Strontium isotopes in selected streams within the Susquehanna River Basin

Abstract

The concentration and isotopic composition of strontium in water samples from the Susquehanna River and its major tributaries between Northumberland, Pennsylvania, and the Chesapeake Bay have been determined to investigate the usefulness of the 87Sr/86Sr ratio as a tracer in hydrologic studies. The87S/86Sr ratios of tributaries, measured just upstream from their confluence with the Susquehanna, show considerable variation, ranging from 0.7091 to 0.7218. These ratios appear to be determined primarily by the local bedrock type and aquifer characteristics. Tributaries whose drainage basins contain important carbonate aquifers have relatively high Sr concentrations and 87Sr/86Sr ratios of about 0.709; where the bedrock is shale with no carbonate rock, the tributaries are characterized by much lower Sr concentrations and 87Sr/86Sr ratios of about 0.716. Tributaries in metamorphic bedrock also have low Sr concentrations and isotopic ratios averaging 0.719. The 87Sr/86Sr ratios of nearly all tributaries in the study area seem to represent a mixture of varying proportions of two distinctly different groundwater types, one with a high Sr concentration and low 87Sr/86Sr ratio, the other characterized by a low Sr concentration and relatively high 87Sr/86Sr ratio. In the northern half of the study area, marked variations in the 87Sr/86Sr ratio of the Susquehanna River indicate incomplete mixing of tributary and mainstream waters, apparently due to the extreme width and small depth‐to‐width ratio of the Susquehanna. Under favorable geologic and hydrologic conditions, strontium isotopes have potential as a tracer of water provenance.