Phosphorous retardation and breakthrough into well water in a soil-aquifer treatment (SAT) system used for large-scale wastewater reclamation.

Retardation and breakthrough of phosphorous in the soil/sediment profiles of a SAT system at the Shafdan wastewater treatment plant, Israel, were investigated in situ. Area-weighted average effluent load to the whole site was 65 m yr(-1). Annual average concentrations of P in the recharged effluent ranged between about 1.5 and 7.7 mg L(-1) during 25 yr of operation, while P in groundwater remained <or= 0.05 mg L(-1) in most wells. Recharge is done through an overlying layer of >40 m sandy soil/sediment formations. By combining results of isotherm tests, long-term monitoring of phosphorous (P) in solid and liquid phases of the recharge site, a simple multi-cell tracer-movement model and measured chloride breakthrough curves to the groundwater we calculated P distribution coefficients and estimated the retardation factor of P. Laboratory measured, isotherm-based distribution coefficient, Kd(I), was about 4-6 L kg(-1) at equilibrium P concentration <6 mg L(-1), while field-based Kd(F) was considerably higher, reaching about 20-55 L kg(-1) after a load of around 1800 m effluent was recharged. Measured P breakthrough times into two shallow observation wells were 19-21 yr. Calculated P breakthrough times using Kd(F) data agreed with observations while those calculated using Kd(I) grossly underestimated retardation and predicted much shorter breakthrough times. This validated the approach and model used. Estimated P breakthrough times to the deeper observation wells and the recovery wells are more than 100 yr and 400-1100 yr, respectively. These estimates show that P contamination of the reclaimed effluents in the Shafdan plant will not be a problem in the foreseeable future.