THE BIOGEOCHEMICAL DYNAMICS OF GROUNDWATER – SURFACE WATER MIXING IN URBAN HEADWATER STREAMS, GREENVILLE, SOUTH CAROLINA

The temporal and spatial biogeochemical dynamics of groundwater, surface water, and hyporheic mixing determine stream water chemistry. In contrast to rural streams, urban streams have flashier discharge and higher dissolved ion concentrations, leading to a distinctive biogeochemical signature. Urban headwaters also appear to be important “hotspots” for nutrient cycling. Recent studies in the Upper Piedmont of South Carolina have shown that pCO₂ and nitrate concentrations in urban streams are highest near the source and decline markedly with distance downstream. This study explores the role groundwater discharge and hyporheic zone mixing in biogeochemical transformations, particularly with regards to nitrogen and carbon, within two urban headwater streams of the Enoree River Basin.

Hyporheic, groundwater, and surface water samples were collected from the two streams during summer 2009. Samples were collected in the upper 2900 m of Brushy Creek and the upper 600 m of Rocky Creek. Groundwater and hyporheic well installation and sampling was often constrained by deep incision, shallow bedrock, and abundant clay substrate. Samples were analyzed for pH, dissolved oxygen, conductivity, DOC, major anion and cation concentrations, and ¹³C isotopic composition.

Groundwater appears to be a significant source of carbon to these two urban streams. Groundwater and hyporheic samples typically had higher concentrations of dissolved organic and inorganic carbon than surface water samples in both streams, and were also isotopically lighter. The relationship between groundwater, hyporheic, and surface water samples is more complex with regards to nitrogen. In Brushy Creek, groundwater and hyporheic waters tended to have lower nitrate concentrations, but higher ammonium concentrations than did surface water. However, in Rocky Creek, groundwater had higher nitrate concentrations than surface water, but ammonium concentrations were variable and not consistently higher or lower than surface water. Spatial heterogeneity of groundwater and hyporheic zone chemical composition in both streams reveals complex, but significant relationships with surface water chemistry. Groundwater and hyporheic systems are therefore significant influences on the distinct biogeochemical signature of urban headwater streams.