Quantifying the impact of septic tank systems on eutrophication risk in rural headwaters.

Abstract

Septic tank systems (STS) are a potential source of nutrient emissions to surface waters but few data exist in the UK to quantify their significance for eutrophication. We monitored the impact of STS on nutrient concentrations in a stream network around a typical English village over a 1-year period. Septic tank effluent discharging via a pipe directly into one stream was highly concentrated in soluble N (8-63 mg L^-1) and P (<1-14 mg L^-1) and other nutrients (Na, K, Cl, B and Mn) typical of detergent and household inputs. Ammonium-N (NH₄-N) and soluble reactive P (SRP) fractions were dominant (70-85% of total) and average concentrations of nitrite-N (NO₂-N) were above levels considered harmful to fish (0.1 mg L^-1). Lower nutrient concentrations were recorded at a ditch and a stream site, but range and average values downstream of rural habitation were still 4 to 10-fold greater than those in upstream sections. At the ditch site, where flow volumes were low, annual flow-weighted concentrations of NH₄-N and SRP increased from 0.04 and 0.07 mg L^-1, respectively upstream to 0.55 and 0.21 mg L^-1 downstream. At the stream site, flow volumes were twice as large and flow-weighted concentrations increased much less; from 0.04 to 0.21 mg L^-1 for NH₄-N and from 0.06 to 0.08 mg L^-1 for SRP. At all sites, largest nutrient concentrations were recorded under low flow and stream discharge was the most important factor determining the eutrophication impact of septic tank systems. The very high concentrations, inter-correlation and dilution patterns of SRP, NH₄-N and the effluent markers Na and B suggested that soakaways in the heavy clay catchment soils were not retaining and treating the septic tank effluents efficiently, with profound implications for stream biodiversity. Water companies, water regulators and rural communities therefore 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 during summer low flow periods.