Sulfate concentration changes with depth

The observed sulfate reduction rates in freshwater sediments cannot be explained by diffusion of sulfate from the lake water into the sediment, because much steeper sulfate concentration gradients should then be observed. Assuming diffusive supply alone, Urban (28) calculated that the change of sulfate concentration with depth should take place within 1 mm instead of several centimeters, which are usually measured. This assumption, however, also means that the sulfate recycling process and the sulfate reduction rate should not be limited by the vertical transport of sulfide to (frequently solid) oxidants or to the oxic boundary layer. 

Figure 8. Bubble diagram of concentrations (milligrams per liter) of chemical components in well 138 (Figure 3) showing changes with time in wells of different depths the upper scale bar (bubble diameter) is the concentration of the species as depicted by bubble, the vertical axis is the elevation of the screened interval of the well, and the horizontal axis is the days since drawdown A, sulfate and B, arsenic.

Figure 7.4 Generalized profiles of concentration and isotope ratio changes for dissolved sulfate and carbon species in anoxic marine sediments. Depth scale is arbitrary with depth units ranging from 10 1 to 102 m. (Reproduced from Claypool, G.E., Kvenvolden, K.A., Ann. Rev. Earth Planet Sci., 11, 299 (1983). With permission from Annual Reviews, Inc.)...

Other processes which may explain changes in DMS concentration with depth, on a short time scale, are those related to bacteria. Studies conducted in sediments (31-39.62) have shown that DMS can be consumed by microorganisms. Based on sulfur requirements of microorganisms, the availability of sulfur from sulfate in these environments far exceeds that of DMS. However, as a carbon source, the concentration of DMS is similar to other compounds of low molecular weight and may be cycled by microorganisms to serve as both a carbon ana sulfur source. Although the utilization of DMS aerobically has been reported (63.64). the extent to which similar processes exist in oceanic environments is not known. For futher discussions of the microbial processes related to DMS the reader is referred to another Chapter of this book (65). 

Bacterial sulfate reduction appeeirs to proceed to considerable depths in marine sediments but rates computed from changes in interstitial water sulfate concentrations, with suitable corrections for diffusion and sedimentation, are generally orders of magnitude below those in surface muds (Goldhaber and Kaplan, 1975). Again, this probably reflects a depletion of utilizable organic matter in the deeper layers by microbial utilization and conversion to more intractable humates and kerogens. 

Anoxic water samples, because they contain little in the way of particles, are far easier than aquifer materials to develop radioassays for the measurement of arsenate reduction. Arsenic speciation quantitatively changes from arsenate to arsenite with vertical transition from the surface oxic waters to the anoxic bottom depths of stratified lakes and fjords (55,56). This also occurs in Mono Lake, California (57), a transiently meromictic, alkaline (pH = 9.8), and hypersaline (salinity = 70-90 g/L) soda lake located in eastern California (Fig. 11). The combined effects of hydrothermal sources coupled with evaporative concentration have resulted in exceptionally high ( 200 fiM) dissolved arsenate concentrations in its surface waters. Haloalkaliphilic arsenate-respiring bacteria have been isolated from the lake sediments (26), and sulfate reduction, achieved with... 

SQM, as Eoote, initially selected a brine extraction location for its well field where the brine had the maximum potassium and the least sulfate for potash and lithium production, and later a location with the maximum sulfate content for potassium sulfate production (Fig. 1.57). Because of this the plants could initially use the simplest processes and have the lowest capital and operating costs. In the initial operation brine with up to 3400 ppm Li was pumped from the Salar in 40 wells, 28 m deep on a 200-500 m grid, which delivered up to 5280 m /hr of brine to the solar ponds. There were also 13 monitoring wells to follow any changes in the brine concentration and its depth from the surface. The ponds were lined with flexible PVC or reinforced hypalon membranes, and the brine flowed through the sections of the pond system in series. The initial salt ponds had an area of 1.16 million m followed by 3.36 million m for the sylvinite ponds, and later 1 million m of ponds were installed for lithium production. The plant employed 184 people, of which 120 were hired from the sparsely populated local area. Contractors were used to drill and maintain the weUs, harvest the salts, transport them to their respective stockpiles, and reclaim the sylvinite to feed the potash plant s conveyor belt. They also provided all of the miscellaneous trucking needed at the Salar, and transported the potash to Coya Sur or Maria Elena and the concentrated lithium chloride brine to the Salar de Carmen. SQM unloaded the brine and potash, and stacked the later material at its nitrate plants (Harben and Edwards, 1997). 

Analyses of trace metal and sulfate in pore water provide evidence of diagenetic change in salt-marsh sediment. Rapidly processed cores from the Indian Neck and Farm River sites showed normalized SO4/CI ratios of greater than 1 at certain depths (Table VI). The cores also contained measurable concentrations of dissolved Mn and, at the Farm River site, Fe. The high Mn concentrations seen in Figs. 9 and 10 coincide with the maximum SO4/CI ratio. Other metals were not detected, with the possible exception of trace amounts of Zn in one Indian Neck core. 

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