Formatted Title
Using Surface Geophysics as a Critical Tool in Development of a Conceptual Site Model for Contaminant Fate and Transport in a Faulted Karst Setting
Background/Objectives
The groundwater system at the Site consists of two aquifers separated by a shale aquitard: a shallow limestone karst system and deeper highly productive limestone/dolomite system which is used for domestic and municipal supply. The hydrogeology of both aquifers is complicated by numerous fracture and fault structures. COCs are present in groundwater of both aquifers, and dense non-aqueous phase liquids (DNAPLs) are present at the Site. It is technically infeasible to fully map DNAPL in bedrock fractures on the Site or delineate each of the pathways for off-Site migration. Sufficient remedial investigation (RI) data are needed for both aquifers to present a credible conceptual site model (CSM) for contaminant fate and transport and remedy evaluations during the Feasibility Study (FS).
Approach/Activities
Historical data collection at the Site included surface geophysics followed by monitoring well installation and sampling. Well data suggested the presence of an unmapped COC source mass on the Site and unmapped migration pathways for COC in both aquifers. The CSM, at that time, had not achieved sufficient credibility to develop a remedy for on-Site conditions or explain observed off-Site conditions. Approximately 10 miles of new electrical datasets were obtained with imaging to depths greater than 600 feet below ground surface (bgs). Data were processed and reviewed using 3-D visualization software. Anomalously highly conductive features were identified as key targets for investigation. Borings were advanced using sonic methods and monitoring wells installed into the shallow karst system both on and off Site. Wells on the Site identified elevated COC concentrations in high yield karst features, and new remediation wells have been installed. Monitoring wells were also installed off Site along a transect transverse to flow direction, which intercepted multiple open void/cave features. Groundwater samples were obtained identifying the presence of COCs, and transducers were used to observe rapid changes in groundwater levels during and following precipitation events. Borings were advanced into the regional aquifer using air rotary methods to 900 feet bgs. Downhole geophysics and high resolution downhole camera surveys were used to map flow zones, and straddle packers were used to collect groundwater samples identifying the presence of COC. Regional geologic data was reviewed in conjunction with boring logs for more than 150 private wells to map the structure of the shale aquitard, providing further insight into the regional geologic framework.
Results/Lessons Learned
Despite complications of groundwater flow and contaminant fate and transport in a karst and fractured rock setting, it is feasible to develop a credible CSM through strategic data collection coupled with regional information compiled by others. Surface geophysics data can be used to map key features. Regional data can be reviewed in concert with surface geophysics information to strategically place a select number of off-Site monitoring wells for focused sampling. The collective dataset is being used to develop the hydrogeologic framework for the fate and transport CSM, covering an area of more than 10 square miles. The work product facilitates completion of the RI and development of the FS.