Contaminant Transport Models Aid in Understanding Trends of Chlorinated Ethenes in Public Supply Wells

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U.S. Geological Survey (USGS) scientists used a mass–balance solute–transport model to enhance an understanding of factors affecting chlorinated ethene (CE) concentrations in a public supply well. They found that long–term simulated and measured CEconcentrations were affected by dense nonaqueous phase liquid (DNAPL) volume, composition, and by the bioavailability of organic carbon that drives biodegradation.

Public supply well

USGS Hydrologist sampling a public supply well for chlorinated ethene contaminants. Photo Credit: Bruce Campbell, USGS

Chlorinated ethenes are one class of chlorinated solvents that are among the most common organic contaminants found in public supply wells in the United States. CEs measured and modeled in this study include tetrachloroethene, commonly referred to as perchloroethene (PCE), and trichloroethene (TCE), cis–1,2–dichloroethene (DCE), and vinyl chloride (VC). The presence of CEs in public supply wells provides evidence that there are one or more contaminant source areas, which may include DNAPLs.

The mass–balance solute–transport model was built by telescoping a calibrated regional three–dimensional MODFLOW model to the capture zone of a public supply well that has a history of CEcontamination. The local model, using the Sequential Electron Acceptor Model in three dimensions (SEAM3D) code, was then used to simulate the interactions between naturally occurring organic carbon that acts as an electron donor, and dissolved oxygen , CEs, ferric iron, and sulfate that act as electron acceptors. This study advances the SEAM3D technology by introducing the concept of using dissolution of a DNAPL source area as boundary and initial conditions for delivering CEs to groundwater.

This study builds upon other USGS studies to refine the understanding of CE transport in groundwater using simulated and measured long–term trends of CE concentrations at a public supply well. Previous USGS efforts using a mass balance modeling approach quantified in model output the effects of assumed particulate organic carbon concentrations, dissolved oxygen concentrations, the source of contamination, and factors affecting water movement.

Public supply wells are required to monitor concentrations of primary and secondary contaminants, including CEs present in treated and finished water that is delivered for distribution to the public. In the case of supply wells that produce groundwater containing CEcontaminants, such as the well described in this study, substantial costs are associated with treating the water. That being the case, having information concerning the expected long–term trends of contaminant concentrations in the raw untreated groundwater can be relevant to deciding whether water treatment costs are likely to increase or decrease over time, and whether it is more cost effective to abandon water production at particular wells rather than to invest in water treatment.

The USGS Toxic Substances Hydrology Program provided the funding for this work.

This article was featured in the USGS GeoHealth Newsletter, Vol. 12, No. 1, 2015

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