Selected Geochemical Processes - Contaminant Geochemistry - page 40

phase, and by environmental conditions (e.g., temperature, pressure, microbio-
logical activity). Rainwater, for example, may affect mineral dissolution paths
differently than groundwater, due to different solution chemistry. When water
comes in contact with a solid surface, a simultaneous process of weathering and
dissolution may occur under favorable conditions. Dissolution of a mineral con-
tinues until equilibrium concentrations are reached in the solution (between solid
and liquid phases) or until all the minerals are consumed.
The initial compositions of both the infiltrating water and the solid materials
may change due to their interaction, which in turn may affect the solubility and the
pathway of processes with time. When a particular component of the dissolved
solution reaches a concentration greater than its solubility, a precipitation process
occurs. Table 2.1 includes the solubility of selected sedimentary minerals in pure
water at 25 C and total pressure of 1 bar, as well as their dissolution reactions. All
of the minerals listed in Table 2.1 dissolve, so that the products of the mineral
dissolution reactions are dissolved species. Figure 2.2 shows the example of
gypsum precipitation with its increasing concentration in an NaCl aqueous

solution.
The presence of organic ligands in the infiltrating water phase may cause the
complexation of some minerals, leading to an increase in their solubility. For
example, oxalate anions enhance the solubility of ZnO in the range of pH = 6-8,
and EDTA (ethylenediamine tetraacetic acid) dissolves hydrous ferric oxide up to
apH= 9 (Stumm and Morgan 1995 ). Fulvic acids in the soil layer may act as
chelating agents, contributing to an increase in solubility of the minerals and, as a
consequence, enhancing their mobility. Laboratory experiments performed by
Bennett et al. ( 1988 ) showed that organic acids can greatly enhance the dissolution
process, especially where reactions take place in an open, flow-through (non-
equilibrium) system.
Carbon dioxide has a dominant effect on the dissolution of carbonate minerals,
such as calcite and dolomite (Table 2.1 ). If a carbonate mineral dissolves in water
that is equilibrated with a constant source of CO 2 , then the concentration of
dissolved carbonic acid remains constant and high. However, when calcite dis-
solution is accompanied by consumption of carbonic acid and a continuous source
of CO 2 is not maintained, the reaction proceeds further to achieve equilibrium.
The subsurface generally is an open system. The presence of CO 2 and other
gases in the atmosphere affects the partial pressure of gas constituents in the
subsurface. For example, carbonate mineral dissolution in a system open to
atmospheric CO 2 does not achieve equilibrium. However, higher local subsurface
CO 2 concentrations can originate from biological activity and other oxidation
processes.
The rate of chemical weathering of minerals in the subsurface depends on a
number of factors, including mineralogy, temperature, flow rate, surface area, the
presence of ligands and CO 2 , and H þ concentrations in the subsurface water
(Stumm et al. 1985 ). Figure 2.3 shows the rate-limiting steps in mineral dissolu-
tion consisting of (a) transport of solute away from the dissolved crystal or