5.1 Atmospheric Pollutants as Contaminant Sources
Atmospheric deposition of contaminants is a pathway for the introduction of contaminants into peatland groundwater systems. Unlike other ecosystems, where toxic or trace metals can create local regions of decreased ecosystem functionality, the large mass of organic material (i.e., peat) limits both the toxicity and mobility of many toxic and trace metals. This is achieved, primarily, through adsorption onto organic matter. Interestingly, this makes peatlands excellent long-term records of metal deposition rates, with clear signals of the Industrial Revolution being captured in European peatlands (Livett et al., 1979; Shotyk et al., 1996).
Due to these sequestration processes, contaminants entering peatlands do not often result in groundwater contamination at larger spatial scales. Despite peatlands having an unparalleled ability to sequester metals over long (centuries) timeframes, changes to the hydrological, geochemical, or climatic conditions can upset the delicate balance of peatland functions that regulate metal sequestration, turning metal sinks into sources. These disruptions can release previously-sequestered metals into more susceptible systems such as drinking water aquifers, rivers, and lakes through direct groundwater inputs and/or surface erosion of peat. For example, Rothwell and others (2007) found Cu, Ni, Pb, Ti, V, and Zn were leached from peat into headwater streams, with Pb, Ti and V being mobilized by the release of dissolved organic carbon (DOC); Pb is also released by erosion (Rothwell et al., 2008).
Atmospheric pollution can be both relatively local sources such as NO3- and SO42-— important components in acid rain—or distal sources as in the case of mercury. More details on mercury in peatlands are provided in Box 6. Atmospheric deposition of these redox-sensitive pollutants can lead to enhanced mineralization (decomposition) of peat (Chapin et al., 2003). This can cause changes to water table variability and soil moisture retention, thus water and solute flows. The specific species of the contaminant will determine its reactivity and toxicity within peatlands, hence its persistence and mobility in peatland groundwater systems and its impact on peatland hydrology.