Ground water preservation by soil protection: Determination of tolerable total Cd contents and Cd breakthrough times

Taking Cd as an example we introduce a procedure to estimate tolerable tota l content of heavy metals in soils with regard to a specific ground water q uality criterion. Furthermore, we present a piston-flow approach to estimat e breakthrough times of a sorptive solute to the ground water. Both procedu res are applied to the sandy soils in the 4300 ha wastewater irrigation are a Braunschweig, Germany. Applicability of these procedures is tested by num erical simulations. The calculated breakthrough times of Cd for an input concentration of 3 mu g 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 ye ars (mean = 141 years). The deviation between the piston-flow approach and the numerical simulation is on the average 1.6%. We determined a mean toler able total Cd content of 0.61 mg kg(-1) with regard to a ground water quali ty criterion of 3 mu g L-1. The limit of the German sewage sludge regulatio n (AbfKlarV, 1992) of 1 mg Cd kg(-1) exceeds the calculated tolerable total content in 90% of the investigated Ap horizons. Moreover, the results of t he numerical simulations show that the limit of 1 mg Cd kg(-1) would lead t o a concentration in seepage water significantly above 8 mu g Cd L-1. We co nclude that in the sandy soils of the wastewater irrigation area the curren t limit of 1 mg Cd kg(-1) is not sufficient to keep the Cd concentration in seepage water below 3 mu g L-1 and, thus, to ensure ground water protectio n in the long run.