Similarities in Storage and Transport of Sulfate in Forested and Suburban Watersheds, Despite Anthropogenically Elevated Suburban Sulfate

Sulfate is a potential pollutant and important nutrient linked with the nitrogen, carbon, and phosphorus cycles. The importance of different anthropogenic sulfate sources in suburban streams (septic systems, fertilizer, road salt, and infrastructure) is uncertain, and the temporal dynamics of stream export sparsely documented. We study sources and export dynamics of sulfate in suburban and forested headwater catchments. Stream baseflow discharge and sulfate concentrations were strongly positively correlated in both watersheds with the highest values in spring. Suburban concentrations and fluxes (2.48-7.5 mg/L or 25.8-78.1 mu M, 16.6 kg/ha/yr) were consistently higher than forested (0.56-2.78 mg/L or 5.8-28.9 mu M, 5 kg/ha/yr). Following precipitation, sulfate concentrations in both forested and suburban streams increased to concentrations above pre-storm values and remained high after peak discharge. These dynamics suggest that both catchments have a large pool of sulfate that can be mobilized under wet conditions. Ridge-top forest soil samples contained 210 kg/ha stored, extractable sulfate. Current atmospheric sulfate deposition rates (5-7 kg/ha/yr) are approximately in balance with sulfate export in the forested stream. In the suburban watershed, we estimated septic fields contribute up to 11 kg/ha/yr (about half from surfactants) and lawn care up to 4.3 kg/ha/yr and are the most likely sources of elevated stream sulfate. Sulfate sulfur (4.9-5.8 parts per thousand forested; 6.1-7.0 parts per thousand suburban) and oxygen isotope values (0.7-2.0 parts per thousand forested; -0.1-4.1 parts per thousand suburban) are consistent with this interpretation, but do not provide strong corroboration due to large variation and overlap in estimated source values. Sulfate can be a nutrient or pollutant that alters stream biogeochemistry depending on dissolved concentrations. For decades, atmospheric sulfur deposition was elevated due to regional anthropogenic emissions. Regulation in the 1990s resulted in decreasing deposition and increasing importance of other sulfur sources, for example, associated with land use. We studied suburban sulfate sources, including infrastructure and activities not widely recognized as sources of stream sulfate. We measured dissolved sulfate concentrations and sulfate tracer isotopes in neighboring forested and suburban watersheds. Sulfate inputs and exports are compared with budgets of possible sulfate contributions to the stream from septic systems, lawn care, road salt, building materials, and historic agriculture. The suburban stream exports about 3x more sulfate a year from the watershed than the forested stream. Extra suburban stream sulfate appears to be largely caused by human waste and cleaning products flowing to septic fields and lawn care. More sulfate is transported to the stream during the wetter spring and after rain. This elevated transport may be because the groundwater table moves upwards during wetter times and transports sulfate that was stored in soil to the stream. Identifying suburban sources of sulfate pollution is important to protecting stream health. The suburban stream has elevated sulfate concentrations and fluxes, likely due to contributions from septic systems and lawn careSulfate mobilized in seasonal- and event-scale wet conditions in both forested and suburban streamsSulfate in the suburban stream is associated with soil storage mobilized in wet conditions, not runoff from impervious surfaces