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Abstract
MTBE is a gasoline additive addressed in Delaware's Risk-Based Corrective Action Program. Due to inconclusive toxicological data, MTBE is difficult to address in a risk-based fashion pertaining to actual cancer risk. MTBE action levels in Delaware's Risk-Based Corrective Action Program (DERBCAP) are based on the low end of the aesthetic range of the contaminant (10 ppb), not a health-based number. Addressing MTBE in this manner can make it very amenable to risk-based decision making. When MTBE concentrations exceed Tier 1 RBSL values, the site must be remediated to those values or modeled in Tier 2 to calculate Site Specific Target Levels (SSTLs) and demonstrate a stable and shrinking plume. A case study is provided to evaluate the applicability of MTBE in DERBCAP at a site where several shallow, private supply wells were contaminated with MTBE following a gasoline release several hundred feet up-gradient. Following well replacement and site characterization, Tier 2 modeling was performed to establish clean-up goals (RBCA Toolkit). By using initial concentrations at the POC wells and forward calculating concentrations down-gradient, clean up goals were established for the 180 and 230 m POCs at 500 and 1100 ppb, respectively. These clean up goals will be protective of the 10 ppb maximum exposure limit (MEL) established at the down-gradient point of exposure (POE). Modflow (WHI) and MT3D (Zheng 1990) were used to determine if the pump and treat remediation schedule set forth by the consultant was sufficient to remediate to the calculated clean up levels. Transient simulations showed that pumping from wells on the leading and trailing edge of the plume “core” for 1 week each month for 2 years resulted in a drop in POE and source concentrations of 40 to 60%. Although the drop is significant, the down-gradient POE would still be in jeopardy of exceeding the MEL. This suggests that additional and/or more frequent remediation may be necessary.