Water quality studies often include the analytical challenge of incorporating censored data and quantifying error of estimation. Many analytical methods exist for estimating distribution parameters when censored data are present. This paper presents a Bayesian-based hierarchical model for estimating the national distribution of the mean concentrations of chemicals occurring in U.S. public drinking water systems using fluoride and thallium as examples. The data used are Safe Drinking Water Act compliance monitoring data (with a significant proportion of left-censored data). The model, which assumes log-normality, was evaluated using simulated data sets generated from a series of Weibull distributions to illustrate the robustness of the model. The hierarchical model is easily implemented using the Markov chain Monte Carlo simulation method. In addition, the Bayesian method is able to quantify the uncertainty in the estimated cumulative density function. The estimated fluoride and thallium national distributions are presented. Results from this study can be used to develop prior distributions for future U.S. drinking water regulatory studies of contaminant occurrence.
Although hydraulic fracturing has a long history, technical improvements made fracturing an industry game‐changer in the early 2000s. This article provides an overview of the present‐day process and where and how it is used.
Recalcitrant groundwater contamination is a common problem at hazardous waste sites worldwide. Groundwater contamination persists despite decades of remediation efforts at many sites because contaminants sorbed or dissolved within low-conductivity zones can back diffuse into high-conductivity zones, and therefore act as a continuing source of contamination to flowing groundwater. A review of the available literature on remediation of plume persistence due to back diffusion was conducted, and four sites were selected as case studies. Remediation at the sites included pump and treat, enhanced bioremediation, and thermal treatment. Our review highlights that a relatively small number of sites have been studied in sufficient detail to fully evaluate remediation of back diffusion; however, three general conclusions can be made based on the review. First, it is difficult to assess the significance of back diffusion without sufficient data to distinguish between multiple factors contributing to contaminant rebound and plume persistence. Second, high-resolution vertical samples are decidedly valuable for back diffusion assessment but are generally lacking in post-treatment assessments. Third, complete contaminant mass removal from back diffusion sources may not always be possible. Partial contaminant mass removal may nonetheless have potential benefits, similar to partial mass removal from primary DNAPL source zones.
In July 2008, the U.S. Environmental Protection Agency (EPA) issued a proposed rule under the authority of the Safe Drinking Water Act (SDWA) regarding the geologic sequestration (GS) of carbon dioxide. The proposed rule recommends the addition of a new class of injection well, Class VI, to EPA’s existing Underground Injection Control (UIC) Program to address the unique nature of carbon dioxide injection and to ensure the protection of underground sources of drinking water (USDWs) from carbon dioxide injection-related activities. The proposed GS Rule builds upon the existing UIC regulatory framework established under the SDWA. The final rule, anticipated in late 2010 or early 2011, will apply to owners and operators of permitted Class VI injection wells for the subsurface injection of carbon dioxide for long-term underground storage. States interested in obtaining approval for primary enforcement responsibility ("primacy") for the new Class VI program will apply for primacy to EPA with either a new UIC program application or a UIC program revision (depending on a particular state’s existing UIC primacy status). Any state that chooses not to apply for primacy, or that has not yet received approval for their proposed Class VI program, will have the UIC Class VI program administered by EPA until such time as the state has an approved Class VI program. EPA is developing multiple technical and programmatic guidance documents for UIC Directors and proposed Class VI injection well owners and operators to assist in meeting the new GS rule requirements. In addition, EPA will be conducting nationwide training workshops for UIC Directors on primacy application and on the technical and programmatic elements of GS Rule and its implementation. EPA is also planning for a series of webcasts focused on technical aspects of the GS rule for interested owners, operators, field service providers, UIC permit reviewers, and others to be involved in GS activities. EPA anticipates that these guidance documents, implementation tools, trainings, and other outreach efforts will assist Class VI injection well owners and operators, as well as state UIC Directors, in meeting the new GS Rule requirements.
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