Geochemical Mixing in Peatland Waters: The Role of Organic Acids

The close interplay between hydraulic and biotic processes controls the evolution of ecosystems in large, circumboreal peatlands and the degree to which they lose, gain or sequester carbon. In peatlands, biota significantly alters surficial acid–base equilibrium and solute chemistry by releasing dissolved organic compounds into surface and pore waters, which then mix through advection and dispersion through pore water flow paths. We report herein the results of new geochemical mixing models that incorporates organic acid dissociation constants to understand how organic acids control the acid–base chemistry in mixtures of dilute acidic bog pore waters and circum-neutral groundwater (typical of carbonate terrains) that upwells or disperses into peat deposits. In our new mixing models, at least twice as much groundwater is required to neutralize bog water acidity when dissolved organic carbon concentrations exceed 10 mmol/L than in their absence. Although it remains uncertain how future climatic change will alter the composition of dissolved organic matter in peatlands, organic acids should remain an important factor with respect to potential neutralization of peatland waters by groundwater sources. The acid base equilibria in large peat basins not only has important implications for vegetation patterning but also for biogeochemical cycles in these globally important reservoirs for carbon.