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