A comparative assessment of the long-term behavior of MtBE and benzene plumes in Florida, USA

The relative long-term behavior of dissolved-phase MtBE and benzene plumes has become a major area of interest in the environmental field in the recent past. This interest stems from the potential for these refined petroleum product constituents to pose a threat to human health and the environment. They have become compounds of concern in the risk-based corrective action process, and the degree to which they are naturally attenuated has been the subject of numerous research projects. This paper presents the results of a database study carried out to ascertain the long-term behavior of dissolved phase MtBE and benzene plumes in Florida and to compare and contrast the results with those of others who have done similar work in other geologic settings. First, a database of monitoring data was assembled from 80 retail petroleum fuel marketing outlets throughout Florida. The information was entered into the relational database and evaluated via time series analysis, via spatial analysis, and via multivariate techniques to ascertain relationships or the lack thereof. Relationships between concentration trends and physical parameters were evaluated for each compound, MtBE vs. benzene plume lengths and areas were compared, and temporally based attenuation rates were estimated and compared. Results of the analyses showed that MtBE and benzene plume geometries are generally more circular and less elongate than plumes in other geographic areas. This difference in geometry is somewhat inconsistent with the findings reported in other studies, and the data from this study suggest that the more circular geometries are likely due to the nearly universally shallow depth to ground water, the relatively high hydraulic conductivities, the relatively high level of precipitation and infiltration, and the relatively flat hydraulic gradient in Florida. MtBE plumes are generally longer than their benzene counterparts and generally larger in area. The results also show that the differences between MtBE and benzene plume behavior are virtually independent of site physical characteristics (e.g. depth to water, lithology, hydraulic gradient, and now velocity). This would seem to suggest that natural attenuation plays a significant role in plume control. Empirically derived attenuation rates for MtBE are slightly smaller than their benzene counterparts, which might account for the differences in plume lengths and areas. In fact, the data show that both MtBE and benzene plumes do stabilize, and given that MtBE plume behavior is independent of site physical characteristics, biodegradation probably plays the dominate role in the natural attenuation process.