06-25-1990 Council Packet - Laserfiche WebLink

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CHAPTER 4 WATER QUALITY Non-point source pollution and Eurasian water mUfoU pose serious threats to the water quality of metropolitan lakes. This chapter examines these threats, their effects on recreational use of tlv: lakes, and their prevention. FINDINGS Overall, recTeanonal use of metropolitan lakes generally has not been affected by declining water quality, '“^le Mu.bar on the t^xt page describes Eurasian water mafoU, an important threat to Minnesota la>.f s.; The Metropolitan Council monitors lakes for water quality measures that influence recreational use, including water clarity, abundance of ?Ugac, dissolved oxygen content, and water tcmpciarj^. V»Tier. water clarity decreases, re creational use sometimes declines. Lakes with poor clarity often have a build-up of algae which leads to green water and unpleasant odors. According to Dick Osgood, limnologist for the Metropolitan Council, boating on lakes in the rcpon gene: »Uy has not been af^l cted by changes in water quality. While the water quality of many lakes m the region has degraded as a result of urban development, the variety and sheer number of lakes in the area usually allow people to find a lake that suits their expectations and recreational needs. Most recreational activities on lakes do not affea water quality. Minor gasoline leaks from toats generaUy are not a problem except in confined areas of lakes. Boat travel can more greatly aff^ the water quality of shadow lakes by disturbing the sediments. This causes nutrients to be released from the sediments into the water and reduces water clarity. Development of lake watersheds inevitably leads to degradation of lake water quality. Since sewage has largely been diverted away from the lakes, non-point source poUution now poses the most Mnous threat to lake water quality. Construction and development activities mcrease runoff and nutrient inputs to the lakes, leading to more algae and decreased water clarity. Various warjrshed treatments, such as stonnwater detention ponds, are used to control runoff and nutrient loading. However, a recent report by the Metropolitan Councu suggests that these treatments cannot completely offset the effects of development near the lakes.^ Nutrient export from a developed watershed, even with appropriate treatment, will probably be greater than from an undeveloped watershed. The MetropoUtan CouncU strongly advocates the continued use of watershed treatment, but suggests that m- lake treatments may be needed in the future to manage nuisances, such as algal growth, associated with nutr.ent loading ^m runoff. 1 Richard A. Osgood. An Evaluation of the Effects of Watershed Treatment Systems on the Summertime Phosphorus Concentration in Metropolitan Area Lakes. Metropolitan Council. 1989. pp. ii-iii.