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 ⩽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, KdI, was about 4–6 L kg−1 at equilibrium P concentration <6 mg L−1, while field-based KdF 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 KdF data agreed with observations while those calculated using KdI 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.