Flux density and breakthrough times for water and tracer in a spatially variable, compacted clay soil

Flux density values, computed from observed infiltration and outflow measurements at 184 locations in a 0.3-m-thick, 9m × 23m layer of compacted clay subsoil, are compared to effective flux density values that are based on breakthrough time distributions for water and Br⁻ tracer over the same area. Results suggest that both water and tracer move at similar rates, but considerably faster than expected, on the basis of flux density alone, and that only a small fraction of the total pore space is involved in active transport. The ramifications of these findings are explored against the background of effective porosity, degree of compaction, and observed changes in bulk density with time.