NEW METHOD FOR SAMPLING GROUNDWATER COLLOIDS UNDER NATURAL GRADIENT FLOW CONDITIONS

This paper describes a new method for the passive sampling of groundwa ter colloids using a multilayer sampler (MLS). It is based on the use of dialysis cells with large pore size (10 mu m) membranes that are in dynamic equilibrium with the mobile colloid and liquid phases in the aquifer. Under laboratory conditions, the dialysis cells reached equil ibrium with a suspension of latex microspheres (5 mg/L) after 44-100 h and with a suspension of kaolinite (16-41 NTU; 20-50 mg/L) after 50-1 80 h. No fractionation was detected in the particle-size distribution between the kaolinite suspensions inside and outside the dialysis cell s. Field profiles, obtained under natural gradient flow conditions in a sand and sandstone aquifer, showed large variability (up to 1 order of magnitude) in the colloid content within profiles (e.g., variation of 7 NTU (similar to 45 mg/L) between cells located at a vertical dist ance of 40 cm) and between them. The predominant colloidal particles f ound in the cells were aluminosilicates, CaCO3, silica, and organic ma tter. The membranes are suitable for sampling groundwater colloids ove r long periods of time, at least 36 days, in very turbid solutions (up to 50 NTU; similar to 550 mg/L).