PRONOUNCED MULTI-ANNUAL SHIFTS IN GROUNDWATER-RESOURCE QUALITY DURING DROUGHT AND RECOVERY PERIODS, SAN JOAQUIN VALLEY, CALIFORNIA

Groundwater-resource quality is commonly assumed to be less sensitive to drought than that of surface waters due to transit times that often exceed drought durations by orders of magnitude. Here, we show that pumping-induced perturbations in aquifer pressure dynamics during drought and subsequent recovery periods can cause dramatic shifts in groundwater-resource quality on a basin-scale. We implement a novel application of time-series clustering on annual nitrate anomalies at >400 public-supply wells (PSWs) across California’s San Joaquin Valley (SJV) from 2000-2022 to group sub-populations of wells with differential water-quality responses to hydrologic stress periods. Results of cluster analysis and step-comparisons of water-quality changes at >3,000 PSWs during an extreme drought (2012-2016) and subsequent recovery (2016-2019) indicate a dominant regional response to pumping stress characterized by increased concentrations of anthropogenic constituents (nitrate, total dissolved solids) and a decreased concentrations of geogenic constituents (arsenic, fluoride), which largely reversed during recovery. Hydrogeologic case studies at wells with long-term (>15 years) age-tracer data show that pumping stress during drought can result in groundwater-age transience and cause pronounced shifts in groundwater-quality on both seasonal and multi-annual timescales. Pumping-induced incursions of modern, oxic groundwater to depth during drought may affect geochemical conditions in deeper parts of regional aquifer systems that control the long-term mobility of geogenic contaminants.