Studies of P accumulation in soil/sediment profiles used for large-scale wastewater reclamation

We studied the long‐term accumulation processes and material balances of phosphorus (P) in the soil/sediment profiles of large‐scale effluent recharge basins used for wastewater reclamation by the soil aquifer treatment (SAT) system. The objective was to quantify and clarify the long‐term performance of soil/sediment in the SAT system as a sorbent to filter out P from the recharged effluent. Total P concentration in the soil/sediment profiles of the Shafdan wastewater treatment plant (WWTP) increased over 25 years of operation (1977–2001) by 20–220 mg kg^?1, as a result of adding loads of 0.17–6.2 kg m^?2 of P. Retained P in the 0–2.0 m soil layer increased from 0.06 to 0.31 kg m^?2 with increasing cumulative load of P while the retained percentage gradually decreased from 19 to 5% of the cumulative P load. Accumulation rate of P in the 0–0.15 m horizon in the basins was inversely proportional to recharge time, decreasing from ～28 mg P kg^?1 year^?1 during the first 3 years of operation, to <2.3 mg P kg^?1 year^?1 between the 20th and 25th years of operation. Thus, P content in this horizon approached a steady state after about 10–15 years of effluent recharge under the operational conditions of the Shafdan WWTP. Phosphorus concentration in deeper horizons increased at constant rates of approximately 7.8, 5.9 and 2.9 mg P kg ^?1 year^?1 in the 0.15–0.30, 0.30–0.60 and 1.80 to 2.10‐m horizons, respectively, over the 25 years of effluent recharge. However, the accumulation front of P appears gradually to have moved deeper in the soil profile. In general, this phenomenon may be explained by kinetic limitations to the achievement of full adsorption equilibrium for P between the flowing solution and the solid phase components of the soil. In addition, both the increase of EPC₀(the equilibrium P concentration in solution at which there is no sorption or desorption to or from the soil under the given conditions), caused by long‐term effluent recharge, and gradual decrease of the annual average concentration of P in the effluent input after 1995, may result in the steady‐state level of P in the topsoil of the basin.