Agenda - Public Works Committee - 09/16/2014

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Figure 4. Secchi readings 2012 & 2013 for Site 201 Depth (meters) Secch Readings N N N N N Ni N N N N N N Ni N CO CO M CO CO M CO CO M CO CO M M %--I Ni %--I %--I %--I Ni %--I %--I %--I Ni %--I %--I %--I %--I Ni %--I %--I %--I M %--I %--I %--I M %--I %--I %--I M %--I %--I %--I M M %--I %--I %--I %--I 0' 0 00' 0 0 0' 0 0 0 0' 0 0 0' 0 0 0' 0 00' 0 0 0%-1, 0 0 0 0 Ni 0 N N N 0 N N N 0 N N N N 0 N N N 0 N N N 0 N N N 0 N N N 0 0 N N N N m. Ll N 0000 . M 0 O N 0000 r\-I VD 0\ LD M 0000 \ M 0 L! \ 0 0000 Ch\\ LD M 0000 0 N 00 rl N N l0 i--I N N M i i r i N (NI i--I r-I (NI N %--I N N LD %--I N N M %--I %--I N N a) %--I N N %--1 \ \ \ \ \ \ \ \ \ \ \ 00 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ L.f) l0 l0 l0 l0 00 00 000 00 00 Q) Q) Q) Q) u) u) u) L.f) l0 l0 l0 l0 00 00 00 00 00 Q) 0 0.5 1 1.5 2 Summary ■ 2012 ■ 2013 Grass Lake 2 year mean NCHF Standard Grass Lake has very good water quality and appears to be fully meeting the thresholds set to protect shallow lakes in the NCHF ecoregion for aquatic recreation (Table 3). Grass Lake's shallow nature, combined with its location, make it susceptible to an influx of nutrients as well as pollutants such as chloride. Engagement at the local level will be required to maintain Grass Lake's high level of water quality. Table 3. A comparison of water quality data from Grass Lake to the shallow lake eutrophication standards for the NCHF ecoregion TP (pg/L) Chl-a (pg/L) Secchi (m) Thresholds set to protect shallow <60 <20 >1.0 lakes in the NCHF ecoregion for aquatic recreation use Grass Lake 2-year summer mean values 13 5.8 1.3 Recommendations: • Continue to participate in the CLMP and regularly collect transparency data to provide the continuous water quality records needed for trend assessment. • Engage with city and county officials to ensure protection of wetlands in the surrounding watershed. Wetlands trap and filter sediments and nutrients, limiting their eventual run-off into Grass Lake. • Best management practices should be used when applying road deicers. Specifically, minimize the salting of roads near the lake, and stockpile snow in upland areas away from the Lakeshore. Grass Lake had chloride concentrations higher than the ecoregion expectations. This is most likely due to "urban" runoff. • Maintain native aquatic plant beds to support fishery habitat. Native aquatic plants also provide natural wave breaks and results in decreased shoreline erosion. Increased wave action stirs lake sediments, clouding the water, making it difficult for new plants to grow. Grass Lake (02-0113) • August 2014 Minnesota Pollution Control Agency 5