HEAVY-METAL UPTAKE BY CROPS FROM POLLUTED RIVER SEDIMENTS COVERED BY NON-POLLUTED TOPSOIL .1. EFFECTS OF TOPSOIL DEPTH ON METAL CONTENTS

Crop uptake of heavy metals from polluted river clay soils is shown to be reduced by covering the polluted soil with a layer of unpolluted clay soil. Plant experiments have been performed to determine the thickness of such a layer required either to comply with permissible levels for metal concentrations in foods and feeds, or to exclude any effect on plant metal levels. The experiments included cover layers up to 0.7m and 1.6m, respectively. Crops grown included cereals, potatoes, sugar beet, maize and various vegetables. Protection of all food crops tested against exceeding permissible levels for cadmium requires a clean topsoil of over 1.6m; for individual crops ranging from zero (no cover layer required) for red cabbage, leek, onion, potato) to 1.2m-1.6m for celery tuber and leaf. Results for feed crops were variable: required topsoil depths for maize range from 0.25-1.2m, and for wheat straw from 0.55 to 1.6m. No-effect depths calculated for Cd, Cu and Zn demonstrate that in many experiments the effect of the polluted soil may be observed at all topsoil depths tested. Heavy-metal concentrations in the soil profile, measured after completion of the experiments, showed no significant migration of metals from the polluted soil into the cover soil.