Applied Geochemistry (v.80, #C)

Laboratory column experiments and transport modeling to evaluate retardation of uranium in an aquifer downgradient of a uranium in-situ recovery site by Martin A. Dangelmayr; Paul W. Reimus; Naomi L. Wasserman; Jesse J. Punsal; Raymond H. Johnson; James T. Clay; James J. Stone (1-13).
The purpose of this study was to determine the attenuation potential and retardation of uranium in sediments taken from boreholes at the Smith-Ranch Highland in-situ recovery (ISR) site. Five column experiments with four different sediments were conducted to study the effects of variable mineralogy and alkalinity on uranium breakthrough. Uranium transport was modeled with PHREEQC using a generalized composite surface complexation model (GC SCM) with one, two, and, three generic surfaces, respectively. Reactive surface areas were approximated with PEST using BET derived surface areas to constrain fitting parameters. Uranium breakthrough was delayed by a factor of 1.68, 1.69 and 1.47 relative to the non-reactive tracer for three of the 5 experiments at an alkalinity of 540 mg/l. A sediment containing smectite and kaolinite retained uranium by a factor of 2.80 despite a lower measured BET surface area. Decreasing alkalinity to 360 mg/l from 540 mg/l increased retardation by a factor of 4.26. Model fits correlated well to overall BET surface area in the three columns where clay content was less than 1%. For the sediment with clay, models consistently understated uranium retardation when reactive surface sites were restricted by BET results. Calcite saturation was shown to be a controlling factor for uranium desorption as the pH of the system changes. A pH of 6 during a secondary background water flush remobilized previously sorbed uranium resulting in a secondary uranium peak at twice the influent concentrations. This study demonstrates the potential of GC SCM models to predict uranium transport in sediments with homogenous mineral composition, but highlights the need for further research to understand the role of sediment clay composition and calcite saturation in uranium transport.
Keywords: Uranium transport modeling; Uranium adsorption; In-situ recovery; Surface complexation modeling;

CO2-induced geochemical reactions in heterogeneous sandstone and potential conditions causing the tight cementation by Leilei Yang; Tianfu Xu; Guanhong Feng; Keyu Liu; Hailong Tian; Bo Peng; Chen Wang (14-23).
The contribution of tight gas, stored in sandstone reservoirs, to hydrocarbon production has been increasing. However, the formation mechanisms of these tight sandstone reservoirs remain unclear and the conditions leading to them are not well-understood. It is generally thought that early CO2 emission may have caused the sandstones of the Permian Shihezi Formation in the Ordos Basin, China, to become tight. In this study, we quantitatively investigate the effects of CO2 intrusion on reservoir evolution, choosing three factors thought to dominate this process (i.e. detrital mineral assemblage, formation water, and intruded CO2 volume) for sensitivity analyses. We constructed eight two-dimensional models to investigate CO2-water-rock reactions and reservoir porosity changes under various conditions. Subsequent to determination of the potential conditions leading to the formation of tight reservoirs, we then constructed a three-dimensional model applying given heterogeneities in porosity (Φ), permeability (K), and mineral assemblages to evaluate porosity distribution. Results show that the processes underlying the formation of tight reservoirs are controlled by various factors rather than any single one. Reservoirs containing Ca-rich minerals (i.e. calcite and anorthite) and Ca2+-rich formation water are apt to become tight as a result of CO2 intrusion because of large-scale carbonate precipitation. In addition, under primary geological and geochemically heterogeneous conditions, CO2-water-rock reactions vary spatially, which further enhances reservoir heterogeneities. Although minerals are heterogeneously distributed, they exhibit some similar characteristics as their common ions link them. Results show that it is possible to predict both high-quality and tight reservoirs. In the study area, favorable reservoirs are found in regions with initially high Φ, high K, and high feldspar mineral contents, while regions that are characterized by low Φ and K as well as Ca2+-rich formation water and a high content of Ca-rich minerals tend to become tight after CO2 intrusion.
Keywords: Tight sandstone reservoir; CO2-water-rock reactions; Porosity; Heterogeneity; The Ordos Basin;

Identification of blind geothermal resources in Surprise Valley, CA, using publicly available groundwater well water quality data by Andrew P.G. Fowler; Nicolas Spycher; Robert A. Zierenberg; Carolyn A. Cantwell (24-48).
Geothermal resource exploration is generally limited to areas with surface expressions of thermal activity (fumaroles and hot springs), or relies on expensive geophysical exploration techniques. In this study, the hidden subsurface distribution of geothermal fluids has been identified using a free and publicly available water quality dataset from agricultural and domestic water wells in Surprise Valley, northeastern California. Thermally evolved waters in Surprise Valley have element ratios that vary in response to Ca carbonate and Mg silicate mineral precipitation, and have elevated total dissolved solids (TDS). The arid climate in Surprise Valley leads to surface water evaporation in a closed basin, producing high TDS Na-Cl-CO3-SO4 brines in three ephemeral alkali lakes and in shallow groundwater under elevated soil CO2 conditions. Evaporated fluids in Surprise Valley follow a chemical divide that leads to Ca carbonate and Mg silicate mineral precipitation. Plots of dissolved element ratios can be used to distinguish groundwater affected by evaporation from water affected by thermal water-rock interaction, however it is challenging to select components for plotting that best illustrate different fluid evolution mechanisms. Here, we use a principal component analysis of centered log-ratio transformed data, coupled with geochemical models of fluid evaporation and thermal mixing pathways, to identify components to plot that distinguish between groundwater samples influenced by evaporation from those influenced by thermal processes. We find that groundwater samples with a thermal signature come from wells that define a coherent, linear geographical distribution that closely matches the location of known and inferred faults. Modification of the general approach employed here provides promise for identifying blind geothermal resources in other locations, by applying low-cost geochemical modeling and statistical techniques to areas where large groundwater quality geochemical datasets are available.
Keywords: Surprise valley; Geothermal; Evaporation; Geochemical modeling; Fluid mixing; Blind geothermal resources;

Studies of phase relations and AFm solid solution formations in the system CaO-Al2O3-CaCl2-CaCrO4-H2O by Xian Zhou; Min Zhou; Xian Wu; Sha Wan; Junjun Gen; Haobo Hou (49-57).
The Friedel's salt (Cl-AFm) as a major hydration phase in chlorinated cement-based materials plays an important role in the immobilization of chromate through ion exchange with chloride. To study the solid solutions of Ca4[Al(OH)6]2CrO4·nH2O (CrO4-AFm) and the composition of coexisting aqueous phase, AFm phases containing Cl and CrO2- 4 were synthesized and characterized with XRD, SEM, FTIR, FAAS and Ion chromatography (IC).Based on the determined total solubility of products and ion concentrations in solution, a solid solution-aqueous solution (SSAS) model for binary mixing of Cl-AFm and CrO4-AFm phases was established to predict aqueous solubility of Ca4[Al(OH)6]2[(CrO4)x(Cl)2-2x]•nH2O as a function of total Cl/Cr ratio in solid solution system. The solubility constant (pK) of pure Cl-AFm is 27.09 and its free energy of formation (ΔGo f) is −7047.80 kJ mol−1 at 27 °C. pK of pure CrO4-AFm is 29.07 and its ΔGo f is −8238.66 kJ mol−1 at 27 °C. Both the experimental and modeled data showed that the formation of solid solutions between Cl-AFm and CrO4-AFm could greatly lower the chromate concentration in aqueous solution. Therefore, this study suggested that (Cl, CrO4)-AFm compounds could efficiently immobilize the trace metal CrO4 2- in cement-stabilized wastes.Display Omitted
Keywords: Chromate; AFm phases; Lippmann diagrams; Solid solution; Aqueous solution;

Freshwater resources on low-lying atoll islands are highly vulnerable to climate change and sea-level rise. In addition to rainwater catchment, groundwater in the freshwater lens is a critically important water resource on many atoll islands, especially during drought. Although many atolls have high annual rainfall rates, dense natural vegetation and high evapotranspiration rates can limit recharge to the freshwater lens. Here we evaluate the effects of land-use/land-cover change and managed aquifer recharge on the hydrogeochemistry and supply of groundwater on Roi-Namur Island, Republic of the Marshall Islands. Roi-Namur is an artificially conjoined island that has similar hydrogeology on the Roi and Namur lobes, but has contrasting land-use/land-cover and managed aquifer recharge only on Roi. Vegetation removal and managed aquifer recharge operations have resulted in an estimated 8.6 × 105 m3 of potable groundwater in the freshwater lens on Roi, compared to only 1.6 × 104 m3 on Namur. We use groundwater samples from a suite of 33 vertically nested monitoring wells, statistical testing, and geochemical modeling using PHREEQC to show that the differences in land-use/land-cover and managed aquifer recharge on Roi and Namur have a statistically significant effect on several groundwater-quality parameters and the controlling geochemical processes. Results also indicate a six-fold reduction in the dissolution of carbonate rock in the freshwater lens and overlying vadose zone of Roi compared to Namur. Mixing of seawater and the freshwater lens is a more dominant hydrogeochemical process on Roi because of the greater recharge and flushing of the aquifer with freshwater as compared to Namur. In contrast, equilibrium processes and dissolution-precipitation non-equilibrium reactions are more dominant on Namur because of the longer residence times relative to the rate of geochemical reactions. Findings from Roi-Namur Island support selective land-use/land-cover change and managed aquifer recharge as a promising management approach for communities on other low-lying atoll islands to increase the resilience of their groundwater supplies and help them adapt to future climate change related stresses.

As coastal lowlands are prone to sea water flooding, sea-level rise might globally increase this risk. To protect its coastline, the Netherlands adds an average of 12 million m3 of sand annually, but more is needed to cope with the expected consequences of global warming. In 2011 a novel approach for coastal protection was applied near The Hague, consisting of a mega beach nourishment of 21.5 million m3 of sand: the Sand Engine – an artificial sand spit rising to 6 m above mean sea level. It uniquely combines coastal engineering construction with environmental, ecological and social considerations. To construct the Sand Engine, material was used from the seafloor, which changed the materials environment from anaerobic to aerobic, triggering two main hydrogeochemical processes: pyrite oxidation and groundwater freshening. The objective of this study was to assess the sediment geochemistry of the Sand Engine and understand the hydrogeochemistry with respect to pyrite oxidation and freshening. When sufficient buffer capacity is lacking, the mobility of metals and metalloids originated from the mineral pyrite, can cause local impacts on ecology and environment. Geochemical and multivariate statistical analyses were performed on 174 sediment samples from the Sand Engine and from material collected from the seafloor prior to its construction, as well as on 86 samples of pore water collected from the Sand Engine. First, a cluster analysis was performed, using model-based (Mclust) and variable clustering. Second, a robust factor analyses (RFA) was used to explain the variation between the groups and discover relationships between elements and/or soil properties within the groups. We distinguished three clusters of sediment samples and two clusters of pore water samples. Sediment cluster 1 was comprised exclusively of surface samples from the Sand Engine; it was differentiated from the other two clusters by its geochemistry, sorting processes and weathering. Sediment clusters 2 consisted of shallow samples from the Sand Engine, as well as deeper autochthone material from the sand pit. Sediment cluster 3 contained deeper samples from the Sand Engine and also shallow autochthone material from the sand pit. Sediment clusters 2 and 3 show differences in carbonate content and, especially, in reactive iron, confirming that in the sand pit area a Holocene marine layer overlies Pleistocene fluvial sand. For the pore water samples, two clusters were estimated: a saline water cluster and a fresh water cluster. Thus the Sand Engine contains source material from two different geological layers, which vary in their reactive iron concentration and carbonate minerals. Pyrite oxidation is seen with depth, resulting in iron oxides and an increase in alkalinity because of calcite dissolution. With the development of the Sand Engine source material is expected to show a decrease in pyrite oxidation, because of low amount of available sulphide minerals. Carbonate dissolution due to pyrite oxidation will then decrease as well. Additional calcite dissolution is caused by freshening processes. With the amount of CaCO3 present in the sand, the Sand Engine contains calcareous-rich sand where acidic conditions are not likely to occur during its life span and therefore create no local environmental risks for acidification. At the surface of the mega beach nourishment, sorting processes are causing locally enrichment of iron oxides and associated elements, as well as a decrease in the stability of heavy minerals like rutile.
Keywords: Cluster analysis; Robust factor analysis; Pyrite oxidation; Coastal management; Sediment; Groundwater;

Tc interaction with crystalline rock from Äspö (Sweden): Effect of in-situ rock redox capacity by Florian Mathias Huber; Yury Totskiy; Rémi Marsac; Dieter Schild; Ivan Pidchenko; Tonya Vitova; Stepan Kalmykov; Horst Geckeis; Thorsten Schäfer (90-101).
The interaction of Tc(VII) with crushed crystalline rock (Äspö diorite; 1–2 mm size fraction) from the Äspö Hard Rock Laboratory (HRL) (Sweden) was studied by laboratory batch sorption and desorption experiments under Ar atmosphere using both natural and synthetic groundwater. The Äspö diorite used in the experiments was drilled, transported and handled as far as possible under anoxic conditions to preserve the in-situ rock redox capacity. For comparison, identical experiments using artificially oxidized Äspö diorite have been carried out to examine the effect of in-situ redox capacity on Tc uptake. According to the batch studies, Tc(VII) uptake on the Äspö diorite is strongly dependent on redox capacity. Uptake on un-oxidized rock is approximately 2 times higher compared to oxidized rock samples, most likely due to higher Fe(II) contents of the un-oxidized rock. Tc redox states and speciation both on the mineral surface and in the bulk were studied using X-ray photoelectron spectroscopy (XPS) and Tc K-edge X-ray absorption near edge structure (XANES) spectroscopy. The spectroscopic results verify a Tc(VII) reduction to Tc(IV) at the rock surface. Distribution coefficients (Kd) and surface normalized distribution coefficients (Ka) were determined and compared to available literature data. The formation of a Tc colloidal phase was not observed under the geochemical conditions prevailing in the experimental studies. Desorption of Tc is very low under anoxic conditions, but after artificial oxidation Tc mobility is strongly increased. The results of this work clearly highlight the effect of in-situ rock redox capacity on Tc retention.

Nano-sized FeS particles have been shown to improve the adsorption performance in permeable reactive barrier (PRB) technologies that are used to treat acid mine drainage. To investigate the coupling effect of Fe3+ and biological nano-sized FeS on removal of redox-sensitive contaminants and the permeability of a limestone system, Fe-reducing bacteria (FRB) and sulfate-reducing bacteria (SRB) were used to form nano-sized FeS coatings on limestone grains. For initial acidic solutions containing 0.5 mg/L As(V), Cr(VI) and Sb(V), retardation factors increase from 26, 5 and 7, respectively, in a limestone-only (pristine) system to »367, 89 and 9, respectively, in an FeS-coated limestone system, and to 345, 367 and 308, respectively, in an FeS-Fe3+ (aq) system. The permeability coefficient of the FeS-coated limestone system is better than that of the pristine limestone system, but declines to two-thirds of the pristine limestone system in the presence of Fe3+ (aq), possibly due to the formation of secondary ferric hydroxides. XPS analysis suggests that the FeS particles are effective at reducing As(V), Sb(V) and Cr(VI) and removing them from solution. These results demonstrate that FeS particles improve retention of redox-sensitive contaminants with and without Fe3+ (aq). These new findings give new insights on the coupling effect of redox systems used in PRBs.
Keywords: Permeable reactive barrier; Biological nano-sized FeS particles; Redox-sensitive contaminants; Permeability; Remediation;

Geochemistry of metals and metalloids in siliceous sinter deposits: Implications for elemental partitioning into silica phases by Camilo Sanchez-Yanez; Martin Reich; Mathieu Leisen; Diego Morata; Fernando Barra (112-133).
Sinter deposits are formed by precipitation of silica from hydrothermal fluids that have reached the surface environment. They are commonly found around hot springs and represent surface expressions of underlying geothermal systems and/or low sulfidation epithermal gold-silver hydrothermal deposits. Several studies have reported ppm to weight percent concentrations of metals (e.g., Au, Ag, Cu) and metalloids (e.g., As, Sb, B) in sinters capping geothermal systems and epithermal gold-silver deposits. However, the relation between the maturity of the siliceous sinter and its metal enrichment remains unknown. Here we use geochemical and mineralogical data that link the silica crystallinity degree with trace metal and metalloid contents in sinter. In this paper, we provide in situ trace element data in metal-rich silica sinter samples from the Puchuldiza geothermal field in the Altiplano of northern Chile that record the complete diagenetic sequence from non-crystalline opal A to microcrystalline quartz. Combined SEM, XRD and LA-ICP-MS data show that the concentration of metals and metalloids in sinters from Puchuldiza display a strong correspondence with silica crystallinity. While arsenic and boron are predominantly enriched in the more amorphous silica phases (opal A/CT), gold and silver show higher concentrations in the more crystalline phases (opal C/quartz). Silica structural, morphological and geochemical transformations from its initial precipitation to its final maturation after diagenesis are responsible for this differential enrichment. During the initial stages, gold and silver are incorporated into silica microspheres as cationic species and/or metal nanoparticles or colloids, while arsenic and boron incorporation is controlled by As-bearing accessory minerals and Fe-oxyhydroxides. As diagenesis progresses and the crystallinity of silica increases, diffusion-driven processes such as Ostwald ripening may progressively enrich gold and silver in the sinter, while metalloids are depleted owing to the low retention of arsenic by silica. These findings indicate that the diagenetic transitions of silica, defined by significant structural changes that involve generation of surface defects and the creation of reactive sites, may play an important role in elemental uptake by silica in near surface environments.
Keywords: Puchuldiza; Altiplano; High-altitude; Siliceous sinter; Silica crystallinity; Precious metals; Metalloids; Nanoparticle; Ostwald ripening;

Chromium isotope fractionations resulting from electroplating, chromating and anodizing: Implications for groundwater pollution studies by Martin Novak; Vladislav Chrastny; Ondrej Sebek; Eva Martinkova; Eva Prechova; Jan Curik; Frantisek Veselovsky; Marketa Stepanova; Barbora Dousova; Frantisek Buzek; Juraj Farkas; Alexandre Andronikov; Nikoleta Cimova; Marie Houskova (134-142).
A number of shallow aquifers in industrial regions have been polluted by toxic Cr(VI). At some sites, spontaneous reduction of dissolved Cr(VI) to insoluble Cr(III) has been observed. Precipitation of non-toxic Cr(III) is accompanied by a Cr isotope fractionation, with the residual Cr(VI) becoming enriched in the heavier isotope 53Cr, and depleted in the lighter isotope 52Cr. Thus far, δ53Cr values of the contamination source have been poorly constrained. These values are needed to quantify the extent of Cr(VI) reduction in the aquifers. We present δ53Cr values of solutions generated during Cr-electroplating, chromating and anodizing at nine industrial sites. The source chemical, CrO3, had a mean δ53Cr of 0.0‰. A small-to-negligible Cr isotope fractionation was observed between the solutions of the plating baths and the source chemical. Across all sample types, the mean δ53Cr(VI) value was 0.2‰. The mean δ53Cr(VI) value of contaminated groundwater in the same region, studied previously, was significantly higher (2.9‰), indicating Cr(VI) reduction. Based on low δ53Cr(VI) values of plating baths and rinsewaters as potential contamination sources, and their low variability, we suggest that most aquifer δ53Cr(VI) values higher than 1.0‰ are a result of in-situ Cr(VI) reduction.
Keywords: Chromium isotopes; Electroplating; Chromating; Isotope fractionations;

Long-term sequestration of nickel in mackinawite formed by Desulfovibrio capillatus upon Fe(III)-citrate reduction in the presence of thiosulfate by Maya Ikogou; Georges Ona-Nguema; Farid Juillot; Pierre Le Pape; Nicolas Menguy; Nicolas Richeux; Jean-Michel Guigner; Vincent Noël; Jessica Brest; Benoit Baptiste; Guillaume Morin (143-154).
In euxinic sediments, the reaction between iron and sulfides results in the formation of Fe(II)-sulfides, which are known to play a key role in trace metal sequestration. The present study investigates the sequestration of nickel during Fe(II)-sulfide formation mediated by the (thio)sulfate-reducing bacterium Desulfovibrio capillatus in the presence of soluble Fe(III)-citrate and thiosulfate as the terminal electron acceptor. XRD, HRTEM and Fe K-edge EXAFS data indicate that biogenic mackinawite (FeS) was the sole mineral formed in our experiments. These data also show that the kinetics of mackinawite crystal growth was significantly accelerated when nickel was present in the starting solution. In addition, the lack of detection of other Fe(II)-sulfides indicates inhibition of the mackinawite (FeS) to greigite (Fe3S4) and/or pyrite (FeS2) transformation that is likely related to (i) the efficiency of Desulfovibrio capillatus in reducing S(0) to H2S and (ii) the absence of O2 during the experiments. Finally, chemical analyses show that 98% of the nickel is associated with biogenic mackinawite and no release of nickel from mackinawite was observed after up to 10 months of incubation under anoxic conditions. This finding is consistent with Ni K-edge EXAFS data which show that Ni(II) substitutes for Fe(II) in the structure of biogenic mackinawite. This study shows that (thio)sulfate-reducing bacteria can efficiently promote the formation of mackinawite in euxinic sedimentary environments and that these Fe(II)-sulfides can act as efficient and long-term trapping minerals for nickel in such settings. Considering the capacity of mackinawite at incorporating other trace metals such as Mn, Co, Cu and Zn, this iron sulfide could also serve as a host for these elements as well. This study suggests that mackinawite likely plays a more important role in the biogeochemical cycles of Fe, S, and associated trace metals than considered up to now.Display Omitted

Organic geochemistry and toxicology of a stream impacted by unconventional oil and gas wastewater disposal operations by William Orem; Matthew Varonka; Lynn Crosby; Karl Haase; Keith Loftin; Michelle Hladik; Denise M. Akob; Calin Tatu; Adam Mumford; Jeanne Jaeschke; Anne Bates; Tiffani Schell; Isabelle Cozzarelli (155-167).
The large volume of wastewater produced during unconventional oil and gas (UOG) extraction is a significant challenge for the energy industry and of environmental concern, as the risks due to leaks, spills, and migration of these fluids into natural waters are unknown. UOG wastewater is often hypersaline, and contains myriad organic and inorganic substances added for production purposes and derived from the source rock or formation water. In this study, we examined the organic composition and toxicology of water and sediments in a stream adjacent to an underground injection disposal facility that handles UOG wastewaters. We sampled water and streambed sediments from an unnamed tributary of Wolf Creek upstream from the disposal facility, near the injection well, and downstream. Two sites downstream from the disposal facility contained organic compounds in both water and sediments that were consistent with a source from UOG wastewater. These compounds included: 2-(2-butoxyethoxy)-ethanol, tris(1-chloro-2-propyl)phosphate, α, α-dimethyl-benzenemethanol, 3-ethyl-4-methyl-1H-pyrrole-2,5-dione, and tetrahydro-thiophene-1,1-dioxide in water, diesel fuel hydrocarbons (e.g. pentacosane, Z-14-nonacosane), and halogenated hydrocarbons (e.g., 1-iodo-octadecane, octatriacontyl trifluoroacetate, dotriacontyl pentafluoropropionate) in sediments. Concentrations of UOG-derived organic compounds at these sites were generally low, typically 4 to <1 μg/L in the water, and <70 μg/g (dry wt.) in the sediment. In addition, water and sediment at a site immediately downstream from the facility contained many chromatographically unresolved and unidentified hydrocarbons. In contrast, sites upstream from the facility or in nearby watersheds not influenced by the disposal well facility contained primarily natural (biologically produced) organic substances from the local environment. Toxicological assays of human cell line exposures to water and sediment showed minimal effects. Results indicate that UOG wastewater has entered the stream and that UOG-derived organic substances are present. The contamination level, however, is low and appears to be restricted to sites immediately downstream from the disposal facility at this time.
Keywords: Unconventional oil and gas production; Marcellus shale; Hydraulic fracturing; Produced water; Wastewater disposal; Class II injection well; Organic substances; Toxicology;

The studied subsurface water samples represent different depositional environments from the Late Miocene lacustrine and fluviatile (delta-slope, delta-plain) facies to the Pliocene and Quaternary lacustrine, fluviatile and terrestrial ones in the upper 2500 m of the sediments, in the central part of the Pannonian Basin. The study area covers about 25,000 km2 and comprises two sub-basins and an area above uplifted basement. Spatial distribution of dissolved arsenic indicates the highest range of the arsenic concentrations (0–180 μg/l) in the groundwater from the Quaternary fluviatile aquifers in both sub-basins. Comparing the measured pH and redox potential to the stability diagrams in the As–O–H, Fe–CO2–H2O, and semiquinone-hydroquinone systems it was observed that in the pe-pH diagram, most samples from the Quaternary and Pliocene sediments appear in the stability field of HAsO4 2−, FeOOH, and semiquinone (Q3−•), and are grouped along a line with a slope similar to those of the redox boundaries between HAsO4 2−and H3AsO3 0, and also semiquinone (Q3−•) and hydroquinone (H2Q2−) species. The samples from the shallowest, unconfined Quaternary aquifers are above the line, and whereas those from the deepest, Upper Miocene aquifers are below the redox boundary of the standard-state systems. The mobilising processes in the different depositional environments are explained on the basis of the position of the samples in the pe-pH stability diagrams. The spatial distribution of arsenic concentrations in groundwater is attributed to the availability of mobilisable arsenic in the sediments rather than to the other elements of the mobilising processes, such as the type of the chemical reactions, redox circumstances, or temperature.
Keywords: Dissolved arsenic; Organic compounds; pe-pH diagram; Redox equilibrium; Sedimentary basin;