Abstract:
The paper presents an algorithm for modeling the vertical water transport of arsenic (As) in contaminated river floodplain deposits using the software product HYDRUS1D. A scenario of river flooding is prepared with sample data from a model plot in the Ogosta River valley, northwestern Bulgaria. Meteorological and hydrological data were also used for the historical flood, which happened in April 1964. The soils in the valley are heavily polluted with arsenic and heavy metals as a result of historical extraction and flotation of Fe- and Au-ores in the region of the town of Chiprovtsi. The study site of P13 is situated in the valley section between the village of Belimel and the Ogosta dam lake at 13 meters from the river bank in the low floodplain with a vertical distance to the river bed of 169 cm. The concentration of arsenic in the soil ranges between 625-11450 mg/kg for individual layers to a depth of 170 cm. The sediments are built of loamy sand, and gravel with loam sandy to sandy filler. The simulation of water and As transport encompasses a seven-day period in which the soil is flooded on the fourth day. The flooding event itself is implemented into the model as a water flux leading to 74 cm flood above the surface. Separate transport simulations of As (V) and As (III) were made for the same soil profile. The values used for the distribution coefficient Kd are determined according to the literature and are respectively Kd As(V) = 2000 l/kg and Kd As(III) = 5 l/kg. The modeling results show that the flood water flow passes for one day across the entire depth of the profile. The soil layers are water-saturated during the flood. The amount of water which passed through the profile for the modeling period is 2500 l/m2 . The transport simulation of As (V) calculates an entry of 800 mg/m2 of the pollutant at a depth of 170 cm for the whole seven day period at an average concentration of As (V) in the porewater of 0.409 mg/l. The calculated amount of As (III) which infiltrated to the bottom of the profile for the same period is 870 g/m2 . It is more than 1000 times higher than the estimated quantity for As (V). Constraints of modeling are the constant groundwater level during the flood, the use of an average Kd distribution coefficient of arsenic for a wider set of soil varieties, and the precondition of only As (V) or As (III) existence in the soil solution during the transport simulation. Despite the limitations and conventions of the As transport modeling with HYDRUS-1D, the results show the significant role of the river floods for the arsenic infiltration from the contaminated soil layers into the groundwater. The presented detailed algorithm for water and mass modeling enables the use of HYDRUS-1D for scenarios with different flooding duration and different depth of ground water table.