Hydropedology of depression-toe slope interaction across a soil unit boundary at the Boreal-Prairie interface

Abstract

The genesis of mineral soil wetlands at the Boreal Plain – Prairie Ecozone interface is not well understood, given the transition to peatlands in this ecotone. Patterns of groundwater depth and hydraulic gradient were investigated in relation to soil development at a forested site at the Boreal Transition Ecoregion in central Alberta, Canada. Depressional and toe slope positions in each of four small catchments (up to 2 ha) oriented along a soil unit boundary were instrumented with wells and piezometers. Groundwater depth was measured manually or by sensor from 2012 to 2017. Groundwater at both slope positions fluctuated commonly 1.5 to 2 m annually and contributed to seasonal inundation in depressions. Direction of vertical hydraulic gradient reversed regularly, but was more commonly downward at toe slopes, indicating recharge dominance. An equal balance of upward and downward flow gradients in depressions was interpreted as flow-through. Lateral hydraulic gradients were commonly positive from toe to depression. Counts of subgroups for depressions were Orthic Humic Gleysol (3), Orthic Luvic Gleysol (2), Terric Mesisol (2), Orthic Gleysol (2) Terric Humisol (1) and Humic Luvic Gleysol (1). Counts of subgroups for toe slopes were Gleyed Dark Gray Chernozem (2), Gleyed Solonetzic Gray Luvisol (2), Gleyed Dark Gray Luvisol (2), Dark Gray Solodized Solonetz (1), Dark Gray Luvisol (1), Gray Solodized Solonetz (1), and Humic Luvic Gleysol (1). The infrequency of eluvial/illuvial soil features in depressions was consistent with their flow-through hydrology. The higher frequency of eluvial/illuvial soil features at toe slopes was also consistent with recharge hydrology. Soil carbonates were more abundant within the weakly saline soil unit. The seasonal hydrologic pattern in depressions included a period of rapid groundwater rise during snowmelt, a period of stability maintained by inputs of precipitation and toe slope flux, and a period of groundwater recession that was probably initiated when forest ET depleted upland soil moisture. Mineral wetlands with Gleysolic soils are found in locations of the landscape where soil saturation is too irregular to allow peatland formation. Mineral soil wetlands in this region may persist despite climate change because of the expected shift towards greater winter precipitation.