Exploring Sulfate Sources and Transport Across and Land Use Gradient in the Piedmont

The Clean Air Act Amendment in 1990 has been successful in limiting sulfate atmospheric deposition, but sulfate stream concentrations remain high, particularly in unglaciated regions.  There are several anthropogenic sources of stream sulfate, including building materials, road salt, septic systems, and agriculture. Beyond acidification, sulfate fluxes have ecological consequences for nutrient cycling through mobilizing phosphate. We explored sulfate sources and transport in catchments across a forested to urban land use gradient in the Maryland Piedmont. Sulfate concentrations increase dramatically with increasingly heavy anthropogenic land use, with ~3 times larger suburban sulfate concentrations compared to a neighboring forested catchment. Despite this, sulfate concentration-discharge (C-Q) behavior is similar between the catchments. Both catchments have positive baseflow C-Q relationships with sulfate mobilized seasonally during wet conditions. Complicating the C-Q relationship, the suburban catchment experiences dilution during the early stage of storms. However, both streams have sulfate enrichment that lags the peak of the storm and persists well after the return to baseflow. Hydrograph separation using stable water isotopes demonstrates that initially both the suburban and forested stream sulfate concentrations follow a simple mixing model between the antecedent stream sulfate concentrations and precipitation. The dilution effect is apparent only in the suburban catchment because of its much higher background sulfate concentrations. Following the initial hour of the storm event, stream sulfate concentrations are enriched beyond the mixing model, in an effect that persists for days. This suggests that storms mobilize sulfate stored above the water table. This stored sulfate may be flushed downwards by precipitation or mobilized by rising water tables. In order to identify the source of anthropogenic sulfate, we measured sulfate sulfur and oxygen isotope concentrations for the forested and suburban streams, as well as a nearby urban stream and several possible sulfate sources. The sulfate isotopes were similar across the land use types and could not be used to distinguish sources, but instead imply that the mixture of sulfate sources result in consistent average stream values across this forested to urban gradient. Improving our understanding of sulfate sources and transport is important to mitigate its environmental impacts.