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4 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 conditionsz shown in Table I 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 1982"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 ATSrM Pilot System. F M�#I (AT ed TSS TP TN MS) I lbs/yr ib yr lbs/ r Pre-Project 33,851 7,400 36,834 R7 Post- Project 16,925 4,998 27,229 w Load 2 Reduction 16,926 2,402 9,607 A . 50% 32.5% 26.1 TSS TP TN lbs/ r lbs/ r lbs/ r 16,925 4,998 27,229 8,463 4,953 27,004 Reduction 8,463 45 225 o; Reduction 50% 0.9% 0.8% TOTAL TSS TP TN lbs/yr lbs/yr lbs/ r y Pre-Project 33,851 7,400 36,834 ------------------- ost- Project 8,462 4,953 27,004 � Load Reduction 25,389 2,447 9,830 a° % Reduction 75.0% 33.1% 26.7% ESTIMATED RESIDENCE TIME OF ANY PONDS,SWALES,ETC.: This Section is not applicable. The pollutant reduction occurs on the algal turf scrubber flow-way. 3 CDM,"Little Wekiva Basin Watershed Management Plan"2005 Orange County,Florida. USGS Station 02253500 Lat 27°36' 11";Long 80°23'24" 41n 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 14 136