| Summary: | The controls of water and O2 availability, microbial activity and temperature on acid (H2SO4) production rates in commercial-scale sulphur (S0) blocks were quantified and recommendations were made for minimizing H2SO4 production in S0 blocks. Acidic drainage from the S0 blocks (pH 0.4-1.0) was attributed to mixing of fresh infiltrating water and low-pH resident water (mean pH=-2.1) with resident water comprising ~4-8% of the drainage. Although clean S0 is strongly hydrophobic, preferential water infiltration occurred rapidly through fractured S0 blocks in which the bulk hydraulic conductivity was estimated to be similar to gravel or clean sand (Ks=1x10-1 to 1x10-3 m/s). Microbial colonization of fracture faces generated localized hydrophilic conditions that helped create preferential pathways for water infiltration. Liquid water contact (compared to water vapour) was essential for S0 oxidation (i.e., H2SO4 production), therefore H2SO4 production in the S0 blocks was limited to fractures and friable S0 through which water flowed. H2SO4 production was greatest in the upper 1 m of the S0 block (70 to >97% of annual H2SO4 production) and the result of autotrophic microbial S0 oxidation.<p>
S0 oxidation rates were very sensitive to temperature and increased by a factor of 4.3 for a temperature increase of 10°C (Q10). Therefore minimizing temperature (<5°C) in S0 blocks would be an effective strategy for controlling H2SO4 production. Heat released during S0 oxidation did have a measurable effect on in situ temperatures and should be considered in the design of insulated cover systems. Although autotrophic microbial activity was insensitive to O2 concentrations when they were >1 vol.%, the total mass production rate of H2SO4 is approximately proportional to the O2 concentration at the surface of the S0 block (assuming in situ O2 concentrations decrease to <1 vol. %). Therefore, cover systems that minimize the surficial O2 concentration are recommended.<p>
Cover systems limiting H2O infiltration would be effective for minimizing the volume of acidic drainage, but may have no impact on H2SO4 production rates within the block. In this study, H2O infiltration through a typical soil cover (~95% efficiency) would easily satisfy the annual H2O demand for H2SO4 production (2.6 mm/m2 in the upper 1 m). Greater near-surface H2SO4 production rates may appear to make surficial biocide application an attractive option for minimizing S0-oxidizing microbial activity, however, this approach might simply drive the zone of H2SO4 production to greater depths and have no affect on the total mass production of H2SO4.
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