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Water flow and solute transport model of potentially toxic elements through unsaturated zone at regional wellfield Kosnica

The subject of this study is the unsaturated zone in the area of the regional water wellfield Kosnica. The objective of this work was to build prognostic water flow model and potentially toxic element (lead, cadmium, zinc) transport model in the unsaturated zone. Research was conducted in the catchment area of the wellfield Kosnica, which is dominated by Fluvisol soil type. In order to better understand water flow and transport of potentially toxic elements in the unsaturated zone, detailed field and laboratory analyses and experiments were made. For numerical simulation HYDRUS-1D program was used. Program solves water flow using Richard's equation. Transport of potentially toxic elements is described with advection and dispersion equation. Based on laboratory analyses of soil properties, the parameters of water flow and solute transport were defined. Bulk density rises with depth while water holding capacity is the largest in the A horizon due to particle size distribution. Minimum values of hydraulic conductivity were determined in A and 4Cl/Cr horizons. Experimental results of sorption processes were used for determination of Freundlich sorption constants and exponents for individual elements. The minimum values of Freundlich's sorption constants and greatest exponents were determined in the first horizons for all three potentially toxic elements. Small differences in sorption were found between the individual elements. Based on chloride tracer experiments on soil columns in the laboratory, dispersivity was determined using Hydrus 1D inverse model. The dispersivity in AC horizon is 1 cm, and in C horizon is 1.7 cm. The correlation coefficient of the measured and simulated values of tracer concentration is 0.43 for AC horizon and 0.74 for C horizon. For the calibration of the water flow model, measured values of water content in the soil, using TDR probes for the period from 1/9/2011 to 31/10/2012, were used. Comparison of simulation results and measured water content was done. Simulated water content was less variable than the measured values. Low correlation (~0.40) coefficient between measured and simulated water content in the soil could be due to use of meteorological data from distant weather station. Hence, by using chloride tracer experiments on soil columns in the laboratory, calibration of the solute transport model was made. Using water flow and transport parameters, transport prognostic model of potentially toxic elements was created at the investigation site. Comparison of potentially toxic elements concentrations obtained by transport prognostic model at the bottom of the soil profile with their maximum allowed concentrations (MAC) limits ordinance indicate a high risk of groundwater contamination at the investigative site. However, it might be concluded that sorption processes have a major impact on the transport of potentially toxic elements in the unsaturated zone.

water flow; transport of potentially toxic elements; unsaturated zone; Kosnica; numerical modeling

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