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A.3 CSIA for TCE in Groundwater for Site Characterization (NJ)

EMD Technology

• Compound Specific Isotope Analysis (CSIA)

Contacts

Patrick McLoughlin

Microseeps, Inc.

412-826-5245

[email protected]

A.3.1 Site Background and Knowledge from Traditional Methods

At the Aviation plaza site in New Jersey, a release of TCE impacted groundwater. The contamination extended into several layers of the aquifer but at no level was the concentration information conclusive. The information showed two distinct “hot spots” but it could not be proven that there were two separate releases. Determining whether two sources were present was critical for site planning and management decisions.

At the site, the presence of intermediate products cis-DCE and VC was evidence of some biodegradationA process by which microorganisms transform or alter (through metabolic or enzymatic action) the structure of chemicals introduced into the environment (USEPA 2011).. The intermediates were present in concentrations much lower than that of the parent TCE, and the parent TCE was present in the source zone at concentrations much greater than 1% of solubility. These concentrations indicate that DNAPL is present and that it replaces any dissolved-phase TCE lost to biodegradation by dissolution. Further, it appeared from the concentrations of cis-DCE and VC that the biodegradation would not overcome the effects of that DNAPL. This information indicates that the isotopic ratioThe concentration of the heavy isotope divided by the concentration of the light isotope. of the TCE in these wells is not changed by biodegradation. As shown in Figure A.3-1, the TCE concentration data did not clearly show which locations were sources and which locations were areas impacted by the up-gradient sources.

A.3.2 EMD Objectives and Approach

The concentration contour maps indicated several “hot spots,” but it was unclear if they were sources or the result of heterogeneous contaminant dissolution, heterogeneous contaminant flow, and heterogeneous biodegradation. Further, it was unclear if intermediate points were a mixing of the contributions from multiple sources. In addition, there are two vertical zones and it was unclear if the contamination in the upper zone originated from the same source as the contamination in the lower zone.

Resolving the unclear issues was crucial to site management. Because robust biodegradation was not occurring at the site, CSIA data could be more clearly evaluated for this purpose. Initially, it was an open question whether both carbon and chlorine CSIA were to be performed to understand the site. In the interest of cost control, it was decided to perform the carbon analyses only first, complementing it with chlorine analyses in a future sampling event based on the results of the carbon CSIA. The carbon CSIA proved to be sufficiently definitive.

Groundwater samples were collected from six site wells as shown in Figure A.3-1.

Figure A.3-1. The TCE concentrations and δ¹³C values for the Aviation Plaza site. The upper zone is portrayed on the top, the lower zone is portrayed on the bottom.

Source: Microseeps, Inc. 2012. Used with permission.

A.3.3 Results

As shown in Table A3-1, the δ¹³C of the TCE in UZ_1 and LZ_1 are very similar in both the upper and lower zones. It can also be seen that the δ¹³C of the TCE in UZ_2 and LZ_2 is very similar in both the upper and lower zones but very different from that in UZ-1 and LZ-1. This suggests that the TCE in UZ-1/LZ-1 is from a source that is different from that of the TCE in UZ-2/LZ-2. Further, because the δ¹³C of the TCE in LZ_3 and LZ_4 are between the results in UZ_1/LZ_1 and UZ_2/LZ_2, it is suspected those wells do not represent a unique source but are impacted by both sources.

A.3.4 Conclusions

The conclusions from this part of the site work include:

• CSIA showed a significant difference in the δ¹³C of the TCE at the two different “hot spots” which indicates there are two different sources.
• Carbon only CSIA was enough to differentiate these sources and further investigation of the chlorine isotopic ratio was not necessary. This provided a cost savings at this site. At other sites, it may be necessary to analyze multiple elements in order to develop a clear understanding of the site.
• The downgradient wells appeared to be influenced by both sources.
• The contamination in the lower zone appears to be from the same source as the contamination in that same geographic location in the upper zone.
• A membrane interface probe (MIP) investigation was performed to confirm the presence of two different sources. Although not typically needed to support the CSIA results, the MIP investigation was performed at this site because the two-source determination had significant site management consequences. The MIP investigation both confirmed the CSIA data and provided other useful information for the conceptual site model (CSM).

A.3.5 Costs

The cost of the CSIA for these six samples was approximately $3,000.

A.3.6 Outcomes and Challenges

CSIA proved to be a powerful tool for forensic purposes. The information gathered through the carbon CSIA results was sufficient to document the existence of two different sources of TCE at this site. Prior to conducting the CSIA study the existing traditional data did not provide sufficient resolution of the two sources. No significant challenges were encountered during the CSIA study at this site.

Permission is granted to refer to or quote from this publication with the customary acknowledgment of the source (see suggested citation and disclaimer).

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