Groundwater Forensics: A Tale of Two Sites

Following the removal of leaking underground storage tanks, Antea Group oversaw the remediation a retail gasoline service station site that was impacted by gasoline range organics. Site remediation was successful at the source location, but methyl tertiary butyl ether (MTBE) concentrations persisted onsite, which was hydraulically upgradient of the historical release zone. Antea Group looked for additional sources of gasoline spills within the area to explain the persistence of this compound.

The search revealed another service station with leaking tanks approximately 800 feet upgradient of the site - a viable suspect as the source of the MTBE concentrations. However, the state’s Department of Environmental Protection (DEP) would not acknowledge the other service station as a potentially responsible party. This was due to two main reasons:

• the historical patterns of groundwater flow between the two sites
• the question of MTBE traveling the distance between the two sites  

Antea Group tackled these 2 issues in the following manner:

Resolving groundwater flow patterns between the two sites:

Antea Group addressed DEP questions regarding groundwater flow on a more localized scale than had been attempted previously. These questions were addressed through a detailed analysis of historical water table elevation data and showed that there was a possibility of groundwater flow from the upgradient site to the client site.

Demonstrating that the MTBE on the two sites had a common origin:

Key elements of this concern included contaminant dilution, dispersion, and biodegradation. Antea Group collected and analyzed groundwater samples for compound-specific stable isotope ratio analysis (CSIRA). The results of this analysis showed that all collected samples exhibited similar carbon isotope ratios with a lack of fractionation. This led to the conclusion that the MTBE both on and offsite was not biodegrading and may have had a common source.

The demonstration that the MTBE present was not biodegrading provided a viable explanation for contaminant transport between the sites. Combined with the other lines of evidence (groundwater flow, timing, similar isotopic ratios), this allowed for a successful argument that the MTBE onsite did not originate from the onsite spill. DEP accepted the justification for onsite closure and closed the site.