Groundwater Development Leads to Decreasing Arsenic Concentrations in the San Joaquin Valley, California
Haugen, E.A., Jurgens, B.C., Arroyo-Lopez, J.A., and Bennett, G.L., 2021, Science of The Total Environment, Volume 771, 2021, 145223, ISSN 0048-9697
Related Study Unit(s): Central Eastside San Joaquin Basin Groundwater Resources Used for Public Supply, Eastern Sacramento Valley and Foothills Groundwater Resources Used for Domestic Supply, Kern County Sub-Basin Groundwater Resources Used for Public Supply, Madera-Chowchilla and Kings Sub-basins Groundwater Resources Used for Domestic Supply, Madera-Chowchilla Basin Groundwater Resources Used for Public Supply, Northern San Joaquin Valley Groundwater Resources Used for Public Supply, Southeast San Joaquin Valley Groundwater Resources Used for Public Supply, Tule-Tulare-Kaweah and Associated Highlands Groundwater Resources Used for Domestic Supply, Western San Joaquin Valley Groundwater Resources Used for Public Supply
In the San Joaquin Valley (SJV), California, about 10% of drinking water wells since 2010 had arsenic concentrations above the US maximum contaminant level of 10 μg/L. High concentrations of arsenic are often associated with high pH (greater than 7.8) or reduced geochemical conditions. Although most wells have low arsenic (<3 μg/L) and do not have changing arsenic concentrations, this study found that most wells with concentrations above 10 μg/L had arsenic trends. Overall, about 24% of wells had time-series trends since 2010 and 59% had paired-sample trends since 2000. Most wells had decreasing arsenic trends, even in wells with higher arsenic concentrations. These wells often had co-detections of increasing nitrate and sulfate trends that reflect oxic groundwater likely derived from agricultural recharge. Wells with increasing arsenic trends were deeper or located in the valley trough where aquifer materials are more fine-grained and where reducing conditions favor arsenic mobility. Wells with arsenic trends also tend to be clustered near areas of higher well density. Groundwater pumping in these areas has likely increased the contribution of younger, more oxic groundwater in wells with declining arsenic or, less frequently, increased the contribution of higher pH or reduced groundwater in wells with rising arsenic. Projections of arsenic trends indicate that 37 wells with high arsenic presently will be below 10 μg/L in ten years. Unfortunately, these improvements will be largely offset by 31 wells that are expected to increase above 10 μg/L in addition to expected rises in nitrate in wells where arsenic decreased. This study shows how human-altered flow systems can impact the natural geochemical character of water in both beneficial and deleterious ways.