Linking hierarchical stratal architecture to plume spreading in a Lagrangian‐based transport model: 2. Evaluation using new data from the Borden site

Abstract

A new data set collected at the well‐documented Borden research site was used in order to evaluate a newly published but as yet untested idea for stochastic modeling (Ramanathan et al., 2008). The new data set reveals the stratal architecture of the Borden aquifer and allowed us to determine how the stratal architecture, at different scales, controlled the macrodispersion observed in the original natural gradient tracer test. The newly published idea for modeling uses a Lagrangian‐based model for the particle displacement variance developed from independent, physically based, quantifiable univariate statistics, including the proportions and mean length of the strata. The method for defining model parameters avoids the often equivocal step of fitting sample bivariate permeability statistics. The model parameters were developed from the new data from the Borden site. The new data included geologic data in much greater abundance than permeability data. There are two scales of unit types delineated in a hierarchy of stratal architecture. Their shapes are complex, not simple layers or lenses. The method facilitated the direct use of both geologic and permeability data, which need not be collocated. The dispersion model developed from these data represents the field‐measured particle displacement variance that occurred in the natural gradient tracer test well. The contributions to time‐dependent macrodispersion by strata at each scale were computed and analyzed independently. This analysis revealed that macrodispersion at the Borden site is primarily controlled by the proportions, and the mean and variance in length of larger‐scale strata of medium sand (M) and strata of fine sand and silt (FZ), with secondary contributions by smaller‐scale strata types occurring within the larger‐scale units. When sampling the pattern in the longitudinal direction, M and FZ couplets repeat at 10 m intervals on average, with a high length variance. To reach a time‐constant macrodispersivity, the stratal length variability must be fully sampled by the plume. The macrodispersivity becomes asymptotic beyond an advective distance of about 60 m, corresponding to about 12 longitudinal transitions between M and FZ unit types.