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Aquatic Geochemistry (v.19, #2)

Hydrogen Cyanide Accumulation and Transformations in Non-polluted Salt Marsh Sediments by A. Kamyshny Jr.; H. Oduro; Z. F. Mansaray; J. Farquhar (pp. 97-113).

While cyanide is known to be produced by many organisms, including plants, bacteria, algae, fungi and some animals, it is generally thought that high levels of cyanide in aquatic systems require anthropogenic sources. Here, we report accumulation of relatively high levels of cyanide in non-polluted salt marsh sediments (up to 230 μmol kg⁻¹). Concentrations of free cyanide up to 1.92 μmol L⁻¹, which are toxic to aquatic life, were detected in the pore-waters. Concentration of total (free and complexed) cyanide in the pore-waters was up to 6.94 μmol L⁻¹. Free cyanide, which is released to the marsh sediments, is attributed to processes associated with decomposition of cord grass, Spartina alterniflora, roots and possibly from other sources. This cyanide is rapidly complexed with iron and adsorbed on sedimentary organic matter. The ultimate cyanide sink is, however, associated with formation of thiocyanate by reaction with products of sulfide oxidation by Fe(III) minerals, especially polysulfides. The formation of thiocyanate by this pathway detoxifies two poisonous compounds, polysulfides and hydrogen cyanide, preventing release of free hydrogen cyanide from salt marsh sediments into overlying water or air.

Keywords: Hydrogen cyanide; Metallo-cyanide complexes; Thiocyanate; Sulfide oxidation intermediates; Inorganic polysulfides; Sulfide

Estimating the Density and Compressibility of Seawater to High Temperatures Using the Pitzer Equations by Carmen Rodriguez; Frank J. Millero (pp. 115-133).

The density and compressibility of seawater solutions from 0 to 95 °C have been examined using the Pitzer equations. The apparent molal volumes (X = V) and compressibilities (X = κ) are in the form $$ X_{phi } = ar{X}^{0} + A_{X} I/(1.2 , m)ln (1 + 1.2 , I^{0.5} ) + , 2{ ext{RT }}m , (eta^{(0)X} + eta^{(1)X} g(y) + C^{X} m) $$ where $$ ar{X}^{0} $$ is the partial molal volume or compressibility, I is the ionic strength, m is the molality of sea salt, A_X is the Debye–Hückel slope for volume (X = V) or adiabatic compressibility (X = κ _s), and g(y) = (2/y ²)[1 − (1 + y) exp(−y)] where y = 2I ^0.5. The values of the partial molal volume and compressibility ( $$ ar{X}^{0} $$ ) and Pitzer parameters (β (0)X, β (1)X and C X) are functions of temperature in the form $$ Y^{X} = sum_{i} a_{i} (T-T_{ ext{R}} )^{i} $$ where a i are adjustable parameters, T is the absolute temperature in Kelvin, and T _R = 298.15 K is the reference temperature. The standard errors of the seawater fits for the specific volumes and adiabatic compressibilities are 5.35E−06 cm³ g⁻¹ and 1.0E−09 bar⁻¹, respectively. These equations can be combined with similar equations for the osmotic coefficient, enthalpy and heat capacity to define the thermodynamic properties of sea salt to high temperatures at one atm. The Pitzer equations for the major components of seawater have been used to estimate the density and compressibility of seawater to 95 °C. The results are in reasonable agreement with the measured values (0.010E−03 g cm⁻³ for density and 0.050E−06 bar⁻¹ for compressibility) from 0 to 80 °C and salinities from 0 to 45 g kg⁻¹. The results make it possible to estimate the density and compressibility of all natural waters of known composition over a wide range of temperature and salinity.

Keywords: Apparent molal volume; Apparent molal compressibility; Density; Seawater; Pitzer equations; Sea salts

Carbon Sequestration and Release from Antarctic Lakes: Lake Vida and West Lake Bonney (McMurdo Dry Valleys) by G. M. Marion; A. E. Murray; B. Wagner; C. H. Fritsen; F. Kenig; P. T. Doran (pp. 135-145).

Perennial ice covers on many Antarctic lakes have resulted in high lake inorganic carbon contents. The objective of this paper was to evaluate and compare the brine and CO₂ chemistries of Lake Vida (Victoria Valley) and West Lake Bonney (Taylor Valley), two lakes of the McMurdo Dry Valleys (East Antarctica), and their potential consequences during global warming. An existing geochemical model (FREZCHEM-15) was used to convert measured molarity into molality needed for the FREZCHEM model, and this model added a new algorithm that converts measured DIC into carbonate alkalinity needed for the FREZCHEM model. While quite extensive geochemical information exists for ice-covered Taylor Valley lakes, such as West Lake Bonney, only limited information exists for the recently sampled brine of >25 m ice-thick Lake Vida. Lake Vida brine had a model-calculated pCO₂ = 0.60 bars at the field pH (6.20); West Lake Bonney had a model-calculated pCO₂ = 5.23 bars at the field pH (5.46). Despite the high degree of atmospheric CO₂ supersaturation in West Lake Bonney, it remains significantly undersaturated with the gas hydrate, CO₂·6H₂O, unless these gas hydrates are deep in the sediment layer or are metastable having formed under colder temperatures or greater pressures. Because of lower temperatures, Lake Vida could start forming CO₂·6H₂O at lower pCO₂ values than West Lake Bonney; but both lakes are significantly undersaturated with the gas hydrate, CO₂·6H₂O. For both lakes, simulation of global warming from current subzero temperatures (−13.4 °C in Lake Vida and −4.7 °C in West Lake Bonney) to 10 °C has shown that a major loss of solution-phase carbon as CO₂ gases and carbonate minerals occurred when the temperatures rose above 0 °C and perennial ice covers would disappear. How important these Antarctic CO₂ sources will be for future global warming remains to be seen. But a recent paper has shown that methane increased in atmospheric concentration due to deglaciation about 10,000 years ago. So, CO₂ release from ice lakes might contribute to atmospheric gases in the future.

Keywords: Gas hydrates; Sediment carbon dioxide; FREZCHEM simulation; Global warming

Contribution of $$ { ext{P}}_{{{ ext{CO}}_{ 2} { ext{eq}}}} $$ and ¹³C_TDIC Evaluation to the Identification of CO₂ Sources in Volcanic Groundwater Systems: Influence of Hydrometeorological Conditions and Lava Flow Morphologies—Application to the Argnat Basin (Chaîne des Puys, Massif Central, France) by Guillaume Bertrand; Hélène Celle-Jeanton; Sébastien Loock; Frédéric Huneau; Véronique Lavastre (pp. 147-171).

Mineralization of groundwater in volcanic aquifers is partly acquired through silicates weathering. This alteration depends on the dissolution of atmospheric, biogenic, or mantellic gaseous CO₂ whose contributions may depend on substratum geology, surface features, and lava flow hydrological functionings. Investigations of $$ { ext{P}}_{{{ ext{CO}}_{ 2} { ext{eq}}}} $$ and δ¹³C_TDIC (total dissolved inorganic carbon) on various spatiotemporal scales in the unsaturated and saturated zones of volcanic flows of the Argnat basin (French Massif Central) have been carried out to identify the carbon sources in the system. Mantellic sources are related to faults promoting CO₂ uplift from the mantle to the saturated zone. The contribution of this source is counterbalanced by infiltration of water through the unsaturated zone, accompanied by dissolution of soil CO₂ or even atmospheric CO₂ during cold periods. Monitoring and modeling of δ¹³C_TDIC in the unsaturated zone shows that both $$ { ext{P}}_{{{ ext{CO}}_{ 2} { ext{eq}}}} $$ and δ¹³C_TDIC are controlled by air temperature which influences soil respiration and soil-atmosphere CO₂ exchanges. The internal geometry of volcanic lava flows controls water patterns from the unsaturated zone to saturated zone and thus may explain δ¹³C heterogeneity in the saturated zone at the basin scale.

Keywords: Volcano; Unsaturated zone; Groundwater; $$ { ext{P}}_{{{ ext{CO}}_{ 2} }} $$ ; Carbon-13

Relationships Between Geochemical Parameters (pH, DOC, SPM, EDTA Concentrations) and Trace Metal (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn) Concentrations in River Waters of Texas (USA) by Kuo-Tung Jiann; Peter H. Santschi; Bobby J. Presley (pp. 173-193).

Water samples from eight major Texas rivers were collected at different times during 1997–1998 to determine the dissolved and particulate trace metal concentrations, expected to show differences in climate patterns, river discharge and other hydrochemical conditions, and human activities along the different rivers. Specifically, two eastern Texas rivers (Sabine, Neches) lie in a region with high vegetation, flat topography, and high rainfall rates, while four Central Texas rivers (Trinity, Brazos, Colorado, and San Antonio) flow through large population centers. Relatively high dissolved organic carbon (DOC) concentrations in the eastern Texas rivers and lower pH led to higher Fe and Mn concentrations in river waters. The rivers that flow through large population centers showed elevated trace metal (e.g., Cd, Pb, Zn) concentrations partly due to anthropogenically produced organic ligands such as ethylenediaminetetraacetic acid (EDTA) present in these rivers. Trace metal levels were reduced below dams/reservoirs along several Texas rivers. Statistical analysis revealed four major factors (suspended particulate matter [SPM], EDTA, pH, and DOC) that can explain most of the observed variability of trace metal concentrations in these rivers. SPM concentrations directly controlled particulate metal contents. Variation in pH correlated with changes of dissolved Co, Fe, Mn, and Ni, and particulate Mn concentrations, while DOC concentrations were significantly related to dissolved Fe concentrations. Most importantly, it was found that, more than pH, EDTA concentrations exerted a major control on dissolved concentrations of Cd and Zn, and, to a lesser extent, Cu, Ni, and Pb.

Keywords: Trace metals; EDTA; Rivers; Watersheds; Texas

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Aquatic Geochemistry (v.19, #2)

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Aquatic Geochemistry (v.19, #2)