SINGLE-WELL INJECTION-DRIFT TEST TO ESTIMATE GROUNDWATER VELOCITY

A simple algebraic equation is presented here to estimate the magnitude of the average linear groundwater velocity based on data from a single-well injection-drift test. A volume of tracer-amended water was injected by forced-gradient into a single-well followed by monitoring of the tracer under natural-gradient conditions as its up-gradient portion drifted back through the well. The breakthrough curve data in the single well during the drift phase was analyzed to determine the mean arrival time of the tracer. The mean up-gradient transport distance back through the single well and the mean arrival time of the tracer was used in a simple algebraic equation to estimate the groundwater velocity. The groundwater velocity based on the single-well injection-drift test was estimated to be 0.52 ± 0.09 feet per day. Two transects of observation wells were used to monitor the tracer transport down-gradient of the injection well and indicated that the true groundwater velocity was 0.53 ± 0.06 feet per day; the multi-well results were in good agreement with those from the single-well and partially validated the simple algebraic equation. Moreover, a numerical transport model showed a good fit of the single-well breakthrough curve data with a slightly higher velocity of 0.58 feet per day and further validated the simple algebraic equation. The new method presented here is relatively simple, rapid, and utilizes tracer data from a single well, as opposed multiple wells, to estimate groundwater velocity.