STREAM WATER CHEMISTRY DURING STORM EVENTS IN THE PIEDMONT REGION OF SOUTH CAROLINA

The Upper Piedmont region of South Carolina, like many areas across the southeastern United States, is experiencing significant urban development. The associated increase in impervious surfaces, addition of stormwater conveyance systems, and decrease in riparian buffers have created rapid and sizable storm responses in the streams. This study compared the chemical response during storm events in two urban and two rural streams in the Piedmont region of South Carolina. The purpose was to determine whether urban streams have a unique biogeochemical signature during storm events.

At each site a pressure transducer provided continuous stream stage measurements. An automated ISCO water sampler collected twenty four water samples during each storm event. Water samples were analyzed in the lab for major cation, anion, dissolved organic carbon, and dissolved oxygen concentrations, pH, and conductivity. Baseflow samples provided a chemical baseline for comparison.

During storm events the urban and rural streams both showed rapid declines in some ion concentrations (silicon, chloride, sodium) presumably from dilution from precipitation runoff. Increases in the concentrations of sulfate and DOC likely reflect acid rain deposition and flushing of riparian soils, respectively. Changes in nitrate concentrations were inconsistent, rising at some sites, while falling at others. Temporally, declines and rises of ion concentrations were in sync with the storm hydrographs along the rising limb, but the recovery to baseflow chemistry lagged behind stage recovery. The urban streams typically had shorter chemical recovery time periods (< 24 hours) compared to the rural streams which often took days. Differences in the behavior between the systems are believed due to differences in the flow paths and proportions of groundwater to surface water mixing. The magnitude of the declines in concentration was larger in the urban streams compared to the rural streams, although the urban streams had higher initial baseflow ion concentrations. The observed differences in stormwater chemistry between the urban and rural streams highlight the importance of flowpath hydrology and have important implications for in stream biogeochemical and ecological processes.