Mean Flow Direction Modulates Non‐Fickian Transport in a Heterogeneous Alluvial Aquifer‐Aquitard System

Regional-scale groundwater quality degradation from nonpoint source pollution threatens the long-term sustainability of major alluvial aquifer-aquitard systems worldwide. Models are needed to simulate regionally upscaled transport processes in heterogeneous geology that produces non-Fickian (anomalous) transport. In this investigation, we show impact nonhorizontal and nonvertical mean flow direction on within a 3D, kilometer-scale, typical system. Notably, vertical anisotropy K seasonal pumping recharge can fundamentally change hydraulic gradients shift between mostly horizontal flow. Detailed 3D simulations under varying directions indicate alterations which control lead increasingly Under flow, diffusion slow advection dominant low-K facies mass transfer rates material, preferential along connected high-K networks causes increased spatial spreading direction. Conversely, predominantly caused by spatially distributed shifts material dominant, resulting vertically oriented particle trajectories compactly migrate through alike, Fickian Thus, transience driven create oscillating patterns, range persistently more Fickian. Results illustrate regional-scale hydrogeologic factors explain why models fail capture effects