Scholarly record

APPLICATION OF HYDRUS-1D FOR EVALUATION OF THE VADOSE ZONE SATURATION STATE IN CONNECTION WITH ARSENIC MOBILIZATION AND TRANSPORT IN CONTAMINATED RIVER FLOODPLAIN - OGOSTA VALLEY CASE STUDY, NW BULGARIA

Dimitar Antonov

Abstract

The Ogosta Valley downstream the town of Chiprovtsi, NW Bulgaria, was subject to long-lasting contamination with arsenic and other toxic elements due to mining and ore-processing activities in the second half of the last century. Although the exploitation of lead-silver and iron ores was ceased in the end of the 90`s, the soil in particular sections of the Ogosta?s river floodplain remained highly contaminated with arsenic, lead and other heavy metals. Arsenic mobilization and transport in the vadose zone is highly dependent on the redox potential. Since the redox reactions are influenced by the degree of water saturation of the soil, the depth of the groundwater level (GWL) and the inflow flux are key factors to determine the conditions in the vadose zone. The aim of the study is to evaluate how much the two factors affect the moisture content in the unsaturated zone and thus also the conditions for arsenic mobilization and transport. The studied area covered a floodplain area of 12.7 km2 where piezometers at nine sites were installed. Three modeling scenarios were implemented for each site: the maximum, minimum and the mean values of the GWL were used as the lower boundary conditions. The annual inflow, calculated by the Turc`s method, was used as upper boundary condition. All the scenario simulations were performed with HYDRUS-1D software. The results showed complete water saturation of the floodplain deposits at some of the arsenic polluted spots, while only the upper layers were saturated at other sites. Thus, temporal anoxic conditions could be expected to appear at certain contaminated sections of the Ogosta?s river floodplain, potentially leading to arsenic reduction and mobilization.

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