David S. Vinson

Elevated mercury concentrations in soils, sediments, water, and fish of the Madeira River basin, Brazilian Amazon: a function of natural enrichments?

Previous site-specific investigations have found that mercury concentrations in water, sediments, and biota of the Brazilian Amazon are elevated above global averages, and that these concentrations are a direct result of widespread mercury amalgamation mining operations conducted by non-organized prospectors. In order to assess the regional impacts of Hg contamination from these non-organized gold mining activities, water, sediments, and fish were systematically collected in 1997 along a 900-km reach of the Madeira River. The sampling program extended from the Amazon River upstream to Porto Velho, the site of historic and ongoing mercury amalgamation mining. Mercury concentrations were found to be elevated above global averages in all sampled media. However, the geochemical data suggest that the high mercury levels are due largely to natural sources and natural biogeochemical processes, and that the impacts of anthropogenically released mercury from mine sites is relatively localized.

Biogeochemistry at the zone of intermittent saturation: Field-based study of the shallow alluvial aquifer, Rio Grande, New Mexico

The Rio Grande in central New Mexico (USA) flows through a semiarid alluvial valley; the river is regulated by levees, riverside drain ditches, irrigation structures, and upstream dams. As a large river in a semiarid region, the Rio Grande experiences large variability in flows and solute concentrations due to riparian evapotranspiration, aquifer recharge, and upstream contributions. In order to characterize biogeochemical processes in this setting, surface water and groundwater from the shallow alluvial aquifer between the river and a parallel drain ditch were sampled from 1 to 13 m depth, including high-resolution multilevel sampling near the fluctuating water table, at a representative site on the middle Rio Grande. The zone of intermittent saturation is a region, ∼50 cm in vertical extent at this site, in which the water table shifts in response to changes in river level and riparian evapotranspiration. Sediment extractions of iron and manganese oxides indicate that these solids are more prevalent in the zone of intermittent saturation, where oxic-anoxic cycling occurs. River water chemistry varies with time, strongly influencing influent waters to the alluvial aquifer. This chemistry evolves significantly in the ∼100 m from the river to four wells. River concentrations of dissolved oxygen (mean 6.9 mg L −1 ) and nitrate (mean 2.8 mg L −1 ) are reduced to near zero in the shallower wells (1–3 m depth), but are less reduced from the river in the deeper wells (5–13 m depth). Mn and Fe increase from near zero in the river to maximum concentrations from 1 to 3 m depth (mean 1.0 mg L −1 and 2.1 mg L −1 , respectively), with lesser increases at 5 and 13 m depth. Sulfate concentration decreases relative to chloride in many samples, especially from 1 to 3 m depth, most likely due to sulfate reduction. Overall, patterns in water and sediment chemistry indicate that waters in the region of the shifting water table are more evolved from the river via terminal electron-accepting processes than deeper waters, including aerobic respiration, denitrification, manganese oxide reduction, iron oxide reduction, and sulfate reduction. This implies that a greater extent of organic carbon metabolism occurs at shallower depths. The influence of sulfate reduction on organic carbon oxidation is facilitated by sulfate concentrations of river water that varied from 31 to 83 mg L −1 during the study. These results illustrate that sulfate reduction may constitute a significant portion of organic carbon metabolism in higher-sulfate shallow alluvial aquifers associated with freshwater rivers. Biogeochemical processes in the shallow alluvial aquifer depend on the river for solute inputs, and may in turn influence large-scale river chemistry.

Solute Concentrations Influence Microbial Methanogenesis in Coal-bearing Strata of the Cherokee Basin, USA.

Microorganisms have contributed significantly to subsurface energy resources by converting organic matter in hydrocarbon reservoirs into methane, the main component of natural gas. In this study, we consider environmental controls on microbial populations in coal-bearing strata of the Cherokee basin, an unconventional natural gas resource in southeast Kansas, USA. Pennsylvanian-age strata in the basin contain numerous thin (0.4–1.1 m) coalbeds with marginal thermal maturities (0.5–0.7% Ro) that are interbedded with shale and sandstone. We collected gas, water, and microbe samples from 16 commercial coalbed methane wells for geochemical and microbiological analysis. The water samples were Na–Cl type with total dissolved solids (TDS) content ranging from 34.9 to 91.3 g L−1. Gas dryness values [C1/(C2 + C3)] averaged 2640 and carbon and hydrogen isotope ratios of methane differed from those of carbon dioxide and water, respectively, by an average of 65 and 183‰. These values are thought to be consistent with gas that formed primarily by hydrogenotrophic methanogenesis. Results from cultivation assays and taxonomic analysis of 16S rRNA genes agree with the geochemical results. Cultivable methanogens were present in every sample tested, methanogen sequences dominate the archaeal community in each sample (avg 91%), and few archaeal sequences (avg 4.2%) were classified within Methanosarcinales, an order of methanogens known to contain methylotrophic methanogens. Although hydrogenotrophs appear dominant, geochemical and microbial analyses both indicate that the proportion of methane generated by acetoclastic methanogens increases with the solute content of formation water, a trend that is contrary to existing conceptual models. Consistent with this trend, beta diversity analyses show that archaeal diversity significantly correlates with formation water solute content. In contrast, bacterial diversity more strongly correlates with location than solute content, possibly as a result of spatial variation in the thermal maturity of the coalbeds.

Response to Comment on "High naturally occurring radioactivity in fossil groundwater from the Middle East".

High levels of naturally occurring and carcinogenic radium isotopes have been measured in low-saline and oxic groundwater from the Rum Group of the Disi sandstone aquifer in Jordan. The combined 228Ra and 226Ra activities are up to 2000% higher than international drinking water standards. Analyses of the host sandstone aquifer rocks show 228Ra and 226Ra activities and ratios that are consistent with previous reports of sandstone rocks from different parts of the world. A compilation of previous data in groundwater from worldwide sandstone aquifers shows large variations in Ra activities regardless of the groundwater salinity. On the basis of the distribution of the four Ra isotopes and the ratios of the short- to long-lived Ra isotopes, we postulate that Ra activity in groundwater is controlled by the balance of radioactive decay of parent Th isotopes on aquifer solids, decay of the dissolved radium isotopes, and adsorption of dissolved Ra on solid surfaces. The availability of surface adsorption sites, which depends on the clay content in the aquifer rocks, is therefore an important constraint for Ra activity in sandstone aquifers. These findings raise concerns about the safety of this and similar nonrenewable groundwater reservoirs, exacerbating the already severe water crisis in the Middle East.