UTILIZING AMBIENT STABLE ISOTOPE DISTRIBUTIONS FOR PUMPING TEST INTERPRETATION IN A GLACIAL OUTWASH AQUIFER IN MINNESOTA

Utilizing an array of monitoring wells and a production well at the University of Minnesota Hydrogeology Field Site, near Akeley Minnesota, a novel pumping test was conducted. Stable isotope samples from a dense array of monitoring wells were collected before the start of a large scale pumping test in the unconfined aquifer. During the pumping test stable isotopes in the produced water were monitored. From previous work we knew that 1) there is a natural variation in ground water isotopes of hydrogen and oxygen due to recharge from nearby Crystal Lake versus meteoric recharge from the the surrounding uplands; 2) the production well is screened near the base of the 12 meter thick aquifer leading to changes in where the produced water is derived from within the aquifer over the course of a pumping test. Pre-pumping initial conditions showed the most evaporatively fractionated ground waters were located in the middle of the aquifer with waters much closer to the local meteoric water line at the base and upper portions. The evaporatively fractionated lake water has δD=-2‰ and δ¹⁸O=-37‰. Ground waters near the meteoric waterline have δD=-11‰ and δ¹⁸O=-80‰. The monitoring wells fall on a simple mixing line between these two end points ranging from 99% meteoric to more than 60% lake water. A pumping rate of 0.01 m³/sec was sustained for a four day period. Initially the produced waters were close to the meteoric water line and rapidly evolved to more evaporatively influenced waters. These results allow us to quantify how much of the produced water is moving laterally versus vertically over the course of the pumping test. The isotopic signatures help separate storativity (S) and specific yield (Sy).