Donald C. Thorstenson

Development of reaction models for ground-water systems

Abstract Methods are described for developing geochemical reaction models from the observed chemical compositions of ground water along a hydrologic flow path. The roles of thermodynamic speciation programs, mass balance calculations, and reaction-path simulations in developing and testing reaction models are contrasted. Electron transfer is included in the mass balance equations to properly account for redox reactions in ground water. The mass balance calculations determine net mass transfer models which must be checked against the thermodynamic calculations of speciation and reaction-path programs. Although reaction-path simulations of ground-water chemistry are thermodynamically valid, they must be checked against the net mass transfer defined by the mass balance calculations. An example is given testing multiple reaction hypotheses along a flow path in the Floridan aquifer where several reaction models are eliminated. Use of carbon and sulfur isotopic data with mass balance calculations indicates a net reaction of incongruent dissolution of dolomite (dolomite dissolution with calcite precipitation) driven irreversibly by gypsum dissolution, accompanied by minor sulfate reduction, ferric hydroxide dissolution, and pyrite precipitation in central Florida. Along the flow path, the aquifer appears to be open to CO 2 initially, and open to organic carbon at more distant points down gradient.

Organic geochemistry and pore water chemistry of sediments from Mangrove Lake, Bermuda

Abstract Mangrove Lake, Bermuda, is a small coastal, brackish-water lake that has accumulated 14 m of banded, gelatinous, sapropelic sediments in less than 10 4 yr. Stratigraphic evidence indicates that Mangrove Lakes sedimentary environment has undergone three major depositional changes (peat, freshwater gel, brackish-water gel) as a result of sea level changes. The deposits were examined geochemically in an effort to delineate sedimentological and diagenetic changes. Gas and pore water studies include measurements of sulfides, ammonia, methane, nitrogen gas, calcium, magnesium, chloride, alkalinity, and pH. Results indicate that sulfate reduction is complete, and some evidence is presented for bacterial denitrification and metal sulfide precipitation. The organic-rich sapropel is predominantly algal in origin, composed mostly of carbohydrates and insoluble macromolecular organic matter called humin with minor amounts of proteins, lipids, and humic acids. Carbohydrates and proteins undergo hydrolysis with depth in the marine sapropel but tend to be preserved in the freshwater sapropel. The humin, which has a predominantly aliphatic structure, increases linearly with depth and composes the greatest fraction of the organic matter. Humic acids are minor components and are more like polysaccharides than typical marine humic acids. Fatty acid distributions reveal that the lipids are of an algal and/or terrestrial plant source. Normal alkanes with a total concentration of 75 ppm exhibit two distribution maxima. One is centered about n -C 22 with no odd/even predominance, suggestive of a degraded algal source. The other is centered at n -C 31 with a distinct odd/even predominance indicative of a vascular plant origin. Stratigraphic changes in the sediment correlate to observed changes in the gas and pore water chemistry and the organic geochemistry.

Equilibrium distribution of small organic molecules in natural waters

Abstract The equilibrium activities of 39 dissolved species in the system C-N-S-H 2 O are computed as a function of pe − and pH at 25°C. For pe-pH conditions encountered in natural waters the predominant dissolved species are CH 4 , H 2 CO 3 , HCO 3 − , CO 3 2− , NH 4 OH, NH 4 + , N 2 , NO 3 − , H 2 S, HS − and SO 4 2− . Organic compounds of higher molecular weight, dissolved or particulate, are thermodynamically unstable in most natural environments. The distribution of dissolved species for the decomposition reactions of carbohydrate (CH 2 O) and alanine (C 3 H 7 O 2 N) in sea water are computed, assuming equilibrium among the decomposition products. The decomposition, in a closed system, of 0.1 gram-atom of organic carbon per liter of solution produces: a. (CH 2 O)— m CH 4 ~ 10 −2 , m HCO 3 − ~ 10 −1 , m HS − ~ 10 −2.5 , m NH 4 + ~ 10 −3 , pH ~ 6.5; b. (C 3 H 7 O 2 N)— m CH 4 ~ 10 −2 , m HCO 3 − ~ 10 −1 , m HS − ~ 10 −1.5 , m HN 4 + ~ 10 −1.5 , pH ~ 7.3. In the course of these reactions, the oxidation potential changes by only 50–60 mV after consumption of the initial oxygen. A mathematical expression for the homogeneous pe buffer capacity, β E , is derived. Data from five reducing marine environments show that the predominant dissolved species tend to approach equilibrium, with the exception that the concentration of dissolved methane in certain environments is considerably higher than the predicted equilibrium concentration. Values of β E range from 10 −3 to 1.2 for the environments studied.

Time variability of pore water chemistry in recent carbonate sediments, Devil's Hole, Harrington Sound, Bermuda

Chemical analyses of pore waters from recent marine carbonate sediments, Devils Hole, Harrington Sound, Bermuda, have been obtained at intervals over a four year period. Interstitial waters were systematically analyzed for pH, titration alkalinity, dissolved sulfides, NH4+, Ca2+, Mg2+ and Na+ or Cl−. Additional analyses on some cores included SO42−, PO43−, dissolved CH4 and N2, and C:N:H ratios in the detrital organic material. The following general trends with depth (to ∼ 1 m) are observed: (1) major cations show little or no change; (2) pH decreases; (3) alkalinity, sulfides, NH4+ and PO43− increase; (4) dissolved CH4 is consistently low. The chemical changes with depth can be modelled theoretically and are consistent with experimental data. Significant seasonal changes in pore water chemistry are observed. The data suggest an annual exchange between the pore waters (to a depth of ∼ 1 m) and the overlying water of Harrington Sound; the exchange occurs between August and January. The nutrient flux out of the sediments during this process may be a major factor in the plankton ecology of Harrington Sound.

Chemistry of unsaturated zone gases sampled in open boreholes at the crest of Yucca Mountain. Nevada: Data and basic concepts of chemical and physical processes in the mountain

Boreholes open to the unsaturated zone at the crest of Yucca Mountain, Nevada, were variously sampled for CO2 (including 13C and 14C), CH4, N2, O2, Ar, CFC-11, CFC-12, and CFC-113 from 1986 to 1993. Air enters the mountain in outcrops, principally on the eastern slope, is enriched in CO2 by mixing with soil gas, and is advected to the mountain crest, where it returns to the atmosphere. The CFC data indicate that travel times of the advecting gas in the shallow Tiva Canyon hydrogeologic unit are ≤5 years. The 14C activities are postbomb to depths of 100 m, indicating little retardation of 14CO2 in the shallow flow systems. The 14C activities from 168 to 404 m in the Topopah Spring hydrogeologic unit are 85–90 pMC at borehole USW-UZ6. The CFC data show that the drilling of USW-UZ6 in 1984 has altered the natural system by providing a conduit through the Paintbrush Nonwelded unit, allowing flow from Topopah Spring outcrops in Solitario Canyon on the west to USW-UZ6, upward in the borehole through the Paintbrush, to the shallow Tiva Canyon flow systems, and out of the mountain.