CHARACTERIZING LONG-TERM CHANGES IN GROUNDWATER CONTAMINATION IN A STREAM VALLEY USING DATA FROM MULTIPLE SOURCES

This paper will describe a data analysis approach that maximizes the value of data collected over a period of years during different phases of site investigation work performed for different purposes. These data were combined to characterize a contamination plume that impacts groundwater underlying a residential area. The residential area is located in a stream valley filled with complex unconsolidated glacial, glaciofluvial, and fluvial deposits up to 100 feet thick which are underlain by bedrock.

During the Remedial Investigation (RI) at a U.S. Environmental Protection Agency (EPA) Superfund site in New York, groundwater quality data were collected by EPA and other agencies at the site over a period of five years. The data, collected under separate phases of work, were combined to characterize changes in groundwater contamination. The RI data collected in 2007 were combined with data collected from 2003 through 2008 by other agencies to characterize long-term plume behavior.

To determine the trends in the concentrations at different locations within the plume, the data were graphed and mapped. Data from a group of wells perpendicular to the direction of groundwater flow were evaluated to characterize plume migration.

Trichloroethene (TCE) and 1,1,1-trichloroethane (1,1,1-TCA) were used to define the plume. In the summer of 2007, the plumes of TCE and 1,1,1-TCA contaminated groundwater in the unconsolidated valley deposits were, respectively, about 8,200 and 8,700 feet long. The plume geometry is primarily controlled by small, discontinuous preferential flow paths in the unconsolidated deposits, which are not easily located and sampled using conventional monitoring wells. To assess future risk to groundwater users, long-term trends in concentrations were analyzed using residential well sample data.The characterization of long-term contaminant changes showed that: 1) concentrations of TCE and 1,1,1-TCE in the core of the plume have steadily dropped over time from 250 micrograms per liter (μg/L) to 90 μg/L; 2) the highest concentrations are dispersing or diffusing and are not being detected downgradient; and 3) concentrations of TCE are beginning to be detected above the maximum contaminant level at some locations at the distal end of the plume, indicating the movement of TCE in groundwater.