UC Davis

2.5 billion people around the world rely on groundwater as their source of drinking water. These people reside primarily in rural areas, where extensive groundwater use by both domestic and agricultural users has caused 20 % of the worlds aquifers to be considered overdrafted. Similar trends can be seen in California’s Central Valley, where a majority of the United States fruits, nuts, and vegetables are produced. The Tulare Lake Basin, occupying the lower one-third of the Central Valley, is home to four of the top ten agricultural counties in the United States. Agricultural production in this region, enabled by a Mediterranean climate, has caused an average groundwater table decline of 0.11 m per year (2.3 km3 per year) over the last six decades. Groundwater table decline in the area furthermore causes issues with water quality as younger, potentially anthropogenic contaminant laden water, is drawn into previously un-contaminated wells. Due to having some of the highest poverty rates in the State of California, communities in the Tulare Lake Basin are often unable to deploy expensive methods, such as well water treatment plants or injection wells, of dealing with these water quality and quantity issues. Managed Aquifer Recharge (MAR) has been proposed as a cost-effective method to remedy both of these concerns by intentionally recharging an aquifer to raise water tables, dilute toxic substances or create other environmental benefit. However, the potential for MAR to mobilize contaminants as recharging water interacts with native aquifer conditions is poorly understood. To address this knowledge gap, we investigated how the source of groundwater recharge, geochemical factors, and geospatial variables (e.g. applied fertilizer amounts, annual precipitation totals etc.) are related to six prevalent or emerging contaminants (arsenic, nitrate, uranium, hexavalent chromium, fluoride, perchlorate) found in drinking water supply wells in the Tulare Lake Basin. Identifying the likely source of water in 786 drinking water supply wells involved using well construction data and concentrations of tritium, stable water isotopes, and dissolved noble gas parameters in groundwater, surface water and precipitation samples in the region. The contaminant distribution of each well water source was then compared by the Tukey-Kramer test, while geochemical and geospatial factors were correlated to contaminants by a matrix of Spearman’s rank correlation coefficient. Results indicate that recharge from un-evaporated surface waters (e.g. stream water) pose no excess risk to drinking water from groundwater and that toxic substances are primarily influenced by geochemical factors and nearby urban sites of chemical production. This suggests that the recharge of clean water through conveyance infrastructure or recharge basins can improve groundwater availability without sacrificing its drinkability, as long as both subsurface geochemistry is properly anticipated and urban manufacturing runoff is minimally present in the source area of a well or potential MAR site. Decision makers can use this information, along with existing demographic based recommendations, to more equitably secure water supplies through cost effective surface recharge technologies.