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

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=34 Resources Conversation Service, North Carolina Cooperative Extension, and the NC Department of Environmental Quality(DEQ), as well as agricultural and environmental interests from within the watershed. Information on the acreage of crops and pasture in the watershed and the amount and timing of nitrogen application was provided by the WOC for each county for inclusion in the watershed model. Assumptions regarding potassium and phosphorus application rates were obtained from the report "Delineating Agriculture in the Neuse River Basin" (Osmond and Neas 2011). For developed lands, pervious surfaces such as lawns also receive nutrient application to support plant growth. Less information is available to develop the modeling assumptions for these areas because the owner types and individual preferences and practices vary widely(homeowners, institutions, parks, etc.). Fortunately, two publications that included local homeowner surveys are available to provide reasonable estimates of nutrient application for these types of areas (Flemin 2013 and Osmond and Hardy 2004). These data were used to help build the watershed model and provide a level of accounting for this source in the watershed that is not normally available. Wastewater Treatment The model also accounts for the treatment of wastewater as either point source discharges or non- point sources. Point sources are wastewater treatment plants (WWTPs)that can be considered major(discharging more than one million gallons per day) or minor(discharging less). Non-point sources are onsite wastewater treatment systems (e.g., septic systems and discharging sand filter systems). There are four minor WWTPs (as defined by EPA) in the Falls Lake Basin that treat less than 1 million gallons per day of wastewater. Effluent data for these four facilities was provided by DWR. Two of these facilities are currently in compliance with the Falls Lake Rules, and two are not. While these systems do not contribute significantly to the nutrient load delivered to Falls Lake (one percent or less), they do represent an opportunity for incremental reduction in nutrient loading. Effluent discharges from the major WWTPs (treating more than 1 million gallons per day of wastewater) were obtained directly from the North Durham Water Reclamation Facility, Town of Hillsborough, and the South Granville Water and Sewer Authority. Over$80 million have been invested to upgrade and optimize these three facilities since 2006. Based on DWR's 2021 Status Report for Falls Lake,these three discharges have reduced their collective total nitrogen and total phosphorus loads by 57 percent and 73 percent, respectively,from the 2006 baseline. Sources of Nutrient Loading Delivered to Falls Lake During the UNRBA study period (2014 to 2018), annual rainfall each year was average to high. Under these conditions, WWTPs contributed approximately 6 percent of the total nitrogen load and 3 percent of the total phosphorus load delivered to Falls Lake (Figure 7). These estimates are based on actual discharges, not permit limits. A lower rainfall condition was also simulated with the model to represent DWR's baseline modeling period more closely(around 20 percent less rainfall than 2014-2018). Under these lower rainfall conditions, the model estimates that WWTPs would contribute approximately 10 percent of the total nitrogen load and 6 percent of the total phosphorus load delivered to Falls Lake. Based on the work of the Scenario Screening Group of the UNRBA and contact with the major system owners, further reducing the nutrient loading from these facilities would be extremely expensive and energy intensive. Additional reductions would also not significantly impact the total loads delivered to Falls Lake. Considering the contributions from watershed sources delivered to the lake, further reductions at WWTPs would have little impact on lake water quality. When looking at the types of actions that 15