SOLID ORGANIC MATTER AS AN ESSENTIAL CONTRIBUTOR TO THE DEVELOPMENT OF REDOX REACTIONS DURING RIVER BANK FILTRATION

River bank filtration is used extensively in Europe to obtain large volumes of good quality water from alluvial aquifers. The occurrence of a reduced zone in the river-water infiltration flow path has both beneficial (denitrification) and detrimental (Mn and/or Fe solubilisation) effects on the quality of the water pumped. Abundant data from the Capdenac-Gare well field site (Southwestern France, object of several publications) is used to understand how redox processes control groundwater quality. This site is extremely interesting because chloride can be used as a natural tracer of Lot River- and ground-water mixing. Organic matter is the “fuel” for redox processes. PHREEQC provides indirect evidence that the amount of dissolved organic matter (DOM) in the infiltrating water is far from sufficient to explain the observed consequences of redox processes (consumption of dissolved oxygen, denitrification, solubilisation of Mn). This strongly suggests the presence of highly reactive solid organic matter (SOM) (as much or more than DOM) in the infiltrating flow path. Interestingly, near the river bank where SOM is most abundant, the major contributor to redox reactions is the easily degradable DOM. Sediment organic matter content should therefore be taken into account for the siting and the management of well fields in alluvial aquifers.