Groundwater movement in heterogeneous, water-limited, low-relief landscapes: understanding interactions of geology, topography, and climate at multiple spatiotemporal scales

The low-relief, sub-humid Boreal Plains (BP) region of Canada, is characterized by pond-peatland-forestland complexes underlain by thick glacial deposits, which result in inter-dependent surface water processes and groundwater flow systems with varying spatiotemporal controls. A fundamental framework of climate, geology, and topography was used to better understand and explain the spatiotemporal variability in hydrologic responses and shallow groundwater flow systems in the BP. A 20 year historic hydrometric dataset (e.g., hydraulic head, vertical hydraulic gradients, geochemical signatures, stable water isotope ratios, and climatological data) collected from a pre-existing network of over 850 shallow and deep monitoring wells was supplemented with contemporary measurements and wells. Two primary spatiotemporal scales were examined to create a holistic, variable-scale conceptual model of groundwater movement in the BP: the large scale (e.g., glacial landforms, regional topography, and decadal climate cycles), and the small scale (e.g., individual landcovers, local hummocks, and annual moisture deficits). At the large spatial scale (i.e., 100 km2) the landscape was delineated into hydrologic response areas (HRAs) based on geologic substrate characteristics (i.e., texture and geologic origin): coarse glaciofluvial outwash, fine-textured hummocky moraine, and clay-till plain. It was found that in the coarse outwash HRA and the hummocky moraine HRA, water tables were recharge (i.e., precipitation) controlled, while in the clay-till plain HRA they are topography controlled. Moreover, groundwater flow (and the controls thereon) is considered “intermediate” in the clay-till plain and “local” in the hummocky moraine; however, the coarse outwash contains multiple scales of flow and is influenced by adjacent HRAs and larger-scale flow systems. These larger-scale flow systems as well as surface water-groundwater interactions were further explored in the coarse outwash by examining the variability of the relative contributions of groundwater to eleven different lakes. It was demonstrated through the use of isotope hydrology, that landscape position is the dominant control over groundwater contributions to lakes; however, surface water connections were also significant and can short circuit groundwater pathways and confound the isotopic signal. Lakes at low landscape positions with large potential groundwater capture areas had relatively higher and more consistent groundwater contributions and low interannual variability thereon. Isolated lakes high in the landscape experienced high interannual variability as they have little to no groundwater input to buffer the volumetric or isotopic changes due to evaporation and precipitation. At the small or local spatial scale, both empirical and numerical modelling approaches were taken. The hydraulic gradients between forested hummocks and adjacent peatlands at sixteen locations, which represent a spectrum of hummock and peatland morphometries, topographic positions, and geologic settings, were monitored over the course of a mesic (non-drought) year. The dominant gradient was found to be from the peatland to the forestland (opposite that of the topographic gradient). Water table depressions under each forested hummock indicate that boreal forestlands are not reliable sources of groundwater recharge, spatially or temporally, which supports previous regional-scale research showing that peatlands are the primary water source for landscape-scale runoff. Variably saturated numerical modelling was undertaken to better understand the source-sink relationship between forested hummocks and adjacent peatlands and the controls on water table dynamics and groundwater recharge at the hummock scale. Hydraulic conductivity played the largest role in determining groundwater recharge rates. It was shown that the sink or source function of a forested hummock is dependent more on hydrogeologic and morphometric controls than climatic variability, where taller and narrower hummocks served as sources of groundwater and longer flatter hummocks served as sinks. Glacial depositional landscapes, such as the BP, are highly complex regions with spatially heterogeneous storage and transmission properties and temporally variable recharge potentials, resulting from the delicate, and often tipped, balance between precipitation and evapotranspiration. These studies demonstrate that in regions such as this, smaller-scale heterogeneities in geology and recharge can be a dominant factor over topography, notably in areas with high conductivity or hummocky terrain. Understanding the natural spatial and temporal variability of, and controls on, water table position, groundwater movement, and groundwater recharge under varying physical and climatic scenarios is important, as water security, ecosystem sustainability, and environmental quality become the focus of land management and reclamation efforts.

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