Towards zero accumulation of heavy metals in soils

Abstract

Sweden has recently introduced new guidelines for the agricultural use of sewage sludge which contain successively more restrictive metal loading rates for soils. At the same time further efforts are made to reduce the input of heavy metals through atmospheric deposition and fertiliser use. The proposed metal loading limits are among the most restrictive in the world, although other scandinavian countries and the Netherlands have proposed similarly low limits. It is the aim of the Swedish Environmental Protection Board to successively reduce the metal load of soils until a situation of zero accumulation is reached. With the possible exception for Cd, there is apparently no scientific evidence at the moment to suggest that zero accumulation of metals in soil is required to adequately protect soil productivity, the environment, and human and animal health. A policy which steers towards zero accumulation may therefore seem excessively cautious. It is, however, also a policy which recognises the practically irreversible nature of elevated heavy metal concentrations and their effects in soil, the deficiencies in the evidence currently available with which to establish safe metal loadings for soils, as well as the need to preserve the agronomic value of soils for many years to come. It is argued that the use of restrictive annual metal loading rates can be used to effectively ensure that maximum soil concentrations or cumulative pollutant loadings, considered to be safe are not reached in the foreseeable future.