Tracer Test Monitoring Using Wells with Long Screened Intervals : Benefits and Disadvantages

Analytical and economic limitations often make fully-penetrating wells an attractive alternative for monitoring tracer test responses in aquifers. Modelling of breakthrough curves generated using long-screened (LS) wells in an unconfined gravel aquifer demonstrated that prolonged tailing could be reproduced with a series of partial breakthrough curves. The results suggested tracer arrived at multiple horizons. Subsequent measurements using a mobile downhole fluorometer demonstrated over 95% of tracer arrival occurred over a 2m depth interval. Simulations of complimentary single-well dilution test results, incorporating vertical flow measurements, indicated that tracer arrival occurred in a zone no thicker than 50cm before being transferred to a shallower level in the aquifer via the well. The probability of encountering this tracer-bearing horizon by installing four 1m long piezometers was approximately 1 in 2. The results highlight the importance of appropriate conceptual models and potential benefits/disadvantages faced by the hydrogeologist when dealing with LS wells in tracer tests.