Defining the sources of low-flow phosphorus transfers in complex catchments

Area of Science:

• Environmental Science
• Water Quality Management
• Eutrophication Studies

Background:

• Land-to-river nutrient transfers can cause persistent eutrophication at low flows.
• Agricultural runoff is a major contributor during storm events, complicating source identification.
• Domestic septic systems are a potential, often overlooked, source of pollution, especially during low-flow periods.

Purpose of the Study:

• To investigate the impact of domestic septic tank systems on river pollution during low flows.
• To quantify the contribution of septic systems to nutrient loads in rural catchments.
• To differentiate septic system pollution from other nutrient sources in small catchments.

Main Methods:

• Conducted septic system surveys in three small rural catchments (3-5 km²).
• Utilized continuous total phosphorus (TP) monitoring and low-flow water quality surveys.
• Employed biochemical fingerprinting techniques, including sterol ratios, protein, microbial (E. coli, enterococci), and boron analysis.

Main Results:

• A correlation was observed between the density of poorly maintained septic systems and frequent total phosphorus (TP) concentrations.
• Biochemical fingerprinting revealed 7-27% of sediments were contaminated with human fecal material.
• Phosphorus fractions positively correlated with markers of fecal and greywater contamination during low flows.

Conclusions:

• Domestic septic systems are a significant source of nutrient pollution in rural catchments during low-flow conditions.
• Biochemical fingerprinting effectively identifies septic system effluent as a contributor to river contamination.
• Targeted management of septic systems is crucial for mitigating eutrophication in affected riverine ecosystems.