Aquatic Geochemistry最新文献

The Florida Everglades has one of the most severe methylmercury (MeHg) contamination issues in the USA, resulting from factors including high rates of atmospheric mercury (Hg) deposition and sulfate inputs from agricultural lands. Sulfate loading stimulates microbial sulfate reduction and production of toxic and bioaccumulative MeHg. Controls on regional Hg emissions have been successful in reducing Hg deposition and MeHg production in wetlands in other areas, but this has not been the case for the Everglades as the Hg deposited here appears to come from unknown global sources of emissions. We posit that reductions in sulfate loading to the Florida Everglades can be an effective alternative approach used to reduce MeHg production. This study tested this hypothesis (1) by evaluating temporal trends in MeHg concentrations in response to a reduction in sulfate loading at a site in central Water Conservation Area (WCA) 3 and (2) using ecosystem-scale models to predict the effects of reductions in sulfate loading on sulfate concentrations in surface water and MeHg Risk. At the WCA site, we report a decline in sulfate concentrations (from about 9?mg/L in the late 1990s to levels of <?1?mg/L by 2001) due to changes in water delivery as part of Everglades restoration. Concurrent with the decline in sulfate, declines in MeHg concentrations in surface water and fish and wading bird tissues were observed at this site. These results suggest the efficacy of reducing MeHg production and bioaccumulation in the ecosystem through a reduction in sulfate loading. A previously developed model was used to predict the effects of reductions in sulfate loading (97%, 33%, and 10% reduction scenarios) on sulfate concentrations in surface water and MeHg Risk in the Everglades. The model identified areas of the ecosystem where MeHg Risk is most sensitive to the reductions in sulfate loading. Results show that reductions of >?33% in sulfate loading will significantly benefit the Everglades by reducing MeHg Risk.

The reported partition coefficients of REE with calcite are reviewed and critically discussed. In some of the reported experimental sets, REE concentrations are found to be supersaturated with respect to individual REE₂(CO₃)₃ but never to REE(OH)₃. Although the solutions are unsaturated with respect to individual REY carbonates, REY including Y are incorporated in calcite surfaces, where they are overgrown by calcite. Charge balances may be obtained by building {REY–Na-(CO₃)₂}_n or by exchange of 2Ca²⁺ against REY³⁺?+?blank space calcite lattice. These surface compounds may either be homogeneously distributed or clustered. Both the size and frequency of clusters increase with [REY]/[Ca] or [ΣREY³⁺]/[Ca²⁺] in solution. If these surface precipitates are removed into solutions saturated with respect to ΣREE₂(CO₃)₃, they start growing in the aqueous phase. In this case, the apparent D_REY and k_REY values decrease with increasing REY concentrations in solution. In previous studies, only the individual distribution coefficients are reported not considering that the entirety of REY determines their behavior in partitioning. Given enough time, these surface clusters equilibrate with the aqueous phase before being overgrown by calcite. In the double logarithmic plots of {REY}/{Ca} versus [REY]/[Ca] or [REY³⁺]/[Ca²⁺], two relationships evolve characterizing the REY distribution in marine calcite and experimental calcites grown in Mg²⁺-free solutions. The double logarithmic plots of partition coefficients of REY³⁺_i in calcite grown from seawater show a slope exceeding unity, whereas those from fluids without Mg²⁺ depict slopes less than unity being both in contrast to the Henderson–Kracek rule.

Dissolved organic matter (DOM) strongly participates in a variety of critical environmental and ecological processes and has a large impact on environmental quality. In this study, ultraviolet–visible absorbance spectroscopy and excitation–emission matrices of fluorescence spectroscopy in combination with parallel factor analysis (EEMs–PARAFAC) were applied to characterize a total of 92 DOM samples extracted from four typical soil types under three different land-use regimes across China. DOC concentrations ranged from 6.52?±?1.09 to 25.62?±?4.83?mg?L^?1 and were generally higher in red soil from Guangdong and Guangxi, especially in paddy soil. Three fluorescence components were identified in soil DOM by EEMs–PARAFAC, including high molecular weight UVA humic-like substances (C1), low molecular weight autochthonous humic-like substances (C2), and protein-like substances (C3). DOM from black soil in Heilongjiang, purple soil from Sichuan, and red soil from Zhejiang had more humic-like substances, whereas DOM from yellow soil in Guizhou and red soil from Guangdong and Guangxi had lower degree of aromaticity and higher proportion of microbial-derived protein-like components (C3). Moreover, DOM from paddy soil tended to be more of protein-like components (C3) than that from other land uses and DOM from dryland soil generally had more autochthonous humic-like substances (C2). Our results demonstrated that soil DOM characteristics both varied significantly by soil type and land use, and EEMs–PARAFAC could be a useful approach to characterize the components and sources of heterogeneous DOM in soils.

This study presents major element geochemistry, mineral magnetism and textural analysis of lake-bottom sediments collected from the Wular Lake located in the Kashmir Valley (northwest Himalaya). Connected to the Jhelum River, Wular Lake basin occupies?~?75% watershed area of the valley, covering?~?10,196?km², and makes it the major depo-centre for Kashmir Valley. The sediments represent dominant silt fraction with TiO₂, MgO, Fe₂O₃ and MnO enrichment; and depletion of Na₂O and P₂O₅ with reference to upper continental crust. The average CIA (64.17%) reflects moderate chemical weathering of the catchment source rocks. The mineral magnetic mapping identified distinct domains of restricted/reducing and?well-ventilated/oxic conditions, illustrating spatial environmental variability within the lake-bottom environments. The geochemical and mineral magnetic mapping therefore provides a baseline reference for emerging climate and anthropogenic changes being experienced in the Kashmir Valley.

Stream water and associated bottom sediments were sampled within the Lom Basin, and their rare earth element (REE) concentrations have been used to decipher their provenance and environment of deposition. Stream waters in the lower Lom Basin have variable Post Archean Australian Shale-normalized REE concentrations (0.24–4978?ng/l), positive Ce anomalies (Ce/Ce* ranges from 1.08 to 8.03), a general positive Eu anomaly (Eu/Eu* varies from 0.9 to 15.2, average?=?4.9) and are slightly enriched in the light rare earth elements (LREE/HREE varies from 1.7 to 10.3). Similarly, the sediments are slightly depleted in heavy rare earth elements (HREE), have a predominant negative Ce anomaly, but show a ubiquitous positive Eu anomaly. The dissolved REE content is controlled by the near-neutral pH of the stream water and adsorption onto Fe and Mn oxyhydroxides. The variable LREE/HREE ratios and Eu anomalies observed in the sediments indicate that these sediments are sourced mainly from felsic rocks with little mafic input. Moreover, Ce anomalies and redox-sensitive trace elemental ratios of Ni/Co, V/(V?+?Ni), V/Cr, Cu/Zn and V/Ni revealed the sediments were deposited under oxic to reducing conditions. Variations in the concentration of REE in the stream water and sediment indicate that Fe and Mn oxyhydroxides are important sinks for the HREE. The newly generated data will guide future studies and environmental policy makers.

The Lake Chad Basin (LCB) is one of the main endorheic basins in the world and has undergone large-level and surface variations during the last decades, particularly during the Sahelian dry period in the 1970s and the 1980s. The Chari–Logone River system covers 25% of the LCB but accounts for up to 82% of the Lake Chad water supply. The aim of this study is to investigate the dissolved phase transported by the Chari–Logone system, in order (1) to elucidate the origin and the behavior of major elements and the weathering processes in the watershed; (2) to estimate the total dissolved flux, its variability over the last decades and the driving factors. To do so, samples were collected monthly between January 2013 and November 2016 at three representative sites of the basin: in the Chari River in “Chagoua,” in the Logone River in “Ngueli” just before the confluence of both rivers, and at a downstream site in “Douguia,” 30?km after the confluence. Concentrations in major elements displayed significant seasonal variations in the Chari and Logone waters. At the seasonal time scale, the comparison between the concentrations of chemical elements and the flow rates showed a hysteresis loop. This hysteresis behavior corresponds to a variable contribution over time of two water bodies, fast surface water, and slow groundwater, the latter carrying higher concentrations and Ca/Na ratio, which may result from the contribution of pedogenic carbonate weathering to the dominant signature of silicate weathering. At the annual time scale,?similar average concentrations are observed in the Chari and Logone Rivers, despite contrasted annual runoff. In addition, an interannual stability of ionic concentrations was observed in the Chari–Logone River during the flood regime, both during the years covered by our monitoring (2013–2016) and during the pre-drought period (1969, 1972 and 1973). This situation corresponds to a chemostatic behavior, where the annual river discharge is the main factor controlling the interannual variation of chemical fluxes.

The aim of this research was to investigate the competition between methanogens and sulfate-reducing bacteria in hypersaline environments. Samples of photosynthetic microbial mats, both soft mats (salinities of 55–126?ppt) and gypsum-hosted endoevaporite mats (salinities of 77–320?ppt), were obtained from hypersaline environments in California, USA, Mexico and Chile. Methane production was determined from the increase in headspace methane concentration within incubation vials containing mat samples. At the end of the incubation period, the δ¹³C values of produced methane were measured. Soft microbial mat vials containing molybdate, a specific inhibitor of bacterial sulfate reduction, exhibited dramatically higher methane production rates and higher (enriched in ¹³C) methane δ¹³C values than the controls. This suggests that the inhibition of sulfate reduction allowed the methanogens at these sites to use the competitive substrates (H₂ and/or acetate) made available. Further, the higher δ¹³C values of the produced methane suggest that substrates (both competitive and non-competitive) were used to near completion. At the endoevaporite sites, which have much higher salinities than the soft mat sites, methane production was not significantly different and the methane δ¹³C values either remained the same or decreased (depleted in ¹³C) with added molybdate. We suggest that substrate availability increased enough to allow for somewhat greater isotopic fractionation resulting in the lower methane δ¹³C values that were observed, but not enough to significantly increase measured production rates. Where no changes in either methane production rates or δ¹³C values occurred, we hypothesize that salinity itself was inhibiting sulfate reduction and thus controlling microbe populations and rates of metabolism.

Reactions between cyanide and compounds, which contain S–S bonds, in aqueous media result in formation of thiocyanate. In this work, we studied the kinetics of reactions of thiosulfate with free cyanide and its complexes under environmental conditions. Rates of reactions between cyanide species and thiosulfate decrease in the following order: CN^??>?HCN?>?[Fe(CN)₆]^3??>?[Fe(CN)₆]^4?. However, at neutral and slightly acidic pH range, reaction of thiosulfate with iron-cyanide complexes outcompetes its reaction with free cyanide, which exists in equilibrium with complexed cyanide. At environmentally relevant conditions, the characteristic time of reaction between free cyanide and thiosulfate was found to be tens of thousands of years, while for iron-cyanide complexes it was found to be hundreds to millions of years. Examples of application of kinetic parameters for calculation of rates of cyanide consumption in industrial (coke oven wastewater) and non-polluted natural aquatic system (Delaware Great Marsh) are provided.

We have studied iron (Fe)-isotope signals in particles (>?0.22?μm) and the dissolved phase (<?0.22?μm) in two subarctic, boreal rivers, their estuaries and the adjacent sea in northern Sweden. Both rivers, the R?ne and the Kalix, are enriched in Fe and organic carbon (up to 29?μmol/L and up to 730?μmol/L, respectively). Observed changes in the particulate and dissolved phase during spring flood in May suggest different sources of Fe to the rivers during different seasons. While particles show a positive Fe-isotope signal during winter, during spring flood, the values are negative. Increased discharge due to snowmelt in the boreal region is most times accompanied by flushing of the organic-rich sub-surface layers. These upper podzol soil layers have been shown to be a source for Fe-organic carbon aggregates with a negative Fe-isotope signal. During winter, the rivers are mostly fed by deep groundwater, where Fe occurs as Fe(oxy)hydroxides, with a positive Fe-isotope signal. Flocculation during initial estuarine mixing does not change the Fe-isotope compositions of the two phases. Data indicate that the two groups of Fe aggregates flocculate diversely in the estuaries due to differences in their surface structure. Within the open sea, the particulate phase showed heavier δ⁵⁶Fe values than in the estuaries. Our data indicate the flocculation of the negative Fe-isotope signal in a low salinity environment, due to changes in the ionic strength and further the increase of pH.

The nature of groundwater recharge along the mountain front (MF) and riparian zone (RZ) was discerned by multiple tools involving rain/water level relationship, geophysical of subsurface, seasonal hydrochemistry and environmental isotopic signatures. The proposed study has been carried out in Courtallam Hills, the north-western part of Tirunelveli District, South India. The study area is a hilly terrain with narrow valleys endowed with steep slopes. The relationship between water-level fluctuation and precipitation were evaluated by observing daily water level in 8-h interval at three piezometer zones and regular rainfall data.?It was inferred that the RZ played a role in storage zone and gets recharged from mountain block (MB) and lateral flow. The seasonal geochemistry of the groundwater was studied to determine the sources of recharge in MF and RZ. Geostatistical treatment of factor analysis revealed that weathering was the dominant recharge process along the foothill. The geophysical studies reveal good quality of groundwater observed in the northern part along with low conductance and high resistivity. The increased level of groundwater conductivity and lower resistivity was noted in southern part of the study area due to the irrigation activities. The isotopic tracers range from ??2.5 to ??12.6‰ for δ¹⁸O and from ??91.2 to ??15.5‰ for δ²H. Moreover, the groundwater recharge was evaluated by source of rainfall moisture. High-altitude recharge from MB along the MF was clearly indicated by depleted isotopic content of the water samples. It was also supported by hydrogeochemical and statistical evidences, showing that rainfall over both MB and MF zones provided the recharge to foothill aquifers, while the RZ zone was mainly recharged by local precipitation with less contribution from regional flows.