5.2 Direct Pollutants as Contaminant Sources
With increasing development pressures globally, particularly in the North, the risk of direct contamination to peatlands has increased. Direct contamination can be from overland exposure such as by wastewater treatment, vehicles traveling across peatland roads, spills associated with train derailment, or by subsurface pollution such as that from buried pipeline leaks. Depending on the vector of pollution, the processes that govern its movement and degradation will be different.
Dissolved contaminants can move quickly in the high near surface hydraulic conductivity layer in many peatlands. This can result in large areas of contamination, coupled with the potential for downgradient impacts. However, the upper layers of peatlands are commonly the most biogeochemically active zones due to the abundant and labile organic matter, high density of vascular roots, access to oxygen, warmer summertime temperatures, and exposure to sunlight. These properties allow for the rapid transformation or sequestration of most contaminants. Thus, many soluble contaminants do not remain long after the contaminant source has been stopped or fixed. The risk of these contaminant sources causing adverse environmental effects depends on the balance between enhanced near surface transport and reactivity of a given contaminant, as well as its toxicity.
In contrast to overland or near surface contaminants, deeper subsurface contaminants often remain in peatlands for longer periods of time because of the commonly low hydraulic conductivity matrix at depth, even after the source has been remedied. Subsurface contaminants introduced into the low hydraulic conductivity layers of the catotelm move slowly. The limited potential for transport must be balanced against the relatively recalcitrant organic matter and the general absence of electron acceptors (e.g., NO3-, SO2-4) at depth in most peatlands. These conditions are not conducive to the biodegradation of most contaminants. Geochemically, the buried peat in the catotelm strongly binds cationic contaminants, further limiting contaminant migration. In most cases, subsurface contaminants will remain in peatlands for long time periods, well beyond direct surface pollution.