Indiana University-Purdue University Indianapolis (IUPUI) / ABSTRACT Dustin Graves ASSESSING THE ROLE OF GEOLOGIC SETTING ON THE HYDROLOGY AND GROUND WATER GEOCHEMISTRY OF FENS IN THE GLACIATED MIDWESTERN UNITED STATES A water quality investigation of several fens located in the temperate glaciated Midwestern United States, near the southern limit of fen occurrence, was conducted to assess the role of geologic setting on the hydrogeochemical signature of fens and to compare hydrogeochemistry of fens located in different geographic and geologic settings. The five studied fens, located in the Central Till Plain physiographic region of Indiana, receive ground water sourced from glacial tills with very similar petrologic composition. These wetlands are hydrogeomorphically classified as slope wetlands with dominant ground water input. More specifically, these sites are inter-till / intra-till type fens (Type Ia and Ib) or outwash terrace type fens (Type II). Shallow ground water was collected just prior to surface interception (source water), and again after discharging into each fen (fen water) and measured for a suite of cations (Ca2+, Mg2+, K+, Na+) and anions (HCO3- SO42-, NO3-, NO2-, PO43-, and Cl-). Fen water hydroperiods showed similar dynamics, despite some variation in the hydrologic input of these systems (source water). Central Indiana fens are recognized as Ca2+, Mg2+, and HCO3- dominated systems. Fen water showed substantial evolution from source water at each study site, evidently the result of carbonate and gypsum dissolution dynamics. However, when only fen water is analyzed, results suggest that ground water of the southern fens represents geochemical similarity, with the exception of anthropogenic influence. The greatest geochemical variation among central Indiana fens can be attributed to Na+ and Cl-, which has been linked to road salt contamination at two of the study sites. This hydrogeochemical study also reveals that fens (slope wetlands) within this particular geologic setting of central Indiana show strong geochemical similarities to fens located throughout the temperate Northern Hemisphere. However, statistical analyses provide evidence that the parameters of Ca2+, HCO3-, and SO42- account for the greatest variation among these wetland communities, suggesting that calcium carbonate and gypsum dissolution dynamics are primarily fen specific while other parameters remain relatively homogenous across a wide geographical range. Lenore P. Tedesco, Ph. D.