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ESTIMATED POLLUTANT LOAD REDUCTION MODEL USED . For estimating watershed loading, exclusive of the RO Concentrate addition, the Spreadsheet Tool for Estimating Pollutant Loads (STEPL) was initially applied, with the land use breakdown as previously noted . In developing these loads, grass swales and dry detention were noted as the existing BMP elements . Event Mean Concentrations used were those applied to Florida conditions2 shown in Table 1 of the Pollutant Load Calculations Attachment, which are similar to the default values within the STEPL model . Load projections for the entire watershed are noted in Table 2 of the Pollutant Load Calculations Attachment. There is available long term USGS flow data for the PC -South Relief Canal which represents the composite influence of the runoff and base flow associated with the majority of the watershed . Data from 1956 to 19824 reveals an annual average flow of 37 . 7 cfs or 24 .4 MGD, with a standard deviation of 13 . 7 cfs ( 8 . 9 MGD) ; a minimum annual average of 15 .2 cfs (9 . 8 MGD) and a maximum annual average of 61 . 6 cfs (39 . 8 MGD) . Using the average flow rates and the STEPL post BMP loading values, the average annual concentrations within the canal can be estimated, as noted in Table 3 of the Pollutant Load Calculations Attachment . Included in this table is the actual concentrations as monitored in the South Canal, using data from USGS from 1972 - 1977 , from Indian River Farms Water Control District from 2000, and from the ongoing PC-South ATSTM Pilot System . BMPs Installed TSS TP TN BMP #1 ATS lbs/yr lbs/yr lbs/yr Pre- Project 339851 79400 36,834 Post- 16,925 49998 279229 Project y Load 169926 21402 99607 Reduction p; % 50% 32. 5% 26. 1 % Reduction BMP #2 TSS TP TN (wetlandpond) lbs/yr lbs/yr lbs/yr Pre- Project 169925 41998 27,229 a Project 89463 41953 27,004 Load 8463 45 225 Reduction p; % 50 % 0. 9% 0. 8 % Reduction TOTAL TSS TP TN lbs/yr lbs/yr lbs/yr Pre- Project 339851 79400 369834 Project 89462 49953 279004 Load 259389 29447 99830 Reduction cam. % F75. 0 % 33 . 1 % 26. 7 % Reduction ESTIMATED RESIDENCE TIME OF ANY PONDS, SWALES, ETC . : This Section is not applicable . The pollutant reduction occurs on the algal turf scrubber flow-way . z CDM, " Little Wekiva Basin Watershed Management Plan" 2005 Orange County, Florida. 3 USGS Station 02253500 Lat 27 ° 36 ' 11 " ; Long 80 ° 23 ' 24" 4 In 1982 RO Concentrate discharge to the canal was initiated and flows reflect this addition after 1982 . DEP Agreement No . G0353 , Attachment A- 1 , Page 4 of 14