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A discussion of Elk River's proposed mitigation steps and why these alternatives are both reasonable and <br />practical follows. <br />REVISIONS TO THE CITY'S WATER QUALITY TREATMENT REQUIREMENT AND IMPLEMENTATION <br />OF WATER QUALITY TREATMENT ORDINANCE <br />The Loading Assessment shows that for Elk River's urban land uses (urban and high density residential, <br />commercial/industrial, Old Town, Mixed Use and public/semi-public) additional water quality treatment is <br />necessary to meet Baseline loading rates. It also shows that a conversion from agricultural land use to rural <br />residential land use is a slight improvement on the Baseline pollutant loads. <br />The City's current Section 2500 Design Standards require sedimentation basins and wet extended detention <br />ponds designed in accordance with the Minnesota Stormwater Manual. This is currently only a design <br />standard and not an adopted ordinance. Options far increasing the water quality treatment required for <br />urban land uses were considered. For comparison, a requirement of 60% total phosphorus removal from <br />urban developments was analyzed to determine whether itwould be adequate to meet mitigate increased <br />loadings. Using an accepted relationship between TP and TSS shown in Figure 1, removal of 60% of TP <br />generally correlates to 93% TSS removal. <br />figure 7 -Stormwater Pollutant Removal Efficiencies Related to Pond Detention Time. <br />(Adapted from OWML,198.y) <br />100 <br /> <br />90 <br /> <br />80 <br /> <br />o ~o <br />60 <br />a <br />50 <br /> <br />40 <br /> <br />~ 30 <br /> <br />C <br />20 <br />° <br /> <br />10 <br /> <br />0 <br /> <br />0 5 10 15 20 25 30 35 40 45 50 <br />Detention Time (hrs) <br />Subwatershed ER04 is primarily urban for 2020'conditions, versus large tracts of agriculture for 1988 <br />conditions. As an example, the 60% TP removal requirement was applied to all urban land uses within <br />ER04 for 2020 conditions to determine if it is adequate to meet the nondegradation goals. Table 1 <br />presents a comparison of loading rates with and without the 60% TP removal requirement. <br /> _ .~ <br />-- <br /> <br />__ <br />. . <br />-- -- <br />~_ <br /> <br />T~ <br />I <br />~ 1 <br /> __ <br />- <br />~! <br />-F TP _ <br /> - -~ - T55 <br /> 1 <br />Elk River 1482-0 700 1 <br />Nondegradation Report Page 3 <br />