A STEPWISE DISCHARGE TRACER DILUTION METHOD FOR DERIVING FRACTURE PROPERTIES IN CRYSTALLINE BEDROCK WELLS

While most of the rural domestic wells in New England derive their water from fractured crystalline bedrock, our knowledge of the hydraulic characteristics of the subsurface water-bearing fractures is limited. In recent years, there have been great advances made in the ability to characterize the hydraulic properties of fractures in bedrock wells. This information is vital to the ability to evaluate ground water supplies, wellhead protection, and in the prevention and remediation of ground water contamination in fractured crystalline rock. Although the United States Geological Survey (USGS) has developed state-of-the-art, downhole techniques to characterize fracture properties, these methods may be cost prohibitive to be applied on a large scale and for use in the private sector. This investigation was undertaken to develop a cost-effective method of downhole fracture characterization using a tracer. This method will be able to identify water bearing fractures that intersect the well, provide information on whether fractures are incoming (producing) or outgoing (receiving) under ambient conditions, determine the transmissivity and hydraulic head of those water bearing fractures, and quantify fracture water quality.

The field research for this study was conducted in three, fractured bedrock wells located at the Branch of Geophysics of the USGS, at the University of Connecticut in Storrs, Connecticut. The tracer (uranine dye) was injected into the well, while at the same time water is pumped out of the well. After steady-state conditions are reached, a tracer concentration profile was developed for the well. Using the dilution of the tracer to determine the flow from a fracture, this flow rate plus the drawdown in the well, can then be used to determine the transmissivity of that fracture by applying equations for a constant head flow test.

The results of this investigation were then compared with the results from the USGS for the same three wells using their methods. This provided a unique opportunity to evaluate the tracer test results with those derived by the USGS methods.