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
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One of the twenty most threatened rivers in
the nation (flow, sediment, nitrogen,
phosphorus, and bacteria)
P loads vary among tributaries
Sources of P pollution diverse
Serious impacts on Lake Pepin
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)
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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
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Identification of phosphorus sources and the
amounts entering POTW’s by
– Size (flow rates)
– Category (household, commercial and industrial)
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Determine the amount of bio-available P from all
point and nonpoint sources
Science Based Approach
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Birr, A. S. and D. J. Mulla. 2001. Evaluation of
phosphorus site index in watersheds at the regional
scale. J. Environ. Quality. 30:2018-2025.
Fang, F., P. L. Brezonik, D. J. Mulla, and L. K. Hatch.
2002. Estimating runoff phosphorus losses from
calcareous soils in the Minnesota River Basin. J.
Environ. Quality 31(6):1918-1929.
Fang, F., P. L. Brezonik, D. J. Mulla, and L. K. Hatch.
2005. Characterization of soil bioavailable
phosphorus in the Minnesota River basin. Soil Sci.
Soc. Am. J. 69:1016-1025.
Minnesota River Slumping River Bluffs
Phosphorus Contributions from
Streambank Erosion
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Total P load from streambanks
calculated using subsoil phosphorus
concentrations (441 mg kg-1) and
streambank erosion estimates (44%
of total sediment load)
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Sekely, A. C., D. J. Mulla, and D. W.
Bauer. 2002. Streambank slumping
and its contribution to the
phosphorus and suspended
sediment loads of the Blue Earth
River, Minnesota. J. Soil Water
Conservation. 57(5):243-250.
Minnesota River Tributary TP Loads
Nonpoint Sources of Phosphorus
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Transported to surface waters by overland
flow and erosion, mainly from soils that are in
close proximity to water
Occurs in two forms:
– Particulate P in eroded sediment
– Soluble P in runoff
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Most important sources:
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Water erosion and runoff (snowmelt, rain)
Streambank erosion
Tile drainage
Atmospheric deposition (wind erosion)
Point Sources of Phosphorus
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Generally more bio-available than nonpoint
sources of phosphorus
Most important sources:
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Commercial/Industrial Process water
Wastewater treatment plants
Food waste and garbage disposal waste
Dishwasher detergents
Example: Redwood watershed
– Over 75% of TP in watershed is from Archer
Daniels processing plant and Marshall wastewater
treatment plant
Conclusions
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Target watersheds that transport the
largest TP loads
Target the largest sources of TP
Streambanks are an important source of
TP loading to surface waters in areas
with slumping stream bluffs
Both point and nonpoint source
reductions are important
Thank You