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Applied Geochemistry
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Item-typ:Veröffentlichung, Geogenic As and Mo groundwater contamination caused by an abundance of domestic supply wellsLacking a connection to a municipal water supply, each household in the municipality of Lithia, approximately 30 km southeast of Tampa, Florida (USA), is responsible for its own supply of drinking water, causing a high-density of private domestic supply wells (DSW) in this area. There, a multilayered aquifer system exists, which can be subdivided into three distinct hydro stratigraphic units, which are, from the top down: the Surficial Aquifer System (SAS), the Intermediate Aquifer System (IAS), and the Upper Floridan Aquifer System (UFA). Despite the relatively small area, the geochemical and hydrogeological setting in Lithia is complex, consisting of: i) extensive cyclical pumping in a municipal well field to the west, ii) large seasonal changes in hydraulic head, ii) multiple aquifers with different hydraulic heads, and iv) a large density of domestic supply wells (DSW). Within the zone of highest As concentrations, there are approximately 100 wells in an area of 2.5 km × 1.5 km. Most of these wells have large open screened intervals, often open to all three aquifers, allowing the downward flow of oxygenated and upward flow of anoxic groundwater. A survey of groundwater quality found that As and Mo concentrations in the DSW were up to 371 μg/L and 4740 μg/L, respectively. To obtain information about the individual aquifers, 5 well clusters with 4 monitoring intervals (approximately 50 m, 65 m, 80 m and 95 m below surface) and 8 push core wells (approximately 9 m below surface) were installed and sampled. In those wells, As and Mo were only elevated in a permeable layer within the IAS at a depth of 50 m. Values were up to 195 μg/L for As and up to 5050 μg/L for Mo. Using the tritium-helium (3H–3He) method, the ages of those samples high in As and Mo were determined to be 40, 30 and 30 years, respectively, while all other samples had ages older than 50 years. This indicated that mixing between young and old groundwater could be responsible for the high As and Mo concentrations. A good negative correlation for the whole data set was also observed between the concentration 3H and δ18O values, which together with hydrogeological modeling confirmed that the increased permeability created by the high density of DSW resulted in flow paths that permitted the perpetual mixing of shallow and deep groundwater. The release of the As and Mo appeared to be a consequence of changes to the physicochemical conditions in the aquifer, either via the introduction of oxygen-rich fluids into the IAS or the mixing of different fluids in the IAS or the introduction of oxygen-depleted fluids into the IAS. While the mobilization of geogenic trace metals is often associated with pumping-induced hydraulic gradient changes, we found that a certain density of multi-aquifer wells can be sufficient to alter hydrologic flow paths and induce the mobilization of geogenic trace metals even in the absence of significant pumping. In Lithia, the DSW effectively increased the local scale permeability of the aquifer, causing the mixing of oxygen-rich surface and deeper anoxic groundwater across a confining unit. Because the alteration to the hydrologic flow paths was a consequence of changes to the physical structure of the aquifer system rather than due to pumping, the alteration is not easily reversible, thus significantly complicating site remediation. Our results provide a cautionary warning of the risks of undue private DSW development in rapidly growing communities.Wissenschaftlicher ArtikelBand:7768 87 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Distribution and mobility of geogenic molybdenum and arsenic in a limestone aquifer matrixTo investigate the potential of Mo and As as possible geogenic contaminants, three sediment cores were examined to evaluate their mineralogical association, distribution and mobility. The cores were described and analyzed for total organic carbon (TOC), Ca, Mg, Si, Al, P, Sr, As, Mo, Fe, and S content. Except in the uppermost segment, limestone was the main lithology with the occasional presence of dolomite and clay. That change in lithology was also observed in the bulk chemical composition, where Ca, Mg and Sr concentrations increased with depth, while Si, Al and P concentrations decreased with depth. Minor minerals included pyrite (FeS2), powellite (CaMoO4) and ferrihydrite. The minimum, maximum, median and standard deviations for all analyzed elements, including As and Mo were comparable for all three cores. Molybdenum and As, however, varied significantly with depth and median As and Mo values were above their respective crustal averages of approximately 1.1 mg/kg and 1.5 mg/kg. The median values for As were 1.9 mg/kg in core DEP-1, 3.3 mg/kg in DEP-2 and 1 mg/kg in DEP-5. The median values for Mo were 2.3 mg/kg in core DEP-1, 2.5 mg/kg in DEP-2 and 2.5 mg/kg in DEP-5. Maximum concentrations for As were 101.9 mg/kg, 47.5 mg/kg and 56.2 mg/kg in cores DEP-1, DEP-2 and DEP-5, respectively. Maximum concentrations for Mo were 880 mg/kg, 123 mg/kg and 225 mg/kg in cores DEP-1, DEP-2 and DEP-5, respectively. Electron microprobe analyses of individual minerals revealed variable concentrations of As ranging from approximately 300 to 9000 mg/kg, in pyrite and up to 17,600 mg/kg in powellite (CaMoO4). The Mo concentration in pyrite was consistently below the detection limit of approximately 100 mg/kg. In powellite the Mo concentration was up to 42 wt%. A subset of 10 samples from different stratigraphic sections and with different As and Mo concentrations was further investigated to assess As and Mo mobility under changing physicochemical conditions. Leaching the aquifer matrix with a 1 M NaOAc solution at a pH of 8.1 removed more than 70% Mo in 8 of the 10 samples. The maximum value was 97%. In contrast to Mo, As was mobilized to a lesser degree. In 8 of the samples less than 30% were removed and the maximum was only 50%. Molybdenum, which seemed to be loosely bound to mineral and organic matter surfaces thus could easily be removed from the aquifer matrix, while As on the other hand should be much less mobile, because it occurred either tightly adsorbed by hydrous ferric oxide or as an impurity in pyrite. Thus, it is advisable to include Mo in the analytical program whenever elevated As concentrations are encountered in groundwater.Wissenschaftlicher ArtikelBand:6370 79 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Understanding arsenic behavior in carbonate aquifers: Implications for aquifer storage and recovery (ASR)Geochemical reactive transport modeling was coupled to bench-scale leaching experiments to investigate and verify the mobilization of geogenic arsenic (As) under a range of redox conditions from an arsenic-rich pyrite bearing limestone aquifer. Modeling and experimental observations showed similar results and confirmed the following: (1) native groundwater and aquifer matrix, including pyrite, were in chemical equilibrium, thus preventing the release of As due to pyrite dissolution under ambient conditions; (2) mixing of oxygen- and nitrate-rich surface water with oxygen-depleted native groundwater changed the redox conditions and promoted the dissolution of pyrite, and (3) the behavior of As along a flow path was controlled by a complex series of interconnected reactions. This included the oxidative dissolution of pyrite and simultaneous sorption of As onto neo-formed hydrous ferric oxides (HFO), followed by the reductive dissolution of HFO and secondary release of adsorbed As under reducing conditions. Arsenic contamination of drinking water in these systems is thus controlled by the re-equilibration of the system to more reducing conditions rather than a purely oxidative process.Wissenschaftlicher ArtikelBand:5261 44 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Enhanced geochemical gradients in a marine shallow-water hydrothermal system: Unusual arsenic speciation in horizontal and vertical pore water profilesThe shallow marine hydrothermal vents near Ambitle Island in eastern Papua New Guinea discharge hot, slightly acidic, As-rich, chemically reduced fluid into cool, slightly alkaline, oxygenated seawater. Gradients in temperature, pH, and total As (AsT), among others, are established as the two aqueous phases mix. The hydrothermal fluid contained ∼900 μg/L AsT, almost exclusively present as the reduced AsIII, while local seawater measured between 1.2 and 2.4 μg/L As, with approximately equal levels of AsIII and AsV. Of particular interest in this study was As speciation and abundance in pore waters as a function of sediment depth and as a function of distance from the area of focused venting. With increasing distance, AsT concentration in the pore water decreased rapidly, but remained elevated up to 300 m from the area of focused venting when compared to a non-hydrothermal control site. As a function of depth (to ∼100 cm) AsT concentration in the pore water profiles was elevated and generally increased with depth. Surprisingly, aqueous AsV far exceeded aqueous AsIII at almost all distances and depths investigated, while at the control site the AsIII concentration exceeded that of AsV. In the Tutum Bay hydrothermal system, chemical disequilibria among As species provide potential metabolic energy for arsenite oxidizing microorganisms where hydrothermal fluid mixes with seawater near the vent orifice, and for arsenate reducing microorganisms with increasing distance and depth from the hydrothermal point source.Wissenschaftlicher ArtikelBand:22Heft:1276 51 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Naturally occurring arsenic in the Miocene Hawthorn Group, southwestern Florida: Potential implication for phosphate miningTo understand the mineralogical association, concentration, and distribution of arsenic (As) in the Hawthorn Group, the chemical and mineralogical composition of 362 samples that were collected from 16 cores in southwestern Florida were examined in detail. In the study area, the Hawthorn Group consisted primarily of a basal carbonate unit (the Arcadia Formation) and an upper siliciclastic unit (The Peace River Formation). The Peace River Formation contains appreciable amounts of phosphate and is currently being exploited for phosphate ore. Samples were taken from cores of each formation at intervals of 7.5 m. In addition, to the interval samples, sections likely to have high As concentrations, such as zones with pyrite crystals, hydrous ferric oxides, green clays, and organic material, were collected and analyzed. Bulk As concentrations were determined by hydride generation-atomic fluorescence spectrometry (HG-AFS) after digestion with aqua regia (3:1 HCl and HNO3). The elements Fe, Al, Si, Mg, Ca, S, and P were measured on the same solutions by inductively coupled plasma optical emission spectrometry (ICP-OES). The identification of discrete minerals was aided by scanning electron microscopy (SEM) and chemical compositions within the sample matrix and in individual minerals were obtained by electron-probe microanalysis (EMPA). This detailed mineralogical and geochemical study demonstrated that: (1) As in the Hawthorn Group varied from formation to formation and was mostly concentrated in trace minerals, such as pyrite; (2) average As concentrations significantly changed from 8.8 mg/kg (σ = 8.6 mg/kg) in the Peace River Formation to 3.0 mg/kg (σ = 3.7 mg/kg) in the Tampa Member of the Arcadia Formation. Arsenic concentrations for all Hawthorn samples varied from 0.1 to 69.0 mg/kg; (3) pyrite, with one exception, occurred as framboids and was unevenly distributed throughout the Hawthorn Group; (4) pyrite framboids were located inside a francolite (carbonate fluorapatite) matrix with As concentrations as high as 3730 mg/kg and as a trace mineral in the sediment matrix concentrations varying from <1 to 8260 mg/kg; (5) hydrous ferric oxides (HFO) contained As concentrations as high as 540 mg/kg; (6) francolite, organic material, and clays contained substantially less As than pyrite; (7) thus, the release of As from pyrite could pose a potential problem for the phosphate industry.Wissenschaftlicher ArtikelBand:22Heft:551 44 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Cadmium in soils and groundwater: A reviewCadmium (Cd) is a non-essential trace element that is widely distributed in the environment. Both geogenic and anthropogenic sources can elevate Cd concentrations in soils and groundwater, which are important for maintaining healthy supplies of food and safe drinking water. Elevated Cd doses are carcinogenic to humans. The WHO Guidelines for Drinking-Water Quality recommend a guideline value for Cd of 3 μg/L. Important anthropogenic Cd sources include mining, atmospheric deposition of combustion emissions, and the use of Cd-containing fertilizers. We document several cases of Cd pollution in soil and groundwater based on worldwide accounts. Besides anthropogenic Cd sources, Cd is also incorporated into sulfides, carbonates, and phosphorites resulting in elevated Cd concentrations in associated rock types. The crustal median Cd content is 0.2 mg/kg. In soils, Cd occurs at concentrations of 0.01 to 1 mg/kg with a worldwide mean of 0.36 mg/kg. Weathering can lead to Cd concentrations up to 5 μg/L in soil water and up to 1 μg/L in groundwater. In aqueous solutions, Cd generally occurs as the divalent Cd2+ and it is mobilized mainly in oxic, acidic conditions. Cadmium sorption is enhanced by the presence of high amounts of hydrous oxides, clay minerals, and organic matter, and its mobility is further influenced by pH, the redox state, and ionic strength of the solution. However, Cd can remain in solution as water-soluble complexes with anions, such as CdCl+ and Cd(SO4)22-, and dissolved organic matter while sorption and precipitation decrease the aqueous concentration of most other heavy metals. As a consequence, Cd is one of the most mobile heavy metals in the environment. The elevated mobilization potential, e.g., through competition and ligand induced desorption, is the reason for faster Cd release from soil into groundwater than other heavy metals. The goal of this study was to present a broad overview of the origin and concentration of Cd in groundwater, and its reaction pathways in aquatic environments. To gain an overview of the hydrochemical behavior of Cd, cases of Cd pollution in soil and groundwater, studies investigating Cd release, and information about the legal framework were compiled.Wissenschaftlicher ArtikelBand:108165 489