Pore water chemistry modeling

Bradbury, M. H. Baeyens, B. 2002. Pore-water Chemistry in Compacted Resaturated MX-Bentonite Physicochemical Characterisation and Geochemical Modelling. Paul Scherrer Institut, Villigen, Switzerland, PSI Berichl Nr. 02-10, 42... 

There has always been some reluctance to assume that the pH in a porous medium is controlled solely by the carbonate buffer system in the pore waters. There are arguments that H ions on particle surfaces can affect pH measurements (Stumm and Morgan, 1981) and, even more importantly, comprehensive models of pore-water chemistry are beginning to demonstrate that H adsorption on mineral surfaces may play an important role in controlling the pH of pore waters. Conclusions of the pH measurements, however, have been confirmed by millimeter-scale measurements of both Pco and calcium concentration in the pore waters (Hales et al., 1997 Cai et al., 1995 Wenzhofer et ah, 2001.)... 

Figure 12 Pore-water chemistry and saturation indices versus depth at the tailings site of the Heath Steele mine. lA represents saturation indices calculated using an ion-association model, and SII represents saturation indices calculated using a specific ion-interaction model (after Ptacek and Blowes, 2000).

Carbon Dioxide (CO2) Cycle. Cenozoic Climate -Oxygen Isotope Evidence. Cenozoic Oceans -Carbon Cycle Models. Ocean Carbon System, Modeling of. Pore Water Chemistry. 

Table 4. Model describing the pore-water chemistry of the eastern gallery shales...

A detailed study of the chemistry of pore waters near the sediment-water interface of sediments from the equatorial Atlantic was conducted by Archer et al. (1989) using microelectrodes that were slowly lowered into the sediment. By modeling the resulting data they were able to confirm that calcite was dissolving above the saturation depth as a result of benthic oxidation of organic matter. The estimated in situ rate constant for calcite dissoluton was 1-100% day1. This rate constant is 10 to 100 times slower than the one used in previous models, which was based on experimental data. If the slower rate constant proves to be correct, then dissolution of calcite by benthic metabolic processes will be of major importance. 

Kosian, P. A., Makynen, E. A., Monson, P. D. et al. (1998). Application of toxicity-based fractionation techniques and structure-activity relationship models for the identification of phototoxic polycyclic aromatic hydrocarbons in sediment pore water. Environmental Toxicology and Chemistry, 17, 1021-33. 

The fundamental parameter in all models for calcium carbonate dissolution in the deep sea is the saturation state of pore waters. In order to determine the samration state, not only must the composition of the pore waters be known, but also the solubility of the calcium carbonate. Therefore, many studies of carbonate chemistry in deep-sea sediment pore waters have focused on the apparent solubility behavior of carbonates in these sediments. 

The composition of pore waters from contaminated cores 1 and 2 were used to initialize the model (Table 2). Concentrations represent leachate collected from the initial half pore volume of each core. Eluent specified in the transport simulations had the composition of uncontaminated ground water in Table 2. Reactions proposed to describe concentration changes for selected constituents within the cores are based on comparisons between eluent and leachate chemistry and analysis of selected constituents in the core samples. Equilibrium constants and kinetic rates for the reactions were adjusted to give the best fit to leachate concentrations from core 1. The same reactions, equilibrium constants, and kinetic rates were then tested by modeling the concentrations of constituents in leachate from core 2. This geochemical model will be used in the future to simulate evolution of contaminated ground water associated with the Area 4 landfill at the aquifer scale. 

Comparisons of modeled concentrations and the chemistry of water (e.g., pH, Si02(aq), Na, K ) and gas samples (CO2) collected over 5 years show that the THC model captures the trends and magnitude of chemical changes. CO2 concentrations in gas samples increase from an initial value of about 1000 ppmv to greater than 25,000 ppmv as a result of exsolution from matrix pore water and transport into fractures. The CO2 dissolves into condensate waters at lower temperatures, resulting in a drop in pH and also dissolution of calcite. 

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