Detecting a hydraulic short circuit along a monitoring well with the recovery curve of a pumping test in a confined aquifer: method and example

Abstract

It is important to verify if a monitoring well was correctly sealed into the ground to avoid preferential flow between aquifer layers (hydraulic short circuit). Previous papers have shown how to check this by variable-head permeability tests interpreted with the velocity-graph method. This paper presents a new method to make this check with the recovery curve of an aquifer test. The theory of this new method is presented. When there is a hydraulic short circuit, the drawdowns and recoveries due to a pumping test in a confined aquifer do not yield unique values of transmissivity and storativity as they should theoretically. An example is provided, that of monitoring well E1 of the Pelican River aquifer test. The pumping phase was interpreted using the methods of Theis and Cooper-Jacob. The recovery phase was interpreted by two methods: (i) the usual graph of sprime versus log (t/tprime) gave transmissivity T and either the Jacob method or the U.S. Department of the Interior Ground Water Manual gave storativity S; and (ii) the method of (sp - sprime) versus log tprime gave both T and S. As a result, four values of T and S were obtained. They differed by 9% for T and by 186% for S. According to the proposed method there was a hydraulic short circuit close to the monitoring well: it gave an initial water level that differed from the real piezometric level by 40 cm. Drawdown and recovery values were corrected for reinterpretation. The new log-log and semilog graphs yielded identical T and S values, thus validating the diagnosis of a hydraulic short circuit and illustrating the detection method.Key words: monitoring well, pumping, recovery, quality control, transmissivity, storativity.