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(SAR = 2.5). The result is an aqueous solution with a sodium adsorption ratio
(SAR) between the volumes of water mixed together. In both cases, with organic
and inorganic contaminants, the chemodynamic properties and the quality of the
resulting water change.
13.2 Selected Contaminant Transformations in Sediments
and Groundwater
Special consideration should be given to the transformation of contaminants in
sediments and groundwater. Under saturated conditions, the solid phase may
function as a sink, reservoir, and reactor for contaminants. Contaminant presence,
persistence, and transformation in the water phase is controlled by the chemistry of
the water body, the surface properties of the materials forming the solid phase
(sediments or suspended particles), and environmental conditions (temperature and
aerobic or anaerobic status).
Pore waters of confining beds in an aquifer (e.g., Black Creek) may contain
relatively high concentrations (*100 lM) of dissolved organic acid anions, such
as acetate and formate, whereas aquifer water contains relatively low concentra-
tions of organic acid anions (*1 lM). In addition, confining bed pore waters also
may contain sulfate in higher concentrations, that is, *100 lM, while sulfate
concentrations in aquifers are only about *5 lM (McMahon and Chapelle 1991 ;
McMahon et al. 1992 ). Discussing these findings for the Black Creek aquifer,
Chapelle ( 2005 ) assumed that diffusion of DIC (dissolved inorganic carbon),
together with DOC (dissolved organic carbon), sulfate, and cations (Mg and Ca)
from confining bed pore waters, provide an electron donor (organic carbon) and an
electron acceptor (sulfate) for microbial metabolism. Additional inorganic carbon
enhances magnesium-calcite dissolution-precipitation, as driven by microbially
produced carbon dioxide. Under these conditions, microbial processes occurring in
the aquifer (sulfate reduction), as well as microbial processes in confining beds
(organic matter fermentation), have an important impact on contaminant trans-
formation in sediments and groundwater.
13.2.1 pH and Hydrolysis Reactions
The pH in ponds and rivers generally is within one pH unit of that of the under-
lying water. Wolfe et al. ( 1990 ) explain these differences by noting that pH values
measured in sediments are a composite of the pH of the interstitial water and the
pH in the vicinity of the charged surface. Because most solids in sediment systems
exhibit a negative charge, the pH near the surface becomes lower than in the bulk
water phase. Partially saturated conditions in sediments favor biological activity,
leading to carbonate formation and a rise in pH.
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