Agenda - Public Works Committee - 09/16/2014

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Chlorophyll -a (a pigment found in algae) is used to estimate the amount of algal production in a lake and, therefore, the lake's response to nutrients. As would be anticipated with the low total phosphorus results found in Grass Lake, the chlorophyll —a summer mean concentration was also quite low. The mean summer concentration of chlorophyll -a was 5.8 ug/L. With concentrations from 10-20 ug/L indicating a mild algal bloom and concentrations greater than 30 ug/L indicating severe nuisance conditions, an algal bloom on Grass Lake in 2012 or 2013 would have been a rare occurrence, if at all. Secchi transparency measures the depth of water clarity in a lake. Grass Lake's average two year transparency is 1.3 meters (4.3 feet). Several transparency readings were at lake bottom and in other instances, the presence of rooted vegetation prevented an "at bottom" reading. In some lakes, high total suspended sediment or high color may also limit transparency. High total suspended sediment may arise from suspended sediments (e.g. from runoff or wind mixing). In the case of Grass Lake, the total suspended sediment is within the typical range for NCHF lakes. Chloride (CI) for Grass Lake was worse than the typical range for NCHF lakes (Table 2). The primary source of CI to Minnesota lakes is winter application of road de-icing (road salt) compounds; however, other potential sources include runoff from agricultural lands, water softeners, treated wastewater effluent, and seepage from septic systems. CI will likely continue to increase in the years to come since it is a conservative pollutant, meaning that it does not break down or leave the lake system over time. Without further examination of the potential sources it is difficult to say which one is the primary contributor for Grass Lake; however, the CI concentration in the lake is far below the water quality standards so there is no immediate concern relative to adverse environmental impacts at these low concentrations. Concentration (ug/L) Total Phosphorus, Chlorophyll -A & Secchi (sia;aw) iyDDaS C H L-A TP Secchi 25 20 15 10 5 0 111 1 1 2 3 4 5 6 7 8 9 10 eeeee CLMP+ data collected for Grass Lake shows that in terms of total phosphorus, chlorophyll and Secchi disk transparency, the water quality of the lake is equal to or better than minimally impacted (reference) lakes in the ecoregion. Trends The primary purpose of CLMP monitoring is to gather water clarity information for as many lakes as possible over a long period of time to determine if the water clarity trend for the lake over time is increasing, declining or remaining stable. At least 20 data points spread over eight years are required for a basic trend analysis, and more data are often needed to see an actual increasing or declining trend. As part of the CLMP, citizens have monitored Site #201 on Grass Lake for 3 separate years (1975, 2012 and 2013). At this time, there are not enough data for Grass Lak to statistically determine if there is a long-term trend in transparency. Based on the three years of data available, summer transparency ranged between 0.61 meters and 1.5 meters (Figure 4). Grass Lake (02-0113) • August 2014 Minnesota Pollution Control Agency 4