VERIFYING AN UNSTEADY STATE TRACER DILUTION METHOD IN FRACTURED BEDROCK WELLS USING PHYSICAL AND COMPUTER MODELS

Characterizing the hydrogeology of fractured crystalline bedrock is both challenging and costly. Nonetheless, it is essential for water resource sustainability and in instituting remediation measures when groundwater becomes contaminated. High costs and the level of effort involved in rigorously charactering bedrock hydrogeologic conditions causes many applied studies to be rather rudimentary in their nature. Although tools are available for rigorously conducting downhole characterization, their application is hindered by costs and logistics. A method introduced by Brainerd and Robbins (2004) demonstrated that a tracer dilution test could be used to greatly reduce the cost of characterizing hydraulic properties within fractured bedrock wells. However, the method required tracer mixing over many hours, continuous monitoring for steady state flow and concentration conditions, and collection of many tracer samples to be analyzed. The objective of this ongoing research is to develop an unsteady state tracer dilution method for characterizing the hydraulic properties and water quality of fractures that will overcome issues plaguing fractured bedrock well characterization. The developing method involves introducing Rhodamine WT tracer dye inside a well, without circulating the well water, using a specially designed mixing tool. An amount of water is then pumped out instantaneously from the well in order to induce a slug test. The recovery of the well, along with the dilution of the tracer from water entering the well from the fractures, is monitored. From the dilution of the tracer dye, the depth of the water contributing fractures can be determined. Based on the rate at which the dilution zone for each fracture advances, relative to the recovery of the water level in the well, the transmissivity, head, and discharge of each fracture can be determined. The water quality of each contributing fracture zone can then be determined by sampling each of the dilution zones. The method is currently being tested in a 4 meter long physical model of a fractured well, field testing will then be conducted in actual bedrock wells.

Brainerd, R.J., Robbins, G.A. 2004. A Tracer Dilution Method for Fracture Characterization in Bedrock Wells. Ground Water 42 No. 5: 774-780.