Hydraulic aspects of riverbank filtration—field studies

The Düsseldorf waterworks have been using riverbank filtration since 1870 with bank filtration as the most important source for public water supply in this densely populated and industrialised region. There have been many threats to this supply in the last few decades—e.g. poor river water quality, heavy clogging of the riverbed, accidental pollution—which had to be overcome. First field studies in the river Rhine were carried out with a diving cabin in 1953 and 1954 to investigate riverbed clogging during high loads of organic contaminants in the river water. In 1987 a second investigation of the riverbed followed in the same area during which time the water quality of the river had improved. After the Sandoz accident in 1986 a joint research project was carried out in the Lower Rhine region to improve knowledge of flow and transport phenomena of riverbank filtration and to develop numerical models for the dynamic simulation of flow and transport. The main objective of the field studies was to gain more insight into the dynamic river–aquifer interactions and the effects of fluctuating river levels. These fluctuations are not only relevant for clogging processes and the velocities and residence times in the subsoil, but can also affect the quality of the well water. Depth-orientated sampling in the adjacent aquifer was employed. One important finding was a marked age-stratification of the bank-filtered water which balances out fluctuating concentrations of dissolved compounds in the river water.