Uptake of heavy metals by food crops from highly-polluted Chernozem-like soils in an irrigation district south of Tbilisi, eastern Georgia

In the middle and lower reaches of the Mashavera valley in SE Georgia, most of the irrigated soils under different agricultural land use display a strong enrichment of heavy metals (HM) that can be traced back to irrigation with water polluted by mining wastes contributed over a period of several decades. The concentrations of total amounts of Cu, Zn and Cd increase with intensity of land use and amount of irrigation in the following sequence: arable fields < occasionally submerged meadows < vegetable gardens < wine gardens and orchards with mixed cropping of vegetables. A high proportion of HM belongs to the supply fraction, which displays the (un-)specifically adsorbed HM, dissolvable in ethylenediamine tetra-acetic acid (EDTA). The narrow correlation of this fraction with the mobile and plant-available fraction of HM indicates a high long-term risk potential for the food chain. Due to the recent high adsorption capacity of the soils for HM, only a small amount of HM in the mobile fraction was found with proportions less than 1 % of the total amounts for Cu and Zn, and a maximum of 1.5 % for Cd. On the other hand, initial investigations of cereals and vegetable species indicate a high uptake of Cu, Zn and Cd, which for Cu and Cd causes concentrations in plants exceeding the tolerance thresholds for plants, animals and human beings. A field experiment established the strong uptake of heavy metals by spinach, which was unexpected due to the weakly alkaline pH as well as the high contents of clay and organic matter of the soils. This result indicates the high risk of soil pollution by heavy metals for the food chain and consumers.