Municipal Infrastructure Impacts

Gasoline vapors detected in a small town municipal sanitary sewer line raised significant health and safety, as well as environmental, concerns. Pollard Environmental, pursuant to Virginia Department of Environmental Quality (VDEQ) directives, was tasked with investigating a potential source.

By virtue of its proximity, a retail convenience store/gas station was considered a possible source, and an investigation indicated some gasoline (in the form of small “beads”) on water in the facility’s fuel underground storage tank (UST) basin. A release was apparent, but available leak detection information and observations indicated that it was minor and no longer actively occurring.

Pollard Environmental evaluated the link between conditions in the tank basin and impacts to the sewer using groundwater depths and flow data, groundwater chemical data, and soil vapor survey results. The distance between the UST basin and the sewer is approximately 100 ft, and although the data suggested that the release area appeared circumstantially connected to the impacted sewer, the widespread nature of the impacts was difficult to reconcile with the timing and apparently limited scale of the release, especially considering the relatively low permeability of the soils in the area.

At the same time, the technical picture remained incomplete. Conditions between the tank basin and the sewer line were not well defined because access for investigation was restricted by a roadway and sidewalks. Those surface features limited subsurface exploration and left open important questions about whether preferential contaminant migration pathways may exist beneath or around the infrastructure. Furthermore, the extent of the impacted sewer system throughout the municipal area introduced the possibility of other sources; in short, the subject facility remained a possible source, but not the only one.

Further investigation and mitigation measures are currently underway for this case. Given that the most important practical issue is risk to human health due to the impacted sewer, the gasoline vapors (also referred to as volatile organic compounds or “VOCs”) are being mitigated via active ventilation while natural attenuation of the contaminants continues.

Pollard Environmental also recommended recovery of the minor amount of gasoline product (aka, light non-aqueous phase liquid or “LNAPL”) from the UST basin and further investigation for subsurface migration pathways between the UST basin and the impacted sewer. These items have been authorized by the VDEQ and the work is underway.

This case is a real-life example of the challenges inherent in identifying and mitigating subsurface petroleum releases, especially when municipal infrastructure may play a significant role in the movement of contaminants to sensitive receptors. The imminent risk to human health emphasizes the importance of safe, expeditious, and effective responses, which in turn requires a strong understanding of the hydrogeological and anthropogenic systems and the chemistry of the materials involved.