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Long-term trends in St. Louis River water quality: 10 years, a historic flood, and some high lake water later
Citation:
Hoffman, J. AND H. Ramage. Long-term trends in St. Louis River water quality: 10 years, a historic flood, and some high lake water later. St Louis River Summit, Duluth, MN, March 03 - 04, 2020.
Impact/Purpose:
Under the EPA Area of Concern (AOC) program, one of the goals of the St. Louis River AOC is to remove the excess nutrient and sediment loadings beneficial use impairment. Here, we merge past state-led water quality monitoring data with contemporary National Estuarine Research Reserve-led water monitoring to provide an up-to-date perspective on the status of sediment and nutrient loads from the river to Lake Superior. We found that the water quality record shows a dynamic evolution from a period strongly affected by unregulated discharges and poor land use practices, to an era of recovery driven by hydrologic cycles.
Description:
Water quality impairments caused by sewage and industrial waste discharge into the St. Louis River have been a primary concern for clean-up efforts throughout the last century. Surveys dating back to 1928 reveal severely degraded water quality in much of the river below Fond du Lac dam. The Minnesota Pollution Control Agency (MPCA) began regular monitoring in 1953 at MN Hwy 23 Bridge and by 1973 were sampling at multiple locations in the lower river, but discontinued monitoring in 2012. Subsequently, the Lake Superior National Estuarine Research began routine monitoring at some of the same stations previously monitored by MPCA. Long-term trends demonstrate a dramatic recovery in water quality throughout the lower river. Beginning in the mid-1970s, there has been a significant increase in dissolved oxygen concentrations, coupled with declines in total suspended solids (TSS) and total phosphorous (TP) concentrations. Notably, annual TP loads have decreased faster in the lower river than the upper river since the mid-1990s, implying a significant, positive change in nutrient dynamics in the lower river. Nevertheless, monitoring data indicate some persistent sources of phosphorous and nitrogen to the river between Hwy 23 Bridge and Blatnik Bridge. Even more recently, dry and wet cycles appear to influence contrasting periods characterized by relatively low (dry) versus high (wet) loadings of TSS and ammonium, albeit not for TP and nitrate. We conclude that the water quality record shows a dynamic evolution from a period strongly affected by unregulated discharges and poor land use practices, to an era of recovery driven by hydrologic cycles.