Evaluation of spray irrigation for on-site wastewater treatment and disposal on marginal soils

Two field sites with soil limitations (shallow depth to restrictive layers) were used in the evaluation of slow rate spray irrigation as an alternative methodology to conventional on-site wastewater treatment and disposal. Secondary treatment and disinfection of the effluent was performed before application to the soil-plant system at rates of 1.25 and 2.5 cm/wk. After 20 months of effluent application at Site 1 and 12 months of application at Site 2, adequate wastewater renovation was occurring at both sites. Nitrogen removal rates by the soil-plant system were between 85% and 95%, with${\rm NO}_{3}{}^{-}\text{-}{\rm N}$concentrations in shallow groundwaters at both field sites less than 0.8 mg/L. Phosphorus removal rates were at least 96%. Stormwater runoff quality measurements from Site 1 indicated no serious threat to nearby surface waters. Small increases in soil${\rm NO}_{3}{}^{-}\text{-}{\rm N}$and${\rm NH}_{4}{}^{+}\text{-}{\rm N}$concentrations to the 30-cm depth occurred only at Site 2. There was no evidence of increased soil${\rm PO}_{4}\text{-}{\rm P}$concentrations as a result of wastewater irrigation. The soil moisture content increased slightly at depths measured from 30 to 150 cm during summer months at both sites. Both sites demonstrated that the renovation capacity of on-site spray irrigation systems in soils with limitations can be very efficient.