EVALUATION OF CHLORIDE AND PESTICIDE TRANSPORT IN A FRACTURED CLAYEY TILL USING LARGE UNDISTURBED COLUMNS AND NUMERICAL MODELING

Saturated groundwater flow and tracer experiments using fluorescent dy e, chloride, and the herbicides mecoprop and simazine were carried out in the laboratory using three large-diameter (0.5 m) undisturbed colu mns of fractured clayey till. Hydraulic conductivity of the columns ra nged from 10(-5) mis in the shallowest column (1 m depth) to 10(-7) mi s in the deepest column (4 m depth) and were similar to field-measured values for these deposits. Results of the tracer experiments are cons istent with a conceptual model of advective transport along the fractu res combined with diffusion into the fine-grained matrix between the f ractures. Arrival of the chloride tracer in the effluent was highly re tarded relative to fracture flow velocities calculated on the basis of the cubic law and measured values of fracture spacing and hydraulic c onductivity. The herbicides were more strongly retarded than the chlor ide at low flow rates, but at higher flow rates the herbicides arrived with the chloride, indicating the influence of nonequilibrium sorptio n of the herbicides to fracture walls and the matrix solids, The colum ns were dismantled following the tracer experiments and mapping under UV light showed that nearly all of the visible, weathered fractures (a nd the few root holes in the case of the shallowest sample) were activ e transport pathways, with the dye appearing mainly on the fracture su rfaces and as a ''rim'' in the adjacent matrix. Concentration profiles measured perpendicular to the fracture surfaces showed that the herbi cides had also moved into the matrix, apparently by diffusion. Simulat ions of solute transport with a discrete fracture flow/matrix diffusio n model showed that the simulations could be ''fit'' to the data if al l of the visible fractures were hydraulically active, but could not be fit if all or most of the flow was channelled through just the primar y fractures (defined by prominent oxidation stains). Simulations with an equivalent porous media (EPM) model could not fit the data using th e measured total porosity as the effective porosity. The simulations c ould likely be fit with a smaller value of effective porosity, but thi s would limit applicability to held situations because fitted effectiv e porosity is expected to change with physical scale and residence tim e of the solute in the soil.