A Conjunctive Use Model for the Tule Groundwater Sub-Basin Area in the Southern-Eastern San Joaquin Valley, California

U.S. Bureau of Reclamation (USBR) | February 4th, 2003

The Tule groundwater sub-basin is an agriculturally-intensive area located in the eastern-central part of the southern San Joaquin Valley, California. Urban and a

The Tule groundwater sub-basin is an agriculturally-intensive area located in the eastern-central part of the southern San Joaquin Valley, California. Urban and agricultural stakeholders in the Tule sub-basin depend on a combination of imported surface water and pumped groundwater to meet their water demands. The water service districts there receive surface water deliveries from the Friant Unit of the Central Valley Project (CVP) (United States Bureau of Reclamation), the State Water Project (SWP) (California Department of Water Resources), the Kings River (United States Army Corps of Engineers), or the Success Reservoir (United States Army Corps of Engineers). All of these surface water sources develop their supplies from run-off and snow melt in the foothills and watersheds of the Sierra Nevada mountain range. The state of California is prone to recurring droughts, some lasting several years. During drought periods, irrigated agriculture depends more heavily on groundwater pumping as surface water supplies are generally less available. To buffer the effects of drought, districts in the Tule sub-basin have cooperatively managed their surface water and groundwater resources conjunctively. During a normal to wet year, excess available surface water supplies (e.g. releases for flood control) are used by some districts to recharge their groundwater reservoirs. However, a prolonged multi-year drought invariably leads to an increased dependence on groundwater pumping and overdraft of the groundwater sub-basin storage. In addition to climate variability, changes in future surface water supplies may also occur due to the passage of the Central Valley Project Improvement Act of 1992, which mandates that 400,000 acre-feet per year of CVP water be released from the Friant Unit into the San Joaquin River for restoration purposes. To better understand the impacts of irrigated agriculture, fluctuating surface water supplies, and groundwater pumping practices on water levels and groundwater storage in the Tule sub-basin area, we developed a GIS-based con- junctive use model to study them. The study area is 541,580 acres in size and contains the entire Tule groundwater sub-basin and parts of the Kaweah and Tulare Lake groundwater sub-basins. The incorporated land in the study area is divided into 26 water service districts: 21 irrigation, water, or public utility districts; 2 major cities; 2 private contractors; and 1 water company. These districts are either completely or partially located within the study area. The study area is further delineated into 9,114 individual land units from a 1985 land use survey of Tulare County. Agriculture is the largest land use, compris- ing 72% of the study area. Native and urban land use comprise 22% and 4% of the study area, respectively. Semi-agricultural and special conditions (i.e. fallow) land use each comprise 1%. Twelve crops account for 95% of the area under agricultural production. Cotton, grain & grass hay, citrus, vineyards, and alfalfa individually represent 20.3, 18.6, 13.6, 13, and 10.3% of the total productive acreage, respectively. Surface water supplies are distributed to the districts and ultimately to the individual land units by a surface water supply system. The surface water supply system in the model is divided into two parts: 1) an inter-district surface water channel network, and 2) an intra-district surface water distribution system. The inter-district channel network consists of the explicitly modeled source and diversion channels which import surface water into the study area and deliver it to individual districts. The intra-district distribution system con- sists of the implicitly modeled district channels (e.g. laterals, ditches, canals, farm turnouts) which deliver surface water to individual land units within each district. The conjunctive use model consists of three loosely-coupled sub-models: 1) a surface water supply (SWS) model, 2) an unsaturated zone water budget (UZWB) model, and 3) a groundwater flow model. The base period of the study covers the fiscal water years of 1970-99. The purpose of the SWS model is to calculate the surface water balance for the source and diversion channels in the inter-district channel network. For each modeled surface water channel, the SWS model computes surface water deliveries from it to each district and conveyance losses from it due to evaporation and channel seepage. The primary model outputs are monthly surface water deliveries to each district and monthly seepage rates from modeled channels. The surface water deliveries became input for the UZWB model. The channel seepage became input for the groundwater flow model as localized aquifer recharge. The allocation of surface water within each district, via the implicitly modeled intra-district surface water distribution system, is estimated by the UZWB model. The total imported surface water for 1970-99 from the CVP and the Success Reservoir are 13,329,262 and 4,653,501 acre-feet (af), respectively. The SWP and the Kings River imported the lesser amounts of 88,625 and 7,332 af, respec- tively. Annual CVP diversions varied from 125,970 af in 1977 to 679,298 af in 1993 with a 30-year annual average of 444,309 af. The Tule River and Pioneer Ditch both receive regulated releases from Success Reservoir. Tule River annual imports varied from 11,034 af in 1977 to 607,154 af in 1983 while the Pioneer Ditch varied from 3,445 af in 1973 to 5,874 af in 1990. The total natural runoff from the Deer Creek and White River from 1970-99 were 703,444 and 219,098 af, respectively. Deer Creek runoff varied from 4,082 af in 1992 to 103,716 af in 1983 while the White River runoff varied from 422 af in 1977 to 37,985 af in 1998. From 1970-99, a total of 15 million af of surface water was applied by the service districts in the study area. The applied surface water varied from a low of 135,482 af in 1977 to a high of 708,293 af in 1996. The Lower Tule River Ir- rigation District and the Delano-Earlimart Irrigation District together account for 59% of the total applied surface water while occupying approximately 40% of the incorporated area in the study area. Over the 30-year base period, an estimated total of 3.5 million af of seepage conveyance loss occurred in all sur- face water channels. Seepage in the Tule River, Deer Creek, and White River accounted for 85% of the total seepage. Total annual seepage varied from a low of 8,128 af in 1977 to 467,084 af in 1983. The UZWB model then calculates the monthly water storage changes in the soil root zone and deep vadose zone of each land unit, where the land unit is the UZWB model scale of resolution. It also models the intra-district surface water distribution system by estimating the monthly allocation of surface water to individual land units within each district. The main model outputs were the recharge to the unconfined aquifer from surface applied water and precipitation, and the groundwater pumping demand from the unconfined and confined aquifers. The recharge and groundwater pumping rates became input for the groundwater flow model. The total annual agricultural and urban consumptive use ranged from 865,800 af in 1970 to 1,246,700 af in 1999. The estimated total pumping ranged from 148,100 af in 1978 to 570,000 af in 1990. As expected, pumping was heaviest during the droughts of 1975-77 and 1987-92, and lightest during the wet years of 1973, 1978, 1982-83, 1995, and 1998. Precipitation totals varied from 177,800 af in 1990 to 974,400 af in 1998. Diffuse recharge from surface applied water ranged from 64,800 af in 1992 to 350,100 af in 1983. The net aquifer recharge for the entire study area was computed by ag- gregating the aquifer recharge and groundwater pumping of each land unit to this scale and adding the contribution to aquifer recharge from channel seepage. The monthly net recharge was then summed to produce a cumulative annual net recharge from 1970 to each fiscal water year from 1971-99. The water balance computed for the entire study area neglects horizontal groundwater inflows and outflows through its vertical boundaries. Groundwater fluxes undoubtedly exist along these boundaries. However, net fluxes are likely small in comparison to the total changes in storage due to vertical stresses applied to the entire study area (e.g. groundwater pumping, evapotranspiration, applied surface water, channel seepage). Horizontal groundwater flow on the inter-land unit and inter-district scales is expected to be more significant. For computing a total water balance, however, we made the simplifying assumption that the study area behaves as a relatively closed system where the net horizontal groundwater inflows through its vertical boundaries are small. Invoking this assumption, we then use the cumulative net recharge as an estimate of the cumulative groundwater storage change in the aquifer system. Ideally, verification of these estimates is performed by comparing them with an objective measure of the study area aquifer storage changes. However, changes in groundwater storage are not directly observable and must always be estimated using non-direct measures. As such, an objec- tive measure for verification does not exist. As an alternative, we compare the water balance model results with those produced by the water-table fluctuation (WTF) method. The trends in cumulative annual groundwater storage changes computed from the water balance and the WTF method from 1970-99 were quite similar. The minimum and maximum differences between them were 2,450 af (1980) and 752,387 af (1991), respectively. From 1970, the maximum amount of ground- water accumulation occurred in the spring of 1987 with the WTF method and the water balance estimating positive storage changes of 1,146,286 and 898,128 af, respectively. The maximum groundwater overdraft occurred in 1993 with the WTF method and the water balance estimating negative storage changes of 1,610,210 and 1,218,566 af, respectively. The 1987 and 1993 fiscal water years marked the beginning and ending of a major 6-year drought in California, respectively. Finally, the groundwater flow model calculates the changes in water levels in the aquifer system subject to transient groundwater recharge and pump- ing stresses. A post-processing routine calculates the cumulative groundwater storage changes over each district and the entire study area for each stress pe- riod. An automated calibration of the groundwater flow model was performed to refine the conceptual model of the hydrogeology and to estimate the spa- tial distributions of the aquifer system horizontal hydraulic conductivity. The calibration period of the groundwater flow model is 1970-85 and the validation period is 1986-99. Three different conceptual models of the aquifer system horizontal hydraulic conductivity, Kh, structure were evaluated in the calibration process: 1) Khas an exponential function of the specific yield, Sy, distribution, 2) Kh as a linear function of the saturated hydraulic conductivity of the soil survey map- ping units, and 3) division of the model domain into square zones of uniform size. The models were calibrated against both spatially distributed hydraulic head targets and cumulative groundwater storage change targets for seven of the largest districts. The discretization of the model domain into uniform square zones provided the most robust Kh structure and produced the most reason- able estimates of hydraulic head and district groundwater storage changes from the three conceptual models over the 1971-85 calibration period. The calibrated model was then used to compute the annual net inter-district groundwater fluxes between adjacent districts. In general, groundwater flux directions were con- sistent with the large-scale hydraulic gradients. Annual inter-district net fluxes between adjacent districts ranged from negligibly small ( < 100 af) to as much as 80,000 af (e.g. net flux from Lower Tule River ID to Pixley ID). Net inter-district fluxes were generally a function of the local transmissivity, the length of the shared border between adjacent districts, and the differences in their surface water supplies.

A database of groundwater wells in the United States

Nature Portfolio (Springer Nature) | April 4th, 2024

Groundwater wells are critical infrastructure that enable the monitoring, extraction, and use of groundwater, which has important implications for the environment, water

Groundwater wells are critical infrastructure that enable the monitoring, extraction, and use of groundwater, which has important implications for the environment, water security, and economic development. Despite the importance of wells, a unified database collecting and standardizing information on the characteristics and locations of these wells across the United States has been lacking. To bridge this gap, we have created a comprehensive database of groundwater well records collected from state and federal agencies, which we call the United States Groundwater Well Database (USGWD). Presented in both tabular form and as vector points, USGWD comprises over 14.2 million well records with attributes, such as well purpose, location, depth, and capacity, for wells constructed as far back as 1763 to 2023. Rigorous cross-verification steps have been applied to ensure the accuracy of the data. The USGWD stands as a valuable tool for improving our understanding of how groundwater is accessed and managed across various regions and sectors within the United States.

A geohydrologic reconnaissance of the Saratoga Spring area, Death Valley National Monument, California

U.S. Geological Survey (USGS) | March 1st, 1966

A global dataset of surface water and groundwater salinity measurements from 1980–2019

Nature Portfolio (Springer Nature) | July 13th, 2020

Salinization of freshwater resources is a growing water quality challenge, which may negatively impact both sectoral water-use and food security, as well as bio

Salinization of freshwater resources is a growing water quality challenge, which may negatively impact both sectoral water-use and food security, as well as biodiversity and ecosystem services. Although monitoring of salinity is relatively common compared to many other water quality parameters, no compilation and harmonisation of available datasets for both surface and groundwater components have been made yet at the global scale. Here, we present a new global salinity database, compiled from electrical conductivity (EC) monitoring data of both surface water (rivers, lakes/reservoirs) and groundwater locations over the period 1980–2019. The data were assembled from a range of sources, including local to global salinity databases, governmental organizations, river basin management commissions and water development boards. Our resulting database comprises more than 16.3 million measurements from 45,103 surface water locations and 208,550 groundwater locations around the world. This database could provide new opportunities for meta-analyses of salinity levels of water resources, as well as for addressing data and model-driven questions related to historic and future salinization patterns and impacts.

A Guide for Using the Transient Ground-Water Flow Model of the Death Valley Regional Ground-Water Flow System, Nevada and California

U.S. Geological Survey (USGS) | November 4th, 2006

This report is a guide for executing numerical simulations with the transient ground-water flow model of the Death Valley regional ground-water flow system, Nevada and Ca

This report is a guide for executing numerical simulations with the transient ground-water flow model of the Death Valley regional ground-water flow system, Nevada and California using the U.S. Geological Survey modular finite-difference ground-water flow model, MODFLOW-2000. Model inputs, including observations of hydraulic head, discharge, and boundary flows, are summarized. Modification of the DVRFS transient ground-water model is discussed for two common uses of the Death Valley regional ground-water flow system model: predictive pumping scenarios that extend beyond the end of the model simulation period (1998), and model simulations with only steady-state conditions.

A Guide to Ordinary High Water Mark (OHWM) Delineation for Non-Perennial Streams in the Western Mountains, Valleys, and Coast Region of the United States

U.S. Army Corps of Engineers (USACE) | August 14th, 2014

Federal regulations define the ordinary high water mark (OHWM) as “that line on the shore established by the fluctuations of water and indicated by physical cha

Federal regulations define the ordinary high water mark (OHWM) as “that line on the shore established by the fluctuations of water and indicated by physical characteristics such as a clear, natural line impressed on the bank, shelving, changes in the character of soil, destruction of terrestrial vegetation, the presence of litter and debris, or other appropriate means that consider the characteristics of the surrounding areas” (U.S. Congress 1986). Under Section 404 of the Clean Water Act (CWA), the OHWM defines the lateral extent of federal jurisdiction in non-tidal waters of the United States (WoUS) in the absence of adjacent wetlands (U.S. Congress 1977). Thus, accurate and consistent OHWM delineation practices are essential for proper implementation of the CWA. The dynamic nature of stream systems and fluvial processes presents challenges for OHWM delineation. Natural sources of variability in river and stream systems (e.g., climate, sediment supply, landscape position, etc.) are compounded by direct and indirect anthropogenic sources of variability (e.g., watershed alteration, dam emplacement and removal, climate change, etc.). Thus, it is challenging to impose a consistent measure of “ordinary” high flow conditions across systems in which the hydrology and geomorphology can vary greatly in both space and time. OHWM delineation in non-perennial (i.e., intermittent and ephemeral) streams can be especially challenging. The U.S. Army Corps of Engineers (USACE) defines intermittent streams as having “flowing water during certain times of the year, when groundwater provides water for stream flow. During dry periods, intermittent streams may not have flowing water. Runoff from rainfall is a supplemental source of water for stream flow” (USACE 2012). Ephemeral streams have “flowing water only during, and for a short duration after, precipitation events in a typical year. Ephemeral stream beds are located above the water table year-round. Groundwater is not a source of water for the stream,” and “[r]unoff from rainfall is the primary source of water for stream flow” (USACE 2012). In contrast to both intermittent and ephemeral streams, perennial streams have “flowing water year-round during a typical year. The water table is located above the stream bed for most of the year,” and “[g]roundwater is the primary source of water for stream flow” (USACE 2012). Given the less persistent streamflow regimes characteristic of non- perennial streams, particularly ephemeral systems, the characterization of ordinary high water flows is perhaps more challenging than in perennially flowing systems. Moreover, depending on climate, vegetation, and other related factors, the appearance of some OHWM indicators may vary greatly between wet and dry seasons or between relatively infrequent flow events, more so than in many perennial streams. Mountainous terrain can present additional challenges to OHWM delineation. For instance, the relatively steep and confined valleys in which mountain streams commonly flow can restrict the development of some alluvial features (e.g., flood-plains, bankfull benches, etc.) that are typical of low-gradient systems and that may help to identify the OHWM. Thus, in non-perennial mountain streams, it is often difficult to determine what constitutes ordinary high water and to interpret the physical and biological indicators established and maintained by ordinary high water flows. Challenges and inconsistencies pertaining to OHWM delineation practices are becoming increasingly relevant in mountainous parts of the western U.S. in light of expanding development. This increased pressure on fluvial systems highlights the need for accurate, consistent, and repeatable OHWM delineation practices in this region. These factors, combined with the particular challenges of OHWM delineation in non-perennial mountain streams, provided the impetus for developing this delineation guide. This guide presents the concepts, field indicators, and methods for assessing, delineating, and documenting the OHWM in non-perennial streams in the Western Mountains, Valleys, and Coast (WMVC) Region of the United States (Figure 1). The information presented here is based on the findings of Mersel et al. (2014) (discussed in Section 1.5) and on years of field observations and data gathering in the WMVC Region and in other regions of the U.S. by the authors and other contributing experts. The remainder of Section 1 provides background information regarding the concept of the OHWM and pertaining to stream hydrology and geomorphology in general. Section 2 discusses and provides examples of the specific field indicators used to identify the OHWM in non-perennial streams in the WMVC Region. Section 3 discusses field methods for delineating the OHWM and addresses additional techniques and lines of evidence that may help in problematic delineation scenarios. The information presented here is technical guidance and does not define, amend, or replace any existing regulations, laws, or legal guidance related to the OHWM or to the regulation of WoUS. Furthermore, determining whether any stream is a jurisdictional WoUS is beyond the scope of this document and involves further assessment in accordance with regulations, case law, and clarifying guidance. This guide pertains to non-perennial streams in the WMVC Region of the U.S., and while the information presented here may have a wider applicability to other regions or to perennial rivers within the WMVC Region, this has not been tested or validated. This manual serves as a companion to A Field Guide to the Identification of the Ordinary High Water Mark (OHWM) in the Arid West Region of the Western United States (Lichvar and McColley 2008) as these two regions—the WMVC and the Arid West—are interspersed with one another. Best professional judgment is required to determine which manual is most appropriate for any given location within these two regions. The technical guidance presented here aims to provide an informed and consistent approach to OHWM delineation within the WMVC Region; however, OHWM delineation is not a precise practice. The OHWM can take on a variety of appearances and characteristics and may change over time due to natural or anthropogenic causes. Best professional judgment and consideration of the unique characteristics of each project site are always required.

A Guide to Water Quality Requirements under the Sustainable Groundwater Management Act

Stanford University | April 17th, 2019

California’s Sustainable Groundwater Management Act (SGMA) requires Groundwater Sustainability Agencies (GSAs) develop Groundwater Sustainability Plans (GSPs) that ach

California’s Sustainable Groundwater Management Act (SGMA) requires Groundwater Sustainability Agencies (GSAs) develop Groundwater Sustainability Plans (GSPs) that achieve sustainability within twenty years of Plan adoption. Meeting this goal requires GSAs to avoid causing any of the six undesirable results that are central to SGMA, including undesirable result number 4 (UR No. 4) – “significant and unreasonable degraded water quality” throughout the basin. Unlike most other undesirable results, UR No. 4 is the subject of robust federal, state and local regulatory regimes carried out by a number of different entities. While the State and Regional Water Resources Control Boards have the broadest responsibilities to protect groundwater quality, many other agencies also have relevant responsibilities. The other distinctive feature of UR No. 4 is that groundwater management that prevents chronic lowering of groundwater levels may be insufficient to prevent significant and unreasonable degradation of groundwater quality. For example, significant changes in groundwater pumping can potentially mobilize naturally occurring constituents or cause migration or expansion of contaminant plumes even when the extracted amount is within the basin’s sustainable yield. This Guide is intended to assist GSAs along with other agencies and stakeholders in understanding the meaning of UR No. 4, and to provide insights into how GSAs can successfully fulfill their legal responsibilities regarding groundwater quality. To avoid UR No. 4 under SGMA, this Guide recommends that GSAs do the following: 1. Understand the existing regulatory regime for groundwater quality and, as necessary, confer with entities that have regulatory authority over groundwater quality in their basins. (See Section 3 of this Guide). 2. Consider existing federal, state and local groundwater quality standards when developing groundwater quality minimum thresholds in their basins. Federal, state or local regulatory requirements were established after careful scientific study, legal review and procedural steps mandated by law. They are designed to protect public health and welfare, and they provide the clearest indication of the point beyond which there is a real risk of an undesirable result. (See Section 3.1). 3. Work closely with their Regional Water Quality Control Boards to address all groundwater quality issues. GSAs working in the Central Valley are especially encouraged to work with the Central Valley Regional Board to address salt and nutrient groundwater contamination. Programs in this region are continuing to evolve and will require that GSAs remain aware of changing regulatory requirements. Additionally, there are many groundwater quality monitoring efforts in this region that are generating a lot of groundwater quality data. These efforts should be aligned with any groundwater monitoring efforts being undertaken by GSAs. (See Sections 3.4.2 and 4.3.2). 4. Use existing groundwater quality data and information to (1) assess and prioritize groundwater quality issues in their basins, (2) meet requirements that GSPs include a description of current and historical groundwater conditions within the basin and (3) serve as either the basis for or a complement to local groundwater quality monitoring programs developed by GSAs. 5. Consider the following approach (discussed in Section 3.2) if they identify groundwater quality problems that either arose or were exacerbated after January 1, 2015: • Where there is a significant groundwater quality problem that is clearly under the purview of another agency, confer with that agency and seek to confirm a reasonable plan to address the groundwater quality problem. If such a plan exists, the water quality problem and the plan should be referenced in the GSP. Section 3 and Table 1 in the Guide provide information on existing regulatory programs and the agencies responsible for their enforcement. • Where a significant groundwater quality problem is not clearly under the purview of another agency, or the responsible agency is unable to confirm a reasonable plan to address the problem, confer with Regional or State Board staff, and perhaps affected parties, to identify a reasonable plan to address the problem. If no reasonable plan is identified, and remediating the problem is impractical, excessively wasteful of resources or otherwise infeasible, the GSA should include in the Plan an explanation of the problem and the reasons why remediation is impractical or infeasible. 6. Consider the following approach (discussed in Section 5) if they are planning projects or actions that may affect groundwater quality: • Assess the potential impacts that the projects or actions are likely to have on groundwater quality over time. If the GSA concludes that the proposed project or action could have unacceptable adverse impacts on water quality, GSAs should consider options such as revising the project to avoid or mitigate the project’s impacts or developing alternative projects that avoid water quality problems. • Ensure regulatory compliance for any GSP management actions (e.g., recharge projects, water banking) that have the potential to negatively impact water quality in their basins or adjacent basins. (Sections 3 and 5 provide information on regulatory requirements and considerations associated with groundwater recharge and active basin management, respectively.) 7. Develop supplemental groundwater monitoring networks where necessary. Many basins being managed under SGMA have significant water quality data gaps. Thus, GSAs in many basins will likely need to develop or expand local groundwater quality monitoring networks. Decisions to do so should be informed by (1) the extent and quality of existing groundwater quality in the region, (2) the level of groundwater development, (3) the severity of existing groundwater quality issues and their proximity to domestic or irrigation wells, public water supply sources or other vulnerable users, (4) current or proposed groundwater management actions or projects, (5) local resources, and (6) the hydrogeologic complexity of the basin. GSAs are not responsible for enforcing existing water quality standards or collecting data to support existing water quality programs. However, they are responsible for avoiding “significant and unreasonable” degradation of water quality in their basins. Working with federal, state and local water quality monitoring and enforcement entities will help GSAs understand groundwater quality issues in their basins, enabling them to develop GSPs that avoid UR No. 4.

A hybrid machine learning model to predict and visualize nitrate concentration throughout the Central Valley aquifer, California, USA

Science of the Total Environment (Elsevier) | June 9th, 2017

Intense demand for water in the Central Valley of California and related increases in groundwater nitrate concentration threaten the sustainability of the groundwater res

Intense demand for water in the Central Valley of California and related increases in groundwater nitrate concentration threaten the sustainability of the groundwater resource. To assess contamination risk in the region, we developed a hybrid, non-linear, machine learning model within a statistical learning framework to predict nitrate contamination of groundwater to depths of approximately 500m below ground surface. A database of 145 predictor variables representing well characteristics, historical and current field and landscape-scale nitrogen mass balances, historical and current land use, oxidation/reduction conditions, groundwater flow, climate, soil characteristics, depth to groundwater, and groundwater age were assigned to over 6000 private supply and public supply wells measured previously for nitrate and located throughout the study area. The boosted regression tree (BRT) method was used to screen and rank variables to predict nitrate concentration at the depths of domestic and public well supplies. The novel approach included as predictor variables outputs from existing physically based models of the Central Valley. The top five most important predictor variables included two oxidation/reduction variables (probability of manganese concentration to exceed 50 ppb and probability of dissolved oxygen concentration to be below 0.5 ppm), field-scale adjusted unsaturated zone nitrogen input for the 1975 time period, average difference between precipitation and evapotranspiration during the years 1971–2000, and 1992 total landscape nitrogen input. Twenty-five variables were selected for the final model for log-transformed nitrate. In general, increasing probability of anoxic conditions and increasing precipitation relative to potential evapotranspiration had a corresponding decrease in nitrate concentration predictions. Conversely, increasing 1975 unsaturated zone nitrogen leaching flux and 1992 total landscape nitrogen input had an increasing relative impact on nitrate predictions. Three-dimensional visualization indicates that nitrate predictions depend on the probability of anoxic conditions and other factors, and that nitrate predictions generally decreased with increasing groundwater age.

A Machine Learning Approach to Predict Groundwater Levels in California Reveals Ecosystems at Risk

Frontiers in Earth Science (Frontiers) | December 17th, 2021

Groundwater dependent ecosystems (GDEs) are increasingly threatened worldwide, but the shallow groundwater resources that they are reliant upon are seldom monitored. In t

Groundwater dependent ecosystems (GDEs) are increasingly threatened worldwide, but the shallow groundwater resources that they are reliant upon are seldom monitored. In this study, we used satellite-based remote sensing to predict groundwater levels under groundwater dependent ecosystems across California, USA. Depth to groundwater was modelled for a 35-years period (1985–2019) within all groundwater dependent ecosystems across the state (n = 95,135). Our model was developed within Google Earth Engine using Landsat satellite imagery, climate data, and field-based groundwater data [n = 627 shallow (< 30 m) monitoring wells] as predictors in a Random Forest model. Our findings show that 1) 44% of groundwater dependent ecosystems have experienced a significant long-term (1985–2019) decline in groundwater levels compared to 28% with a significant increase; 2) groundwater level declines have intensified during the most recent two decades, with 39% of groundwater dependent ecosystems experiencing declines in the 2003–2019 period compared to 27% in the 1985–2002 period; and 3) groundwater declines are most prevalent within GDEs existing in areas of the state where sustainable groundwater management is absent. Our results indicate that declining shallow groundwater levels may be adversely impacting California’s groundwater dependent ecosystems. Particularly where groundwater levels have fallen beneath plant roots or streams thereby affecting key life processes, such as forest recruitment/succession, or hydrological processes, such as streamflow that affects aquatic habitat. In the absence of groundwater monitoring well data, our model and findings can be used to help state and local water agencies fill in data gaps of shallow groundwater conditions, evaluate potential effects on GDEs, and improve sustainable groundwater management policy in California.

A multi-isotope (B, Sr, O, H, and C) and age dating (3H–3He and14C) study of groundwater from Salinas Valley, California: Hydrochemistry, dynamics, and contamination processes

American Geophysical Union (AGU) | January 31st, 2002

The chemical and isotope (11B/10B, 87Sr/86Sr, 18O/16O, 2H/H, 13C/12C, 14C, and 3He/3H) compositions of groundwater from the upper aquifer system of the Salinas Valley in

The chemical and isotope (11B/10B, 87Sr/86Sr, 18O/16O, 2H/H, 13C/12C, 14C, and 3He/3H) compositions of groundwater from the upper aquifer system of the Salinas Valley in coastal central California were investigated in order to delineate the origin and processes of groundwater contamination in this complex system. The Salinas Valley has a relatively deep, confined ‘‘400- foot’’ aquifer, overlain by a ‘‘180-foot’’ aquifer and a shallower perched aquifer, all made up of alluvial sand, gravel and clay deposits. Groundwater from the aquifers have different 14C ages: fossil (14C = 21.3 percent modern carbon (pmc) for the 400-foot aquifer and modern (14C = 72.2–98.2 pmc) for the 180-foot aquifer. Fresh groundwater in all aquifers is recharged naturally and artificially through the Salinas River. The two modes of recharge can be distinguished chemically.We identified several different saline components with distinguishable chemical and isotopic fingerprints. Saltwater intrusion in the northern basin has C1 concentrations up to 1700 mg/L, a Na/Cl ratio less than seawater, a marine Br/Cl ratio, a Ca/Cl ratio greater than seawater, d11B between +17 and +38% and 87Sr/86Sr between 0.7088 and 0.7096. Excess dissolved Ca, relative to the expected concentration for simple dilution of seawater, correlates with 87Sr/86Sr ratios, suggesting base exchange reaction with clay materials. (2) Agriculture return flow is high inNO3 and SO4, with a 87Sr/86Sr = 0.7082,d11B = 19% and d13C between 23 and 17%. The 3H–3He ages (5–17 years) and 14C data suggest vertical infiltration rates of irrigation water of 3–10 m/yr. (3) Nonmarine saline water in the southern part of the valley has high total dissolved solids up to 3800 mg/L, high SO4, Na/Cl ratio >1, d11B between +24 and +30%, and 87Sr/86Sr = 0.70852. This groundwater may have acquired its geochemical signature from leaching of sedimentary rocks associated with the Coast Range marine deposits of Mesozoic to early Cenozoic age. The combination of different geochemical and isotopic fingerprints enables us to delineate the impact of salt sources in different areas of the valley and to reconstruct the origin of the SO4-enriched NO3-depleted saline plume that is located west of the city of Salinas. We suggest that the latter is derived from a mixture of different natural saline waters rather than from anthropogenic contamination.