H24C-01 16:00h
Geochemical and Microbiological Controls on Arsenic Cycling in Geothermal Springs of Yellowstone National Park
Many geothermal springs in Yellowstone National Park (YNP) contain high concentrations of arsenic ranging from 1 to as high as 12 mg L-1, and discharge into neighboring streams and rivers resulting in a significant flux of As to regional watersheds such as the Madison River (southwestern Montana). In many cases, the predominant valence of As in geothermal source waters is As(III), but oxidation reactions occurring down gradient of geothermal discharge can result in rapid conversion of As(III) to As(V), with important ramifications for the composition of biomineralized solid phases and the fate of As in these systems. Furthermore, changes in source water chemical composition such as pH and the concentration of dissolved sulfide have a pronounced affect on rates of As(III) oxidation and the microbial populations which mediate As transformation. Several geothermal springs with variable source water composition have been sampled and characterized with respect to aqueous and solid phase geochemistry, and 16S rDNA sequence distribution within the outflow channels. Geochemical attributes of thermal springs are an important determinant controlling the distribution of microorganisms; simultaneously, microbial processes play an important role in dictating aqueous and solid phase chemical speciation within geothermal systems. Functional relationships between As redox cycling and specific microbial populations are being established through both molecular and cultivation approaches.
H24C-02 INVITED 16:15h
Microbial Transformation of Arsenic
Whether the source is natural or anthropogenic, it has become evident that arsenic is readily transformed by a great diversity of microbial species and has a robust biogeochemical cycle. Arsenic cycling primarily involves the oxidation of As(III) and the reduction of As(V). Over thirty arsenite oxidizing prokaryotes have been reported and include alpha, beta, and gamma Proteobacteria , Deinocci and Crenarchaeota. At least twenty species of arsenate-respiring prokaryotes are now known and include Crenarchaeota, thermophilic bacteria, low and high G+C gram positive bacteria, and gamma, delta, and epsilon Proteobacteria. These organisms are metabolically diverse, and depending on the species, capable of using other terminal electron acceptors (e.g., nitrate, selenate, fumarate, sulfate). In addition to inorganic forms (e.g., sodium arsenate) organoarsenicals can be utilized as a substrate. The feed additive roxarsone (3-nitro-4-hydroxyphenyl arsonic acid) has been shown to readily degrade leading to the release of inorganic arsenic (e.g., As(V)). Degradation proceeds via the cleavage of the arsenate functional group or the reduction of the nitro functional group and deamination. The rapid degradation (within 3 days) of roxarsone by Clostridium sp. strain OhILAs appears to follow the latter pathway and may involve Stickland reactions. The activities of these organisms affect the speciation and mobilization of arsenic, ultimately impacting water quality.
H24C-03 INVITED 16:30h
Points Along the Reaction Path: Complementary Approaches to Understanding Arsenic Transitions in Two Surficial Environments Impacted by Mining Activities
In natural settings that exhibit chemical and hydrologic complexity, As concentration and speciation depend on system characteristics that respond to seasonal events and/or anthropogenic influences. As arsenic-bearing minerals decompose, As may be introduced to aqueous solutions with potential for human exposure or ecological impacts. This transformation may be attenuated by sorption, or precipitation of other phases. Two examples illustrate how laboratory experiments, field observations, spectroscopy, and geochemical modeling synergistically contribute to predictions of temporal change in mine-impacted environments. In both cases X-ray absorption spectroscopy (XAS) has proven valuable for determining As-mineral associations, which are critical to As release and transport pathways. Field and analytical/spectroscopic observations at a high-As pit lake at the Jamestown Mine (Tuolumne County, California) have informed development of an integrated hydrogeochemical model of pit lake evolution. Water fluxes in the pit lake exhibit strong seasonal variation, and pit wall characteristics also change as efflorescent salts build up and are eroded in response to seasonal differences in temperature and rainfall. The model incorporates lake stratification and mixis events as well as interaction with the pit wall rock in the context of water fluxes from ground water, precipitation, and evaporation. The use of XAS to determine As associations in smelter-impacted soils on Vashon-Maury Island, Washington has provided insight into the potential for As mobilization in the vadose zone. Arsenic associations with Al-oxyhydroxides and/or phyllosilicates such as clinochlore imply that As initially released from the smelter as particulate As(III) and As(V) oxides was oxidized, dissolved, and adsorbed onto soil minerals and colloids. Physical transport of As oxide particles and As adsorbed on soil colloids may account for limited downward migration of As within the soil column.
H24C-04 INVITED 16:45h
Groundwater Flow and Arsenic Biogeochemistry in Bangladesh
Although groundwater in Bangladesh is severely contaminated by arsenic, little is known about the complex transient patterns of groundwater flow that flush solutes from aquifers and carry solutes into the subsurface. Hydrologic modeling results for our field site in the Munshiganj district indicate that groundwater flow is vigorous, flushing the aquifer over time-scales of decades and also introducing solute loads into the aquifer with recharge from rice fields, ponds and rivers. The combined hydrologic and biogeochemical results from our field site imply that the biogeochemistry of the aquifer system may not be in steady-state, and that the net effect of competing processes could either increase or decrease arsenic concentrations over the next decades. Modeling results suggest that irrigation has greatly changed the location, timing and chemical content of recharge to the aquifer, drawing large fluxes of anoxic water into the aquifer during the dry season that may mobilize arsenic from oxides in near-surface sediments.
H24C-05 17:00h
Regional and Local Control of Arsenic Concentrations in Shallow Aquifers by the Permeability of Surface Soils
One of the bewildering aspects of the current arsenic crisis in Bangladesh and other South Asian countries is the extreme degree of spatial variability of groundwater As concentrations. This presentation focuses on the origin of this variability in the top 20 meters of shallow aquifers by combining surface geophysical measurements (EM31) with groundwater and sediment properties obtained by modifying a local hand-drilling method in three contrasting areas of Bangladesh: (1) Birganj, in northwestern Bangladesh, where groundwater As concentrations rarely exceed 50 ug/L, (2) Araihazar, a central portion of the country where shallow groundwater As concentrations are highly variable, and (3) Lakshmipur, where essentially all shallow wells are elevated in As. Comparison with a series of auger cores collected in Araihazar indicates that the EM31 signal combines contributions related to the ionic strength of soil water as well as the proportion of fine-grained sediment. The combined set of observations shows a rather consistent relation between the conductivity of surface soils measured by induction and shallow groundwater As. In Birganj, EM31 conductivities rarely exceed 10 mS/m and can be below the detection limit of the instrument (~0.1 mS/m) over distances of 100s of meters. In Araihazar, areas with EM31 conductivities ranging form 10-15 mS/m are intermixed with regions with EM31 readings of 20-30 mS/m. The lower and higher EM31 conductivity ranges are generally associated with low and elevated As concentrations, respectively. In contrast, EM31 readings in the portion of Lakshmipur that was surveyed are consistently high and range from 30-50 mS/m. Overall, therefore, it appears that high groundwater As concentrations are typically associated with aquifers capped by fine-grained sediment whereas low groundwater As concentrations prevail in aquifers overlain by sandy deposits. This association, combined with gradual downstream fining of surface deposits of the Ganges-Brahmputra-Meghna delta, helps explain the spatial variability of shallow groundwater As variations down to the ~1 km scale (i.e. within Araihazar) as well as broader patterns throughout the country at the ~100 km scale.
H24C-06 17:15h
The Arsenic Cycle in Searles Lake, California: An Arsenic-Rich, Salt-Saturated Soda Lake: I. Sediment Experiments.
Searles Lake is a residual playa of what once was the end-member of a series of soda lakes that were connected during the Pleistocene. Brinewaters are saturated (300 g/L), alkaline (pH = 9.8), and rich in arsenic (3.9 mM). Porewater profiles (max. depth = 25 cm.) show the speciation of arsenic changes from arsenate [As(V)] in the slightly oxic (DO = 6.2 uM) surface to predominantly arsenite [As(III)] in the sediments. Porewaters also contained ammonia (0.4 - 1.2 mM), sulfide (0.1 - 0.2 mM), and methane (0.05 - 0.6 uM). Sediment slurries incubated with artificial brinewater (salinity = 346 g/L) demonstrated reduction of As(V) to As(III). The rate of As(V) reduction (40 umol/L/day) increased 50 percent with addition of lactate. Addition of sulfide or hydrogen to slurries stimulated this rate by 2.4- and 4.1-fold, respectively. This suggests that chemoautotrophs are important agents of As(V)-respiration in this system. The rate of As(V)-reduction responded inversely with increased salinity, decreasing from 470 umol/L/day at 50 g/L to 40 umol/L/day at 346 g/L. A similar relationship to salinity was found for methane production. Slurries incubated under aerobic conditions showed a rapid biological oxidization of As(III) (rate = 220 umol/L/day). These results show that a microbiological arsenic cycle occurs in this extreme environment. The anaerobes in this ecosystem, however, seem best adapted to lower salinities. Nonetheless, significant dissimilatory As(V) reduction still occurs at the condition of salt-saturation.
H24C-07 17:30h
The Arsenic Cycle in Searles Lake, California: An Arsenic-Rich, Salt-Saturated Soda Lake. II. Isolation of Arsenic-Metabolizing Microbes.
The motivation for isolating arsenic-metabolizing prokaryotes from Searles Lake was to characterize the physiology of microbes that can cope simultaneously with at least 3 environmental extremes: saturating salt concentration, high pH, and high dissolved inorganic arsenic. A secondary motivation was to find extremely halophilc Archaea that could respire As(V), as this has only been reported for the Crenarchaea. Enrichment cultures of arsenate [As(V)]-respirers were established by inoculating Searles Lake mud into an anaerobic, alkaline (pH = 9.8) artificial medium containing 346 g/L dissolved salts, with lactate as the electron donor and As(V) as the electron acceptor. After about 6 months of bi-weekly transfers, the enrichment was purified by serial dilution, with the highest growth-positive dilution tube exhibiting motile cells having uniform morphology (curved rods). This culture, strain SLAS-1, grew by oxidizing lactate to acetate plus carbon dioxide while reducing As(V) to arsenite [As(III)]. The doubling time was 48 hours at 346 g/L salinity, and nearly equivalent growth rates were observed over a salinity range of 200 to 346 g/l, with no growth evident below 200 g/L. The pH range was 8.5 to 10, with an optimum at 9.5. Strain SLAS-1 has an unusual motility that can be characterized as a "fish-like" swimming motion. Thin section electron micrographs revealed the presence of an internal cytoplasmic filament that runs the full length of the microorganism. We suggest that this filament may be involved in cellular motility. However, taxonomic classification of SLAS-1 made by 16S rRNA gene sequences aligned it in the order Haloanaerobacteriales of the Domain Bacteria. In a further effort to isolate haloalkaliphilic Archaea, a similar enrichment strategy was employed as above, but cell-wall antibiotics were added to the medium to discourage the growth of Bacteria. An enrichment culture, designated Serl-Ab, was established that oxidized lactate to acetate plus carbon dioxide. Preliminary evidence suggests that the culture consists of a lactate-oxidizing sulfate-reducer growing in synthrophy with a chemoautotrophic, sulfide-oxidizing As(V)-respirer. Terminal restriction length polymorphism analysis has indicated the presence of both bacterial and archaeal components in the Serl-Ab enrichment, although it is not yet known which is responsible for the observed As(V)-reduction and sulfate-reduction. Efforts are ongoing to resolve Serl-Ab by using classical isolation procedures for a heterotrophic sulfate reducer and an autotrophic As(V)-respirer. In addition, new efforts are being undertaken to isolate hydrogen-oxidizing As(V)-respirers, as well as aerobic As(III)-oxidizers from the extreme environment of Searles Lake.