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1.
To investigate the possible variations of Rn concentration in crystalline rocks as a function of flow conditions, a field study was carried out of a fractured aquifer in granite. The method is based on the in situ measurement of Rn in groundwater, aquifer tests for the determination of hydraulic characteristics of the aquifer and laboratory measurement of Rn exhalation rate from rocks. A simple crack model that simulates the Rn concentration in waters circulating in a fracture intersecting a borehole was also tested. The Rn concentrations in groundwaters from boreholes of the study site ranged from 192 to 1597 Bq L−1. The Rn exhalation rates of selected samples of granite and micaschist were determined from laboratory experiments. The results yielded fluxes varying from 0.5 to 1.3 mBq m−2 s−1 in granite and from 0.5 to 0.9 mBq m−2 s−1 in micaschists. Pumping tests were performed in the studied boreholes to estimate the transmissivity and calculate the equivalent hydraulic aperture of the fractures. Transmissivities ranged from 10−5 to 10−3 m2 s−1. Using the cubic law, hydraulic equivalent fracture apertures were calculated to be in the range of 0.5–2.3 mm.  相似文献   

2.
Activity concentrations of the naturally occurring, short-lived and highly particle-reactive radionuclide tracer 234Th in the dissolved and particulate phase were determined at 7 shallow-water stations (maximum depths: 30 (S.1 and S.2), 65 (S.3), 97 (S.5), 105 (S.6) and 220 m (S.4 and S.7) in Saronikos Gulf and Elefsis Bay (central Aegean Sea, Greece) during 3 seasonal cruises (summer 2008, autumn 2008 and winter 2009) to assess the time scales of the dynamics and the depositional fate of particulate matter (POC, particulate 234Th). For that reason, in situ filtrating systems were deployed in several depths of the water column consisting of GF/A disc prefilters to scavenge particulate fraction of 234Th and organic carbon and impregnated cartridges to adsorb dissolved 234Th.The obtained data showed average particulate 234Th activity concentrations of 3.7 ± 0.4 Bq m−3 in summer, 2.1 ± 0.2 Bq m−3 in autumn and 2.4 ± 0.2 Bq m−3 in winter. The respective average dissolved 234Th activity concentrations were 30.1 ± 2.8 Bq m−3 in summer, 30.2 ± 2.9 Bq m−3 in autumn and 27.4 ± 3.0 Bq m−3 in winter. The activity ratios of total 234Th and its long-lived conservative parent 238U were below unity in most of the stations indicating radioactive disequilibrium throughout the water column, thus very dynamic trace-metal scavenging and particle export from the water column. These profiles (234Th and 238U) were used to estimate the export fluxes and scavenging rates of 234Th, as well as their residence times in the water column. The average cumulative export fluxes of particulate 234Th were estimated to be 33 ± 4 Bq m−2 d−1 in summer, 35 ± 5 Bq m−2 d−1 in autumn and 45 ± 6 Bq m−2 d−1 in winter, whereas the respective average cumulative scavenging rates of dissolved 234Th were 39 ± 5, 33 ± 5 and 50 ± 7 Bq m−2 d−1. Moreover, the cumulative average residence times of 234Th were 25 ± 4 d in summer, 45 ± 6 d in autumn and 64 ± 7 d in winter 2009 for the dissolved fraction and 4 ± 1, 3 ± 1 and 4 ± 1 d for the particulate one, respectively.POC/ ratio profiles decreased versus depth showing a variety of marine processes, such as loss of POC due to dissolution after biological activity, impact of minerals in particle sinking and microbial remineralization. Average cumulative export fluxes of POC were 162 ± 18 mmol m−2 d−1 in summer, 107 ± 19 mmol m−2 d−1 in autumn and 157 ± 25 mmol m−2 d−1 in winter 2009. The seasonal data of POC fluxes certified the existence of phytoplankton bloom in winter for Saronikos Gulf. In addition, after evaluating the maxima of POC fluxes in Elefsis Bay (a small embayment in northern Saronikos Gulf) during summer, potential bloom of phytoplankton also concluded; this approach is in agreement with previous data of the same area. Finally, the elevated POC concentrations and fluxes in the region certify that the Gulf is still one of the most organic polluted in the Mediterranean Sea.  相似文献   

3.
We have performed holographic interferometry measurements of the dissolution of the (0 1 0) plane of a cleaved gypsum single crystal in pure water. These experiments have provided the value of the dissolution rate constant k of gypsum in water and the value of the interdiffusion coefficient D of its aqueous species in water. D is 1.0 × 10−9 m2 s−1, a value close to the theoretical value generally used in dissolution studies. k is 4 × 10−5 mol m−2 s−1. It directly characterizes the microscopic transfer rate at the solid-liquid interface, and is not an averaged value deduced from quantities measured far from the surface as in macroscopic dissolution experiments. It is found to be two times lower than the value obtained from macroscopic experiments.  相似文献   

4.
The Xunyang Hg mine (XMM) situated in Shaanxi Province is an active Hg mine in China. Gaseous elemental Hg (GEM) concentrations in ambient air were determined to evaluate its distribution pattern as a consequence of the active mining and retorting in the region. Total Hg (HgT) and methylmercury (MeHg) concentrations in riparian soil, sediment and rice grain samples (polished) as well as Hg speciation in surface water samples were measured to show local dispersion of Hg contamination. As expected, elevated concentrations of GEM were found, ranging from 7.4 to 410 ng m−3. High concentrations of HgT and MeHg were also obtained in riparian soils, ranged from 5.4 to 120 mg kg−1 and 1.2 to 11 μg kg−1, respectively. Concentrations of HgT and MeHg in sediment samples varied widely from 0.048 to 1600 mg kg−1 and 1.0 to 39 μg kg−1, respectively. Surface water samples showed elevated HgT concentrations, ranging from 6.2 to 23,500 ng L−1, but low MeHg concentrations, ranging from 0.022 to 3.7 ng L−1. Rice samples exhibited high concentrations of 50–200 μg kg−1 in HgT and of 8.2–80 μg kg−1 in MeHg. The spatial distribution patterns of Hg speciation in the local environmental compartments suggest that the XMM is the source of Hg contaminations in the study area.  相似文献   

5.
Mercury fluxes from air/surface interfaces in paddy field and dry land   总被引：3，自引：0，他引：3  
In order to provide insight into the characteristics of Hg exchange in soil/water-air surface from cropland (including paddy field and dry land), Hg fluxes were measured in Chengjiang. Mercury fluxes were measured using the dynamic flux chamber method, coupled with a Lumex® multifunctional Hg analyzer RA-915+ (Lumex Ltd., Russia). The Hg fluxes from paddy field and dry land were alternatively measured every 30 min. Data were collected for 24-48 h once per month for 5 months. Mercury fluxes in both fields were synchronously measured under the same conditions to compare Hg emissions between paddy field and dry land over diurnal and seasonal periods and find out what factors affect Hg emission on each surface. These results indicated that air Hg concentrations at the monitoring site was double the value observed at the global background sites in Europe and North America. The Hg release fluxes were 46.5 ± 22.8 ng m−2 h−1 in the warm season, 15.5 ± 18.8 ng m−2 h−1 in the cold season for dry land, and 23.8 ± 15.6 ng m−2 h−1 in the warm season, 6.3 ± 11.9 ng m−2 h−1 in the cold season for paddy field. Solar radiation is important in the emission of Hg over both sites. Hg exchange at the soil/air and water/air interfaces showed temporal variations. The amount of Hg emission from dry land was higher than that from the paddy field, and the emission in daytime was higher than that at night. Moreover, Hg emissions from land covered by crops, was lower than that for bare land.  相似文献   

6.
Many water-supply systems in South America utilize the waters of the Guarani aquifer at least as part of their networks. However, there is little present knowledge in Brazil of the factors affecting Rn presence in the water supplied for end-users, despite the economic importance of Guarani aquifer. 222Rn analyzes of 162 water samples were performed at 8 municipalities in São Paulo State, Brazil, with the aim of investigating the major factors affecting its presence in solution. The 222Rn activity concentration ranged from 0.04 up to 204.9 Bq/L, with three samples exceeding the World Health Organization maximum limit of 100 Bq/L. Aeration was confirmed as the most important factor for Rn release, as expected due to its gaseous nature. Accumulation in pipes and stratification in the water column were other significant factors explaining the data obtained in some circumstances. The Rn daughters 214Pb and 214Bi were also determined in a set of selected samples and their presence was directly related to the occurrence of Rn dissolved in water.  相似文献   

7.
Extreme U and Pb isotope variations produced by disequilibrium in decay chains of 238U and 232Th are found in calcite, opal/chalcedony, and Mn-oxides occurring as secondary mineral coatings in the unsaturated zone at Yucca Mountain, Nevada. These very slowly growing minerals (mm my−1) contain excess 206Pb and 208Pb formed from excesses of intermediate daughter isotopes and cannot be used as reliable 206Pb/238U geochronometers. The presence of excess intermediate daughter isotopes does not appreciably affect 207Pb/235U ages of U-enriched opal/chalcedony, which are interpreted as mineral formation ages.Opal and calcite from outer (younger) portions of coatings have 230Th/U ages from 94.6 ± 3.7 to 361.3 ± 9.8 ka and initial 234U/238U activity ratios (AR) from 4.351 ± 0.070 to 7.02 ± 0.12, which indicate 234U enrichment from percolating water. Present-day 234U/238U AR is ∼1 in opal/chalcedony from older portions of the coatings. The 207Pb/235U ages of opal/chalcedony samples range from 0.1329 ± 0.0080 to 9.10 ± 0.21 Ma, increase with microstratigraphic depth, and define slow long-term average growth rates of about 1.2-2.0 mm my−1, in good agreement with previous results. Measured 234U/238U AR in Mn-oxides, which pre-date the oldest calcite and opal/chalcedony, range from 0.939 ± 0.006 to 2.091 ± 0.006 and are >1 in most samples. The range of 87Sr/86Sr ratios (0.71156-0.71280) in Mn-oxides overlaps that in the late calcite. These data indicate that Mn-oxides exchange U and Sr with percolating water and cannot be used as a reliable dating tool.In the U-poor calcite samples, measured 206Pb/207Pb ratios have a wide range, do not correlate with Ba concentration as would be expected if excess Ra was present, and reach a value of about 1400, the highest ever reported for natural Pb. Calcite intergrown with opal contains excesses of both 206Pb and 207Pb derived from Rn diffusion and from direct α-recoil from U-rich opal. Calcite from coatings devoid of opal/chalcedony contains 206Pb and 208Pb excesses, but no appreciable 207Pb excesses. Observed Pb isotope anomalies in calcite are explained by Rn-produced excess Pb. The Rn emanation may strongly affect 206Pb-238U ages of slow-growing U-poor calcite, but should be negligible for dating fast-growing U-enriched speleothem calcite.  相似文献   

8.
Before the Kobe earthquake, an anomalous increase in atmospheric Rn concentration was observed. By separating the measured concentration of atmospheric Rn into three components according to the distance from the monitoring station, the variation of Rn exhalation rate can be estimated for the respective area using the daily minimum and maximum concentrations. The mean rate of Rn exhalation gradually increased in an area of 20 km around the monitoring station, becoming five times higher than normal in the period between October 1994 and the date of the earthquake. This area had a large co-seismic displacement of up to 30 cm, which roughly corresponds to the crustal strain of 10−6-order, and it is considered the main source for the atmospheric Rn prior to the Kobe earthquake. Analyses revealed that the pre-seismic change in the atmospheric Rn concentration exhibited an anomalous pattern which would yield information on the spatial distribution of the mechanical response of the ground.  相似文献   

9.
The solubility of cyclooctasulfur in water and sea water at various temperatures in the range between 4 and 80 °C was determined. Cyclooctasulfur in equilibrium with rhombic sulfur reacted with hot acidic aqueous potassium cyanide to form thiocyanate anion which was measured by anion chromatography. Sulfur solubility in pure water was found to increase with temperature by more than 78 times: from 6.1 nM S8 at 4 °C to 478 nM S8 at 80 °C. The following thermodynamic values for solubilisation of S8 in water were calculated from the experimental data: K° = 3.01 ± 1.04 × 10−8, ΔGr° = 42.93 ± 0.73 kJ mol−1, ΔHr° = 47.4 ± 3.6 kJmol−1, ΔSr° = 15.0 ± 11.7 J mol−1 K−1). Solubility of cyclooctasulfur in sea water was found to be 61 ± 13% of the solubility in pure water regardless of the temperature.  相似文献   

10.
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   

11.
Despite reduced anthropogenic deposition during the last decades, deposition sulphate may still play an important role in the biogeochemical cycles of S and many catchments may act as net sources of S that may remain for several decades. The aim of this study is to elucidate the temporal and spatial dynamics of both SO42− and δ34SSO4 in stream water from catchments with varying percentage of wetland and forest coverage and to determine their relative importance for catchment losses of S. Stream water samples were collected from 15 subcatchments ranging in size from 3 to 6780 ha, in a boreal stream network, northern Sweden. In forested catchments (<2% wetland cover) S-SO42− concentrations in stream water averaged 1.7 mg L−1 whereas in wetland dominated catchments (>30% wetland cover) the concentrations averaged 0.3 mg L−1. A significant negative relationship was observed between S-SO42− and percentage wetland coverage (r2 = 0.77, p < 0.001) and the annual export of stream water SO42− and wetland coverage (r2 = 0.76, p < 0.001). The percentage forest coverage was on the other hand positively related to stream water SO42− concentrations and the annual export of stream water SO42− (r2 = 0.77 and r2 = 0.79, respectively). The annual average δ34SSO4 value in wetland dominated streams was +7.6‰ and in streams of forested catchments +6.7‰. At spring flood the δ34SSO4 values decreased in all streams by 1‰ to 5‰. The δ34SSO4 values in all streams were higher than the δ34SSO4 value of +4.7‰ in precipitation (snow). The export of S ranged from 0.5 kg S ha−1 yr−1 (wetland headwater stream) to 3.8 kg S ha−1 yr−1 (forested headwater stream). With an average S deposition in open field of 1.3 kg S ha−1 yr−1 (2002-2006) the mass balance results in a net export of S from all catchments, except in catchments with >30% wetland. The high temporal and spatial resolution of this study demonstrates that the reducing environments of wetlands play a key role for the biogeochemistry of S in boreal landscapes and are net sinks of S. Forested areas, on the other hand were net sources of S.  相似文献   

12.
Wetlands are significant sources and sinks for arsenic (As), yet the geochemical conditions and processes causing a release of dissolved arsenic and its association with the solid phase of wetland soils are poorly known. Here we present experiments in which arsenic speciation was determined in peatland mesocosms in high spatiotemporal resolution over 10 months. The experiment included a drought/rewetting treatment, a permanently wet, and a defoliated treatment. Soil water content was determined by the TDR technique, and arsenic, iron and sulfate turnover from mass balancing stocks and fluxes in the peat, and solid phase contents by sequential extractions. Arsenic content ranged from 5 to 25 mg kg−1 and dissolved concentrations from 10 to 300 μg L−1, mainly in form of As(III), and secondarily of As(V) and dimethylated arsenic (DMA). Total arsenic was mainly associated with amorphous iron hydroxides (R2 > 0.95, α < 0.01) and deeper into the peat with an unidentified residual fraction. Arsenic release was linked to ferrous iron release and primarily occurred in the intensely rooted uppermost soil. Volumetric air contents of 2-13 % during drought eliminated DMA from the porewater and suppressed its release after rewetting for >30 d. Dissolved As(III) was oxidized and immobilized as As(V) at rates of up to 0.015 mmol m−3 d−1. Rewetting mobilized As(III) at rates of up to 0.018 mmol m−3 d−1 within days. Concurrently, Fe(II) was released at depth integrated rates of up 20 mmol m−3 d−1. The redox half systems of arsenic, iron, and sulfur were in persistent disequilibrium, with H2S being a thermodynamically viable reductant for As(V) to As(III). The study suggests that rewetting can lead to a rapid release of arsenic in iron-rich peatlands and that methylation is of lesser importance than co-release with iron reduction, which was largely driven by root activity.  相似文献   

13.
A promising technique for the removal of heavy metal ions from wastewater streams involved firstly the ions adsorption on a colloidal precipitate (carrier) and then the separation of the loaded flocs (coagula) by a modified column flotation. Here, the effluent feed and the carrier (ferric hydroxide) enter smoothly by the top of the column through a special diffuser, in counter current with rising bubbles (100–600 μm diameter) generated by using recycled water, surfactant and air suction through a venturi. High separation values of the column flotation of the carrier precipitates were achieved, despite the high superficial flow rate and the high Fe+ 3 concentration utilized (> 60 mg L− 1 Fe). No rupture of colloidal carrier aggregates was observed and a low split was ensured by monitoring the concentrate (floated product) flow rate. Results indicated that best separation was attained by controlling the medium pH (for best heavy metal ion adsorption onto the carrier), followed by sodium oleate, used as “collector” and optimizing operating parameters (conditioning, flow rates, etc.). The column throughput reached 43 m h− 1 (m3 m− 2 h− 1), which is about 4 times the normal capacity of DAF-dissolved air flotation unit, the most used floater in wastewater treatment. Various metals (Cu, Ni, Pb, etc.) and molybdate ions present in synthetic and real effluent were successfully removed based on this colloidal adsorbing flotation principle. The process was also applied in a pilot scale to treat an industrial electroplating wastewater. Most of toxic metals (Cu, Ni and Zn) were reduced from initial concentrations of about of 2 to 10 mg L− 1, to below 0.5 to 1.0 mg L− 1, meeting local municipal discharge limits (but Cd ions). It is believed that flotation separation using medium-sized bubbles has great potential as a clean water and wastewater treatment technology.  相似文献   

14.
Soils in the McMurdo Dry Valleys, Antarctica contain ice and considerable amounts of salt. Ice often occurs at shallow depth throughout the Dry Valleys and other areas of hyperarid permafrost, notably on Mars. This common occurrence of shallow ice is enigmatic; however, since according to published sublimation models it should disappear relatively quickly (at rates of order 0.1 mm a−1) due to vapor loss to the atmosphere. This loss may be offset by recharge from snowmelt infiltrating and freezing in the soil. Herein, we present a first quantitative estimate of this recharge based on measured vertical profiles of δD and δ18O that reveal considerable detail about the sources and sinks of ice. We model these profiles, taking into account the salt content and a soil temperature record along a 1.6 m depth profile of ∼10 ka old ice-cemented soils in Victoria Valley, Antarctica. The stable isotopes of ice are enriched in heavy isotopes at the top of the ice cement (20 cm depth); both δD and δ18O values plotted against depth exhibit a concave upward curve. At depth, the isotope composition is similar to that of Lake Victoria and modern meteoric water. The concave shape of the isotope profile is suggestive of downward advection-dispersion of snowmelt water enriched in heavy isotopes into the ice cement. Our advection-dispersion model, coupled with field data, enables us to quantify the advective flux and dispersion of melt water into the ice. The advective velocity and dispersion coefficient depend on the time since advection began and the ice-to-brine ratio; they are, respectively, of the order of 10−11-10−10 m s−1 and 10−12-10−11 m2 s−1. These values suggest that over the ∼10 ka time period, a total of 190 mm water infiltrated into the ice-cemented ground. The isotope composition and deuterium excess values of the uppermost ice cement can be modeled from snowmelt water enriched in salts using open system-Rayleigh fractionation. To develop the isotopic signature of the upper ice cement requires evaporation of ∼95% of the snowmelt water. Based on 190 mm brine infiltrating into the soil requires an initial total of ∼4 m of snowmelt water. This corresponds to ∼0.4 mm a−1 suggesting that, under the current climate condition, water from snowmelt is sufficient to compensate modeled sublimation rates, and therefore conserve ground ice in Victoria Valley.  相似文献   

15.
This study focused on the analysis of As levels in human hair samples collected from six villages in the Kandal Province of Cambodia. Of interest were the influence of, and interactions among, certain factors affecting As intake into the human body: As concentrations in groundwater, period of groundwater consumption, age and gender. The results revealed As levels in human hair ranging from 0.06 to 30 μg g−1 with median and arithmetic mean values of 0.61 and 3.20 μg g−1 (n = 68), respectively. Furthermore, a linear relationship was found between As concentrations in human hair and in the local groundwater. Arsenic (III) is the dominant species in Kandal groundwater, constituting in most cases at least 60% of the total As. Arsenic concentration ranged from 5 to 1543 μg L−1, with the median value 348 μg L−1 and arithmetic mean 454 μg L−1. In large rural, poor areas holding most of Kandal’s 1.1 million people, up to 2 in 1000 people are believed to be at risk of cancer through the As-enriched water they drink. A toxicity risk assessment provides a hazard quotient (HQ) equaling 5.12, also a clear indication of non-carcinogenic exposure risk. On the authors’ visit to Kampong Kong commune, Kandal Province, cases of arsenicosis were diagnosed in patients as a result of drinking As-enriched groundwater.  相似文献   

16.
Lake Constance is one of Europe’s largest oligotrophic lakes and provides a water source for more than 4.5 million people in Germany and Switzerland. We present here a 12 month study on iodine concentrations, speciation and fluxes to and from the lake to gain a quantitative understanding of the limnic iodine cycle. Monthly water samples were obtained from all major tributaries (14) and the outflow to construct a mass-balance model. Sediment traps were also deployed in the lake for two years at two different stations. Total soluble iodine (TSI) in aqueous samples were analysed by ICP-MS and speciation (iodide, iodate and soluble organically bound iodine, SOI) by ion chromatography-ICP-MS. Iodine concentrations in the Alpine tributaries (1-2 μg l−1) decreased over the summer months due to increasing proportions of snow and glacial melt water from the Alps, while iodine levels in the lowland rivers (∼2-10 μg l−1) increased over the summer. Deposition of TSI to the catchment (16,340 kg I yr−1) was similar to the TSI out-flux by rivers (16,000 kg I yr−1). By also including the particulate riverine iodine flux out of the catchment (∼12,350 kg I yr−1) it is shown that the catchment is a net source of iodine, with the highest particulate fluxes coming from the Alpine rivers. The total TSI flux to the lake was 16,770 kg I yr−1, the largest proportion coming from the Alpenrhein (43%), followed by the Schussen (8%) and Bregenzer Ach (7.7%). Overall the mass-balance for TSI in the lake was negative, with more iodine flowing out of the lake than in (−2050 kg I yr−1; 12% of TSI in-flux). To maintain mass-balance, 8.8 μg I m−2 d−1 from the Obersee and 23 μg I m−2 d−1 from the Untersee must be released from the sediments into the water column. Thus, in comparison with the total iodine flux to the sediments measured by the sediment traps (4762-8075 kg I yr−1), up to 39% of the deposited iodine may be mobilised back into the lake. SOI was the dominant iodine fraction entering the lake, with a total flux of 10,290 kg I yr−1 (64% of TSI input), followed by iodate (3120 kg I yr−1) and iodide (2760 kg I yr−1). Net formation of SOI from iodide and iodate was also noted within the lake, with an estimated production of 6560 kg I yr−1, suggesting a strong role for biology in iodine cycling. In conclusion, organically bound iodine was the dominant iodine species in aqueous and solid phases in Lake Constance, despite low DOC concentrations (<2 mg l−1), and thus is expected to play an important role in iodine cycling in most freshwater environments.  相似文献   

17.
Kinetics of arsenopyrite oxidative dissolution by oxygen   总被引：1，自引：0，他引：1  
We used a mixed flow reactor system to determine the rate and infer a mechanism for arsenopyrite (FeAsS) oxidation by dissolved oxygen (DO) at 25 °C and circumneutral pH. Results indicate that under circumneutral pH (6.3-6.7), the rate of arsenopyrite oxidation, 10−10.14±0.03 mol m−2 s−1, is essentially independent of DO over the geologically significant range of 0.3-17 mg L−1. Arsenic and sulfur are released from arsenopyrite in an approximate 1:1 molar ratio, suggesting that oxidative dissolution by oxygen under circumneutral pH is congruent. Slower rates of iron release from the reactor indicate that some of the iron is lost from the effluent by oxidation to Fe(III) which subsequently hydrolyzes and precipitates. Using the electrochemical cell model for understanding sulfide oxidation, our results suggest that the rate-determining step in arsenopyrite oxidation is the reduction of water at the anodic site rather than the transfer of electrons from the cathodic site to oxygen as has been suggested for other sulfide minerals such as pyrite.  相似文献   

18.
Mercury air/surface exchange was measured over litter-covered soils with low Hg concentrations within various types of forests along the eastern seaboard of the USA. The fieldwork was conducted at six forested sites in state parks in South Carolina, North Carolina, New Jersey, Pennsylvania, New York and Maine from mid-May to early June 2005. The study showed that the Hg air/surface exchange was consistently very low and similar (overall daytime mean flux = 0.2 ± 0.9 ng m−2 h−1, n = 310, for all six sites monitored) with the various forest types. These flux values are comparable with those found in a year-long study in Tennessee (yearly daytime mean = 0.4 ± 0.5 ng m−2 h−1), but lower than many previous flux results reported for background soils. The Hg fluxes at all sites oscillated around zero, with many episodes of deposition (negative fluxes) occurring in both daytime and nighttime. While there were particular days showing significant correlations among the Hg air/surface exchange and certain environmental parameters, perhaps because of the low fluxes encountered, few significant correlations were found for any particular day of sampling between the Hg flux and environmental parameters such as solar radiation, soil temperature, air temperature (little variability seen), relative humidity, and ambient air Hg concentrations. Factors driving the Hg exchange as previously found for enriched soils may not hold for these background litter-covered forest soils. The results suggest that spatial variations of the Hg air/surface exchange were small among these different forest types for this particular time of year.  相似文献   

19.
The occurrence of mining areas in the vicinities of salt marshes may affect their ecological functions and facilitate the transfer of pollutants into the food chain. The mobilisation of metals in salt marsh soils is controlled by abiotic (pH, redox potential) and biotic (influence of rhizosphere) factors. The effect of the rhizosphere of two plant species (Sarcocornia fruticosa and Phragmites australis) and different flooding regimes on potentially harmful metals and As mobilisation from salt marsh soil polluted by mining activities were investigated (total concentrations: 536 mg kg−1 As, 37 mg kg−1 Cd, 6746 mg kg−1 Pb, 15,320 mg kg−1 Zn). The results show that the changes in redox conditions (from 300 mV to −100 mV) and pH after flooding and rewetting periods may mobilise the contaminant elements into soil solution (e.g., 100 μg L−1 Cd, 30 μg L−1 Pb, 7 mg L−1 Zn), where they are available for plants or may be leached from the soil. Drying periods generated peaks of concentrations in the soil solution (up to 120 μg L−1 Cd and 50 μg L−1 Pb). The risk assessment of As and metal-polluted salt marshes should take into account flood dynamics in order to prevent metal(loid) mobilisation.  相似文献   

20.
The shallow aquifer beneath the Western Snake River Plain (Idaho, USA) exhibits widespread elevated arsenic concentrations (up to 120 μg L−1). While semi-arid, crop irrigation has increased annual recharge to the aquifer from approximately 1 cm prior to a current rate of >50 cm year−1. The highest aqueous arsenic concentrations are found in proximity to the water table (all values >50 μg L−1 within 50 m) and concentrations decline with depth. Despite strong vertical redox stratification within the aquifer, spatial distribution of aqueous species indicates that redox processes are not primary drivers of arsenic mobilization. Arsenic release and transport occur under oxidizing conditions; groundwater wells containing dissolved arsenic at >50 μg L−1 exhibit elevated concentrations of O2 (average 4 mg L−1) and NO3 (average 8 mg L−1) and low concentrations of dissolved Fe (<20 μg L−1). Sequential extractions and spectroscopic analysis of surficial soils and sediments indicate solid phase arsenic is primarily arsenate and is present at elevated concentrations (4–45 mg kg−1, average: 17 mg kg−1) relative to global sedimentary abundances. The highest concentrations of easily mobilized arsenic (up to 7 mg kg−1) are associated with surficial soils and sediments visibly stained with iron oxides. Batch leaching experiments on these materials using irrigation waters produce pore water arsenic concentrations approximating those observed in the shallow aquifer (up to 152 μg L−1). While As:Cl aqueous phase relationships suggest minor evaporative enrichment, this appears to be a relic of the pre-irrigation environment. Collectively, these data indicate that infiltrating irrigation waters leach arsenic from surficial sediments to the underlying aquifer.  相似文献   

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