10.25338/B8H920
Pauloo, Rich
0000-0002-6231-9530
University of California, Davis
Mean flow direction modulates non-Fickian transport in a heterogeneous
alluvial aquifer-aquitard system
Dryad
dataset
2020
FOS: Earth and related environmental sciences
2020-09-10T00:00:00Z
2020-09-10T00:00:00Z
en
https://doi.org/10.1016/S0022-1694(99)00160-2
4952913 bytes
3
CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
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.