Septic tank systems (STS) are often ignored in national or catchment source apportionment because of a lack of data on, and understanding of, their eutrophication impact.  About 2 million such systems discharge nutrient laden waste effluent into potentially sensitive surface waters across the UK and Ireland. Recent catchment studies in the UK indicate they are an important and underestimated source of nutrients (N and P) to headwater streams in rural areas contributing to downstream eutrophication.  Using boron (B) as a tracer for detergent inputs, positive relationships between B and soluble reactive P (SRP) under both low flow and high flow conditions, in areas with no known sewage treatment works, have implicated septic tanks as a significant nutrient source. Direct monitoring of surface waters in one rural catchment showed 4-10 fold increases in average stream ammonium N (NH₄-N) and SRP concentrations downstream of domestic dwellings. The very high concentrations, inter-correlation and dilution patterns of SRP, NH₄-N and the effluent markers sodium (Na) and B suggested that septic tank soakaways in the heavy clay catchment soils were not retaining and treating the septic tank effluents efficiently. Stream discharge was the most important factor determining the eutrophication impact of these STS with greatest impact during summer low flow. In another catchment, removal of STS resulted in a dramatic improvement in river water quality. Although individually STS may contribute small nutrient loads because they generate only a small amount of annual flow, their collective contribution to rural P loads can be significant depending on STS density. Rural communities, water regulators and the water industry need to be made more aware of the potential impacts of STS on water quality so that their management can be optimised to reduce the risk of potential eutrophication and toxicity to aquatic ecosystems.