01-24-2000 Council Work Session - Laserfiche WebLink

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I I I * »t *■ t i • M M M It is a documented fact that the amount of phosphorus increases with urbanization. When urban development occurs, the function of the natural drainageways and filtration system is adversely affected in three ways: The speed and quantity of the storm water runoff are increased as the absorption ability of the land is eliminated with hardcover (houses, driveways, roads, etc.). This increase in the speed and quantit>' of storm water runoff causes the third adverse effect, which is the increase of the amount of nutrients which are contained in the storm water runoff which now has less time in the natural marsh filtration system to be purified before entering the lake. Nutrients contained in dying vegetation, lawn fertilizers, and from other urban sources are discharged into the lake faster and in greater quantities as the level of urbanization increases. The Hickok report entitled "Storm Water Impact Investigation for the Metropolitan Council", November, 1972, revised February, 1973, shows showed that in an urbanized area, as much as 3.68 pounds of phosphorus per acre is generated versus 0.3 pounds per acre of discharge from a two acre minimum lot size zone as exists in rural Orono. Orono's Surface Water Management Plan of 1974, also done by Hickok, documented that Orono's existing rural wetlands could assimilate the rural phosphorus load but would be insufficient to handle an urban phosphorus load. In fact, Orono's existing urban wetlands were already overloaded. Thus, if rural Orono were to become urbanized, even at relatively low urban densities, phosphorus loading of Lake Minnetonka would increase to dangerous levels. Orono. however, must not rely solely on the ability of the natural filtration system of the marshes and drainageways to purify storm water so ncccssar>' for the preservation of Lake Minnetonka. Several recent studies, one of which is by the University of Wisconsin entitled "Effect of Marshes on Water Quality", concludes that because of our freezing winter temperatures much of the filtration value of the natural marsh process is lost during the annual spring snow melt. It is commonly known that it is the cellular structure of aquatic vegetation that assimilates nutrients from storm water running through marshland in the summer. These studies point out, however, that this cellular structure freezes and ruptures during the winter, thereby releasing all the stored-up nutrients to flush-out into the lake in one mass loading during the first prolonged thaw. There tore. Orono is advised to keep the basic level of urbanization low. particularly in the rural service area, so as to minimize the adverse effect on Lake Minnetonka it this spring thaw thcor>‘ is in time proven to be valid. CMP 3A - 8 c