Application of a Darcian approach to estimate liquid flux in a deep vadose zone

Approaches for estimating liquid flux in the shallow (0-2 m) vadose zone are hindered by the high degree of spatial and temporal variability present near the land surface. It is hypothesized that high-frequency variations in flux will be damped with depth. This study was conducted to estimate deep liquid flux using the Darcian approach at a waste disposal site in south-central Idaho that is underlain by a complex sequence of unsaturated basalt flows intercalated with thin sedimentary layers. Flux is estimated by combining in situ water potential measurements from sedimentary interbeds located at depths of 34 and 73 m below land surface (bls) with laboratory estimates for the unsaturated hydraulic conductivity. Tensiometer data at seven locations indicated nearly constant conditions for 30 mo, while nine of the other 10 sites showed small gradual trends. Assumption of a unit hydraulic gradient led to flux estimates ranging from 0.2 to 10000 cm yr-1. Estimates in the 34-m interbed ranged across four orders of magnitude while flux estimates for the 73-m interbed ranged three orders of magnitude. While the tensiometer data appear to reflect in situ conditions and are a sensitive indicator of hydrologic conditions in the deep vadose zone, the laboratory-developed hydraulic properties introduce a high degree of uncertainty, potentially affecting predictions by orders of magnitude. There is a need to develop techniques for assessing flux rates for the range of applicable field conditions to improve the confidence in deep flux estimates.