Active Layer Groundwater Flow: The Interrelated Effects of Stratigraphy, Thaw, and Topography

Abstract

The external drivers and internal controls of groundwater flow in the thawed “active layer” above permafrost are poorly constrained because they are dynamic and spatially variable. Understanding these controls is critical because groundwater can supply solutes such as dissolved organic matter to surface water bodies. We calculated steady‐state three‐dimensional suprapermafrost groundwater flow through the active layer using measurements of aquifer geometry, saturated thickness, and hydraulic properties collected from two major landscape types over time within a first‐order Arctic watershed. The depth position and thickness of the saturated zone is the dominant control of groundwater flow variability between sites and during different times of year. The effect of water table depth on groundwater flow dwarfs the effect of thaw depth. In landscapes with low land‐surface slopes (2–4%), a combination of higher water tables and thicker, permeable peat deposits cause relatively constant groundwater flows between the early and late thawed seasons. Landscapes with larger land‐surface slopes (4–10%) have both deeper water tables and thinner peat deposits; here the commonly observed permeability decrease with depth is more pronounced than in flatter areas, and groundwater flows decrease significantly between early and late summer as the water table drops. Groundwater flows are also affected by microtopographic features that retain groundwater that could otherwise be released as the active layer deepens. The dominant sources of groundwater, and thus dissolved organic matter, are likely wet, flatter regions with thick organic layers. This finding informs fluid flow and solute transport dynamics for the present and future Arctic.

Empirical Models for Predicting Water and Heat Flow Properties of Permafrost Soils

“…Although the distributions of properties among categories have some overlap, the data show that acrotelm soils are substantially more porous and permeable, less dense and thermally conductive, and retain less porewater than do catotelm and especially than mineral soils (Figure 3). Using a subset of our data, studies predicted that as permafrost thaws, the water table depth will exert greater control on fluid flux than will thaw depth (O'Connor et al, 2019) and that the main sources of water and dissolved carbon will be in riparian areas with thick organic layers (Neilson et al, 2018; O'Connor et al, 2019). Our findings suggest that such local predictions apply to much larger spatial areas of the Alaskan tundra.…”

Water Tracks Enhance Water Flow Above Permafrost in Upland Arctic Alaska Hillslopes

“…Journal of Geophysical Research: Earth Surface the newly thawed deeper layers are less than that of the higher layers and the water table depth decreases (O'Connor et al, 2019). This change in groundwater discharge has wide-reaching implications for solute fluxes, as near-surface water flowing through higher peat layers tends to be the primary source of downstream solutes in permafrost-rich terrains (Frey et al, 2007).…”

Guidelines for cold‐regions groundwater numerical modeling

“…The model domain size and mesh discretization should be carefully chosen to best represent the system scale and to allow accurate reproduction of heterogeneities and hydraulic and thermal gradients while keeping in mind the study's objectives. For example, recent publications have shown the importance of shallow supra‐permafrost groundwater on cold‐regions hydrology (Lamontagne‐Hallé et al, 2018; O'Connor, Cardenas, Neilson, Nicholaides, & Kling, 2019; Rey, Walvoord, Minsley, Rover, & Singha, 2019; Toohey, Herman‐Mercer, Schuster, Mutter, & Koch, 2016). To simulate these local processes included in a larger groundwater system, while limiting computational time, a finer mesh size near the surface and a coarser mesh size at greater depths can be used (e.g., McKenzie & Voss, 2013).…”