Duluth Metropolitan Area Streams Snowmelt Runoff Study

The streams of the North Shore of Lake Superior are typically high gradient, and exhibit high flows coincident with snowmelt and spring rains. The streams within the Duluth Metropolitan Area (DMA) are no exception. Urbanization of watersheds can have dramatic impacts on both the hydrology and water quality of streams. During snowmelt these impacts can be severe, because of the variety of contaminants concentrated in the snowpack –including trace metals, road salt, sediment, and nutrients. Four of the larger streams in the DMA were sampled to collect baseline water quality data during the snowmelt runoff period to improve our understanding of impact that may occur during or as a result of snowmelt. Amity, Miller, Keene, and Kingsbury Creeks were each sampled at three locations along the watershed's urbanization gradient. Samples were collected on the rise, peak, and fall of the snowmelt hydrograph, and during summer baseflow for comparison. Samples were analyzed for pollutants commonly found in urban runoff: fecal coliform bacteria, biochemical oxygen demand, total suspended solids, total dissolved solids, chloride, nitrogen, phosphorus, and zinc. Typically, concentrations of nutrients and sediment mirrored the pattern of discharge over the hydrograph: highest at the peak, lower but similar concentrations on the rise and fall, and lowest concentrations during summer baseflow. Concentrations were compared to water quality standards where available (e.g., chloride, fecal coliform bacteria, zinc). The total chloride and total zinc standards were not exceeded in any samples. Although the fecal coliform standard was not exceeded, high (>200) values were found in the Keene and Kingsbury Creek watersheds. For those parameters without standards (suspended sediment, nitrogen, and phosphorus), data were compared to the typical range (interquartile range) of concentrations for the Northern Lakes and Forests (NLF) ecoregion. This range was derived from data collected at minimally-impacted streams in the NLF. Phosphorus concentrations were below the 75th percentile of the NLF stream data in most samples. Nitrogen concentrations exceeded the 75th percentile in all snowmelt samples, and during baseflow in Keene and Kingsbury Creeks. Total suspended sediment concentrations were typically greater than the 75th percentile during snowmelt, but not during baseflow. Biochemical oxygen demand (BOD) concentrations exceeded the 4 mg/L detection limit only in the Miller Creek watershed, and some values exceeded minimum secondary treatment limits assigned to wastewater treatment facility discharges. Yields (amount per unit area of watershed per time; e.g. kg/mi2/day) of nutrients and sediments during snowmelt were approximately an order of magnitude higher than yields during baseflow. It was hypothesized that yields of nitrogen, phosphorus, and suspended sediment would increase as urbanization in the watershed increased. This was not always the case. Yields of these parameters were lowest in Amity and Miller Creeks, the watersheds with the lowest, and highest, percentage of urban land use. Kingsbury Creek was the only watershed where yields increased at sites from the headwaters to the confluence. A summary of recommendations and best management practices for the protection of surface waters during winter and snowmelt conditions is also included