Subsurface Transport of Heavy Metals Mediated by Biosolid Colloids in Waste-Amended Soils

Abstract

Land application of agricultural and industrial solid waste products (biosolids) is being widely promoted as a cost-effective nutrient management alternative with dramatic increases worldwide in recent years (Linden, 1995). Although many biosolid materials are considered excellent sources of plant macronutrients, some contain high levels of heavy metals in labile forms that may pose a significant threat to soil and groundwater quality (McBride et al., 1997, Scancar et al, 2001). There is a general perception that many hydrophobic contaminants, such as heavy metals, readily sorb to the soil matrix and are considered to be virtually immobile, posing little danger to groundwater supplies (McCarthy and Zachara, 1989). However, the risk may be greater than anticipated considering that a significant metal load may be associated with dispersed biosolid colloid particles, which may be mobilized through soil macropores to lower soil depths and greater distances (Kretzschmar et al., 1995; Gove et al., 2001). The high organic carbon content and increased surface reactivity of mobile biosolid colloids may out-compete the soil matrix in heavy metal sorption affinity, thus facilitating metal transport (Karathanasis, 1999; Karathanasis and Ming, 2002). This colloid-mediated metal transport may explain the negative metal mass balances found by some researchers trying to account for the losses of biosolid-applied heavy metals in soils (McGrath and Lane, 1989; Baveye et al., 1999).