Critical Aquifer Overdraft Accelerates Degradation of Groundwater Quality in California's Central Valley During Drought

Abstract

Groundwater provides approximately one third of fresh water used by humans on the planet, but can be vulnerable to depletion during drought-particularly in large, regional aquifers that support irrigated agriculture (Aeschbach-Hertig & Gleeson, 2012;Taylor et al., 2013). Aquifer overdraft occurs where net outflows due to pumping exceed inflows from precipitation, surface-water recharge, and lateral flow, causing declines in groundwater levels and, in some extreme cases, land subsidence (Scanlon et al., 2012;Whittemore et al., 2016). Although physical relationships between pumping and groundwater-level decline have been observed and modeled in a variety of hydrogeologic settings (Butler et al., 2016;Faunt, 2009), effects of aquifer exploitation on water quality are often highly localized, difficult to extrapolate on regional scales, and have only been addressed in a handful of studies (e.g., Blaszyk & Gorski, 1981;Lambrakis & Kallergis, 2001;Nunes et al., 2021). California's Central Valley is a large (51,800 km 2 ), agricultural region that maintains high primary productivity under a semi-arid climate by irrigation from extensive surface-water diversions and groundwater pumpage (Faunt, 2009). Long-term groundwater depletion rates on the order of 2 km 3 yr −1 over the latter part of the 20th century can double during drought as pumpage increases to meet demand shortfalls from diminished surface-water flows (Famiglietti et al., 2011;Faunt et al., 2016). Drought-induced pumpage has precipitated dramatic groundwater-level declines and land subsidence in critically overdrafted groundwater basins within the San Joaquin Valley (SJV) subregion over the past 30 yr (Figure 1a; DWR, 2016DWR, , 2020. Recent work in the Central Valley has focused on vulnerability of shallow, domestic wells to supply failure during drought (Jasechko & Perrone, 2020;Pauloo et al., 2020), but little has been done to connect

Modern groundwater reaches deeper depths in heavily pumped aquifer systems

“…Our pan-US statistical analyses are consistent with local-scale research in California’s Central Valley 26 , 27 , where groundwater pumping draws young and shallow groundwater deeper into the aquifer system; our results suggest that pumping-induced downwelling is not unique to California’s Central Valley and is likely occurring in other heavily pumped US aquifer systems. Though we find that modern groundwater tends to reach deeper depths in heavily pumped aquifer systems (Fig.…”

Drivers of Spatiotemporal Variability in Drinking Water Quality in the United States

“…For example, increasing ambient temperatures cause faster chlorine decay, subsequently requiring a higher chlorine dosage to achieve free chlorine residual targets and potentially increasing DBP formation . Similarly, prolonged droughts may accelerate degradation of groundwater quality by nitrate and increase arsenic and GWR violations (Figure ). Recent studies have highlighted the importance of other climate hazards that threaten water quality including post-wildfire increases in hexavalent chromium concentrations in soils, and volatile and semivolatile organic compounds (VOCs and SVOCs), and arsenic, nitrate, and DBPs in public drinking water systems …”

Widespread and increased drilling of wells into fossil aquifers in the USA

“…The processes that lead to mixtures of fossil groundwater and recent recharge remain poorly understood, but may include (i) mixing along converging flow paths (e.g., Arava Valley in Israel 25 ); (ii) cross-formational mixing (upconing and downwelling) induced as a result of borehole drilling and subsequent pumping (e.g., Diass Aquifer System in Senegal 26 and the North China Plain 27 ); (iii) fast downward flow of recent precipitation along defective well casings to deeper depths where the well is perforated (e.g., Bohemian Cretaceous Basin in the Czech Republic 28 and the Aleppo and Steppe Basins in Syria 29 ); (iv) pumping from wells with long perforated intervals that simultaneously draw groundwater from both shallow and deep depths (e.g., Malm Limestone Aquifer System in Poland 30 ); (v) leakage through gaps in impermeable layers (‘windows’ in aquitards) separating fossil and modern groundwater; or (vi) relatively rapid vertical groundwater flow and mixing along geologic faults that may serve as conduits that connect fossil and modern groundwater. Because shallow contaminated groundwater can be drawn downward by pumping from deeper wells 31 , developing data products that quantify not only total groundwater withdrawals in a region but also quantify the depths of the wells (and the depths of their screened intervals) from which groundwater is withdrawn will be key to understanding fossil aquifer contamination risk. Geogenic contamination can arise as groundwater interacts with the mineral skeleton of the geologic formations that it flows through and resides within.…”