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Integrated Methods for Site Characterization and Conceptual Model Development for a Contaminated Fractured-Bedrock Aquifer

C.D. Johnson (cjohnson@usgs.gov)
U.S. Geological Survey, OGW Branch of Geophysics, 11 Sherman Place, Unit 5015, Storrs, CT 06269 United States

J.R. Kastrinos (jrk@haleyaldrich.com)
Haley and Aldrich, 465 Medford Street, Suite 2200, Boston, MA 02129 United States

F.P. Haeni (fphaeni@snet.net)
FP Haeni, LLC, Deep River, CT 06417 United States

Abstract

A multi-disciplined and team-based approach was used to integrate geophysical, hydrologic, and chemical data to characterize lithology, fractures, and hydraulic properties of fractured crystalline bedrock and to determine the nature and extent of ground-water contamination from a landfill and former chemical-waste disposal pits at the University of Connecticut. Detection of volatile organic compounds (VOCs) in domestic bedrock wells in the mid-1980s led to this investigation, in which a team comprised of hydrologists, engineers, geophysicists, geologists, chemists, toxicologists, and community-involvement personnel collected, analyzed, and evaluated data; developed and refined a conceptual model of the ground-water flow and contaminant distribution at the site; and evaluated alternatives and implemented a final remediation plan. The characterization phase began in 1999 and the remediation phase is currently ongoing. An integrated and iterative approach of using multiple methods in phases was important for corroborating the interpretation of individual methods and essential for guiding the design and implementation of additional testing at the site. The use of geophysical data early in the investigation allowed the study team to obtain detailed subsurface information using a minimum of boreholes. Surface geophysical methods were used to target potential discharge of contaminants from the landfill for further investigation. Borehole geophysical methods were used to investigate the anomalies identified by surface geophysical methods, the location and orientation of fractures that intersect and surround each well, the direction and magnitude of ambient flow in the wells, and the transmissive fractures that could provide pathways for contaminant migration. Borehole geophysical and hydraulic data were used to design discrete-zone monitoring systems for the collection of hydraulic head and chemical data and to prevent cross contamination through the boreholes. The results of the geophysical surveys together with hydraulic and geochemical data from discrete-zone monitoring systems in bedrock wells, chemical data from private water supply wells, surface-water-quality data, and drive-point profiling data were used to develop a conceptual model of the ground-water flow and contaminant distribution at the site. The conceptual model was refined iteratively as additional data were obtained including transient head and chemical data. The conceptual model was used to assess contaminant migration from the landfill and chemical-waste disposal pits and to evaluate remedial alternatives. The University has initiated remedial actions on both the landfill and the chemical-waste disposal pits and has a long-term monitoring program, results of which will be evaluated against the conceptual model. This investigation demonstrated (1) the importance of a multi-disciplinary approach for characterizing contamination in a complex fractured-rock aquifer; (2) the development and use of an iteratively updated site conceptual ground-water flow model; and (3) the value of discrete-zone isolation and long-term monitoring in fractured-rock aquifers to prevent cross-contamination while permitting head measurements and water-quality sampling. This allowed characterization of contaminant migration over time, which might not have been identified without the discrete-interval data.

Final copy as submitted to the American Geophysical Union for publication as: Johnson, C.D., Kastrinos, J.R., and Haeni, F.P., 2005, Integrated methods for site characterization and conceptual model development for a contaminated fractured-bedrock aquifer [abs.]: EOS Transactions, American Geophysical Union, Fall Meeting Supplement, v. 86, no. 52, abstract H41B-0405 INVITED.