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

• Published in: Journal of hydrology (Amsterdam) 2001-03, Vol.243 (3), p.162-179
• Main Authors: Nilsson, B, Sidle, R.C, Klint, K.E, Bøggild, C.E, Broholm, K
• Format: Article
• Language: English
• Subjects: aquifers; Breakthrough data; Clayey till; Denmark; Earth sciences; Earth, ocean, space; Exact sciences and technology; filters; Fractures; Hydraulic tests; Hydrogeology; Hydrology. Hydrogeology; macropore flow; Mass balance; mass transfer; saturated hydraulic conductivity; Scale dependence; solutes
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/S0022-1694(00)00416-9

A forced gradient tracer experiment indicated a rapid transport of a solute tracer, chloride, through a 13-m thick fractured till–silty sand lens system to an underlying sandy aquifer in Ringe, Denmark. Chloride was applied to a 4×4.8 m 2 area on the ground surface and chloride breakthrough was monitored 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 extension of the research undertaken by Sidle et al. (Water Resources Research, 34 (1998) 2515) and focuses only on the chloride breakthrough data from the silty sand lens (embedded in the till) and the sandy aquifer, and on the scale 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 start of the tracer injection and the “peak concentration” was reached after 3 weeks. In addition, slug tests, free flow tests, and large-scale infiltration tests were conducted to assess the spatial variability of the hydraulic characteristics of the interconnected aquitard/silty sand lens system and the underlying aquifer. The large-scale infiltration tests and the free flow tests yielded substantially higher K values than the slug tests, where the slug tests represented a small-scale sampling volume and only a localized fracture network. The infiltration tests captured the decrease in bulk K from the uppermost, bioturbated till to the underlying fracture-dominated till. K values varied in the till with depth by up to 3 orders of magnitude from 1×10 −7 to 1×10 −4 m s −1 with the highest permeabilities in the 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 significantly influence the solute transport, particularly in the non-weathered till.