Formatted Title
Optimizing Remediation in Fractured and Weathered Bedrock: Lessons from Successful Injection Projects
Background/Objectives
Background/Objectives. In situ remediation of contamination in weathered bedrock and fractured bedrock is a challenge facing remediation practitioners. Common drilling methods like direct-push and auger encounter refusal. More expensive drilling results in less investigation points/wells and injection points that are farther apart. Due to drilling refusal and different drilling equipment, at many contaminated sites investigation and subsequent remediation is divided into overburden and bedrock components. Bedrock sampling generally commences in competent bedrock; however, at many sites a layer of weathered/fractured bedrock exists at the overburden bedrock interface. This weathered bedrock zone can be five feet or greater in thickness, and for sites with glacially compacted overburden hydraulic conductivity can be higher in weathered bedrock than in the overlying overburden or the competent bedrock below. Therefore, the weathered bedrock may contain elevated concentrations of groundwater contaminants and serve as a major migratory pathway for dissolved phase contaminants and non-aqueous phase liquids.
Approach/Activities
Approach/Activities. This presentation will focus on two case studies where remedial design and injection events were optimized through specifically targeting a weathered bedrock zone between overburden and competent bedrock. At Site 1, a multi-acre PCE plume exists within aquifers in overburden and underlying granite bedrock. At Site 2, TCE remediation was performed through a dual phase extraction system with extraction screen focused on shallow bedrock and a pump and groundwater extraction wells in schist bedrock. Injection of enhanced reductive dechlorination amendments were performed at both sites using different injection methods to optimize reagent delivery for the geologic properties at each site: weathered bedrock injection wells at Site 1 and direct-push injection points at Site 2.
Results/Lessons Learned
Results/Lessons Learned. Injection at both sites resulted in significant reduction in contaminant concentrations along with overall shrinking of the impacted plume. In addition, contaminant concentrations in underlying competent bedrock were observed following injection efforts targeting the weathered bedrock unit. The presentation will highlight the interpretation of remedial investigations at each site and how available site data were utilized to optimize remedial outcomes.