| Abstract: | Taking Cd as an example we introduce a procedure to estimate tolerable total content of heavy metals in soils with regard to a specific ground water quality criterion. Furthermore, we present a piston‐flow approach to estimate breakthrough times of a sorptive solute to the ground water. Both procedures are applied to the sandy soils in the 4300 ha wastewater irrigation area Braunschweig, Germany. Applicability of these procedures is tested by numerical simulations. The calculated breakthrough times of Cd for an input concentration of 3 μg L?1 and a mean water flux density of 570 mm yr?1 varies, as a function of depth of water table and sorption characteristics, between 10 and 805 years (mean = 141 years). The deviation between the piston‐flow approach and the numerical simulation is on the average 1.6%. We determined a mean tolerable total Cd content of 0.61 mg kg?1 with regard to a ground water quality criterion of 3 μg L?1. The limit of the German sewage sludge regulation (AbfKlärV, 1992) of 1 mg Cd kg?1 exceeds the calculated tolerable total content in 90% of the investigated Ap horizons. Moreover, the results of the numerical simulations show that the limit of 1 mg Cd kg?1 would lead to a concentration in seepage water significantly above 8 μg Cd L?1. We conclude that in the sandy soils of the wastewater irrigation area the current limit of 1 mg Cd kg?1 is not sufficient to keep the Cd concentration in seepage water below 3 μg L?1 and, thus, to ensure ground water protection in the long run. |
