Strontium isotope systematics of base flow in Piedmont Province watersheds, Georgia (USA)

Abstract

87Sr/ 86Sr ratio variations were analyzed in rainfall, shallow ground water and base flow collected from 4 Piedmont streams within the Middle Oconee River basin in northeastern Georgia during the period between March, 2003 and March, 2004. They Sr isotope ratio analyses were accompanied by measurements of stream discharge, rainfall, stable O isotope ratios and major ion and 3H concentrations. The average Sr ion concentration and 87Sr/ 86Sr ratio for the terminal stream basin (the Middle Oconee River) were 23.6 μg/L and 0.7172, respectively. The average 87Sr/ 86Sr ratios of the rainwater and shallow ground water were below 0.7125, indicating that most of the Sr in this stream water is input by weathering reactions in deeper ground water, rather than by ion exchange in shallow soil horizons. This is consistent with the higher alkalinity concentrations (∼23–47 mg/L) and specific conductance values (60–113 μS/cm) that characterize stream base flow. Piedmont streams are characterized by lower concentrations of Sr and higher 87Sr/ 86Sr ratios than average global stream flow. Base flow rates decreased by a factor of 2–3 during the summer months and this is accompanied by increased alkalinity concentrations. 87Sr/ 86Sr ratios, however, were temporally invariant for a given stream basin and were independent of season, antecedent rainfall, and discharge. 87Sr/ 86Sr ratios were unique for each of the 4 basins and a general trend toward higher ratios with increasing basin area was apparent. The inferred contribution from minerals with high Rb contents such as K feldspar and muscovite may have resulted from the greater integration of flow from mineralogically diverse pathways afforded by a larger basin area. The basin specificity and temporal or seasonal invariability make 87Sr/ 86Sr ratios an invaluable hydrological tracer that can be readily employed in mass balance studies of stream flow within the Piedmont Province.