Document Belgrade_doc_004386d648

Source Specific Mixing Zone Report Source Specific Mixing Zone Report Belgrade Water Reclamation Facility Belgrade Water Reclamation Facility Western Groundwater Services, LLC 6595 Bear Claw Lane Bozeman, MT 59715 (406) 579-1493 City of Belgrade, Montana March 2023 ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page i Table of Contents 1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2. CONCEPTUAL MODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 Effluent Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Effluent Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.3 Subsurface Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4 Groundwater Level and Flow Direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.5 Aquifer Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.6 Groundwater Recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.7 Groundwater Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.8 Groundwater Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3. MODEL SETUP AND CALIBRATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.1 Software and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.2 Flow Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.3 Transport Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4. Source Specific Mixing Zone Delineation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.1 BWRF Discharge Projection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.2 Denitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.3 Initial Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.4 Nitrogen Transport Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.5 Source Specific Mixing Zone (SSMZ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 5. MONITORING PLAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.1 Existing Monitoring Wells and Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.2 Proposed Monitoring Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 6. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 List of Tables Table 1. Groundwater Recharge Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Table 2. Groundwater Discharge Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Table 3. Regional Model Mass Balance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Table 4. Calibrated Dispersivity Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Table 5. Estimated Storage and Porosity Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Table 6. Groundwater Mixing Zone Criteria from ARM 17.30.518. . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Table 7. Sampling and Analysis for Existing Permit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Table 8. Proposed Sampling Wells and Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page ii Table of Contents (Continued) List of Appendices Appendix A – Well Logs and Data List of Figures Figure 1. Location Map. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Figure 2. Effluent Discharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 4. Effluent Nitrogen Box Plots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 3. Effluent Chloride. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 5. Effluent Nitrogen Concentrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 6. Regional Geology Map. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 7. Local Geology Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 8. Cross Section A-A’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 9. Cross Section B-B’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 10. Cross Section C-C’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 11. Water Table Map. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 12. BWRF Well Location Map. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 13. Well Location Map. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 14. BWRF Monitor Well Water Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 15. Well Water Levels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 16. Aquifer Testing Hydrographs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 17. IP Discharge During Aquifer Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 18. Aquifer Testing Distance-Drawdown Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 19. Surface Water Map. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 20. Water Rights Map. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 21. BWRF Monitoring Well Time-History Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Figure 22. IP-A Cl/NOx Plots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Figure 23. IP-B Cl/NOx Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Figure 24. IP-C Cl/NOx Plots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Figure 25. Model Finite-Difference Grid and Boundaries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Figure 26. Flow Calibration Objective Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Figure 27. Calibration Target Plots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Figure 28. Calibrated Horizontal Hydraulic Conductivity, Layer 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Figure 29. Calibrated Vertical Hydraulic Conductivity, Layer 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Figure 30. Flooded and Dry Model Cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Figure 31. Modeled Groundwater Elevation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Figure 32. Refined Mesh (Layers 1 - 6) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Figure 33. Chloride Transport Calibration Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Figure 34. Simulated Chloride Concentrations, Layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Figure 35. Nitrogen Transport Residual Plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Figure 36. Simulated Nitrogen Concentrations, Layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Figure 37. Projected BWRF Discharge as Daily Average . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Figure 38. Simulated Nitrogen Concentrations, Model Layer 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Figure 39. Simulation Time History Plot Locations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Figure 40. Simulated Nitrogen Time History Plots Near IP Beds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Figure 41. Simulated Nitrogen Time History Plots Near IP-B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Figure 42. Simulated Nitrogen Time History Plots Down Gradient. . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Figure 43. Proposed Source Specific Mixing Zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Figure 44. Existing Monitoring Well Network. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Figure 45. Proposed Monitoring Well Network. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Figure 46. Monitoring Well and Sampling Pump Illustration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 ---PAGE BREAK--- Western Groundwater Services Page 1 03/24/2023 1. INTRODUCTION The City of Belgrade has completed upgrades to the wastewater treatment plant, referred to as the Belgrade Water Reclamation Facility (BWRF). The upgraded facility includes oxidation ditch treatment technology that achieves total nitrogen treatment levels of 5- to 8-milligrams nitrogen per liter (mg N/L).1 The City has operated the upgraded plant since August 2022. The upgraded facility discharge is regulated by Montana Ground Water Pollution Control System Permit Number MTX000116, renewed 10/1/2018 and expiring 9/30/2023. The City will be renewing this permit in the first part of 2023.2 The permit is required for discharge of the treated effluent from the plant to groundwater. During the summer months, a portion of the treated effluent is also discharged onto the land as irrigaiton. The discharge to groundwater occurs through three groups of Infiltration/Percolation (IP) beds B, and C) in proximity to the irrigation area (Figure For the present permit, the Department of Environmental Quality (DEQ) has determined that groundwater discharge from the BWRF qualifies for a standard groundwater mixing zone. The mixing zone allows for a land area immediately from the discharge location where exceedances of applicable water quality standards are allowed. The standard mixing zone has been established administratively in the permitting process allowing for a down-gradient length of 750 ft.3 The permit specifies maximum daily loads for total nitrogen, which are intended to prevent exceedance of 10 milligrams nitrogen per liter (mg N/L) in groundwater at the down-gradient end of the mixing zone. The limit of 10 mg N/L is the human health standard and is applied to groundwater in the State. A source specific mixing zone (SSMZ) can be requested by the City based on site specific data. This mixing zone can be incorporated into discharge permitting and is intended to more accurately represent the actual discharge than a standard mixing zone. This report conducts technical analysis to determine an SSMZ for the newly upgraded BWRF. A mixing zone study for the BWRF prior to the upgrade was completed in 2020.4 The basic data from this earlier report have been retained for the current report. The current work upgrades groundwater modeling software and provides analysis for the upgraded BWRF considering the increased discharge capacity and the improvement in nitrogen treatment. Modeling work included migrating the existing model to updated software, recalibration of the model to flow and concentration data, and simulation of plant performance at design capacity and effluent quality. Through this analysis, the SSMZ is defined with respect to horizontal and vertical extent. 1 Ovivo Carrousel(R) System, 2 permits are renewed every five years. 3 The depth of a standard mixing zone extends from the water table beneath the IP beds down 15 ft. The width is approximately equal to the width of the IP beds normal to the direction of groundwater flow. A width of 906 ft has been specified for IP-A and IP-B. A width of 1,292 ft has been specified for IP-C. 4 Western Groundwater Services, LLC (2020) Source Specific Mixing Zone Report, Wastewater Treatment Plant Discharge, City of Belgrade, Montana, prepared for TDH Engineering, Inc. ---PAGE BREAK--- Page 2 Western Groundwater Services IP-B IP-A IP-C Spray Irrigation BWRF Treatment Cells Standard Mixing Zone (approx.) Treatment Upgrades (construction in-progress) ± 0 1,500 3,000 Feet Figure 1. Location Map ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 3 2. CONCEPTUAL MODEL The conceptual model documents information and interpretations of the physical-chemical system related to discharge from the IP beds to groundwater, and subsequent transport in groundwater to the discharge locations. This information establishes calibration data for modeling, and the requirements for the computer model described in the next section. 2.1 Effluent Discharge Historic effluent discharge occurs intermittently to the three IP beds and the irrigation system (Figure There is one flow meter for the combined discharge to the IP-A and IP-B beds, and a second flow meter for discharge to the IP-C bed and the irrigation system. Since September 2018 IP-B has been put back into service. Of the IP-A/IP-B total discharge, 1/3 now goes to IP-A and 2/3 to IP-B. IP-C and the irrigation system are run individually. In 2018, 45% of the discharge was through the irrigation system; 28% was through IP-C; 21% was through IP-A and 6% was through IP-B. The daily average discharge rate to the IP beds was 224 gallons per minute (gpm), or 333,000 gallons per day (gpd). 2.2 Effluent Water Quality Parameters of interest for the mixing zone model are chloride (Cl) and nitrogen species. The data record extends from January 2010 into 2019, with some data gaps (Figure 3, Figure 4, Figure Nitrogen sample data include ammonia (NH3), total Kjeldahl (TKN)5, and nitrate plus nitrite (ΣNOx). Total nitrogen (TN) is calculated from the species as TN = TKN – NH3 + ΣNOx. Organic nitrogen (OrgN) was also estimated as OrgN = TKN – NH3. For some sampling dates, TN and OrgN could not be calculated as above, and were estimated from available specie data (hence use of TN* and OrgN* on the plot axis labels). Both chloride and nitrogen species exhibit seasonality. Chloride concentrations change in relation to dilution effects whereas nitrogen concentrations are affected by increased biomass growth during warm weather. Using data from 2010 through August 2019, annual average concentration for chloride is 146 mg/L. From the same dataset, annual averages for nitrogen species (as mg N/L) were: 1) total nitrogen – 24.8; organic nitrogen – 5.2; ammonia – 15.9; and nitrate plus nitrite 4.9. The average daily loading for total nitrogen in 2018 based on flows and water quality averages ranged from 5- to 65-pounds per day per IP bed.6 Lower total IP loading rates to 78-lbs/d) are realized during the summer months due to irrigation discharge; higher total IP loading rates (47- to 131-lbs/d) occur in the non-irrigation months. At least two IP beds are operating outside the irrigation season, except that three IP beds were operated during the fourth quarter of 2018. One IP bed was operated during the irrigation season (late April – early October). 5 TKN is a measurement of organic and ammonia nitrogen. 6 Calculated as the average concentration multiplied by the total discharge. ---PAGE BREAK--- Page 4 Western Groundwater Services 1/1/10 1/1/11 1/1/12 1/1/13 1/1/14 1/1/15 1/1/16 1/1/17 1/1/18 1/1/19 0 200,000 400,000 600,000 800,000 IP-B (gpd) 1/1/10 1/1/11 1/1/12 1/1/13 1/1/14 1/1/15 1/1/16 1/1/17 1/1/18 1/1/19 0 200,000 400,000 600,000 800,000 IP-A (gpd) 1/1/10 1/1/11 1/1/12 1/1/13 1/1/14 1/1/15 1/1/16 1/1/17 1/1/18 1/1/19 0 400,000 800,000 1,200,000 1,600,000 IP-C (gpd) 1/1/10 1/1/11 1/1/12 1/1/13 1/1/14 1/1/15 1/1/16 1/1/17 1/1/18 1/1/19 0 400,000 800,000 1,200,000 1,600,000 IRR (gpd) Data are daily totals from effluent flow meters. Figure 2. Effluent Discharge ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 5 100 110 120 130 140 150 160 170 180 190 200 Cl (mg/L) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Plots based on grab samples of effluent. 1/1/10 1/1/11 1/1/12 1/1/13 1/1/14 1/1/15 1/1/16 1/1/17 1/1/18 1/1/19 0 20 40 60 80 100 120 140 160 180 200 Cl (mg/L) Figure 3. Effluent Chloride 0 10 20 30 40 50 SNOx (mg N/L) 0 10 20 30 40 50 NH3 (mg N/L) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0 3 6 9 12 15 OrgN* (mg N/L) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0 10 20 30 40 50 TN* (mg N/L) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 4. Effluent Nitrogen Box Plots ---PAGE BREAK--- Page 6 Western Groundwater Services 1/1/10 1/1/11 1/1/12 1/1/13 1/1/14 1/1/15 1/1/16 1/1/17 1/1/18 1/1/19 0 5 10 15 20 25 30 SNOx (mg N/L) 1/1/10 1/1/11 1/1/12 1/1/13 1/1/14 1/1/15 1/1/16 1/1/17 1/1/18 1/1/19 0 5 10 15 OrgN* (mg N/L) 1/1/10 1/1/11 1/1/12 1/1/13 1/1/14 1/1/15 1/1/16 1/1/17 1/1/18 1/1/19 0 5 10 15 20 NH3 (mg N/L) 1/1/10 1/1/11 1/1/12 1/1/13 1/1/14 1/1/15 1/1/16 1/1/17 1/1/18 1/1/19 0 10 20 30 40 50 60 TN* (mg N/L) Data are from grab samples of effluent. Figure 5. Effluent Nitrogen Concentrations ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 7 2.3 Subsurface Geology The BWRF is located on alluvial sediments mapped as Quaternary (Qabo) braided river deposits.7 Younger braided river deposits (Qab), also of Quaternary age, occur to the east and north (Figure 6, Figure The BWRF area appears to be somewhat of an anomaly in that the shallow sediments are highly permeable. Away from the BWRF these sediments include some fraction of clay and have lower permeability. Based on well log data, the older alluvium (Qabo) was separated into a “clay section” and a “clean section”, although this distinction is not well defined throughout the area (Figure 8, Figure 9, and Figure 10). The base of the alluvium is not defined by well logs below the BWRF, however wells to the north intercept Tertiary bedrock (Tscr)8 at about 190 ft below ground. There is a postulated fault designated the Central Park Fault mapped north of the BWRF and is used to explain greater thickness of alluvium to the south. Well data used in the cross sections and elsewhere in the report are provided in Appendix A. 7 Geological surface mapping data were obtained from the Montana Bureau of Mines and Geology (MBMG) in GIS format (seamless database). The MBMG GIS is primarily a compilation of 1:100,000 quadrangle maps prepared by MBMG staff and the U.S. Geological Survey. 8 Six Mile Creek Formation, Reese Creek Member (Tsrc). ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 8 A A' B B' Qabo Qab Qal Tscr Qafo Central Park Faul t (pos tulate d ) IP C IP B IP A 9670 133162 135735 186839 185586 252338 252336 252339 267822 268896 #4 #2 #1 #3 #5 #6 #7 ± 0 4,000 8,000 2,000 Feet Private well locations Cross section line Belgrade water supply wells Figure 6. Regional Geology Map ---PAGE BREAK--- Page 9 Western Groundwater Services IP C IP B IP A Dry Creek Road C C' Powers Blvd Baseline Road Qabo Qal Qab Qab 1A 1C 2A 2B 2C 3A 3B 3C 4A 4B 4C 6C 5A 5B 6A 6B 7A 7C 8A 7B ± 0 900 1,800 450 Feet Cross section line IP Beds BWRF monitoring wells Figure 7. Local Geology Map ---PAGE BREAK--- Page 10 Western Groundwater Services Belgrade Well #7 Dringle McDowell Qabo (Clay Section) Qabo (Clean Section) Qab 4410 4320 4230 4140 4050 3960 3870 3780 4320 4230 4140 4050 3960 3870 3780 0 Qal Qal Tscr (Reese Cr. Mbr.) 6000 12000 18000 ? ? ? ? ? ? IP Beds A & B A (WEST) A' (EAST) Elev. (ft MSL) Grid Dimensions (H x 150 ft x 30 ft Vertical Exaggeration: 10X 0 1,500 3,000 Feet Horizontal Scale Clay, silt or formation with clay/silt content Clean formation without clay or silt Figure 8. Cross Section A-A’ ---PAGE BREAK--- Page 11 Western Groundwater Services 4290 4200 4110 4020 3930 3840 0 3000 6000 9000 12000 15000 18000 21000 24000 27000 30000 33000 4290 4200 4110 4020 3930 3840 4470 4380 USGS (Morgan) Qabo (Clay Section) Qabo (Clean Section) Qab Qal Tscr (Reese Cr. Mbr.) ? IP Beds A & B Qab Qal Dringle Gallatin Airport ? ? ? ? ? Belgrade Well #6 IP Bed C (2200' NE) ? B (NORTH) B' (SOUTH) Elev. (ft MSL) Grid Dimensions (H x 150 ft x 30 ft Vertical Exaggeration: 10X 0 1,500 3,000 Feet Horizontal Scale Clay, silt or formation with clay/silt content Clean formation without clay or silt Figure 9. Cross Section B-B’ ---PAGE BREAK--- Page 12 Western Groundwater Services 1A Qabo (Clay Section) 4400 4430 4340 4370 4310 4400 4340 4370 4310 Qab 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 3B 4C 7C 2B 5B 6B 6C 1C 4B C (NORTH) C' (SOUTH) Elev. (ft MSL) Grid Dimensions (H x 50 ft x 10 ft Vertical Exaggeration: 10X 0 500 1,000 Feet Horizontal Scale IP Beds A & B IP Bed C Clay, silt or formation with clay/silt content Clean formation without clay or silt Figure 10. Cross Section C-C’ ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 13 2.4 Groundwater Level and Flow Direction Groundwater elevation mapping was completed using estimated and measured groundwater levels for the area surrounding the BWRF (Figure 11).9 Measured groundwater levels were primarily obtained from the BWRF monitoring wells and from the Gallatin Local Water Quality District monitoring well network for the two year period of 2017 and 2018. From these data, the average groundwater elevation was estimated for each location. Additional points were established along surface waters using the 2015 aerial photo maps and the U.S. Geological Survey (USGS) digital elevation model (DEM) for the area.10 Point data were mapped to a raster image using a kriging algorithm within the Arc GIS software (Spatial Analyst). Contours were subsequently drawn using the raster data and a contouring algorithm within the same software. The mapped area spans groundwater elevations starting at about 4,150 feet mean sea level (ft MSL) where the West and East Gallatin Rivers join and rises to about 4,530 ft MSL to the south of the BWRF. Flow direction, inferred to be normal to the contours, is northwesterly. Groundwater flow from the BWRF is estimated to discharge to primarily the East Gallatin River about 4.5 miles to the north. The hydraulic gradient to the south of the BWRF is approximately 0.007 ft/ft. To the north of the BWRF is flattens to 0.003 ft/ft. These gradients are consistent with the BWRF discharge permit (NW, 0.004 ft/ft). Selected monitoring wells at the BWRF and from the monitoring well network are routinely measured for static water level and provide time-history data for groundwater elevation at the well sites (Figures 12, 13, 14 and 15). These data were plotted relative to mean sea level (MSL) based on the ground surface elevation taken from the USGS DEM.11 The BWRF monitoring wells have a depth to water of about 20- to 30-ft, and exhibit fluctuation less than 10-ft over the existing data record from 1/1/2017 to mid-2019. monitoring wells exhibit similar fluctuation except for Yukon Subdivision (235475), which exhibits nearly 20-ft. 9 Mapping by Slagle (1995) was not suitable for the site and local-area scale of the modeling project. 10 Elevations of both wells and surface water points were referenced to the USGS DEM. 11 This datum is being used to ensure consistency with the groundwater model. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 14 4486 4451 4411 4403 4401 4391 4381 4378 4376 4380 4360 4358 4311 4211 4256 4223 4223 4203 4196 4180 4187 4170 4157 4151 4308 4334 4380 4341 4406 4370 4357 4328 4269 4341 4318 4298 4285 4282 4416 4459 4495 4557 4459 4170 4530 4530 4200 4230 4260 4290 4500 4320 4470 4440 4350 4410 4380 ± 0 8,000 16,000 Feet Contour data point and elevation (ft) Groundwater Contour (C.I. 30 ft) BWRF Monitoring Well Monitoring Well Surface Water Point Figure 11. Water Table Map ---PAGE BREAK--- Page 15 Western Groundwater Services IP-B IP-A IP-C 1A 1C 2A 2B 2C 3A 3B 3C 4A 4B 4C 6C 5A 5B 6A 6B 7A 7C ± 0 800 1,600 Feet Quarterly Sampling Sampling Not Sampled Monitoring Well Location and ID Figure 12. BWRF Well Location Map ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 16 BWRF 235475 135735 133176 [PHONE REDACTED] 265132 266826 296728 ± 0 6,000 12,000 Feet Well and ID Figure 13. Well Location Map ---PAGE BREAK--- Page 17 Western Groundwater Services 1/17 4/17 7/17 10/17 1/18 4/18 7/18 10/18 1/19 4/19 7/19 Month/Year 0 5 10 15 20 25 30 35 40 45 50 Depth to Water (Ō bMP) MW-1A MW-3A MW-5A MW-6A IP-A Monitor Wells 1/17 4/17 7/17 10/17 1/18 4/18 7/18 10/18 1/19 4/19 7/19 Month/Year 4370 4375 4380 4385 4390 4395 4400 4405 4410 4415 4420 Groundwater ElevaƟon (Ō MSL) MW-1A MW-3A MW-5A MW-6A IP-A Monitor Wells 1/17 4/17 7/17 10/17 1/18 4/18 7/18 10/18 1/19 4/19 7/19 Month/Year 0 5 10 15 20 25 30 35 40 45 50 Depth to Water (Ō bMP) MW-4B MW-5B MW-6B IP-B Monitor Wells 1/17 4/17 7/17 10/17 1/18 4/18 7/18 10/18 1/19 4/19 7/19 Month/Year 4370 4375 4380 4385 4390 4395 4400 4405 4410 4415 4420 Groundwater ElevaƟon (Ō MSL) MW-4B MW-5B MW-6B IP-B Monitor Wells 1/17 4/17 7/17 10/17 1/18 4/18 7/18 10/18 1/19 4/19 7/19 Month/Year 0 5 10 15 20 25 30 35 40 45 50 Depth to Water (Ō bMP) MW-1C MW-3C MW-6C IP-C Monitor Wells 1/17 4/17 7/17 10/17 1/18 4/18 7/18 10/18 1/19 4/19 7/19 Month/Year 4370 4375 4380 4385 4390 4395 4400 4405 4410 4415 4420 Groundwater ElevaƟon (Ō MSL) MW-1C MW-3C MW-6C IP-C Monitor Wells Figure 14. BWRF Monitor Well Water Levels ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 18 A. 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 4200 4210 4220 4230 4240 4250 4260 4270 4280 4290 4300 4310 4320 4330 4340 4350 4360 4370 4380 4390 Groundwater ElevaƟon (Ō MSL) 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 4370 4380 4390 4400 4410 4420 4430 4440 4450 4460 4470 4480 4490 4500 4510 4520 4530 4540 4550 Groundwater ElevaƟon (Ō MSL) B. Figure 15. Well Water Levels ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 19 2.5 Aquifer Properties Two pumping tests were conducted at the BWRF in 2018 (TD&H 2019b). Six-inch diameter test wells were installed for testing in proximity to the IP-A and IP-C beds, and designated 7A and 7C, respectively (Figure 16). The wells were screened from about 42 to 58 ft below ground, and both had static water levels of about 26 ft. The electronic data were analyzed for the modeling project using AQTESOLV and a distance- drawdown method.12 There is some likelihood that testing data could have been influenced by the IP bed discharges occurring during testing (Figure 17). There also appeared to be a generally declining groundwater level that could affect the tests. The tests yielded extremely high values for aquifer transmissivity (Figure 18). With assumptions regarding the aquifer thickness affected by the test (7A – 47 ft, 7C – 48 ft), the hydraulic conductivity is estimated at 4,800 ft/d (7A test), and 1,300 ft/d (7C test). The 7A test is indicating exceptionally high transmissivity for the Belgrade area. Discharge permitting for the IP beds is based on a hydraulic conductivity of 600 ft/d. For comparison, the City’s new Well #7 constructed and tested in 2017 was estimated to have a transmissivity of 17,000 ft2/d and hydraulic conductivity in the range from 90- to 170-ft/d, also considered very high values. This well is located about 1.25 miles west from the BWRF. The upper 135 ft of material at this well location was of lower permeability and did not produce appreciable water during air-rotary drilling. The Airport Well (Well located about 1.35 miles south of the IP-C beds is the lowest producer of the City’s wells at 450 gallons per minute (gpm). The City’s Source Water Assessment completed by Montana DEQ reported based on a literature review that aquifer transmissivity ranged from about 5,100- to 90,000-ft2/d. These data are indicating a substantially heterogeneous aquifer with highly variable transmissivity and conductivity, and which are poorly mapped at this time. The aquifer tests at the BWRF were both indicating generally low storativities of 0.02 and 0.04, respectively. It is not uncommon to obtain a low storativity for a short-term test that does not result in appreciable aquifer drawdown. The materials of the Belgrade area would be expected to have a specific yield of about 0.1 to 0.3 and a storativity of about 0.0001- to 0.001. 12 Data were provided in electronic format by TD&H Engineering. AQTESOLV is a well hydraulics program by HydroSOLVE, Inc. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 20 0 25 50 75 100 125 150 175 200 225 250 ‐0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 12:00 AM 12:00 PM 12:00 AM 12:00 PM 12:00 AM 12:00 PM 12:00 AM 12:00 PM Discharge Rate (gpm) Drawdown (ft) Time (12/14/18 ‐ 12/17/18) TW‐7A MW‐5A MW‐6A Discharge 0 25 50 75 100 125 150 175 200 225 250 ‐0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 12:00 PM 12:00 AM 12:00 PM 12:00 AM 12:00 PM Discharge Rate (gpm) Drawdown (ft) Time (12/12/18 ‐ 12/14/18) TW‐7C MW‐2C MW‐6C Discharge A. B. Figure 16. Aquifer Testing Hydrographs ---PAGE BREAK--- Page 21 Western Groundwater Services 0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 450,000 12/5 12/7 12/9 12/11 12/13 12/15 12/17 12/19 Daily Discharge Volume (gal) Month/Day 2018 IP‐C IP‐A IP‐B 7A Pumping Phase 7C Pumping Phase Figure 17. IP Discharge During Aquifer Testing ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 22 10 -1 10 0 10 1 10 2 10 3 -0.5 -0.2 0.1 0.4 0.7 1. Radial Distance (ft) Displacement (ft) Obs. Wells TW-7A MW-5A MW-6A Aquifer Model Unconfined Solution Theis Parameters T = 2.256E+5 ft2/day S = 0.02458 Kz/Kr = 1. b = 47. ft 10 -1 10 0 10 1 10 2 10 3 -0.5 0. 0.5 1. 1.5 2. 2.5 Radial Distance (ft) Displacement (ft) Obs. Wells TW-7C MW-2C MW-6C Aquifer Model Unconfined Solution Theis Parameters T = 6.258E+4 ft2/day S = 0.04465 Kz/Kr = 1. b = 48. ft A. B. Figure 18. Aquifer Testing Distance-Drawdown Analysis ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 23 2.6 Groundwater Recharge Recharge to the groundwater in relation to the BWRF occurs from: 1) regional groundwater inflow; 2) leakage from surface waters, including irrigation ditches; 3) excess irrigation; 4) precipitation through the land surface; and 5) discharge through the IP beds. Estimated groundwater recharge in the general area of the BWRF and extending north to the confluence of the East and West Gallatin Rivers is summarized in Table 1. Figure 19 illustrates surface water features within the area considered for recharge analysis. Table 1. Groundwater Recharge Summary Recharge Source Quantity (acre-feet per year) Groundwater Inflow 32,000 – 116,000 Leakage from Surface Water 9,200 Excess Irrigation 4,900 Infiltration from Precipitation 700 BWRF Beds 200 – 400 TOTAL 47,000 – 131,200 Based on the 7C pumping test, the regional groundwater inflow is estimated at about 3,300 gallons per day per foot of aquifer width (1 ft *62,600 ft2/d *0.007 ft/ft*7.48 gal/ft3). The cross-width length of the aquifer is about 6 miles to the south of the BWRF. Across this length regional inflow is estimated at 116,000 acre-feet per year (af/yr). If the average transmissivity is lower, such as was measured at Belgrade Well #7 – 17,000 ft2/d, this volume would be reduced to about 32,000 af/yr. The Montana Bureau of Mines and Geology (MBMG) measured leakage from irrigation ditches in the Four Corners area, south of Belgrade (provisional unpublished data). For the Mammoth Ditch, which extends to the north passing through Belgrade and the model area, a leakage rate of 0.41 cubic feet per second per mile (cfs/mi) was measured in May 2010. The rate of leakage increased to 0.61 cfs/mi in August 2010, averaging 0.51 cfs/mi for the two readings. The model area includes 25 miles of ditches and 25 miles of tributary streams. The northern segments of these waterways are most likely gaining flow, whereas the southern segments are losing. Assuming a total of 25 miles of losing ditches and tributary streams and applying the average leakage from Mammoth ditch – 0.51 cfs/mi, leakage to groundwater is estimated at 12.75 cfs. This recharge quantity equates to about 9,200 af/yr. Irrigated farmland within the model area of interest occurs primarily to the north of the BWRF and has an estimated total area of about 13,000 acres. Assuming an irrigation requirement of 1.5 ft/yr and irrigation efficiency of 80% for sprinklers, which predominate for the area, excess irrigation is estimated at 0.375 ft/yr. Total excess irrigation volume recharging groundwater is estimated at 4,900 af/yr. Infiltration from precipitation was calculated using a water-balance method for the near surface soil. By this method and using data from the Gallatin Field weather monitoring station, it was estimated that 0.26 inches/year, or equivalently 0.02 ft/yr, of infiltration depth could occur. The area from about 1.75 miles south of the BWRF to the confluence of the East and West Gallatin Rivers is estimated at 30,235 acres. Groundwater recharge from precipitation onto this area has an estimated annual volume of about 700 af/yr. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 24 Middle Thompson Bull Run Story East Gallatin River Spain Ferris Spain Ferris Fork Mammoth Durham West Gallatin River ± 0 8,000 16,000 Feet Irrigation ditches Tributary streams Trunk streams Figure 19. Surface Water Map Hoffman Weaver ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 25 Groundwater recharge from the BWRF through the IP beds has averaged 304 af/yr from 2010 to 2019. The maximum occurred in 2013 and was 390 af/yr. In 2018 the total discharge from the IP beds was 332 af/yr. The source of this water is groundwater produced from City of Belgrade wells (plus infiltration into sewers). The BWRF discharge into the IP Beds is a return flow of these withdrawals and does not constitute a recharge source to the aquifer. 2.7 Groundwater Discharge Discharge of groundwater occurs to surface waters, including ditches, to withdrawals by pumping wells and springs, and by evapotranspiration from wetland areas. Some fraction of pumped groundwater will be returned; only the consumptive use fraction results in a groundwater discharge. Evapotranspiration directly from groundwater (and represented in the model) is assumed to occur from the confluence area of the East and West Gallatin Rivers south to the Central Park fault. Air photos show that wetland vegetation occurs through this area and then transitions to dry land areas. A presumed negligible flow of groundwater may pass through the confluence area, otherwise it is assumed the entire recharge flow discharges into surface water. Some of this discharge will occur into ditches and tributary streams upstream from the confluence area. The total flow reported for recharge sources less the consumptive part of groundwater diversions and evapotranspiration will be discharged into surface waters. Table 2 summarizes the groundwater discharge. Table 2. Groundwater Discharge Summary Discharge Receiver Quantity (acre-feet per year) Surface Water 37,600 – 121,800 Groundwater Withdrawals (Wells, Springs) (Consumptive Use Only) 6,900 Evapotranspiration 7,200 - 14,400 TOTAL 51,700 – 143,100 An estimate of consumptive use from groundwater was made using the State databases for water rights (Figure 20). There were 1,905 water right diversions for wells and springs located within the recharge area. Of these, 1,300 have a designated maximum volume that can be used to evaluate consumptive use of groundwater. Most water rights are listed with a maximum volume that is larger than actual use. As a rough estimate of consumptive use from groundwater diversions, 50% of the maximum volume was assumed to be withdrawn. Of this withdrawal, 33% was assumed to be consumptive.13 Consumptive use over the recharge area considering 1,300 diversions was 3.6 af/yr per diversion. Extending this unit consumptive rate to the 1,905 diversions on record, the annual consumptive use is estimated as 6,900 af/yr. Actual evapotranspiration (AET) was estimated as part of the water balance based on Gallatin Field climate data.14 AET was estimated at 13.7 inches per year. The area where riparian vegetation and wetlands are present includes approximately 12,600 acres based on review of air-photo mapping. It is estimated that in this area evapotranspiration is deriving water supply directly from saturated groundwater. Maximum evapotranspiration of groundwater in this area is estimated at 14,400 af/yr. A lower bound of 1/2 this volume, or 7,200 af/yr is also estimated, as some fraction of evapotranspiration will be sourced from unsaturated zone groundwater recharged by precipitation. 13 Consumptive Use = Max Volume * 0.5 * 0.33 14 AET is calculated from a soil water balance model that includes precipitation, run-off, deep percolation, and soil moisture storage. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 26 ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! 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! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ± 0 9,000 18,000 Feet ! Water Right Point of Diversion (Groundwater Only) ! MBMG Database Well Location Figure 20. Water Rights Map ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 27 2.8 Groundwater Quality Groundwater quality data are obtained from sampling of the BWRF monitoring wells. Selected wells are sampled and quarterly as part of the BWRF operations. Samples are collected as per a procedure that includes purging of groundwater prior to sample collection. Analysis of water samples has been conducted at Energy Laboratories, Inc. in Billings. Water quality data evaluated for the mixing zone analysis include chloride and nitrogen. The nitrogen parameters are total, ammonia, Kjeldahl, and nitrate plus nitrite. Of these, nitrate plus nitrite is the most significant in groundwater while ammonia and Kjeldahl are normally non-detectable. Under this condition, the total nitrogen is equal to the nitrate plus nitrate analysis. Time history plots for the three groups of monitoring wells corresponding to IP beds A, B and C exhibit seasonal fluctuation, which was also shown in the effluent water quality (Figure 21). High values on these plots measured prior to about 2011 reflect sampling and analysis from the 2(A, B, C) wells, which are within 60 ft of the IP beds, are screened at the water table, and are not presently sampled (monitoring wells 2A and 2B have been abandoned as part of the BWRF upgrade). With the exception of 4A, 3B, and 4C, monitoring wells are screened from the water table to about 20 feet below. Monitoring wells 4A, 3B, and 4C also were sampled only prior to 2011. These wells, however, are screened from about 65 ft to 80 ft below ground, providing some measure of the depth to which effluent is transported vertically in the groundwater. Monitoring well 1C is the most up-gradient well, measuring ambient water quality. By comparison, monitoring wells 4A and 3B exhibit significantly greater concentrations of chloride and nitrate plus nitrite than is measured at 1C. Monitoring well 4C also appears to have greater chloride concentration. These data are indicating a mixing zone depth of 65- to 80-ft. Comparison of the ratio of chloride to nitrate plus nitrite provides an indication of denitrification.15 The IP-A monitoring wells are not providing a definitive measure of denitrification (Figure 22). Although a comparison of monitoring wells 2A and 4A would suggest very substantial denitrification, the deeper monitoring well (4A) has a ratio similar to the upgradient background well, 1A. The IP-B and IP-C plots, however, may be indicating denitrification occurring deeper in the aquifer (Figure 23, Figure 24). Based on comparison of data from monitoring wells 2B and 3B, there could be about 30% of nitrogen mass reduction at depth and that could be attributed to denitrification. Similarly, based on data from monitoring wells 2C and 4C, a mass reduction of 22% is estimated and also could be attributed to denitrification. The IP-C data are more definitive than the IP-B data due to less chance for interference from upgradient groundwater quality. 15 Denitrification is an anoxic biological process that results in conversion of nitrate to nitrogen gas, thereby reducing nitrate concentration in groundwater. The chloride:nitrate ratio will increase where denitrification is present, as chloride mass is conserved and nitrate mass is depleted. ---PAGE BREAK--- Page 28 Western Groundwater Services 1/02 1/03 1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 1/14 1/15 1/16 1/17 1/18 1/19 Month/Year 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 Cl (mg/L) MW-1A MW-2A MW-3A MW-4A MW-5A MW-6A IP-A Monitor Wells 1/02 1/03 1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 1/14 1/15 1/16 1/17 1/18 1/19 Month/Year 0 5 10 15 20 25 SNOx (mg N/L) MW-1A MW-2A MW-3A MW-4A MW-5A MW-6A IP-A Monitor Wells 1/02 1/03 1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 1/14 1/15 1/16 1/17 1/18 1/19 Month/Year 0 10 20 30 40 50 60 70 80 90 100 Cl (mg/L) MW-2B MW-3B MW-4B MW-5B MW-6B IP-B Monitor Wells 1/02 1/03 1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 1/14 1/15 1/16 1/17 1/18 1/19 Month/Year 0 5 10 15 20 SNOx (mg N/L) MW-2B MW-3B MW-4B MW-5B MW-6B IP-B Monitor Wells 1/03 1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 1/14 1/15 1/16 1/17 1/18 1/19 Month/Year 0 10 20 30 40 50 60 Cl (mg/L) MW-1C MW-2C MW-3C MW-4C MW-6C IP-C Monitor Wells 1/03 1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 1/14 1/15 1/16 1/17 1/18 1/19 Month/Year 0 1 2 3 4 5 6 7 8 9 10 SNOx (mg N/L) MW-1C MW-2C MW-3C MW-4C MW-6C IP-C Monitor Wells Figure 21. BWRF Monitoring Well Time-History Plots ---PAGE BREAK--- Page 29 Western Groundwater Services 1.5 2 2.5 3 3.5 4 4.5 5 5.5 SNOx (mg N/L) 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Cl (mg/L) MW-6A: Cl/NOx = 2.6 Screen: 27-47 ft 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 SNOx (mg N/L) 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Cl (mg/L) MW-5A: Cl/NOx = 1.1 Screen: 28-48 ft 1 1.25 1.5 1.75 2 2.25 SNOx (mg N/L) 5 10 15 20 25 30 Cl (mg/L) MW-4A: Cl/NOx = 10.5 Screen: 64-76 ft 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 SNOx (mg N/L) 5 10 15 20 25 30 35 40 45 50 55 Cl (mg/L) MW-3A: Cl/NOx = -3.5 Screen: 28-49 ft 0 5 10 15 20 25 SNOx (mg N/L) 0 20 40 60 80 100 120 140 160 180 Cl (mg/L) MW-2A: Cl/NOx = 1.2 Screen: 28-48 ft 0 0.5 1 1.5 2 2.5 3 3.5 SNOx (mg N/L) 0 5 10 15 20 25 30 35 40 Cl (mg/L) MW-1A: Cl/NOx = 10.9 Screen: 29-49 ft Figure 22. IP-A Cl/NOx Plots ---PAGE BREAK--- Page 30 Western Groundwater Services 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 SNOx (mg N/L) 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 Cl (mg/L) MW-6B: Cl/NOx = 4.4 Screen: 28-48 ft 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 SNOx (mg N/L) 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Cl (mg/L) MW-5B: Cl/NOx = 4.0 Screen: 27-48 ft 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 SNOx (mg N/L) 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Cl (mg/L) MW-4B: Cl/NOx = 2.3 Screen: 28-48 ft 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 SNOx (mg N/L) 0 5 10 15 20 25 Cl (mg/L) MW-3B: Cl/NOx = 6.6 Screen: 69-80 ft 1 2 3 4 5 6 7 8 9 10 11 SNOx (mg N/L) 0 10 20 30 40 50 60 70 80 90 Cl (mg/L) MW-2B: Cl/NOx = 4.6 Screen: 28-48 ft Figure 23. IP-B Cl/NOx Plots ---PAGE BREAK--- Page 31 Western Groundwater Services 0 1 2 3 4 5 6 SNOx (mg N/L) 0 5 10 15 20 25 30 35 Cl (mg/L) MW-6C: Cl/NOx = 4.8 Screen: 18-50 ft 0.5 0.75 1 1.25 1.5 SNOx (mg N/L) 3 4 5 6 7 8 9 10 11 12 Cl (mg/L) MW-4C: Cl/NOx = 8.7 Screen: 65-79 ft 0 1 2 3 4 5 6 7 8 SNOx (mg N/L) 0 5 10 15 20 25 30 35 40 45 Cl (mg/L) MW-3C: Cl/NOx = 4.2 Screen: 18-49 ft 0 1 2 3 4 5 6 7 8 9 SNOx (mg N/L) 0 10 20 30 40 50 60 Cl (mg/L) MW-2C: Cl/NOx = 6.8 Screen: 18-50 ft 0.5 0.75 1 1.25 1.5 1.75 SNOx (mg N/L) 1 2 3 4 5 6 7 Cl (mg/L) MW-1C: Cl/NOx = 2.2 Screen: 25-49 ft Figure 24. IP-C Cl/NOx Plots ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 32 3. MODEL SETUP AND CALIBRATION 3.1 Software and Methods Computer modeling was completed using the Ground Water Vistas 8)16 user interface, Modflow-USG (Transport Version 1.10.0, U.S. Geological Survey), PEST and BeoPEST 17.3, Watermark Computing), and mod-PATH3DU 2, S.S. Papadopolous and Associates, Inc.). Modflow-USG is a groundwater flow and transport simulator that allows use of an unstructured grid (USG) and provides fully coupled solution of the flow and transport equations. PEST and BeoPEST are a suite of programs used for numerical calibration. The BeoPest code provides for parallel processing of model runs to reduce calibration runtime for large parameter models. Mod-PATH3DU uses the flow output from Modflow-USG to draw groundwater pathlines. 3.2 Flow Model 3.2.1 Setup Modeling work first converted the existing three-dimensional regional flow model to Modflow-USG (from Modflow 2005) (Figure 25). Grid cells are of uniform horizontal dimensions of 100- by 100-meters but have varying vertical thickness.17 Total model thickness is 100 m, and is intended to include parts of the groundwater system that would be below the region affected by the BWRF discharge. The 100 m thickness was distributed over nine layers varying from 6- to 28-m each.18 The entire model was rotated 20-degrees counterclockwise in order to align flow direction with the y-axis of grid cells. The U.S. Geological Survey digital elevation model (DEM) for the project area with 30 m node spacing was used as the top of layer 1. Deeper layers were specified based on the DEM land surface elevations (by raster math calculations in ESRI Arc Map). North of the Central Park fault, the deepest two layers (8 and 9) extending from 54- to 100-m were set to no-flow cells representing low permeability bedrock. It was assumed the permeable alluvium south of the fault terminates at the fault. The East and West Gallatin Rivers were located on the east and west sides of the model as boundaries. The East Gallatin River also forms the north boundary. The southern border of the modeled area was represented with a general head boundary type that specifies both conductivity and hydraulic head for regional inflow to the model area. This general head boundary replaced a constant head boundary that was used in the earlier modeling work to enable calibration of conductivity at this model border. Tributary streams and irrigation ditches were also located within the model domain. These surface waters were represented using the river boundary type of Modflow-USG. 16 Environmental Simulations, Inc. 17 SI units are used in the model to match the units used in GIS datasets that are used in model setup land elevation, surface waters). 18 Layer 1 (top) – 12 m; Layers 2 thru 6 – 6 m; Layer 7 – 12 m; Layer 8 – 18 m; Layer 9 – 28 m (bottom). ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 33 ± 0 7,000 14,000 Feet BWRF IP Beds General Head Boundary River Boundary (Ditches) River Boundary (Tributary Streams) River Boundary (East, West Gallatin Rivers) Figure 25. Model Finite-Difference Grid and Boundaries ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 34 3.2.2 Calibration of Flow Parameters Flow model calibration provides numerical estimates of hydraulic conductivity throughout the model domain, including boundary conditions. Calibration was based on measured groundwater levels in wells and estimates of fluxes from groundwater to surface water. The calibrated parameters included conductances and hydraulic conductivity values. There were a total of 374 adjustable parameters of which 369 were horizontal and vertical hydraulic conductivity pilot points and five corresponded to conductance values for surface waters and the general head boundary (single value for each reach). The IP beds were assigned a total injection rate of 1,222 m3/d (224 gpm or 362 af/yr) into the top layer of the model (layer corresponding to the 2017-2018 average discharge. Areal recharge was used to represent both infiltration from precipitation and withdrawals from wells and springs using the estimates from above.19 Evapotranspiration was included in the modeled area north of the Central Park fault using estimated actual evapotranspiration (13.7 in/yr) from the water balance and a 2-m extinction depth. There was some adjustment to flux targets in an iterative calibration process. Final targets for the W. and E. Gallatin Rivers were specified as -35,000 m3/d and -75,000 m3/d, respectively (negative sign indicates discharge from groundwater to surface water). The flux target for tributary streams was -25,000 m3/d, and for ditches was 1,000 m3/d (ditches are adding recharge to groundwater). These calibration results indicate that most groundwater in the area of the BWRF discharges to the East Gallatin River. There were 21 hydraulic head targets for measured groundwater elevation at monitoring wells specified for calibration. These targets included City of Belgrade supply wells, BWRF and monitoring wells, and selected wells from the Montana Bureau of Mines and Geology (MBMG) Ground Water Information Center (GWIC) database. City supply wells were assigned static water levels available from City data. BWRF and monitoring well targets were assigned the average elevation for the period 2017 through 2018. The GWIC wells were selected from a subset constructed after 2009 and were assigned the static water level from the well log. The model tended to overestimate hydraulic head values. For the flow component calibration, initial and final objective function values were 8964 and 47, respectively (Figure 26).20 The iterative solution was generally successful at determining a minimum objective function value for the model. The match of the model to calibration targets is shown on Figure 27. The mass balance for the model is within the estimated range presented above (Table Table 3. Regional Model Mass Balance Source/Sink In (af/yr) Out (af/yr) IP Beds 362 0 General Head 55,267 0 Rivers 36,013 60,013 Withdrawals-Precip. 0 7,304 Evapotranspiration 0 9,988 TOTAL 91,641 91,643 Difference (In-Out)/In -0.002% 19 By this method, withdrawals are distributed uniformly over the model domain rather than at discrete locations. 20 The objective function minimum was set to 1.0 for the analysis. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 35 1 2 3 4 5 6 7 Iteration 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 ObjecƟve FuncƟon Objective Function, Total Head Contribution Flux Contribution Max: 8694 Min: 47 Optimal solution: Iteration 6 Figure 26. Flow Calibration Objective Function ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 36 Figure 27. Calibration Target Plots 4175 4200 4225 4250 4275 4300 4325 4350 4375 4400 4425 Target Head (ft) -15 -12 -9 -6 -3 0 3 6 9 12 15 Target - Modeled 4175 4200 4225 4250 4275 4300 4325 4350 4375 4400 4425 Modeled Head Heads Residuals -25000 -20000 -15000 -10000 -5000 0 5000 Target Flux (af/yr) -900 -600 -300 0 300 600 900 Target - Modeled (af/yr) -25000 -20000 -15000 -10000 -5000 0 5000 Modeled Flux (af/yr) Fluxes Residuals A. B. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 37 A raster image of the calibrated horizontal hydraulic conductivity (Kx) values for model layer 1 is provided on Figure 28. Similar distributions occur in the other model layers. A raster image of vertical hydraulic conductivity (Kz), also for model layer 1, is provided on Figure 29. The tight range for Kz values may reflect insensitivity to calibration targets—the values are clustered around the initial estimate of 10 m/d. Anisotropy ratios (Kx/Kz) ranged from 1.3 to 49, with a mean of 7.4, over all layers. The calibrated flow model exhibits dry and flooded cells at various locations (Figure 30). Dry cells occur where the modeled water table falls below the bottom elevation of the layer. The large dry cell area west of the BWRF resulted from groundwater levels below the bottom of layer 1. Flooded cells are identified where the model calculates a water table above the top elevation of layer 1. These cells are primarily north of the Central Park fault in the area where groundwater is known to be at shallow depth below land surface. Groundwater hydraulic head contours for model layer 1 match the estimated contours with reasonable accuracy (Figure 31). Hydraulic gradient at IP-C is 0.007 and decreases to 0.004 at IP-A/B. The direction of the gradient is N 30° W. A value of 0.004 and direction of N 45° W has been used for the BWRF discharge permit. Pathlines of the model flow field are marked with arrowheads at a 5-year time-of-travel (TOT) interval. Travel time from the BWRF to the East Gallatin River ranges from 22 to 27 years. These travel times are for purely advective transport, which does not include hydrodynamic dispersion or chemical reactions. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 38 BWRF ± 0 7,000 14,000 Feet Layer 1 Kx meters/day 13 - 50 50 - 100 100 - 150 150 - 200 200 - 250 250 - 300 300 - 350 350 - 400 400 - 450 450 - 492 Central Park Fault Figure 28. Calibrated Horizontal Hydraulic Conductivity, Layer 1 ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 39 Central Park Fault ± 0 7,000 14,000 Feet Layer 1 Kz meters/day 7 - 9 9 - 11 11 - 13 13 - 15 15 - 17 BWRF Figure 29. Calibrated Vertical Hydraulic Conductivity, Layer 1 ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 40 463 500 167 000 Cent ral P ark Fault BWRF ± 0 7,000 14,000 Feet BWRF IP Beds Groundwater above land surface (flooded cel) Groundwater below bottom layer 1 (dry cell) Figure 30. Flooded and Dry Model Cells ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 41 4190 4400 422 0 4250 42 80 4 310 4 340 437 0 4430 ± 0 7,000 14,000 Feet BWRF IP Beds Groundwater Pathlines (arrowheads at 5-yr TOT) General Head Boundary River Boundary (Ditches) River Boundary (Tributary Streams) River Boundary (East, West Gallatin Rivers) Groundwater Elevation (ft MSL) (C.I. 10 ft) 4230 Figure 31. Modeled Groundwater Elevation 2024 2019 2014 2009 2004 1999 1994 2007 ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 42 3.3 Transport Model The transport model is simply the flow model run with transport added in, as the transport solution is now integral to Modflow-USG.21 Quadtree mesh refinement was added to the model domain in the region of the BWRF for the tranport calibration and simulation (Figure 32). The horizontal dimensions of the mesh cells were reduced to 25- by 25-meters in a region surrounding the IP beds and extending down gradient several thousand feet. A transition zone with cells of 50- by 50-meters in the horizontal dimension occurs along the boundary of the refined mesh. The thickness of the cells was not changed. 3.3.1 Calibration of Dispersivity There are three dispersivity values determined for the longitudinal, transverse horizontal, and transverse vertical directions. One set of parameters was assumed for the entire model domain. The calibration was completed using chloride concentrations in effluent and groundwater. The same steady-state discharge rate as for flow parameter calibration (224 gpm) was used for dispersivity calibration. Chloride concentration of the treated effluent was set at the average value of 146 mg/L. Chloride concentrations at target locations were set at the average value measured from 2002 through 2015 less a background concentration of 4 mg/L, as determined for BWRF monitoring well 1C. A transport period of 5,000 days was determined to be the minimum time period to establish steady concentrations at monitoring well locations. The objective function was reduced from 547 to 176 in the fitting process (Figure 33A). The modeled concentrations as compared to the target concentrations had a residual mean of -0.16 mg/L and residual range of -7.63 to 8.63 mg/L. The residual root mean squared error was 2.1 mg/L (Figure 33B). Chloride contours show the simulated horizontal extent of the discharge plume in Layer 1 at 5,000 days of transport simulation (Figure 34). Estimated dispersivity values shown in Table 4 provided the best fit to model data. The calibration results indicate the model is increasing longitudinal and horizontal dispersivity to widen and lengthen the plume to match the targets. Table 4. Calibrated Dispersivity Values Longitudinal (ft) Transverse Hoizontal (ft) Transverse Vertical (ft) 1,132 863 1.3 21 Panday, S. (2022) USG-Transport Version 1.10.0: Transport and Other Enhancements to MODFLOW-USG, GSI Environmental, Feb. 2022. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 43 BWRF ± 0 3,000 6,000 Feet BWRF IP Beds General Head Boundary River Boundary (Ditches) River Boundary (Tributary Streams) River Boundary (East, West Gallatin Rivers) Figure 32. Refined Mesh (Layers 1 - 6) ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 44 1 2 3 4 Iteration 160 200 240 280 320 360 400 440 480 520 560 ObjecƟve FuncƟon Max: 547 Min: 176 Optimal solution: Iteration 4 0 10 20 30 40 50 Target Concentration (mg/L) -10 -8 -6 -4 -2 0 2 4 6 8 10 Target - Modeled (mg/L) 0 10 20 30 40 50 Modeled ConcentraƟon (mg/L) Concentrations Residuals A. B. Figure 33. Chloride Transport Calibration Plots ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 45 1 1 3 5 7 10 7 3 0.1 1 1 ± 0 3,000 6,000 Feet BWRF IP Beds General Head Boundary River Boundary (Ditches) River Boundary (Tributary Streams) River Boundary (East, West Gallatin Rivers) Chloride Concentration Contours (C.I. 0.1, 1, 3, 5, 7, 10 mg/L) 3 Figure 34. Simulated Chloride Concentrations, Layer 1 ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 46 3.3.2 Simulated Nitrogen Concentrations The calibrated flow and transport model was used to simulate the current nitrogen concentrations in groundwater. These simulation results were compared to measured concentrations as a means to assess model accuracy. Target nitrogen concentrations were estimated for 11 monitoring wells using data for the period from 2002 to 2015. The background concentration of 1 mg N/L determined at monitoring well 1C was subtracted from down-gradient targets. The discharge rate was 224 gpm and the source concentration was 24.8 mg N/L. The simulation was run for 5,000 days to reach a steady condition. Simulated nitrogen concentrations were always greater than target values (Figure 35). The residual mean (target - modeled) was -1.3 mg N/L and the residual range was -2.93 to -0.34 mg N/L for the eleven targets. The root mean square error was 1.53. These results are reasonably close but exhibit a bias to higher values the model is overestimating nitrogen concentrations as compared to the target data set. Contouring of the simulation results is shown on Figure 36. 0 1 2 3 4 5 6 7 8 9 10 Target Concentration (mg N/L) -4 -2 0 2 4 Target - Modeled (mg N/L) 0 1 2 3 4 5 6 7 8 9 10 Modeled ConcentraƟon (mg N/L) Concentrations Residuals Figure 35. Nitrogen Transport Residual Plot ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 47 1 1 2 3 4 5 1 2 3 2A 2B 2C 3A 3B 3C 4A 4C 6C 6A 6B ± 0 1,000 2,000 Feet Nitrogen Concentration Contours (C.I. 1 mg N/L) 3 Figure 36. Simulated Nitrogen Concentrations, Layer 1 ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 48 4. SOURCE SPECIFIC MIXING ZONE DELINEATION This section applies the computer model to assess nitrogen transport in groundwater and to determine a Source Specific Mixing Zone (SSMZ), as defined in ARM 17.30.518, for the BWRF. The model was used to simulate nitrogen transport to year 2038, coinciding with the planning period used for design of the BWRF upgrades. Simulated nitrogen concentrations in groundwater were used to delineate the horizontal and vertical extent of the SSMZ. 4.1 BWRF Discharge Projection Prior to upgrades completed in 2022, the BWRF discharge contained an average total nitrogen concentration of 24.8 mg N/L. This value is used for nitrogen simulation before 2023. The upgraded BWRF will treat nitrogen to much lower levels. The estimated maximum is 13.5 mg N/L, as published in the Disposal Design Report.22 This value was used in simulations of nitrogen transport starting in year 2023. The BWRF technology supplier for the upgraded treatment system estimates total nitrogen in the range of 5- to 8-mg N/L.23 It is most likely these lower values will be realized during 2023. Annual average design capacity of the upgraded BWRF is 1.74 million gallons per day (mgd). Over the course of a year, some fraction of this discharge will be land applied as irrigation. Based on the Disposal Design Report, it is estimated that at peak capacity in year 2038, 87% of the discharge will be through the IP beds and 13% will be land applied. These percentages were applied to each year from 2023 to 2038, although it is likely that a greater proportion of discharge will be land applied in the earlier years. Of the IP bed total discharge it was distributed 30% to IP-A, 35% to IP-B, and 35% to IP-C, and was applied uniformly over the area of the bed. Discharge time history plots are provided on Figure 37, and show the maximum discharge to groundwater in year 2038 as 1.51 mgd. The remaining 0.23 mgd is allocated to land application. Land applied discharge is not included in the model as it is considered to be fully used by crop growth. 4.2 Denitrification Denitrification was not included in the simulation. This process reduces the amount of nitrogen in groundwater by reduction of nitrate to nitrogen gas and degassing to the atmosphere. There may be denitrification occurring, although it is expected to have a small effect. 4.3 Initial Conditions Transient simulation of both flow and transport requires initial conditions for nitrogen concentration and hydraulic head in groundwater. These initial conditions were established by simulating a 60-year period using the 2022 estimated discharge (1.003 mgd) and annual average nitrogen concentration of 24.8 mg N/L.24 These values will exceed the mass loading that occurred in the prior 60-years resulting in a conservative starting point for subsequent simulations. This simulation and subsequent simulations were run with estimated storage and porosity values (Table These values were estimated based on typical values for alluvium materials. The 60-year simulation generated stable hydraulic head values over the entire model domain. Simulated concentrations in the general region of the IP beds also were stable. 22 TD&H Engineering, Inc. (2020) Belgrade Water Reclamation Facility Upgrades, Disposal Design Report, February, p. 4-4. 23 Ovivo USA product video (2-Stage): 24 Initial concentrations for this simulation assume background level is 0 mg N/L. ---PAGE BREAK--- Page 49 Western Groundwater Services Figure 37. Projected BWRF Discharge as Daily Average 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Discharge Rate (mgd) IP Total IP-A IP-B IP-C ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 50 Table 5. Estimated Storage and Porosity Values Specific Storage (ft-1)A Specific Yield Porosity 0.001 0.25 0.30 4.4 Nitrogen Transport Simulation Nitrogen transport simulation was run for years 2023 to 2038. The initial conditions applied at the start of year 2023 were as described above. The discharge rate was varied according to the projected discharge discussed above, however, the nitrogen source concentration remained fixed at 13.5 mg N/L. The discharge was represented in the model by the Modflow-USG recharge property assigned to the area of the three IP beds, and which changed each year according to projected discharge rates.25 Simulation output for selected years shows the extent of nitrogen concentrations in the near surface groundwater (Figure 38). Contours show the plume expanding through time for the lower concentrations of 0.1- and 1-mg N/L contours. At the same time, there is a contraction of the contours related to higher concentrations, which are also nearer to the IP beds. The contraction occurs to 2033, and then switches to a slight expansion by year 2038. A 10 mg N/L contour exists only for year 2023 directly overlying IP-B. The occurrence of transport to the south in the up-gradient direction is attributed to mounding of the discharge, and also to numerical dispersion.26 Several monitoring locations were created to extract time history data from the simulation output (Figure 39). The IP-A, IP-B, and IP-C wells (MW-IPA, etc.) show the decline in nitrogen that occurs starting in 2023 and continues to about 2028, at which time it starts to increase (Figure 40). The concentrations remain substantially lower in 2038 than in 2023. Time history plots were also created for selected model layers at the location designated as MWZ in proximity to IP-B (similar results occur for IP-A and IP-C) (Figure 41). These plots show that nitrogen is present to model layer 8, which has a depth range of 177 to 236 ft below ground. The concentrations are low below model layer 3, which has a maximum depth of 79 ft below ground. An increasing trend is shown for model layers 1, 3, and 6, showing that discharge from the upgraded BWRF could have an effect at these depths. Static groundwater at the IP beds is about 25 ft below ground surface. Time history plots were also created for selected down-gradient locations, identifed as DG-1, DG-2, and DG-3 (Figure 42). As shown by DG-1, nitrogen increases out to about year 2029 and then begins to decrease in response to the lower nitrogen concentrations of the BWRF discharge. The more distant locations show low concentrations prevail with increasing trends. The trends appear to flatten near to 2038. 25 To facilitate this representation of discharge as a recharge source, precipitation recharge and groundwater extraction from domestic groundwater withdrawals was eliminated. 26 Up-stream weighting was applied to reduce/eliminate numerical dispersion. This method was found to provide equal results to the time variation diminishing scheme. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 51 1 3 5 7 1 0.1 0.1 0.1 1 3 5 ± 0 3,000 6,000 Feet 2023 2028 2033 2038 BWRF IP Beds Nitrate contours (mg N/L) 0.1, 1, 3, 5, 7, 10) Figure 38. Simulated Nitrogen Concentrations, Model Layer 1 ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 52 MW-IPB MW-IPA MW-IPC MW-DG1 MW-DG2 MW-DG3 MWZ ± 0 3,000 6,000 Feet BWRF IP Beds Simulation Monitoring Well 2038 Nitrate Contours (for reference) Figure 39. Simulation Time History Plot Locations ---PAGE BREAK--- Page 53 Western Groundwater Services 1/1/2023 1/1/2028 1/1/2033 1/1/2038 0 1 2 3 4 5 6 7 8 9 10 11 Simulated Nitrate ConcentraƟon (mg N/L) IP-A IP-B IP-C Figure 40. Simulated Nitrogen Time History Plots Near IP Beds ---PAGE BREAK--- Page 54 Western Groundwater Services 1/1/2023 1/1/2028 1/1/2033 1/1/2038 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Simulated Nitrate ConcentraƟon (mg N/L) MWZ(Layer 1) MWZ(Layer 3) MWZ(Layer 6) MWZ(Layer 8) Figure 41. Simulated Nitrogen Time History Plots Near IP-B ---PAGE BREAK--- Page 55 Western Groundwater Services 1/1/2023 1/1/2028 1/1/2033 1/1/2038 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Simulated Nitrate ConcentraƟon (mg N/L) DG1 DG2 DG3 Figure 42. Simulated Nitrogen Time History Plots Down Gradient ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 56 4.5 Source Specific Mixing Zone (SSMZ) This section presents the proposed SSMZ for the BWRF and responds to criteria specified in ARMs 17.30.518 and 506. The proposed SSMZ and land parcels are provided on Figure 43. The vertical extent of the SSMZ is determined as 80-ft below ground, coinciding with the base of model layer 3 (see Figure 37). The saturated thickness of the SSMZ is determined as 54 feet. The mapped SSMZ is based on the results of nitrogen transport modeling presented above. The mapped SSMZ also considers the land ownership in the area, and the practical limits of the mixing zone that will avert impacts to other water users. The SSMZ as presented herein is establishing mixing zone areas to meet present and future populations. The mixing zone boundary establishes the down-gradient boundary beyond which nitrogen concentration must be less than 10 mg N/L. This is the human health limit and is the groundwater quality standard for nitrogen that is applied in the existing permit. The human health limit will also apply to the permit renewal as the nitrogen loading for the upgraded BWRF will not be increased. The existing present nitrogen loading rates are 71 pounds per day (lbs/d) for IP-A, 72 lbs/d for IP-B, and 84 lbs/d for IP-C. ARM 17.30.518 includes several conditions that must be met for DEQ to approve a mixing zone. These conditions and responses are provided in Table 6. ---PAGE BREAK--- Page 57 Western Groundwater Services STATE OF MONTANA STATE OF MONTANA STATE OF MONTANA GALLATIN AIRPORT AUTHORITY GALLATIN AIRPORT AUTHORITY GALLATIN AIRPORT AUTHORITY GALLATIN AIRPORT AUTHORITY GALLATIN BEEF PRODUCTION ASSOC IP-B IP-C IP-A 2,940 ft 3,380 ft 1,210 ft 1,430 ft Z = 54 ft Z = 54 ft ± 0 1,000 2,000 Feet Horizontal Boundary Land parcels Z = 54 ft Saturated Thickness Figure 43. Proposed Source Specific Mixing Zone ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 58 Table 6. Groundwater Mixing Zone Criteria from ARM 17.30.518 ARM 17.30.518(5)(a)-(l) a) quantity, toxicity, and persistence of the pollutant; The pollutant is nitrate, which exists as an anion in groundwater. It has toxicity to humans at concentrations above 10 mg N/L. It can be degraded to nitrogen gas and liberated to the atmosphere where groundwater is anoxic. b) water-bearing characteristics of subsurface materials; Groundwater in the area of the SSMZ occurs in unconsolidated alluvium deposits of clay, silt, sand and gravel. Horizontal hydraulic conductivity is estimated to average 252 ft/d with a range from 42- to 1,600- ft/d. The hydraulic gradient in the area of the IP beds is 0.004 ft/ft and N30°W. Porosity of the media is estimated at 0.30. c) rate and direction of ground water flow; Groundwater seepage velocity in the SSMZ is about 7 ft/d and in the direction of approximately N30°W. (K = 540 ft/d; i = 0.004; n = 0.30) d) pollutant migration; Nitrate undergoes mixing and dilution during transport as a result of local variations in pore water velocity. Nitrate may degrade to nitrogen gas and liberate to the atmosphere where it migrates through anoxic conditions. e) volume of ground water and area available for mixing; The proposed mixing zone for IP-A+B is 221 acres. It is 47 acres for IP-C. Both mixing zones are determined to have a saturated thickness of 54 ft. f) concentration of pollutants within the mixing zone; Nitrate concentration is estimated to range from 3 mg N/L to less than 10 mg N/L in year 2038. g) length of time pollutants will be present; Nitrate will be present as long as the BWRF operates, however, future upgrades to the BWRF will continue to reduce nitrate concentration of the discharge. h) proposed boundaries of the mixing zone; The proposed boundaries are mapped on Figure 43. A GIS shapefile can also be provided. i) potential impacts to water uses; There are no groundwater users known within the mixing zone boundary. j) compliance monitoring; The City will monitor groundwater concentrations of several parameters including nitrate in monitoring wells and the BWRF discharge. These data are submitted to DEQ. k) contingency plan if pollutants migrate beyond the mixing zone at concentrations greater than the allowed limits; Monitoring data will be reviewed with sufficient time frames to forecast if concentrations of nitrate will exceed 10 mg N/L beyond the mixing zone boundary. The City will have sufficient time to plan, design and construct BWRF improvements to reduce nitrate concentrations to below 10 mg N/L. l) specific explanation as to why the proposed mixing zone is the smallest practicable size and why it will have a minimum practicable effect on water users. The proposed mixing zone is within the limits of available land parcels where it can be defined without impact to other water users. Considering future growth for Belgrade, these land area limits are considered the smallest practicable size for the mixing zone. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 59 ARM 17.30.506(2)(h) addresses the conditions that may require a mixing zone to also be established in receiving surface waters. The BWRF discharge plume has greatest hydraulic connection to the East Gallatin River. Ditches and tributary streams in proximity to the IP beds are losing water to groundwater, but do not receive groundwater recharge, as there is an unsaturated zone between the surface water and the groundwater. The BWRF discharge eventually enters the East Gallatin River at a distance of about 4.5-miles from the IP beds. Discharge from IP beds A and B occurs from IP bed C. Travel time over this distance is approximately 22- to 27-years. Concentrations of nitrogen estimated at the surface water discharge were approximately 0.1 mg N/L based on transport modeling in year 2038. Denitrification, which was not included in the transport modeling, can occur in wetland areas at the point of discharge resulting in further reduction in nitrogen concentration. This surface water is neither close to the IP beds nor is it reached by the discharge in a short time. Consequently, the discharge should not require a surface water mixing zone as per ARM 17.30.506(2)(h). ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 60 5. MONITORING PLAN Monitoring wells are required as part of the permit. This section proposes an updated groundwater monitoring plan for the IP bed discharge. The purpose of groundwater monitoring is to measure pollutants and other parameters to assess if standards are being met at the mixing zone boundary. These data are also used to assess water quality trends and to support analytical analyses, such as transport modeling of pollutants in groundwater. 5.1 Existing Monitoring Wells and Sampling The existing monitoring wells are located downgradient from IP beds about 450- to 550-ft (Figure 44). Each IP bed also has one monitoring well that is 60- to 90-feet north of the IP bed (2A, 2B, and 2C). Of these, monitoring wells, 2A and 2B were abandoned as part of the upgrade construction (they were not being used for required sampling). There are also two background monitoring wells located up-gradient to the south of the IP beds (1A and 1C). In 2018 there were also two additional wells constructed for aquifer testing (7A and 7C), which are not part of the monitoring well network identified in the existing permit. Sampling frequency varies from to quarterly, as listed in Table 7. Sampling includes parameters for laboratory and field analysis. Laboratory analysis is completed at a state-certified lab. Field parameters are measured using calibrated, portable instruments. Data are provided to DEQ in the discharge monitoring report. Table 7. Sampling and Analysis for Existing Permit Monitoring Wells Parameters Reported Quantity Sampling Frequency 1A, 3A, 5A 4B, 5B 1C, 3C Lab: Cl, NOx, TKN Field: SC, SWL, Temp, TD Quarterly Average Quarterly 6A 6B 6C Lab: Cl, NOx, TKN, E. Coli Field: SC, SWL, Temp, TD Average Coli - also Daily Max) 2A, 4A 2B, 3B 2C, 4C Reserve (sampling not required) NA NA 7A, 7C Not sampled NA NA Cl - Chloride (mg/L); NOx - Nitrate plus Nitrite (mg N/L); TKN - Total Kjeldhal Nitrogen (mg N/L), E. Coli - Escherichia Coli (CFU/100 mL) SC - Specific Conductance (uS/cm); SWL - Static Water Level (ft bmp); Temp - Water Temperature TD - Total Depth (ft bmp) ---PAGE BREAK--- Page 61 Western Groundwater Services (abandoned) (abandoned) 1A 1C 2A 2B 2C 3A 3B 3C 4A 4B 4C 6C 5A 5B 6A 6B 7A 7C ± 0 800 1,600 Feet Not applicable Quarterly Reserve Sampling Frequency Figure 44. Existing Monitoring Well Network ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 62 5.2 Proposed Monitoring Plan The proposed monitoring plan is intended to apply to the upgraded BWRF and through the planning period to year 2038. The monitoring plan recommends that four new wells be constructed and that a subset of the existing wells also be used (Figure 45). There are 10 wells proposed for monitoring, matching the same number of wells presently used. New monitoring wells are located only on State land and should not require access permission from the Airport Authority. The same parameter list would be used, except that total depth (TD) would not be required (Table Sample frequencies would be at all locations. A subset of existing wells would also be used for reserve sampling that may be requested by DEQ or could be used by the City for additional information gathering. The sampling frequency extended to all wells is a significant increase in monitoring and will provide more defensible data sets for assessment of trends and future planning. New monitoring wells should be constructed at 4-inch diameter and to screen and sample shallow groundwater (Figure 46). Screens will be located from approximately the water table to 20 feet below. Total well depth will be about 45- to 55-feet. Existing and new wells should be sampled through dedicated 4-inch stainless steel submersible pumps, powered from a portable generator 5 kW), and constructed to discharge to land surface at the wellhead. These pumps preclude measurement of well depth, but provide a substantial time savings and improve the quality of the sample as compared to a manually bailed sample or a portable pumping system. A disinfected tee fitting with sampling tap and control valve can be threaded onto the wellhead for each sampling event and used for all of the wells. Table 8. Proposed Sampling Wells and Parameters Monitoring Wells Parameters Reported Quantity Sampling Frequency 1A (background) 8A, 9A 7B, 8B 1C (background) Lab: Cl, NOx, TKN Field: SC, SWL, Temp Average 5A 5B 3C, 6C Lab: Cl, NOx, TKN, E. Coli Field: SC, SWL, Temp Average Coli - also Daily Max) 3A, 4A, 6A, 7A 3B, 4B, 6B 2C, 4C, 7C Reserve (sampling not required) NA NA Cl - Chloride (mg/L); NOx - Nitate plus Nitrite (mg N/L); TKN - Total Kjeldhal Nitrogen (mg N/L), E. Coli - Escherichia Coli (CFU/100 mL) SC - Specific Conductance (uS/cm); SWL - Static Water Level (ft bmp); Temp - Water Temperature TD - Total Depth (ft bmp) ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 63 1A 1C 3C 6C 5A 5B 8A 9A 7B 8B ± 0 1,000 2,000 Feet Abandoned Sampling Frequency Reserve Figure 45. Proposed Monitoring Well Network ---PAGE BREAK--- Page 64 Western Groundwater Services A. B. Figure 46. Monitoring Well and Sampling Pump Illustration ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 65 6. REFERENCES Panday, S. (2022) USG-Transport Version 1.10.0: Transport and Other Enhancements to MODFLOW-USG, GSI Environmental, Feb. 2022. Slagle, S. E. (1995) Geohydrologic Conditions and Land Use in the Gallatin Valley, Southwestern Montana, 1992-1993. U.S. Geological Survey Water-Resources Investigations Report 95-4034. TD&H Engineering (2019a) Belgrade, Montana Preliminary Engineering Report, report to City of Belgrade (March). TD&H Engineering (2019b) Water Transport Study Report, Belgrade, Montana, report to the City of Belgrade (September). TD&H Engineering, Inc. (2020) Belgrade Water Reclamation Facility Upgrades, Disposal Design Report, February, p. 4-4. Western Groundwater Services, LLC (2020) Source Specific Mixing Zone Report, Wastewater Treatment Plant Discharge, City of Belgrade, Montana, prepared for TDH Engineering, Inc. ---PAGE BREAK--- Source Specific Mixing Zone Report Western Groundwater Services Page 66 Appendix A – WELL LOGS AND DATA ---PAGE BREAK--- BWTP Modeling Report Western Groundwater Services WELL LOGS CROSS SECTION A-A’ ---PAGE BREAK---     

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