RELATIONSHIPS BETWEEN URBAN LAND COVER AND NITROGEN BIOGEOCHEMISTRY IN STREAMS OF THE SOUTH CAROLINA PIEDMONT, PART II: THE BRUSHY CREEK WATERSHED
From June to August 2004, we collected water samples for chemical analyses from 15 sites which had been sampled in 2000 and from an additional 11 sites which were located near headwaters, constructed ponds, and suspected point-sources of contamination. Nitrate concentrations in 2004 ranged from 0.36 mg/L to 9.28 mg/L, with a watershed average of 3.75 mg/L. At most localities, nitrate concentrations were greater in 2004 than in 2000 (mean increase=0.30 mg/L), perhaps related to higher rainfall (and presumably N inputs) in 2004. Six sites sampled in the upper headwaters in 2004 had an average nitrate concentration of 5.80 mg/L and included the highest nitrate concentrations in the watershed. Four sites sampled upstream of ponds had an average nitrate concentration of 4.18 mg/L, whereas sites immediately downstream of the ponds had an average nitrate concentration of only 1.29 mg/L. Localities further downstream showed decreases in nitrate concentrations. We hypothesize that denitrification and dissimilatory nitrate reduction in the ponds accounted for this decline. Ammonium concentrations were typically less than 0.10 mg/L but at a few sites were as high as 0.38 mg/L. Total dissolved nitrogen generally followed the same spatial pattern as nitrate concentrations.
High concentrations of nitrate associated with heavily urbanized headwaters suggest that nitrogen from atmospheric deposition has saturated the system. Fertilizer may also contribute to stream nitrogen, however. Downstream, nitrate concentrations are modified by dilution along with denitrification and dissimilatory nitrate reduction in ponds.