STABLE ISOTOPE SIGNATURES OF GROUNDWATER IN THE INGLEFIELD SANDSTONE AQUIFER AND PRECIPITATION IN SOUTHWESTERN INDIANA: A DECADE OF CHANGE

Recent and long-term changes to water levels within the Inglefield Sandstone aquifer of Southwestern Indiana prompted an investigation of the stable isotope ratios of Oxygen and Hydrogen found in groundwater and precipitation of the Evansville, Indiana area. Stable isotope data from the same system in 2004 – 2006 permit identification of decadal isotopic differences and similarities.

The University of Southern Indiana Groundwater Monitoring Lab houses a deep-shallow piezometer nest within the Pennsylvanian Inglefield Sandstone, screened at 33.5 m and 15 m respectively. Both the shallow and deep groundwater were sampled monthly for stable isotope analyses, along with event-based precipitation sampling. Analyses show that in nearly every month, deep groundwater has a more enriched δ¹⁸O signature than shallow groundwater, and all samples fall in the δ¹⁸O range -6.17 to -6.90 ‰. In 2004-2006 samples, deep groundwater was also more enriched than shallow groundwater, but all groundwater samples are now more depleted than the 2004-2006 samples. This depletion over the last 10 years is as much as 0.98 ‰ in shallow groundwater in April and as little as 0.35 ‰ in July. Although data are limited, stable isotope ratios of late-winter and early spring precipitation show depletion from 2004-2006 to 2015-2016 with δ¹⁸O being more depleted by as much as 2.8‰ in March. The local meteoric water line in 2006 was δD = 6.4 δ¹⁸O + 6.8, and in 2015 is δD = 7.9 δ¹⁸O + 11.3.

The changes in local groundwater stable isotope signatures may result from long-term changes in seasonal precipitation patterns, and changes in groundwater recharge. Local changes in groundwater recharge are likely driven in part by a recent shift from domestic groundwater use to a public surface water supply, and also correspond to a long-term increase in shallow groundwater levels.