Assessment of the deposition of polluted sediments transferred by a catastrophic flood and related changes in groundwater quality

In the second half of the 20th century, industry and agriculture in the mountainous part of the Czech reaches of the Elbe caused much water pollution and a high suspended load. A reservoir with a total volume of 92,00,000 m3 was built in 1914 to close the mountainous part of the watershed and to prevent floods endangering the cities downstream. Approximately 20% of its volume has now been filled with polluted sediments. These could potentially be transported downstream and settled in the flood plain, and could thus also influence the quality of the ground water. This paper sets out to model such an event. The authors carried out a radar investigation to find where the sediments were located and to compute the amounts of sediments. Samples were taken for quality analyses. The re-suspension conditions in the reservoir were computed using hydraulic methods, and a hydrograph of a catastrophic 500-year flood (Q500) was constructed by stochastic hydrology. The upgraded FAST 2D dynamic flow model was used to compute a map of the sediments settled in the flood plain, and the movement of the pollution below the ground was then described using the MT3DMS model. The principal result is a simulated prediction of the concentration of zinc (as a general representative of heavy metals) in different aquifer layers after 10 and 20 years. The results do not show a severe risk according to the Czech national guideline for remediation.