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.

Population growth and climate variability highlight the need to enhance freshwater security and diversify water supplies. Subsurface storage of water in depleted aquifers is increasingly used globally to alleviate disparities in water supply and demand often caused by climate extremes including floods and droughts. Managed aquifer recharge (MAR) stores excess water supplies during wet periods via infiltration into shallow underlying aquifers or direct injection into deep aquifers for recovery during dry seasons. Additionally, MAR can be designed to improve recharge water quality, particularly in the case of soil aquifer treatment and riverbank filtration. While there are many potential benefits to MAR, introduction of recharge water can alter the native geochemical and hydrological conditions in the receiving aquifer, potentially mobilizing toxic, naturally occurring (geogenic) contaminants from sediments into groundwater where they pose a much larger threat to human and ecosystem health. On the basis of the present literature, arsenic poses the most widespread challenge at MAR sites due to its ubiquity in subsurface sediments and toxicity at trace concentrations. Other geogenic contaminants of concern include fluoride, molybdenum, manganese, and iron. Water quality degradation threatens the viability of some MAR projects with several sites abandoning operations due to arsenic or other contaminant mobilization. Here, we provide a critical review of studies that have uncovered the geochemical and hydrological mechanisms controlling mobilization of arsenic and other geogenic contaminants at MAR sites worldwide, including both infiltration and injection sites. These mechanisms were evaluated based on site-specific characteristics, including hydrological setting, native aquifer geochemistry, and operational site parameters (e.g., source of recharge water and recharge/recovery cycling). Observed mechanisms of geogenic contaminant mobilization during MAR via injection include shifting redox conditions and, to a lesser extent, pH-promoted desorption, mineral solubility, and competitive ligand exchange. The relative importance of these mechanisms depends on various site-specific, operational parameters, including pretreatment of injection water and duration of injection, storage, and recovery phases. This critical review synthesizes findings across case studies in various geochemical, hydrological, and operational settings to better understand controls on arsenic and other geogenic contaminant mobilization and inform the planning and design of future MAR projects to protect groundwater quality. This critical review concludes with an evaluation of proposed management strategies for geogenic contaminants and identification of knowledge gaps regarding fate and transport of geogenic contaminants during MAR.

Approximately 10% of community water systems in the United States experience a health-based violation of drinking water quality; however, recently allocated funds for improving United States water infrastructure ($50 billion) provide an opportunity to address these issues. The objective of this study was to examine environmental, operational, and sociodemographic drivers of spatiotemporal variability in drinking water quality violations using geospatial analysis and data analytics. Random forest modeling was used to evaluate drivers of these violations, including environmental (e.g., landcover, climate, geology), operational (e.g., water source, system size), and sociodemographic (social vulnerability, rurality) drivers. Results of random forest modeling show that drivers of violations vary by violation type. For example, arsenic and radionuclide violations are found mostly in the Southwest and Southcentral United States related to semiarid climate, whereas disinfection byproduct rule violations are found primarily in Southcentral United States related to system operations. Health-based violations are found primarily in small systems in rural and suburban settings. Understanding the drivers of water quality violations can help develop optimal approaches for addressing these issues to increase compliance in community water systems, particularly small systems in rural areas across the United States.

Rainwater harvested from four pilot-scale roofs (concrete tile, green, metal, and asphalt-fiberglass shingle) was batch-chlorinated to a target total chlorine 10 min residual of 2 mg/L as Cl 2 and passed through an activated carbon filter after 24 h to simulate treatment in a residential system. Total coliforms (TCs) were not detected, and the total trihalomethane (TTHM) concentration was typically (>85% of samples) below the US Environmental Protection Agency maximum contaminant level (MCL) of 80 μg/L in chlorinated-unfiltered rainwater harvested from conventional roofing materials. In contrast, TCs were always detected, and the TTHM concentration was 3-5 times higher than the MCL in chlorinated-unfiltered rainwater harvested from the green roof. The filter lowered the turbidity and TTHM concentration, but it also shed coliforms. Batch chlorination and filtration are more suitable potable treatment techniques for rainwater harvested from conventional roofs than from a green roof, but concerns regarding disinfection and turbidity remain.

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