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.

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.