Objective Monitoring of Remediation in Fractured Rock

The complexity of fractured rock provides substantial challenges in evaluating the effectiveness of remediation technology applied to contaminated groundwater.  The large range in the hydraulic conductivity of fractures, coupled with complex fracture connectivity, yields convoluted, three-dimensional flow paths over lengths of meters to kilometers. In addition, investigations in fractured rock are often characterized by sparse monitoring locations. In many instances, implementing remediation relies on the application of remediation amendments in boreholes used for monitoring. Monitoring in boreholes where amendments have been added yields a biased interpretation. Monitoring groundwater remediation requires detailed knowledge of the groundwater flow regime, including groundwater residence times. Such information provides the basis for the objective selection of monitoring locations. In addition, objective monitoring of groundwater remediation requires the characterization of the spatial distribution of the contaminant mass prior to the start of the remediation. The roles of different types of porosity in fractured rock aquifers must be considered in identifying the contaminant distribution; some remediation technologies will not access portions of the contaminant mass. The contaminant mass residing in fractures having a range of hydraulic properties must be identified along with the distribution of contaminant mass in the primary porosity of the rock and sorbed to rock surfaces. The implementation of bioaugmentation in conjunction with an ongoing pump-and-treat operation is used as an example of objective remediation monitoring in TCE-contaminated groundwater of fractured mudstones underlying the former Naval Air Warfare Center, West Trenton, NJ. The characterization of the mudstone units demonstrated that monitoring wells along flow paths between injection and pumping locations were required to assess the evolution of biogeochemical conditions. In addition, the initial TCE mass in both the rock matrix and fractures was characterized prior to the start of the bioaugmentation to assess the percentage of the mass removed.