Millions of Californians access drinking water via domestic wells, which are vulnerable to drought and unsustainable groundwater management. Groundwater overdraft and the possibility of longer drought duration under climate change threatens domestic well reliability, yet we lack tools to assess the impact of such events. Here, we leverage 943 469 well completion reports and 20 years of groundwater elevation data to develop a spatially-explicit domestic well failure model covering California’s Central Valley. Our model successfully reproduces the spatial distribution of observed domestic well failures during the severe 2012–2016 drought (n = 2027). Next, the impact of longer drought duration (5–8 years) on domestic well failure is evaluated, indicating that if the 2012–2016 drought would have continued into a 6 to 8 year long drought, a total of 4037–5460 to 6538–8056 wells would fail. The same drought duration scenarios with an intervening wet winter in 2017 lead to an average of 498 and 738 fewer well failures. Additionally, we map vulnerable wells at high failure risk and find that they align with clusters of predicted well failures. Lastly, we evaluate how the timing and implementation of different projected groundwater management regimes impact groundwater levels and thus domestic well failure. When historic overdraft persists until 2040, domestic well failures range from 5966 to 10 466 (depending on the historic period considered). When sustainability is achieved progressively between 2020 and 2040, well failures range from 3677 to 6943, and from 1516 to 2513 when groundwater is not allowed to decline after 2020.

Population growth, climate uncertainties, and unsustainable groundwater pumping challenge aquifer sustainability worldwide. Efficient and data-driven groundwater supply management is a necessity to maintain essential water-dependent functions. Currently, managers lack the cost-effective, scalable, and reliable groundwater monitoring systems needed to collect vital groundwater data. Existing automated groundwater monitoring systems tend to be cost-prohibitive, and manual methods lack the spatial or temporal resolution to sufficiently meet critical water modeling, management, and policy objectives. In this study, we developed a fully automated, open source, low cost wireless sensor network (LCSN) for real-time groundwater data acquisition, processing, and visualization in the South American Subbasin Groundwater Observatory (GWO), located in California, USA. We demonstrate the steps taken to create the GWO, including field, hardware, software, and data pipeline components so that it may be easily reproduced in new areas. We find that the GWO is comparable in cost to manual measurements at a weekly measurement frequency, and costs between three and four times less than comparable commercially available telemetry and dashboard systems, largely due to the use of free open source software to acquire, clean, store, and visualize data. The open source-powered GWO thus lowers the financial and technical barrier of entry for real-time groundwater monitoring, creating the potential for more informed water management worldwide, particularly in regions whose managers are restricted by the high capital costs of commercial monitoring systems.

Growing demands on water supply worldwide have resulted in aquifer overdraft in many regions, especially in alluvial basins under intensive irrigation. This further leads to serious deterioration of groundwater quality. Managed aquifer recharge (MAR) has been shown to mitigate groundwater overdraft, but whether MAR can actually stabilize or reverse the ongoing declines in regional groundwater quality caused by non‐point sources has not been demonstrated. This study was intended to address the question by investigating impacts of different MAR strategies on regional groundwater quality. A geostatistical model was first used to characterize a heterogeneous alluvial aquifer system in a portion of the Tulare Lake Basin. Three‐dimensional numerical models were then employed to simulate groundwater flow and mass transport. Next, MAR strategies were applied in locations with different geological conditions or joint with different irrigation activities, and their performances were evaluated. Results demonstrate the potential of significant, long‐term benefits for regional groundwater quality by applying strategic, high‐intensity recharge operations on geologically favorable subregions. Siting MAR above the incised valley fill (IVF) deposit, a near‐surface paleochannel containing unusually coarse, high‐conductivity hydrofacies, leads to more extensive improvement in the groundwater quality in terms of salinity due to significant vertical flow and lateral outward flow from the IVF. Overall, decades would be required to alleviate groundwater quality concerns in the studied 189 km² region. The simulations indicate that the deep concentrations remain below the secondary maximum contaminant level as the solute mass migrates downward with the prominent contribution from the attenuation via dispersion and matrix diffusion.

Regional‐scale groundwater quality degradation from nonpoint source pollution threatens the long‐term sustainability of major alluvial aquifer‐aquitard systems worldwide. Models are needed to simulate regionally upscaled transport processes in heterogeneous geology that produces non‐Fickian (anomalous) transport. In this investigation, we show the impact of nonhorizontal and nonvertical mean flow direction on non‐Fickian transport within a 3D, kilometer‐scale, typical alluvial aquifer‐aquitard system. Notably, vertical anisotropy in K and seasonal pumping and recharge can fundamentally change hydraulic gradients and shift the mean flow direction between mostly horizontal and mostly vertical flow. Detailed 3D flow and transport simulations under varying mean flow directions indicate that alterations to hydraulic gradients which control the mean flow direction can lead to increasingly non‐Fickian transport. Under mostly horizontal flow, diffusion and slow advection dominant low‐K facies slow mass transfer rates from low‐K material, and preferential flow along connected high‐K networks causes increased spatial spreading along the mean flow direction. Conversely, predominantly vertical flow caused by spatially distributed pumping and recharge shifts mass transfer processes in low‐K material from diffusion and slow advection dominant to advection dominant, resulting in vertically oriented particle trajectories that compactly migrate through high‐K and low‐K facies alike, and increasingly Fickian transport. Thus, mean flow direction transience driven by vertical anisotropy in K and seasonal pumping and recharge can create oscillating transport patterns, that range from persistently non‐Fickian to more Fickian. Results illustrate the regional‐scale hydrogeologic factors that explain why upscaled transport models fail to capture non‐Fickian effects resulting from mean flow direction transience

Groundwater pumping from wells, together with water uses such as agricultural irrigation have been converting formerly open groundwater basins into closed systems that accumulate total dissolved solids (TDS). This process of anthropogenic basin closure and salinization (ABCSal) would appear to pose a threat to groundwater sustainability that is at least as formidable as groundwater overdraft and contamination from the surface, yet has been little explored. Models of groundwater flow and solute transport herein show that groundwater basin openness itself should be considered a primary determinant of sustainability. Results show that groundwater basin closure is a threshold condition that sets the aquifer system on a path of increasing salinity that can only be halted by opening the basin. Further, the magnitude of groundwater pumping and degree of basin closure significantly influence the spatial distribution of salinity. In open basins, salinity approaches dynamic equilibrium over long‐term conditions. Stratification of higher‐TDS groundwater overlying lower‐TDS groundwater occurs below farmlands whose irrigation‐supplying wells are impacted by irrigation return flow from upstream farmlands, and act to redistribute relatively saline groundwater to the land surface. More intensive pumping leads to groundwater basin closure and more vertically‐oriented groundwater flow toward pumping wells. TDS retainment in the basin and repeated well capture, re‐distribution as irrigation water, and evapoconcentration lead to progressive salinization. Regardless of basin closure status, fresh recharge protects nearby downstream portions of the basin from salinization, indicating that managing or limiting the spread of contaminated groundwater may be achieved via managed aquifer recharge of good quality water.