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HomeMy WebLinkAboutNC0069841_Rocky River Model Application Scenarios Memo_20240326 4000 Sancar Way, Suite 200 • PO Box 14409 Research Triangle Park, NC 27709 Tel 919-485-8278 MEMORANDUM 1 To: NC DEQ DWR Cc: Black & Veatch, WSACC From: Trevor Clements, Hillary Yonce, Will Hicks (Tetra Tech) Date: March 26, 2024 Subject: Rocky River Model Scenario Application to Support Speculative 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 Memorandum – Rocky River Scenario Application March 26, 2024 2 (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 20161, Weaver and Fine 20032). 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 (gage or permit) Drainage Area (sq mi) Summer 7Q10 Flow (cfs) Winter 7Q10 Flow (cfs) Summer 7Q10 Unit Area Flow (cfsm) Winter 7Q10 Unit Area Flow (cfsm) West Branch Rocky River (South Prong) near Cornelius2 02123932 4.98 0.50 0.90 0.100 0.181 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 Concord2 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 Faggarts Crossroads2 02124374 45.5 3.10 8.20 0.068 0.180 Dutch Buffalo Creek at NC 49 near Mount Pleasant2 02124471 45.1 0.70 2.20 0.016 0.049 Goose Creek at Fairview2 02124692 24.0 0.30 1.00 0.013 0.042 Crooked Creek3 N/A N/A N/A N/A 0.001 0.010 1 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/sir20155001/. 2 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/WRIR_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. Memorandum – Rocky River Scenario Application March 26, 2024 3 Location of Estimate Source ID (gage or permit) Drainage Area (sq mi) Summer 7Q10 Flow (cfs) Winter 7Q10 Flow (cfs) Summer 7Q10 Unit Area Flow (cfsm) Winter 7Q10 Unit Area Flow (cfsm) Long Creek WWTP NC0024244 64.0 1.60 9.50 0.025 0.148 Little Bear Creek at Saint Martin2 02124944 12.4 0 (Zero) 0.30 0 (Zero) 0.024 Big Bear Creek near Richfield2 02125000 55.6 0 (Zero) 0.20 0 (Zero) 0.004 Richardson Creek near Marshville2 02125500 163 0.50 1.60 0.003 0.010 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 Table 2. Estimated 7Q10 low flow for boundary conditions of the Rocky River QUAL2K model application. QUAL2K Model Boundary Condition Drainage Area (sq mi) Applied Unit Area Discharge from Table 1 Calibration Model Flow (cfs) Summer 7Q10 Flow (cfs) Winter 7Q10 Flow (cfs) Dye Creek headwaters 3.96 West Branch Rocky 1.06 0.40 0.72 Rocky River as a tributary 3.72 West Branch Rocky 0.33 0.37 0.67 West Branch Rocky River 22.9 West Branch Rocky 2.11 2.30 4.14 Clarke Creek 28.1 Clarke Creek 1.26 1.28 2.95 Mallard Creek headwaters 34.7 Mallard Creek WWTP 3.53 0.59 1.94 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 Creek 98.7 Dutch Buffalo Creek 2.17 1.53 4.81 Clear Creek 24.5 Goose Creek 1.82 0.31 1.02 Goose Creek 42.3 Goose Creek 0.92 0.53 1.76 Memorandum – Rocky River Scenario Application March 26, 2024 4 QUAL2K Model Boundary Condition Drainage Area (sq mi) Applied Unit Area Discharge from Table 1 Calibration Model Flow (cfs) Summer 7Q10 Flow (cfs) Winter 7Q10 Flow (cfs) 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 headwaters1 32.0 Long Creek WWTP 0.05 0.11 0.67 Little Long Creek1 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 1 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. 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 Memorandum – Rocky River Scenario Application March 26, 2024 5 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. 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 3). Included in this table are the more typical effluent discharge volumes in summer 2023 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 3. Modeled WWTP typical, permitted design average, and speculative design average flows. WWTP Owner Receiving Water NPDES ID Mean Flow (MGD) Permit Date Current DAF (MGD) Future ATC or Speculative DAF (MGD) Rocky River Mooresville Dye Br NC0046728 3.95 10/7/2019 7.5 No change Mallard Creek Charlotte Water Mallard Cr NC0030210 7.22 12/17/2019 12.0 16.0 (ATC issued) Rocky River Regional WSACC Rocky Riv NC0036269 19.70 9/29/2021 26.5 30.0, 34.0 (ATC issued) 40.0, 50.0 (speculative) Muddy Creek WSACC Rocky Riv NC0081621 0.14 5/10/2021 0.30 1.0 (no ATC) West Stanly Stanly Co. Rocky Riv NC0043532 0.43 12/11/2023 1.2 2.5 (no ATC) Long Creek Albemarle Long Cr NC0024244 1.78 3/23/2023 12.0 16.0 (no ATC) Norwood Norwood Rocky Riv NC0021628 0.07 2/6/2024 0.75 No change New Crooked Creek1,2 Union Co. Crooked Cr TBD1 Implicit2 TBD1 1.91 6, 12.0 1 Grassy Branch2 Union Co. Crooked Cr NC0085812 Implicit2 4/5/2022 0.05 0.12 (no ATC) Monroe2 Monroe Richardson Cr NC0024333 Implicit2 12/17/2021 12.5 15.4 (no ATC) 1 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 Memorandum – Rocky River Scenario Application March 26, 2024 6 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 4). 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 20134. A summary of these calculations is included in Appendix A. 4 https://www.epa.gov/sites/default/files/2015-08/documents/aquatic-life-ambient-water-quality-criteria-for-ammonia- freshwater-2013.pdf Memorandum – Rocky River Scenario Application March 26, 2024 7 Table 4. Scenario application and simulation of WWTPs for the Rocky River QUAL2K model. Permit Limits Scenario Mooresville RR WWTP Mallard Creek WWTP WSACC RRRWWTP Muddy Creek WWTP West Stanly WWTP Long Creek WWTP Norwood WWTP New CC WWTP1,4 Grassy Branch WWTP1 Monroe WWTP2 DAF Flow (MGD) Existing 7.5 16.0 34.0 1.0 2.5 16.0 0.75 6.0 0.120 15.4 Future Interim 40.0 Future 50.0 12.0 Mean DO (mg/L) Existing ≥ 6.0 ≥ 6.0 ≥ 6.0 ≥ 5.0 ≥ 6.0 ≥ 5.0 Monitor only ≥ 6.0 ≥ 6.0 ≥ 6.0 Future Summer NH33 (mg/L) Existing 1.0 1.0 1.6 1.0 1.8 1.1 Monitor only 1.0 2.0 1.0 Future Interim 1.5 Future 1.5 Winter NH3 (mg/L) Existing 2.0 2.0 3.5 2.0 2.4 Monitor only 1.9 4.0 1.8 Future Interim 3.4 Future 3.2 Summer BOD5 (mg/L) Existing 5.0 4.2 as CBOD5 10.0 as CBOD5 5.0 5.0 10.0 30.0 5.0 5.0 5.0 Future Winter BOD5 (mg/L) Existing 10.0 8.3 as CBOD5 20.0 as CBOD5 10.0 20.0 10.0 10.0 Future TN and TP (mg/L) Existing Monitor only Monitor only Monitor only Monitor only Monitor only (+TKN, NOX) Monitor only (+TKN, NOX) Monitor only Monitor only N/A Monitor only Future 1 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. 4 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 Memorandum – Rocky River Scenario Application March 26, 2024 8 5.0 SCENARIO APPLICATION RESULTS 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 5). 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 1.07 mg/L), while DO concentrations improve from current limits (5.47 mg/L DO instream at 34 MGD increasing to 5.76 and 5.80 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 15.88 and 1.11 mg/L, respectively. The increased flow tiers of 40 and 50 MGD from RRRWWTP increase concentrations at the terminus to 17.31 and 18.37 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 5. Water quality results downstream of RRRWWTP by scenario. # Scenario RRRWWTP Flow (MGD) Minimum DO below RRRWWTP (mg/L) Maximum NH3 below RRRWWTP (mg/L) TN at Rocky River Outlet (mg/L) TP at Rocky River Outlet (mg/L) 0 Baseline Calibration Model 19.7 6.24 0.03 9.37 0.62 Existing Maximum Permit Limits 1 Summer Critical Conditions 34 5.74 1.02 15.96 1.13 2 Winter Critical Conditions 7.27 2.06 18.30 1.12 Intermediate Permit Limits1 3 Summer Critical Conditions 40 5.76 1.01 17.31 1.34 4 Winter Critical Conditions 7.24 2.12 19.71 1.32 Final Permit Limits1 5 Summer Critical Conditions 50 5.80 1.07 18.37 1.44 6 Winter Critical Conditions 7.18 2.15 20.81 1.42 1 New Crooked Creek WWTP flow limits were 12.0 MGD for scenario 3 – 6. Memorandum – Rocky River Scenario Application March 26, 2024 9 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 6). 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 6. Speculative limits for Rocky River Regional WWTP based on assimilative capacity evaluation. Phased Flow Tier Flow (MGD) DO (mg/L) NH3 (mg/L) CBOD5 (mg/L) Summer Winter Summer Winter 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 TP 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. Memorandum – Rocky River Scenario Application March 26, 2024 10 5.1 SUMMER SCENARIOS MODELING RESULTS BY PARAMETER Figure 1. Rocky River simulated baseline and expansion streamflow, summer critical conditions. Figure 2. Rocky River simulated baseline and expansion instream DO, summer critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 11 Figure 3. Rocky River simulated baseline and expansion instream ammonia, summer critical conditions. Figure 4. Rocky River simulated baseline and expansion instream conductivity, summer critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 12 Figure 5. Rocky River simulated baseline and expansion instream total nitrogen, summer critical conditions. Figure 6. Rocky River simulated baseline and expansion instream total phosphorus, summer critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 13 5.2 WINTER SCENARIOS MODELING RESULTS BY PARAMETER Figure 7. Rocky River simulated expansion streamflow, winter critical conditions. Figure 8. Rocky River simulated expansion instream DO, winter critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 14 Figure 9. Rocky River simulated expansion instream ammonia, winter critical conditions. Figure 10. Rocky River simulated expansion instream conductivity, winter critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 15 Figure 11. Rocky River simulated expansion instream total nitrogen, winter critical conditions. Figure 12. Rocky River simulated expansion instream total phosphorus, winter critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 16 Memorandum – Rocky River Scenario Application March 26, 2024 A-1 Appendix A APPENDIX A: SITE-SPECIFIC NH3 CRITERIA FOR RRRWWTP 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 × ൬ 0.0278 1 + 10 (଻.଺଼଼ି௣ு)+1.1994 1 + 10 (௣ுି଻.଺଼଼)൰ × ൫2.126 × 10଴.଴ଶ଼×(ଶ଴ିெ (்,଻))൯ 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 7. Site-specific NH3 wasteload allocation calculations by seasonal flow scenario. Parameter Summer Winter Existing Permit Maximum Proposed Interim Proposed Future Existing Permit Maximum Proposed Interim Proposed Future 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 Memorandum – Rocky River Scenario Application March 26, 2024 B-1 Appendix B APPENDIX B: MODEL RESULTS FOR MALLARD AND LONG CREEKS 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. B.1 MALLARD CREEK SCENARIO GRAPHICS Figure 13. Mallard Creek simulated baseline and expansion streamflow during summer critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-2 Appendix B Figure 14. Mallard Creek simulated baseline and expansion instream DO during summer critical conditions. Figure 15. Mallard Creek simulated baseline and expansion instream ammonia during summer critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-3 Appendix B Figure 16. Mallard Creek simulated baseline and expansion instream conductivity during summer critical conditions. Figure 17. Mallard Creek simulated baseline and expansion instream total nitrogen during summer critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-4 Appendix B Figure 18. Mallard Creek simulated baseline and expansion instream total phosphorus during summer critical conditions. Figure 19. Long Creek simulated baseline and expansion streamflow during summer critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-5 Appendix B Figure 20. Long Creek simulated baseline and expansion instream DO during summer critical conditions. Figure 21. Long Creek simulated baseline and expansion instream ammonia during summer critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-6 Appendix B Figure 22. Long Creek simulated baseline and expansion instream conductivity during summer critical conditions. Figure 23. Long Creek simulated baseline and expansion instream total nitrogen during summer critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-7 Appendix B Figure 24. Long Creek simulated baseline and expansion instream total phosphorus during summer critical conditions. Figure 25. Mallard Creek simulated expansion streamflow during winter critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-8 Appendix B Figure 26. Mallard Creek simulated expansion instream DO during winter critical conditions. Figure 27. Mallard Creek simulated expansion instream ammonia during winter critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-9 Appendix B Figure 28. Mallard Creek simulated expansion instream conductivity during winter critical conditions. Figure 29. Mallard Creek simulated expansion instream total nitrogen during winter critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-10 Appendix B Figure 30. Mallard Creek simulated expansion instream total phosphorus during winter critical conditions. B.2 LONG CREEK SCENARIO GRAPHICS Figure 31. Long Creek simulated expansion streamflow during winter critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-11 Appendix B Figure 32. Long Creek simulated expansion instream DO during winter critical conditions. Figure 33. Long Creek simulated expansion instream ammonia during winter critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-12 Appendix B Figure 34. Long Creek simulated expansion instream conductivity during winter critical conditions. Figure 35. Long Creek simulated expansion instream total nitrogen during winter critical conditions. Memorandum – Rocky River Scenario Application March 26, 2024 B-13 Appendix B Figure 36. Long Creek simulated expansion instream total phosphorus during winter critical conditions.