Toxic Substances Hydrology Program
Predicting Plume Growth Rates
A Simple Method for Calculating Growth Rates of Petroleum Plumes
Predicting the rate that contaminant plumes will grow beneath oil and gasoline spill sites is an important component of evaluating the potential success of monitored natural attenuation as a remediation option. Reliably predicting plume growth rates can be very difficult and requires knowledge of many complex processes and factors that control the natural degradation and migration of contaminants. U.S. Geological Survey (USGS) researchers working at two sites (the Bemidji Crude Oil Spill Research Site, Bemidji, Minnesota, and the Laurel Bay Gasoline Spill Research Site, South Carolina) have developed a new method to predict the rate that benzene and other volatile organic compounds might spread in a contaminant plume undergoing natural attenuation. The rate that a plume grows (or shrinks, if natural processes destroy contaminants) is dependent on the contaminant concentration, how fast ground water is moving, and the abundance of other dissolved chemicals (electron acceptors) that control the flow of electrons critical to contaminant degradation.
What They Found
As electron acceptors such as iron oxides are consumed, many plumes progress to methanogenic conditions as the dominant contaminant degradation process. Methanogenic degradation is characterized by the production of methane as a reaction product. This was observed at both sites as the available iron oxides in the aquifer sediments were depleted. The scientists further observed that under methanogenic conditions, degradation of benzene became negligible. Consequently, the benzene plumes at the sites again began to expand downgradient. This potential expansion of benzene plumes has important implications for remediation plans that rely on monitored natural attenuation as the clean-up method (see side bar). Most applications of monitored natural attenuation assume that the plume will continue to shrink with time. These results show that for some cases this might not be a good assumption. Appropriate monitoring can indicate when these conditions occur, and the new method provides a simple approach to estimate when these conditions may occur so that appropriate action can be taken.
USGS scientists developed a simple method to estimate the growth rates of contaminant plumes as methanogenic conditions develop. By considering the reactions that occur at the leading edge of the plume (the reaction front), the amount of benzene and iron in aquifer sediments, and the site’s ground-water flow field, the growth rate of a benzene plume at a hydrocarbon contamination site can be estimated. This simple method enables clean-up professionals and regulators to assess the potential for reexpansion of benzene plumes. Remediation professionals can then assess the long-term performance of monitored natural attenuation as a remediation alternative.