USGS Groundwater Information: Branch of Geophysics
Since 1998, the USGS has cooperated with the University of Connecticut as part of a research team using a multi-disciplinary, integrated approach to characterize the nature and extent of contamination in the soil, ground water, and surface water in the area of a landfill and former chemical-waste disposal pits at the University of Connecticut (UConn), Storrs, Connecticut. The site investigation was the result, in part, of sampling of domestic wells in the mid 1980's that had indicated the presence of volatile organic compounds (VOCs) in ground water in the area. To characterize the fractured-rock aquifer and help assess remediation alternatives, the team developed a conceptual model using integrated multidisciplinary data.
As part of this investigation, the USGS Office of Ground Water, Branch of Geophysics (OGW BG) collected and analyzed surface and borehole geophysical data and hydraulic data at the landfill study area. Surface and borehole geophysical methods together with hydraulic and geochemical data were used to characterize lithology, fractures, and hydraulic properties of the crystalline bedrock; to determine the nature and extent of ground-water contamination from the landfill and former chemical-waste disposal pits in the study area; and to develop a site conceptual groundwater flow model. This applied research was conducted as part of ongoing OGW BG efforts to develop and test field techniques and interpretive methods for characterizing groundwater flow in fractured-rock aquifers. Previous research at sites such as the USGS Toxic Substances Hydrology Program Mirror Lake research site in New Hampshire showed that data from multiple hydrogeologic disciplines were required to identify fractures, their hydraulic significance, and their hydraulic connectivity. Multidiscipline data collected over a variety of scales are required to understand and characterize groundwater flow and chemical migration in fractured rock (Shapiro and others, 1995; Shapiro and Hsieh, 1998).
Research and Results
Surface geophysics: At the UConn landfill site, surface geophysical methods were used to characterize the hydrogeology and potential discharge of contaminants from the landfill. Azimuthal square-array resistivity soundings indicated a dominant bedrock fracture strike direction ranging from 285° to 30° east of true north, consistent with local geologic maps. Additional soundings verified by aerial photography were used to help interpret the orientation of waste disposal cells in the landfill. Two-dimensional (2-D) resistivity profiles indicated a landfill thickness of 32 to 49 feet. Seismic refraction surveys at the northern and southern ends of the landfill confirmed the presence of shallow bedrock at 25 feet below land surface. Electromagnetic (EM) inductive terrain-conductivity profiles combined with 2-D resistivity surveys detected conductive anomalies interpreted to be leachate plumes near two surface-water discharge areas: (1) a shallow plume that dissipates about 148 feet north of the landfill, and (2) a deeper plume in overburden and shallow bedrock along an intermittent drainage area southwest of the landfill. Two sheet-like conductive anomalies were detected in bedrock and were targeted for further investigation by drilling, borehole geophysical logging, and water sampling.
Borehole geophysics: Boreholes were installed to depths of 30 feet in the glacial deposits and 125 to 300 feet in the bedrock. Conventional geophysical logging; acoustic and optical imaging; single-hole directional radar reflection logging; flowmeter logging under ambient and pumped conditions; and discrete-interval hydraulic testing, sampling, and monitoring were conducted in 11 bedrock boreholes. Collectively, these surveys were used to investigate
Integrated interpretation of the borehole data indicated that one of the two sheet-like, conductive anomalies identified by the surface geophysical investigations was caused by a lithologic change rather than by conductive leachate within the fractures. The second anomaly was interpreted to be caused by both conductive fluid in the fracture and conductive minerals in the rock. The acoustic- and optical-imaging and radar logs were used to investigate the location and orientation of fractures that intersect and surround each well and to identify specific zones for flowmeter and hydraulic testing.
Heat-pulse flowmeter data (fig. 1) collected under ambient and pumped conditions identified several transmissive fractures and ambient vertical flow between some of the fractures. These results indicate a hydraulic potential for cross-contamination within open boreholes that are open to multiple fractures. Ambient flow in the background bedrock well, upgradient of the landfill, was about 340 gallons per day. Five of the bedrock wells had no ambient flow, and the other five had ambient flow of 20 to 35 gallons per day. Specific capacity and open-hole transmissivity were determined in eight of the bedrock boreholes. The specific capacity estimates ranged from 0.03 to 1.6 gallons per minute per foot. The values for open-hole transmissivity ranged over two orders of magnitude, and when proportioned, individual fracture transmissivity ranged from 1 to 340 feet squared per day.
Hydraulic testing: A multifunctional bedrock aquifer transportable testing tool (BAT3), recently developed by the USGS, was used temporarily to isolate sections of a borehole to collect discrete-interval groundwater samples, identify hydraulic head as a function of depth, and conduct single-hole hydraulic tests. Semi-permanent discrete-zone monitoring (DZM) systems, developed and tested by OGW BG as part of this research, were installed in the boreholes to prevent cross contamination, obtain water samples, measure hydraulic head as a function of depth, conduct single-hole hydraulic tests, and assess hydraulic gradients between the fractured bedrock and surficial aquifers. Monitoring of the DZM systems in 11 wells provided a method for long-term measurement of hydraulic head and water quality of the aquifer at specific zones. Discrete-interval head data were collected monthly for over two years. Water samples were collected quarterly by University consultants and analyzed for the known chemical signatures of the landfill leachate and the contamination from the former chemical pits. These data were used to help define the conceptual site model for groundwater flow and to determine and explain the distribution of contamination.
Results: The results of these investigations were evaluated in an iterative and integrated manner to develop a conceptual model of groundwater flow at the site (fig. 2). Collectively, results of the geophysical and hydrogeologic investigations identified two contaminant migration pathways characterized by landfill leachate -- one pathway discharges into a wetland and ultimately dissipates to background levels; and the other seeps into bedrock, where it can be traced based on water chemistry and EM conductivity. The migration pathways are oriented north-south, coinciding with the direction of a dominant fracture set identified by the geophysical surveys. Discrete-interval head data from individual fractures or fractured zones in the bedrock showed that over most of the study area, the groundwater flow direction remained constant seasonally. However, in an area west of the former chemical-waste disposal pits, data from seven boreholes showed seasonal variations in groundwater flow directions. The University and the State are using these results to evaluate and implement remedial alternatives for the site.
This integrated approach to data collection, analysis, and interpretation allowed the conceptual model to be refined iteratively as additional geophysical and hydraulic data were obtained and time-dependent head and chemical data were collected and analyzed.
For more information about Branch of Geophysics applied research at the University of Connecticut landfill study area, contact Carole Johnson (cjohnson or (860) 487-7402 x17) or see the related publications listed below.
References & Related USGS Publications:
Haeni, F.P., Johnson, C.D., and Soloyanis, S., 2003, Multiple field techniques for site characterization and conceptual model development at a landfill and former chemical-waste disposal pits in fractured rock [abs.], in University Consortium Solvents-in-Groundwater Research Program annual meeting, June 17-19, 2003, Orangeville, Ontario, Canada, Proceedings: Waterloo, Ontario, University of Waterloo, Department of Earth Sciences.
Haeni, FP, Lane, J.W. Jr., Williams, J.W., and Johnson, C.D., 2001, Use of a geophysical toolbox to characterize groundwater flow in fractured rock, in Fractured Rock 2001 Conference, Toronto, Ontario, March 26-28, 2001, Proceedings: Toronto, CD-ROM.
Johnson, C.D., 2002, Acoustic and optical imaging tools for fractured-rock aquifer investigations [abs.], in Geological Society of America 2002 Annual Meeting Abstracts with Programs: Denver, Colo., Geological Society of America, v. 34, no. 6, p. 228.
Johnson, C.D., Dawson, C.B., Belaval, Marcel, and Lane, J.W., Jr., 2002, An integrated surface-geophysical investigation of the University of Connecticut landfill, Storrs, Connecticut -- 2000: U.S. Geological Survey, Water-Resources Investigations Report 02-4008, 39 p.
Johnson, C.D., Haeni, F.P., and Lane, J.W., Jr., 2001, Importance of discrete-zone monitoring systems in fractured-bedrock wells - a case study for the University of Connecticut landfill, Storrs, Connecticut, in Fractured Rock 2001 Conference, Toronto, Ontario, March 26-29, 2001, Proceedings: Toronto, CD-ROM.
Johnson, C.D., Haeni, F.P., Lane, J.W., Jr., and White, E.A., 2002, Borehole-geophysical investigation of the University of Connecticut landfill, Storrs, Connecticut: U.S. Geological Survey, Water-Resources Investigations Report 01-4033, 187 p.
Johnson, C.D., Joesten, P.K., and Mondazzi, R.A., 2005, Borehole-geophysical and hydraulic investigation of the fractured-rock aquifer near the University of Connecticut landfill, Storrs, Connecticut, 200 to 2001: U.S. Geological Survey, Water-Resources Investigations Report 03-4125, 133 p.
Johnson, C.D., and Kastrinos, J.R., 2002, Use of geophysical, hydraulic, and geochemical methods to develop a site conceptual ground-water flow model in central Connecticut, in Fractured Rock 2002, Denver, Colorado, March 13-15, 2002, Proceedings [abs.]: Westerville, Ohio, National Ground Water Association.
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.
Johnson, C.D., Kochiss, C.K., and Dawson, C.B., Use of discrete-zone monitoring systems for hydraulic characterization of a fractured-rock aquifer at the University of Connecticut landfill, Storrs, Connecticut, 1999 to 2002: U.S. Geological Survey, Water-Resources Investigations Report 03-4338, 105 p.
Johnson, C.D., Lane, J.W., Jr., Williams, J.H., and Haeni, F.P., 2001, Application of geophysical methods to delineate contamination in fractured rock at the University of Connecticut landfill, Storrs, Connecticut, in Symposium on the Application of Geophysics to Engineering and Environmental Problems, Denver, Colorado, March 4-7, 2001, Proceedings: Wheat Ridge, Colo., Environmental and Engineering Geophysical Society, CD-ROM.
Powers, C.J., Wilson, Joanna, Haeni, F.P., and Johnson, C.D., 1999, Surface-geophysical investigation of the University of Connecticut landfill, Storrs, Connecticut: U.S. Geological Survey Water-Resources Investigations Report 99-4211, 34 p.
Shapiro, A.M., 2001, Characterizing Ground-Water Chemistry and Hydraulic Properties of Fractured Rock Aquifers Using the Multifunction Bedrock Aquifer Transportable Testing Tool (BAT3): U.S. Geological Survey Fact Sheet FS-075-01, 4 p.
Shapiro, A.M., and Hsieh, P.A., 1998, How good are estimates of transmissivity from slug tests in fractured rock?: Ground Water, v. 36, no. 1, p. 37-48.
Shapiro, A.M., Hsieh, P.A., and Winter, T.C., 1995, The Mirror lake Fractured-Rock Research Site -- A multidisciplinary research effort in characterizing ground-water flow and chemical transport in fractured rock: U.S. Geological Survey Fact Sheet FS-138-95, 2 p.