THE STABLE ISOTOPIC SIGNATURE OF HURRICANE PRECIPITATION IN SOUTHWESTERN INDIANA AND ITS USE AS AN ENVIRONMENTAL TRACER
|By: Elizabeth Curtis-Robinson|
The Evansville, Indiana region typically receives a portion of it’s total yearly rainfall from moisture which originates in the Gulf of Mexico. During 2005, four individual hurricanes (Arlene, Dennis, Katrina and Rita) delivered a large proportion of summer rainfall (24.2 cm of 43.4 cm) from June through September. Rain from these four hurricanes delivered approximately 23% of the local annual precipitation for 2005. Our hypothesis is that the precipitation from these four storms will contain a distinctly heavier 18O/16O and 2H (Deuterium) signature than “normal” summer rainfall. A distinctive isotopic signature may prove useful as an environmental tracer, specifically to trace groundwater flow paths to the Inglefield Sandstone Aquifer which underlies the University of Southern Indiana campus. Isotopic analysis of precipitation events were also used to create a meteoric water line (MWL unique to the Evansville, Indiana region. '
A stable isotope is a non-radioactive atom of a specific element, such as oxygen, hydrogen, or nitrogen, which has the same atomic number but different atomic mass. This difference in mass causes isotopic fractionation during human activities, biological processes, evaporation, and precipitation. For example, the more common and lower mass 16O isotope is preferentially evaporated out of a solution (for example, tropical ocean water) containing both 16O and 18O. Conversely, the heavier 18O isotope is preferentially concentrated into precipitation. This fractionation process occurs globally, but because equatorial and tropical regions receive more solar radiation, the greater concentration of heavier 18O isotope is taken into the vapor phase.
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 15m, respectively.
We would like to thank Chad Evans, Meteorologist for ABC News, for providing us with valuable meteorlogical data. We are also grateful to Laura Bordelon for her involvement in improving the Ground Water Monitoring Lab at the USI campus. We also thank the USI RISC Grant program, the Geological Society of America, as well as the committee for the Victor H. and Elizabeth A. Barnett Research Award, for research and travel support.
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