Transport characteristics of heavy metals, metalloids and pesticides through major agricultural soils of Bangladesh as determined by TDR

Abstract

The transport parameters of soluble chemicals through soils are needed to assess the pollution risks of soil and groundwater resources. The transport parameters of seven heavy metal/metalloid compounds (NaAsO2, Cd(NO3)2, Pb(NO3)2, Ni(NO3)2, ZnCl2, CuSO4 and Co(NO3)2), two pesticides (cartap and carbendazim) and an inert salt (CaCl2) were determined in repacked columns of eight agricultural soils of Bangladesh. The relationships between physicochemical properties of the soils and solutes and the solute-transport parameters were investigated by time-domain reflectometry (TDR) using bulk soil electrical conductivity as a proxy. The transport velocity (V) of the solutes decreased linearly with the increasing clay content of the soils. Of the nine reactive solutes, cartap and As moved with the largest and smallest V values, respectively. Dispersivity of the solutes decreased with increasing clay content linearly but following power law with increasing median grain diameter of the soils. It also increased significantly (P<0.05) with the increasing length of travel path. The solute retardation factor (R) increased with increasing clay content following power law, implying that clay content is a determining factor for heavy metal retention in soils. Parameter R increased non-linearly with increasing travel length except for cadmium, for which the increase was linear. Based on R, the general adsorption affinity of the solutes in the soils was As>Co>Cd>carbendazim>Pb>Cu>cartap>Zn>Ni>Ca. In some soils, however, Cd and carbendazim showed a lower adsorption than Pb and Co, and Zn and cartap were more strongly adsorbed than Ni. The relationships between physicochemical properties of the soils and solutes and the solute transport parameters contribute to the improved interpretation of TDR data as a preliminary assessment of pollutant mobility in soils with different textures.