.50 0.90 0.100 0.181
(South Prong) near Cornelius2
Clarke Creek near Harrisburg2 02124080 21.9 1.00 2.30 0.046 0.105
Mallard Creek WWTP NC0030210 37.5 0.64 2.1 0.017 0.056
Coddle Creek near Concorde 02124230 57.9 5.60 9.00 0.097 0.155
Reedy Creek at Rocky River2 02124320 30.9 1.60 3.00 0.052 0.097
Irish Buffalo Creek near 02124374 45.5 3.10 8.20 0.068 0.180
Faggarts Crossroads2
Dutch Buffalo Creek at INC 49 02124471 45.1 0.70 2.20 0.016 0.049
near Mount Pleasant2
Goose Creek at Fairview2 02124692 24.0 0.30 1.00 0.013 0.042
Crooked Creek N/A N/A N/A N/A 0.001 0.010
Long Creek WWTP NC0024244 64.0 1.60 9.50 0.025 0.148
Little Bear Creek at Saint 02124944 12.4 0 (Zero) 0.30 0 (Zero) 0.024
Martine
Big Bear Creek near Richfield' 02125000 55.6 0 (Zero) 0.20 0 (Zero) 0.004
Richardson Creek near 02125500 163 0.50 1.60 0.003 0.010
Marshville2
Lanes Creek near Trinity2 02125696 87.7 0 (Zero) 0.05 0 (Zero) 0.001
cfsm=cubic feet per second per square mile,sq mi=square mile
Weaver,J.C.,2016, Low-flow characteristics and flow-duration statistics for selected USGS continuous-record streamgaging
stations in North Carolina through 2012(ver. 1.1, March 2016): U.S.Geological Survey Scientific Investigations Report 2015-5001,
89 p., http://dx.doi.org/10.3133/sir20l55001/.
z Weaver,J.C.and J.M. Fine.2003. Low-flow characteristics and profiles for the Rocky River in the Yadkin-Pee Dee River Basin,
North Carolina,through 2002. U.S.Geological Survey Water-Resources Investigations Report 03-4147.
https://pubs.usgs.gov/wri/wri034147/pdf/W RI R_03-4147-Revised_2012Oct.pdf
3 Direct communications with Curtis Weaver(USGS)via email on September 25,2019,based on nine total nearby selected USGS
sites from Crooked Creek and Richardson Creek basins,six partial-record sites,and three continuous-record stream gages.
aTETRA TECH 2

Memorandum- Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
The seasonal low-flow unit area discharges from Table 1 were applied to the total drainage area for each
tributary to determine what the associated summer and winter 7Q10 flows in Table 2. These flows were
then supplemented with relatively small additional flows that are attributed to the intervening drainage
areas between modeled tributaries. These supplemental flows were weighted using a water balance
approach so that known 7Q10 flow conditions at the two mainstem USGS gages were adequately
estimated (Table 3). Without accounting for the total intervening drainage area in the watershed (over 200
square miles)the total flow throughout the system by water balance approach with the mainstem USGS
gages would be underpredicted by nearly 20 cfs.
Table 2. Estimated 7Q10 low flows developed for exclusively tributaries as a portion of the Rocky River
QUAL2K model application for boundary conditions (excluding interim flows).
Model . . 7Q10
Tributary Drainage Area Discharge from Table I Model 7Q1 0 Flow Flow(cfs)
Dye Creek 3.96 West Branch Rocky 1.06 0.40 0.72
headwaters
Rocky River as a 3.72 West Branch Rocky 0.33 0.37 0.67
tributary
West Branch 22.9 West Branch Rocky 2.11 2.30 4.14
Rocky River
Clarke Creek 28.1 Clarke Creek 1.26 1.28 2.95
Mallard Creek 34.7 Mallard Creek WWTP 3.53 0.59 1.94
headwaters
Coddle Creek 74.3 Coddle Creek 6.72 7.19 11.55
Back Creek 15.5 Reedy Creek 0.04 0.80 1.50
Reedy Creek 43.1 Reedy Creek 5.15 2.23 4.18
Irish Buffalo Creek 110 Irish Buffalo Creek 8.86 7.49 19.82
Dutch Buffalo 98.7 Dutch Buffalo Creek 2.17 1.53 4.81
Creek
Clear Creek 24.5 Goose Creek 1.82 0.31 1.02
Goose Creek 42.3 Goose Creek 0.92 0.53 1.76
Crooked Creek 50.4 Crooked Creek 2.37 0.05 0.50
Island Creek 21.9 Crooked Creek 0.48 0.02 0.22
Long Creek 32.0 Long Creek WWTP 0.05 0.11 0.67
headwaters'
Little Long Creek' 29.1 Long Creek WWTP 0.63 1.48 8.79
Little Bear Creek 12.5 Little Bear Creek 0.27 0.00 0.30
Big Bear Creek 96.1 Big Bear Creek 2.09 0.00 0.35
Richardson Creek 234 Richardson Creek 7.54 0.72 2.30
Cribs Creek 19.5 Richardson Creek 0.42 0.06 0.19
Lanes Creek 138 Lanes Creek 1.93 0.00 0.08
The 7Q10 estimate for Long Creek is located downstream of the confluence of Long Creek and Little Long Creek.The 7Q10
estimate for each boundary condition was calculated by applying the combined estimate to the ratio of each calibration flow over the
combined calibration flow.
OTETRA TECH 3

Memorandum- Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
As mentioned, intervening drainage area flows were calculated using the unit drainage area approach.
Unit area discharge for interim drainage area upstream of RRRWWTP were determined from the
RRRWWTP NPDES permit fact sheet, while unit discharges from RRRWWTP to the confluence was
based on the USGS gage at Norwood. The sum of all interim flows and additional intervening drainage
areas relative to benchmark 7Q10 flows at the USGS gages were overestimated during winter by 3-8%,
and underestimated during summer by 11 - 13%, so intervening drainage area flows were weighted
based on accurately capturing the water balance downstream. Since Mallard Creek and Long Creek were
simulated separately, the interim flow that would have been applied to them was applied to the next
upstream tributary.
Table 3. Total estimated 7Q10 low flows developed for boundary conditions for Rocky River QUAL2K
model application (including interim drainage area flow contributions).
ModelTributary Flows Intervening Drainage Area Flows Boundary Condition
QUAI-21K Summer Winter Spatial Extent Drainage Weighted Weightedo Total Total
..eled 7e i 7Q1 i Area(sq Summer Winter Summer Winter
Tributary Flow Flow mi) 7010 7Q1e i
(cfs) (Cfs) Flow(cfs) Flow(cfs) Flow(cfs) Flow(cfs)
Dye HW 0.40 0.72 Dye HW to RR N/A 0.15 0.11 0.54 0.82
RR trib 0.37 0.67 Dye/RR to WB RR 1.17 0.66 0.49 1.03 1.16
WB RR 2.30 4.14 WB RR to Clarke 5.25 1.41 1.05 3.71 5.19
Clarke 1.28 2.95 Clarke to Mallard 11.30 2.87 2.14 4.15 5.09
Coddle 7.19 11.55 Coddle to Back 9.70 0.06 0.05 7.25 11.60
Back 0.80 1.50 Back to Reedy 0.50 0.24 0.18 1.04 1.68
Reedy 2.23 4.18 Reedy to RRRWWTP 1.90 0.15 0.23 2.38 4.41
ater Balance Check Point(WSACC Permit 7Q10): Interim 20.70
Summer 20.70 cfs,Winter 31.90 cfs Sum:
Irish BuffaloPL7.49
19.82 Irish to Dutch 1.00 W1.19 0.26 8.69 20.09
Dutch Buffalo 1.53 4.81 Dutch to Clear 15.30 3.83 0.85 5.36 5.67
Clear 0.31 1.02 Clear to Goose 52.50 0.001 0.0002 0.31 1.02
Goose 0.53 1.76 Goose to Crooked 0.01 0.47 0.10 0.99 1.87
Crooked 0.05 0.50 Crooked to Island 6.60 2.49 0.55 2.54 1.06
Island 0.02 0.22 Island to Long 34.10 2.92 10.651 2.95 0.87
Richardson 0.72 2.30 Richardson to Cribs 11.00 0.04 0.01 0.75 2.31
Cribs 0.06 0.19 Cribs to Lanes I 0.50 0.94 0.39
Lanes 0.00 0.08 Lanes to Plank Rd 12.00 1.37 2.30 1.37 2.38
Water Balance Check Point(Norwood 7Q10): Interim 47.27 79.46
Summer 47.27 cfs,Winter 79.46 cfs Sum:
OTETRA TECH 4

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
2.2 WARM TEMPERATURES
Critical low flow conditions can be paired with conservative warm-weather conditions when preparing
model conditions for assimilative capacity evaluation modeling. Although 7Q10 low flows are not likely to
occur in tandem with the warmest weather conditions, the conservative approach provides a margin of
safety in the evaluation.
Long-term monitoring data by the Yadkin Pee Dee River Basin Association at a site on the Rocky River at
Flowes Store Road (Q7780000) maintains a biweekly to monthly record of water temperature
observations. Monitoring records from 1998 to present indicate that the month with the highest average
water temperatures occur in the month of July. Long-term mean water temperature for July is 25.0 °C,
while the coldest month on average is typically January at 6.5 'C. An evaluation of monitoring data for all
July dates in the period of record at Flowes Store Road revealed a 75th percentile water temperature of
approximately 26.4 'C. The warmest winter month (based on the NPDES permit seasonal delineation as
November 1 — March 31) based on the same evaluation at the Flowes Store Road site on the Rocky
River indicates that the warmest month is typically November at 11.9 'C. The 75th percentile water
temperature for all Novembers on record is approximately 13.1 C.
The 75th percentile summer warm water temperature was applied to each point source WWTP. Point
source tributary temperatures from the calibration model were utilized, as values aligned with the 75th
percentile temperature. The 75th percentile winter warm temperature was applied to each point source
WWTP and tributary, with the exception of Crooked Creek, for which the temperature was derived from
model simulation specific to that creek.
Associated with warm seasonal water temperatures, dissolved oxygen (DO) concentrations for headwater
and tributary boundary conditions were modified for the winter scenario to reflect the same relative
percent DO saturation observed during the summer.
aTETRA TECH 5

Memorandum- Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
3.0 PERMITTED AND SPECULATIVE WWTP EXPANSIONS
The following municipal WWTP NPDES permittees are included in the baseline QUAL2K model for the
Rocky River system (Table 4). Included in this table are the more typical effluent discharge volumes in
summer 2022 which were used in the calibrated model, as well as currently permitted and speculative
design average flow (DAF)volumes. The status of Authorization to Construct(ATC)from the North
Carolina Division of Water Resources is provided in the last column with regard to future effluent flows.
NPDES permitted and speculative limits for water chemistry characteristics are detailed in Section 4.
Table 4. Modeled WWTP typical, permitted design average, and speculative design average flows.
WWTP Owner Receiving NPIDES • .
ModelWater Date DAF Speculative DAF
(MGD) 2022 Flow Flow for (MGD) (MGD)
Rocky Mooresville Dye Branch NCO046728 3.95 5.07 10/7/2019 7.5 No change
River
Mallard Charlotte Mallard NCO030210 7.22 9.83 12/17/2019 12.0 16.0(ATC issued)
Creek Water Creek
Rocky WSACC Rocky River NCO036269 19.70 21.10 9/29/2021 26.5 30.0, 34.0(ATC issued)
River
Regional 40.0, 50.0(speculative)
Muddy WSACC Rocky River NCO081621 0.30 0.17 5/10/2021 0.30 1.0 (no ATC)
Creek
West Stanly Co. Rocky River NCO043532 0.30 0.55 12/11/2023 1.2 2.5(no ATC)
Stanly
Long Albemarle Long Creek NCO024244 1.78 4.09 3/23/2023 12.0 16.0 (no ATC)
Creek
Norwood Norwood Rocky River NCO021628 0.07 0.38 2/6/2024 0.75 No change
New Union Co. Crooked TBD' Implicitz N/A TBD' 1.9' 6, 12.0 '
Crooked Creek
Creek',2
Grassy Union Co. Crooked NCO085812 Implicitz 0.04 4/5/2022 0.05 0.12 (no ATC)
Branchz Creek
Monroez Monroe Richardson NCO024333 Implicitz 6.16 12/17/2021 12.5 15.4 (no ATC)
Creek
Speculative limits for a new outfall to Crooked Creek(CC)are currently under evaluation by DWR.A new outfall is anticipated to
incorporate flows from existing CC#2(NC0069841)which discharges to Crooked Creek at 1.9 MGD.
2 Effluent flows from these WWTPs were modeled implicitly based on simulation of their receiving waters as boundary conditions to
the mainstem Rocky River model segments.
MGD=million gallons per day
aTETRA TECH 6

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
4.0 SCENARIO DETAILS
The six model scenarios developed included simulation of critical conditions (low flow and warm
temperatures)to capture existing and speculative WWTP permitted limits for flow and water chemistry.
For these scenarios, each WWTP was modeled at permit limits for flow and various water quality
constituents to capture an extremely conservative theoretical scenario under which assimilative capacity
of the Rocky River is evaluated (Table 5).
Scenarios 1 and 2 simulate summer and winter critical conditions respectively, with all WWTPs at existing
permitted limits. For the WWTPs modeled, effluent simulation was characterized by approved limits for
flow, DO, ammonia (NH3), and 5-day biochemical and carbonaceous biochemical oxygen demand
(BOD5, CBOD5).
Scenarios 3, 4, 5, and 6 simulate critical summer and winter conditions with all WWTPs at existing
permitted limits again, with the following two exceptions:
a. New Crooked Creek WWTP was conservatively included at the maximum flow rate in discussion
between Union County and NC DEQ of 12.0 MGD. Inputs to the Rocky River model for the
Crooked Creek tributary are pulled directly from Union County's Crooked Creek critical conditions
QUAL2K model, simulating the 12.0 MGD discharge located around the Grassy Branch area.
b. Rocky River Regional WWTP modeled at speculative permit limit flows of 40 MGD (scenarios 3
and 4) and 50 MGD (scenarios 5 and 6).
Model inputs for the Rocky River Regional Wastewater Treatment Plant (RRRWWTP)of NH3 permit
limits were calculated by season and flow tier using the same methodology documented in the
RRRWWTP NPDES fact sheet dated 9-29-2021. NH3 limits were established using site-specific WLA
calculations put forth for ammonia criteria by the U.S. Environmental Protection Agency in 20131. A
summary of these calculations is included in Appendix A.
https://www.epa.gov/sites/default/files/2015-08/documents/aquatic-life-ambient-water-quality-criteria-for-ammonia-
freshwater-2013.pdf
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Memorandum- Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
Table 5. Scenario application and simulation of WWTPs for the Rocky River QUAL2K model.
Permit Scenario Mooresville Mallard WSACC Muddy West Long Norwood New CC Grassy Monroe
Limits RR WWTP Creek RRRWWTP Creek Stanly Creek WWTP WWTP1 4 Branch WWTp2
Existing 34.0 6.0
DAF Flow 7.5 16.0 1.0 2.5 16.0 0.75 0.120 15.4
(MGD) Future Interim 40.0 12.0
Future 50.0
Existing
Mean DO >_6.0 >_6.0 >_6.0 >_5.0 >_ Monitor 6.0 >_5.0 >_6.0 >_6.0 >_6.0
(mg/L) Future only
Summer Existing 1.6
NH33 1.0 1.0 1.0 1.1 Monitor 1.0 2.0 1.0
(mg/L) Future Interim 1.5 only
Future 1.5
1.8
Winter Existing 3.5
NH3 2.0 2.0 2.0 2.4 Monitor 1.9 4.0 1.8
(mg/L) Future Interim 3. only
Future 3.2
Summer Existing 4.2 as 10.0 as
BOD5 5.0 CBOD5 CBOD5 5.0 10.0 5.0 5.0
(mg/L) Future
5.0 30.0 5.0
Winter Existing 8.3 as 20.0 as
BOD5 10.0 CBOD5 CBOD5 10.0 20.0 10.0 10.0
(mg/L) Future
Existing Monitor Monitor
TN and Monitor only Monitor Monitor only only Monitor Monitor N/A Monitor
TP (mg/L) Monitor only only only (+TKN, (+TKN, only only only
Future NOX) NOX)
Modeled in separate QUAL2K of Crooked Creek(CC).
2 Modeled Implicitly as assimilated along Richardson Creek,speculative limits.
3 NH3 limits for WSACC RRRWWTP are determined based on site-specific chronic toxicity,see Appendix A.
^All future limits for new CC flow and water quality are TBD.
mg/L=milligram per liter, NOX=nitrogen dioxide,TKN=total Kjeldahl nitrogen,TN=total nitrogen,TP=total phosphorus
NTETRA TECH 8

Memorandum- Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
5.0 SCENARIO APPLICATION
Model results for all six scenario applications indicate that existing water quality standards are met
downstream of the RRRWWTP discharge under expanded flow tiers from 34 MGD up to 40 and 50 MGD.
The observed conditions that were modeled as part of the baseline calibration condition indicated very
low instream NH3 concentrations along the Rocky River at 0.03 mg/L and DO concentrations staying
above the instream DO water quality standards as well, at 6.24 mg/L (Table 6).
Results for summer critical conditions with WWTPs simulated at currently permitted limits for flow and
water quality indicate that maximum instream concentrations hit about 1.0 mg/L where the outfall enters
the river(allowable by site-specific NH3 toxicity criteria), which rapidly declines. When RRRWWTP
effluent flows increase to 40 and 50 MGD, model results indicate that NH3 concentrations increase very
slightly (to 1.01 and 1.07 mg/L for 40 and 50 MGD respectively), while DO concentrations improve from
current limits (5.75 mg/L DO instream at 34 MGD increasing to 5.78 and 5.81 mg/L for 40 and 50 MGD
respectively).
TN and TP concentrations at the terminus of the Rocky River under typical summer conditions were
simulated when calibrated to be 9.37 and 0.62 mg/L respectively, while existing permit limits bring these
values up to 15.97 and 1.13 mg/L, respectively. The increased flow tiers of 40 and 50 MGD from
RRRWWTP increase concentrations at the terminus to 17.30 and 18.36 mg/L TN respectively, and 1.34
and 1.44 mg/L TP, respectively.
These scenario results indicate that increased flow capacity from RRRWWTP is not likely to exacerbate
low DO concentrations instream. While NH3 concentrations do increase in the immediate vicinity of the
outfall, site-specific permit limits for NH3 would have to decrease from the currently permitted 1.6 mg/L
NH3 at 34 MGD down to 1.5 mg/L NH3 for both the 40 and 50 MGD flow tiers. Total nutrient
concentrations increase at the terminus of the Rocky River under RRRWWTP expanded flow tiers due to
increased loading under both seasonal critical conditions.
Table 6. Water quality results downstream of RRRWWTP by scenario.
Rocky# Scenario RRRWWTP Minimum Maximum TN at Rocky TP at
Flow DO below NH3 below River Outlet River Outlet
0 Baseline Calibration Model 19.7 6.24 0.03 9.37 0.62
Existing Maximum Permit Limits
1 Summer Critical Conditions 5.75 1.02 15.97 1.13
34
2 Winter Critical Conditions 7.29 2.06 18.30 1.12
Intermediate Permit Limits'
3 Summer Critical Conditions 5.78 1.01 17.30 1.34
40
4 Winter Critical Conditions 7.25 2.12 19.71 1.32
Final Permit Limits'
5 Summer Critical Conditions 5.81 1.07 18.36 1.44
50
6 Winter Critical Conditions 7.20 2.15 20.81 1.42
New Crooked Creek WWTP flow limits were 12.0 MGD for scenario 3-6.
aTETRA TECH 9

Memorandum- Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
Based on the model results, existing DO water quality standards and site-specific NH3 toxicity criteria
should maintain aquatic habitat integrity based on current regulations (Table 7). Instream condition
requirements are met downstream of the RRRWWTP if the increased flow tiers of 40 and 50 MGD:
1. Maintain existing CBOD5 limits of 10 mg/L in summer and 20 mg/L in winter,
2. Maintain existing DO daily average of greater than or equal to 6.0 mg/L, and
3. Decrease NH3 limits from 1.6 to 1.5 mg/L in summer(both 40 and 50 MGD tiers), and
4. Decrease NH3 limits from 3.5 to 3.4 and 3.2 mg/L for the 40 and 50 MGD tiers, respectively.
Table 7. Speculative limits for Rocky River Regional WWTP based on assimilative capacity evaluation.
Phased Flow Tier Flow D• • :OD
Constructed Permitted (Current Operation) 26.5 >_6.0 1.7 3.9 15.4 22.6
Intermediate Permitted (ATC) 30 >_6.0 1.6 3.7 10 20
Final Permitted (ATC) 34 >_6.0 1.6 3.5 10 20
Intermediate Proposed 40 >_6.0 1.5 3.4 10 20
Final Proposed 50 >_6.0 1.5 3.2 10 20
Summer and winter period model results for flow, DO, ammonia, conductivity, TN, and TIP are presented
graphically below in Section 5.1 (Figures 1 -6) and Section 5.2 (Figures 7- 12), respectively. For the
summer results, the baseline calibration model results are included for comparison since they represent
close to critical summer conditions.
aTETRA TECH 10

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
5.1 SUMMER SCENARIOS MODELING RESULTS BY PARAMETER
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Figure 2. Rocky River simulated baseline and expansion instream DO, summer critical conditions.
aTETRA TECH 11

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
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Figure 3. Rocky River simulated baseline and expansion instream ammonia, summer critical conditions.
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Figure 4. Rocky River simulated baseline and expansion instream conductivity, summer critical
conditions.
OTETRA TECH 12

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
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20 r -�-_ -- -
JC �'
C 15
Z
10
N
C N
5 N M C N
y O
m y O uWf m Y U N H -
0
O LL W in Z a
12
0 10 20 30 40 50 60 70 80 90
Distance from Headwaters(mi)
Baseline 34 MGD — -40 MGD 50 MGD
Figure 5. Rocky River simulated baseline and expansion instream total nitrogen, summer critical
conditions.
a
v y w F- c
0
v T 3= m 3 v
Gl m L > 0 0 oD m N
¢� �p y u W I L aU O O C t C
p 0 3 v UCO IX a� o l7 U ° s
4.5 1
4.0 . .
3.5
3.0 _
m 2.5 _
E
a 2.0
1.5 —
I
1.0 _ c
N
O ^ C =
0.5 > a) - 3 a w � c u► E
O VI a+ N Y U N VI y
O O Vr IIA Z a
0.0 —
0 10 20 30 40 s0 60 70 80 90
Distance from Headwaters(mi)
Baseline 34 MGD — -40 MGD 50 MGD
Figure 6. Rocky River simulated baseline and expansion instream total phosphorus, summer critical
conditions.
OTETRA TECH 13

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
5.2 WINTER SCENARIOS MODELING RESULTS BY PARAMETER
a
v a w o
v v > 3m m 3 N Y
i m Y — -0 v ?�L 2 y O O o0 ro N
v O to f0 -0 U v v O O c L C
G o 3 U v cc �'� o C7 U R
300
250
200 --_----
lr
3 150
0
100
c
C N N j
50 0 c
O1 3 d £ w c n E
O N ++ Y U
0 LL C7 n z a F-
0 10 20 30 40 50 60 70 80 90
Distance from Headwaters(mi)
34MGD —•-40MGD - 50MGD
Figure 7. Rocky River simulated expansion streamflow, winter critical conditions.
a
a o
Co
0) Y Y 3 �- O O ticf0 QJ
y o CO a -0U KN _UO O s c
p 0 U U ccD:•L 3 C7 U J ro
13
12 1
11
_ --_ - _-_----------- ---- ------ - --------
10
�00 9 I�
O 8
0
7
6
c �
5 -0 N .O W O Y rV CC
E C
M o N a Y U
_ ,.
A ------------- -----------------i-LL---'�- ----. C7 z --------'
0 10 20 30 40 50 60 70 40 90
Distance from Headwaters(mi)
—34MGD — • -40MGD ---50 MGD - WQ54.0 mg/L DO Saturation
Figure 8. Rocky River simulated expansion instream DO, winter critical conditions.
aTETRA TECH 14

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
a r-
0 ~4- 7 0
.N a! f0 T COm N Y -2 N
v ca �Y 3 y5.
0 0 U U Cocc= 0 C U U J cc J
2.5 C L
2.0 DWR allowable
r site-specific NH3 standard — —
o� 1.5
E - - - - - _
E
0
1.0
a
0.5
c
o I 0
LO
�i lO dd (U
LL � L N N z d a/
0.0
0 10 20 30 40 50 60 70 80 90
Distance from Headwaters(mi)
34MGD —•—40MGD SOMGD NH3 Toxicity Limit
Figure 9. Rocky River simulated expansion instream ammonia, winter critical conditions.
a
v a w t 0
5 U
d' �j Co coD a) Y "6 N
QJ m Y "O�[ O 3 L U .a O O 00 N a
a) O 16 /c0 -0 U a) a. H y. .a O O C L C
0 0 U C U Co J J
600 C L
r
500
E 400
u
i
300
0
0 U 200
c
100 N N m 00 N
C a! N c u1 E
0 G :) LL (D Ln Z a
0 10 20 30 40 50 60 70 8o 90
Distance from Headwaters(mi)
34 MGD — • -40 MGD 50 MGD
Figure 10. Rocky River simulated expansion instream conductivity, winter critical conditions.
OTETRA TECH 15

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
a a w C
3
-10, -6
d OC Y f4 -0 Y 'O 3 m L 3 O O oD M aj
N O CIOM N 17 U N CC H aU-. O O C L C
3 lJ U co �= UJO C J
30
f
25
20
J
C
G 15
Z
H
10
Y
C N
C f0
5 O 0 CO U) C
O1 3 Ca E a, L E
D
0 10 20 30 40 50 60 70 80 90
Distance from Headwaters(mi)
34 MGD —• -40 MGD - -50 MGD
Figure 11. Rocky River simulated expansion instream total nitrogen, winter critical conditions.
a
a F C
3 0
NCU
� a, T m CO 3 N Y _ N
UJ � Y 'O Y U 3 L > O O OD rC a1
> O m rp tC O U a.H U 9 O O C L C
❑ O 3 U V CO OC'�
4.5 I
4.0
1
3.5 -
3.0 -
2.5 `-♦_ -
2.0
1.5
1.0 --
c
o 0 c
m O N C
0.5 W to N E N N C Ln E
M Ln O N f0 Y V N W
0.0 D c` �. U N Z a
0 10 20 30 40 50 60 70 80 90
Distance from Headwaters(mi)
34MGD — • -40MGD -50MGD
Figure 12. Rocky River simulated expansion instream total phosphorus, winter critical conditions.
aTETRA TECH 16

Memorandum- Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
APPENDIX
NC DWR calculated the site-specific NH3 criteria for RRRWWTP in August 2021 based on the equation
for the chronic criterion magnitude:
CCC = 0.8876 x ( 0.0278 + 1.1994 1 x (2.126 x 100.02
l 8x(20-MAX(T,7))
1 + 10(7.688-pH) 1 + 10(pH-7.688)
For each increasing flow tier, the proportion of wasteflow relative to 7Q10 streamflow increases
incrementally, and the relative mix is used to calculate the site-specific instream NH3 concentration which
was determined to be 1.2 mg/L in summer, and 2.3 mg/L in winter. Applicable for each scenario are
seasonal background water quality concentrations and 7Q10 seasonal low flows.
Table 8. Site-specific NH3 wasteload allocation calculations by seasonal flow scenario.
Parameter Summer Winter
Existing Proposed Proposed Existing ' Proposed Proposedo
Permit Interim Future ermit Interim Future
Maximum Maximun,6�
Instream Conditions
7Q10 Flow(cfs) 20.7 31.9
Instream Temperature(°C) 25.75 13.54
Instream pH 7.6 7.4
Instream NH3(mg/L) 0.22 0.22
Effluent Conditions
Effluent Flow(MGD) 34 40 50 34 40 50
Effluent Temperature(°C) 27 20
Effluent pH 6.8 6.5
Mixed Instream Conditions
Mixed Temperature(°C) 26.6 26.7 26.7 17.6 17.8 18.1
Mixed pH 7.0 7.0 7.0 6.8 6.8 6.8
Mixed NH3(mg/L) 1.2 2.3
Proposed NH3 Effluent Concentration
Calculated Effluent NH3 (mg/L) 1.6 1.5 1.5 3.5 3.4 3.2
aTETRA TECH A-1 Appendix A

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
APPENDIX • • ' MALLARD AND LONG
Rocky River modeling and scenario application both included explicit simulation of the mainstems of both
Mallard Creek and Long Creek. Model results for these streams are included below, with graphical
presentation showing scenario conditions, with comparison to the baseline calibration model results
during summer(Figure 13- Figure 36). Scenario results represent seasonal critical conditions with the
wastewater treatment plant that discharges to each stream (Mallard Creek WWTP and Long Creek
WWTP) at currently permitted limits for flow and water quality. Although RRRWWTP flow tiers were
increased to 40 and 50 MGD, those scenarios do not produce any changes to these two tributaries, so
those results were not included below.
13.1 MALLARD CREEK SCENARIO GRAPHICS
5
r
30
25
20
3 15
0
S
10
M M �
c
5 L C
N
> O
0 Ilk
A ~
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Headwaters(mi)
-Baseline -Scenario Results
Figure 13. Mallard Creek simulated baseline and expansion streamflow during summer critical conditions.
OTETRA TECH B-1 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
T.
8
6
5
00
E 4 -
O
C)
3
2
M
o -
c T w -
1 O s C
> `0
E
0
a r
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Headwaters(mi)
Baseline Scenario Results ------- WQS 4.0 mg/L
Figure 14. Mallard Creek simulated baseline and expansion instream DO during summer critical
conditions.
a
r
a
m
M
1.2
1
0.8
E
E 0.6
0
E
E
a 0.4
M
0.2 v
L C
O N •�
> O
0
a)H
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Headwaters(mi)
-Baseline -Scenario Results -NH3 Toxicity Limit
Figure 15. Mallard Creek simulated baseline and expansion instream ammonia during summer critical
conditions.
aTETRA TECH B-2 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
v
m
io
700
600
500
E
V
400
300
v
c
O
CJ 200
100 p
0
a� r
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Headwaters(mi)
Baseline —Scenario Results
I
Figure 16. Mallard Creek simulated baseline and expansion instream conductivity during summer critical
conditions.
3
a
m
30
25
20 I
ou
E 15
F
10
v W
5 C L C
O y E
> O
0 2 ~
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Headwaters(mi)
—Baseline —Scenario Results
Figure 17. Mallard Creek simulated baseline and expansion instream total nitrogen during summer critical
conditions.
OTETRA TECH B-3 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
a
6
5
4
J
E3
a
H
2
E
d H
0
0 0.5 1 1.5 2 2.5 3 IS 4 4.5 S
Distance from Headwaters(mi)
Baseline -Scenario Results
Figure 18. Mallard Creek simulated baseline and expansion instream total phosphorus during summer
critical conditions.
a
3
0 3 ccy
J
cu CC)
0 1: UZ
N J m
J J
35
30
25
-j� 20
u
3
LL 15
10
E
c
Y = CO <-
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
—Baseline —Scenario Results
Figure 19. Long Creek simulated baseline and expansion streamflow during summer critical conditions.
aTETRA TECH B-4 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
o_
c � v
0 3 Co v
v
ccUD
J O J m
9
8
7
6
m 5
E
O 4
0
3
2
N — 7
4/ 00 C_
1 c r U E
Y = Co
t-
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
Baseline Scenario Results WQS 4.0 mg/L
Figure 20. Long Creek simulated baseline and expansion instream DO during summer critical conditions.
Q.
no F
c v
_9 3 co v Cu N m
c Wo
J O J m
1.2
1
:7 0.8
J
£
E 0.6
0
E
Q 0.4
N
0.2 G — 7
N N 00 C
40 L E
C �
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
-Baseline -Scenario Results -NH3 Toxicity Limit
Figure 21. Long Creek simulated baseline and expansion instream ammonia during summer critical
conditions.
aTETRA TECH B-rj Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
L
oo � m v
v Do
:i] U
J O J m
500
450
400
—350
E
u
n 300
250
u
200
c
O
u 150
100 — w
N t co C_
50 to t= s .1 E
c_
Y = Co Z H
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
—Baseline —Scenario Results
Figure 22. Long Creek simulated baseline and expansion instream conductivity during summer critical
conditions.
00
3
L
3 m
„ v CO
t C _U
J .0 m
35
30
25
20
to
E
F 15
10
T v
5 N N v oo c
0o rCU
E
c_
Y = CoZ H
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
—Baseline —Scenario Results
Figure 23. Long Creek simulated baseline and expansion instream total nitrogen during summer critical
conditions.
aTETRA TECH B-6 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
3 L
3 m v
0 — m
J � J
1.4
1.2
1
3 0.8
E
0.6
0.4
0.2 Y v m .00
co r ti E
c
y � N U N
0 = Co Z H
0 1 2 3 4 S 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
Baseline —Scenario Results
Figure 24. Long Creek simulated baseline and expansion instream total phosphorus during summer
critical conditions.
a
r
a
ro
30
25
20
u
3 15
0
LL
10
1�
f0 �
5 O L C
o E
0
a 2cu
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Headwaters(mi)
—Scenario Results
Figure 25. Mallard Creek simulated expansion streamflow during winter critical conditions.
aTETRA TECH B-7 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
T.
12
10
- - - - - - - - - -
8
m
E 6
O
4
M
fo �
2 c w
O L C
w E
> `o
0
a r
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Headwaters(mi)
Scenario Results --- ---WQS 4.0 mg/L - - -DO Saturation
Figure 26. Mallard Creek simulated expansion instream DO during winter critical conditions.
a
r
2
1.8
1.6
1.4
J
CD 1.2
E
f0 1
'c
O
0.8
E
0.6
0.4 °o v
O L C_
0.2 — E
> O
� � H
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Headwaters(mi)
-Scenario Results -NH3 Toxicity Limit
Figure 27. Mallard Creek simulated expansion instream ammonia during winter critical conditions.
aTETRA TECH B-8 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
n
m
700
600
_500
E
u
Y 400
u 300
v
c
O
u 200
100 o L c
w E
0
a
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Headwaters(mi)
—Scenario Results
Figure 28. Mallard Creek simulated expansion instream conductivity during winter critical conditions.
a
7E
30
25
20
J
OD
E 15
Z
F-
10
C 3
5 O -C C_
E E
c r
O G
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Headwaters(mi)
—Scenario Results
Figure 29. Mallard Creek simulated expansion instream total nitrogen during winter critical conditions.
aTETRA TECH B-9 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
H
3
a
m
m
5
4.5
4
3.5
3
J
C�
c 2.5
a
~ 2
1.5
1 20
C
2 L _C
0.5
.� 2 -
0
0 0.5 1 1.5 2 2.5 3 IS 4 4.5 5
Distance from Headwaters(mi)
-Scenario Results
Figure 30. Mallard Creek simulated expansion instream total phosphorus during winter critical conditions.
B.2 LONG CREEK SCENARIO GRAPHICS
a
00 r
c 3 v
J
3 m
Gl m N Cl]
y m_
J J J
40
35
30
25
0
3 20
0
LL
15
10
� N
5 u�i N co 'C
S M L
Y I = m Z H
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
-Scenario Results
Figure 31. Long Creek simulated expansion streamflow during winter critical conditions.
aTETRA TECH B-10 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
3
.0, 3 cc
m v
J J on
12
10
8
J
E 6
O
t7
4 ----------------------------------------------------------------------------------- ----------------------------------- ----- ---
2 N — 00
c: r U E
Y = Co H
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
Scenario Results ------- WQS4.0mg/L — — —DO Saturation
Figure 32. Long Creek simulated expansion instream DO during winter critical conditions.
a
3 v
co
3 m
C Wo
J J co
2
1.8
1.6
1.4
:7
;Z 1.2
E
E 1
O
E 0.8
Q
0.6
0.4IU
w ran C
0.2 on r ti E
C �
Y = Co H
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
—Scenario Results —NH3 Toxicity Limit
Figure 33. Long Creek simulated expansion instream ammonia during winter critical conditions.
aTETRA TECH B-11 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
L
oo � m v
v Do
:i] U
J O J m
450
400
350
E 300
u
Y 250
200
v
c
u 150
100
w —
w
50 on N y co c
t= s 1 E
Y = Co Z H
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
—Scenario Results
Figure 34. Long Creek simulated expansion instream conductivity during winter critical conditions.
CO
c
3
3 m
v m v
C Wo
J J J CO
30
25
20
J
to
1 15
Z
F-
10
5 IU V
t00
V
to # s ti E
Y = co H
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
—Scenario Results
Figure 35. Long Creek simulated expansion instream total nitrogen during winter critical conditions.
aTETRA TECH B-1 2 Appendix B

Memorandum— Rocky River Scenario Application March 26, 2024 (revised June 7, 2024)
a
W 3
J
3 Co v
Q1 m 41 CO
J J CO
1
0.9 IT V
0.8
0.7
0.6
J
m
0.5
C-
0.4
0.3
0.2 — w
coon r v c
0.1 r U E
Y = Co
H
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Distance from Headwaters(mi)
—Scenario Results
Figure 36. Long Creek simulated expansion instream total phosphorus during winter critical conditions.
aTETRA TECH B-13 Appendix B