Black Lake Algae Control Plan, Alum Treatments, and Septic Source Tracking

Black Lake is a large lake near Olympia, Washington with high nutrient levels resulting in frequent lake closures due to toxic algae blooms. Herrera prepared an algae control plan in 2015 for the Black Lake Special District that used an HSPF model of the watershed with lake data to prepare detailed water and phosphorus budgets showing the primary cause of the blooms was the release of phosphorus from sediments during the summer. Herrera limnologists collected sediment cores for analysis of phosphorus fractions and used those results to calculate the amount of aluminum sulfate (alum) and Phoslock required to inactivate sediment phosphorus and prevent algae blooms. Herrera prepared an alum treatment plan refining the alum dose and specifying application methods, permit requirements, and water quality monitoring procedures. Herrera engineers prepared contractor specifications and provided oversight, monitoring, and reporting of the alum application over a 5-day period in the spring of 2016. Although the alum treatment eliminated toxic algae blooms through 2020, the alum dose was not sufficient to meet water clarity goals so the District conducted a second alum treatment at a quadrupled dose over a 17-day period in the spring of 2021. Herrera provided contractor oversight, water quality monitoring, and public and permitting agency notifications documented in a detailed report of the 2021 alum treatment.

To also address watershed sources of phosphorus, Herrera designed a study locating sources of phosphorus and fecal coliform bacteria from on-site septic systems. For this initial pollutant monitoring project, Herrera monitored flow, total phosphorus, E. coli bacteria, and optical brightener fluorescence from laundry detergents at 15 key drainage sites during base flow and storm events. The results were used to identify contaminant sources and guide education of watershed residents on proper best management practices for animal waste disposal, fertilizer application, and septic system maintenance. Herrera then conducted a second phase of pollutant source tracking to verify the observed septic system inputs and to survey the entire lake shoreline for other inputs. Three contaminated streams were identified and reported to the Health Department for enforcement of septic system regulations and the District notified residents of the potential health risks and septic system maintenance needs.