Monitoring heavy metal concentrations in leachates of an acid forest soil during repeated applications of sewage sludge

Metal transfer from sludge to soil and subsequently to groundwater represents one of the most critical long-term hazards related to the application of these wastes to soils. Some studies working with soils with a pH ranging from 4.5 to 8.1 held that this was practically insignifi cant as metals remained at the site of input (Dowdy et al., 1991; Sukkariyah et al., 2005). However, several fi eld trials and column studies using soils with a pH ranging between 3.7 and 8.4 (Egiarte et al., 2006; Toribio and Romanyà, 2006) have evidenced the migration of metals with depth, highlighting the concern of groundwater contamination with these pollutants. Here in this study, the mobility of sewage sludge-derived heavy metals (Zn, Cd, Pb, Cr and Ni) in acid sandy soils under pine stands was monitored during 2.5 years after two consecutive applications of this waste at three different loading rates. Only those of Zn, Pb and Ni will be discussed here.