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

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

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 CE concentrations were affected by dense nonaqueous phase liquid (DNAPL) volume, composition, and by the bioavailability of organic carbon that drives biodegradation.

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 CE contamination. 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 CE contaminants, 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.

References

Modeling long–term trends of chlorinated ethene contamination at a public supply well: JAWRA Journal of the American Water Resources Association, 2014, doi:10.1111/jawr.12230 (Advanced Web release).

Modeling the effects of naturally occurring organic carbon on chlorinated ethene transport to a public supply well: Groundwater, 2014, v. 52, no. S1, p. 76–89, doi:10.1111/gwat.12152.

A framework for assessing the sustainability of monitored natural attenuation: U.S. Geological Survey Circular 1303, 2007, 46 p.

More Information

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

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