Pahrump Groundwater Plan Evaluation in Regards to Identifying Projects for Preliminary Engineering Reports
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Nye County Water District Pahrump Groundwater Plan Evaluation in Regards to Identifying Projects for Preliminary Engineering Reports Prepared by: Shaw Engineering Interflow Hydrology Paul Winkelman, P.E. Dwight Smith, P.E., P.G. Scott Benedict, P.E. June 2nd 2017 Nye County Water District Pahrump GW Plan Evaluation in June 2017 Regards to Identifying Projects for PER Abbreviations used in Report AFA Acre-Feet Annually AMSL Above Mean Sea Level AS Aquifer Storage ASR Aquifer Storage and Recovery AR Artificial Recharge BOCC Nye County Board of Commissioners DUI Desert Utilities Inc USEPA US Environmental Protection Agency FMP Flood Master Plan GBWC Great Basin Water Company GPCD, gpcd Gallons Per Capita per Day GPD, gpd Gallons Per Day GPM, gpm Gallons Per Minute HGL Hydraulic Grade Line IRP Integrated Resource Plan ISDS Individual Sewer Disposal System (septic) LVVWD Las Vegas Valley Water District MGD, mgd Million Gallons per Day Mg/l, mg/l Milligrams Per Liter NCPW Nye County Public Works NCWD Nye County Water District NDEP Nevada Department of Environmental Protection NDWR Nevada Division of Water Resources PUCI Pahrump Utility Co. Inc. PER Preliminary Engineering Report PUC Public Utility Company PUCN Public Utilities Commission of Nevada RI Rapid Infiltration RIBS Raised Infiltration Basins WWTP Wastewater Treatment Plant Shaw Engineering i Nye County Water District Pahrump GW Plan Evaluation in June 2017 Regards to Identifying Projects for PER Acknowledgments The assistance of Susan Rybarski and Greg Pohll with the Desert Research Institute on the utilization of the ground water flow model was greatly appreciated. In addition, Jimmy Hou’s and Patrick Johnson’s contributions in the analysis of geographical data and preparation of exhibits was extremely valuable. Shaw Engineering ii Nye County Water District Pahrump GW Plan Evaluation in June 2017 Regards to Identifying Projects for PER Pahrump Groundwater Plan Evaluation in Regards to Identifying Projects for Preliminary Engineering Reports Table of Contents Page Abbreviations i Acknowledgements ii Table of Content iii Section 1.0 Executive Summary 1-1 2.0 Introduction 2-1 3.0 Review of Basin 162 Hydrology 3-1 4.0 Existing Basin Water Balance 4-1 5.0 DRI Model Utilization and Limitations 5-1 6.0 Re-Distribution of Production Wells 6-1 7.0 Flood Detention Rapid Infiltration Basin Conversion 7-1 8.0 Wastewater Treatment and Individual Sewage Disposal Systems Potential Return Flows 8-1 9.0 Aquifer Storage and Recovery and Artificial Recharge in Regards to Basin 162 9-1 10.0 Potential Future Basin Water Balance 10-1 11.0 Backbone Infrastructure and Create Incentives to Voluntarily Connect to Public Water Systems 11-1 12.0 Wastewater Treatment and Individual Sewage Disposal Systems Water Quality Considerations 12-1 13.0 Conclusion and Recommendations 13-1 14.0 References 14-1 Shaw Engineering iii Nye County Water District Pahrump GW Plan Evaluation in June 2017 Regards to Identifying Projects for PER List of Tables Page Table 2-1: Committed Water Rights – Basin 162 2-1 Table 4-1: Pahrump Valley Water Balance Summary 4-3 Table 5-1: Water Balance Components in the DRI Model (2017 Updated Version) 5-1 Table 6-1: Re-Distribution of 2,000 AFA of Pumping to Fan – Facility Cost Opinion 6-7 Table 6-2: Highway 160 Corridor Well Construction – Facility Cost Opinion 6-9 Table 6-3: Stateline 8,000 AFA of Deep Wellfield Pumping – Facility Cost Opinion 6-12 Table 7-1: Pahrump Flood Plan – Proposed Detention Basins with Incremental Construction and O&M Cost Increase to Convert a Portion Located on the Fan to Infiltration Basins 7-4 Table 7-2: Summary of Flood Control Basins Simulated for AR Infiltration 7-6 Table 8-1: WWTP Influent Flow and Flow per Customer, 2016 8-1 Table 8-2: WWTP Effluent Disposal, 2016 8-1 Table 9-1: Cost Opinion to Utilize a RIB to Recharge Manse Spring 9-4 Table 10-1: Estimate of Current Consumptive Use of Groundwater 10-1 Table 10-2: Estimate of Possible Consumptive Use of Groundwater by 2060 10-2 Table 10-3: Water Rights Commitments Summary for Current Conditions 10-4 Table 10-4: Water Rights Commitments Estimate for Future Conditions (2065) 10-5 Table 11-1: Existing Water System Approximate Service Elevation Ranges 11-3 Table 11-2: 24-inch Valley-Utility Inter-Connect Transmission Main Cost Opinion 11-2 Shaw Engineering iv Nye County Water District Pahrump GW Plan Evaluation in June 2017 Regards to Identifying Projects for PER List of Tables-cont. Page Table 12-1: WWTP Effluent Total Nitrogen Discharged 12-2 Table 12-2: Estimated ISDS Effluent Total Nitrogen Discharged 12-3 Table12-3: Estimated Cost to Add Filtration and Reverse Osmosis and Water Lost 12-7 Table 13-1: Cost per AFA for Artificial Recharge of Stormwater and Manse Spring 13-1 List of Figures Figure 3-1: Hydrographic Basin Map 3-2 Figure 4-1: Predevelopment Water Balance Schematic 4-3 Figure 4-2: Current Water Balance Schematic 4-4 Figure 5-1: Simulated Drawdown for Current Conditions and in a 50-year Future Scenario with Population Growth Rate of 1.5% 5-2 Figure 6-1: Northern Pahrump Alluvial Fan Pumping Redistribution Scenario (2,000 AFA) in 50 years at 1.5% Growth 6-2 Figure 6-2: Pumping Redistribution Scenarios into Clark County at 8,000 AFA, at 50 years with 1.5% Growth 6-3 Figure 6-3: Pumping Redistribution Scenario to a Deep Stateline Wellfield 6-4 Figure 6-4: Proposed Facilities to Re-Distribute 2,000 AFA of Pumping to the Fan 6-6 Figure 6-5: Proposed Facilities to Develop 8,000 AFA of Pumping Along Highway 160 Southeast from Pahrump 6-8 Figure 6-6: Proposed Facilities to Develop 8,000 AFA of Pumping for Deep Wellfield Pumping Stateline 6-10 Figure 7-1: Flood Plan – Proposed Detention Basins with Potential Infiltration Basin Modifications to Promote Recharge 7-2 Figure 7-2: Simulated Drawdown in 50 Years at 1.5% Growth with AR Of Stormwater Runoff and Manse Spring Recharge 7-7 Shaw Engineering v Nye County Water District Pahrump GW Plan Evaluation in June 2017 Regards to Identifying Projects for PER List of Figures-cont. Page Figure 7-3: Simulated Drawdown in 50 Years at 1.5% Growth with Combined Implementation of Flood Detention Basin AR, Manse Spring and 2,000 AFA Pahrump Fan Pumping Redistribution 7-8 Figure 8-1: Wastewater Treatment Plant Locations 8-1 Figure 9-1: Water System Boundaries and Corresponding Well Productions by Location 9-2 Figure 9-2: Potential Facility Improvements to Utilize Manse Spring for RIB Recharge 9-5 Figure 10-1: Simulated Drawdown with Combined Stormwater and Manse Spring Recharge, Pahrump Fan Pumping Redistribution, and Southern Highway 160 Wellfield Pumping Redistribution 10-7 Figure 10-2: Simulated Drawdown for No Action with 1.5% Growth, and with AR and Pumping Redistribution to Highway 160 Wellfield 10-8 Figure 10-3: Difference in Predicted Drawdown between the 50-year No Action Scenario and Cumulative Actions of AR and Pumping Redistributions, for the 1.5% Growth Scenario 10-9 Figure 11-1: Existing Domestic Wells, Municipal Water System Boundaries With Proposed 24-inch Transmission Main Inter-Connect 11-2 Figure 12-1: Septic System (ISDS) Schematic 12-1 Appendices Appendix A – GWMP Appendix B – Soils Maps for Pahrump Valley Appendix C – Long Term Water Level Trends in Pahrump Valley, Figure 5-1 Taken from “Nye County Water Resource Plan”, April 2017 Appendix D – Public Comments and Responses to Draft Report Dated May 8th 2017 Shaw Engineering vi Nye County Water District Pahrump GW Plan Evaluation in June 2017 Regards to Identifying Projects for PER 1.0 Executive Summary The Nye County Water District (NCWD) contracted with Shaw Engineering to identify potentially viable groundwater resource projects to advance to the preliminary engineering report (PER) level. The primary goal of this investigation was to identify physical infrastructure projects that support the GWMP that can be prioritized from the standpoint of cost versus benefit. The projects evaluated should either increase the sustainable yield, or enhance the capturability of the perennial yield. Secondary objectives included review of potential groundwater quality concerns with individual sewage disposal systems (ISDS) and pharmaceuticals discharged into the environment from WWTP and ISDS. The Pahrump groundwater Basin 162 is over-appropriated by approximately 43,400 AFA. The sustainable pumping rate based on the present well configuration is on the order of 10,000 AFA to 12,000 AFA, while depletion in storage in the basin fill aquifer is currently estimated at 5,000 AFA. Uncaptured deep carbonate outflow is estimated to be 6,000 AFA to 8,000 AFA. The results of meetings with NCWD, Pahrump area utilities, NDWR, review of previous studies, and the analysis contained in this report resulted in the following recommended actions (1-deemed essential, 2-required, but not a potential public health concern, 3- potentially needed to support the GWMP objectives): Rank Description 1 A nitrate sampling program is recommended to determine if elevated levels are occurring. Elevated nitrate levels in groundwater can pose a public health threat therefore it is deemed essential to pursue nitrate sampling. 2 The purchase of the Manse Springs water rights by Nye County is in the best interest of the groundwater users in Basin 162 and it is recommended to initiate negotiations for their acquisition. o If the Manse Spring and water rights can be purchased, then at that time it is recommended to proceed with the planning, design and construction of a rapid infiltration basin (RIB) project to artificially recharge to the Valley basin fill.