Mean flow direction modulates non-Fickian transport in a heterogeneous alluvial aquifer-aquitard system
Pauloo, Rich (2020), Mean flow direction modulates non-Fickian transport in a heterogeneous alluvial aquifer-aquitard system, Dryad, Dataset, https://doi.org/10.25338/B8H920
Regional-scale groundwater quality degradation from nonpoint source pollution threatens the long-term sustainability of major alluvial aquifer-aquitard systems worldwide. Upscaled models can efficient represent nonpoint source transport, but fail to accurately characterize non-Fickian (anomalous) transport caused by mean flow direction transience. In this study, we demonstrate that hydrogeologic factors explain this failure. Specifically, vertical anisotropy in K and seasonal pumping and recharge in typical alluvial aquifer systems can fundamentally change hydraulic gradients and shift the mean flow direction between mostly horizontal and mostly vertical flow. Detailed 3D flow and transport simulations in a heterogeneous alluvial aquifer under varying mean flow directions indicate that alterations to hydraulic gradients which control the mean flow direction can lead to increasingly non-Fickian transport. Under mostly horizontal flow, diffusion and slow advection dominant low-K facies slow mass transfer rates from low-K material, and preferential flow along connected high-K networks causes increased spatial spreading along the mean flow direction. Conversely, predominantly vertical flow caused by spatially distributed pumping and recharge shifts mass transfer processes in low-K material from diffusion and slow advection dominant to advection dominant, which results in vertically oriented particle trajectories that compactly migrate through high- and low-K facies alike, leading to increasingly Fickian transport. Thus, mean flow direction transience driven by vertical anisotropy in K and seasonal pumping and recharge can create oscillating transport patterns, ranging from persistently non-Fickian to more Fickian. Results illustrate the hydrogeologic factors that explain why upscaled transport models fail to capture non-Fickian effects resulting from mean flow direction transience.
This research uses three models, detailed in the methods of the manuscript, and a brief description of the models and the data they rely on are provided below:
- hydraulic conductivity field: a T-PROGS (transition probability geostatistics) heterogeneous hydrofacies model of the Kings River Alluvial Fan. Model dimensions are 15 km 12.6 km x 100.5 m. The model was generated by a former study (Weissmann et al., 1999) and used data from well completion reports, borehole logs, and other geophysical logs.
- groundwater flow model: a MODFLOW-2000 groundwater flow model.
- a particle transport model: an RW3D model that solves the advection dispersion equation.
The initial and boundary contditions of the models are specified in input files within the provided datasets, and detailed in the manuscript.
Detailed README.md files herein explain how to use the input files to re-produce the flow and transport models in this study.