BIOGEOCHEMICAL AND GEOGRAPHIC FACTORS CONTROLLING FLUVIAL NITRATE HYDROCHEMISTRY IN THE UPPER SALUDA RIVER BASIN

Grab samples were collected from 28 sites in the Upper Saluda river basin in upstate South Carolina throughout the 2001 summer season. These sites represented mountainous headwaters, agricultural areas, and semi-urban areas. The drainage area of each site was calculated using the gravimetric method, with the entire drainage area of the basin totaling 744.114 km². The pH, dissolved oxygen level, conductivity, and temperature of each water sample were measured on site. A portion of the water was then analyzed for turbidity, while the remainder was filtered and analyzed for alkalinity levels, dissolved oxygen levels, and the concentrations of anions and cations. Charge balance analysis confirms that only tested ions contribute significantly to charge balances of the samples.

Nitrate levels were found to be directly proportional to both drainage basin area and the approximate distance as measured from headwater regions. This indicates that nitrates are being introduced downstream through weathering and anthropogenic sources, and that the decrease in tree cover in downstream developed areas is causing reduced nitrogen uptake. Nitrate concentrations were found to be well above pristine levels even in headwater sites, but were particularly high downstream of commercial agriculture operations and a sewage treatment plant. Initial cation analysis of the Upper Saluda samples reveals predominately hard-weathered primary and secondary Sodium plagioclase and Potassium feldspar mineral geochemistry. These silicate rocks are weathering most prominently to gibbsite (bauxite), less so to kaolinite, and very little to smectite, the least-weathered secondary mineral of this group. Additional field work is necessary to confirm the geologic nature of the sampling region, as the few extant geologic maps for the region are not reliable.