A Case Study of Traditional and Alternative Monitoring Techniques for Solvent Contamination within Fractured Bedrock

Multiple plumes of chlorinated organic compounds (PCE, TCE, and cis-1,2-DCE) are present above drinking water standards in groundwater underlying Camp Stanley Storage Activity (CSSA). One such plume has migrated off-post and is now impacting more than 20 drinking water wells within the Middle Trinity Aquifer. The off-post groundwater plume is defined by a series of traditional monitoring wells that monitor the Lower Glen Rose, Bexar Shale, and Cow Creek members of the Middle Trinity Aquifer. In the vicinity of CSSA, these geologic formations have been breached by stepwise normal displacements associated with the Balcones Fault Zone. As expected, the presence of faulting, fractures, and minor karstic features has resulted in a sometimes erratic and fluctuating distribution of contaminants. The hydrogeologic model is further complicated by a seasonally-dependent groundwater level swing of more than 150 feet, and the presence of a 60-foot thick confining unit within the Middle Trinity Aquifer. In an effort to characterize the occurrence of solvents associated with this plume, CSSA has adopted a unique monitoring strategy to optimize the amount of both geologic and contaminant data available from standard boreholes. The most recent study incorporated the traditional methods of continuous rock coring, borehole geophysical and video surveys, and discrete interval packer testing along with less traditional characterization tools such as optical televiewers, borehole flow zone determinations, and multi-port wells. These innovative tools were used to maximize data collection at each point location; thereby accelerating the characterization process and saving money. As part of this study, the effects of the open borehole completion typically used by groundwater consumers were investigated at an off-post location of known contamination. The effort included three boreholes continuously cored in the vicinity of the presumed source area, and one borehole adjacent to the off-post well with the greatest concentration of solvent contamination. Standard geophysical methods and borehole video were used for geologic correlation, the identification of porous features, and selection of discrete interval groundwater sampling depths using the straddle packer approach. Optical televiewer and hydrophysical technologies were used to further refine the borehole characterization approach. The results of all these data types were integrated to design and implement a WestbayTM multi-level monitoring system at both onand offpost locations. The characterization approach was successful in identifying structural features and intervals of preferential flow which conduct contaminants away from the source area. Both the multi-level monitoring approach and well construction study indicate that significant contamination exists chiefly in shallow bedrock pathways, and that concentrations attenuate by dilution within the main body of the aquifer. These results may indicate that simple changes in the construction of off-post drinking water wells may significantly improve groundwater quality and reduce or eliminate contaminant exposure. Cost savings resulted from maximizing the use of single boreholes as multi-level observation points to monitor the entire thickness of the aquifer in lieu of more costly traditional well clusters. Findings of this study can be used to guide the local public utilities and well installers in preventative well construction techniques that minimize the effects of aquifer contamination.