Metrics for Assessing the Quality of Groundwater Used for Public Supply, CA, USA: Equivalent-Population and Area

Abstract

Data from 11,000 public supply wells in 87 study areas were used to assess the quality of nearly all of the groundwater used for public supply in California. Two metrics were developed for quantifying groundwater quality: area with high concentrations (km(2) or proportion) and equivalent-population relying upon groundwater with high concentrations (number of people or proportion). Concentrations are considered high if they are above a human-health benchmark. When expressed as proportions, the metrics are area-weighted and population-weighted detection frequencies. On a statewide-scale, about 20% of the groundwater used for public supply has high concentrations for one or more constituents (23% by area and 18% by equivalent-population). On the basis of both area and equivalent-population, trace elements are more prevalent at high concentrations than either nitrate or organic compounds at the statewide-scale, in eight of nine hydrogeologic provinces, and in about three-quarters of the study areas. At a statewide-scale, nitrate is more prevalent than organic compounds based on area, but not on the basis of equivalent-population. The approach developed for this paper, unlike many studies, recognizes the importance of appropriately weighting information when changing scales, and is broadly applicable to other areas.

Identifying areas of degrading and improving groundwater-quality conditions in the State of California, USA, 1974–2014

“…Areas of degrading or improving groundwater quality conditions were assessed at different spatial scales within the State of California using a network of 2135 equalarea grid cells , covering an area of about 105,312 km 2 . The grid cell network is the same used by Belitz et al (2015) and is available digitally from Johnson et al (2018) (Fig. 1).…”

“…The California Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP) recently completed a statewide assessment of the status of water quality in groundwater resources used for public drinking water (Belitz et al 2015). The GAMA-PBP is part of the California State Water Resources Control Board (SWRCB) GAMA program (SWRCB 2018).…”

“…In this paper, the assessment of Belitz et al (2015) is extended to incorporate information about trends in groundwater quality in aquifers used for public drinking water supply. The purpose of this paper is to (1) provide a new method for scoring wells based on constituent concentrations and trend direction and magnitude and…”

Assessment of Groundwater Nitrate Pollution Potential in Central Valley Aquifer Using Geodetector-Based Frequency Ratio (GFR) and Optimized-DRASTIC Methods

“…Harter [4] estimated more than 80 pounds of nitrogen per acre per year leaching into the groundwater. The nitrate contamination level in some public, domestic, and monitoring wells of the CV have exceeded the EPA maximum contaminant level (MCL) of 10 mg/L [5][6][7]. Several studies have attributed the groundwater nitrate concentration level to agricultural activities of the region [7][8][9].…”

Nitrate vulnerability projections from Bayesian inference of multiple groundwater age tracers

“…In many key groundwater basins in California, a majority of deep supply wells have nitrate concentrations that are presently above natural, background levels but below the United States Environmental Protection Agency (USEPA) regulatory limit of 45 mg/l (as NO 3 À ) (Belitz et al, 2015). The likelihood that nitrate resides in the vadose zone and shallow aquifer zones, but will gradually be transported to deeper aquifers, makes prediction of future trends in nitrate concentration of paramount interest.…”