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

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�n=41 Implications for a Revised Nutrient Management Strategy Given the vast amount of data collected in the watershed and lake, and the unavoidable constraints on further reductions in nutrient loading, a more reasonable implementation program with achievable water quality standards is the most correct path. A long-term, continued program based on investment in water quality management in the watershed and lake is needed. This approach will provide continued progress on protecting and improving water quality. The information developed during this evaluation process shows a reservoir that is eutrophic but meeting its designated uses. It also shows a lake that is subject to elevated algal activity and ongoing development in the watershed. To mitigate the impacts of development, implementation of the New Development Rule is an important management component of the strategy. The full impact of new development includes additional loading to the lake at flows exceeding SCM design standards. Land conservation and promoting the retention and creation of natural areas in the watershed are also essential components of an effective, ongoing strategy. Actions to improve watershed health and address current nutrient loading will provide protection of this important water resource. Pump-and-Treat Scenario As discussed in previous sections, one model scenario revealed that removal of all human-generated nutrient inputs and conversion of all land to forests would not meet the chlorophyll-a standard in Falls Lake. This means that all sources in the watershed would have to be treated to levels below that of a wooded watershed to comply with chlorophyll-a standards. This level of treatment would require a pump-and-treat system. To illustrate the cost, tremendous challenges, and economic impact of applying pump-and-treat systems to potentially meet the chlorophyll-a standard, a hypothetical implementation of an algal harvesting system called an algal floway is described below. System Description The City of Durham pilot tested a small-scale algal floway nutrient removal system that pumps water out of Falls Lake, releases the water to flow across shallow, open conveyances that grow algae, and then discharges the treated water back into the lake. The cultivated algae consume (remove) nitrogen and phosphorus as the water flows across the conveyance. These algae are periodically harvested along with the nutrients stored in their cells. The harvested material is transported to another location and treated to convert it into a soil amendment or to be disposed of alternatively (landfill, etc.). Reports associated with the pilot test, conceptual design, and 2021 construction cost estimates are available online at https://www.durhamnc.gov/4678/Algal-Floway. Based on the pilot study results, the City of Durham evaluated the feasibility and operational aspects of a full-scale algal floway which could treat 10 million gallons of water per day. Approximately 2,000 wet tons of cultivated algae would be removed from the floway each year. The amount of nitrogen removed by the algae would range from 3,000 to 6,000 pounds each year. This range assumes the water level at the withdrawal point(either a tributary to Falls Lake or the lake itself) would always be high enough to pump water into the treatment system and that the concentrations of nutrients from the pumped water were like those observed in the pilot study. The cost to construct this facility was estimated at$7.9 million with annual expenses over a 20-yr service life of $170,000 per year. These costs do not include the cost of land or the effort of successfully siting the system. Reductions Required to Meet the Chlorophyll-a Standard Ninety Percent of the Time As noted in previous sections, the UNRBA watershed modeling estimates the current delivered total nitrogen load to Falls Lake averages 1.65 million pounds per year. The UNRBA EFDC lake model estimates that a 50 percent reduction in this total nitrogen load would be required to meet the 22