05/06/1996 Park Board Packet - Laserfiche WebLink

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Section 4 - Reshanau Lake According to Heiskary (1985), lakes with TSIs of 0 to 40 are oligotrophic, 41 to 50 are mesotrophic, 51 to 70 are eutrophic, and greater than 70 are hypereutrophic. According to this classification, Reshanau Lake is highly eutrophic. Eutrophic lakes have high nutrient content and ' high biological productivity. Tro hic State Indices can also be us ed sed to determine suitability for recreational uses. The MPCA (1990) classifies lakes as fully supporting swimming, partially supporting, or nonsupporting. A TSI of less than 60 indicates a lake fully supporting swimming. Reshanau Lake has a TSI of 68 and would be classified as nonsupporting swimming. INFLOW/OUTFLOW CONDITIONS Inflow water quality conditions were monitored at RL-1 and RL-2 as shown in Figure 4-1. Inflow RL-2 drains the largest portion of the watershed(2,090 acres)while RL-1 drain smaller residential portions of the watershed of 512 acres, respectively. Table 4-3 presents the inflow and outflow water quality data. uiflows for RL-1 and RL-2 were increasingly dependent on wet weather as the summer _ progressed. Site RL-2 with its larger drainage area had flow during all sampling times, although in August and September this flow was supplemented by well water from an adjacent well that was being developed. l - Total phosphorus concentrations in all of the inflows were almost always above the Metropolitan Council's guidelines of 100 µg/1 during the 1995 season except on the November 8th sampling date. The highest TP concentrations observed at site RL-1 generally corresponded with peak flow dates, and the flow-weighted mean concentration was higher than the arithmetic average concentration. This means that most of the pollutant load was contributed by larger storm events. The TSS concentrations were always below the 30 mg/1 guideline recommended by the Metropolitan Council. The DO in the inflows remained above 5 mg/l during sampling except during late July and August. These periods of low DO can be attributed to elevated stream temperatures and very low flow conditions. With higher temperatures, the ability of oxygen to dissolve in water decreases. Stream T site RL-2 was also under construction during the late summer months. A well was being drilled and developed during August and September. The groundwater was pumped overland into the 'MONTGOMERY WATSON Page 4b