A Composite Plume Approach for the Analysis of Dissolved Contaminants in Ground Water vs. Distance from Source Areas

The analysis of dissolved contaminant transport away from source areas containing nonaqueous phase liquids is an impor- tant component in the evaluation of the risk posed to downgradient receptors, of monitoring plan development, and of remedial alternative selection. Typically, individual source-plume systems are evaluated on a site-specific basis; however, much can be learned by looking at the collective data from multiple sites. This has been done with leaking underground storage tank sites i n California, Texas, and Florida, where the analyses led to important conclusions concerning dissolved MTBE (methyl tertiary butyl ether) and benzene plume behaviors that could not be drawn from individual site data sets. In these previous works, each site data set was reduced to a plume length and then a statistical analysis of plume lengths was performed. In this paper, the use of multiple-site collective data analyses is also pursued; however, rather than use a single metric plume length analysis, a co m- posite plume data analysis approach is proposed and examined. This alternate approach provides a means for making use of data from ground water plumes that are incompletely defined. In addition, few of the assumptions required by single plume studies for interpolation of data are necessary. The composite plume approach is especially well suited to the analysis of large sites or regions with multiple source areas within similar hydrogeochemical settings. A case study of petroleum hydrocarbon transport at a large former oil field with more than 90 source areas illustrates the application and benefits of this approach.