Terrestrial ecosystem health under long-term metal inputs: modeling and risk assessment

Metal contamination of soils may pose long-term risks to ecosystem health if not properly managed. Future projection of contamination trends, coupled with ecological assessment, is needed to assess such risks. This can be achieved by coupling dynamic models of soil metal accumulation and loss with risk assessment on the basis of projected metal levels. In this study, we modeled the long-term dynamics of Cu, Zn, and Cd in agricultural topsoils of a northern Chinese catchment (Guanting reservoir) and related projected metal levels to 2060 to ecological risk. Past metal dynamics were simulated using historical metal inputs from atmospheric deposition, irrigation, fertilizers, and animal manures. Modeling future dynamics was done using scenarios of projected metal input rates. Ecological risk assessment was done using the Potentially Affected Fraction (PAF) approach to estimate the combined toxic pressure due to the three metals. Modeled labile soil metals agreed well with measurements from monitoring in 2009 following adjustment of the porewater dissolved organic concentration. Metals were predicted to be largely retained in the topsoil. Projections were sensitive to changes in imposed soil pH, organic matter, and porewater dissolved organic carbon. Modeling suggests that decreases in input rates to between 5% and 7.5% of 2009 levels are required to prevent further accumulation. Computed PAFs suggest zinc makes the greatest contribution to ecological risk. Under the most conservative estimate of PAF, the threshold of potential ecological risk was reached before 2060 in two of the three future input scenarios.