Herrera’s Newly Developed Microbial Source Tracking Method

On-site sewage systems, referred to as septic systems, are used extensively across Washington State to treat wastewater from homes and businesses. Old and unmaintained septic systems pose a substantial threat to public and environmental health when drain fields cease to effectively adsorb the wastewater pollutants. In terms of water quality, septic systems can be a significant source of phosphorus, fueling toxic cyanobacteria blooms in lakes. Traditionally, health departments rely solely on review of submitted inspection records, but occasionally conduct inspections themselves and use dye testing methods to identify failing sewage systems, which requires an extensive amount of effort and homeowner cooperation.

In 2017, Herrera’s team developed a simple microbial source tracking method using optical brightener fluorescence and quantitative PCR analysis of Bacteroidetes DNA biomarkers to determine whether septic systems were contaminating lakes with phosphorus, fecal bacteria, and other constituents. Initially developed for Lake Whatcom, Herrera applied this method to other Washington lakes, conducting lake shoreline and outfall surveys by boat on Lake Tapps near Auburn and Black Lake in Olympia.

The team surveyed the three lakes between 2017 and 2020, executing each phase in wet, winter weather when the water table was high and soils were saturated. In Phase 1, the team used an optical brightener meter to successfully identify hot spots at multiple locations along the shoreline. Once identified, water samples were collected for analysis of phosphorus, fecal indicator bacteria, and two human biomarkers. In Phase 2, significant septic system inputs identified by high concentrations of human biomarkers in lake and drainage samples were sampled again to verify the Phase 1 results.

Herrera’s data collected from these surveys prompted health departments in Whatcom County, King County, and Thurston County to further investigate septic systems sources and reduce nutrient and human pathogen inputs to the lakes from the polluting basins. As of September 2021, the Regional On-Site Sewage System Loan Program has extended loans for septic repairs and replacement to the entire state, helping homeowners fix failing septic systems to keep local waters clean. Herrera looks forward to further providing limnological solutions to our clients across the Pacific Northwest, improving water quality and supporting the overall health of our local watersheds for future generations.