Identification of Bacteria in Groundwater Used for Domestic Supply in the Southeast San Joaquin Valley, California, 2014

U.S. Geological Survey (USGS) | June 22nd, 2021

Groundwater is an important source of drinking water in California. Water-borne diseases caused by microbial contamination are a growing concern. The MI test, a membrane

Groundwater is an important source of drinking water in California. Water-borne diseases caused by microbial contamination are a growing concern. The MI test, a membrane filtration method for the chromogenic / fluorogenic detection of total coliforms and Escherichia coli, was used for samples collected January to April 2014 from 42 domestic wells in the southeastern San Joaquin Valley. The wells were sampled as part of the Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP), a cooperative study between the U.S. Geological Survey and the California State Water Resources Control Board. Polymerase chain reaction analysis and sequencing of deoxyribonucleic acid (DNA) were used for 34 target and nontarget colonies that grew on the MI media from samples collected from 13 of the domestic wells to identify what genera of bacteria could exist in groundwater used by domestic wells. Gene sequences obtained using the Sanger method were entered into the basic local alignment search tool (BLAST) database, and 17 genera of bacteria were identified. Of these, 13 genera contain species that are human pathogens or opportunistic human pathogens. All the genera that include human pathogens are naturally present in soil, plants, or water; one of the pathogens also can be found in fecal matter. Six of the human pathogens were from non-target colony growth on the MI media. Target and non-target microbial growth on MI media are indicators of the possible presence of pathogenic bacteria even if the bacteria naturally are from soil rather than from a fecal source.

Identification of fresh submarine groundwater off the coast of San Diego, USA, using electromagnetic methods

Hydrogeology Journal (Springer) | March 1st, 2022

Climate change has a pronounced effect on water resources in many semiarid climates, causing populated areas such as San Diego County (USA), to become more vulnerable to

Climate change has a pronounced effect on water resources in many semiarid climates, causing populated areas such as San Diego County (USA), to become more vulnerable to water shortages in the coming decades. To prepare for decreased water supply, San Diego County is adopting policies to decrease water use and to develop additional local sources of water. One new local source of freshwater is produced by a desalination facility that purifies brackish groundwater from the coastal San Diego Formation. This formation has been studied extensively onshore, but little is known about the geology or groundwater quality offshore in the adjacent continental shelf. Because most groundwater systems are interconnected and complex, further analysis is needed to identify offshore geology, possible sequestration of freshwater in the shelf, and potential pathways for saltwater intrusion. This comprehensive understanding is important because seawater intrusion may limit use of the San Diego Formation and longevity of desalination facilities. Controlled-source electromagnetic methods are uniquely suited to detecting offshore groundwater as they are sensitive to changes in pore fluids such as the transition from fresh to brackish groundwater. This paper describes results from surface-towed electromagnetic surveys that mapped the pore-fluid salinity and possible fluid pathways in the continental shelf off the coast of San Diego. The results indicate a considerable volume of fresh-to-brackish groundwater sequestered in the shelf, both in continuous lenses and isolated pockets, that appear influenced by fault systems and shallow stratigraphy.

Identifying the location and population served by domestic wells in California

Journal of Hydrology: Regional Studies (Elsevier) | November 17th, 2014

Study region: California, USA. Study focus: Identification of groundwater use is an important stepin the regional-scale assessment of groundwater quality. In Califo

Study region: California, USA. Study focus: Identification of groundwater use is an important stepin the regional-scale assessment of groundwater quality. In California, 1990 US Census data indicate that domestic wells provide drinking-water to about 1.2 million people. However, the location of these domestic well users of groundwater is poorly identified because the census tracts can be quite large (up to 20,000 km2). The purposes of this paper are to present methods used for (1) estimating the location of domestic wells, (2) estimating the location of households using domes-tic well water; and (3) identifying where in California groundwater isan important source of domestic drinking supply.New hydrological insights for the region: Aggregating the results indicates that three hydrogeologic provinces contain nearly 80% of all domestic wells and also have the highest density of domestic well-users: Central Valley (31.6%), Sierra Nevada (31.5%), and NorthernCoast Ranges (16.6%). Results were also aggregated into groundwater basins and highland areas, collectively called Groundwater Units(GUs). Twenty-eight of the 938 GUs contain more than 50% of the total population served by domestic wells, 70 GUs contain more than 75%, and 150 GUs contain 90%. The 28 GUs are mostly located in the eastern and southern San Joaquin Valley (11), the Sacramento Valley (7), and the western foothills of the Sierra Nevada province (5). Using the information presented in this research along with other information about domestic-well use, the US Geological Survey has begun sampling high-use GUs for the Shallow Aquifer Assessment component of the Groundwater Ambient Assessment (GAMA) program.

Implementing California’s Sustainable Groundwater Management Act (SGMA): Implementing California’s Sustainable Groundwater Management Act (SGMA)

Ostrom Workshop Colloquium Presentation | February 29th, 2016

This research effort is guided by the following overarching research question: What can be learned from existing groundwater management arrangements in Californ

This research effort is guided by the following overarching research question: What can be learned from existing groundwater management arrangements in California — adjudications, special act districts, and select AB 3030 planning processes and county ordi nances — to inform the creation of durable, sustainable groundwater management plans under California’s new Sustainable Groundwater Management Act? The current and planned research described in this presentation is being conducted with a team of researche rs at Stanford University’s Woods Institute on the Environment, Water in the West Program, and Gould Center on Dispute Resolution. Preliminary work is being supported by a grant from Stanford’s Environmental Ventures Program (Janet Martinez, PI).

Implementing SGMA: Results from a Stakeholder Survey

University of California, Davis (UC Davis) | August 26th, 2019

The Sustainable Groundwater Management Act (SGMA) of 2014 represents a historic transition to collective groundwater resource management and has the potential to signif

The Sustainable Groundwater Management Act (SGMA) of 2014 represents a historic transition to collective groundwater resource management and has the potential to significantly reduce groundwater overdraft in California. A total of 260 groundwater sustainability agencies (GSAs) recently formed to collectively manage groundwater resources in the 127 high and medium priority groundwater basins of the state. The simultaneous formation of hundreds of new governing agencies is an unprecedented institutional effort with very few examples to learn from. As GSAs move towards the design and deliberation of their groundwater sustainability plans (GSPs), assessments on the process up until now can directly inform development processes that are still taking place. This report summarizes the results from a state-wide survey that targeted SGMA participants.

Implications of projected climate change for groundwater recharge in the western United States

Journal of Hydrology (Elsevier) | March 1st, 2016

Existing studies on the impacts of climate change on groundwater recharge are either global or basin/location-specific. The global studies lack the specificity to inform

Existing studies on the impacts of climate change on groundwater recharge are either global or basin/location-specific. The global studies lack the specificity to inform decision making, while the local studies do little to clarify potential changes over large regions (major river basins, states, or groups of states), a scale often important in the development of water policy. An analysis of the potential impact of climate change on groundwater recharge across the western United States (west of 100 longitude) is presented synthesizing existing studies and applying current knowledge of recharge processes and amounts. Eight representative aquifers located across the region were evaluated. For each aquifer published recharge budget components were converted into four standard recharge mechanisms: diffuse, focused, irrigation, and mountain-systems recharge. Future changes in individual recharge mechanisms and total recharge were then estimated for each aquifer. Model-based studies of projected climate-change effects on recharge were available and utilized for half of the aquifers. For the remainder, forecasted changes in temperature and precipitation were logically propagated through each recharge mechanism producing qualitative estimates of direction of changes in recharge only (not magnitude). Several key patterns emerge from the analysis. First, the available estimates indicate average declines of 10–20% in total recharge across the southern aquifers, but with a wide range of uncertainty that includes no change. Second, the northern set of aquifers will likely incur little change to slight increases in total recharge. Third, mountain system recharge is expected to decline across much of the region due to decreased snowpack, with that impact lessening with higher elevation and latitude. Factors contributing the greatest uncertainty in the estimates include: (1) limited studies quantitatively coupling climate projections to recharge estimation methods using detailed, process-based numerical models; (2) a generally poor understanding of hydrologic flowpaths and processes in mountain systems; (3) difficulty predicting the response of focused recharge to potential changes in the frequency and intensity of extreme precipitation events; and (4) unconstrained feedbacks between climate, irrigation practices, and recharge in highly developed aquifer systems.

Improved Imaging of the Large-Scale Structure of a Groundwater System with Airborne Electromagnetic Data

American Geophysical Union (AGU) | March 17th, 2022

A groundwater model is a representation of a groundwater system used to both understand the properties and processes of the subsurface, and to manage the groundwater reso

A groundwater model is a representation of a groundwater system used to both understand the properties and processes of the subsurface, and to manage the groundwater resource. An important step in developing a groundwater model is describing the large-scale structure, the "plumbing system" that controls the groundwater flow. Given the size of many groundwater models, obtaining an accurate description of the large-scale structure can be a challenging task. The traditional approach has involved the drilling of wells. While wells can provide accurate information at the location of the wells, the spatial sampling of the subsurface that can be accomplished through the drilling of wells is limited. An airborne electromagnetic (AEM) method provides an alternative by using a geophysical measurement to map out the electrical resistivity to depths of hundreds of meters. Working with AEM data acquired in the Kaweah Subbasin in the Central Valley of California, we were able to image the large-scale structure - the bedrock surface and a thick clay layer. The key improvement was made by incorporating prior knowledge into the inversion of the data. Once we had obtained the large-scale structure, we found that this could be used to improve the imaging of the smaller-scale structure.

Improving Groundwater Model Calibration with Repeat Microgravity Measurements

National Groundwater Association (NGWA) | December 23rd, 2021

Groundwater-flow models depend on hydraulic head and flux observations for evaluation and calibration. A different type of observation—change in storage measured using

Groundwater-flow models depend on hydraulic head and flux observations for evaluation and calibration. A different type of observation—change in storage measured using repeat microgravity—can also be used for parameter estimation by simulating the expected change in gravity from a groundwater model and including the observation misfit in the objective function. The method is demonstrated using new software linked to MODFLOW input and output files and field data from the vicinity of the All American Canal in southeast California, USA. Over a 10-year period following lining of the previously highly permeable canal with concrete, gravity decreased by over 100 μGal (equivalent to about 2.5 m of free-standing water) at some locations as seepage decreased and the remnant groundwater mound dissipated into the aquifer or was removed by groundwater pumping. Simulated gravity from a MODFLOW model closely matched observations, and repeat microgravity data proved useful for constraining both hydraulic conductivity and specific yield estimates. Specific yield estimated using the infinite-horizontal slab approximation agreed well with model-derived values, and the departure from the linear, flat-water-table approximation was small, less than 2%, despite relatively large and dynamic water-table slope. First-order second-moment parameter uncertainty analysis shows reduction in uncertainty for all hydraulic conductivity and specific yield parameter estimates with the addition of repeat microgravity data, as compared to drawdown data alone.

Improving Groundwater Security in the United States

The White House | December 14th, 2024

The U.S. is facing a serious and unprecedented groundwater challenge. In many aquifers, groundwater withdrawal has outpaced natural recharge, which is exacerbated by the

The U.S. is facing a serious and unprecedented groundwater challenge. In many aquifers, groundwater withdrawal has outpaced natural recharge, which is exacerbated by the changing climate and precipitation variability. Much of the water in the major aquifers in the U.S. is fossil water, recharged over 10,000 years ago, and will not be replaced naturally on human timescales. In the western U.S., groundwater resources are being depleted at alarming rates, mostly from agricultural withdrawal. The depletion has caused land subsidence and earth fissures as well as permanent reduction of storage capacity. The depletion has national and global consequences as non-renewable groundwater in the west is embedded in agricultural products transported to the rest of the U. S. and in agricultural commodities exported globally. Many federal agencies have programs to measure the quality and quantity of groundwater, as well as conservation programs for sustainable use of groundwater. PCAST applauds and endorses their efforts as well as those by state governments, non-governmental organizations, professional organizations, local communities, private citizens, and others, to manage their groundwater resources. To ensure the sustainability of this indispensable resource and mitigate risks from its depletion, it is imperative that we enhance our understanding and management of groundwater resources.

Incomplete Determination for Groundwater Sustainability Plan submitted for Owens Valley Subbasin (Basin No. 6-012.01)

California Department of Water Resources (DWR) | April 14th, 2025