Nitrate pollution is a big hazard around industrial pig farms. It is possible to manage this problem by the help of soil filtering plots. The SOLEPUR treatment represents a soil purifying system for treatment of pig slurry using soil physical and chemical capacities succeeded by denitrification of nitrogen in leachates. The SOLEPUR process was described in detail in a previous paper (Martinez, 1997). After intensive application of pig slurry, pollution by Cu and Zn is observed. High concentrations of heavy metals can perturb the soil nitrogen cycle (Wilson, 1977). Premi and Cornfield (1969) reported that soil Zn concentrations of 100, 1000 and 10 000 μg Zn g-1 (as ZnSO4) showed no, partial and complete inhibition of nitrification, respectively. The aim of our study was to determine the threshold of heavy metal (Cu, Zn) concentrations in soil of the SOLEPUR treatment facility beyond which nitrification will be inhibited, thereby interrupting the purifying capacity of this soil based system.