Average Soil Water Retention Curves Measured by Neutron Radiography.

Water retention curves are essential for understanding the hydrologic behavior of partially saturated porous media and modeling flow and transport processes within the vadose zone. We directly measured the main drying and wetting branches of the average water retention function obtained using two-dimensional neutron radiography. Flint sand columns were saturated with water and then drained and rewetted under quasi-equilibrium conditions using a hanging water column setup. Digital images (2048 by 2048 pixels) of the transmitted flux of neutrons were acquired at each imposed matric potential (∼10-15 matric potential values per experiment) at the National Institute of Standards and Technology Center for Neutron Research BT-2 neutron imaging beam line. Volumetric water contents were calculated on a pixel-by-pixel basis using Beer-Lambert's law after taking into account beam hardening and geometric corrections. To account for silica attenuation and remove scattering effects at high water contents, the volumetric water contents were normalized (to give relative saturations) by dividing the drying and wetting sequences of images by the images obtained at saturation and satiation, respectively. The resulting pixel values were then averaged and combined with information on the imposed basal matric potentials to give average water retention curves. The average relative saturations obtained by neutron radiography showed an approximate one-to-one relationship with the average values measured volumetrically using the hanging water column setup. There were no significant differences (P < 0.05) between the parameters of the van Genuchten equation fitted to the average neutron radiography data and those estimated from replicated hanging water column data. Our results indicate that neutron imaging is a very effective tool for quantifying the average water retention curve.

Novel corresponding-states principle approach for the equation of state of isotopologues: H2(18)O as an example.

The corresponding-states principle (CSP) has been considered for the development of the equations of state (EOS) of minor isotopologues that are usually unknown. We demonstrate that, for isotopologues of a given molecular fluid, a general extended multi-parameter corresponding-states EOS can be reduced to the three-parameter EOS, utilizing the critical parameters (temperature and density) and Pitzer's acentric factor as correlation parameters. Appropriate general CSP mathematical formalism and equations for constructing the EOS of minor isotopologues are described in detail. The formalism and equations were applied to isotopologues of water and demonstrated that the isotopic effect on the critical parameters and the acentric factor of H(2)(18)O can be successfully calculated from the EOS of H2O and experimental data on the isotope effects (liquid-vapor isotope fractionation factor and molar volume isotope effect). We have also shown that the experimental data on the vapor pressure isotope effect (VPIE) for 18O-substituted water are inconsistent within the framework of thermodynamics with the liquid-vapor oxygen isotope fractionation factor. The novel approach of CSP to isotopologues developed in this study creates a new opportunity for constructing the EOS of minor isotopologues for many other molecular fluids.

Isotopic self-exchange reactions of water: evaluation of the rule of the geometric mean in liquid-vapor isotope partitioning.

Deviations from the random distribution of hydrogen isotopes among isotopic species of liquid and vapor water (the rule of the geometric mean) were critically assessed theoretically and experimentally from the triple to critical point of water. A third-order polynomial equation of the classical near-critical expansion was used to accurately describe the liquid-vapor isotope fractionation of H2O and D2O on the basis of their equations of state. It was found that experimental data for the enthalpy of mixing of H2O-D2O can be used to calculate accurately the deviation from the rule of the geometric mean in liquid and vapor water, ln(KD(v)/KD(l)). A new equation obtained in this study shows that the value of ln(KD(v)/KD(l)) smoothly decreases from +0.009 to 0 with increasing temperature from the triple to critical temperature of water. In contrast, the equation available in the literature and that derived from mass spectrometric measurements of liquid-vapor partitioning of H2O and HDO show complex behavior, including maximum, minimum, and crossover.

Pressure Effect on Hydrogen Isotope Fractionation Between Brucite and Water at Elevated Temperatures.

Experimental evidence for a pressure effect on isotopic partitioning at elevated temperatures demonstrates that equilibrium deuterium-protium fractionation between the mineral brucite [Mg(OH)(2)] and pure water systematically increases by 12.4 per mil as pressure increases from 15 to 800 megapascals at 380 degrees C. A linear relation is observed between the measured fractionation factor and the density of water (0.070 to 1.035 grams per cubic centimeter). The trend of the isotope pressure effect is the same as that of recent theoretical studies, but the magnitude is smaller. The pressure effect must be accounted for in the interpretation of isotopic data of geologic systems involving water (paleotemperature, source of fluids).

Abiogenic methane formation and isotopic fractionation under hydrothermal conditions

Recently, methane (CH(4)) of possible abiogenic origin has been reported from many localities within Earth's crust. However, little is known about the mechanisms of abiogenic methane formation, or about isotopic fractionation during such processes. Here, a hydrothermally formed nickel-iron alloy was shown to catalyze the otherwise prohibitively slow formation of abiogenic CH(4) from dissolved bicarbonate (HCO(3)-) under hydrothermal conditions. Isotopic fractionation by the catalyst resulted in delta(13)C values of the CH(4) formed that are as low as those typically observed for microbial methane, with similarly high CH(4)/(C(2)H(6) + C(3)H(8)) ratios. These results, combined with the increasing recognition of nickel-iron alloy occurrence in oceanic crusts, suggest that abiogenic methane may be more widespread than previously thought.

Analysis of Acanthurus triostegus for marine toxins by the stick enzyme immunoassay and mouse bioassay.

The herbivorous convict tang or surgeon fish, Acanthurus triostegus, collected at different locations in the Hawaiian islands, was analyzed for toxicity using a mouse bioassay and a stick enzyme immunoassay (S-EIA) developed to detect ciguatoxin and closely related polyethers. Results of the S-EIA test indicated that about 94% of the samples gave negative readings, while 6% of the fish were positive and thus considered toxic. Fish samples from each location were pooled and the flesh and viscera were successively extracted with hexane, methanol, and water and tested in a mouse bioassay. About one-third of the methanol soluble fractions killed mice within 20 min, 70% killed within 4 hr, and 85% killed within 48 hr. About 40% of the hexane-soluble fractions killed mice within 24 hr and 55% killed within 48 hr. The water-soluble extracts of the flesh and viscera of fish taken from three locations showed relatively high toxicity. Common symptoms for all fractions included convulsions and jumping (especially just prior to death), respiratory distress, hind leg to complete paralysis, loss of body tone, and tremors. Analysis of the data suggests that in the flesh and viscera of A. triostegus there are at least three different nervous system-type toxins, most of which did not appear to react to the S-EIA test.

The composition of Permian seawater.

Forty-nine brine inclusions in marine halite from the Ochoan Salado Formation in the Delaware Basin and fifteen inclusions in halite from the Leonardian Wellington Formation in the Kansas Basin were extracted, and their chemical compositions were determined. The brines are of the Na-K-Mg-Cl-SO4 type; their compositions resemble those of evaporated modern seawater. The values of (mCl(-) - mNa+)/mBr- and (mMg(2+) + mCa(2+) - mSO4(2-) - 1/2mHCO3-)/mBr- of the inclusion brine from the two formations are equal to or slightly higher than those of modern seawater. The original mNa+/mBr- and mCl-/mBr- ratios of the inclusion brines were probably equal to or slightly larger than those of modern seawater. The values of mMg2+/mBr- of the inclusion brines from the Salado Formation are very close to that of modern seawater; the ratios of inclusion brines from the Wellington Formation are slightly lower, probably due to the formation of dolomite/magnesite. The mMg2+/mBr- ratio in the initial seawater was probably close to the parent seawater of the Salado brines. The values of (mSO4(2-) - mCa(2+) + 1/2mHCO3-)/mBr- of the inclusion brines appear to be reduced by the formation of dolomite/magnesite, and the value of this ratio in Permian seawater was probably similar to that of modern seawater. The mK+/mBr- ratios of the inclusion brines are variable, but the original ratios are probably close to or slightly larger than that of modern seawater. If the Br- concentration of Permian seawater was equal to that of modern seawater, the composition of Permian seawater can be narrowly constrained; in mmol/kg H2O, 460 < or = mNa+ < 630, 550 < or = mCl- < 730, mMg2+ = 54 +/- 6, mK+ approximately equal to 11, (mSO4(2-) - mCa(2+) + 1/2mHCO3-) > or = 17, 20 < mSO4(2-) < 45, 5 < mCa2+ < 20, and 0.15 < mHCO3- < 5. The composition of Permian seawater was therefore quite similar to that of modern seawater.