Barr prepared a report titled Detailed Assessment of Phosphorus Sources to Minnesota Watersheds in conjunction with University of Minnesota researchers, Dr. David Mulla and Dr. Prasanna Gowda, and LimnoTech, under the TMDL (total maximum daily load) master contract for the MPCA. Concerns from the State Legislature and other parties about the phosphorus content of dishwasher detergents resulted in legislation requiring a study of all of the sources and amounts of phosphorus entering publicly owned treatment works (POTWs) and Minnesota surface waters. The MPCA retained the Barr team to assess and inventory the sources (point and non-point) and amounts of phosphorus entering: 1) surface waters for 10 major watershed basins and for the entire state during low (dry), average, and high (wet) flow conditions; and 2) three different sizes and categories of POWTs. The assessment also examined statewide variations in these annual loadings. Barr also looked at the effect of source-reduction options on receiving-water quality and wastewater-treatment-facility performance.
The assessment found that, under average flow conditions, the point-source total phosphorus contribution was 31 percent of the loadings to surface waters statewide, while non-point sources represented 69 percent. It also found that, statewide, 43 percent of phosphorus entering POTWs was from human waste and 57 percent was from non-ingested sources—with only 2.8 percent of that from automatic dishwasher detergents. The detailed report, including study description, findings, and recommended concepts for lowering phosphorus amounts, can be found on the MPCA’s website at https://www.pca.state.mn.us/water/phosphorus
Senior Water Resources Engineer