Column elution experiments on volcanic ash: Geochemical implications for the main Ethiopian rift waters

Column laboratory experiments were employed to assess the leaching behaviour of pyroclastic glassy ash deposits collected in the central Main Ethiopian Rift, where surface and groundwater resources are affected by fluoride (F-) pollution, which is the cause of an endemic disease (fluorosis) in the local community. To elucidate the source of F- and simulate the water-rock interaction processes, as well as quantify its distribution within different grain sizes, the pyroclastic ash was analysed by XRF, XRD and SEM and separated into coarse and fine fractions. Three columns were filled with raw (unsieved), coarse (63 mu m-2 mm) and fine (< 63 mu m) fraction, respectively, and flushed with synthetic rain water in saturated conditions. Very fast F- leaching was observed in the fine fraction column at the start of the experiment, while in the other two columns, F- was slowly released; in addition, a strong accumulation of F- was found in the fine fraction. The effect was more pronounced in the fine fraction column due to the available effective adsorbing surface area. Subsequent to elution experiments, the columns were characterised via moment analysis of tracer test. Finally, flow and transport modelling (MODFLOW-2000 and MT3DMS) was employed to compute the amount of F- adsorbed onto the solid phase, comparing the calculated conservative transport of F- and the observed concentrations. The results of this study suggest that fluoride is a fundamental constituent of the glass phase (about 0.3 wt.%) and that it is released during the incongruent dissolution of glassy particles. Dissolution of coatings on glass particles could provide an additional contribution to the geochemistry of the interacting fluids. These processes are more effective in the fine fraction due to a much higher reactive (specific) surface area.