GEOCHEMICAL CONSTRAINTS ON FLOODPLAIN GROUNDWATER SOURCE, FLOW PATHS, AND SURFACE WATER MIXING AT CONGAREE NATIONAL PARK, SOUTH CAROLINA

The temporal and spatial dynamics of floodplain hydrogeology are complex and involve the exchange of water and solutes between streams and groundwater systems. Congaree National Park protects >26,000 acres of forested floodplain along the Congaree River. The Congaree Observation Well Network (COWN) includes ten piezometers screened 4-7 m deep and installed along a 1.9 km transect near the northern floodplain margin. This surficial aquifer, which sits on a deep Cretaceous aquifer, consists of a basal sand overlain by a variable top-stratum of peat, clay and sand. The COWN is intended to evaluate the evolution and dynamics of surface and groundwater interactions within the floodplain.

In June 2009, ten COWN groundwater samples and two related surface water samples were collected. Field measurements included temperature, conductivity, dissolved oxygen (DO), and pH. Lab analyses included major inorganic ions, alkalinity, dissolved organic carbon (DOC), total dissolved nitrogen (TDN) and dissolved organic nitrogen (DON). All samples were dilute, with ionic strengths <0.0002, conductivities <100 micro-S/cm, pH ranging from 4.5 - 5.8, and DO concentrations ranging from 3.4 - 6.5 mg/L. Groundwater had low pH and DO relative to surface water. Ion compositions were mixed, but groundwater had high silicon relative to surface water. DOC concentrations were greater in surface water (>10 mg C/L) than in groundwater (<0.5 mg C/L). All samples had bicarbonate concentrations <3 mg C/L and pCO2 was 20 to 400 times oversaturated relative to atmospheric CO₂. In groundwater, nearly all nitrogen was present as nitrate, while in surface water it was present as DON. Groundwater nitrate concentrations were relatively high, with 5 of 10 samples >4 mg/L.

These data provide an initial snapshot of the groundwater and surface water geochemistry at this site. They pose many questions about groundwater source, age, and flow paths as well as questions about land use impacts and lacustrine ecology, particularly in light of some distinct differences in the concentrations and speciation of C and N between the groundwater and surface water samples. This collaborative effort is the beginning of an ongoing project to develop a conceptual model of the water balance and groundwater-surface water dynamics of this floodplain forested system.