Phosphorus Loads from Streambank Erosion to Surface Waters in the Minnesota River Basin D. J. Mulla Professor, Dept. Soil, Water, Climate University of Minnesota

Minnesota River Basin Pollution l One of the twenty most threatened rivers in the nation (flow, sediment, nitrogen, phosphorus, and bacteria) l P loads vary among tributaries l Sources of P pollution diverse l Serious impacts on Lake Pepin l TMDL for a 40% reduction in BOD in the Lower Minnesota River, goal for a TP concentration of 0.17 mg/L, nearly half of current levels

Assessment of P Sources to Surface Waters Study (2005) l Determine phosphorus contributions to surface waters within each of the ten major Minnesota drainage basins –Point and Nonpoint Sources –Low, Average, and High Flow Conditions l Identification of phosphorus sources and the amounts entering POTW’s by –Size (flow rates) –Category (household, commercial and industrial) l Determine the amount of bio-available P from all point and nonpoint sources

Science Based Approach l Birr, A. S. and D. J. Mulla Evaluation of phosphorus site index in watersheds at the regional scale. J. Environ. Quality. 30: l Fang, F., P. L. Brezonik, D. J. Mulla, and L. K. Hatch Estimating runoff phosphorus losses from calcareous soils in the Minnesota River Basin. J. Environ. Quality 31(6): l Fang, F., P. L. Brezonik, D. J. Mulla, and L. K. Hatch Characterization of soil bioavailable phosphorus in the Minnesota River basin. Soil Sci. Soc. Am. J. 69:

Minnesota River Slumping River Bluffs

Phosphorus Contributions from Streambank Erosion l Total P load from streambanks calculated using subsoil phosphorus concentrations (441 mg kg -1 ) and streambank erosion estimates (44% of total sediment load) l Sekely, A. C., D. J. Mulla, and D. W. Bauer Streambank slumping and its contribution to the phosphorus and suspended sediment loads of the Blue Earth River, Minnesota. J. Soil Water Conservation. 57(5):

Minnesota River Tributary TP Loads

Nonpoint Sources of Phosphorus l Transported to surface waters by overland flow and erosion, mainly from soils that are in close proximity to water l Occurs in two forms: –Particulate P in eroded sediment –Soluble P in runoff l Most important sources: –Water erosion and runoff (snowmelt, rain) –Streambank erosion –Tile drainage –Atmospheric deposition (wind erosion)

Point Sources of Phosphorus l Generally more bio-available than nonpoint sources of phosphorus l Most important sources: –Commercial/Industrial Process water –Wastewater treatment plants –Food waste and garbage disposal waste –Dishwasher detergents l Example: Redwood watershed –Over 75% of TP in watershed is from Archer Daniels processing plant and Marshall wastewater treatment plant

Conclusions l Target watersheds that transport the largest TP loads l Target the largest sources of TP l Streambanks are an important source of TP loading to surface waters in areas with slumping stream bluffs l Both point and nonpoint source reductions are important

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