RELATIONSHIPS BETWEEN STREAM SOLUTE CONCENTRATIONS AND WATERSHED TOPOGRAPHICAL CHARACTERISTICS IN THE BLUE RIDGE AND PIEDMONT PROVINCES OF SOUTH CAROLINA

The Blue Ridge and Piedmont regions of the southeastern United States are experiencing rapid land transformation, especially with the expansion of urban lands. Such transformations can alter water quality in headwater streams. However, natural topographic variations within the landscape may also influence solute concentrations in streams. Our goal was to determine whether solute concentrations in headwater streams were related to several geomorphic factors: sample site elevation, mean watershed slope, floodplain width, and adjacent valley slope. During May-July 2021, water samples were collected under baseflow conditions from 21 first to third-order streams draining mostly forested watersheds at varying elevations in the Blue Ridge and Piedmont regions of South Carolina. Samples were analyzed for concentrations of all major ions, ammonium, dissolved iron, dissolved silicon, and dissolved organic carbon. Floodplain width and adjacent valley slope at the base of the watershed were determined for each stream using digital elevation data. Concentrations of most solutes were negatively correlated with sample site elevation, mean basin slope, and adjacent valley slope. Streams with adjacent valley slopes of 10 degrees or higher tended to have low solute concentrations that were less variable than in streams with adjacent valley slopes of 10 degrees or lower. Correlations between floodplain width and solute concentrations tended to be positive but weak. Also, solute concentrations tended to be more variable in streams with total floodplain widths greater than 75 meters. We hypothesize that these relationships result from the influence of groundwater residence time on stream solute concentrations. Higher relief within the watershed and steeper adjacent valley slopes facilitate higher hydrologic gradients and hence lower groundwater residence time, which results in lower stream solute concentrations. In addition, we speculate that in floodplains that are wider than 75 meters, more complex groundwater flow paths might result in higher variability in stream solute concentrations. Further studies on soil hydrologic characteristics, groundwater flow paths, and local variations in bedrock geology are necessary to understand these relationships more clearly.