Theme: 11. Advanced Diagnostic Tools

Topic Area: 11a. Environmental Forensics: Site Characterization and Source Determinations

Formatted Title:

Identifying and Delineating Hidden Sources within a Larger Chlorinated Solvent Plume

Background/Objectives:

Chlorinated solvents, including tetrachloroethene (PCE) and trichloroethylene (TCE), are among the most commonly-detected chemicals at cleanup sites and it is not uncommon in industrial settings for groundwater plumes to be commingled with releases from multiple sites. In particular, large plumes covering wide swaths of urban industrial areas may mask multiple sources along groundwater flow paths. Identifying sources and disentangling multiple potential sources in commingled groundwater plumes are critical yet challenging components of forensic investigations to identify, and where appropriate, allocate sources to a plume. Herein, we propose a methodology for disentangling large, chlorinated solvent plumes by identifying and delineating the impacts of smaller downgradient sources hidden within a larger plume emanating from an upgradient source or sources. This methodology allowed us to identify multiple previously unidentified downgradient sources within a larger commingled TCE and PCE plume, and to delineate the extent of the upgradient TCE plume.

Approach/Activities:

Our approach involved the application of three methodologies in two steps. A first step used chlorinated solvent “fingerprints” consisting of “pie-charts” of PCE and its daughter products to identify unique downgradient fingerprints which cannot be explained by the upgradient source per Robrock and Mesard (2018). We then applied a second methodology previously proposed by Dai and Chau (2008) to distinguish contributions between two commingled upgradient and downgradient sources by plotting contaminant concentrations along the centerline of a plume. This methodology modeled the TCE concentrations of the upgradient source migrating beneath the downgradient source by fitting a “most likely” decay curve to observed data from the upgradient source. However, Dai and Chau (2008) did not establish what the “most likely” decay curve should be. Based on the U.S. EPA (2002), naturally-occurring degradation along the centerline of a plume under natural attenuation conditions is best described as a first order decay. We therefore applied a first-order decay curve to delineate the full downgradient extent of the upgradient TCE plume.

Results/Lessons Learned:

The first step of plotting chlorinated solvent fingerprints within the urban industrial area downgradient of the source of a mixed TCE/PCE chlorinated solvent plume allowed us to identify a number of different fingerprints (e.g., different ratios of PCE and its daughter products) which could not be explained by the upgradient source, and most likely represented previously unidentified sources of TCE/PCE releases. A number of these different fingerprints matched the locations on historical Sanborn maps of industrial operations that, based on historical operations and chemical uses, reasonably would have the potential to have used chlorinated solvents, and released them into the environment, indicating the presence of new potential sources of chlorinated solvents within the larger upgradient plume. The second step, involving modeling the plume decay along the centerline of the plume, allowed us to delineate the extent of the upgradient plume that was masked by additional downgradient sources. In conclusion, we established that there were at least two previously unknown TCE-contributing sites within the larger chlorinated solvent plume and were able to delineate the extent of the impacts from the upgradient source.

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Peter Mesard