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

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�n=42 chlorophyll-a standard 90 percent of the time in the upper part of the lake. A 50 percent reduction in total nitrogen load from 2014 to 2018 levels translates to a reduction of 825,000 pounds per year on average. This is a massive reduction goal on top of the reductions already achieved since 2006. Multiple pump-and-treat type systems would be needed to achieve this level of reduction. Many, if not all of these systems, would be treating nutrients from unmanaged lands including forests and wetlands that do not receive watershed-scale human inputs (however, nutrients are "applied"to these areas from atmospheric deposition). Implementation Assuming the highest estimate of nitrogen removal (6,000 pounds per year, which is unlikely), removing 825,000 pounds of nitrogen per year would require 138 full-scale algal floways. These floways would generate an estimated 276,000 wet tons of harvested algae, which would need to be transported to an offsite treatment facility. A rough extrapolation of the pilot study estimates the cost of 138 algal floways at$1.1 billion for construction and $23.4 million per year for operation and maintenance. These facilities have an assumed life span of 20 years, meaning that life-time costs would be approximately$1.6 billion (construction plus 20 years of operational costs). Treatment beyond 20 years would require additional construction or repair costs. This is a conservative cost estimate as site selection, land costs, and legal costs are not included. These costs assume the maximum amount of treatment based on the pilot study. Assuming the low end of the treatment range (3,000 pounds of nitrogen removed per year per system), construction costs would double to $2.2 billion, and operation and maintenance would double to $46.9 million for a total of$2.7 billion for the 20-year life span of the systems. Feasibility While hypothetical costs can be estimated for 138 systems,there are many logistical reasons that warrant this approach infeasible: • The City of Durham has faced challenges siting even one of these systems, so siting 138 would be unachievable. • This number of systems would require treatment of 1.38 billion gallons of water per day (10 million gallons per day times 138 systems). This is more water than entered Falls Lake during the average to high precipitation conditions that were evaluated by the UNRBA(only 239 million gallons per day to 476 million gallons per day). If sufficient water is not present in the tributary or lake where pumping occurs,the systems cannot fully operate, and pumps may be stressed. • More than 276,000 wet tons of algae would have to be harvested, transported, and composted each year. This would likely require construction of a dedicated composting facility. • The effectiveness of the floways is estimated based on the results of a pilot study. For the sake of illustration, constant removal efficiencies for all 138 systems are assumed. However, cumulative effects for multiple systems are unknown, and removal efficiency for a given floway could decrease significantly if incoming water had already been treated by an upstream floway. Similarly, some of these systems would have to be placed in areas that were predominately forested. Inflow concentrations from these areas would likely be lower than the pilot study, and removal efficiencies would be lower. • Processing that volume of water and removing that level of nutrients from the system could have unknown environmental and ecological impacts. Addition of 138 algal floway footprints and algal harvest transportation could have negative environmental impacts such as increased impervious area and greenhouse gas emissions. 23