Abstract

A pilot plant study on the behaviour of ethylene diamino- tetraacetic acid (EDTA) during groundwater recharge of river water confirmed that EDTA is nondegradable under anaerobic and subtoxic conditions and partially degradable under aerobic conditions. The degree of aerobic degradation showed to be dependent on EDTA concentrations and temperature. At concentrations of about 500 mug EDTA/l, at a retention time of 15 days in the recharge bed, the removal was approx. 60% at a temperature >10 degrees C and approx. 30% at a temperature < 10 degrees C. At concentrations of tens of mug/l, i.e. the present maximum concentration level in the water of the rivers Rhine and Meuse, the removal was approx. 33% at a temperature >10 degrees C and approx. 15% at a temperature <10 degrees C. At concentrations of 7 mug/l no removal was found. The most serious consequence of the pollution of river water by EDTA for the drinking water supply using dune or bank recharge appears to be the adverse effect on the removal of heavy metals, respectively their mobilisation during the recharge. From the studied heavy metals (Cd, Cu, Ni, Pb, Zn) the most profound mobilisation effects have been found for Pb, Ni and Cu. The mobilisation of Pb has been found by the recharge in the dune sand contaminated by suspended solids from river water. The mobilisation was substantially higher during winter than during summer. In winter at a EDTA concentration of 500 mug/l the mean Pb concentration in the effluent of the recharge tank exceeded the WHO drinking water quality guideline of 10 mug/l. Based on the varations of phosphate concentrations in the effluent of the tank, it has been suggest that the seasonal differences in the mobilisation effects were caused by the formation of highly insoluble Pb5(PO4)3Cl. Mobilisation effects for Cu and Ni were found already at a concentration as low as 90 mug EDTA/l.