Hydrogeology Research Group

Danish Tracer Experiment

A series of groundwater flow and tracer experiments were performed on an undistrubed columns of fractured clay-rich till, in collaboration with researchers from the Geological Survey of Danmark and Greenland with the Danish Geotechnical Institute and the University of Copenhagen. The experiments show that groundwater flow and contaminant migration through the sample is primarily controlled by fractures and root holes. Tracer experiments using chloride, pesticides, colloid-sized bacteriophage (PRD-1 and MS-2) and uncharged latex microspheres, indicated very fast transport rates of 4 to 360 m/day. These rates are similar to fracture flow velocities calculated on the basis of the measured bulk hydraulic conductivity of the column and measured fracture spacing, using the cubic law for flow through parallel-walled fractures. A second study showed that solute transport was more accurately simulated using a discrete-fracture/matrix-diffusion model than by using an equivalent porous media model.

A field experiment carried out in collaboration with the Geological Survey of Danmark and Greenland shows that rapid downward migration of solutes and microorganisms can occur in a fractured till. A solute tracer, chloride, and a bacteriophage tracer, PRD-1, were added to groundwater and allowed to infiltrate downwards over a 4 x 4 m area. Chloride was detected in horizontal filters at 2.0 m depth within 3-40 days of the start of the tracer test, and PRD-1 was detected in the same filters within 0.27-27 days. At 2.8 m depth chloride appeared in all the filters, but PRD-1 appeared in only about one-third of the filters. At 4.0 m depth chloride appeared in about one-third of the filters and tracer amounts of PRD-1 were detected in only 2 of the 36 filters. Transport rates and peak tracer concentrations decreased with depth, but at each depth there was a high degree of variability. The transport data is generally consistent with expectations based on hydraulic conductivity measurements and on the observed density of fractures and biopores, both of which decrease with depth.

Recent journal publications resulting from this research include:

• McKay, L.D., J. Fredericia, M. Lenczewski, J. Morthorst, and K.E.S. Klint, Spatial variability of contaminant transport in a fractured till, Avedore, Denmark, Nordic Hydrology, in press.
• Jorgensen, P.R., L. D. McKay, and N. Spliid, Evaluation of chloride and pesticide transport in a fractured clayey till using large undistribued columns and numberical modeling, Water Resources Research, 34(4), 539-553, 1998.
• Hinsby, K., L.McKay, P.Jorgensen, M.Lenczewski, and C. Gerba, Fracture aperture measurements and migration of solutes, viruses and immiscible creosote in a column of clay-rich till, Ground Water, 34(6), 1065-1075, 1996.