Spatially varying hydraulic and solute transport characteristics of a fractured till determined by field tracer tests, Funen, Denmark

Abstract

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 fractured till in Funen, Denmark, to determine both bulk and spatially distributed 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 horizontal screens at depths of 2.5 and 4 m. Arrival times of half of the input Cl- concentrations (C/c0 = 0.5) ranged about sevenfold among various screens at each sampling depth; 5-35 hours at 2.5 m and 11-68 hours 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 indicate differences in transport characteristics of the till with depth. At the 2.5 m depth the EPM model fit the initial part of the breakthrough curves quite well for most samplers but did not predict the long tails adequately. The converse was generally true for the discrete fracture model. Thus it appears that transport in the upper 2.5 m of till is controlled by a complex interconnected system of fractures and macropores. Field measurements of fractures and macropores at this site substantiate these results. At the 4 m depth the discrete fracture model described solute transport in all samplers better than the EPM model, indicating that vertical fracture flow dominates in the lower portion of the block. Fracture apertures and spacings were estimated based on the cubic law and fitting the breakthrough data to the discrete fracture model. Fitted fracture spacings at the 4 m depth (0.025-0.08 m) were slightly smaller than the minimum spacing (0.09 m) mapped in the field. Calculated apertures at the 4 m depth were large compared with other studies and ranged from 65 to 127 μm (average value of 94 μm).