IN SITU MEASUREMENTS OF DIFFUSION COEFFICIENTS IN A CLAY-RICH AQUITARD USING A DEUTERIUM TRACER

Apparent diffusion coefficients (D_a) for solutes in aquitards are traditionally determined on core-sized samples (typically < 50 cm³) or estimated empirically. Here we developed an in situ field method to determine D_a of solutes, and tested it in a thick clay-rich till aquitard with a hydraulic conductivity of 9x10^-10 m/s. The field testing system consisted of a packer, pressure transducer, and circulation and sampling peristaltic pump, which were installed in four purpose-build piezometers completed at 15 m below ground surface. All piezometers were located within a 10 m² area. Three tests were conducted using formation water spiked with a deuterium tracer, and one using formation water to act as a control. All three tests yielded very similar results, with C/C_o values attaining 0.1 at the end of the 264 day test period. The best-fit D_a value for these data was determined using a 3D-numerical model. All three tests yielded similar D_a values. These field derived values compared well with laboratory determined diffusion tests using core samples collected from the piezometer screen intake zones and the results of the field-scale transport modeling at the test site. Our findings revealed that D_a was not scale dependant, and was spatially constant.