THE STABLE ISOTOPIC SIGNATURE OF HURRICANE PRECIPITATION IN SOUTHWESTERN INDIANA AND ITS USE AS AN ENVIRONMENTAL TRACER

The Evansville, Indiana region typically receives a portion of its annual precipitation from moisture originating in the Gulf of Mexico. However, during 2005, just four individual hurricanes (Arlene, Dennis, Katrina, and Rita) delivered an anomalously high proportion of local summer rainfall (24.2 cm of 43.4 cm from June through September, 56%). Rain from these four hurricanes delivered approximately 23% of the local annual precipitation for 2005. We hypothesize that the precipitation from these storms will contain a distinctively heavier ¹⁸O/¹⁶O and ²H (Deuterium) signature than “normal” summer rainfall, and therefore might prove useful as an environmental tracer.

Isotopic analyses of eighteen water samples included three rain samples from two hurricanes (Katrina and Rita), four “typical” rainfall events, and two snow samples. Additionally nine bedrock ground water samples were collected from a deep – shallow piezometer nest in the Inglefield Sandstone located on the University of Southern Indiana campus. The Inglefield Sandstone Member is the lowest unit of the Pennsylvanian Patoka Formation, and is a thinly bedded, fine - medium grained, micaceous quartz arenite that forms a locally important domestic water supply. The deep – shallow piezometers are screened at depths of 33.5 m and 15 m, respectively.

Stable isotopic ratios of precipitation yielded a meteoric water line (δD = 6.4 δ¹⁸O + 6.8) specific to the Evansville, Indiana region. The δD ratios for typical rain range from –33 to –18 ‰, and δ¹⁸O ranges from –6 to –4 ‰. Some sampled hurricane precipitation has a distinctive, heavier isotopic signature, with a range of δD from +11 to –31 ‰, and δ¹⁸O from + 0.4 to –4.4 ‰. Isotopic ratios of snow “anchor” the light extreme of sampled meteoric water with δD from –51 to –48 ‰, and δ¹⁸O at –9.3 ‰. Stable isotope ratios of both deep and shallow ground water generally fall on the local meteoric line, and cluster closely near a δD = –37 to –36 ‰, and δ¹⁸O at –6 ‰. Deep ground water consistently displays slightly lighter ratios than shallow ground water. Given that 2005 hurricane rainfall appears to be isotopically and volumetrically distinct, it may assist in characterizing the local ground water flow system, and recharge to, the Inglefield Sandstone aquifer. Sampling and analyses for isotopic ratios is ongoing.