Water quality in the St. Louis River Area of Concern, Lake Superior: Historical and current conditions and delisting implications

Water quality in the St. Louis River Area of Concern (AOC) was assessed at two stations over a 60 year period (1953-2013) and system-wide for 2012-2013 to determine if the AOC beneficial use impairment (BUI) of “Excessive loading of sediment and nutrients” should be considered for removal. Based on the time-series analysis, concentration and loading of total suspended solids and total phosphorus to Lake Superior from the St. Louis River have decreased over time, and episodic hypoxia in the mainstem of the estuary was eliminated after 1975. Detection of temporal patterns in nitrogen concentration and loading, particularly in the lower estuary, were complicated by Lake Superior nitrogen inputs and changes in wastewater treatment practices. For the system-wide assessment, sample locations were based on a probabilistic survey design. In 2012 and 2013, there was significant monthly (May-October) variability in water quality constituents. Based on area-weighted estimates, 60-85% of the estuary surface area was below BUI criterion for total phosphorus, total suspended solids, and chlorophyll a. Water quality in the western arm of Lake Superior in 2013 was indicative of oligotrophic conditions, satisfying delisting requirements. The long-term improvements in water quality followed improvements in watershed land-use practices and treatment of wastewater. The stratified system-wide survey provided unbiased estimates of spatial and temporal condition and identified some outlier sites. The data from this study supports the BUI removal process for the St. Louis River AOC.