Mean Flow Direction Modulates Non‐Fickian Transport in a Heterogeneous Alluvial Aquifer‐Aquitard System. Issue 3 (26th March 2021)

• Record Type: Journal Article
• Authors: Pauloo, Richard A.; Fogg, Graham E.; Guo, Zhilin; Henri, Christopher V.
• Is Part Of: Water resources research. Volume 57:Issue 3(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-03-26
• Subjects: hydrogeology -- non‐Fickian -- simulation -- transport; Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2020WR028655
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: 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 the impact of nonhorizontal and nonvertical mean flow direction on non‐Fickian transport within a 3D, kilometer‐scale, typical alluvial aquifer‐aquitard system. Notably, vertical anisotropy in K and seasonal pumping and recharge can fundamentally change hydraulic gradients and shift the mean flow direction between mostly horizontal and mostly vertical flow. Detailed 3D flow and transport simulations 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, resulting in vertically oriented particle trajectories that compactly migrate through high‐K and low‐K facies alike, and increasingly Fickian transport. Thus, mean flow direction transience driven by vertical anisotropy in K and seasonal pumping and recharge can create oscillating transport patterns, that range from persistently non‐Fickian to more Fickian. Results illustrate the regional‐scale hydrogeologic factors that explain why upscaled transport models fail to capture non‐Fickian effects resulting from mean flow direction transience Plain Language Summary: Regional‐scale groundwater quality degradation from nonpoint source pollution threatens long‐term aquifer sustainability and demands accurate contaminant transport models. These models must effectively represent non‐Fickian (i.e., anomalous) transport, which is widely observed in nature, and characterized by non‐Gaussian travel time and spatial spreading of subsurface plumes. In this research, we simulate groundwater flow and contaminant transport in a geostatistical representation of a regional‐scale alluvial aquifer‐aquitard system. We vary the vertical and horizontal hydraulic gradients, and thus, the mean direction of fluid flow and the hydraulic forcings on different sediments that control mass transfer processes. Results demonstrate that as the mean flow direction is increasingly parallel to the direction of laterally extensive aquifer stratigraphy, increasingly non‐Fickian transport is observed. These results suggest that strong vertical anisotropy in hydraulic conductivity and seasonal pumping and recharge can create strong vertical gradients and increasingly vertical flow that lead to more compact plumes, and when fields are fallow, ambient horizontal hydraulic gradients produce more non‐Fickian transport. 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 parameters that may be used to improve existing transport modeling in regional aquifer‐aquitard systems Key Points: Mean flow direction modulates anomalous transport in a heterogeneous alluvial aquifer‐aquitard Transport is more non‐Fickian under horizontal flow with diffusion dominant facies Strong vertical gradients can switch low‐K facies from diffusion to advection dominant