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Groundwater Contamination (1984) / Chapter Skim
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3. Contaminants in Groundwater: Chemical Processes
Pages 46-64

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From page 46...
... Field and laboratory studies have established that various toxic heavy metals, transition metals, metalloids, radionuclides, and other inorganic species can be mobile or immobile in the groundwater zone, depending on the hydrogeochemical conditions represented by the pH, the redox condition, the ionic strength, the mineralogy, the solid-phase surface area, and the complexing capacity. Although the importance of chemical reactions in the attenuation of contaminants is widely recognized, the capabilities for attenuation predictions are not well developed.
From page 47...
... This approach has the objective of predicting the advance rate and the shape of the front of a contaminant zone emanating from a continuous or a temporary source. The second approach has the objective of predicting the contaminant concentrations that will occur in the zone of contamination after chemical mass transfer has caused equilibrium concentrations to be achieved by precipitation, dissolution, oxidation, or reduction.
From page 48...
... for the advection-dispersion equation for nonreactive contaminants in saturated homogeneous porous media can be used for predictive purposes. In these models the velocity of the advancing front becomes the effective contaminant velocity, VIR, and the dispersion coefficient becomes the effective dispersion coefficient, DIR.
From page 49...
... In addition, this zone contains a variety of dissolved constituents including the competitor cation A+, which exists in the leachate at concentrations different than the ambient groundwater. Therefore, each segment of the porous medium contacted by X+ has been contacted previously by the major cations (including A+)
From page 50...
... Breakthrough curves that are simulated using the advection-dispersion model with linear isotherms are symmetrical or nearly symmetrical, whereas curves obtained from laboratory-column experiments are asymmetrical with extended tails. Reynolds (1978)
From page 51...
... The first describes the advance of the front of the contaminant zone as chemical mass transfer continuously occurs in response to a changing solution-phase concentration, whereas the second describes the equilibrium concentrations that occur after chemical mass transfer in a reaction zone has caused equilibrium to be achieved. According to this conceptualization, chemical mass transfer occurs in the reaction zone, and in front of the reaction zone the contaminants are transported at the equilibrium concentrations established previously in the reaction zone.
From page 52...
... Nearly all of the thermodynamic data for solid phases is derived for chemically pure manifestations of the minerals, whereas in groundwater impurities are the norm. When solids precipitate in the groundwater zone, amorphous or poorly crystalline forms commonly exist at first and then slowly undergo conversion to more crystalline forms.
From page 53...
... For some species there is considerable uncertainty in the values that are currently used to represent the equilibrium constants. Nevertheless, attempts at evaluating the species distributions are essential in investigations of the behavior of most inorganic contaminants in groundwater.
From page 54...
... As dispersion occurs, the redox and pH conditions may change, and with these and other effects of dispersion various chemical reactions involving mineral and amorphous solids take place. These reactions can cause further changes in pH, the redox condition, the gross water chemistry, and other factors.
From page 55...
... Field and laboratory research indicates that the sorption of organic compounds is dominantly onto particulate organic matter in the sediments. This partitioning of the solute between the aqueous phase and solid organic matter generally appears to reach equilibrium rapidly and is reversible; thus it can be introduced into the chemical mass-transfer term of the transport equation as a K`~.
From page 56...
... Another problem may occur in or ganic-rich groundwaters such as landfill leachates. Trace or ganic compounds can become associated with high-molecular weight, dissolved organic matter, often humic or fulvic acids (Schnitzer and Khan, 1972~.
From page 57...
... This activity is dominantly bacterial, although yeasts, fungi, and viruses may also be present. We have measured, by direct microscopic counting, 106 bacteria per gram of dry, sandy aquifer material in a number of landfill-leachate-contaminated groundwater systems.
From page 58...
... , but the applicability of these models for application to contaminated-groundwater systems has yet to be assessed. FIELD STUDIES OF CONTAMINATED GROUNDWATER Although most of the literature on the chemical behavior of contaminants in groundwater is based on batch and column experiments and on thermodynamic models, insight pertaining to contaminant behavior is also obtained from field investigations of zones of contamination.
From page 59...
... However, field studies have established that it will occur in groundwater zones where labile organic matter and denitrifying bacteria exist even if the groundwater contains low but measurable concentrations of dissolved oxygen (Gillham and Cherry, 1978~. Although groundwater from wells in zones where denitrification apparently occurs contains detectable quantities of dissolved oxygen, it is likely that the reduction of NO3- takes place in microenvironments in small pores or on grain surfaces where the redox conditions are most suitable.
From page 60...
... It is expected that toxic organic contaminants are mobile in groundwater at many municipal landfills situated on permeable deposits. The dissolved organic fraction in contaminated groundwater at landfills has a much greater potential to cause severe contamination of groundwater resources than dissolved inorganic contaminants.
From page 61...
... Research pertaining to the chemical and biochemical behavior of contaminants in fractured porous media Is In its Infancy. SUMMARY AND CONCLUSIONS Most of what is known about the chemical behavior of contam61 behavior of inorganic contaminants in groundwater involve the use of the distribution coefficient, which is incorporated in a simple retardation term into the advection-dispersion equation, and the use of thermodynamics-based chemical equilibrium models.
From page 62...
... Considerable success has been achieved in estimating, from solubility data, the distribution coefficients for adsorption of halogenated hydrocarbons by solid organic matter in porous geologic materials that have appreciable organic matter. The other processes that can cause attenuation of dissolved organic compounds are much less amenable to quantification, particularly for anaerobic groundwater where biological transformations are poorly understood.
From page 63...
... . Methods for determining radionuclide retardation factors: Status report, Battelle Pacific Northwest Laboratory, PNL-3349/UC-70, Richland, Wash.
From page 64...
... . Contaminant transport in fractured porous media: Analytical solution for a single fracture, Water Resour.


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