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LAKE ST. CROIX TMDL UPDATE
John Erdmann, Minnesota Pollution Control Agency
The interstate, intergovernmental St. Croix Basin Water Resources Planning Team has labored long—since the early 1990s—on behalf of the St. Croix's stewardship. Now the Basin Team has a TMDL study for Lake St. Croix that is all but complete. The TMDL study began in 2009, in response to Wisconsin's and Minnesota's designations of the lake as "impaired"— for excessive phosphorus (P), with attendant high algal abundance and low water clarity—the year before. However, the St. Croix Basin Water Resources Planning Team had actually determined the TMDL in 2004: the TMDL is a limit on the loading, or annual mass, of P that enters the lake from all sources; and through extensive sediment core analyses and other studies, the Basin Team quantified that limit as 360 metric tons/yr (t/yr). So the main job of the TMDL study was to allocate the 360-t/yr allowable P load among all the sources.M.o
Quick explanation: a TMDL is literally a "Total Maximum Daily Load," a term in the Clean Water Act that was coined in 1972, when pollution cleanup was aimed primarily at wastewater discharged into rivers; but a daily load limit is inappropriate for lakes, which generally respond to longer-term pollutant loadings, and which usually receive most of their loadings from watershed runoff, not wastewater discharges.
Since Lake St. Croix is the lowermost 22 miles of the St. Croix River, extending from Stillwater to Prescott, its TMDL includes all of the P sources in the entire St. Croix Basin. The loading from all of these sources is 460 t/yr under "current" conditions (actually, 1990s conditions, used as the baseline for the TMDL study). So the TMDL requires an overall P load reduction of100 t/yr.
Natural background sources account for 166 t/yr of the current and future P loads, forcing the TMDL's load reduction to come out of the remaining 294-t/yr anthropogenic loading. The studies mentioned above are also the source of the current and background loads given here.
By accounting for Lake St. Croix's background P loading and other known P loads in a rational manner, the existing load was reasonably attributed to the sources in the Basin, and the TMDL was reasonably allocated among them. Non-regulated sources account for 85% of the existing total load to Lake St. Croix, and 82% of the TMDL. The non-regulated load is roughly half background load and half anthropogenic. Agricultural land use accounts for 70% to 90% (depending on how much grassland is actually pasture) of the anthropogenic non-regulated load. Regulated sources of P include approximately four dozen municipal and industrial wastewater treatment facilities and some two dozen regulated municipal separate storm sewer systems (MS4s). Together these and a few very minor additional sources are 15% of the total existing load; the wastewater facilities represent ~11%, the MS4s ~3%. The TMDL mandates P load reductions of typically 33% from both wastewater facilities and MS4s. The TMDL requires non-regulated load reductions of ~25% averaged over the Basin, but substantially more than that from agricultural areas and smaller cities and towns. Because the TMDL incorporates an 18-t/yr margin of safety (5%) and reserve capacity of ~4 t/yr, the actual anthropogenic P load reduction under the TMDL is 122 t/yr.