Water is a critical resource, but ensuring it is available faces challenges from climate extremes and human intervention. In this Review, we evaluate the current and historical evolution of water resources, considering surface water and groundwater as a single, interconnected resource. Gravity Recovery And Climate Experiment (GRACE) satellite data show declining, stable, and rising trends in total water storage over the past two decades in various regions globally. Groundwater monitoring provide longer term context over the past century, showing rising water storage in Northwest India, Central Pakistan, and Northwest United States and declining water storage in the US High Plains and Central Valley. Climate variability causes some changes in water storage but human intervention, particularly irrigation, is a major driver. Waterresource resilience can be increased by diversifying management strategies. These approaches include green solutions, such as forest and wetland preservation, and gray solutions, including increasing supplies (desalination, wastewater reuse), enhancing storage in surface reservoirs and depleted aquifers, and transporting water. A diverse portfolio of these solutions, in tandem with managing groundwater and surface water as a single resource, can address human and ecosystem needs while building a resilient water system.Water is a critical resource, but ensuring it is available faces challenges from climate extremes and human intervention. In this Review, we evaluate the current and historical evolution of water resources, considering surface water and groundwater as a single, interconnected resource. Total water storage trends varied among regions over the past century. Some areas, including Northwest India, Central Pakistan, and Northwest United States, have seen rises in water storage over the past century. Others, including the US High Plains and Central Valley, have experienced net declines. Climate variability causes some changes in water storage but human intervention, particularly irrigation, is a major driver. Waterresource resilience can be increased by diversifying management strategies. These approaches include green solutions, such as forest and wetland preservation, and gray solutions, including increasing supplies (desalination, wastewater reuse), enhancing storage in surface reservoirs and depleted aquifers, and transporting water. A diverse portfolio of these solutions, in tandem with managing groundwater and surface water as a single resource, can address human and ecosystem needs while building a resilient water system.

Mobilization of arsenic and other trace metal contaminants during managed aquifer recharge (MAR) poses a challenge to maintaining local groundwater quality and to ensuring the viability of aquifer storage and recovery techniques. Arsenic release from sediments into solution has occurred during purified recycled water recharge of shallow aquifers within Orange County, CA. Accordingly, we examine the geochemical processes controlling As desorption and mobilization from shallow, aerated sediments underlying MAR infiltration basins. Further, we conducted a series of batch and column experiments to evaluate recharge water chemistries that minimize the propensity of As desorption from the aquifer sediments. Within the shallow Orange County Groundwater Basin sediments, the divalent cations Ca(2+) and Mg(2+) are critical for limiting arsenic desorption; they promote As (as arsenate) adsorption to the phyllosilicate clay minerals of the aquifer. While native groundwater contains adequate concentrations of dissolved Ca(2+) and Mg(2+), these cations are not present at sufficient concentrations during recharge of highly purified recycled water. Subsequently, the absence of dissolved Ca(2+) and Mg(2+) displaces As from the sediments into solution. Increasing the dosages of common water treatment amendments including quicklime (Ca(OH)2) and dolomitic lime (CaO·MgO) provides recharge water with higher concentrations of Ca(2+) and Mg(2+) ions and subsequently decreases the release of As during infiltration.

The spatial distribution of reactive minerals in the subsurface is often a primary 24 factor controlling the fate and transport of contaminants in groundwater systems. 25 However, direct measurement and estimation of heterogeneously distributed minerals are 26 often costly and difficult to obtain. While previous studies have shown the utility of using 27 hydrologic measurements combined with inverse modeling techniques for tomography of 28 physical properties including hydraulic conductivity, these methods have seldom been 29 used to image reactive geochemical heterogeneities. In this study, we focus on As-30 bearing reactive minerals as aquifer contaminants. We use synthetic applications to 31 demonstrate the ability of inverse modeling techniques combined with mechanistic 32 reactive transport models to image reactive mineral lenses in the subsurface and quantify 33 estimation error using indirect, commonly measured groundwater parameters.

Managed aquifer recharge (MAR) enhances freshwater security and augments local groundwater supplies. However, geochemical and hydrological shifts during MAR can release toxic, geogenic contaminants from sediments to groundwater, threatening the viability of MAR as a water management strategy. Using reactive transport modeling coupled with aquifer analyses and measured water chemistry, we investigate the causal mechanisms of arsenic release during MAR via injection in the Orange County Groundwater Basin. Here, injection water is oxygenated, highly purified recycled water produced by advanced water treatment. Injection occurs via a well screened at several depth intervals ranging from 160−365 m, allowing recharge into multiple confined horizons (zones) of the aquifer system. However, these zones are characterized by varying degrees of prior oxidation due to historic, long-term infiltration from the overlying aquifer. The resulting sediment geochemical heterogeneity provides a critical control on the release (or retention) of arsenic. In zones with prior oxidation, As mobilization occurs via arsenate desorption from Fe-(hydr)oxides, primarily associated with shifts in pH; within zones that remain reduced prior to injection, As release is attributed to the oxidative dissolution of As-bearing pyrite. We find that As release can be attributed to various geochemical mechanisms within a single injection well owing to geochemical heterogeneity across the aquifer system.