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Water volumes in the calibrated model were calculated to within 23 percent and 24 percent of those measured at the outfalls of each of the basins. These water volumes were applied to the suspended-solids and phosphorus concentrations that were used to calibrate SLAMM for suspended-solids and phosphorus loads. Suspended-solids loads were calculated to be within 4 percent and 17 percent, total-phosphorus loads within 24 percent and 28 percent, and dissolved-phosphorus loads within 9 percent and 10 percent of those measured at the storm-sewer outfall at the Monroe and Harper basins, respectively.
Lawns and streets are the largest sources of total and dissolved phosphorus in the basins. Their combined contribution was approximately 80 percent, with lawns contributing more than the streets. Streets were the largest source of suspended solids.
Relative inputs of water and phosphorus to Harper and Monroe Basins from various source areas
Street-dirt samples were collected using industrial vacuum equipment. Leaves in these samples were separated out and the remaining sediment was sieved into >250 mm, 250-63 mm, 63-25 mm, <25 mm size fractions and were analyzed for total phosphorus. Approximately 75 percent of the sediment mass resides in the >250 mm size fractions. Less than 5 percent of the mass can be found in the particle sizes less than 63 mm. The >250 mm size fraction also contributed nearly 50 percent of the total-phosphorus mass and the leaf fraction contributed an additional 30 percent. In each particle size, approximately 25 percent of the total-phosphorus mass is derived from leaves or other vegetation.
Photos of samplers used to collect samples of runoff from streets, driveways, lawns, and roofs