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Aquatic Geochemistry (v.17, #6)

Arsenic and Antimony in Groundwater Flow Systems: A Comparative Study by Stephanie S. Willis; Shama E. Haque; Karen H. Johannesson (pp. 775-807).

Arsenic (As) and antimony (Sb) concentrations and speciation were determined along flow paths in three groundwater flow systems, the Carrizo Sand aquifer in southeastern Texas, the Upper Floridan aquifer in south-central Florida, and the Aquia aquifer of coastal Maryland, and subsequently compared and contrasted. Previously reported hydrogeochemical parameters for all three aquifer were used to demonstrate how changes in oxidation–reduction conditions and solution chemistry along the flow paths in each of the aquifers affected the concentrations of As and Sb. Total Sb concentrations (Sb_T) of groundwaters from the Carrizo Sand aquifer range from 16 to 198 pmol kg⁻¹; in the Upper Floridan aquifer, Sb_T concentrations range from 8.1 to 1,462 pmol kg⁻¹; and for the Aquia aquifer, Sb_T concentrations range between 23 and 512 pmol kg⁻¹. In each aquifer, As and Sb (except for the Carrizo Sand aquifer) concentrations are highest in the regions where Fe(III) reduction predominates and lower where SO₄ reduction buffers redox conditions. Groundwater data and sequential analysis of the aquifer sediments indicate that reductive dissolution of Fe(III) oxides/oxyhydroxides and subsequent release of sorbed As and Sb are the principal mechanism by which these metalloids are mobilized. Increases in pH along the flow path in the Carrizo Sand and Aquia aquifer also likely promote desorption of As and Sb from mineral surfaces, whereas pyrite oxidation mobilizes As and Sb within oxic groundwaters from the recharge zone of the Upper Floridan aquifer. Both metalloids are subsequently removed from solution by readsorption and/or coprecipitation onto Fe(III) oxides/oxyhydroxides and mixed Fe(II)/Fe(III) oxides, clay minerals, and pyrite. Speciation modeling using measured and computed Eh values predicts that Sb(III) predominate in Carrizo Sand and Upper Floridan aquifer groundwaters, occurring as the Sb(OH) ₃ ⁰ species in solution. In oxic groundwaters from the recharge zones of these aquifers, the speciation model suggests that Sb(V) occurs as the negatively charged Sb(OH) ₆ ⁻ species, whereas in sufidic groundwaters from both aquifers, the thioantimonite species, HSb₂S₄ ⁻ and Sb₂S₄ ²⁻, are predicted to be important dissolved forms of Sb. The measured As and Sb speciation in the Aquia aquifer indicates that As(III) and Sb(III) predominate. Comparison of the speciation model results based on measured Eh values, and those computed with the Fe(II)/Fe(III), S(-II)/SO₄, As(III)/As(V), and Sb(III)/Sb(V) couples, to the analytically determined As and Sb speciation suggests that the Fe(II)/Fe(III), S(-II)/SO₄ couples exert more control on the in situ redox condition of these groundwaters than either metalloid redox couple.

Keywords: Arsenic; Antimony; Groundwater; Carrizo Sand; Upper Floridan; Aquia

An Equation of State for Hypersaline Water in Great Salt Lake, Utah, USA by David L. Naftz; Frank J. Millero; Blair F. Jones; W. Reed Green (pp. 809-820).

Great Salt Lake (GSL) is one of the largest and most saline lakes in the world. In order to accurately model limnological processes in GSL, hydrodynamic calculations require the precise estimation of water density (ρ) under a variety of environmental conditions. An equation of state was developed with water samples collected from GSL to estimate density as a function of salinity and water temperature. The ρ of water samples from the south arm of GSL was measured as a function of temperature ranging from 278 to 323 degrees Kelvin (^oK) and conductivity salinities ranging from 23 to 182 g L⁻¹ using an Anton Paar density meter. These results have been used to develop the following equation of state for GSL (σ = ± 0.32 kg m⁻³): $$ ho - ho^{0} = { 184}.0 10 6 2 { } + { 1}.0 4 70 8*{ ext{S}} - 1. 2 10 6 1*{ ext{T }} + { 3}. 1 4 7 2 1 { ext{E}} - 4*{ ext{S}}^{ 2} + , 0.00 1 9 9 { ext{T}}^{ 2} - 0.00 1 1 2*{ ext{S}}*{ ext{T}}, $$ where ρ ⁰ is the density of pure water in kg m⁻³, S is conductivity salinity g L⁻¹, and T is water temperature in degrees Kelvin.

Keywords: Density; Conductivity; Great Salt Lake; Salinity; Terminal lake

Determination of Total Free Sulphides in Sediment Porewater and Artefacts Related to the Mobility of Mineral Sulphides by Kristina A. Brown; Eric R. McGreer; Bernie Taekema; Jay T. Cullen (pp. 821-839).

A field and laboratory study of the accuracy of a method commonly used to determine free sulphide concentrations in the porewater of marine sediments is presented. The method uses an ion-selective electrode (ISE), sensitive to the sulphide ion (S²⁻), in sediments buffered to high pH (>12) and is commonly used in regulatory monitoring programs to assess the impacts of open net-pen finfish aquaculture on local marine habitats. Here we report that on the timescale of field measurements, the accepted protocol can lead to significant bias of free sulphide measurements, with orders of magnitude higher concentration detected in the buffered sediment–porewater slurry than in porewater samples isolated and analysed separately. Laboratory experiments with model marine sediments and analysis of sediment composition indicate that this bias is likely introduced by the dissolution of particulate sulphides and/or sulphur present in the sediments under the intense alkaline conditions of the protocol. Recommendations for the modification and continued use of this commonly applied field methodology are discussed.

Keywords: Free sulphide; Aquaculture; Environmental monitoring; Method; Toxicity

Diel Aquatic CO₂ System Dynamics of a Bermudian Mangrove Environment by John A. Zablocki; Andreas J. Andersson; Nicholas R. Bates (pp. 841-859).

Mangrove ecosystems play an important, but understudied, role in the cycling of carbon in tropical and subtropical coastal ocean environments. In the present study, we examined the diel dynamics of seawater carbon dioxide (CO₂) and dissolved oxygen (DO) for a mangrove-dominated marine ecosystem (Mangrove Bay) and an adjacent intracoastal waterway (Ferry Reach) on the island of Bermuda. Spatial and temporal trends in seawater carbonate chemistry and associated variables were assessed from direct measurements of dissolved inorganic carbon, total alkalinity, dissolved oxygen (DO), temperature, and salinity. Diel pCO₂ variability was interpolated across hourly wind speed measurements to determine variability in daily CO₂ fluxes for the month of October 2007 in Bermuda. From these observations, we estimated rates of net sea to air CO₂ exchange for these two coastal ecosystems at 59.8 ± 17.3 in Mangrove Bay and 5.5 ± 1.3 mmol m⁻² d⁻¹ in Ferry Reach. These results highlight the potential for large differences in carbonate system functioning and sea-air CO₂ flux in adjacent coastal environments. In addition, observation of large diel variability in CO₂ system parameters (e.g., mean pCO₂: 390–2,841 μatm; mean pH_T: 8.05–7.34) underscores the need for careful consideration of diel cycles in long-term sampling regimes and flux estimates.

Keywords: Mangrove; CO₂ ; Diel; Gas flux; Coastal ocean

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Aquatic Geochemistry (v.17, #6)

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Aquatic Geochemistry (v.17, #6)