Mass transport and scale-dependent hydraulic tests in a heterogeneous glacial till-sandy aquifer system

A forced gradient tracer experiment indicated a rapid transport of a solute tracer, chloride, through a 13-m thick fractured till-silty sand lens syst em to an underlying sandy aquifer in Ringe, Denmark. Chloride was applied t o a 4 x 4.8 m(2) area on the ground surface and chloride breakthrough was m onitored in horizontal filters at depths of 2.5 m (till), 4 m (till), 5.5 m (silty sand lens), and 20-20.5 m (sandy aquifer). This paper is an extensi on of the research undertaken by Sidle ct al. (Water Resources Research, 34 (1998) 2515) and focuses only on the chloride breakthrough data from the s ilty sand lens (embedded in the till) and the sandy aquifer, and on the sca le dependency of saturated hydraulic conductivity tests. The rapid chloride transport shows that the fracture and macropore flow is significant in the non-weathered till between the silty sand lens and the sandy aquifer. The first arrival of chloride in the sandy aquifer occurred 8 days after the st art of the tracer injection and the "peak concentration" was reached after 3 weeks. In addition, slug tests, free flow tests, and large-scale infiltra tion tests were conducted to assess the spatial variability of the hydrauli c characteristics of the interconnected aquitard/silty sand lens system and the underlying aquifer. The large-scale infiltration tests and the free fl ow tests yielded substantially higher K values than the slug tests, where t he slug tests represented a small-scale sampling volume and only a localize d fracture network. The infiltration tests captured the decrease in bulk K from the uppermost, bioturbated till to the underlying fracture-dominated t ill. K values varied in the till with depth by up to 3 orders of magnitude from 1 x 10(-7) to 1 x 10(-4) m s(-1) with the highest permeabilities in th e uppermost 2.5 m. The high permeabilities explain the fast breakthrough of the solute in the underlying aquifer. The present study indicates that at high flow rates diffusive exchange of solute tracer mass did not significan tly influence the solute transport, particularly in the non-weathered till. (C) 2001 Elsevier Science B.V. All rights reserved.