Soil testing for heavy metals

Abstract

Soil testing for metal contaminants is a continually evolving process aimed at improving the assessment of environmental and human health hazards associated with heavy metals in soils and plants. A number of challenges present themselves before accurate, reliable and precise contaminant hazard assessment criteria for soils and plants can be made. These include: sampling, extraction and analytical obstacles associated with the determination of trace levels of metals in environmental media; quality assurance and quality control issues associated with both extraction and analytical procedures (especially for metals where non‐compliance with regulatory standards may be penalised); and confounding environmental effects (e.g. rooting depth, soil salinity, Eh, pH, plant species, metal species) which limit the usefulness of the relationship between the current tests and actual hazards. These difficulties have combined to produce soil tests for heavy metals often poorly correlated with hazardwhether this be crop uptake of a contaminant (e.g. Cd), or the adverse effects of metals or metalloids on human or environmental health (e.g. As, Cr, Cu, Hg, Ni, Se, Pb, Zn). Assessment of an “available” fraction of a particular soil nutrient is the accepted norm of soil testing for crop nutrition. In many countries, assessment of metal hazard is still inappropriately based on the total soil metal concentration, despite increasing recognition that the concept of elemental availability is just as relevant for environmental hazard as for crop nutrition. Tests that aim to assess metal “bioavailability” are now gaining widespread acceptance by regulators as a means to characterise hazards from contaminants in soil. While a significant advance on the use of total metal concentrations, the concept raises difficulties in providing an adequate assessment of potential risk, due to changes in environmental conditions which affect bioavailability, e.g. soil pH, soil organic matter content. This chapter summarises current soil testing methodologies for metal contaminants and examines new concepts and procedures for assessing hazards from metal contamination of soils.