Pore water equilibrators

Figure 5. Data from the literature (56, 80, 99, 164, 195, 220, 222, 223, 243) indicate that diffusive fluxes of sulfate (calculated from 40 pore-water profiles measured with pore-water equilibrators) are linearly related to concentrations of sulfate in the overlying lake water. The correlation is significant (p < 0.05) both with (r2 = 0.991) and without (r2 = 0.42) the two lakes with high sulfate concentrations. The strong correlation suggests that variations in the depth interval within which sulfate is consumed and in the minimum sulfate concentration defining the gradient are relatively unimportant in determining the flux, compared to variations in sulfate concentrations defining the upper end...

The in situ pore water sampler also called pore water equilibrator or dialyzer is based on the principle that given enough time, a contained quantity of water in the sampler and adjacent pore water will diffuse and equilibrate through a dialysis membrane, or other materials such as porous Teflon, with the surrounding water and its dissolved solutes. The in situ equilibrator can either be removed from the soil to collect pore water or the pore water can be collected through an attached tubing while the device remains in the sediment. 

The exchange of solutes such as nutrients, dissolved organic substances, and metals and their associated processes at soil-floodwater interfaces are measured using laboratory-incubated cores or in situ pore water equilibrators as has been described. But these methods often underestimate fluxes because they do not account for processes such as bioturbation and bioirrigation at the soil-floodwater interface. To overcome these limitations, autonomous benthic chambers installed on top... 

FIGURE 14.11 Pore water equilibrators used to determine in situ solute concentration gradients. 

Diffusion flux can be made by measuring solute gradients using pore water equilibrator, benthic flux chamber, and core incubation. 

Equation (6.20) determines the maximum degree of swelling and the maximum pore radius of a liquid-equilibrated membrane. This relation suggests that the external gas pressure over the bulk water phase, which is in direct contact with the membrane, controls membrane swelling. The observa-hon of different water uptake by vapor-equilibrated and by liquid water-equilibrated PEMs, denoted as Schroeder s paradox, is thus not paradoxical because an obvious disparity in the external conditions that control water uptake and swelling lies at its root cause. 

Diagrams of benthic sampiing equipment, (a) Pore water peepers used for collecting pore water in situ via moiecuiar diffusion across a 0.4- j,m polycarbonate membrane. These probes were 36 inches iong. Each of the six ports on the peeper held 6mL of equilibration fluid. The probes were deployed for 1 to 2.5 y to ensure equilibrium had been reached with the pore waters, (b) Benthic flux chamber. This chamber covered a bottom area of SSScm. Source From Aller, R. C., et al. (1998). Deep-Sea Research, 45, 133-165. 

Experiments conducted on the solubility behavior of the odlitic sediments indicated that they were not in metastable equilibrium with their pore waters. On the basis of equilibration rates, Bernstein and Morse (1985) estimated that the residence time of the pore waters in these sediments was only a few days or less. 

Powers and Brownyard (P20) called the water lost from the C S-H on passing from the saturated to the D-dry condition gel water . It comprises part of the pore water plus an arbitrarily defined fraction of the interlayer water. Whatever procedure is used to dry or equilibrate a paste, COj-free conditions are essential, and the amount of water retained can be obtained accurately only if the ignition loss and COj content are also determined. Calculations based on the initial w/s ratio are often unreliable due to changes in the water content during curing. 

Plots of alkalinty versus Tqo2 and Ca " " versus alkalinity demonstrate conclusively the occurrence of metabolic dissolution. However, they do not show the ratio of the rates of dissolution and carbon oxidation well. The reason is that calcite dissolution is rapid relative to organic matter oxidation. Therefore, pore waters that have become undersaturated due to oxic metabolism re-equilibrate with sedimentary calcite very rapidly. The sedimentary layer in which the release of metabolic acids is not matched by dissolution is expected to be thin, so that the slope... 

The lA Pliocene, Ranzano and Antognola formations were cemented by meteoric water the Bismantova Formation was cemented in part by water with a meteoric component the Loiano and the Borello formations were cemented by slightly modified marine pore water and all the foreland basin units (except the Borello) were cemented by water variably enriched in 0 (8 0 = -2 to +8) generated from silicate reactions. The most 0-enriched values for S 0, a,er are compatible with depths and temperatures of cementation of the three deepest formations, but not for the less deeply buried Loiano and upper part of the Mamoso-arenacea formations. 0-enriched fluids in these latter formations were more probably derived from underlying, more deeply buried rocks apd expelled by compaction. Possibly, the calcite in the deepest buried formations re-equilibrated with hot water after precipitation. 

Carothers, 1992 Spiro et al., 1993 Morad et a ., 1994), The mineral is most suitable for the study of pore-water evolution during sediment subsidence because, unlike other carbonate minerals, siderite probably does not undergo recrystallization and isotope re-equilibration during burial diagenesis, as it has no unstable precursors or polymorphs (Curtis etai, 1975 Gautier, 1982 Pearson, 1985 Curtis Coleman, 1986). Therefore, stable isotope data of siderite cements can provide a powerful tool for the interpretation of diagenetic events in geological... 

Taylor 1981 Alt et al. 1986 Muehlenbachs 1986 Stakes 1991 Staudigel et al. 1995 see Fig. 7a). It is also worth noting that pore-waters in the oceanic crust are initially low in 5 0 (0 near the sediment-water interface to -3 %o at depths of several hundred meters Perry et al. 1976), although it would be reasonable to assume that these waters are quantitatively expelled and/or isotopically equilibrated with co-existing rocks before the oceanic lithosphere is deeply subducted. Because lower crustal gabbros (and pore fluids) on one hand and upper crustal basalts and sediments on the other deviate in o from typical peridotites (5 0 = 5.5 0.2 detailed below) in opposite directions, the oxygen isotope system has the potential to discriminate between subducted components derived from different parts of the oceanic crust. Of course, it is also possible that a mixture of ocean crust components could balance to have no contrast in with the mantle. However, this case would require fortuitous mixtures of these materials and is not to be generally expected. 

---