Evidence of Heavy Metal Movement Down the Profile of a Heavily-Sludged Soil

Abstract

Two sites in central England where sewage sludge has been deposited for decades were studied to measure the heavy metal distribution in the soil profiles. The first site (S1) was a field receiving heavy loads sludge from a nearby wastewater treatment plant, and the second (S2) was a farm applying ‘normal’ sludge rates of 8 t ha−1 y−1 of the same sludge. Soil samples were also taken by a near-by untreated control site. In S1 the movement of heavy metals was significant even down to 80 cm depth compared to the control. In S2, the concentrations of lead (Pb) and zinc (Zn) and the organic matter content were higher than the control down to 20 cm, while nickel (Ni) moved significantly down to 80 cm. This underlies the possibility that the metals bound onto organic surfaces moved along with organic matter down to that depth. The movement of metals in S2 points out the potential risks of applying sewage sludge for a long time.