SPATIALLY VARYING HYDRAULIC AND SOLUTE TRANSPORT CHARACTERISTICS OF AFRACTURED TILL DETERMINED BY FIELD TRACER TESTS, FUNEN, DENMARK

A natural gradient tracer test that simulated one-dimensional flow was conducted in a large (4 m x 4.8 m surface area) isolated block of fra ctured till in Funen, Denmark, to determine both bulk and spatially di stributed hydraulic properties. Hydraulic conductivity (based on free- flow tests) was higher in the upper 2.5 m (5.81 x 10(-5) m s(-1)) than in the upper 4 m (8.00 x 10(-6) m s(-1)) of till, indicating the more highly fractured and bioturbated nature of the upper part of the till . Chloride was introduced at the surface of the block as NaCl solution (490 mg L-1 Cl-); and breakthrough of Cl- was monitored in 13 horizon tal screens at depths of 2.5 and 4 m. Arrival times of half of the inp ut Cl- concentrations (C/c(0) = 0.5) ranged about sevenfold among vari ous screens at each sampling depth; 5-35 hours at 2.5 m and 11-68 hour s at 4 m. Chloride breakthrough data for the two depths were fitted to both an equivalent porous media (EPM) model, based on the convective- dispersive equation, and a discrete fracture model. These results indi cate differences in transport characteristics of the till with depth. At the 2.5 m depth the EPM model fit the initial part of the breakthro ugh curves quite well for most samplers but did not predict the long t ails adequately. The converse was generally true for the discrete frac ture model. Thus it appears that transport in the upper 2.5 m of till is controlled by a complex interconnected system of fractures and macr opores. Field measurements of fractures and macropores at this si ie s ubstantiate these results. At the 4 m depth the discrete fracture mode l described solute transport in all samplers better than the EPM model , indicating that vertical fracture flow dominates in the lower portio n of the block. Fracture apertures and spacings were estimated based o n the cubic law and fitting the breakthrough data to the discrete frac ture model. Fitted fracture spacings at the 4 m depth (0.025-0.08 m) w ere slightly smaller than the minimum spacing (0.09 m) mapped in the f ield. Calculated apertures at the 4 m depth were large compared with o ther studies and ranged from 65 to 127 mu m (average value of 94 mu m) .