Groundwater remediation technologies are designed, installed, and operated based on the conceptual models of contaminant hydrogeology that are accepted at that time. However, conceptual models of remediation can change as new research, new technologies, and new performance data become available. Over the past few years, results from multiple-site remediation performance studies have shown that achieving drinking water standards (i.e., Maximum Contaminant Levels, MCLs) at contaminated groundwater sites is very difficult. Recent groundwater research has shown that the process of matrix diffusion is one key constraint. New developments, such as mass discharge, orders of magnitude (OoMs), and SMART objectives are now being discussed more frequently by the groundwater remediation community. In this paper, the authors provide their perspectives on the existing "reach MCLs" approach that has historically guided groundwater remediation projects, and advocate a new approach built around the concepts of OoMs and mass discharge.

The groundwater remediation field has been changing constantly since it first emerged in the 1970s. The remediation field has evolved from a dissolved-phase centric conceptual model to a DNAPL-dominated one, which is now being questioned due to a renewed appreciation of matrix diffusion effects on remediation. Detailed observations about contaminant transport have emerged from the remediation field, and challenge the validity of one of the mainstays of the groundwater solute transport modeling world: the concept of mechanical dispersion (Payne et al. 2008). We review and discuss how a new conceptual model of contaminant transport based on diffusion (the usurper) may topple the well-established position of mechanical dispersion (the status quo) that is commonly used in almost every groundwater contaminant transport model, and evaluate the status of existing models and modeling studies that were conducted using advection-dispersion models.

Different points of view have emerged concerning how to best consider and address the largely unexamined ancillary environmental impacts, and more particularly the social and economic impacts, of remediation activities. These views are generally categorized as “green remediation” and “sustainable remediation.” This article dissects the commonalities and differences between “green” and “sustainable” remediation approaches. Several key obstacles to the broader implementation of sustainable remediation practices are identified. Similarities identified among the two concepts offer a common ground and areas of collaboration. The objective of this article is to support maturation of the remediation industry by addressing the opposition to and supporting the implementation of sustainable remediation practices, including offering recommendations for a path forward. ©2015 Wiley Periodicals, Inc.

First off, we are honored that Dr. Neuman has chosen to engage over our paper. One of our key objectives for submitting the subject paper was to initiate a dialogue between remediation professionals and groundwater modelers. Dr. Neuman's input-as well as the kind words we have received directly from groundwater remediation professionals-suggests that to at least some degree we have accomplished this important objective.We would agree with Dr. Neuman that viewing deterministic models with a stochastic perspective provides an improvement. However, we disagree with his assertions that stochastic modeling is the answer to the historical shortcomings of the predictions made concerning groundwater remediation. Our rebuttal will not contain the elegant mathematics of Dr. Neuman's comment, but rather applies words, anecdotes, facts, and ideas to reaffirm our suggestion that it is time for a new path.

Thousands of leaking underground gasoline tanks have been found throughout California from which a tremendous amount of gasoline has leaked into ground water over the last half century. The most water‐soluble constituent of gasoline is benzene, and it typically contaminates ground water below leaking underground tanks. In a state‐mandated program 7,167 wells serving water‐supply systems throughout California were tested for a broad panel of organic contaminants. Of the wells tested, 812 (11.3%) had detectable concentrations of at least one of the contaminants tested for. Detectable concentrations of benzene were reported for only 10 wells. Similar findings were reported in surveys focused on San Gabriel and San Fernando Basins. While many processes influence the environmental fates of organic contaminants in ground water, the most likely explanation for the nonoccurrence of benzene is that it is destroyed near its source by biodegradation.