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California Groundwater Wells Receive Grades for Improvement and Degradation

In California, groundwater is a major source for drinking and other uses. Identifying where groundwater quality is getting better or worse is essential for managing groundwater resources.

A new study conducted by a team from the California Water Science Center, led by Research Hydrologist Bryant Jurgens, assessed areas of improving and degrading groundwater-quality by using a new metric for scoring. The scoring was based on how high chemical concentrations were and whether they were getting better or worse and how rapidly or slowly they were changing. This work was conducted in nine hydrogeologic provinces throughout California. The location of the provinces generally corresponded to groundwater basins identified by the California Department of Water Resources.

Each province was divided into a network of grid cells that cover 99% of the area of groundwater used for public supply in the state. The area covered by the grid cells contain more than 13,000 public-supply wells statewide. At each well, data for 38 inorganic chemical constituents were analyzed to determine time series trends in water quality. For each constituent at a well, the concentrations were scored against water-quality benchmarks (low, moderate, high) and the trends were scored based on the direction (increasing or decreasing) and magnitude of the rate of change. The individual scores at a well were aggregated to score cell areas and used to determine percentages of areas in hydrogeologic provinces and the state that were impaired and improving or were impaired and degrading.

Well Map
Map of California showing boundaries of cells (a) and wells (b) located within nine hydrogeologic provinces of the state assessed in this study.
Hydrologist Michael Land is shown sampling a public supply well in the upper Santa Ana watershed.
Hydrologist Michael Land is shown sampling a public supply well in the upper Santa Ana watershed.

Results from the research showed that concentrations of nitrate and total dissolved solids* were the most frequently detected constituents in areas used for California’s public water supply. For these constituents, provinces where agriculture is or was once predominant had the largest concentrations. The results also showed that, where agricultural land has been urbanized, a significant portion of those areas had decreasing concentrations.

Arsenic was the only constituent that showed decreasing trends statewide. This could potentially be because many wells are increasingly capturing groundwater that contains higher levels of nitrate and oxygen that promote arsenic immobilization.

The full results from this research, have recently been published in the journal Environmental Monitoring and Assessment. An interactive web map also has been created that allows users to view results for grid cell areas and wells across the state and obtain the datasets for each individual well.

* Total dissolved solids comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water.

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