RISK ASSESSMENT AND MANAGEMENT OF CADMIUM IN AGRICULTURAL SYSTEMS
M.J. McLaughlin, D.P. Stevens, and R.E. Hamon
CSIRO Land and Water/University of Adelaide, PMB 2, Glen Osmond, SA 5064, Australia

Compared to other heavy metals, cadmium (Cd) is relatively mobile and bioavailable in soils, so that transfer through the food chain is a major risk pathway. Cadmium can be added to soils in various forms - fertilizer, atmospheric aerosols or particulates, animal manures, and urban/industrial waste streams (Fig. 1).

Fig. 1. Cadmium inputs and chemical pools in soil.	Fig. 2. Relationship between extractable P and Cd in Australian topsoils (Tiller et al., 1997).

Threshold values for soil Cd concentrations may be derived to protect groundwater quality (risk through Cd leaching), ecosystem health (risk to soil or terrestrial organisms) or human health (risk from soil or food ingestion). Generally, ingestion of Cd-contaminated food is the most sensitive risk pathway, and is the pathway used to derive trigger or remediation values in soil. Threshold soil Cd concentrations to protect food chain exposure can be derived either using food Cd standards as the driving variable (e.g. Australian soil Cd limits for land application of biosolids), or may be more complex human-health based risk assessments incorporating dietary variability, food production variability as well as soil-to-crop transfer factors (e.g., US EPA 503 Biosolid Regulations).

Soil chemical factors play a major role in controlling Cd accumulation by food crops, with soil pH, soil salinity, soil micronutrient status, fertilization strategy, and crop cultivar all affecting food chain accumulation. In the short term, manipulation of these factors will have a much larger mitigating effect on crop Cd concentrations than merely decreasing Cd inputs to soil, and many of these factors are currently used by farmers to reduce food chain Cd accumulation. However, as the soil's ability to sorb or fix Cd into non-bioavailable pools is limited, the long-term strategy to control food chain accumulation of Cd must eventually be to achieve mass balance in agricultural systems, where Cd inputs = Cd outputs. It is unwise to allow soil Cd concentrations to increase significantly, as removal of Cd from soils, while often claimed to be feasible, remains an economically and practically elusive goal.

References

• McLaughlin, M.J. and Singh, B.R. 1999. Cadmium in soils and plants. A global perspective. Pp. 1-9. In "Cadmium in Soils and Plants". Eds. M.J. McLaughlin and B.R. Singh. Kluwer Academic Publishers, Dordrecht, The Netherlands.
  
• Tiller, K.G., McLaughlin, M.J. and Roberts, A.N. 1999. Environmental impacts of heavy metals in agroecosystems and amelioration strategies in Oceania. Pp. 1-41. In "Soils and Groundwater Pollution and Remediation. Asia, Africa and Oceania." Ed. P.M. Huang and I.K. Iskandar. Lewis Publishers, Boca Raton.