Orange County Water District Salt and Nitrate Projections for Orange County Management Zone Final Technical Memorandum Prepared by Greg D. Woodside, P.G., C.Hg. July 18, 2016 Table of Contents SECTION 1 INTRODUCTION .................................................................................... 1-1 1.1 Background Discussion ..................................................................................... 1-1 1.2 General Description of Orange County Management Zone ............................... 1-3 1.3 Types and Sources of Data Used in the Model ................................................. 1-4 SECTION 2 BASIN HYDROGEOLOGY ..................................................................... 2-1 2.1 Description of Basin Hydrogeology.................................................................... 2-1 2.1.1 Forebay and Pressure Areas .................................................................. 2-3 2.1.2 Groundwater Subbasins, Mesas, and Gaps ........................................... 2-5 2.1.3 Coastal Plain of Orange County: Areas outside OCWD Boundaries ..... 2-6 2.2 Determination of Total Basin Volume ................................................................ 2-7 2.3 Water Budget .................................................................................................... 2-8 2.3.1 Measured Recharge ............................................................................... 2-9 2.3.2 Unmeasured Recharge ......................................................................... 2-11 2.3.3 Groundwater Production ....................................................................... 2-13 2.3.4 Subsurface Outflow .............................................................................. 2-13 2.3.5 Evaporation ........................................................................................... 2-14 2.4 Calculation of Change in Groundwater Storage .............................................. 2-14 2.5 Elevation Trends.............................................................................................. 2-18 2.6 Basin Model ..................................................................................................... 2-23 SECTION 3 MODELING APPROACH........................................................................ 3-1 3.1 Conceptual Model.............................................................................................. 3-1 3.2 Model Methodology ........................................................................................... 3-2 3.3 Model Input ........................................................................................................ 3-3 3.4 Model Verification ............................................................................................ 3-10 3.5 TDS Predictions Using the Model .................................................................... 3-11 3.6 Sensitivity Analysis .......................................................................................... 3-12 3.7 Nitrate Projection for Orange County Management Zone ................................ 3-15 SECTION 4 SUMMARY AND CONCLUSION ............................................................ 4-1 SECTION 5 REFERENCES ....................................................................................... 5-1 Salt and Nitrate Projections for Orange County Management Zone Final Technical Memorandum i Table of Contents List of Tables Table 2-1: Estimated Basin Groundwater Storage by Hydrogeologic Unit ................... 2-8 Table 2-2: Example Annual Basin Water Budget ......................................................... 2-9 Table 3-1: Summary of Inflows to Orange County Management Zone ......................... 3-3 Table 3-2: Delivered Water Quality Summary .............................................................. 3-4 Table 3-3: Recent TDS Concentrations at Monitoring Wells in Shallow Semi-Perched Interval ........................................................................................................ 3-5 Table 3-4: Summary of Subsurface Inflow Components .............................................. 3-8 Table 3-5: Summary of TDS Estimates for Model Cases ........................................... 3-13 Table 3-6: Comparison of Amount of Imported Water Recharge and Projected TDS Concentrations for Cases 1-4 ................................................................... 3-14 Table 3-7: Summary of Inflows to OC Management Zone ......................................... 3-16 Table 3-8: Summary of Subsurface Inflow Components ............................................ 3-17 List of Figures Figure 1-1: Boundaries of the Orange County Management Zone .............................. 1-4 Figure 2-1: Coastal Plain of Orange County Groundwater Basin, Basin 8-1 ................ 2-1 Figure 2-2: Geologic Cross-Section, Orange County Groundwater Basin .................... 2-3 Figure 2-3: Orange County Groundwater Basin ........................................................... 2-4 Figure 2-4: Basin 8-1 and OCWD Boundaries ............................................................. 2-7 Figure 2-5: OCWD Recharge Facilities in Anaheim and Orange ............................... 2-10 Figure 2-6: Seawater Intrusion Barriers ...................................................................... 2-11 Figure 2-7: Estimated Subsurface Recharge ............................................................. 2-12 Figure 2-8: Distribution of Groundwater Production, Water Year 2012-14 ................. 2-13 Figure 2-9: Relationship between OCWD Basin Storage and .................................... 2-14 Figure 2-10: Schematic Cross-Section of the Basin Showing Three Aquifer Layers .. 2-16 Figure 2-11: Groundwater Level Contour Map, June 2014 ........................................ 2-17 Figure 2-12: Groundwater Level Changes, June 2013-14 .......................................... 2-17 Salt and Nitrate Projections for Orange County Management Zone Final Technical Memorandum ii Table of Contents Figure 2-13: Change in Groundwater Storage, WY 1974-75 to 2013-14.................... 2-18 Figure 2-14: Principal Aquifer Groundwater Elevation Profiles, 1969 and 2013 ......... 2-19 Figure 2-15: Location of Long-Term Groundwater Elevation Hydrograph .................. 2-19 Figure 2-16: Water Level Hydrographs of Wells SA-21 and GG-16 in Pressure Area 2-20 Figure 2-17: Water Level Hydrographs of Well A-27 in Forebay ................................ 2-21 Figure 2-18: Water Level Hydrographs of Wells SAR-1 and OCWD-CTG1 ............... 2-22 Figure 2-19: Basin Model Extent ................................................................................ 2-23 Figure 2-20: Basin Model Extent ................................................................................ 2-24 Figure 3-1: TDS Concentration in SAR at Below Prado Dam ....................................... 3-5 Figure 3-2: Estimation of Stormflow TDS ..................................................................... 3-6 Figure 3-3: Monitoring Well AM-50 TDS Concentration ............................................... 3-7 Figure 3-4: Location of Miraloma Basin and Monitoring Well AM-50 ............................ 3-7 Figure 3-5: Subsurface Inflow into the Orange County Management Zone.................. 3-9 Figure 3-6: Wells Used To Estimate Subsurface Inflow Concentrations .................... 3-10 Figure 3-7: Estimated TDS Concentration in Base Case for 30-year Period .............. 3-11 Figure 3-8: TDS Estimates for Model Cases .............................................................. 3-14 Figure 3-9: Estimated Nitrate (as N) Concentration for 30-year Period ...................... 3-18 Appendices Appendix A – Amount and Quality of Imported Water Recharged, Orange County Management Zone Calendar Years 2012-2014 Appendix B – 2013-14 Delivered Water Quality Summary Appendix C – Septic System Locations Appendix D – TDS Projection Spreadsheets Appendix E - Nitrate Projection Spreadsheets Salt and Nitrate Projections for Orange County Management Zone Final Technical Memorandum iii Section 1 Introduction SECTION 1 INTRODUCTION 1.1 Background Discussion The purpose of this report is to present results of an evaluation of future total dissolved solids (TDS) and nitrate concentrations in the Orange County (OC) Management Zone. The evaluation consists of estimating future TDS and nitrate concentration based on estimated future inflow and outflow from the basin. Management of salt and nitrate concentrations in groundwater is important to maintaining the long-term sustainable use of groundwater supplies. An important aspect of managing salt and nitrate concentrations in groundwater is managing the salt and nitrate concentrations of water used to recharge the groundwater basin. Removing poor-quality groundwater is another component of successful management. The evaluation of future salt and nitrate concentrations is important to the Orange County Water District (OCWD). OCWD was established by the State of California in 1933 to manage the Orange County Groundwater Basin. OCWD’s programs to manage water quality include monitoring, remediation of contaminated groundwater, and recharging high-quality recycled water. OCWD also operates the Prado Wetlands to remove nitrate from Santa Ana River water that is recharged into the groundwater basin. These efforts help provide high-quality groundwater to water users in Orange County. This evaluation provides OCWD with an understanding of the factors affecting TDS