VARIATIONS IN REE SIGNATURES AS INDICATORS OF RECHARGE AREA AND FLOW PATH LENGTH IN KARST SPRINGS

Geochemical tracers are one of the tools used to try to understand the heterogeneous nature of recharge and flow within karst aquifers. Rare earth elements (REE) have been used in previous studies to differentiate aquifer source rock and weathering processes, as their concentrations are affected by sorption, oxidation and complexation, but they haven’t been used as much in karst settings. Fifteen karst springs in three Pennsylvania river basins were analyzed for REE concentrations in an attempt to better understand source areas, residence times, and flow paths. Two of the basins are in central Pennsylvania and previously studied by Shuster and White (1971); one of the basins is in urban karst outside Philadelphia. These springs were previously classified based on their physiochemical properties (major ions, temperature, saturation indices) as having no seasonal variation and higher dissolved ions (indicative of matrix flow contribution) or high seasonal variations and more variable dissolved ions (indicative of more contribution from conduits). This grouping did not provide a basis for interpreting the REE results. Comparing behavior of light rare earth elements (LREE) and heavy rare earth elements (HREE) showed both low ratios and high ratios for springs previously classified as conduit-dominated. The high ratio seemed to be related to higher discharge. Springs with both seasonal variation and little seasonal variation displayed Ce and Eu anomalies. When overall REE concentrations were low, springs showed more pronounced Ce and Eu anomalies. The Ce and Eu anomalies along with the low LREE ratios can be indicative of shorter flow paths. These results indicate that REE concentrations in spring water are more affected by recharge area and residence time (which affects sorption, oxidation and complexation) rather than contributions from conduit or matrix flow. This discrepancy between REE results and flow classification could provide a useful complementary means for further understanding karst spring behavior.