Geochemical Transactions最新文献

{"title":"Fe(II) reduction of pyrolusite (β-MnO2) and secondary mineral evolution","authors":"Michael V. Schaefer,&nbsp;Robert M. Handler,&nbsp;Michelle M. Scherer","doi":"10.1186/s12932-017-0045-0","DOIUrl":"https://doi.org/10.1186/s12932-017-0045-0","url":null,"abstract":"<p>Iron (Fe) and manganese (Mn) are the two most common redox-active elements in the Earth’s crust and are well known to influence mineral formation and dissolution, trace metal sequestration, and contaminant transformations in soils and sediments. Here, we characterized the reaction of aqueous Fe(II) with pyrolusite (β-MnO₂) using electron microscopy, X-ray diffraction, aqueous Fe and Mn analyses, and ⁵⁷Fe M?ssbauer spectroscopy. We reacted pyrolusite solids repeatedly with 3?mM Fe(II) at pH 7.5 to evaluate whether electron transfer occurs and to track the evolving reactivity of the Mn/Fe solids. We used Fe isotopes (56 and 57) in conjunction with ⁵⁷Fe M?ssbauer spectroscopy to isolate oxidation of Fe(II) by Fe(III) precipitates or pyrolusite. Using these complementary techniques, we determined that Fe(II) is initially oxidized by pyrolusite and that lepidocrocite is the dominant Fe oxidation product. Additional Fe(II) exposures result in an increasing proportion of magnetite on the pyrolusite surface. Over a series of nine 3?mM Fe(II) additions, Fe(II) continued to be oxidized by the Mn/Fe particles suggesting that Mn/Fe phases are not fully passivated and remain redox active even after extensive surface coverage by Fe(III) oxides. Interestingly, the initial Fe(III) oxide precipitates became further reduced as Fe(II) was added and additional Mn was released into solution suggesting that both the Fe oxide coating and underlying Mn phase continue to participate in redox reactions when freshly exposed to Fe(II). Our findings indicate that Fe and Mn chemistry is influenced by sustained reactions of Fe(II) with Mn/Fe oxides.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"18 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2017-12-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-017-0045-0","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4201178","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Interactions between magnetite and humic substances: redox reactions and dissolution processes","authors":"Anneli Sundman,&nbsp;James M. Byrne,&nbsp;Iris Bauer,&nbsp;Nicolas Menguy,&nbsp;Andreas Kappler","doi":"10.1186/s12932-017-0044-1","DOIUrl":"https://doi.org/10.1186/s12932-017-0044-1","url":null,"abstract":"<p>Humic substances (HS) are redox-active compounds that are ubiquitous in the environment and can serve as electron shuttles during microbial Fe(III) reduction thus reducing a variety of Fe(III) minerals. However, not much is known about redox reactions between HS and the mixed-valent mineral magnetite (Fe₃O₄) that can potentially lead to changes in Fe(II)/Fe(III) stoichiometry and even dissolve the magnetite. To address this knowledge gap, we incubated non-reduced (native) and reduced HS with four types of magnetite that varied in particle size and solid-phase Fe(II)/Fe(III) stoichiometry. We followed dissolved and solid-phase Fe(II) and Fe(III) concentrations over time to quantify redox reactions between HS and magnetite. Magnetite redox reactions and dissolution processes with HS varied depending on the initial magnetite and HS properties. The interaction between biogenic magnetite and reduced HS resulted in dissolution of the solid magnetite mineral, as well as an overall reduction of the magnetite. In contrast, a slight oxidation and no dissolution was observed when native and reduced HS interacted with 500?nm magnetite. This variability in the solubility and electron accepting and donating capacity of the different types of magnetite is likely an effect of differences in their reduction potential that is correlated to the magnetite Fe(II)/Fe(III) stoichiometry, particle size, and crystallinity. Our study suggests that redox-active HS play an important role for Fe redox speciation within minerals such as magnetite and thereby influence the reactivity of these Fe minerals and their role in biogeochemical Fe cycling. Furthermore, such processes are also likely to have an effect on the fate of other elements bound to the surface of Fe minerals.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"18 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2017-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-017-0044-1","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4774629","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A potential nitrogen sink discovered in the oxygenated Chukchi Shelf waters of the Arctic","authors":"Jian Zeng,&nbsp;Min Chen,&nbsp;Minfang Zheng,&nbsp;Wangjiang Hu,&nbsp;Yusheng Qiu","doi":"10.1186/s12932-017-0043-2","DOIUrl":"https://doi.org/10.1186/s12932-017-0043-2","url":null,"abstract":"<p>The western Arctic Shelf has long been considered as an important sink of nitrogen because high primary productivity of the shelf water fuels active denitrification within the sediments, which has been recognized to account for all the nitrogen (N) removal of the Pacific water inflow. However, potentially high denitrifying activity was discovered within the oxygenated Chukchi Shelf water during our summer expedition. Based on ¹⁵N-isotope pairing incubations, we estimated denitrification rates ranging from 1.8?±?0.4 to 75.9?±?8.7?nmol?N₂ L^?1?h^?1. We find that the spatial pattern of denitrifying activity follows well with primary productivity, which supplies plentiful fresh organic matter, and there was a strong correlation between integrated denitrification and integrated primary productivity. Considering the active hydrodynamics over the Chukchi Shelf during summer, resuspension of benthic sediment coupled with particle-associated bacteria induces an active denitrification process in the oxic water column. We further extrapolate to the whole Chukchi Shelf and estimate an N removal flux from this cold Arctic shelf water to be 12.2 Tg-N?year^?1, which compensates for the difference between sediment cores incubation (~?3 Tg-N?year^?1) and geochemical estimation based on N deficit relative to phosphorous (~?16 Tg-N?year^?1). We infer that dynamic sediment resuspension combined with high biological productivity stimulates intensive denitrification in the water column, potentially creating a nitrogen sink over the shallow Arctic shelves that have previously been unrecognized.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"18 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2017-09-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-017-0043-2","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4806233","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Salinization and arsenic contamination of surface water in southwest Bangladesh","authors":"John C. Ayers,&nbsp;Gregory George,&nbsp;David Fry,&nbsp;Laura Benneyworth,&nbsp;Carol Wilson,&nbsp;Leslie Auerbach,&nbsp;Kushal Roy,&nbsp;Md. Rezaul Karim,&nbsp;Farjana Akter,&nbsp;Steven Goodbred","doi":"10.1186/s12932-017-0042-3","DOIUrl":"https://doi.org/10.1186/s12932-017-0042-3","url":null,"abstract":"<p>To identify the causes of salinization and arsenic contamination of surface water on an embanked island (i.e., polder) in the tidal delta plain of SW Bangladesh we collected and analyzed water samples in the dry (May) and wet (October) seasons in 2012–2013. Samples were collected from rice paddies (wet season), saltwater ponds used for brine shrimp aquaculture (dry season), freshwater ponds and tidal channels (both wet and dry season), and rainwater collectors. Continuous measurements of salinity from March 2012 to February 2013 show that tidal channel water increases from ~0.15 ppt in the wet season up to ~20 ppt in the dry season. On the polder, surface water exceeds the World Health Organization drinking water guideline of 10?μg As/L in 78% of shrimp ponds and 27% of rice paddies, raising concerns that produced shrimp and rice could have unsafe levels of As. Drinking water sources also often have unsafe As levels, with 83% of tubewell and 43% of freshwater pond samples having &gt;10?μg As/L. Water compositions and field observations are consistent with shrimp pond water being sourced from tidal channels during the dry season, rather than the locally saline groundwater from tubewells. Irrigation water for rice paddies is also obtained from the tidal channels, but during the wet season when surface waters are fresh. Salts become concentrated in irrigation water through evaporation, with average salinity increasing from 0.43 ppt in the tidal channel source to 0.91 ppt in the rice paddies. Our observations suggest that the practice of seasonally alternating rice and shrimp farming in a field has a negligible effect on rice paddy water salinity. Also, shrimp ponds do not significantly affect the salinity of adjacent surface water bodies or subjacent groundwater because impermeable shallow surface deposits of silt and clay mostly isolate surface water bodies from each other and from the shallow groundwater aquifer. Bivariate plots of conservative element concentrations show that all surface water types lie on mixing lines between dry season tidal channel water and rainwater, i.e., all are related by varying degrees of salinization. High As concentrations in dry season tidal channel water and shrimp ponds likely result from groundwater exfiltration and upstream irrigation in the dry season. Arsenic is transferred from tidal channels to rice paddies through irrigation. Including groundwater samples from the same area (Ayers et al. in Geochem Trans 17:1–22, 2016), principal components analysis and correlation analysis reveal that salinization explains most variation in surface water compositions, whereas progressive reduction of buried surface water by dissolved organic carbon is responsible for the nonconservative behavior of S, Fe, and As and changes in Eh and alkalinity of groundwater.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"18 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2017-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-017-0042-3","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4478518","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Weakly bound water structure, bond valence saturation and water dynamics at the goethite (100) surface/aqueous interface: ab initio dynamical simulations","authors":"Ying Chen,&nbsp;Eric J. Bylaska,&nbsp;John H. Weare","doi":"10.1186/s12932-017-0040-5","DOIUrl":"https://doi.org/10.1186/s12932-017-0040-5","url":null,"abstract":"<p>Many important geochemical and biogeochemical reactions occur in the mineral/formation water interface of the highly abundant mineral, goethite [α-Fe(OOH)]. Ab initio molecular dynamics (AIMD) simulations of the goethite α-FeOOH (100) surface and the structure, water bond formation and dynamics of water molecules in the mineral/aqueous interface are presented. Several exchange correlation functionals were employed (PBE96, PBE96?+?Grimme, and PBE0) in the simulations of a (3?×?2) goethite surface with 65 absorbed water molecules in a 3D-periodic supercell (a?=?30??, FeOOH slab ~12?? thick, solvation layer ~18?? thick).</p><p>The lowest energy goethite (100) surface termination model was determined to have an exposed surface Fe³⁺ that was loosely capped by a water molecule and a shared hydroxide with a neighboring surface Fe³⁺. The water molecules capping surface Fe³⁺ ions were found to be loosely bound at all DFT levels with and without Grimme corrections, indicative that each surface Fe³⁺ was coordinated with only five neighbors. These long bonds were supported by bond valence theory calculations, which showed that the bond valence of the surface Fe³⁺ was saturated and surface has a neutral charge. The polarization of the water layer adjacent to the surface was found to be small and affected only the nearest water. Analysis by density difference plots and localized Boys orbitals identified three types of water molecules: those loosely bound to the surface Fe³⁺, those hydrogen bonded to the surface hydroxyl, and bulk water with tetrahedral coordination. Boys orbital analysis showed that the spin down lone pair orbital of the weakly absorbed water interact more strongly with the spin up Fe³⁺ ion. These weakly bound surface water molecules were found to rapidly exchange with the second water layer (~0.025?exchanges/ps) using a dissociative mechanism.</p><p>Water molecules adjacent to the surface were found to only weakly interact with the surface and as a result were readily able to exchange with the bulk water. To account for the large surface Fe–OH₂ distances in the DFT calculations it was proposed that the surface Fe³⁺ atoms, which already have their bond valence fully satisfied with only five neighbors, are under-coordinated with respect to the bulk coordination.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"18 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2017-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-017-0040-5","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5180548","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Global hydrogen reservoirs in basement and basins","authors":"John Parnell,&nbsp;Nigel Blamey","doi":"10.1186/s12932-017-0041-4","DOIUrl":"https://doi.org/10.1186/s12932-017-0041-4","url":null,"abstract":"<p>Hydrogen is known to occur in the groundwaters of some ancient cratons. Where associated gases have been dated, their age extends up to a billion years, and the hydrogen is assumed also to be very old. These observations are interpreted to represent the radiolysis of water and hydration reactions and migration of hydrogen into fracture systems. A hitherto untested implication is that the overwhelming bulk of the ancient low-permeability basement, which is not adjacent to cross-cutting fractures, constitutes a reservoir for hydrogen.</p><p>New data obtained from cold crushing to liberate volatiles from fluid inclusions confirm that granites and gneiss of Archean and Palaeoproterozoic (&gt;1600?Ma) age typically contain an order of magnitude greater hydrogen in their entrained fluid than very young (&lt;200?Ma) granites. Sedimentary rocks containing clasts of old basement also include a greater proportion of hydrogen than the young granites.</p><p>The data support the case for a global reservoir of hydrogen in both the ancient basement and in the extensive derived sediments. These reservoirs are susceptible to the release of hydrogen through a variety of mechanisms, including deformation, attrition to reduce grain size and diagenetic alteration, thereby contributing to the hydrogen required by chemolithoautotrophs in the deep biosphere.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"18 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2017-03-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-017-0041-4","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4801315","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A survey of photogeochemistry","authors":"Timothy A. Doane","doi":"10.1186/s12932-017-0039-y","DOIUrl":"https://doi.org/10.1186/s12932-017-0039-y","url":null,"abstract":"<p>The participation of sunlight in the natural chemistry of the earth is presented as a unique field of study, from historical observations to prospects for future inquiry. A compilation of known reactions shows the extent of light-driven interactions between naturally occurring components of land, air, and water, and provides the backdrop for an outline of the mechanisms of these phenomena. Catalyzed reactions, uncatalyzed reactions, direct processes, and indirect processes all operate in natural photochemical transformations, many of which are analogous to well-known biological reactions. By overlaying photochemistry and surface geochemistry, complementary approaches can be adopted to identify natural photochemical reactions and discern their significance in the environment.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"18 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2017-02-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-017-0039-y","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4416571","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"How long do natural waters “remember” release incidents of Marcellus Shale waters: a first order approximation using reactive transport modeling","authors":"Zhang Cai,&nbsp;Li Li","doi":"10.1186/s12932-016-0038-4","DOIUrl":"https://doi.org/10.1186/s12932-016-0038-4","url":null,"abstract":"<p>Natural gas production from the Marcellus Shale formation has significantly changed energy landscape in recent years. Accidental release, including spills, leakage, and seepage of the Marcellus Shale flow back and produced waters can impose risks on natural water resources. With many competing processes during the reactive transport of chemical species, it is not clear what processes are dominant and govern the impacts of accidental release of Marcellus Shale waters (MSW) into natural waters. Here we carry out numerical experiments to explore this largely unexploited aspect using cations from MSW as tracers with a focus on abiotic interactions between cations released from MSW and natural water systems. Reactive transport models were set up using characteristics of natural water systems (aquifers and rivers) in Bradford County, Pennsylvania. Results show that in clay-rich sandstone aquifers, ion exchange plays a key role in determining the maximum concentration and the time scale of released cations in receiving natural waters. In contrast, mineral dissolution and precipitation play a relatively minor role. The relative time scales of recovery τ_rr, a dimensionless number defined as the ratio of the time needed to return to background concentrations over the residence time of natural waters, vary between 5 and 10 for Na, Ca, and Mg, and between 10 and 20 for Sr and Ba. In rivers and sand and gravel aquifers with negligible clay, τ_rr values are close to 1 because cations are flushed out at approximately one residence time. These values can be used as first order estimates of time scales of released MSW in natural water systems. This work emphasizes the importance of clay content and suggests that it is more likely to detect contamination in clay-rich geological formations. This work highlights the use of reactive transport modeling in understanding natural attenuation, guiding monitoring, and predicting impacts of contamination for risk assessment.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"17 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2016-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-016-0038-4","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4527401","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Oxidation of arsenite to arsenate on birnessite in the presence of light","authors":"Samantha L. Shumlas,&nbsp;Soujanya Singireddy,&nbsp;Akila C. Thenuwara,&nbsp;Nuwan H. Attanayake,&nbsp;Richard J. Reeder,&nbsp;Daniel R. Strongin","doi":"10.1186/s12932-016-0037-5","DOIUrl":"https://doi.org/10.1186/s12932-016-0037-5","url":null,"abstract":"<p>The effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. X-ray absorption spectroscopy (XAS) was used to determine the nature of the adsorbed product on the surface of the birnessite. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (k_obs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04?h^?1, respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Post-reaction XAS analysis of As(III) exposed birnessite showed that arsenic was present on the surface as As(V). Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"17 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2016-10-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-016-0037-5","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4277928","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Sources of salinity and arsenic in groundwater in southwest Bangladesh","authors":"John C. Ayers,&nbsp;Steven Goodbred,&nbsp;Gregory George,&nbsp;David Fry,&nbsp;Laura Benneyworth,&nbsp;George Hornberger,&nbsp;Kushal Roy,&nbsp;Md. Rezaul Karim,&nbsp;Farjana Akter","doi":"10.1186/s12932-016-0036-6","DOIUrl":"https://doi.org/10.1186/s12932-016-0036-6","url":null,"abstract":"<p>High salinity and arsenic (As) concentrations in groundwater are widespread problems in the tidal deltaplain of southwest Bangladesh. To identify the sources of dissolved salts and As, groundwater samples from the regional shallow Holocene aquifer were collected from tubewells during the dry (May) and wet (October) seasons in 2012–2013. Thirteen drill cores were logged and 27 radiocarbon ages measured on wood fragments to characterize subsurface stratigraphy.</p><p>Drill cuttings, exposures in pits and regional studies reveal a?&gt;5?m thick surface mud cap overlying a?~30?m thick upper unit of interbedded mud and fine sand layers, and a coarser lower unit up to 60?m thick dominated by clean sands, all with significant horizontal variation in bed continuity and thickness. This thick lower unit accreted at rates of?~2?cm/year through the early Holocene, with local subsidence or compaction rates of 1–3?mm/year. Most tubewells are screened at depths of 15–52?m in sediments deposited 8000–9000 YBP. Compositions of groundwater samples from tubewells show high spatial variability, suggesting limited mixing and low and spatially variable recharge rates and flow velocities. Groundwaters are Na–Cl type and predominantly sulfate-reducing, with specific conductivity (SpC) from 3 to 29 mS/cm, high dissolved organic carbon (DOC) 11–57?mg/L and As 2–258?ug/L, and low sulfur (S) 2–33?mg/L.</p><p>Groundwater compositions can be explained by burial of tidal channel water and subsequent reaction with dissolved organic matter, resulting in anoxia, hydrous ferric oxide (HFO) reduction, As mobilization, and sulfate (SO₄) reduction and removal in the shallow aquifer. Introduction of labile organic carbon in the wet season as rice paddy fertilizer may also cause HFO reduction and As mobilization. Variable modern recharge occurred in areas where the clay cap pinches out or is breached by tidal channels, which would explain previously measured ¹⁴C groundwater ages being less than depositional ages. Of samples collected from the shallow aquifer, Bangladesh Government guidelines are exceeded in 46?% for As and 100?% for salinity.</p>","PeriodicalId":12694,"journal":{"name":"Geochemical Transactions","volume":"17 1","pages":""},"PeriodicalIF":2.3,"publicationDate":"2016-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1186/s12932-016-0036-6","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4460155","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}