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Phosphorus Removal by Plant Harvesting on Lake Minnetonka October 22, 2004 John M. Barten, Three Rivers Park District Table 2. Plant weight and percent phosphorus. Average 10002 11.81 0.232 2.642 The amount of phosphorus in an average truckload of plants harvested from Lake Minnetonka was calculated at 2.6 pounds. In 2004, 196 truckloads of plants were removed from the lake. Thus, approximately 510 pounds of phosphorus were removed from the lake by the harvesting program. The minimum and maximum pounds of phosphorus removed from the lake using the largest and smallest truckloads was calculated as 333 and 757 pounds/year respectively. The Minnehaha Creek Watershed District estimates an annual phosphorus inflow to Lake Minnetonka of approximately 20,000 pounds. The harvesting program thus removes between 2 and 4 percent of the annual phosphorus load to the lake. The annual cost of the harvesting program is estimated at $104,000. The estimated removal cost of phosphorus with the harvesting program averages $204 per pound, significantly less than removal rates for most watershed BMPs. For instance, the estimated phosphorus removal cost of a street sweeping program is $300/ pound, and for many detention ponds is $1,000 per pound. Although the harvesting program removes only three percent of the annual phosphorus load to Lake Minnetonka, the value of that removal can be viewed in terms of the difficulty of removal of phosphorus from stormwater. For instance, it would require the installation of 1500 rain gardens in the watershed to remove a comparable amount of C. I Usersl GabriellDownloadslMinnetonka Premovharvesting.doc p. 5 of 6 Truck Fuel Plant % Dry %P/ Lbs. Weight wt. dry P/truck Sample Date Weight (lbs.) Weight load 1 8/3/04 27580 7/8 11609 10.3 0.161 1.930 2 8/10/04 25540 3/8 9924 10.4 0.218 2.250 3 7/27/04 26800 3/4 10918 14.0 0.195 2.980 4 8/12/04 30000 3/4 14118 14. 0.154 3.120 5 7/22/04 23600 3/4 7718 13.7 0.163 1.720 6 7/7/04 24820 1/8 9381 9.7 0.388 3.530 7 6/29/04 22260 Full 6200 10.0 0.228 1.410 8 7/20/04 25900 1/2 10195 13.1 0.285 3.810 9 7/18/04 24400 Full 8340 11.6 0.370 3.560 10 7/29/04 25280 Full 9220 12.6 0.117 1.330 11 7/13/04 28100 1/2 12395 10.1 0.273 3.420 Average 10002 11.81 0.232 2.642 The amount of phosphorus in an average truckload of plants harvested from Lake Minnetonka was calculated at 2.6 pounds. In 2004, 196 truckloads of plants were removed from the lake. Thus, approximately 510 pounds of phosphorus were removed from the lake by the harvesting program. The minimum and maximum pounds of phosphorus removed from the lake using the largest and smallest truckloads was calculated as 333 and 757 pounds/year respectively. The Minnehaha Creek Watershed District estimates an annual phosphorus inflow to Lake Minnetonka of approximately 20,000 pounds. The harvesting program thus removes between 2 and 4 percent of the annual phosphorus load to the lake. The annual cost of the harvesting program is estimated at $104,000. The estimated removal cost of phosphorus with the harvesting program averages $204 per pound, significantly less than removal rates for most watershed BMPs. For instance, the estimated phosphorus removal cost of a street sweeping program is $300/ pound, and for many detention ponds is $1,000 per pound. Although the harvesting program removes only three percent of the annual phosphorus load to Lake Minnetonka, the value of that removal can be viewed in terms of the difficulty of removal of phosphorus from stormwater. For instance, it would require the installation of 1500 rain gardens in the watershed to remove a comparable amount of C. I Usersl GabriellDownloadslMinnetonka Premovharvesting.doc p. 5 of 6