Agenda 11-02-23; 4-a - Resolution Endorsing Consensus Principles II for Revised Falls Lake Rules

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�n=40 forest"scenario does not achieve the Stage II nutrient allocations prescribed in the Falls Lake Rules or compliance with the chlorophyll-a criterion at all lake monitoring stations. Figure 12 shows the simulated and observed chlorophyll-a concentrations using the UNRBA Falls Lake WARMF model developed for 2014 to 2018. The model was developed and calibrated to observations collected from 2015 to 2018 while 2014 was used to initialize the model. The orange line shows the calibrated model, and the dots show the observations. The top half of the figure shows results at Interstate 85 in the upper part of the lake. The bottom half of the figure shows results in the lower part of the lake near the dam. As with the box and whisker plot shown 150 Calibrated — All Forest in Figure 5, the upper lake has higher and more variable chlorophyll-a concentrations 100 than the lower lake. While the "all forest" no watershed-scale human inputs scenario (green ' 40 -- line) has lower chlorophyll-a concentrations than the calibrated model, it still exceeds the -_ 0 40 pg/L chlorophyll-a standard (red dashed line) in the upper lake. At Interstate 85, 75 approximately 32 percent of the simulated o chlorophyll-a concentrations exceed 40 pg/L J 50 under this hypothetical scenario. For the 40 ------- ---- ---� ,�--.- ---- calibrated model (2015 to 2018 conditions), 25 38 percent of the simulated chlorophyll-a 0 values exceed 40 pg/L at this location. 2015 2016 2017 2018 2019 Therefore, while the percent exceedance Figure 12.Simulated Chlorophyll-a Concentrations in Upper Falls decreases, not even this hypothetical scenario Lake at Interstate 85(top)and Lower Falls Lake near the Dam can meet the chlorophyll-a standard (bottom) everywhere, all the time in Falls Lake. Therefore, it is not possible to achieve the chlorophyll-a standard in Falls Lake as currently applied. Significant reductions in nutrient loading to Falls Lake have been achieved since the baseline period. Another scenario was evaluated with the EFDC model to determine if these reductions from the watershed would eventually lead to reductions in the amount of nutrients cycled into and out of the Falls Lake sediments and whether this would lower simulated chlorophyll-a concentrations. While changes to the internal nutrient releases were predicted, the simulated chlorophyll-a concentrations did not change significantly. This finding indicates that algae and chlorophyll-a levels will be very resistant to changes in nutrient loading or other conditions of the lake. A question that was frequently asked by stakeholders is the importance of USACE lake operations on nutrient storage, algal growth, and chlorophyll-a concentrations in Falls Lake. A scenario was evaluated that simulates an outflow structure at normal pool elevation so that water is not retained for flood control purposes. Because the USACE already targets this elevation in their operations,this scenario did not significantly affect simulated water quality in Falls Lake. Maximum values of some parameters either increased or decreased or shifted in time. However, the percent of simulated chlorophyll-a concentrations at 1-85 exceeding 40 pg/L(37 percent) was nearly the same as the calibrated model (38 percent). Scenarios were also evaluated to determine the percent reduction in nitrogen and phosphorus loading to Falls Lake that would be required to meet the chlorophyll-a standard at least 90 percent of the time in the upper part of the lake where concentrations are usually highest. This analysis shows that an additional 50 percent reduction in total nitrogen loading beyond what has already been achieved would be required. 21