Use of vertical profiles of δ¹⁸O to constrain estimates of hydraulic conductivity in a thick, unfractured aquitard

Vertical distributions of δ 18 O were used to constrain hydraulic conductivity ( K ) in a thick, unfractured aquitard. Previous investigations utilizing permeability tests at a variety of scales [ Keller et al. , 1989] indicated that bulk K at the site ranges from 3.5× 0 −11 m/s to 1.2×10 −10 m/s. Advective displacements calculated on the basis of these K values lead to a wide range of predicted distributions of natural tracers after 11,000 years. Comparison of δ 18 O distributions and simulated advection‐diffusion profiles provide a more precise constraint on advective displacement and hence on K . The simulations, cast in terms of a Peclet number (P = V 2 T/D * ) ranging in value from 0 to 12, indicate that transport is diffusion controlled and that K is actually less than 1.2×10 −11 m/s. P greater than 0.1 ( K > 1.2×10 −11 m/s) overestimates transport. This result indicates that conventional permeability tests in very tight media tend to overestimate K . In practical terms, advective transport at the site is negligible, and transport can be simulated with diffusion only ( P = O).