Phosphorus in soil treatment systems : 4 accumulation and mobility 5

17 Septic tanks with subsequent soil treatment systems (STS) are a common treatment technique 18 for domestic wastewater in rural areas. Phosphorus (P) leakage from such systems may pose a 19 risk to water quality (especially if they are located relatively close to surface waters). In this 20 study, six STS in Sweden (11 to 28 years old) were examined. Samples taken from the 21 unsaturated subsoil beneath the distribution pipes were investigated by means of batch and 22 column experiments, and accumulated phosphorus were characterized through X-ray 23 absorption near edge structure (XANES) analysis. At all sites the wastewater had clearly 24 influenced the soil. This was observed through decreased pH, increased amounts of oxalate 25 extractable metals and at some sites altered P sorption properties. The amount of accumulated 26 P in the STS were found to be between 0.32 and 0.87 kg m, which in most cases was just a 27 fraction of the estimated P load (< 30%). Column studies revealed that high P concentrations 28 (up to 6 mg L) were leached from the material when deionized water was applied. However, 29 the response to deionized water varied between the sites. As evidenced by XANES analysis, 30 aluminium phosphates or P adsorbed to aluminium (hydr)oxides, as well as organically bound 31 P, were important sinks for P. Generally soils with a high content of oxalate-extractable Al 32 were also less vulnerable to P leakage. 33