Groundwater sulfate reduction

Fig. 33.3. Steady-state distribution of microbial activity and groundwater composition in an aquifer hosting acetotrophic sulfate reduction and acetoclastic methanogenesis, obtained as the long-term solution to a reactive transport model.

Kirk, M.F., T.R. Holm, J. Park, Q. Jin, R. A. Sanford, B.W. Fouke and C. M. Bethke, 2004, Bacterial sulfate reduction limits natural arsenic contamination of groundwater. Geology 32,953-956. 

You work in a research laboratory and your job is to investigate the microbial degradation of organic pollutants in laboratory aquifer column systems. You supply a column continuously with a synthetic groundwater containing 0.3 mM 02, 0.5 mM NOj, 0.5 mM SO -, and 1 mM HCOj, as well as 0.1 mM benzoic acid butyl ester, which is easily mineralized to C02 and H20. The temperature is 20°C and the pH is 7.3 (well buffered). Would you expect sulfate reduction or even methanogenesis to occur in this column Establish an electron balance to answer this question. 

Microbial mat formation may also stimulate metal removal through sulfate reduction. Barnes, Scheeren Buisman (1994) have developed a process that specifically uses sulfate-reducing bacteria to treat metal-contaminated groundwater. In this process, as groundwater is pumped through the water treatment plant, sulfide produced by sulfate-reducing bacteria precipitates the metals in the water. Metal concentrations in the treated water were reportedly reduced to fig/l quantities and the water was suitable for release into the environment. 

Several whole-lake ion budgets have shown that internal alkalinity generation (IAG) is important in regulating the alkalinity of groundwater recharge lakes and that sulfate retention processes are the dominant source of IAG (3-5)1 and synoptic studies (6-9) have shown that sulfate reduction occurs in sediments from a wide variety of softwater lakes. Baker et al. (10) showed that net sulfate retention in lakes can be modeled as a first-order process with respect to sulfate concentration and several "whole ecosystem" models of lake acidification recently have been modified to include in-lake processes (11). 

For example, and 5 0 of SO was used to assess mixing between a vertically stacked aquifer system in contact with a salt dome located in northern Germany (Berner era/., 2002). Two major sulfate pools were identified based upon their isotopic compositions (i) SO from the dissolution of evaporite minerals, and (ii) SO derived from atmospheric deposition and from the oxidation of pyrite. Using both the and of the sol , zones of variable groundwater mixing and significant amounts of bacterial sulfate reduction were identified. The SOl derived from the dissolution of the rock salt in the highly saline deep brine showed nearly constant values between +9.6%c and 11.9%o and between +9.5%c and 12.1%o, consistent with the Permian evaporite deposits. Sulfate in near-surface... 

The chief utility of this hypothesis testing is that it indicates that microbial processes occurring in the aquifer (sulfate reduction) as well as microbial processes in confining beds (organic matter fermentation) have an important impact on the geochemistry of groundwater in adjacent aquifers. 

Sulfate reduction permeable reactive barriers to treat acidity, cadmium, copper, nickel, and zinc two case studies. In Handbook of Groundwater Remediation Using Permeable Reactive Barriers—Applications to Radionuclides, Trace Metals, and Nutrients (eds. D. L. Naftz, S. J. Morrison, J. A. Davis, and C. C. Puller). Academic Press, San Diego, CA, pp. 495-522. 

Finally, seawater intrusion is also characterized by high 6 Ss ifa,e (>20%o) and S B (>39%o) values that are associated with low SO4/CI and B/Cl ratios below the marine ratios (0.05 and 8 X 10 , respectively). The relative depletion of sulfate and S enrichment is attributed to sulfate reduction along the salt-freshwater interface (Krouse and Mayer, 2000). The removal of dissolved boron is explained via adsorption onto clay minerals in which " B is adsorbed preferentially onto the clays and the residual saline groundwater becomes enriched in "B (Vengosh et al., 1994). 

Figure 8 Depth profile (meters below land surface) of the stratigraphy, sulfate reduction rates, sulfate concentration, iron sulfide content of sediments, and dissolved organic carbon in groundwater obtained from an...

Sulfate reduction to sulfide may occur over a significant depth range in groundwater. Sulfide formation may be small scale as in vugs or sulfide species may enter streams to precipitate in a quiet bay or lagoon. An example of the latter appears to be the copper sulfides in.Pematty lagoon in South Australia (Lambert et al., 1971 Donnelly et al., 1972). Wallhauser and Puchelt (1970) identified D. desulfuricans in 42 samples of connate water from the Carboniferous of the Ruhr Basin. 

Bacterial sulfate reduction and sulfide oxidation in groundwaters of sulfur deposits (After Ivanov, 1964)... 

The concentrations of sulfate were generally lowest in the south-western and southern parts of Bangladesh as well as in the Sylhet region in the northeast. Low concentrations of sulfate can be found under strongly reducing conditions as a result of sulfate reduction. The deep groundwaters from the southern coastal region had mostly low sulfate concentrations (<4 mg L ). 

In the Floridan aquifer, traces of gypsum are present in the carbonate rock. The high calcium concentration from gypsum dissolution exceeds its value at saturation with calcite, leading to precipitation of the carbonate and the production of additional CO2 (reaction 8). Concurrently, anaerobic decay of buried organic matter (reaction 3) and sulfate reduction (reaction 6) take place. The combination of these processes has caused an increase in the CO2 pressure of the groundwater from 10" bar in the recharge zone to 10" bar downdip as the pH decreases from 8.0 to 7.4 over a map distance of 115 km (Fig. 5.3) (cf. Back and Hanshaw 1970 Plummer et al. 1983). 

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