Decadal-scale water-quality trends in California's public-supply aquifers: Pilot study in two priority basins

Science Center Objects

This project will contribute to the USGS Strategic Science Plan objective of developing a Water Census to inform decision-makers and the public about the status of groundwater resources and how they are changing, and forecasts of future changes in groundwater quality, and supports the Cooperative Water Program priority of assessing the quality of drinking water supplies for the protection of human health. The SWRCB has explicitly identified statewide monitoring of trends in groundwater quality in aquifers used for drinking water supply by the GAMA Priority Basin Project as essential to the State's groundwater quality management programs.

map of California showing the GAMA province boundaries

Map showing areas of California where the Groundwater Ambient Monitoring and Assessment Program (GAMA) has done assessments.

(Public domain.)

Assessing groundwater quality in aquifers used for drinking-water supplies across California is a top priority of the California State Water Resources Control Board (SWRCB). The USGS completed a statewide comprehensive assessment of aquifers used for public supply for the SWRCB's Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project (PBP), sampling 2,400 wells in 2004-2012. The original design of the GAMA PBP specified that trends monitoring be accomplished by resampling approximately 10% of the baseline network every 3 years and resampling the entire baseline network every 10 years. However, this original design is costly. This proposal is a pilot of an alternative plan for statistically reliable monitoring of trends in California's public-supply aquifers at a reduced cost compared to the original design. Two areas were selected for this pilot study: the Salinas-Monterey valleys and the Kings and Madera-Chowchilla subbasins of the San Joaquin Valley. These areas were selected because: (1) they have heavy use of groundwater for both public and domestic supplies, (2) GAMA PBP assessments in 2005 found constituents of concern (including nitrate, uranium, arsenic, and DBCP) at concentrations greater than maximum contaminant levels for drinking water in up to 20% of the public-supply aquifer system, and (3) previous work suggests decadal-scale trends will be discernible.

Objective: (1) Develop methodology for monitoring and statistical evaluation of water-quality trends in aquifers used for public supply that can be used for a future statewide trends monitoring program. (2) Monitor and evaluate decadal-scale water-quality trends in public-supply aquifers in two priority areas. (3) Forecast future decadal-scale trends in the public-supply aquifers. (4) Communicate results in formats that contribute the GAMA Program objectives of monitoring statewide groundwater quality and increasing the availability of information about groundwater quality to the public.

A total of 50 public-supply wells will be sampled in the Salinas-Monterey and the Kings/Madera-Chowchilla areas, corresponding to approximately 25% of the wells sampled in 2005. Water-quality data from the California Department of Public Health (CDPH) database for public-supply wells will be compiled. Decadal-scale trends will be evaluated statistically in both datasets. Future decadal-scale trends will be forecast using data about water-quality conditions in the shallow aquifer and groundwater age distributions.

This project will contribute to the USGS Strategic Science Plan objective of developing a Water Census to inform decision-makers and the public about the status of groundwater resources and how they are changing, and forecasts of future changes in groundwater quality, and supports the Cooperative Water Program priority of assessing the quality of drinking water supplies for the protection of human health. The SWRCB has explicitly identified statewide monitoring of trends in groundwater quality in aquifers used for drinking water supply by the GAMA Priority Basin Project as essential to the State's groundwater quality management programs.