5. Basin Evaluation Salt and Nitrate Balance

Transcription

1 SNMP Attachment B, Section B (originally Section 5 in CV-SALTS Phase 2 Conceptual Model Task 5 deliverables) 5. As part of the evaluation of water quality the SNMP requires loading estimates and fate and transport analysis for salt and nitrate. Development of this information considers a number of components that affect the salt/nitrate balance in any given area. This section broadly describes the mass balance components at the Central Valley and regional hydrologic unit scales, along with summaries of mass balance components at the Initial Analysis Zone (IAZ) scale. Mass balance components such as groundwater recharge, surface water/groundwater interaction (stream leakage), horizontal movement from one area to another, vertical movement downward toward deeper aquifer units, and groundwater pumping all play roles in the movement and management of salt and nitrate into and out of areas of interest. Understanding the movement of salt and nitrate is critical for an SNMP, as these assessments provide the basis from which to determine the overall increasing/decreasing trends of total mass in groundwater. When discussing management practices, the movement of mass in the context of individual balance components becomes critical for sustaining good quality groundwater sources and/or improving impaired groundwater. The Phase I Initial Conceptual Model (LWA et. al. 2013) was used to evaluate salt and nitrate balances for 22 water balance subregions (also referred to as Initial Analysis Zones, or IAZs) that collectively make up the Central Valley Floor. These areas are based on hydrologic boundaries, and for the purposes of this section, they are grouped and summarized geographically into the Northern Central Valley, Middle Central Valley, and Southern Central Valley. Additional detail on the mass balance information for each of the 22 IAZ water balance subregions individually is provided in Appendix G (see SNMP Section B Appendix). 5.1 CONCEPTUAL MODEL Salt and nitrate management requires an understanding of water movement on and beneath the land surface. Section 4 and Appendix F (see SNMP Section B.2.1.2, text and appendix, respectively) of this SNMP provide a summary of the water movement for each of the 22 IAZs. Groundwater flows were based on the 2009 Central Valley Hydrologic Model (CVHM) developed by the US Geological Survey (USGS); the model provided the flow paths for determining salt (TDS 1 ) and nitrate mass balance. These flows served as the framework for determining where TDS and nitrate mass was entering and leaving each IAZ. Concentrations of nitrate and TDS were assigned to each flow component in order to move mass in, out, and between IAZs. 1 The amount of salt contained in water is estimated through the amount of total dissolved solids (TDS); the term TDS is used throughout this report to represent the concentration of salt in water. Central Valley SNMP 5-1 May 2016

2 Horizontal Boundaries: CVHM defines 21 hydrologically-delineated areas in the Central Valley floor as Water Balance Subregions (WBS). The determination was made to use these 21 areas as the boundaries for ICM IAZs, with one exception: the CVHM Delta- Mendota Basin was subdivided, so 22 IAZs were ultimately used to split up the Central Valley Floor for the ICM technical analyses of water, salt, and nitrate movement. The horizontal boundaries of the 22 IAZs are hydrologically based using surface water features and alluvial aquifer boundaries for defining extents. The IAZ areas are also directly related to the model structure of the 2009 CVHM and corresponding WBS used by the California Department of Water Resources (DWR). DWR has compiled substantial information on water deliveries and diversions for subregions of the Central Valley Floor and has used these subregions as water supply planning areas. Therefore, the IAZs were used for the boundaries of the initial analysis of water, salt, and nitrate movement in the Central Valley Floor for the purposes of SNMP development. Vertical Boundaries: The vertical dimensions of the IAZs are significant to the conceptualization of movement of water, salt, and nitrate for management purposes. The ICM approach included using a defined depth to the upper part of the aquifer system beneath each IAZ. The water, salt, and nitrate balance calculations in the ICM were performed for a 20-year period. To estimate the groundwater affected by activities over a 20-year period, the vertical travel distance must be calculated. The vertical distance represents the distance that the water, at the water table, would travel downward or upward over a 20-year period. This 20-year travel zone defines the ICM shallow portion of the subsurface where the ICM water, salt, and nitrate balance analyses were performed. Mass Balance Methodology: The methodology developed for the ICM mass balance calculations is based on the simulated water budget components in CVHM on a quarterly basis for a 20-year period between 1983 and The 20-year period was selected based on the most recent 20-year period in the CVHM simulation, which provided all of the water balance component flow values. Calculations utilizing hundreds of complicated queries on the data to add or subtract masses of salt and nitrate for each IAZ were performed to determine the movement of salt and nitrate over the 20-year period on a quarterly basis. The calculations of adding and subtracting masses enabled the determination of mass movement simultaneously between each IAZ for the entirety of the Central Valley Floor, as conceptualized in Figure 5-1. Central Valley SNMP 5-2 May 2016

3 IA Z Vadose 20-year Travel o Mixing Model o o Lower Portion of the Figure 5-1. Conceptual Model of Mass Balance Calculations for Each IAZ 5.2 SALT AND NITRATE SOURCE IDENTIFICATION AND LOADING ESTIMATES Source Loading Identification Nitrate and TDS mass loading at the surface originates from various sources. Both diffuse and point sources were considered in the loading estimates and include several forms: Dissolved constituents in applied water; Atmospheric deposition; Point source discharges; Permitted land application of dissolved or suspended constituents in municipal or industrial wastewater or solids; and Other materials applied directly to land to grow plants, or added to water during irrigation as a water-borne application method. For an in depth discussion of the many sources included in calculating mass loading, refer to LWA et al. (2013). Central Valley SNMP 5-3 May 2016

4 5.2.2 Loading Estimates Mass loading was estimated using the Watershed Analysis Risk Management Framework (WARMF) program, which simulates mass uptake in the root zone and mass moving past the root zone considering soil, crop, and climate variables. The mass moving past the crop root zone was used to estimate loading to the groundwater in the mixing model. Several scenarios were developed to look at how different levels of loading would affect the mass balances for both nitrate and TDS (LWA et al. 2013). Because actual nitrate application and uptake rates are not precisely known for the entire Central Valley Floor, six different scenarios were developed based on the efficiency of nitrate uptake by the crops. A high nitrate use efficiency means that more nitrate has been taken up by the crop and less mass is available to move past the root zone and into groundwater, whereas a low nitrate use efficiency means that less nitrate is taken up by the crop and more mass is available to move past the root zone and into groundwater. Initial scenarios developed included high, moderate and low nitrate use efficiencies. Three subsequent runs utilized the low nitrate loading to assess impacts of greater nitrate loading amounts. These consisted of efficiencies based on 90%, 75%, and 60% of the low nitrate use efficiencies (NUE) to increase nitrate loading (Table 5-1). Appendix G (see SNMP Section B Appendix) provides the mass balance components based on the moderate NUE run. For TDS, three different scenarios were developed (Table 5-1). These consist of the original loading calculations from WARMF and two additional scenarios (50% of the original loading and 200% of the original loading). For a detailed discussion and explanation of the development of mass loading calculations, please see LWA et al Table 5-1. Mass Loading Scenarios for Nitrate and TDS Mass Load Scenarios for ICM Nitrate High NUE Moderate NUE Low NUE 90% of Low NUE 75% of Low NUE 60% of Low NUE TDS 50% of Original Loading 100% of Loading (Original Load) 200% of Original Loading Central Valley SNMP 5-4 May 2016

5 5.3 IMPORT/EXPORT Nitrate and TDS are imported into the Central Valley largely in the form of irrigation water and fertilizer for crops and associated crop amendments. There are many other sources of nitrate and TDS related to activities in the Central Valley. Small amounts of naturally occurring nitrate and TDS exist in most the Central Valley. However, one exception is the western portion of the San Joaquin basin, where elevated levels of TDS occur naturally due to the presence of marine sediments in the Coastal Range Mountains. Precipitation also imports small amounts of nitrate and TDS mass. Aside from the export of nitrate and TDS via the production of agriculture and manufactured goods, the only natural export of nitrate and TDS from the Central Valley occurs through the Sacramento/San Joaquin Delta. Where gaining stream conditions exist, groundwater provides water (and therefore mass) to surface water, which is then transported to the Sacramento River in the north or the San Joaquin River in the south, eventually reaching the Delta and exiting to the Pacific Ocean. The exception is the Tulare Lake Hydrologic Region in the southern end of the Central Valley. Due to a natural groundwater divide, the region has no natural export of water (other than crop evapotranspiration and evaporation from surface water bodies, neither of which contains nitrate and TDS). The region has approximately the same boundary as IAZs ESTIMATED ASSIMILATIVE CAPACITY (EXISTING AND PROJECTED) Existing Assimilative Capacity Existing assimilative capacity was assessed using the ambient groundwater quality for shallow groundwater (20-year travel zone) as well as the results from the loading scenarios. Shallow ambient groundwater conditions were based on recent groundwater quality test data from 2003 to The final groundwater quality concentrations for the loading scenarios were used to calculate assimilative capacity. Assimilative capacity is obtained by subtracting the difference between ambient conditions and a determined threshold (Figure 5-2). For nitrate, the threshold is the regulatory drinking water standard, which is 10 mg/l nitrate as nitrogen (or NO3-N). For TDS, there is no primary drinking water standard which to compare ambient conditions. Therefore, three thresholds for TDS were chosen for assimilative capacity calculations: 500, 700, and 1000 mg/l. Figure 5-2. Example Assimilative Capacity Calculation Central Valley SNMP 5-5 May 2016

6 The left portions of Table 5-2 and Table 5-3 below show the calculated ambient concentrations for both shallow and deep groundwater in addition to the results for each modeling scenario. Assimilative capacity was calculated separately for each scenario and is found on the right side of the tables. Color coding indicates the relative amount of assimilative capacity available, spanning from green (indicating assimilative capacity is available) to red (no assimilative capacity is available). Table 5-2. Ambient and Simulated Nitrate Concentrations with Assimilative Capacity Calculations Central Valley SNMP 5-6 May 2016

7 Table 5-3. Ambient and Simulated Nitrate Concentrations with Assimilative Capacity Calculations 5.5 FATE AND TRANSPORT Nitrate and TDS mass is moved throughout the groundwater systems of the Central Valley via various hydrologic pathways. The hydrologic components of the ICM provided the framework in which mass was transferred in, out, and between IAZs. Mass is transported: Along vertical boundaries between the bottom boundaries of IAZs and the lower aquifer through local groundwater flow conditions; Along horizontal boundaries between IAZs; and To surface water where gaining stream conditions exist. Mass is introduced: Through recharge from the surface; and From surface water where losing stream conditions exist. Nitrate and TDS mass leaves the groundwater aquifer system through groundwater pumping and discharge to surface water. Surface water (and associated nitrate and TDS mass) leaves the Central Valley Floor via the Sacramento-San Joaquin River Delta; however, it should be noted that the Tulare Lake Hydrologic Region in the southern portion of the Central Valley has no natural outflows of water and therefore acts as a sink where mass does not leave the area. Both nitrate and TDS mass were modeled conservatively in groundwater, i.e., no precipitation or denitrification chemical processes were modeled in the ICM (2013) mixing models (LWA et al. 2013). Central Valley SNMP 5-7 May 2016

8 5.6 SALT AND NITRATE BALANCE CONDITIONS Summaries of the nitrate and TDS mass balance conditions for the three major geographic areas of the Central Valley Floor are provided below in this section. For more details on each IAZ s specific salt and nitrate conditions, please refer to Appendix G (see SNMP Section B Appendix). Nitrate and TDS mass balance components (Figure 5-3) in this analysis include: Groundwater pumping of nitrate and TDS mass; Stream leakage with nitrate and TDS mass transport (from gaining stream conditions or losing stream conditions); Net recharge with nitrate and TDS mass transport (deep percolation out of the root zone); Horizontal groundwater flow with nitrate and TDS mass transport to/from neighboring IAZs; and Vertical groundwater flow with nitrate and TDS mass transport to/from the aquifer below the starting unit mass (here, the lower aquifer refers to the part of the aquifer system below the 20-year travel zone). Figure 5-3. Mass Balance Components and Mass Transport due to Groundwater Movement The subsections below provide figures and tables that summarize mass balance information for each IAZ within the three areas of the Central Valley (Northern, Middle, and Southern) during the ICM study period ( ) within the 20-year travel zone. The figures and tables included in the following subsections provide the details of annual average nitrate and TDS mass balance components with the following signage: Negative numbers indicate that salt or nitrate is being exported from the IAZ via one or more of the following components of the mass balance: Central Valley SNMP 5-8 May 2016

9 o Negative values for pumping indicate mass being removed from the IAZ through wells; o Negative mass balance values due to stream leakage indicate mass leaving the IAZ groundwater body by entering streams (during gaining stream conditions); o Negative mass balance values due to groundwater recharge do not occur 2 ; o Negative mass balance values due to vertical flow indicate mass leaving the IAZ by mass transport downwards into the lower aquifer below the 20-year travel zone; and o Negative mass balance values due to horizontal flow indicate mass leaving the IAZ horizontally to travel into neighboring IAZs. Positive numbers indicate that salt or nitrate is being imported into the IAZ via one or more of the following components of the mass balance: o Positive mass balance values due to stream leakage indicate mass entering the IAZ groundwater body from losing stream conditions; o Positive mass balance values due to groundwater recharge indicate mass entering the IAZ via deep percolation mass transport past the root zone; o Positive mass balance values due to vertical flow are rare, but indicate mass entering the IAZ from the lower aquifer (this can occur in areas of groundwater discharge, where lower portions of the groundwater aquifer are actually pushing water (and therefore mass) upwards into the 20-year travel zone); and o Positive horizontal flow values indicate mass entering the IAZ horizontally from neighboring IAZs. The following subsections summarize nitrate and TDS mass balance data or IAZs. Appendix G (see SNMP Section B Appendix) contains more details for each of the IAZs in the three areas of the Central Valley, including: 1) time-series plots showing quarterly and annual patterns of each mass balance component over time, 2) details about the average mass balance components described above, 3) pie charts showing the different mass balance components, and 4) summaries of the mass balance conditions for each IAZ over the 20-year study period. Appendix G (see SNMP Section B Appendix) provides local entities with nitrate and TDS mass balance details for their area which may be used to support development of a local SNMP Summary of Northern Central Valley IAZs IAZs in the Northern Central Valley (IAZs 1 through 7) represent the Sacramento Valley Groundwater Basin, where gaining streams are prevalent and groundwater recharge is high. The 20-year travel zones for IAZs 1 through 7 represent an average range of nitrate mass between about 4 thousand tons to about 35 thousand tons (Figure 5-4) and an average range of TDS mass between 1 million tons to about 5.5 million tons (Figure 5-5). A comparison of average annual nitrate and TDS budget components 2 While negative values for recharge do sometimes occur in the flow budget (as a result of water evaporating from the soil where shallow groundwater conditions exist), mass does not leave through this pathway as evaporated water does not contain solutes. Central Valley SNMP 5-9 May 2016

10 (between 1983 and 2003) gives insight to the differences between the IAZs in the Northern Central Valley, and also some similarities (Table 5-4, Table 5-5, Figure 5-6, and Figure 5-7). IAZ 4 stands out as behaving differently from the other IAZs: o Vertical flow and mass transport from groundwater deeper than the 20-year travel zone (from the lower aquifer ) is upward instead of downward; and o IAZ 4 is also a sink with respect to horizontal flow, receiving more nitrate and TDS mass from adjacent IAZs compared to all others. Every IAZ except IAZ 2 resulted in an increase in nitrate mass over the 20-year period (Figure 5-8). Every IAZ except IAZ 6 resulted in a decrease in TDS mass over the 20-year period (Figure 5-9). Figure 5-4. Average Nitrate Mass Present in the 20-Year Travel Zone for Northern Central Valley IAZs ( ) (Tons) Central Valley SNMP 5-10 May 2016

11 Figure 5-5. Average TDS Mass Present in the 20-Year Travel Zone for Northern Central Valley IAZs ( ) (Tons) Table 5-4. Annual Average Nitrate Mass Balance Components for the 20-Year Travel Zone of Each IAZ in the Northern Central Valley ( ) (Tons) Central Valley SNMP 5-11 May 2016

12 Table 5-5. Annual Average TDS Mass Balance Components for the 20-Year Travel Zone of Each IAZ in the Northern Central Valley ( ) (Tons) Figure 5-6. Average Nitrate Mass Balance Components for the 20-Year Travel Zone for IAZs in the Northern Central Valley ( ) (Tons) Central Valley SNMP 5-12 May 2016

13 Figure 5-7. Average TDS Mass Balance Components for the 20-Year Travel Zone for IAZs in the Northern Central Valley ( ) (Tons) Figure 5-8. Starting, Net Change, and Final Nitrate Mass for the 20-Year Travel Zone for IAZs in the Northern Central Valley ( ) (Tons) Central Valley SNMP 5-13 May 2016

14 Figure 5-9. Starting, Net Change, and Final TDS Mass for the 20-Year Travel Zone for IAZs in the Northern Central Valley ( ) (Tons) Summary of Middle Central Valley IAZs IAZs in the Middle Central Valley (IAZs 8 through 13 plus 22) represent the central portion of the Central Valley, including the Delta, where vertical groundwater flow and mass transport downward to the saturated sediments below the 20-year travel zone is prevalent. The 20-year travel zones for IAZs in this area represent an average range of nitrate mass between about 8 thousand tons to about 37 thousand tons (Figure 5-10) and an average range of TDS mass between 3 million tons to about 20 million tons (Figure 5-11). A comparison of average annual nitrate and TDS budget components (between 1983 and 2003) gives insight to the differences between the IAZs in the Middle Central Valley IAZs and also some similarities (Table 5-6, Table 5-7, Figure 5-12, and Figure 5-13). IAZ 9 stands out as behaving differently from the other IAZs in that the stream leakage component dominates the influx of TDS mass into the IAZ. IAZ 9 is the only IAZ contributing groundwater from the 20-year travel zone to the Delta. IAZs 8, 13, and 22 have similarly high amounts of vertical groundwater flow leaving the 20-year travel zone to transport a large proportion of nitrate and TDS mass downward. IAZs 8 and 22 have the largest influxes of nitrate and TDS mass respectively due to their groundwater recharge components. Central Valley SNMP 5-14 May 2016

15 IAZs 8, 9, and 10 resulted in an increase in nitrate mass over the 20-year period, whereas IAZs 11, 12, 13, and 22 resulted in a decrease in nitrate mass (Figure 5-14). IAZs 8 and 13 resulted in a decrease in TDS mass over the 20-year period, whereas IAZs 9-12 and 22 resulted in an increase in TDS mass (Figure 5-15). Figure Average Nitrate Mass Present in the 20-Year Travel Zone for Middle Central Valley IAZs ( ) (Tons) Central Valley SNMP 5-15 May 2016

16 Figure Average TDS Mass Present in the 20-Year Travel Zone for Middle Central Valley IAZs ( ) (Tons) Table 5-6. Annual Average Nitrate Mass Balance Components for the 20-Year Travel Zone of Each IAZ in the Middle Central Valley ( ) (Tons) Central Valley SNMP 5-16 May 2016

17 Table 5-7. Annual Average TDS Mass Balance Components for the 20-Year Travel Zone of Each IAZ in the Middle Central Valley ( ) (Tons) Figure Average Nitrate Mass Balance Components for the 20-Year Travel Zone for IAZs in the Middle Central Valley ( ) (Tons) Central Valley SNMP 5-17 May 2016

18 Figure Average TDS Mass Balance Components for the 20-Year Travel Zone for IAZs in the Middle Central Valley ( ) (Tons) Figure Starting, Net Change, and Final Nitrate Mass for the 20-Year Travel Zone for IAZs in the Middle Central Valley ( ) (Tons) Central Valley SNMP 5-18 May 2016

19 Figure Starting, Net Change, and Final TDS Mass for the 20-Year Travel Zone for IAZs in the Middle Central Valley ( ) (Tons) Summary of Southern Central Valley IAZs IAZs in the Southern Central Valley (IAZs 14 through 21) represent the southern portion of the Central Valley, including portions of the San Joaquin Valley and the Tulare Basin, where major outflow components are usually vertical flow downward to the saturated sediments below the 20-year travel zone and groundwater pumping. The greatest inflow components are typically groundwater recharge, water from storage depletion, and stream leakage via losing stream conditions. The 20-year travel zones for IAZs 14 through 21 represent an average range of nitrate mass between about 20 thousand tons to about 220 thousand tons (Figure 5-16) and an average range of TDS mass between 1.5 million tons to about 105 million tons (Figure 5-17). A comparison of average annual nitrate and TDS mass balance components (between 1983 and 2003) gives insight to the differences between the IAZs in the Southern Central Valley IAZs and also some similarities (Table 5-8, Table 5-9, Figure 5-18, and Figure 5-19). IAZs 15 and 21 show the greatest influx of nitrate mass due to the groundwater recharge component, while IAZs 14 and 19 show the greatest influx of TDS mass due to the groundwater recharge component. IAZs 14 and 19 also show the greatest outflux of TDS mass due to the vertical flow downward out of the shallow 20-year travel zone component, while IAZ 15 shows the greatest nitrate mass outflux due to the vertical flow downward out of the shallow 20-year travel zone. Central Valley SNMP 5-19 May 2016

20 IAZs 15, 18, and 19 show the greatest outflux of nitrate and TDS mass due to the groundwater pumping component. IAZs 14, 16, and 18 resulted in a decrease in nitrate mass over the 20-year period, whereas IAZs 15, 17, and resulted in an increase in nitrate mass (Figure 5-20). All IAZs resulted in a decrease in TDS mass over the 20-year period (i.e. the salt moved deeper into the aquifer system beneath the IAZs) (Figure 5-21). Central Valley SNMP 5-20 May 2016

21 Figure Average Nitrate Mass Present in the 20-Year Travel Zone for Southern Central Valley IAZs ( ) (Tons) Figure Average TDS Mass Present in the 20-Year Travel Zone for Southern Central Valley IAZs ( ) (Tons) Central Valley SNMP 5-21 May 2016

22 Table 5-8. Annual Average Nitrate Mass Balance Components for the 20-Year Travel Zone of Each IAZ in the Southern Central Valley ( ) (Tons) Table 5-9. Annual Average TDS Mass Balance Components for the 20-Year Travel Zone of Each IAZ in the Southern Central Valley ( ) (Tons) Central Valley SNMP 5-22 May 2016

23 Figure Average Nitrate Mass Balance Components for the 20-Year Travel Zone for IAZs in the Southern Central Valley ( ) (Tons) Central Valley SNMP 5-23 May 2016

24 Figure Average TDS Mass Balance Components for the 20-Year Travel Zone for IAZs in the Southern Central Valley ( ) (Tons) Figure Starting, Net Change, and Final Nitrate Mass for the 20-Year Travel Zone for IAZs in the Southern Central Valley ( ) (Tons) Central Valley SNMP 5-24 May 2016

25 Figure Starting, Net Change, and Final TDS Mass for the 20-Year Travel Zone for IAZs in the Southern Central Valley ( ) (Tons) Central Valley SNMP 5-25 May 2016