2 Groundwater Controls on Peatland Form and Function

Peat consists of organic soil that accumulates on terrains where the water table is sufficiently close to the ground surface for long enough time to inhibit oxidation and decomposition of plant material. Under water saturated conditions, oxygen diffusion into the soil profile is slow, thus bacterial activity that drives decomposition is hindered, resulting in the accumulation of partially decomposed plant remains (peat). Wetlands that have accumulated a sufficient thickness of organic soil (commonly ~40 cm) are peatlands, and how they form and function reflect their hydrogeomorphic setting and climate (Brinson, 1993). Climate favors peatlands in northern boreal and subarctic latitudes where precipitation exceeds or closely matches potential evaporation, and in tropical areas where particularly high rainfall occurs (Figure 1).

Figure 1 – Global peatland distribution (from Xu et al., 2018). The black shading classes are applicable to Canada and some smaller areas as shown. Elsewhere, black = peat and white = no peat. Data for this map can be accessed in more detail here.

Classifications of peatlands commonly reflect the origin, character, and degree of groundwater input. The porewater concentration of dissolved minerals and nutrients introduced by groundwater strongly affects their trophic status and geochemical and ecological function. Commonly, peatlands are categorized and named on the basis of the extent of groundwater interaction; and this convention is used herein. In the context of peatlands, groundwater consists of water below the water table that may be present at or near the ground surface. This book also discusses the variably saturated vadose zone that— from time to time—contains groundwater.