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941.
942.
To understand the biogeochemical cycles of trace metals (Cd, Cu, Fe, Mn, Ni and Zn) in a hypersaline subtropical marsh, geochemical studies of both interstitial and solid phases were conducted on sediment cores from Chiricahueto marsh, SE Gulf of California. The sequential extraction procedure proposed by Tessier was used to estimate the percentages of the metals present in each geochemical phase of the sediment. Metal concentrations in the solid phase were found to be enriched in the upper layers and mainly associated with reactive fractions such as organic matter, Fe–Mn oxyhydroxides and carbonates (46–74% of Ni, Mn and Cd, and 11–19% of Cu and Zn). Principal factor analysis (PFA) and Spearman correlation analysis revealed a strong positive association of metals and their reactive phases with OC (the diagenetic component), and a negative or non-association with the mud content, Al, Fe and Li (the lithogenic component). Diagenetically released metals are mainly mobilized within hypersaline sediments by buoyancy transport (>90% of total flux) in response to an extreme salinity gradient by input of fresh groundwater (3–6 psu cm−1). The molecular diffusion due to the gradient of metals in porewater (maximum and higher levels at 5–7 and below 20 cm depth, respectively) is significantly less important to the advective transport. Most of the metals mobilized by diffusion–advection processes are re-precipitated in the sediments by authigenic minerals, only <10% of most metals are extruded out to the overlying water column. Authigenic accumulation rates were estimated as 1.42–7.09 mg m−2 a−1 for Cd; 58.8–378 for Cu; 6922–17,985 for Fe; 38.2–345 for Mn; 20.8–263 for Ni; and 282–2956 mg m−2 a−1 for Zn. The Mn–Fe oxyhydroxides (40–85% of reactive metals) in the upper oxic–suboxic layers (<5 cm below surface) and sulfide minerals (75–97%) in anoxic sediment layers (7–18 cm) constitute the main scavengers for metals. 相似文献
943.
944.
945.
Rice is a major exposure route for arsenic in Chakdaha block,Nadia district,West Bengal,India: A probabilistic risk assessment 总被引:2,自引:0,他引:2
The importance or otherwise of rice as an exposure pathway for As ingestion by people living in Bengal and other areas impacted by hazardous As-bearing groundwaters is currently a matter of some debate. Here this issue is addressed by determining the overall increased cancer risk due to ingestion of rice in an As-impacted district of West Bengal. Human target cancer health risks have been estimated through the intake of As-bearing rice by using combined field, laboratory and computational methods. Monte Carlo simulations were run following fitting of model probability curves to measured distributions of (i) As concentration in rice and drinking water and (ii) inorganic As content of rice and fitting distributions to published data on (i) ingestion rates and (ii) body weight and point estimates on bioconcentration factors, exposure duration and other input variables. The distribution of As in drinking water was found to be substantially lower than that reported by previous authors for As in tube wells in the same area, indicating that the use of tube well water as a proxy for drinking water is likely to result in human health risks being somewhat overestimated. The calculated median increased lifetime cancer risk due to cooked rice intake was 7.62 × 10−4, higher than the 10−4–10−6 range typically used by the USEPA as a threshold to guide determination of regulatory values and similar to the equivalent risk from water intake. The median total risk from combined rice and water intake was 1.48 × 10−3. The contributions to this median risk from drinking water, rice and cooking of rice were found to be 48%, 44% and 8%, respectively. Thus, rice is a major potential source of As exposure in the As-affected study areas in West Bengal and the most important exposure pathway for groups exposed to low or no As in drinking water. 相似文献
946.
J.A. Saunders M.-K. Lee M. Shamsudduha P. Dhakal A. Uddin M.T. Chowdury K.M. Ahmed 《Applied Geochemistry》2008
Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Bangladesh was amended with labile water-soluble organic C (molasses) and MgSO4 to stimulate metabolism of indigenous SO4-reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO4, and contained <10 μg/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Bangladesh, groundwater was under moderately reducing conditions and contained ∼10 mg/L Fe and ∼100 μg/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 μg/L) under geochemical conditions consistent with bacterial Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO4 reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS2. By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO4 reduction commenced but increased to pre-experiment level once SO4 reduction ended. In the ongoing experiment, both SO4 and Fe(II) were amended to groundwater to evaluate if FeS/FeS2 formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist’s Workbench, its stability field completely dominates in reducing Eh–pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions. 相似文献
947.
Kathleen A. Radloff Anya R. Manning Brian Mailloux Yan Zheng M. Moshiur Rahman M. Rezaul Huq Kazi M. Ahmed Alexander van Geen 《Applied Geochemistry》2008
Microbial Fe reduction is widely believed to be the primary mechanism of As release from aquifer sands in Bangladesh, but alternative explanations have been proposed. Long-term incubation studies using natural aquifer material are one way to address such divergent views. This study addresses two issues related to this approach: (1) the need for suitable abiotic controls and (2) the spatial variability of the composition of aquifer sands. Four sterilization techniques were examined using orange-colored Pleistocene sediment from Bangladesh and artificial groundwater over 8 months. Acetate (10 mM) was added to sacrificial vials before sterilization using either (1) 25 kGy of gamma irradiation, (2) three 1-h autoclave cycles, (3) a single addition of an antibiotic mixture at 1× or (4) 10× the typical dose, and (5) a 10 mM addition of azide. The effectiveness of sterilization was evaluated using two indicators of microbial Fe reduction, changes in diffuse spectral reflectance and leachable Fe(II)/Fe ratios, as well as changes in P-extractable As concentrations in the solid phase. A low dose of antibiotics was ineffective after 70 days, whereas autoclaving significantly altered groundwater composition. Gamma irradiation, a high dose of antibiotics, and azide were effective for the duration of the experiment. 相似文献
948.
949.
Flood hazard delineation combining geomorphological and hydrological methods: an example in the Northern Iberian Peninsula 总被引:2,自引:1,他引:1
Flood mapping requires the combination and integration of geomorphological and hydrological-hydraulic methods; however, despite
this, there is very little scientific literature that compares and validates both methods. Two types of analysis are addressed
in the present article. On the one hand, maps of flood plains have been elaborated using geomorphological evidence and historical
flood data in the mountainous area of northwestern Spain, covering an area of more then 232 km2 of floodplains. On the other hand, a hydrometeorological model has been developed (Clark semidistributed unit hydrograph)
in the Sarria River basin (155 km2, NW Spain). This basin is not gauged, hence the model was subjected to a goodness-of-fit test of its parameter (curve number)
by means of Monte Carlo simulation. The peak flows obtained by means of the hydrological model were used for hydraulic modeling
(one-phase, one-dimensional and steady flow) in a 4 km2 urban stretch of the river bed. The delineation of surface areas affected by floods since 1918, as well as those analyzed
subsequent to the geomorphological study, reveals a high degree of reliability in the delineation of the flooded areas with
frequent recurrence intervals (<50 years). If we compare these flooded surface areas with the estimate obtained by the hydrological-hydraulic
method we can see that the latter method overestimates the extent of the surface water by 144% for very frequent recurrence
intervals (>10 years) and underestimates it as the recurrence interval increases, by up to 80% less floodplain for exceptional
events (>500 years). Finally, a management map is put forth combining the most reliable results available by integrating both
methods.
Originally presented at the Sixth International Conference on Geomorphology. 相似文献
950.
Vittorio Zanon Fátima Viveiros Catarina Silva Ana Rita Hipólito Teresa Ferreira 《Natural Hazards》2008,45(1):19-31
A detailed study was carried out on a piece of land that had been struck by lightning during the violent rainstorm that raged
over the Island of S?o Miguel (Azores Archipelago) in late October 2006. Temperature and gas measurements (CO2, CO, H2S and CH4) were performed in four study trenches, dug in an area of ∼3 m2, where an underground fire had been initiated by the impact with a lightning stroke, followed by the emission of a column
of gases and smoke. The soil under study was originally a well-pedogenized about 80 cm thick bed, made of volcanic clayey
to silty tephra fallouts and contained 5.5–9.7% of organic matter. The underground fire was monitored for one week and revealed a peak release of
404 ppm CO and 3.4% CO2 originating from a horizon located about 45 cm under the soil surface. Measurements of temperature, performed one week after
the impact, indicated a maximum value of 326°C inside the soil, while 516.5°C were measured on the surface of a lava block
interred about 20 cm under the surface. Subsequently, a stratigraphic and sedimentologic study proved the role of the grain-size
of the soil and of the organic matter content of the different horizons of the impact area, in determining the ratio between
anoxic/oxidised combustion conditions and in the progress of the process itself. It was also noticed that combustion was not
total all over in the soil bed and that the process had slightly migrated toward SW during the observation period. The combustion
process went on for about ten days, in spite of several other violent rainstorms, until it was artificially extinguished through
the excavations made to obtain study trenches. This particular circumstance evidenced the potential natural hazard represented
by this kind of atmospheric event, especially in a land where the volcanic nature of the soil may easily mislead inexperienced
observers and, consequently, delay proper action. 相似文献