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Groundwater remediation and no‐further action decision making at petroleum underground storage tank (UST) sites has largely been based on an understanding of plume length, plume stability, and attenuation rates for key hydrocarbon constituents. Regulatory guidance to support and guide such decisions is based in part on plume studies involving individual hydrocarbon constituents, namely benzene and methyl tert‐butyl ether (MTBE). Questions remain regarding whether current guidance is applicable to chemical mixtures such as gasoline range organics (GRO), diesel range organics (DRO), and oxygen containing organic compounds (OCOCs) resulting from hydrocarbon biodegradation. To help address this concern, data from California's GeoTracker database were used to estimate maximum plume lengths, plume stability, and attenuation rates of DRO (which can be used as an analytical surrogate for OCOCs) and GRO relative to benzene and MTBE. The distributions of maximum plume lengths were similar for the four constituents with medians ranging from 27 to 32 m. The fraction of monitoring wells with a decreasing concentration trend ranged from 19% for DRO to 40% for MTBE, while fewer than 7% of the wells had an increasing concentration trend for any of the constituents. Median attenuation rates ranged from 0.10% day−1 for DRO to 0.17% day−1 for MTBE. The results suggest attenuation based risk management is appropriate for DRO and GRO plumes at most petroleum UST sites. Article impact statement: This paper compares the attenuation behavior of DRO to other gasoline constituents and considers the risk management implications. [ABSTRACT FROM AUTHOR]

Preferential flow through weathered bedrock leads to rapid rise of the water table after the first rainstorms and significant water storage (also known as 'rock moisture') in the fractures. We present a new parameterization of hydraulic conductivity that captures the preferential flow and is easy to implement in global climate models. To mimic the naturally varying heterogeneity with depth in the subsurface, the model represents the hydraulic conductivity as a product of the effective saturation and a background hydraulic conductivity Kbkg, drawn from a lognormal distribution. The mean of the background Kbkg decreases monotonically with depth, while its variance reduces with the effective saturation. Model parameters are derived by assimilating into Richards' equation 6 years of 30 min observations of precipitation (mm) and water table depths (m), from seven wells along a steep hillslope in the Eel River watershed in Northern California. The results show that the observed rapid penetration of precipitation and the fast rise of the water table from the well locations, after the first winter rains, are well captured with the new stochastic approach in contrast to the standard van Genuchten model of hydraulic conductivity, which requires significantly higher levels of saturated soils to produce the same results. 'Rock moisture,' the moisture between the soil mantle and the water table, comprises 30% of the moisture because of the great depth of the weathered bedrock layer and could be a potential source of moisture to sustain trees through extended dry periods. Furthermore, storage of moisture in the soil mantle is smaller, implying less surface runoff and less evaporation, with the proposed new model. [ABSTRACT FROM AUTHOR]

Tetrafluoromethane (CF 4 ) concentrations were measured in 14 groundwater samples from the Cuyama Valley, Mil Potrero and Cuddy Valley aquifers along the Big Bend section of the San Andreas Fault System (SAFS) in California to assess whether tectonic activity in this region is a significant source of crustal CF 4 to the atmosphere. Dissolved CF 4 concentrations in all groundwater samples but one were elevated with respect to estimated recharge concentrations including entrainment of excess air during recharge ( C r e ; ∼30 fmol kg −1 H 2 O), indicating subsurface addition of CF 4 to these groundwaters. Groundwaters in the Cuyama Valley contain small CF 4 excesses (0.1–9 times C r e ), which may be attributed to an in situ release from weathering and a minor addition of deep crustal CF 4 introduced to the shallow groundwater through nearby faults. CF 4 excesses in groundwaters within 200 m of the SAFS are larger (10–980 times C r e ) and indicate the presence of a deep crustal flux of CF 4 that is likely associated with the physical alteration of silicate minerals in the shear zone of the SAFS. Extrapolating CF 4 flux rates observed in this study to the full extent of the SAFS (1300 km × 20–100 km) suggests that the SAFS potentially emits ( 0.3 – 1 ) × 10 − 1 kg CF 4 yr −1 to the Earth's surface. For comparison, the chemical weathering of ∼ 7.5 × 10 4 km 2 of granitic rock in California is estimated to release ( 0.019 – 3.2 ) × 10 − 1 kg CF 4 yr −1 . Tectonic activity is likely an important, and potentially the dominant, driver of natural emissions of CF 4 to the atmosphere. Variations in preindustrial atmospheric CF 4 as observed in paleo-archives such as ice cores may therefore represent changes in both continental weathering and tectonic activity, including changes driven by variations in continental ice cover during glacial–interglacial transitions. [ABSTRACT FROM AUTHOR]

Quantifying the overall progress in remediation of contaminated groundwater has been a significant challenge. We utilized the GeoTracker database to evaluate the progress in groundwater remediation from 2001 to 2011 at over 12,000 sites in California with contaminated groundwater. This paper presents an analysis of analytical results from over 2.1 million groundwater samples representing at least $100 million in laboratory analytical costs. Overall, the evaluation of monitoring data shows a large decrease in groundwater concentrations of gasoline constituents. For benzene, half of the sites showed a decrease in concentration of 85% or more. For methyl tert-butyl ether ( MTBE), this decrease was 96% and for TBE, 87%. At remediation sites in California, the median source attenuation rate was 0.18/year for benzene and 0.36/year for MTBE, corresponding to half-lives of 3.9 and 1.9 years, respectively. Attenuation rates were positive (i.e., decreasing concentration) for benzene at 76% of sites and for MTBE at 85% of sites. An evaluation of sites with active remediation technologies suggests differences in technology effectiveness. The median attenuation rates for benzene and MTBE are higher at sites with soil vapor extraction or air sparging compared with sites without these technologies. In contrast, there was little difference in attenuation rates at sites with or without soil excavation, dual phase extraction, or in situ enhanced biodegradation. The evaluation of remediation technologies, however, did not evaluate whether specific systems were well designed or implemented and did not control for potential differences in other site factors, such as soil type. [ABSTRACT FROM AUTHOR]

Treated wastewater discharged from more than 400 onsite wastewater treatment systems ( OWTS) near the Civic Center area of Malibu, California, 40 km west of downtown Los Angeles, composes 28% of the recharge to a 3.4 km2 alluvial aquifer. On the basis of δ18O and δD data, the fraction of wastewater in some samples was >70%. Ammonium and nitrate concentrations in water from 15 water-table wells sampled in July 2009 and April 2010 ranged from <0.01 to 12 milligrams per liter as nitrogen (mg/L as N), and from <0.01 to 11 mg/L as N, respectively. Chemical and isotopic data ( δ15N of ammonium and nitrate, and δ18O of nitrate) show two processes remove nitrogen discharged from OWTS. Where groundwater was reducing, sorption of ammonium resulted in 30 to 50% nitrogen removal. Where groundwater was initially oxic, nitrification with subsequent denitrification as reducing conditions developed, resulted in up to 60% nitrogen removal. Nitrogen removal through sorption dominated during the cooler April sample period, and denitrification dominated during the warmer July sample period. The combination of mixing and nitrogen removal due to denitrification, sorption, and volatilization produces a δ15N apparent fractionation factor (εapp = −5), that can be explained using laboratory-derived fractionation factors (ε) for the individual processes. Phosphate concentrations ranged from < 0.04 to 2 mg/L as phosphorous. Sorption to iron oxides on the surfaces of mineral grains at near-neutral pH's removed some phosphate; however, little removal occurred at more alkaline pH's (>7.3). [ABSTRACT FROM AUTHOR]

The article looks at the California's Water Code and Government Code that supplement the common law and act to regulate the management, allocation, and use of the state groundwater resources. Topics discussed include the chapter 346 and 347 of the state government code, the U.S. Sustainable Groundwater Management Act, and legislative framework for prioritizing groundwater basins in the state.

Groundwater recharge with reclaimed municipal wastewater presents a wide spectrum of technical and health challenges that must be carefully evaluated prior to undertaking a project. This review will provide a discussion of groundwater recharge and its management with special reference to health and regulatory aspects of groundwater recharge with reclaimed municipal wastewater. At present, some uncertainties with respect to health risk considerations have limited expanding use of reclaimed municipal wastewater for groundwater recharge, especially when a large portion of the groundwater contains reclaimed wastewater that may affect the domestic water supply.The proposed State of California criteria for groundwater recharge are discussed as an illustration of a cautious approach. In addition, a summary is provided of the methodology used in developing the World Health Organization''s Guidelines for Drinking Water Quality to illustrate how numerical guideline values are generated for contaminants that may be applicable to groundwater recharge. [Copyright &y& Elsevier]

Perchlorate has been widely used as a propellant in solid rocket fuel, and has recently been identified as a contaminant in both groundwater and surface waters. Perchlorate is recognized by the U.S. Environmental Protection Agency (USEPA) as a potential health risk, and the State of California has set a drinking water action level of 18 µg L−1. Incidents of groundwater contamination have been associated with industrial sites in California and Nevada that have been involved in the manufacturing or testing of solid rocket propellants. Microorganisms have been shown to be capable of reducing perchlorate (Cl O−4) to chloride (Cl−) and oxygen, thus transforming perchlorate into innocuous end‐products. Bioreactor processes for the remediation of perchlorate contaminated wastewater have previously been established. However, these systems were optimized for perchlorate concentrations in the grams per liter range, while groundwater contamination can be a million‐fold lower but still exceed the water quality action level. This literature review will focus on microbial‐mediated perchlorate reduction, and discuss issues of importance to the remediation of perchlorate‐contaminated groundwater. [ABSTRACT FROM AUTHOR]

Discusses growing concern over the contamination of groundwater with toxic chemicals. Water industry which has a major stake in the safe disposal of toxic wastes; Partnership between the Environmental Defense Fund and the Metropolitan Water District of Southern California; Findings regarding the use of solvents by firms; Source-reduction options.

Nitrate in drinking water may cause serious health problems for consumers. Agricultural activities are known to be the main source of groundwater nitrate contaminating rural domestic and urban public water supply wells in farming regions. Management practices have been proposed to reduce the amount of nitrate in groundwater, including improved nutrient management practices and "pump and fertilize" with nitrate-affected irrigation wells. Here, we evaluate the feasibility and long-term impacts of agricultural managed aquifer recharge (Ag-MAR) in the source area of public water supply wells. A numerical model of nitrate fate and transport was developed for the Modesto basin, part of California's Central Valley aquifer system. The basin is representative of semi-arid agricultural regions around the world with a diversity of crop types, overlying an unconsolidated sedimentary aquifer system. A local public supply well in an economically disadvantaged community surrounded by farmland was the focus of this study. Model scenarios implemented include business as usual, alternative low-impact crops, and Ag-MAR in the source area of the public supply well. Alternative nutrient management and recharge practices act as remediation tools in the area between farmland and the public supply well. Improved agricultural source area management practices are shown to be an effective tool to maintain or even enhance groundwater quality in the targeted supply well while remediating ambient groundwater. Best results are obtained when lowering nitrate load while also increasing recharge in the source area simultaneously. This scenario reduced nitrate in the supply well's drinking water by 80% relative to the business as usual scenario. It also remediated ambient groundwater used by domestic wells between the source area farmlands and the supply well and showed 60% more reduction of nitrate after 60 years of application. Increasing recharge led to shorter initial response time (five years) and showed the most sustainable impact. Our analysis further suggests that Ag-MAR in a highly discontinuous, wide-spread pattern leads to slow water quality response and may not yield sufficient water quality improvements., (Copyright © 2019 The Authors. Published by Elsevier B.V. All rights reserved.)

The Groundwater Replenishment (GWR) System in Orange County, California will receive $2.25 million in federal funding for the fiscal year 2006, according to an appropriation bill recently signed into law by President George W. Bush. [Copyright &y& Elsevier]