GEOMORPHIC CONTROLS OVER NITRATE CONCENTRATIONS IN URBAN HEADWATER STREAMS OF THE SOUTH CAROLINA PIEDMONT

Headwater streams contribute about 70% of total flow volume and are critical biogeochemical hotspots in river basins. The relationship between urbanization, channel incision, and nitrate concentration is complex, but previous studies have suggested a positive correlation between incision and nitrate concentrations. Other studies have found direct correlation between urban land cover and incision. However, little research has focused on headwater streams, particularly those that are urbanized and drain land dominated by impervious cover. Urbanized headwater streams in the Piedmont region of South Carolina have high (>1 mg/L) but variable nitrate concentrations. The objective of this study was to test the hypothesis that incision is positively correlated with nitrate concentrations.

Stream cross-sections and bankfull widths were measured and used to calculate incision for 19 localities from longitudinal stream transects in the Brushy Creek and Rocky Creek watersheds near Greenville, SC. Both streams are tributaries of the Enoree River. Surface water samples were collected at each locality along both transects, and paired shallow groundwater samples were collected along the Brushy Creek transect.

The drainage area for the Brushy Creek transect had 76% urban land cover and 36% impervious land surface, and the drainage area for the Rocky Creek transect had 61% urban land cover and 20% impervious land surface. Incision ratios ranged from 4.2 to 9.1 in Brushy Creek and from 1.5 to 6.5 in Rocky Creek. Nitrate concentrations in surface waters ranged from 9.3 to 11.3 mg/L in Brushy Creek and from 3.7 to 4.1 mg/L in Rocky Creek. The Brushy Creek transect had significantly higher incision ratios and surface nitrate concentration than the Rocky Creek transect. The relationship between incision and nitrate concentrations within each stream was not statistically significant. At Brushy Creek, nitrate concentrations in surface water exceeded concentrations in groundwater at five of six locations. Within each transect, nitrate concentrations showed high spatial variability associated with stream flow and depth variations. Our results suggest that higher incision ratios are associated with higher nitrate concentrations in urbanized headwaters, though additional data are needed to further test this hypothesis.