EVALUATING CONTROLS ON NUTRIENT, ROAD SALT AND SEDIMENT EXPORT IN A SUBURBAN WATERSHED: INSIGHTS FROM THE MILL CREEK

The Mill Creek Watershed, located in Montgomery County, PA, has been previously documented as one of the most biologically-impaired tributaries in the Schuylkill River watershed. This is surprising as the watershed is largely characterized by low intensity residential development and wooded land with light commercial activity limited to its headwaters. Therefore, this study evaluated the effect of suburban land use practices on the sediment, nutrient, and road salt export to the Mill Creek. Monthly base flow samples were collected from several locations along the mainstem and major tributaries from September 2013 to July 2014 and supplemented with samples collected during select storm events. Water samples were analyzed for total suspended sediments (TSS), chloride, and nitrate, and the analytical results were compared to existing GIS-based land use data in an effort to evaluate controls on solute and sediment export. The results show concentrations of chloride and nitrate from several locations in excess of USEPA Surface Water and/or Drinking Water Standards. Elevated levels of both contaminants observed throughout the year indicate the strong likelihood of inputs from contaminated shallow groundwater. Although no relationship was observed between chloride yields and any of the land use parameters, base-flow chloride concentrations exhibit a strong relationship with high-intensity land use. Nitrate yields exhibit the strongest relationship with open space developed land use, suggesting fertilizer from lawns is the primary source of contamination. Sediment yields show no correlation with any land use parameter during any given sampling date. However, storm yields for all three contaminants exhibit a strong positive correlation with the number of storm drains above the sampling point indicating their role as a primary conduit of delivery. In addition, the strong correlation of sediment yields with storm drains suggests excess water is eroding the channels of the Mill Creek and consequently is the main source of sediment to the system. These results suggest the number of storm drains within a watershed may play an equally, if not more important role, than land use practices with regards to contaminant inputs, emphasizing the need for sustainable suburban storm water management practices.