In 2018 to 2022, domestic and municipal Delta drinking water wells continue to have elevated nitrate and arsenic levels, which can impact human health.

The Measure

Groundwater meets drinking water quality standards in the Delta for nitrate and arsenic. 

  • Expectations

    Drinking water quality for residents in the Delta is improved to meet standards for nitrate and arsenic.

  • Performance Metrics

    • Number of groundwater wells used for drinking water supply that exceed arsenic and/or nitrate drinking water limits, evaluated every five years.

Domestic and Municipal Groundwater Wells Sampled Within the Delta

The charts below show groundwater wells sampled within the Delta for nitrate (left chart) and arsenic (right chart). The number of sampled public groundwater* wells in the Delta exceeding or that have exceeded nitrate and arsenic standards (MCL) should be less than 50 percent of the 2001—2013 baseline values** by 2034. 

Next Data Update: These graphs will be updated with new data in 2028.

*Total number of wells sampled differ from year to year due to a variety of factors. Raw data and information can be retrieved via the Groundwater Ambient Monitoring and Assessment Program website

**The performance measure baseline usually uses the 2001–2013 timeframe, but the current analysis used for the graphs above uses 2001-2012. 2001-2012 was used because it aligns with the five-year increments that Delta Plan reviews are conducted. This allows the well data to be compiled and analyzed in five-year increments (2013-2017 and 2018-2022).

Groundwater wells used for domestic and municipal water supply that exceed arsenic and/or nitrate drinking water limits in the Delta are water quality issues and are possible indicators of other more serious contaminants. This is especially true for small water systems and disadvantaged communities that are highly dependent on groundwater sources that may not meet certain drinking water quality standards. Sources of nitrate and arsenic contamination can come from both natural and man-made sources. Consumption of high levels of nitrate and arsenic have various long and short term health effects.

Each chapter of the Delta plan includes strategies to achieve the goals of the plan. These strategies are general guidance on achieving the objective laid out in the plan and in the Delta Reform Act of 2009. Associated with these strategies are recommendations. The recommendations describe more specific and implementable actions to support the achievement of Delta Plan strategies. Strategies and recommendations may also have associated performance measures. Delta Plan performance measures track progress in achieving desired outcomes for the Delta Plan. Below are the strategies and recommendations associated with this performance measure.

Delta Plan Strategy

  • Improve drinking water quality

Delta Plan Recommendations

  • Complete central valley drinking water policy
  • Complete North Bay Aqueduct Alternative Intake project
  • Protect groundwater beneficial uses
  • Participation in CV-SALTS
  • Number of groundwater wells used for drinking water supply that exceed arsenic and/or nitrate drinking water limits, evaluated every five years.
  • Number of wells within the Delta which exceed 2008 California water quality standards for levels of nitrate not to exceed (10 ppm NO3-N) and arsenic not to exceed (10 ppb As) between the years of 2001 and 2013
  • A 50 percent reduction in number of wells exceeding nitrate and arsenic standards from baseline levels using historical data (2001-2013), by 2034.

Data is retrieved from the State Water Resources Control Board's Groundwater Ambient Monitoring and Assessment Program (GAMA). The tool can filter different types of contaminants and their concentrations, time of sampling, and locations. Data is then filtered by municipal and domestic categories and collecting the locations of wells within the Delta using geoprocessing software (ArcGIS).

Data Source: Groundwater Ambient Monitoring and Assessment Program (GAMA)

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