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\begin{document}

\title{Hydraulic gradients modulate anomalous transport in an alluvial aquifer
system}



\author[1]{Rich Pauloo}%
\author[2]{Graham Fogg}%
\author[3]{Zhilin Guo}%
\author[4]{Christopher Henri}%
\affil[1]{University of California Davis}%
\affil[2]{Univ California Davis}%
\affil[3]{University of Arizona}%
\affil[4]{University of California, Davis}%


\vspace{-1em}



  
  \date{May 15, 2024}


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\begin{abstract}
Upscaled transport models are needed to address regional-scale
groundwater quality degradation in major aquifers worldwide, and these
models must accurately characterize anomalous (non-Fickian) transport
(e.g., early solute arrival due to preferential flow, and late time
tailing). Although it is well known that seasonal pumping and recharge
can cause vertical hydraulic gradient values to exceed horizontal
gradients by 10-100 times, the relationship between large shifts in
vertical to horizontal gradient ratio (VHGR) and plume migration is less
understood. We simulate flow and transport of a conservative solute in a
heterogeneous alluvial aquifer under varying VHGR representative of
typical seasonal hydraulic gradient fluctuations and find that VHGR
strongly impacts anomalous transport, plume migration, and mass transfer
rates between hydrofacies. At low VHGR (e.g., ambient hydraulic gradient
conditions corresponding to a more horizontal flow direction), low-K
facies are diffusion-dominated, resulting in low mass transfer rates out
of low-K material, increased spatial spreading along the mean flow
direction, and tortuous particle trajectories. In contrast, at high VHGR
(e.g., representative of pumping and recharge, and corresponding to an
increasingly vertical mean flow direction), results in increasingly
Fickian transport, explained by a shift from diffusion- to
advection-dominated mass transfer in low-K facies, and illustrated by
relatively non-tortuous, vertically oriented particle trajectories. We
conclude that seasonal fluctuations in VHGR in a typical alluvial
aquifer system may create oscillating transport behavior, with
implications for contaminant transport and cleanup. The strong
dependence of anomalous transport on VHGR implies that approaches to
upscale anomalous transport may benefit from incorporating information
about the VHGR, and highlights specific hydrogeologic forcings that
cause upscaled transport methods to fail under transient boundary
conditions.%
\end{abstract}%



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