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

The U.S. Geological Survey, in cooperation with the University of Connecticut, used a suite of hydraulic methods to characterize the hydrogeology of a fractured-rock aquifer near the former landfill and chemical-waste disposal pits at the University of Connecticut, Storrs, Connecticut. Multiple methods were used to determine head, driving potential, and transmissivity, including manual open-hole water-level and discretezone water-level measurements from 11 boreholes; continuous discrete-zone water-level measurements from 6 of the boreholes; estimated head and transmissivity for 11 boreholes using heat-pulse flowmeter profiles and pumping records; and differential head testing using a straddle-packer apparatus from 4 boreholes. These data were analyzed to identify and characterize relations between long-term water-level patterns and precipitation, topographic setting, contaminant distribution at the site, and a conceptual ground-water flow model.

Data collected using the heat-pulse flowmeter, the straddle-packer apparatus, and discrete-zone monitoring (DZM) systems helped to establish, refine, and verify a conceptual model of ground-water flow in the study area. Monitoring of DZM systems installed in 11 boreholes provided a method for longterm monitoring of hydraulic head and water quality of the aquifer at fracture zones of different depths. These data were used to help define the conceptual site model for ground-water flow and to determine and explain the distribution of contamination.

Hydrographs constructed for discretely isolated zones in the boreholes showed the magnitude of seasonal changes of water levels and driving potential in response to precipitation and drought. Heads in discrete zones and in different boreholes varied both in magnitude of response and in timing of response to precipitation. Water levels in open boreholes and in DZM systems showed a semi-diurnal pattern that coincides with gravimetric tidal plots generated for this area. No fluctuations that might indicate pumping were identified in the continuous water-level records. Lack of hydraulic response between boreholes during cross-hole testing in the area of the former chemical-waste disposal pits indicates poor hydraulic connection between the boreholes that were tested. In general, data indicated the presence of downward driving potentials in the recharge areas and in the area of the ground-water divide, and upward driving potentials in discharge areas north and south of the landfill.

The results of this study illustrate the importance of discrete-zone isolation and monitoring in fractured-rock aquifers to prevent cross contamination while permitting head measurements and water-quality sampling that can be used to identify and characterize contamination or pathways for contaminant migration in a fractured-rock aquifer. Without DZM systems installed in the boreholes, only open-hole heads can be measured. The open-hole heads may be misleading when determining potential flow directions at contamination sites, because they are a composite of the heads associated with each of the fractures intersecting the borehole. The flowmeter tool and straddle-packer apparatus are effective screening tools for generating a snapshot of the hydraulic conditions, including vertical flow, transmissivity, and heads; however, they cannot prevent flow and potential cross-contamination and cannot easily be used to monitor long-term conditions.

This work was conducted as part of a larger multidisciplinary investigation to characterize the nature and extent of contamination in the soil, surface water, and ground water in the overburden and fractured bedrock in the area of the landfill and former chemical-waste disposal pits near the University of Connecticut. The methods and hydraulic data presented in this report were used along with surface- and borehole-geophysical data and geochemical data to understand and characterize the ground-water flow in overburden and fractured bedrock; to assess possible chemical migration; to develop a site conceptual ground-water flow model; and to assess remediation alternatives.