4 Influence of Microorganisms on Groundwater Chemistry

The chemical composition of groundwater is regulated by heterogeneous interactions between the solid, liquid, and gas phase (Glynn and Plummer, 2005). These reactive systems feature multiple contributions from acid-base, surface and aqueous complexation, and oxidation-reduction reactions, as well as frequent involvement of prokaryotic microorganisms. Mass transport processes such as advection and dispersion also come into play, which give rise to changes in groundwater chemistry over a wide range of time scales and flow path lengths.

Prokaryotes contribute to chemical reactions in groundwater systems in two ways. First, metabolic enzyme activity can speed up (catalyze) slow reactions and force corresponding reaction quotients to shift rapidly towards or away from equilibrium. Second, prokaryotic cells behave as microscopic reactive solids owing to the chemical reactivity of functional groups, such as carboxyl or phosphoryl substituents, in the macromolecular components of cell walls, external sheaths, and EPS. The metabolic intervention of microbes in chemical reactions affects many aspects of groundwater chemistry, including pH, redox conditions, mineral dissolution and precipitation processes, and the chemical speciation of solutes (Chapelle, 2000). As reactive solids, bacteria not only contribute to the sorption (surface complexation) of dissolved ions but also serve as heterogenous nucleation templates for mineral precipitation.