In a recent article, TP Clement (2010, hereafter referred to as TPC) discusses the complexities and limitations of "hindcasting" models and criticizes the use of complex models when undertaking investigations of subsurface reactive transport processes. TPC implies that complex numerical models that stimulate reactive transport processes in groundwater are likely if not always an inappropriate tool to apply to "hindcasting" investiagtions and that scientists and engineers who implement these investiagtions using such models are somehow not aware of teh technical and scientific complexities and limitations of such methods and approaches. To illustrate his point of view, TPC uses a case study of an ongoing health study of exposure to volatile organic compounds (VOCs) in drinking water at US Marine Corps Base Camp Lejeune, North Carolina (hereafter referred to as the case-control health study at Camp Lejeune). The article presents some thought-provoking points-of-view. However, we believe there is a lack of detail on several key issues that require specificity and clarification, particularly with respect to modleing approaches and methods, the physics of containment occurrence and reactive transport in teh subsurface, and agency policies for the review and dissemination of data and reports.

Analytical models, when used in stochastic analysis mode, may provide an effective tool for making informed management decisions for simplified environmental systems. This approach was used to evaluate migration of an organochlorine pesticide plume in a shallow, unconfined aquifer underlying a barrier island in coastal Georgia, USA. The contaminant plume at the site consists of four isomers of benzene hexachloride (BHC), also known as hexachlorocyclohexane (HCH). The deterministic analysis conducted at the site, which used calibrated, single-value input parameters, indicates that the contaminant plume will not reach wetlands that are downgradient of the source. Given the uncertainties involved in the deterministic analysis, this outcome was not considered to be sufficient to make effective management decisions at the site. Subsequently, probabilistic analysis using a range of input parameter values was conducted to estimate the risk that the pesticide plume would reach the downgradient wetlands. The two-stage Monte Carlo analysis that was conducted indicates the probability that contaminant levels will exceed the detection limit of BHC (0.044 micrograms per liter) at the wetlands increases from 1 percent to a maximum of 13 percent during the period 2005-2065. This represents an 87% or greater confidence level that the pesticide plume will not reach the wetlands. This outcome was used to inform environmental management decisions at the site. The modeling analysis was conducted using the publicly available analytical contaminant transport analysis system (ACTS) software.

Two of three water-distribution systems that have historically supplied drinking water to family housing at U.S. Marine Corps Base Camp Lejeune, North Carolina, were contaminated with volatile organic compounds (VOCs). Scientific data relating to the harmful effects of VOCs on a child or fetus are limited. Therefore, the U.S. Agency for Toxic Substances and Disease Registry (ATSDR) is conducting an epidemiological study to evaluate potential associations between in utero and infant (up to 1 year of age) exposures to VOCs in contaminated drinking water at Camp Lejeune and specific birth defects and childhood cancers. The study includes births that occurred during the period 1968-1985 to women who were pregnant while they resided in family housing at Camp Lejeune. To support the epidemiological study, water-modeling techniques are being used to reconstruct historical conditions of groundwater flow, contaminant fate and transport, and the distribution of drinking water contaminated with VOCs. Results for this phase of the study indicate that PCE concentrations in drinking water, delivered to the Tarawa Terrace housing area, exceeded the current maximum contaminant level for PCE of 5 micrograms per liter (µg/L) for 346 months-November 1957-February 1987; the maximum simulated PCE concentration in Tarawa Terrace drinking water was 183 µg/L during March 1984 compared to a measured concentration of 215 µg/L during February 1985.