Interactions between groundwater and surface water in a virginia coastal plain watershed. 2. Acid—base chemistry

Abstract

AbstractAn intensive study was conducted on the biogeochemical processes controlling the acid‐base chemistry of Reedy Creek, a wetland‐influenced coastal plain stream located near Ashland, Virginia. In the context of the hydrological processes determined by Eshleman et al., the present research sought to: (1) identify the sources and sinks of anthropogenic and organic acidity within the watershed, (2) determine the seasonal and spatial variability in the stream base flow chemistry, (3) determine the episodic variability in stream stormflow chemistry and (4) quantify a watershed sulphate budget for the 1990 water year. Sulphate is the primary acidic anion in groundwater and surface water at the Reedy Creek watershed. Dissolved organic carbon (DOC) and anion deficit data suggest that organic acids are less important than sulphate in contributing acidity to the stream. In groundwater sampled from wells installed along two transects perpendicular to the stream, sulphate concentrations were higher and alkalinity lower in the groundwater in the hill‐slope than in the stream. Sulphate concentrations and alkalinity observed in groundwater in the wetland were usually between those of the hillslope and stream. These data suggest that the wetland is a sink for sulphate and acidity; sulphate reduction may be an important mechanism for generating alkalinity in the wetland. The DOC concentrations were higher in the stream and wetland groundwater than in hillslope groundwater. No consistent spatial patterns in sulphate concentrations were observed in surface water chemistry under base flow conditions. Anion deficit, DOC and the sum of base cation concentrations were usually higher at upstream sites than downstream sites. At all sites, sulphate concentrations were higher and alkalinity lower in winter and spring than during summer and autumn. Calculated anion deficits were lower in winter and spring than during summer and autumn, reflecting an increase in the contribution of organic acidity during the growing season. Seasonality was not observed in DOC concentrations. Stream discharge was found to be positively correlated with base flow sulphate concentrations and inversely correlated with alkalinity. A sulphate mass balance indicated that approximately 30% of the estimated 24.9kg SO2−2/ha yr wet atmospheric input was exported from the watershed as sulphate in stream runoff in the water year 1990.