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1 ' <br /> CMP Part 3A. Environmental Protection Plan <br /> It is commonly known that it is the cellular structure of aquatic vegetation that <br /> assimilates nutrients from storm water running through marshland in the <br /> summer. These studies point out, however, that this cellular structure freezes and <br /> ruptures during the winter, thereby releasing all the stored-up nutrients to <br /> flush-out into the lake in one mass loading during the first prolonged thaw. <br /> Therefore, Orono is advised to keep the basic level of urbanization low, <br /> particularly in the rural service area, so as to minimize the adverse effect on Lake <br /> Minnetonka if this spring thaw theory is in time proven to be valid. <br /> Supplementary Stormwater Pondin� & Alternatives While Orono's �- <br /> a�e�e� Surface Water Management Plan adopted in.2002 places a significant <br /> emphasis on preservation of wetlands, it also provides for the establishment of <br /> supplementary stormwater management ponds to help counteract the impacts of <br /> development within the watershed. <br /> The City of Orono in October 1993 amended the 1980 Comprehensive Plan by <br /> adoption of the use of NURP (National Urban Runoff Program) standards for the <br /> design of man-made stormwater ponds. T::�;, :�;��� � r* ��� �* * � <br /> , NURP ponds have <br /> since been required for stormwater management within new developments in <br /> accordance with standards of the Minnehaha Creek Watershed District. <br /> William W. Walker, Jr., an environmental engineer from Concord, <br /> Massachusetts, is credited with the 'model' which evaluated the design criteria <br /> for NURP ponds. He used phosphorus retention data from 60 reservoirs and <br /> compared it to phosphorus retention data for a number of ponds and lakes, <br /> including many in the Twin Cities area. He concluded that his 'model' could be <br /> used to predict the phosphorus retention capability of urban lakes and wet <br /> detention basins based on a number of design parameters. Conversely, it is then <br /> possible to design ponds which should retain a given amount of phosphorus. <br /> Phosphorus inflow from a watershed to a basin is reasonably predictable given <br /> the characteristics of the watershed. <br /> NURP pond design criteria are primarily based on the removal of suspended <br /> particles such as sand, silt, etc. Secondary design criteria enhance the capability <br /> of NURP ponds to remove other pollutants such as trace metals, hydrocarbons, <br /> nutrients (such as phosphorus) and pesticides. Phosphorus is mainly removed <br /> through biological uptake in algae and aquatic plants. When the algae die, the <br /> nutrients fall to the bottom of the pond and become part of the sediments. <br /> Ponds in the Twin Cities area designed according to NURP standards have total <br /> phosphorus removal efficiencies of 47 to 68 percent according to Walker. <br /> Possibilities for improving this include (1) increasing mean pond depth; (2) <br /> promoting infiltration; (3) promoting plug flow conditions(i.e. multiple cell <br /> City of Orono Community Management Plan 2008-2030 Page 3A-7 <br />