Transport of haematite and silica colloids through sand columns eluted with artificial groundwaters

The passage of haematite and silica colloids through columns of washed glacial sand was monitored by enumerating particles in the inflow and outflow, in order to provide a basis for the design of a field experiment to measure groundwater colloid mobility. The effects of flow velocity (0.5 and 2 m day-1) and concentrations of humic substances and calcium in the eluting solution were investigated. Transport of both colloids was greater at the higher flow velocity. Under all conditions, silica was transported to a greater extent than haematite, and the transport of haematite was more sensitive to solution composition, reflecting its greater tendency to absorb humic substances. The results are consistent with the differing electrophoretic mobilities and homoaggregation rates of the two colloids. It was calculated that 99.9% removal of both types of colloid would occur over distances of the order of a metre. However, there is evidence that attachment of the colloids to the sand grains is reversible, so that the removal may not be permanent. (Author)