HYDROGEOCHEMICAL CONTROLS ON MINERAL WEATHERING AND MASS TRANSPORT IN A FORESTED WATERSHED, UPPER JAMES RIVER, VIRGINIA

Global climate change is linked to increased atmosphere CO₂ from anthropogenic sources. Elevated CO₂ levels and the associated warmer temperatures contributes to the enhancement of mineral weathering. Investigations into watershed scale hydrogeochemistry is an important component to the study of mineral weathering fluxes from rivers to oceans. The Upper James River (7938 km²)of Virginia is the headwaters for the James River watershed, which discharges into the Chesapeake Bay. The humid subtropical climate helps support the heavily forested high relief landscape that drains the Paleozoic carbonate and siliclastic bedrock of the Valley and Ridge Province. Soils rich in iron hydroxides and clays are abundant. In addition, karst features, such as caves and springs occur in the carbonate bedrock.

The initial focus of this research is to identify the geochemical regularities and differences in the carbonic acid system, elemental concentrations, and mineral equilibria between surface waters and shallow groundwaters along with soil weathering contributions to the drainage. Streams were sampled for field parameters and aliquots were preserved for laboratory analysis of cations (ICP), anions (IC), and for titration alkalinity. Discharge was measured and pulled from USGS gage locations on selected streams to characterize the elemental fluxes from subcatchments of the Upper James River watershed. Seasonal changes and long term trends in the hydrogeochemistry of the Upper James River are investigated by comparing historical data for the watershed obtained from USGS gages. Soil samples were collected from forested regions over representative lithologic units in the watershed and leached to determine the readily soluble hydroxides and carbonates. This allows for endmember characterization of the siliclastic and carbonate lithologies and in the determination of their contribution to the hydrogeochemistry and elemental fluxes from the watershed.

Preliminary data show that smaller streams within the drainage have HCO₃ concentrations and elemental chemistries that are consistent with the catchment lithologies. The larger streams show evidence of anthropogenic pollution such as high Na, Cl and SO₄ concentrations. Long term data from USGS gauging stations show an inverse relationship between elemental concentrations and discharge.