Spatial variability of contaminant transport in a fractured till, Avedore Denmark

A field experiment shows that rapid downward migration of solutes and micro organisms can occur in a fractured till. A solute tracer, chloride, and a b acteriophage tracer, PRD-1, were added to groundwater and allowed to infilt rate downwards over a 4 x 4 m area. Chloride was detected in horizontal fil ters at 2.0 m depth within 3-40 days of the start of the tracer test, and P RD-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 trace amounts of PRD-1 were detected in only 2 of the 36 filters. Transport rates and peak tracer concentrations decreased with dep th, but at each depth there was a high degree of variability. The transport data is generally consistent with expectations based on hydraulic conducti vity measurements and on the observed density of fractures and biopores, bo th of which decrease with depth. Transport of chloride was apparently retar ded by diffusion into the fine-grained matrix between fractures, but the ra pid transport of PRD-1, with little dispersion, indicates that it was trans ported mainly through the fractures.