Unprecedented 34 S-enrichment of pyrite formed following microbial sulfate reduction in fractured crystalline rocks.

In the deep biosphere, microbial sulfate reduction (MSR) is exploited for energy. Here, we show that, in fractured continental crystalline bedrock in three areas in Sweden, this process produced sulfide that reacted with iron to form pyrite extremely enriched in 34 S relative to 32 S. As documented by secondary ion mass spectrometry (SIMS) microanalyses, the δ34 Spyrite values are up to +132‰V-CDT and with a total range of 186‰. The lightest δ34 Spyrite values (-54‰) suggest very large fractionation during MSR from an initial sulfate with δ34 S values (δ34 Ssulfate,0 ) of +14 to +28‰. Fractionation of this magnitude requires a slow MSR rate, a feature we attribute to nutrient and electron donor shortage as well as initial sulfate abundance. The superheavy δ34 Spyrite values were produced by Rayleigh fractionation effects in a diminishing sulfate pool. Large volumes of pyrite with superheavy values (+120 ± 15‰) within single fracture intercepts in the boreholes, associated heavy average values up to +75‰ and heavy minimum δ34 Spyrite values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture-specific δ34 Spyrite variability and overall average δ34 Spyrite values (+11 to +16‰) lower than the anticipated δ34 Ssulfate,0 support this hypothesis. The superheavy pyrite found locally in the borehole intercepts thus represents a late stage in a much larger fracture system undergoing Rayleigh fractionation. Microscale Rb-Sr dating and U/Th-He dating of cogenetic minerals reveal that most pyrite formed in the early Paleozoic era, but crystal overgrowths may be significantly younger. The δ13 C values in cogenetic calcite suggest that the superheavy δ34 Spyrite values are related to organotrophic MSR, in contrast to findings from marine sediments where superheavy pyrite has been proposed to be linked to anaerobic oxidation of methane. The findings provide new insights into MSR-related S-isotope systematics, particularly regarding formation of large fractions of 34 S-rich pyrite.

An assessment of health risks associated with arsenic exposure via consumption of homegrown vegetables near contaminated glassworks sites.

The health risk posed by arsenic in vegetables grown in private gardens near 22 contaminated glassworks sites was investigated in this study. Firstly, vegetable (lettuce and potato) and soil samples were collected and arsenic concentrations measured to characterize the arsenic uptake in the selected crops. Secondly, a probabilistic exposure assessment was conducted to estimate the average daily intake (ADIveg), which was then evaluated against toxicological reference values by the calculation of hazard quotients (HQs) and cancer risks (CRs). The results show that elevated arsenic concentrations in residential garden soils are mirrored by elevated concentrations in vegetables, and that consumption of these vegetables alone may result in an unacceptable cancer risk; the calculated reasonable maximum exposure, for example, corresponded to a cancer incidence 20 times higher than the stated tolerance limit. However, the characterization of risk depends to a great extent on which toxicological reference value is used for comparison, as well as how the exposure is determined. Based on the assumptions made in the present study, the threshold levels for chronic non-carcinogenic or acute effects were not exceeded, but the cancer risks indicated highlight the need for further exposure studies, as dietary intake involves more than just homegrown vegetables and total exposure is a function of more than just one exposure pathway. In addition, glassworks sites--and contaminated sites in general--contain multiple contaminants, affecting the final and total risk.

Metal uptake by homegrown vegetables - the relative importance in human health risk assessments at contaminated sites.

Risk assessments of contaminated land often involve the use of generic bioconcentration factors (BCFs), which express contaminant concentrations in edible plant parts as a function of the concentration in soil, in order to assess the risks associated with consumption of homegrown vegetables. This study aimed to quantify variability in BCFs and evaluate the implications of this variability for human exposure assessments, focusing on cadmium (Cd) and lead (Pb) in lettuce and potatoes sampled around 22 contaminated glassworks sites. In addition, risks associated with measured Cd and Pb concentrations in soil and vegetable samples were characterized and a probabilistic exposure assessment was conducted to estimate the likelihood of local residents exceeding tolerable daily intakes. The results show that concentrations in vegetables were only moderately elevated despite high concentrations in soil, and most samples complied with applicable foodstuff legislation. Still, the daily intake of Cd (but not Pb) was assessed to exceed toxicological thresholds for about a fifth of the study population. Bioconcentration factors were found to vary more than indicated by previous studies, but decreasing BCFs with increasing metal concentrations in the soil can explain why the calculated exposure is only moderately affected by the choice of BCF value when generic soil guideline values are exceeded and the risk may be unacceptable.