Core-scale solute transport model selection using Monte Carlo analysis

TL;DR

This study evaluates core-scale solute transport models using Monte Carlo analysis on experimental breakthrough data, demonstrating that the multirate model provides unbiased predictions and better captures the complex porosity characteristics of Culebra Dolomite.

Contribution

The paper introduces a null-space Monte Carlo approach for objective model selection, highlighting the superiority of the multirate model over simpler models for core-scale solute transport.

Findings

Single- and double-porosity models show structural deficiencies with late-time bias.

The multirate model yields unbiased, accurate predictions.

Multirate model aligns with observed multiscale porosity characteristics.

Abstract

Model applicability to core-scale solute transport is evaluated using breakthrough data from column experiments conducted with conservative tracers tritium (H-3) and sodium-22, and the retarding solute uranium-232. The three models considered are single-porosity, double-porosity with single-rate mobile-immobile mass-exchange, and the multirate model, which is a deterministic model that admits the statistics of a random mobile-immobile mass-exchange rate coefficient. The experiments were conducted on intact Culebra Dolomite core samples. Previously, data were analyzed using single- and double-porosity models although the Culebra Dolomite is known to possess multiple types and scales of porosity, and to exhibit multirate mobile-immobile-domain mass transfer characteristics at field scale. The data are reanalyzed here and null-space Monte Carlo analysis is used to facilitate objective model selection. Prediction (or residual) bias is adopted as a measure of the model structural error. The analysis clearly shows single- and double-porosity models are structurally deficient, yielding late-time residual bias that grows with time. On the other hand, the multirate model yields unbiased predictions consistent with the late-time -5/2 slope diagnostic of multirate mass transfer. The analysis indicates the multirate model is better suited to describing core-scale solute breakthrough in the Culebra Dolomite than the other two models.