Risk-Based Site Characterizations
Risk-based site characterizations:
For contaminant release sites falling under regulatory agency requirements, LWS personnel have conducted over 400 site characterizations to assess the extent of contaminant impacts and risks to nearby potential receptors. In addition to determining the current areal extent of impacts and locations of nearby potential receptors (water wells, basements, springs, etc.), these risk-based site characterizations often require collecting hydrogeologic, physical, and biologic data to assess the contaminants’ migration and attenuation potential.
Leaded Petroleum Release in Michigan:
A release of petroleum in tight, glacial till soils was characterized via monitoring well installations and determined that the petroleum impact to ground water was minimal and did not extend off-site, so the organic petroleum constituents were not a concern. However, based on the age of the gas station it was likely that leaded fuels were released and it was found that the concentrations of lead in ground water from monitoring wells were above the regulated standard. Therefore, the finding of lead in ground water was a potential concern. However, based on LWS personnel’s correspondence with local consultants it was learned that lead in shallow ground water is not uncommon in northern Michigan where glacial deposits had migrated across ore deposits in the Canadian Shield. Subsequently, LWS personnel installed additional monitoring wells in the upgradient and cross-gradient directions and determined that the release case could be closed as the concentrations were not elevated above ambient lead concentrations in ground water.
Petroleum Release in a City in Illinois:
A release of petroleum was characterized and extended approximately 400 feet offsite. The potential exposure pathways for indoor inhalation, and soil ingestion/dermal contact for offsite utility workers, were eliminated based on the depth to ground water (well below utility trench depths) and petroleum concentrations in ground water below screening level for the indoor inhalation pathway. However, the offsite contaminant plume exceeded drinking water standards. Based on LWS personnel’s conversations with State and Local agencies, it was determined that a city ordinance prohibited the installation of new drinking water wells within the areas served by the municipal water supply. Therefore, the ground water ingestion pathway could be eliminated and the release case closed.
Petroleum Release in North Carolina:
After the leaking tank and secondary source (clayey deltaic soils) were excavated, residual impacts of petroleum in onsite ground water remained above drinking water standards. Collection of soil samples for grain-size, permeability, and organic carbon content allowed LWS personnel to use a simple fate and transport model to demonstrate low risk of significant further plume migration. However, the soils had a low count of heterotrophic bacteria to break down the petroleum so, while the plume was not likely to migrate further, it was not likely to attenuate quickly. Rather than continuing to monitor the concentrations in ground water, likely for decades based on the low migration potential and slow degradation rate, LWS personnel evaluated other means to reduce exposure pathways onsite. It was determined that the case could be closed with a deed restriction on the property to prevent residential use of the property and to prohibit drinking water well installation on the property.