I am trying to understand what mechanism(s) account for the fact that the [SO42-]:[Cl-] ratio in water can be indicative of the level of activity of sulfate-reducing bacteria in aquatic ecosystems.
What type of aquatic system are you working with (e.g., seawater, saltmarsh, river, or lake)? Do you know that the initial [SO4]:[Cl] was (or should be)? The key with sulfate reduction is that the [SO4]:[Cl] should decline as SO4 is reduced to HS- or H2S, depending on pH. Also, since sulfate is reduced in the presence of organic matter, alkalinity tends to increase during sulfate reduction, so the [HCO3]:[Cl] should increase. So, for example, if you are working in a stratified lake, sulfate concentrations should decline with depth, with an increased rate of decline at the chemocline. Alkalinity concentrations should increase at the same depth sulfate decreases. If you have the ability, check other solutes such as nitrate, ammonium, and dissolved oxygen. Nitrate declines, ammonium may increase, and dissolved oxygen should decrease as the redox boundaries are crossed. An excellent resource for this reaction, and other bacterially-mediated redox reactions, would be Stumm and Morgan "Aquatic Chemistry." Chameides and Perdue "Biogeochemical Cycles" also has good summaries of the reaction chemistry and bacteria involved.
Thanks very much for this. I am working with lake data (some of the lakes are naturally acidic, pH ~5 - 6), and unfortunately don't have temporal data so can't compare to an initial [SO4]:[Cl] ratio.
Meredith, my guess would be that any sulfate reduction would occur at depth when the lake is stratified. So, I don't think you really need temporal data, rather collect samples along a depth profile. If you have a DO meter with a long cord, you can check changes in dissolved oxygen concentrations to figure out where to sample more intensely to capture the change. I would hypothosize that surface water would have a higher SO4:Cl ratio than waters at depth, particularly during the summer when the lake should be most stratified. Of course, some this depends on the amount of organic productivity in the lake.
Since sulphate is the fuel of the reaction it is not surprising that the more fuel the more reaction? Sulphate reduction is depending on organic carbon, did you also check for that? And reduced S can escape as gas at low pH, but stay in solution at high.
Hi Meredith, I relate to C. Brannon Andersen's answer. Having some understanding of the [Cl] would be very useful. If there is data from the lake from other studies you could compile an average or median [Cl]. Here in New Zealand we see considerable variation in freshwater Cl concentrations depending on the distance from the coast. However, under stable conditions the Cl concentration is usually very consistent for a given lake. With respects to redox assignments I use a field portable colorimeter to measure Total Sulfide (TS), a handheld meter for D.O., Eh, SpC, pH as well as taking samples for the main terminal electron accepting species (TEAS: NO3, Mn, Fe, SO4). If you were interested the redox assignment workbook of Jurgen et al. (USGS) is useful. Measuring total sulfide helps to discriminate as to whether FeIII or SO4 reduction is the dominant terminal electron accepting process. Kind regards, Clint
What information do I have if I know the Sulfate/chloride?
Which is the meaning of this operation?
Why should I need the Sulfate/chloride value?
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