Strontium isotope ratios in stream base flow within the Georgia Piedmont province

Abstract

Strontium isotope ratios, strontium ion concentrations, major ion concentrations, and stream discharge were measured within the Middle Oconee River basin upstream of Arcade, Georgia during the period 2003- 2004. Strontium isotope ratios (87Sr/86Sr) in stream base flow were between 0.7126 and 0.7172, considerably higher than rainfall (0.7117) and shallow ground water (0.7120). This indicates that Sr and by inference other weathering products in base flow are derived principally from weathering, rather than ion exchange in the shallow soil zone. Unlike all other parameters, strontium isotope ratios were temporally invariant within a given watershed, independent of seasonal variations with respect to base flow discharge. Strontium isotope ratios were significantly different in all four watersheds and therefore provide the best geochemical “tracer” for base flow. 87Sr/86Sr ratios increased as a function of basin area, independent of Sr ion concentrations, probably as the result of the increased contribution from rubidium-bearing minerals such as K-feldspar, muscovite, and biotite.