Mean flow direction modulates non-Fickian transport in a heterogeneous alluvial aquifer-aquitard system

Abstract:

Regional-scale groundwater quality degradation from nonpoint source pollution threatens long-term aquifer sustainability worldwide, and demands accurate contaminant transport models. Upscaled regional transport models presently fail to accurately characterize non-Fickian (anomalous) transport caused by transience in the mean flow direction. 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. To understand how large shifts in mean flow direction modulate transport phenomena, we simulate 3D flow and transport of a conservative solute in a heterogeneous alluvial aquifer under varying mean flow directions representative of typical seasonal hydraulic gradient fluctuations. We examine tailing, plume spreading, and the spatial distribution of final particle positions. Results indicate that changes in the mean flow direction can lead to increasingly non-Fickian transport. When flow is mostly horizontal, diffusion and slow advection dominant low-K facies slow mass transfer rates out of low-K material, and preferential flow along connected high-K networks increase spatial spreading along the mean flow direction. In contrast, 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, resulting in vertically oriented trajectories that compactly migrate through high and low K facies alike, and lead to increasingly Fickian transport. Thus, fluctuations in mean flow direction driven by vertical anisotropy of K and seasonal pumping and recharge in a typical alluvial aquifer-aquitard system create oscillating transport patterns, ranging from persistently non-Fickian to more Fickian. These findings have important implications for efforts to upscale transport, explain why some boundary conditions remain difficult to solve with existing methods, and reveal the hydrogeologic characteristics of groundwater flow systems that may be used to improve existing upscaled transport models.