Application of Dye Tracing for Simulated Sewage Spill Management in Agricultural Landscapes

Water sources used for agricultural irrigation can be intermittently contaminated with human fecal material from municipal wastewater discharges, combined sewer overflows, leaking sewage infrastructure, or failing onsite wastewater systems. In August 2018, a pipeline carrying raw sewage ruptured beneath a roadway crossing the main canal of the Roza Irrigation District (RID) in the Yakima Valley, Washington, resulting in the release of untreated sewage into irrigation water. To better understand contaminant transport following infrastructure failures and to simulate the 2018 spill, the USFDA and the Western Center for Food Safety at the University of California, Davis, conducted a hydrologic dye tracer study in August 2019. The study was designed to characterize water transport and mixing relevant to fecal contamination, using Escherichia coli as a regulatory indicator of fecal contamination while employing Rhodamine WT as a conservative tracer to measure the transport, dilution, and mixing of a pollution source introduced into the canal. Moored fluorometers deployed within the RID canal, combined with towed fluorometers traversing longitudinal and cross-sectional transects, were used to evaluate pollutant fate and transport. The dye plume reached the most downstream station (73.2 km) in 30 h 30 min. Under the 2019 tracer-study conditions, the field-derived peak 3 h mean steady-state dilution ranged from 132,301:1 to 189,221:1. After adjustment to the reported 2018 canal flow and sewage-discharge rate of 22.7 L/min, the corresponding scenario-specific dilution ranged from 66,143:1 to 94,599:1, producing estimated E. coli concentrations of 153 to 219 CFU/100 mL. Findings demonstrated that dye tracing, mass-balance calculations, and empirical travel-time analysis can inform sewage-spill response and provide critical decision-support for agricultural water risk management.