"Memory" effects of heavy metals on forest soils

By irrigating a forest soil with a solution containing a certain concentration of Cd,Cr and Pb several times,"memory"effect on the soil,i.e.decrease in the heavy metal adsorption rate of the soil as was affected by sewaging in the past,was studied.Results show that the soil has a high environmental capacity for Cr and Pb,so the "memory" effect of the soil is not significant.After irrigation with CrCl₃ solution several times,about 99% of the Cr in the soil below 20 cm was adsorbed by the soil.After irrigation with PbCl₂ three times,Pb concentrations at various sampling points declined but remained over 85%.If more irrigation was done,the Pb adsorption rate would further go down.The soil is rather limited in environmental capacity for Cd.The surface soil layer would easily get saturated in adsorption and the adsorption zone would move downwards,posing a threat to the groundwater.