THE EFFECT OF WATERSHED TOPOGRAPHY ON SOLUTE CONCENTRATIONS IN FORESTED HEADWATERS: IMPLICATIONS FOR UNDERSTANDING PIEDMONT URBAN BIOGEOCHEMISTRY

Land use and land cover are changing rapidly in the Piedmont and the Blue Ridge regions of South Carolina as urban land cover has expanded into rural areas covered by forest, pasture, and cropland. One approach to assess the influence of different types of land cover on water quality in streams is to compare streams in watersheds with minimal human influence to streams in watersheds modified extensively by human activity. However, because of changes in elevation from the lower Piedmont to the Blue Ridge Escarpment, the water quality of streams may vary naturally. For example, with increasing elevation, average air temperature, evapotranspiration rates, and soil depth all decrease, but precipitation increases and topography generally becomes steeper. These environmental variations may affect the residence time of groundwater and the supply of solutes from bedrock and soils to streams. During June-July 2018, we sampled 21 first- to third-order streams draining watersheds of 0.7 to 16.1 km². Forest covered >70% of the area of each watershed. Elevations at sample locations ranged from 87 m to 590 m, and watershed maximum elevations ranged from 150 m to 1060 m. We measured pH, water temperature, dissolved oxygen, and conductivity in the field, and we collected water samples for analysis of all major ions, dissolved iron, dissolved silicon, dissolved organic carbon, ammonium, and total dissolved nitrogen. We also estimated instantaneous discharge using stream velocity and cross-sectional area. For most solutes, concentrations were highest at low elevation and decreased with increasing elevation. This relationship was especially strong for solutes generated by mineral weathering (e.g., calcium, magnesium, potassium, sodium, iron, silicon, and bicarbonate).These biogeochemical patterns reflect differences in soil structure and hydrology between higher and lower elevation watersheds. Overall, our results suggest that when comparing watersheds with different land covers, variation in elevation and topography makes interpreting the influence of urbanization more difficult. This consideration is especially important given that much urban development in the South Carolina Piedmont has occurred at elevations with few forested watersheds for comparison.