01-24-2000 Council Work Session - Laserfiche WebLink

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I I I r r [ r r r \ I I I -M The 208 Study concluded that urban stormwater runoff pollution is potentially a significant portion of total annual loading for the Metropolitan Area. Further water pollution abatement programs must consider this source of pollutant input and change to a balanced approach from the historic practice of considering only point sources of pollution if future water quality goals are to be reached. Future upgrading of municipal treatment plants and industrial dischargers should be viewed in light of potential large capital e.xpenditures for treatment structures versus reduced expenditures for minimum-structural or source control of nonpoint sources of pollution. The study recommendation was that "abatement of nonpoint pollution from urban runoff should receive high priority in future water pollution programs". The Minnehaha Creek Watershed District's overall plan (1969) noted that: "Urban development rapidly increases the amount of storm run-off in an area because the hard-surfacing of streets and construction of buildings reduces the amount of water absorbed by the soil. In addition, urban areas are usually graded and drainage provided to prevent accumulation of surface water. Since ground water aquifers are normally recharged by seepage of surface water, increased urbanization may be expected to bring not only greater surface water run-off but a decrease in the natural recharge of ground water". This finding lead to the MCWD policy that each municipality prepare a drainage plan for surface water resulting from urban runolTand that "each community should identify swamps and marshes to be left in their natural state", thereby acting as settling ponds and filters and avoiding direct runolTinto Lake Minnetonka. The "Harza Study", a program for prescr\ing the quality' of Lake Minnetonka, (1971) determined that the limiting nutrient for algal growth in Lake Minnetonka was phosphorus and that the natural function of marshland and wetland was to retard surface water flow and remove nutrients, especially the phosphorus. And, stomi water, especially that portion which runs off from urban lands, is expected to become an ever more important source of phosphorus pollution. The Harza Study noted that presets ing wetlands will play an important part in the future handling of storm water. As the watershed continues to increase in population, the rate of flow following stomis will tend to increase. The existing urbanized areas were found to be approximately 25% imper\ious while new urbanization tended to be 35% impcr\ious. Ihis factor plus the general rate of urbanization was at that time having the effect of increasing the water resources of Lake Minnetonka by an equivalent of 4.25 inches per year. This accelerated flow rate was causing pressure for the construction of storm sewers or other structures to convey the stomi water through the low areas and to reduce the w ater level increase. But. the construction of storm sewers or paved channels allows stomi water to CMP 3A- 16