Long-term accumulation and material balance of organic matter in the soil of an effluent infiltration basin

The fate of organic matter (OM) in large-scale infiltration basins used for wastewater treatment by the soil aquifer treatment (SAT) system was investigated. Measured changes in the organic matter concentrations in the soil profiles of the infiltration basins and detailed long-term records of OM concentrations in the recharged effluent and in the observation wells and recovery wells water, were used to calculate OM material balances in the SHAFDAN wastewater treatment plant, serving the City of Tel-Aviv, Israel, since 1977. The average annual total organic matter (TOM) load delivered by the effluents to the soil was ~ 5 kg m− 2 y− 1. Soil OM concentrations increased from 0.11% in the pristine soil to ~ 0.8% and ~ 0.6%, in the 0–0.15 m and 0.15–0.30 m soil layers, respectively, after ~ 20 y of effluent recharge, but did not change significantly in the 1.80–2.10 m deep layer. The OM accumulation rates in the top two soil layers were fast initially, then declined slowly and the OM concentrations approached a steady state following 10–15 y of effluent recharge. This suggests that stabilization of the ‘active biofilm’ layer in the infiltration basins' soils is a relatively slow process. Material-balance calculations showed, that accumulated OM in the top 0–2.1 m soil layer amounted to only ~ 4% of the TOM added by the effluents during ~ 20 y of recharge. Along the flow pathway of the effluent through the vertical 50–100 m thick soil-sediment column, DOC concentrations decreased by 70–90% (from ~ 18.9 mg L− 1 to ~ 3.7 mg L− 1). Continued flow in the aquifer from the observation wells to the recovery wells further decreased DOC concentrations by about 50% (from ~ 3.7 to ~ 1.5 mg L− 1).