Theme: 10. Characterization, Fate and Transport

Topic Area: 10a. Advanced Investigation Tools and Techniques

Formatted Title:

Molarity of a Chlorinated Groundwater Solution and CVOC Trends to Identify a cis-1,2-DCE Source

Background/Objectives:

Chlorinated volatile organic compounds (CVOCs) in groundwater, including PCE, TCE and cis-1,2-DCE, were identified at concentrations greater than screening levels at a former dry cleaner facility. The facility is in a mixed-use commercial/industrial area of Dallas, Texas, and operated from circa 1960 to 1987. Ten groundwater monitor wells have been installed into the shallow alluvial aquifer at the facility. The initial six wells were installed in January 2021 and four additional wells installed in December 2021. Five semi-annual sampling events have been performed between January 2021 and August 2023.

The groundwater hydraulic gradient is to the west-northwest. The distribution of CVOCs in groundwater relative to the location of the former dry cleaner indicates: PCE, TCE and cis-1,2-DCE exceedances in wells located downgradient to the northwest, cross gradient to the north and upgradient to the east and wells located to the west, and southwest had no exceedances. The highest PCE, TCE and cis-1,2-DCE concentrations were detected in wells MW-7 and MW-10. MW-7 is located cross gradient to the northwest and MW-10 is upgradient to the east. This distribution indicates a potential off-site source of CVOCs.

Approach/Activities:

Initially off-site access to the east was requested to delineate the eastern/upgradient area of the CVOC plume, and to identify the potential off-site source with new wells, however access was denied. Therefore, an alternative approach was devised using only the data on hand to determine if an off-site source was contributing to the observed CVOCs in groundwater at the facility.

Observing the relationship between the total mass of CVOC in solution, i.e. the molarity of the CVOCs compared to the PCE, TCE, and cis1,2-DCE concentration trends provides an indication of whether an increase in cis-1,2-DCE concentrations are from PCE/TCE degradation or from a new CVOC input. The molarity for each CVOC constituent was calculated as the solute concentration [grams(g)/liter] divided by the solute molar mass [g/mole]. The total molarity of the CVOCs in solution was then calculated as the sum of the PCE, TCE, cis-1,2-DCE, and VC molarities.

Results/Lessons Learned:

Wells MW-1, 4, 5, 6, and 9 are located at the on-site source area or downgradient to the southwest and show a recent decrease in CVOC mass and a corresponding decreasing trend in PCE, TCE, and cis1,2-DCE concentrations indicating CVOC degradation and no new CVOCs. A recent uptick in CVOC mass at wells MW-2 and MW-7 with a modest PCE/TCE decrease; indicates that the cis-1,2-DCE increase is potentially from a combination of PCE/TCE degradation and new cis-1,2-DCE entering the system. The uptick of CVOC mass observed at wells MW-3 and MW-10 with a corresponding cis-1,2-DCE increase but with no or very little TCE/PCE degradation suggests that the cis-1,2-DCE increase is from cis-1,2-DCE entering the system and not from TCE/PCE degradation. This along with the relative upgradient locations of MW-3 and MW-10 indicates that an off-site and upgradient CVOC source is located to the east of the site and to the east of MW-3 and MW-10.

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Jeff Gibbs