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1.
This study reports on the behavior of two redox-sensitive elements, As and Sb, along the turbidity gradient in the freshwater reaches of the turbid Gironde Estuary. During a 17-month survey, surface water and suspended particulate matter (SPM) were sampled monthly at six sites representing both fluvial branches of the Gironde Estuary. Additionally, two longitudinal high resolution profiles were sampled along the fluvial estuary of the Garonne Branch during two contrasted seasons, i.e. with and without the presence of the maximum turbidity zone (MTZ). Seasonal variability and spatial distribution of dissolved (<0.2 μm; <0.02 μm) and particulate As, Sb and Fe were measured and combined with SPM data to understand metalloid behavior in the estuarine freshwater turbidity gradient.At the two main fluvial entries of the Gironde Estuary, dissolved As and Sb concentrations showed strong (by a factor of 2–4) seasonal variations, that were only partly controlled by discharge-related dilution. Seasonal addition of dissolved As and Sb was attributed to the degradation of particulate As and Sb carrier phases in bottom sediment and/or in the adjacent aquifers, rather than release from SPM. In the surface freshwater reaches of the Gironde Estuary, Sb behaved conservatively under all hydrological conditions. In contrast, As was strongly reactive in the presence of the MTZ, with opposite behaviors in the two fluvial branches of the estuary: in the Garonne Branch As was removed from the dissolved phase, whereas in the Dordogne Branch As was added. Redistribution of As between the dissolved and the particulate phases along the turbidity gradient in estuarine freshwater only affected the <0.02 μm fraction, as the 0.02–0.2 μm fraction remained constant (300 ng L−1 in September 2005). Accordingly, As removal seemed to be decoupled from concomitant “colloidal” (0.02–0.2 μm) Fe flocculation in the turbidity gradient. The contrasting behavior of dissolved As in the fluvial estuaries of the Garonne and Dordogne Branches was attributed to sorption processes during equilibration of river-borne dissolved As with estuarine SPM forming the MTZ. This equilibrium, described by a distinct distribution coefficient Kd(As)  11,000 L kg−1 in the MTZ, resulted in either As release (desorption; Dordogne Branch) or removal (adsorption; Garonne Branch) in the respective fluvial estuaries. A mixing experiment under controlled laboratory conditions tended to support that equilibration between the dissolved phase and MTZ particles may induce both As release and removal in the estuarine freshwater reaches, with As distribution evolving towards a distinct Kd value for increasing SPM concentrations. The long-term survey allowed estimating annual (2004) dissolved fluxes of As and, for the first time Sb, at the main fluvial entries of the Gironde Estuary at 30.7 t a−1 and 3.2 t a−1 (Garonne River) and at 8.0 t a−1 and 2.3 t a−1 (Dordogne River), respectively.  相似文献   

2.
Mining residues from the Sb mine of Bournac in the upper Orb River valley (Southern France), constitute an important source of As and Sb pollution. Arsenic concentrations are as high as 78 μg/L and Sb reaches 32 μg/L in the small creek draining the tailings impoundment. Although both metalloids occur mainly in oxidized form in the creek water, their behaviour differs significantly. Iron oxides are the main carrier phases for both elements in the suspended particulate matter. In oxic conditions the two elements are mainly present in water in oxidised form As(V) and Sb(V) and both field studies and laboratory experiments indicate a higher affinity of As(V) than Sb(V) for the solid phase. In the pool, which receives the water from Bournac Creek, the reductive dissolution of Fe-oxides is linked to the oxidation of small pyrite grains transported from the tailings dump. In oxic conditions Sb is released to solution more efficiently than As. Conversely, in anoxic conditions, mobilisation of As is greater than that of Sb. This is attributed to the reduction of As, which favours its mobility. Whatever the conditions, the activity of bacteria naturally present in the sediments enhances the remobilization of Sb in oxidizing conditions and that of As in reducing conditions.  相似文献   

3.
The Mokrsko Stream in the central Czech Republic is an oxic and slightly alkaline stream that drains a natural As geochemical anomaly. Although long-term monitoring has characterized the general seasonal trends in trace element concentrations (i.e., As, Mo, Cu, Zn) in this stream, little is known about solubility controls and sorption processes that influence diel cycles in trace element concentrations. Trace elements (including As species, Cu, Mn, Mo, Pb and Sb) and other parameters were monitored over two 24-h periods in unfiltered and filtered (0.1 μm) samples collected in August 2010 and June 2011. Copper and Pb were predominantly (>92% of the mass) associated with the particulate fraction (>0.1 μm). Arsenic, Mo and Sb were predominantly (>88% of the mass) in the “dissolved” (<0.1 μm) form. Particulate-associated elements displayed up to a factor of 13 differences between minimum and maximum concentrations, most likely due to increased streamflow related to rainfall events. Dissolved concentrations of the trace metal cations (Cu, Fe, Mn and Pb) were consistently low and displayed no diel trends. Dissolved As(V), Mo and Sb varied on a diel cycle, with increased concentrations (up to 36%) during the late afternoon and decreased concentrations during the nighttime. Diel trends in trace anionic elements are explained by temperature-dependent sorption, as the diel changes in pH during base flow were very small (0.07 std. units). Very low concentrations of As(III), which have been shown to vary in a diel cycle, were attributed to enhanced hydraulic exchange with As(III)-rich hyporheic water during rainfall events.  相似文献   

4.
Dissolved and particulate concentrations of metals (Fe, Al, Mn, Co, Ni, Cu, Zn, Cd, Tl, Pb) and As were monitored over a 5 year period in the Amous River downstream of its confluence with a creek severely affected by acid mine drainage (AMD) originating from a former Pb–Zn mine. Water pH ranged from 6.5 to 8.8. Metals were predominantly in dissolved form, except Fe and Pb, which were in particulate form. In the particulate phase, metals were generally associated with Al oxides, whereas As was linked to Fe oxides. Metal concentrations in the dissolved and/or particulate phase were generally higher during the wet season due to higher generation of AMD. Average dissolved (size < 0.22 μm) metal concentrations (μg/L) were 1 ± 4 (Fe), 69 ± 49 (Al), 140 ± 118 (Mn), 4 ± 3 Co, 6 ± 4 (Ni), 1.3 ± 0.8 (Cu), 126 ± 81 (Zn), 1.1 ± 0.7 (Cd), 0.9 ± 0.5 (Tl), 2 ± 3 (Pb). Dissolved As concentrations ranged from 5 to 134 μg/L (30 ± 23 μg/L). During the survey, the concentration of colloidal metals (5 kDa < size < 0.22 μm) was less than 25% of dissolved concentrations. Dissolved metal concentrations were generally higher than the maximum concentrations allowed in European surface waters for priority substances (Ni, Cd and Pb) and higher than the environmental quality standards for other compounds. Using Diffusion Gradient in Thin Film (DGT) probes, metals were shown to be in potentially bioavailable form. The concentrations in Leuciscus cephalus were below the maximum Pb and Cd concentrations allowed in fish muscle for human consumption by the European Water Directive. Amongst the elements studied, only As, Pb and Tl were shown to bioaccumulate in liver tissue (As, Pb) or otoliths (Tl). Bioaccumulation of metals or As was not detected in muscle.  相似文献   

5.
Vertical profiles from the water column, including the maximum turbidity zone (MTZ) to the consolidated sediment were sampled in September 2000 in the freshwater reaches of the Gironde Estuary during a complete neap tide-spring tide cycle. The vertical distributions of dissolved major redox parameters and metals (Mn, Fe, Cd, Cu, V, Co, Ni, Mo, and U) were determined. Reactive particulate metal fractions were also determined from selective leaching. The studied system is characterized by density layers functioning at different time-scales, consisting of two mobile layers, i.e., the liquid (LM) and the soft mud (SM), overlying consolidated sediments (CS). This results in a three-zone diagenetic regime where (1) O2 dynamics are fast enough to show depletion in the rapidly mixed LM sequence (tidal time-scale), (2) denitrification occurs on the weekly time-scale mixing SM sequence, and (3) the Mn, Fe, and sulfate cycling occurs in the CS layer (annual time-scale). The studied trace metals show differential behavior during early diagenesis: (1) Cd, Cu, and V are released into pore water preferentially from organic matter in the SM, (2) Co, Ni, and U are released in the CS from Mn and Fe oxides during reductive dissolution, and (3) Mo from both processes. Transient conditions (i.e., oscillations of redox fronts and reoxidation processes), due to the dynamics of the mobile layers, strongly influence the trace metal distributions as inducing resolubilization (Cd, Cu, and Mo). In the CS, authigenic metal phases accumulate, either by direct precipitation with sulfides (Cu, Cd) or co-precipitation with Fe-sulfides (Mo). Microbially mediated reduction of Fe oxides is proposed to control U removal from pore water by reduction of U(VI) to U(IV) at depth. However, a significant fraction of the trace metals is trapped in the sediment in exchangeable forms, and therefore is susceptible to be mobilized due to resuspension of estuarine sediment during strong river flood periods and/or dredging activities.  相似文献   

6.
This paper presents the results of a study on the geochemistry of waters circulating in the mineralised area of the south-eastern sector of Mt. Peloritani (north-eastern Sicily, Italy), aimed at basic understanding of the geochemical processes influencing their chemical composition. Chemico-physical parameters and data on 26 major and minor chemical elements are reported for 103 water samples. Water chemistry is mainly dominated by dissolution of carbonates and hydrolysis of aluminosilicate minerals. Total dissolved salts (TDS) range from 80 to 1398 mg/L. All the waters exhibit EH characteristic of an oxygenated environment. Excluding two samples, which show very high H+ activity (pH = 3.0 and 2.7), all the waters have pH values in the range 6.2–8.6. Cluster analysis based on major ion contents defined three main chemical water types, reflecting different hydrochemical processes. The first, group I, has low salinity (average TDS = 118 ± 30 mg/L) and abundance orders (meq/L) Na > Ca ≈ Mg > K and Cl ≈ HCO3 > SO4. With increased water–rock interaction, waters in groups II and III become more saline, changing composition towards SO4–Cl-alkaline earth and HCO3-alkaline earth types. Weathering of carbonate minerals causes waters to become saturated with respect to calcite and dolomite, whereas the incongruent dissolution of aluminosilicate minerals causes the solution to reach equilibrium with kaolinite and to form smectites. Trace element geochemistry in the analysed waters reflects interactions between waters and existing mineralisation, with elemental concentrations showing highly variable values, and higher concentrations of As, Pb, Sb and Zn near known mineralisation. Lead–Zn and As–Sb statistical associations, probably distinguishing interactions with different mineralogical phase paragenesis, were revealed by factor analysis. The main aqueous chemical forms of trace elements predicted by chemical speciation calculations are also reported. As most of the analysed spring waters provide the main source of freshwater for domestic purposes, attention should be given to As and Sb, whose concentrations exceed the recommended limits.  相似文献   

7.
Non-conservative behaviour of dissolved Mo was observed during specific time periods in the water column of the Wadden Sea of NW Germany. In July 2005 dissolved Mo declined within 36 h from a level only slightly below seawater (82 nM) to a minimum value of 30 nM, whereas in August 2002 dissolved Mo revealed a tidal cyclicity with maximum values up to 158 nM at low tide. In contrast, cruises in August 2003 and 2004 displayed an almost conservative behaviour of Mo. The decrease in dissolved Mo during July 2005 and elevated values in August 2002 were accompanied by Mo enrichments on aggregates in the water column of the Wadden Sea. Along with Mo, dissolved Mn showed unusual concentration patterns in July 2005, with values distinctly below the common summer level (by a factor of five). A direct relation between the loss of Mo and scavenging by freshly formed MnOx phases could not be inferred from our data because both metals revealed inverse patterns. Parallel to decreasing dissolved Mo concentrations dissolved Mn showed an increasing trend while particulate Mn decreased. Such finding is compatible with the formation of oxygen-depleted zones in aggregates, which provide suitable conditions for the rapid fixation of Mo and parallel release of Mn by chemically and/or microbially mediated processes. Our assumption is supported by biological (e.g. number of aggregate-associated bacteria) and sedimentological (e.g. aggregate abundance and size) parameters. The production of organic components (e.g. TEP) during breakdown of an algae bloom in July 2005 led to the formation of larger Mo-enriched aggregates, thus depleting the water column in dissolved Mo. After deposition on and incorporation into sandy tidal flats these aggregates are rapidly decomposed by microbial activity. Pore water profiles document that during microbial decomposition of these aggregates, substantial amounts of Mo are released and may replenish and even enrich Mo in the open water column. We postulate a conceptual model for the observed non-conservative behaviour of Mo in coastal waters, which is based on the tight coupling of geochemical, biological, and sedimentological processes.  相似文献   

8.
This study investigates Sb speciation in sediments along the drainage of the Upper Peter adit at the Bralorne Au mine in southern British Columbia, Canada, and compares the behavior of Sb with that of As. The Upper Peter mineralization consists of native Au in quartz-carbonate veins with 1 wt.% sulfides dominated by pyrite and arsenopyrite although stibnite, the primary Sb-bearing sulfide mineral, can be locally significant. Dissolved Sb concentrations can reach up to 349 μg L−1 in the mine pool. Sediments were collected for detailed geochemical and mineralogical characterization at locations along the 350-m flow path, which includes a 100-m shallow channel within the adit, a sediment settling pond about 45 m beyond the adit portal and an open wetland another 120 m farther downstream. From the mine pool to the wetland outlet, dissolved Sb in the drainage drops from 199 μg L−1 to below the detection limit due to the combined effect of dilution and removal from solution. Speciation analyses using X-ray absorption near-edge structure (XANES) spectroscopy indicate that Sb(III)–S accounts for around 70% of total Sb in the sediments in the main pool at the far end of the adit. At a short distance (24 m) downstream of the main adit pool, however, Sb(III)–O and Sb(V)–O species represent ?50% of total Sb in the bulk sediments, indicating significant oxidation of the primary sulfides inside the adit. Although Sb appears largely oxidized in the bulk samples collected near the portal, Sb(III)–S species are nevertheless present in the <53-μm fraction, suggesting a higher oxidation rate for stibnite in the coarser grains, possibly due to galvanic interaction with pyrite. Secondary Sb species released from the sulfide oxidation are most likely sorbed/co-precipitated with Fe-, Mn-, and Al-oxyhydroxides along the flow channel in the adit and in the sediment settling pond, with the Fe phase being the dominant sink for Sb.  相似文献   

9.
Environmental contamination with As and Sb caused by past mining activities at Sb mines is a significant problem in Slovakia. This study is focused on the environmental effects of the 5 abandoned Sb mines on water, stream sediment and soil since the mines are situated in the close vicinity of residential areas. Samples of mine wastes, various types of waters, stream sediments, soils, and leachates of the mine wastes, stream sediments and selected soils were analyzed for As and Sb to evaluate their geochemical dispersion from the mines. Mine wastes collected at the mine sites contained up to 5166 mg/kg As and 9861 mg/kg Sb. Arsenic in mine wastes was associated mostly with Fe oxides, whereas Sb was present frequently in the form of individual Sb, Sb(Fe) and Fe(Sb) oxides. Waters of different types such as groundwater, surface waters and mine waters, all contained elevated concentrations of As and Sb, reaching up to 2150 μg/L As and 9300 μg/L Sb, and had circum-neutral pH values because of the buffering capacity of abundant Ca- and Mg-carbonates. The concentrations of Sb in several household wells are a cause for concern, exceeding the Sb drinking water limit of 5 μg/L by as much as 25 times. Some attenuation of the As and Sb concentrations in mine and impoundment waters was expected because of the deposition of metalloids onto hydrous ferric oxides built up below adit entrances and impoundment discharges. These HFOs contained >20 wt.% As and 1.5 wt.% Sb. Stream sediments and soils have also been contaminated by As and Sb with the peak concentrations generally found near open adits and mine wastes. In addition to the discharged waters from open adits, the significant source of As and Sb contamination are waste-rock dumps and tailings impoundments. Leachates from mine wastes contained as much as 8400 μg/L As and 4060 μg/L Sb, suggesting that the mine wastes would have a great potential to contaminate the downstream environment. Moreover, the results of water leaching tests showed that Sb was released from the solids more efficiently than As under oxidizing conditions. This might partly explain the predominance of Sb over As in most water samples.  相似文献   

10.
Trace metals (Mn, Fe, Mo, U, Cr, V) were studied in pore waters of an intertidal flat located in the German Wadden Sea. The study system is an example of a permeable tidal flat system where pore water exchange is affected by tidal driven pressure gradients besides diffusion. Permanently installed in situ samplers were used to extract pore waters down to 5 m depth throughout one year. The samplers were either located close to the tidal flat margin or in central parts of the tidal flat. Despite dynamic sedimentological and hydrological conditions, the general trends with depth in deep tidal flat pore waters are remarkably similar to those observed in deep sea environments. Rates of trace metal cycling must be comparably large in order to maintain the observed pore water profiles. Trace metals further show similar general trends with depth close to the margin and in central parts of the tidal flat. Seasonal sampling revealed that V and Cr vary concurrent with seasonal changes in dissolved organic carbon (DOC) concentration. This effect is most notable close to the tidal flat margin where sulphate, DOC, and nutrients vary with season down to some metres depth. Seasonal variations of Mn, Fe, Mo, and U are by contrast limited to the upper decimetres of the sediment. Their seasonal patterns depend on organic matter supply, redox stratification, and particulate matter deposited on sediment surfaces. Pore water sampling within one tidal cycle provides evidence for pore water advection in margin sediments. During low tide pore water flow towards the creekbank is generated by a hydraulic gradient suggesting that deep pore waters may be seeping out of creekbank sediments. Owing to the enrichment of specific elements like Mn in pore water compared to sea water, seeping pore waters may have an impact on the chemistry of the open water column. Mass balance calculations reveal that the impact of deep pore waters on the Mn budget in the open water column is below 4%. Mn deep pore water discharge of the whole Wadden Sea is estimated to be about 9% of the total dissolved riverine Mn input into the Southern North Sea.  相似文献   

11.
The distribution of redox sensitive elements (U, As,Sb, Mo and V) has been investigated in Lake Balatonand two man-made reservoirs (Lake Kis-Balaton-1 andKis-Balaton-2) built along the River Zala, the mainriver discharging into it. These elements underwentremoval in Lake Kis Balaton-2 (KB-2) during anoxicconditions (May and September).Similarities were found between the distributions ofdissolved As and Sb. Both elements had elevateddissolved concentrations in Lake Kis-Balaton-1 (KB-1)and in Lake Balaton in September. This increasedmobility could be due to a change of speciation (viareduction and the formation of methylated species).Such changes in speciation are well documented in theliterature and are generally enhanced during periodsof warmth and high primary productivity.Dissolved V and Mo distributions also showedsimilarities with higher concentrations in LakeBalaton than in other parts of the system.Although removal is not clearly detected from thedissolved concentrations, U, As, Sb and Mo aresignificantly enriched in the organic-rich sedimentsof Lake KB-1, while V is mainly associated with thealumino-silicate fraction. In Lake KB-1, theconcentrations of all elements in SPM (suspendedparticulate matter) are lower than in the depositedsediments (except for As) suggesting that enrichmentprocesses are occuring in the deposited sediments.Additionnally the cycling of As and Mo can be alsopartly controlled by uptake and scavenging processesin the water column, as suggested by the elevated Asand Mo concentrations observed in SPM seasonnally forAs in Lake KB-1 and at Z.mouth-1 station and for Mo atZ.mouth-1 station and Lake Balaton.  相似文献   

12.
A mass balance has been calculated for the elements Li, B, and Sr in the Gulf of Papua from sampling undertaken during 1993 to 1999. Parameters measured included Fly, Kikori, and Purari River inputs of dissolved and particulate phases, removal flux to sediment traps at the base of the continental shelf slope, and century-scale accumulation rates in shelf and slope sediments (derived from excess 210Pb profiles in sediment cores). About 91% of river input Li was in particulate form, and there was conservative behavior of dissolved Li in the salinity gradient of the estuaries. Li accumulation rate in inner-shelf sediments was slightly less than river inputs, suggesting that more than 90% of Li river inputs were trapped in rapid aluminosilicate mud accumulation zones of the inner shelf (<50-m depth). Li removal rate to sediment traps at the base of the slope at ∼1000-m water depth was an order of magnitude smaller than the inner-shelf sedimentation. Export of Li to deep water Coral Sea was estimated to be 1.2 × 108 mol yr−1, and this amount is equivalent to the riverine dissolved Li annual supply rate. About 66% of river input of B was in the particulate phase, and low dissolved B concentrations in freshwater were conservatively mixed with higher concentrations of B in seawater across the salinity gradient. Removal of B to inner-shelf sediments was about 83% of the total river input, indicating a small export of B (1.2 × 108 mol yr−1) to the Coral Sea. About half of the dissolved B input from rivers is sorbed to particles and trapped in inner-shelf sediments. Only 24% of river input of Sr was in particulate form, and low freshwater concentrations of dissolved Sr were conservatively mixed with higher concentrations of Sr in seawater across the salinity gradient. Only 20% of total river inputs of Sr were buried in shelf sediments, and there was a large export (7.3 × 108 mol yr−1) of Sr off the shelf to the Coral Sea. A sediment core from a rapidly accumulating mud deposition zone of the inner shelf shows twofold sympathetic variations in Li, B, and Sr/Ca supply rates over 200- to 1000-yr time intervals.  相似文献   

13.
The chemical status of ∼40 major and trace elements (TE) and organic carbon (OC) in pristine boreal rivers draining the basaltic plateau of Central Siberia (Putorana) and interstitial solutions of permafrost soils was investigated. Water samples were filtered in the field through progressively decreasing pore size (5 μm → 0.22 μm → 0.025 μm → 10 kDa → 1 kDa) using cascade frontal filtration technique. Most rivers and soil porewaters exhibit 2-5 times higher than the world average concentration of dissolved (i.e., <0.22 μm) iron (0.03-0.4 mg/L), aluminum (0.03-0.4 mg/L), OC (10-20 mg/L) and various trace elements that are usually considered as immobile in weathering processes (Ti, Zr, Ga, Y, REEs). Ultrafiltration revealed strong relationships between concentration of TE and that of colloidal Fe and Al. According to their partition during filtration and association with colloids, two groups of elements can be distinguished: (i) those weakly dependent on ultrafiltration and that are likely to be present as truly dissolved inorganic species (Li, Na, K, Si, Mn, Mo, Rb, Cs, As, Sb) or, partially (20-30%) associated with small size Fe- and Al-colloids (Ca, Mg, Sr, Ba) and to small (<1-10 kDa) organic complexes (Co, Ni, Cu, Zn), and (ii) elements strongly associated with colloidal iron and aluminum in all ultrafiltrates largely present in 1-100 kDa fraction (Ga, Y, REEs, Pb, V, Cr, Ti, Ge, Zr, Th, U). TE concentrations and partition coefficients did not show any detectable variations between different colloidal fractions for soil porewaters, suprapermafrost flow and surface streams. TE concentration measurements in river suspended particles demonstrated significant contribution (i.e., ?30%) of conventionally dissolved (<0.22 μm) forms for usually “immobile” elements such as divalent transition metals, Cd, Pb, V, Sn, Y, REEs, Zr, Hf, Th. The Al-normalized accumulation coefficients of TE in vegetation litter compared to basalts achieve 10-100 for B, Mn, Zn, As, Sr, Sn, Sb, and the larch litter degradation is able to provide the major contribution to the annual dissolved flux of most trace elements. It is hypothesized that the decomposition of plant litter in the topsoil horizon leads to Fe(III)-, Al-organic colloids formation and serves as an important source of elements in downward percolating fluids.  相似文献   

14.
The fate of arsenic in groundwater depends largely on its interaction with mineral surfaces. We investigated the kinetics of As(III) oxidation by aquifer materials collected from the USGS research site at Cape Cod, MA, USA, by conducting laboratory experiments. Five different solid samples with similar specific surface areas (0.6-0.9 m2 g−1) and reductively extractable iron contents (18-26 μmol m−2), but with varying total manganese contents (0.5-3.5 μmol m−2) were used. Both dissolved and adsorbed As(III) and As(V) concentrations were measured with time up to 250 h. The As(III) removal rate from solution increased with increasing solid manganese content, suggesting that manganese oxide is responsible for the oxidation of As(III). Under all conditions, dissolved As(V) concentrations were very low. A quantitative model was developed to simulate the extent and kinetics of arsenic transformation by aquifer materials. The model included: (1) reversible rate-limited adsorption of As(III) onto both oxidative and non-oxidative (adsorptive) sites, (2) irreversible rate-limited oxidation of As(III), and (3) equilibrium adsorption of As(V) onto adsorptive sites. Rate constants for these processes, as well as the total oxidative site densities were used as the fitting parameters. The total adsorptive site densities were estimated based on the measured specific surface area of each material. The best fit was provided by considering one fast and one slow site for each adsorptive and oxidative site. The fitting parameters were obtained using the kinetic data for the most reactive aquifer material at different initial As(III) concentrations. Using the same parameters to simulate As(III) and As(V) surface reactions, the model predictions were compared to observations for aquifer materials with different manganese contents. The model simulated the experimental data very well for all materials at all initial As(III) concentrations. The As(V) production rate was related to the concentrations of the free oxidative surface sites and dissolved As(III), as with apparent second-order rate constants of and for the fast and the slow oxidative sites, respectively. The As(III) removal rate decreased approximately by half for a pH increase from 4 to 7. The pH dependence was explained using the acid-base behavior of the surface oxidative sites by considering a surface pKa = 6.2 (I = 0). In the presence of excess surface adsorptive and oxidative sites, phosphate diminished the rate of As(III) removal and As(V) production only slightly due to its interaction with the oxidative sites. The observed As(III) oxidation rate here is consistent with previous observations of As(III) oxidation over short transport distances during field-scale transport experiments. The model developed here may be incorporated into groundwater transport models to predict arsenic speciation and transport in chemically heterogeneous systems.  相似文献   

15.
Due to the widespread contamination of groundwater resources with arsenic (As), controls on As mobility have to be identified. In this study we focused on the distribution of As in the dissolved, colloidal and particulate size fraction of experimental solutions rich in ferric iron, dissolved organic matter (DOM) and As(V). Size fractions between <5 kDa and >0.2 μm were separated by filtration and their elemental composition was analyzed. A steady-state particle size distribution with stable element concentration in the different size classes was attained within 24 h. The presence of DOM partly inhibited the formation of large Fe-(oxy)hydroxide aggregates, thus stabilized Fe in complexed and colloidal form, when initially adjusted molar Fe/C ratios in solution were <0.1. Dissolved As concentrations and the quantity of As bound to colloids (<0.2 μm) increased in the presence of DOM as well. At intermediate Fe/C ratios of 0.02-0.1, a strong correlation between As and Fe concentration occurred in all size fractions (R2 = 0.989). At Fe/C ratios <0.02, As was mainly present in the dissolved size fraction. These observations indicate that As mobility increased in the presence of DOM due to (I) competition between As and organic molecules for sorption sites on Fe particles; and (II) due to a higher amount of As bound to more abundant Fe colloids or complexes <0.2 μm in size. The amount of As contained in the colloidal size fractions also depended strongly on the initial size of the humic substance, which was larger for purified humic acids than for natural river or soil porewater samples. Arsenic in the particle size fraction >0.2 μm additionally decreased in the order of pH 4 ? 6 > 8. The presence of DOM likely increases the mobility of As in iron rich waters undergoing oxidation, a finding that has to be considered in the investigation of organic-rich terrestrial and aquatic environments.  相似文献   

16.
This study describes the hydrogeochemistry and distributions of As in groundwater from a newly investigated area of Burkina Faso. Groundwaters have been sampled from hand-pumped boreholes and dug wells close to the town of Ouahigouya in northern Burkina Faso. Although most analysed groundwaters have As concentrations of less than 10 μg L−1, they have a large range from <0.5 to 1630 μg L−1. The highest concentrations are found in borehole waters; all dug wells analysed in this study have As concentrations of <10 μg L−1. Skin disorders (melanosis, keratosis and more rare skin tumour) have been identified among the populations in three villages in northern Burkina Faso, two within the study area. Although detailed epidemiological studies have not been carried out, similarities with documented symptoms in other parts of the world suggest that these are likely to be linked to high concentrations of As in drinking water. The high-As groundwaters observed derive from zones of Au mineralisation in Birimian (Lower Proterozoic) volcano-sedimentary rocks, the Au occurring in vein structures along with quartz and altered sulphide minerals (pyrite, chalcopyrite, arsenopyrite). However, the spatial variability in As concentrations in the mineralised zones is large and the degree of testing both laterally and with depth so far is limited. Hence, concentrations are difficult to predict on a local scale. From available data, the groundwater appears to be mainly oxic and the dissolved As occurs almost entirely as As(V) although concentrations are highest in groundwaters with dissolved-O2 concentrations <2 mg L−1. The source is likely to be the oxidised sulphide minerals and secondary Fe oxides in the mineralised zones. Positive correlations are observed between dissolved As and both Mo and W which are also believed to be derived from ore minerals and oxides in the mineralised zones. The discovery of high As concentrations in some groundwaters from the Birimian rocks of northern Burkina Faso reiterates the need for reconnaissance surveys in mineralised areas of crystalline basement.  相似文献   

17.
The distribution, partition and speciation of mercury (Hg) were studied along the redox gradient of an anthropogenically perturbed tropical estuary, the Sinnamary Estuary in French Guiana. This system is a partially mixed estuary characterized by an anoxic freshwater end-member, while the marine end-member consists of the Amazon Plume.The set up of an artificial oxygenation system in the anoxic freshwater end-member generates sharp gradients of major chemical species (iron, sulfides, etc.) coupled with intense organic matter (OM) turnover. The coexistence of oxygenated waters and dissolved sulfides in an organic rich environment depicts the Upper Sinnamary Estuary (USE: part of Sinnamary Estuary under the tidal influence but upstream of the salt intrusion) as a potential site for Hg methylation. The concentrations of all mercury compounds (HgT) in the unfiltered samples (HgTUNF), in the dissolved (HgTD) and particulate (HgTP) phases of the USE average 11 ± 3, 6 ± 2 and 5 ± 3 (i.e. 600 ± 200 pmol g−1) pmol L−1, respectively. Average concentrations of monomethylmercury (MMHg) in the unfiltered (MMHgUNF), dissolved (MMHgD) and particulate (MMHgP) phases were 3.7 ± 1.0, 2.0 ± 0.9 and 1.8 ± 1.2 (i.e. 220 ± 130 pmol g−1) pmol L−1, respectively. Water oxygenation and sulfides concentrations emerged to play a critical role in controlling MMHg levels. Additionally, iron cycling, acid-base mechanisms, and redox-dependent processes were involved in the MMHg partitioning between phases.Overall, the USE constitutes a biogeochemical reactor that gathers partitioning and methylation processes. The permanent MMHg inputs from the anoxic freshwater end-member combined with the intense endogenous Hg methylation ensures high-MMHg levels in both dissolved and particulate phases. To illustrate, the USE exports 60 ± 20% more MMHgUNF than it imports: 5.5 ± 0.7 vs. 3.5 ± 1.2 kg year−1.  相似文献   

18.
The distribution of several trace elements in different aqueous fractions has been studied in running waters from Sardinia (Italy). Trace elements and major components were determined in water samples collected at high- and low-discharge from rivers (90 samples) and streams (70 samples). At selected sites, total (non filtered samples) and dissolved (0.4 μm and 0.015 μm pore-size filtered samples) amounts of trace elements were determined, and the composition of the solid matter retained on the filters was investigated for estimating the eventual interrelationship. The elements B, Li, Rb, Sr, Ba, As, Sb, Mo, Tl and U in the studied waters showed small differences between total and dissolved amounts; dissolved concentrations were higher under low flow conditions, when the contribution of rainwater to the rivers was minimum; their concentrations were often correlated with total dissolved solids (TDS), and appeared to be related to the intensity of water-rock interaction processes. The elements Al, Fe, Mn, Pb, Zn, Cd, Cu, Co, Ni, Cs, Y, REE and Th were not related to TDS and/or major ions; they showed higher concentrations under high flow conditions; marked differences occurred between total and dissolved amounts; much lower concentrations were generally observed in the water filtered through 0.015 μm than in the water filtered through 0.4 μm, especially when sampling was carried out after heavy rain events that enhanced the load of solid matter in the water. These observations indicate an aqueous transport via sorption processes on very fine particles, such as Fe-oxide/hydroxide and clay mineral particles, which have been inferred by SEM-EDX analyses of the matter retained on the filters.  相似文献   

19.
The Guadalupe Valley aquifer is the only water source for one of the most important wine industries in Mexico, and also the main public water supply for the nearby city of Ensenada. This groundwater is monitored for major ion, N-NO3, P-PO4, Fe, As, Se, Mo, Cd, Cu, Pb, Zn and Sb concentrations, as well as TDS, pH, dissolved oxygen and temperature. High concentrations of N-NO3 (26 mg l−1), Se (70 μg l−1), Mo (18 μg l−1) and Cu (4.3 μg l−1) suggest that groundwater is being polluted by the use of fertilizers only in the western section of the aquifer, known as El Porvenir graben. Unlike the sites located near the main recharge area to the East of the aquifer, the water in El Porvenir graben has low tritium concentrations (<1.9 TU), indicating a pre-modern age, and thus longer water residence time. No significant variations in water quality (generally <10%) were detected throughout 2001–2002 in the aquifer, suggesting that reduced rainfall and recharge during this dry period did not significantly affect water quality. However, the wells nearest to the main recharge area in the Eastern aquifer show a slight but constant increase in TDS with time, probably as a result of the high (∼200 L S−1) uninterrupted extraction of water at this specific recharge site. Relatively high As concentrations for the aquifer (10.5 μg l−1) are only found near the northern limit of the basin associated with a geological fault.  相似文献   

20.
A nutrient mass balance for the tidal freshwater segment of the James River was used to assess sources of nutrients supporting phytoplankton production and the importance of the tidal freshwater zone in mitigating nutrient transport to marine waters. Monthly mass balances for 2007–2010 were based on riverine inputs, local point sources (including sewer overflow events), ungauged inputs, riverine outputs, and tidal exchange. The tidal freshwater James River received exceptionally high areal loads (446 mg TN m?2 day?1 and 55 mg TP m?2 day?1) compared to other estuaries in the region and elsewhere. P inputs were principally from riverine sources (84 %) whereas point sources contributed appreciably (54 %) to high N loads. Despite high loading rates and short water residence time, areal mass retention was high (143 mg TN m?2 day?1 and 33 mg TP m?2 day?1). Retention of particulate fractions occurred during high discharge, whereas dissolved inorganic fractions were retained during low discharge when chlorophyll-a concentrations were high. On an annualized basis, P was retained more effectively (59 %) than N (32 %). P was retained by abiotic mechanisms via trapping of particulate forms, whereas N was retained through biological assimilation of dissolved inorganic forms. Results from a limited suite of stable isotope determinations suggest that DIN from point sources was preferentially retained. Combined inputs from diffuse and point sources accounted for only 20 % and 36 % (respectively) of estimated algal N and P demand, indicating that internal nutrient recycling was important to sustaining high rates of phytoplankton production in the tidal freshwater zone.  相似文献   

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