Field investigation of the waste isolation pilot plant (WIPP) site (New Mexico) using a nonstationary stochastic model with a trending hydraulic conductivity field

Verification of a stochastic model which models the phenomena of flow and transport in a nonstationary conductivity field [Rubin and Seong, 1994] is attempted with field data from the Waste Isolation Pilot Plant (WIPP) site near Carlsbad, New Mexico. The nonstationarity is manifested as a spatial linear trend in the mean log conductivity field. Analysis of the log conductivity and the hydraulic head data shows that the site corresponds to Rubin and Seong's α2 model, where the mean flow direction and the gradient of mean log conductivity are normal to each other. The experimental semivariogram of the log conductivity obtained from field data strongly suggests that it is nonstationary and can be fit with a Gaussian covariance function. The α2 model is successful in predicting the experimental semivariogram of the head obtained from the field data, especially in capturing the finite sill characteristics. The stationary model predicts an unbounded semivariogram at large distances. Solute travel times are also investigated by comparing the travel time cumulative distribution function (CDF) resulting from the stationary and the nonstationary hydraulic conductivity models. The stationary model predicts a higher CDF at early times and hence overestimates the very early travel times to the regulatory plane of compliance than the α2 model.