Chromium,chromium isotopes and selected trace elements,western Mojave Desert,USA

Chromium(VI) concentrations in excess of the California Maximum Contaminant Level (MCL) of 50 μg/L occur naturally in alkaline, oxic ground-water in alluvial aquifers in the western Mojave Desert, southern California. The highest concentrations were measured in aquifers eroded from mafic rock, but Cr(VI) as high as 27 μg/L was measured in aquifers eroded from granitic rock. Chromium(VI) concentrations did not exceed 5 μg/L at pH < 7.5 regardless of geology. δ⁵³Cr values in native ground-water ranged from 0.7 to 5.1‰ and values were fractionated relative to the average δ⁵³Cr composition of 0‰ in the earth’s crust. Positive δ⁵³Cr values of 1.2 and 2.3‰ were measured in ground-water recharge areas having low Cr concentrations, consistent with the addition of Cr(VI) that was fractionated on mineral surfaces prior to entering solution. δ⁵³Cr values, although variable, did not consistently increase or decrease with increasing Cr concentrations as ground-water flowed down gradient through more oxic portions of the aquifer. However, increasing δ⁵³Cr values were observed as dissolved O₂ concentrations decreased, and Cr(VI) was reduced to Cr(III), and subsequently removed from solution. As a result, the highest δ⁵³Cr values were measured in water from deep wells, and wells in discharge areas near dry lakes at the downgradient end of long flow paths through alluvial aquifers. δ⁵³Cr values at an industrial site overlying mafic alluvium having high natural background Cr(VI) concentrations ranged from −0.1 to 3.2‰. Near zero δ⁵³Cr values at the site were the result of anthropogenic Cr. However, mixing with native ground-water and fractionation of Cr within the plume increased δ⁵³Cr values at the site. Although δ⁵³Cr was not necessarily diagnostic of anthropogenic Cr, it was possible to identify the extent of anthropogenic Cr at the site on the basis of the δ⁵³Cr values in conjunction with major-ion data, and the δ¹⁸O and δD composition of water from wells.     

Distribution and bioavailability of Cr in central Euboea, Greece

Plants and soils from central Euboea, were analyzed for Cr_(totai), Cr(VI), Ni, Mn, Fe and Zn. The range of metal concentrations in soils is typical to those developed on Fe-Ni laterites and ultramafic rocks. Their bioavailability was expressed in terms of concentrations extractable with EDTA and 1 M HNO₃, with EDTA having a limited effect on metal recovery. Cr(VI) concentrations in soils evaluated by alkaline digestion solution were lower than phytotoxic levels. Chromium and Ni — and occasionally Zn — in the majority of plants were near or above toxicity levels. Cr(VI) concentrations in plants were extremely low compared to total chromium concentrations. Cr_(total) in ground waters ranged from <1 μg.L^?1 to 130 μg.L^?1, with almost all chromium present as Cr(VI). With the exception of Cr_(total) and in some cases Zn, all elements were below regulatory limits for drinking water. On the basis of Ca, Mg, Cr_(total) and Si ground waters were classified into three groups: Group(I) with Cr concentrations less than 1 μg.L^?1 from a karstic aquifer; Group(II) with average concentrations of 24 μg.L^?1 of Cr and relatively high Si associated with ophiolites; and Group(III) with Cr concentrations of up to 130 μg.L^?1, likely due to anthropogenic activity. Group(III) is comparable to ground waters from Assopos basin, characterized by high Cr(VI) concentrations, probably due to industrial actrivities.     

Delta Chromium-53/52 isotopic composition of native and contaminated groundwater,Mojave Desert,USA

Chromium(VI) concentrations in groundwater sampled from three contaminant plumes in aquifers in the Mojave Desert near Hinkley, Topock and El Mirage, California, USA, were as high as 2600, 5800 and 330 μg/L, respectively. δ^53/52Cr compositions from more than 50 samples collected within these plumes ranged from near 0‰ to almost 4‰ near the plume margins. Assuming only reductive fractionation of Cr(VI) to Cr(III) within the plume, apparent fractionation factors for δ^53/52Cr isotopes ranged from ε_app = 0.3 to 0.4 within the Hinkley and Topock plumes, respectively, and only the El Mirage plume had a fractionation factor similar to the laboratory derived value of ε = 3.5. One possible explanation for the difference between field and laboratory fractionation factors at the Hinkley and Topock sites is localized reductive fractionation of Cr(VI) to Cr(III), with subsequent advective mixing of native and contaminated water near the plume margin. Chromium(VI) concentrations and δ^53/52Cr isotopic compositions did not uniquely define the source of Cr near the plume margin, or the extent of reductive fractionation within the plume. However, Cr(VI) and δ^53/52Cr data contribute to understanding of the interaction between reductive and mixing processes that occur within and near the margins of Cr contamination plumes. Reductive fractionation of Cr(VI) predominates in plumes having higher ε_app, these plumes may be suitable for monitored natural attenuation. In contrast, advective mixing predominates in plumes having lower ε_app, the highly dispersed margins of these plumes may be difficult to define and manage.     

Effects of indigenous bacteria on Cr(VI) reduction in Cr-contaminated sediment with industrial wastes

Black, clay-like sediments have been obtained from the area of the pigment manufacturing factories in Dongducheon city, Korea. These sediments were contaminated by heavy metals, especially chromium (700 mg/kg). Indigenous bacteria in the sediments were isolated to investigate their ability to reduce Cr(VI) to Cr(III). The enriched bacterial consortium reduced over 99% of dissolved Cr(VI) in 96 h from the onset of the experiments under anaerobic condition, while there was no change in Cr(VI) concentration until 300 h in abiotic controls. Total amount of dissolved Cr decreased simultaneously when Cr(VI) was reduced, which was likely due to precipitation of Cr(OH)₃ after microbial reduction of Cr(VI) to Cr(III). Under aerobic condition, only 30% of dissolved Cr(VI) was reduced by indigenous bacteria until 900 h. The reduction of Cr(VI) did not accompany bacterial growth since the amount of protein did not show a significant change with time both in the presence and absence of O₂. These indigenous bacteria may play a role in the treatment of Cr(VI)-contaminated sediments.     

Geochemical modeling for predicting potential solid phases controlling the dissolved molybdenum in coal overburden,Powder River Basin,WY, U.S.A.

Surface coal mining processes may increase the dissolved Mo concentrations in overburden (mine spoils) and in nearby soils. Spoil and soil samples were subjected to solubility studies for determining potential solid phases controlling the dissolved Mo. Additionally, native soil samples were included in the study for comparison. Ion activity products (IAPs) were calculated from the total elemental concentrations of aqueous extracts and compared with solubility products (K_sps) of Mo solid phases. Effects of dissolved organic carbon (DOC) on metal ion complexation were also evaluated. The pH of aqueous extracts ranged from 7.14 to 8.31. DOC in aqueous extracts varied from 17.3 to 58.7 mg/l. Molybdenum concentrations in aqueous extracts ranged between 11.5 and 35.6 μg/l for spoils, 13.5 and 20.0 μg/l for soils near mine site and 14.5 and 29.6 μg/l for native soils. Initially, IAPs suggested varying degrees of supersaturation with respect to wulfenite (PbMoO₄). However, when DOC-Pb²⁺ complexes were considered, the IAPs indicated a close approach to saturation with respect to PbMoO₄. Furthermore, the IAPs suggested a high degree of undersaturation with respect to powellite (CaMoO₄) and ferrimolybdite (FeMoO₄). Results in this study suggest the dissolved Mo concentrations in mine spoils and soils that are near coal mine may be controlled by PbMoO₄ solid phase.     

Antioxidants and chromium reductases by <Emphasis Type="Italic">Penibacillus</Emphasis> species enhance the growth of soybean under chromium stress

Hexavalent chromium [Cr(VI)] is used in various industries, but its improper and uncontrolled discharge contaminates the environment. In order to circumvent chromium toxicity, several physicochemical and biological strategies have been employed. Among biological approach, microbes convert toxic Cr(VI) to less soluble Cr(III) form and hence can be used to detoxify/remove Cr(VI) from contaminated environment. Considering these, present study was designed to assess the effect of chromium reductases and antioxidants secreted by Penibacillus species to detoxify Cr(VI) and concurrently to augment soybean growth. Bacterial strains (MAI1 and MAI2) were identified as Penibacillus sp. using 16S rRNA gene. Penibacillus species reduced Cr(VI) significantly at pH 7. Maximum Cr(VI) was reduced at 50 and 100 µg/ml of Cr(VI) concentrations. Penibacillus sp. also reduced Cr(VI) significantly at 25 and 35 °C as well as 1 g sodium alginate in 1 g polyvinyl alcohol. Bacterial strains reduced Cr(VI) into Cr(III) which were detected as 33 ± 1 and 35 ± 1 µg/ml in supernatant and 67 ± 2.5 and 65 ± 1 µg/ml in cell debris, respectively, after 120 h. Chromium reductase found in cell-free extract reduced almost all Cr(VI) compared to those observed in cell debris. Both malondialdehyde and antioxidant levels were increased with gradual increase in Cr(VI) concentration. Penibacillus species inoculated soybean plants had better growth and photosynthetic pigments under Cr(VI) stress.     

Natural hexavalent chromium in groundwaters interacting with ophiolitic rocks

Thirty of the 58 groundwaters sampled in September-October 2000 in the study area (La Spezia Province, Italy) have Mg-HCO₃ to Ca-HCO₃ composition, undetectable Cr(III) contents, and virtually equal concentrations of total dissolved Cr and Cr(VI). Therefore, dissolved Cr is present in toto as Cr(VI), with concentrations of 5-73 ppb. These values are above the maximum permissible level for drinking waters (5 ppb). Local ophiolites, especially serpentinites and ultramafites, are Cr-rich and represent a Cr source for groundwaters. However, since Cr is present as Cr(III) in rock-forming minerals, its release to the aqueous solution requires oxidation of Cr(III) to Cr(VI). This can be performed by different electron acceptors, including Mn oxides, H₂O₂, gaseous O₂, and perhaps Fe(III) oxyhydroxides. Based on this evidence and due to the absence of anthropogenic Cr sources, the comparatively high Cr(VI) concentrations measured in the waters of the study area are attributed to natural pollution.     

Redox control of arsenic mobilization in Bangladesh groundwater

Detailed hydrochemical measurements, δ³⁴S_SO4 and ³H analyses were performed on 37 groundwater samples collected during February 1999, January and March 2000 from 6 locations in eastern and southeastern Bangladesh to examine redox processes that lead to As mobilization in groundwater. The study sites were chosen based on available nation-wide As surveys to span the entire spectrum of As concentrations in Bangladesh groundwater, and to represent 3 of 5 major geological units of the Ganges-Brahmaputra Delta: uplifted Pleistocene terrace, fluvial flood plain and delta plain. Arsenic was found to be mobilized under Fe-reducing conditions in shallow aquifers (<35 m depth), presumably of Holocene age. It remained mobile under SO₄-reducing conditions, suggesting that authigenic sulfide precipitation does not constitute a significant sink for As in these groundwaters. The redox state of the water was characterized by a variety of parameters including dissolved O₂, NO₃⁻, Mn²⁺, Fe²⁺ concentrations, and SO₄²⁻/Cl⁻ ratios. High dissolved [As] (> 50 μg/l; or > 0.7 μM ) were always accompanied by high dissolved [HCO₃⁻] (> 4 mM), and were close to saturation with respect to calcite. Groundwater enriched in As (200–800 μg/l; or 2.7–10.7 μM) and phosphate (30–100 μM) but relatively low in dissolved Fe (5–40 μM) probably resulted from re-oxidation of reducing, As and Fe enriched water. This history was deduced from isotopic signatures of δ³⁴S_SO4 and ³H₂O (³H) to delineate the nature of redox changes for some of the reducing groundwaters. In contrast, As is not mobilized in presumed Pleistocene aquifers, both shallow (30–60 m) and deep (150–270 m), because conditions were not reducing enough due to lack of sufficient O₂ demand.     

Treatment of Cr(VI) contaminated water with Pannonibacter phragmitetus BB

Pannonibacter phragmitetus BB was utilized to treat hexavalent chromium [Cr(VI)] contaminated water. Cr(VI) concentration of the contaminated water (pH 10.85) was 534 mg/L. With the inoculum size ranging from 1 to 20 %, P. phragmitetus BB completely reduced Cr(VI) within 27 h when the initial medium concentration exceeded 20 g/L. The lag time of bio-reduction by Cr(VI)-induced cells was 24 h, which was longer than the non-Cr(VI)-induced cells. Under the agitation condition, an obvious bio-reduction lag phase existed and Cr(VI) was completely reduced within 24 h. However, the lag phase was not observed under the static condition, Cr(VI) was reduced continuously after inoculation and Cr(VI) was completely reduced after 27 h incubation. The main chromium components after Cr(VI) reduction were Cr(OH)₃, Cr₂O₃ and CrCl₃. The results of this study are fundamentally significant to the application of P. phragmitetus BB in the treatment of Cr(VI) contaminated water.     

A regional-scale study of chromium and nickel in soils of northern California, USA

A soil geochemical survey was conducted in a 27,000-km² study area of northern California that includes the Sierra Nevada Mountains, the Sacramento Valley, and the northern Coast Range. The results show that soil geochemistry in the Sacramento Valley is controlled primarily by the transport and weathering of parent material from the Coast Range to the west and the Sierra Nevada to the east. Chemically and mineralogically distinctive ultramafic (UM) rocks (e.g. serpentinite) outcrop extensively in the Coast Range and Sierra Nevada. These rocks and the soils derived from them have elevated concentrations of Cr and Ni. Surface soil samples derived from UM rocks of the Sierra Nevada and Coast Range contain 1700–10,000 mg/kg Cr and 1300–3900 mg/kg Ni. Valley soils west of the Sacramento River contain 80–1420 mg/kg Cr and 65–224 mg/kg Ni, reflecting significant contributions from UM sources in the Coast Range. Valley soils on the east side contain 30–370 mg/kg Cr and 16–110 mg/kg Ni. Lower Cr and Ni concentrations on the east side of the valley are the result of greater dilution by granitic sources of the Sierra Nevada.Chromium occurs naturally in the Cr(III) and Cr(VI) oxidation states. Trivalent Cr is a non-toxic micronutrient, but Cr(VI) is a highly soluble toxin and carcinogen. X-ray diffraction and scanning electron microscopy of soils with an UM parent show Cr primarily occurs within chromite and other mixed-composition spinels (Al, Mg, Fe, Cr). Chromite contains Cr(III) and is highly refractory with respect to weathering. Comparison of a 4-acid digestion (HNO₃, HCl, HF, HClO₄), which only partially dissolves chromite, and total digestion by lithium metaborate (LiBO₃) fusion, indicates a lower proportion of chromite-bound Cr in valley soils relative to UM source soils. Groundwater on the west side of the Sacramento Valley has particularly high concentrations of dissolved Cr ranging up to 50 μg L⁻¹ and averaging 16.4 μg L⁻¹. This suggests redistribution of Cr during weathering and oxidation of Cr(III)-bearing minerals. It is concluded that regional-scale transport and weathering of ultramafic-derived constituents have resulted in enrichment of Cr and Ni in the Sacramento Valley and a partial change in the residence of Cr.     

Kinetics of Mn(II) oxidation by Leptothrix discophora SS1

The kinetics of Mn(II) oxidation by the bacterium Leptothrix discophora SS1 was investigated in this research. Cells were grown in a minimal mineral salts medium in which chemical speciation was well defined. Mn(II) oxidation was observed in a bioreactor under controlled conditions with pH, O₂, and temperature regulation. Mn(II) oxidation experiments were performed at cell concentrations between 24 mg/L and 35 mg/L, over a pH range from 6 to 8.5, between temperatures of 10°C and 40°C, over a dissolved oxygen range of 0 to 8.05 mg/L, and with L. discophora SS1 cells that were grown in the presence of Cu concentrations ranging from zero to 0.1 μM. Mn(II) oxidation rates were determined when the cultures grew to stationary phase and were found to be directly proportional to O₂ and cell concentrations over the ranges investigated. The optimum pH for Mn(II) oxidation was approximately 7.5, and the optimum temperature was 30°C. A Cu level as low as 0.02 μM was found to inhibit the growth rate and yield of L. discophora SS1 observed in shake flasks, while Cu levels between 0.02 and 0.1 μM stimulated the Mn(II) oxidation rate observed in bioreactors. An overall rate law for Mn(II) oxidation by L. discophora as a function of pH, temperature, dissolved oxygen concentration (D.O.), and Cu concentration is proposed. At circumneutral pH, the rate of biologically mediated Mn(II) oxidation is likely to exceed homogeneous abiotic Mn(II) oxidation at relatively low (≈μg/L) concentrations of Mn oxidizing bacteria.     

Natural Cr(VI) contamination of groundwater in the Cecina coastal area and its inner sectors (Tuscany,Italy)

A detailed hydrogeochemical study of groundwater in the Cecina coastal plain (Livorno province, Italy) and its inner sectors was undertaken in 2008, as chemical analyses carried out on groundwater since 2006 have revealed Cr(VI) concentrations of up to 49 μg/L (well above the permissible limit of 5 μg/L). Ophiolite outcrops are present throughout the study area, and their fragments likely represent a significant portion of the existing multilayered aquifer skeleton. Waters delivered by the serpentinite outcrops have a typically Mg–HCO₃ composition, whereas those of the coastal plain are prevailingly of the Ca/Mg–HCO₃ type with significant Mg contents. Significant NO₃ contamination characterises the studied coastal plain, and an interesting negative correlation exists between Cr(VI) and both NO₃ and SO₄ deriving from the widespread use of (NH₄)₂SO₄ as a farm fertilizer. Chromium speciation calculations carried out using the EQ3NR code reveal that the prevailing Cr(VI) species in solution is CrO₄ ^2?; however, CaCrO₄° and MgCrO₄° neutral complexes represent significant percentages (up to 42 %). These findings suggest that the mobility and consequently the bioavailability of Cr(VI) can be significantly enhanced by these neutral complexes, which are not considered to be affected by adsorption/desorption processes. The Cr(VI) source, investigated by means of the Mg/SiO₂ molar ratio, seems to be represented mainly by Mg-bearing minerals of the chlorite group. Petrographic observations confirm the occurrence of this mineral group. The interaction between rainwater and the local serpentinite rock was simulated at different $P_{{{\text{CO}}_{ 2} }}$ and $P_{{{\text{O}}_{ 2} }}$ conditions by reaction path modelling using the EQ3/6 software package. $P_{{{\text{O}}_{ 2} }}$ was varied in accordance with the assumption that redox conditions are determined in part by NO₃. Results are in good agreement with experimental data on spring waters and subordinately with data on some coastal plain groundwater, which plot in a rather wide $P_{{{\text{CO}}_{ 2} }}$ and $P_{{{\text{O}}_{ 2} }}$ field. Although the dissolved Cr content is mostly of natural origin, fertilization may affect its fate.     

Chromium speciation and mobility in a high level nuclear waste vadose zone plume

Radioactive core samples containing elevated concentrations of Cr from a high level nuclear waste plume in the Hanford vadose zone were studied to asses the future mobility of Cr. Cr(VI) is an important subsurface contaminant at the Hanford Site. The plume originated in 1969 by leakage of self-boiling supernate from a tank containing REDOX process waste. The supernate contained high concentrations of alkali (NaOH ≈ 5.25 mol/L), salt (NaNO₃/NaNO₂ >10 mol/L), aluminate [Al(OH)₄⁻ = 3.36 mol/L], Cr(VI) (0.413 mol/L), and ¹³⁷Cs⁺ (6.51 × 10⁻⁵ mol/L). Water and acid extraction of the oxidized subsurface sediments indicated that a significant portion of the total Cr was associated with the solid phase. Mineralogic analyses, Cr valence speciation measurements by X-ray adsorption near edge structure (XANES) spectroscopy, and small column leaching studies were performed to identify the chemical retardation mechanism and leachability of Cr. While X-ray diffraction detected little mineralogic change to the sediments from waste reaction, scanning electron microscopy (SEM) showed that mineral particles within 5 m of the point of tank failure were coated with secondary, sodium aluminosilicate precipitates. The density of these precipitates decreased with distance from the source (e.g., beyond 10 m). The XANES and column studies demonstrated the reduction of 29-75% of the total Cr to insoluble Cr(III), and the apparent precipitation of up to 43% of the Cr(VI) as an unidentified, non-leachable phase. Both Cr(VI) reduction and Cr(VI) precipitation were greater in sediments closer to the leak source where significant mineral alteration was noted by SEM. These and other observations imply that basic mineral hydrolysis driven by large concentrations of OH⁻ in the waste stream liberated Fe(II) from the otherwise oxidizing sediments that served as a reductant for CrO₄²⁻. The coarse-textured Hanford sediments contain silt-sized mineral phases (biotite, clinochlore, magnetite, and ilmenite) that are sources of Fe(II). Other dissolution products (e.g., Ba²⁺) or Al(OH)₄⁻ present in the waste stream may have induced Cr(VI) precipitation as pH moderated through mineral reaction. The results demonstrate that a minimum of 42% of the total Cr inventory in all of the samples was immobilized as Cr(III) and Cr(VI) precipitates that are unlikely to dissolve and migrate to groundwater under the low recharge conditions of the Hanford vadose zone.     

Influence of soil minerals on chromium(VI) reduction by sulfide under anoxic conditions

The effects of soil minerals on chromate (Cr^VIO₄ ^2-, noted as Cr(VI)) reduction by sulfide were investigated in the pH range of 7.67 to 9.07 under the anoxic condition. The examined minerals included montmorillonite (Swy-2), illite (IMt-2), kaolinite (KGa-2), aluminum oxide (γ-Al₂O₃), titanium oxide (TiO₂, P-25, primarily anatase), and silica (SiO₂). Based on their effects on Cr(VI) reduction, these minerals were categorized into three groups: (i) minerals catalyzing Cr(VI) reduction – illite; (ii) minerals with no effect – Al₂O₃; and (iii) minerals inhibiting Cr(VI) reduction- kaolinite, montmorillonite, SiO₂ and TiO₂ . The catalysis of illite was attributed primarily to the low concentration of iron solubilized from the mineral, which could accelerate Cr(VI) reduction by shuttling electrons from sulfide to Cr(VI). Additionally, elemental sulfur produced as the primary product of sulfide oxidation could further catalyze Cr(VI) reduction in the heterogeneous system. Previous studies have shown that adsorption of sulfide onto elemental sulfur nanoparticles could greatly increase sulfide reactivity towards Cr(VI) reduction. Consequently, the observed rate constant, k _obs, increased with increasing amounts of both iron solubilized from illite and elemental sulfur produced during the reaction. The catalysis of iron, however, was found to be blocked by phenanthroline, a strong complexing agent for ferrous iron. In this case, the overall reaction rate at the initial stage of reaction was pseudo first order with respect to Cr(VI), i.e., the reaction kinetics was similar to that in the homogeneous system, because elemental sulfur exerted no effect at the initial stage prior to accumulation of elemental sulfur nanoparticles. In the suspension of kaolinite, which belonged to group (iii), an inhibitive effect to Cr(VI) reduction was observed and subsequently examined in more details. The inhibition was due to the sorption of elemental sulfur onto kaolinite, which reduced or completely eliminated the catalytic effect of elemental sulfur, depending on kaolinite concentration. This was consistent with the observation that the catalysis of externally added elemental sulfur (50 μM) on Cr(VI) reduction would disappear with a kaolinite concentration of more than 5.0 g/L. In kaolinite suspension, the overall reaction rate law was:

ANC,BNC and mobilization of Cr from polluted sediments in function of pH changes

During the manufacturing of chromate salts (1972–1992) large quantities of Chromite Ore Processing Residue (COPR) were released into a decantation pond east of the former chemical plant of Porto-Romano (Durres, Albania), giving rise to yellow colored pond sediments. These Cr(VI) bearing sediments were deposited upon Quaternary silty-clay lagoonal sediments rich in iron oxides and organic matter. The pH values in these lagoonal sediments vary around 6.6, while in the pond sediments, it is mainly acidic (due to the presence of the sulfur stock piles in the area and the release of the H₂SO₄ from the activity of the former chemical plant), varying between 1.4 and 3.8. Continuous leaching of the COPR waste resulted in yellow-colored surface water runoff. The prediction of pH changes in the different types of sediments based upon acid/base neutralizing capacity (ANC/BNC) jointly with the quantitative data on release of heavy metals and especially Cr is considered an important advantage of the pH_stat leaching test if compared to conventional leaching procedures. Thus, factors controlling the leaching of Cr(VI), Cr(III), Ca, Al, Fe, Mg from the COPR were investigated by means of pH_stat batch leaching tests and mineralogical analysis. Moreover, mathematical and geochemical modeling complemented the study. The COPR in the area contain very high concentrations of chromium 24,409 mg/kg, which mainly occurs as Cr(III) (75–90%) as well as Cr(VI) (25–10%). The leaching of Cr(VI) occurs in all the range (2–10) of the tested pH values, however, it decreases under acidic conditions. Beside some reduction of Cr(VI) to Cr(III), the Cr(VI) content of the leachtes remains relatively high in the acidic environment, while the limning of Cr(VI) pond sediments will increase the release of the latter specie. The leaching of the Cr(III) occurs strictly under acidic conditions, whereby limning of these sediments will give rise to the lower solubility of Cr(III). The key mineral phases responsible for the fast release of the Cr(VI) are: the chromate salts (i.e. sodium chromate and sodium dichromate), while sparingly soluble chromatite (CaCrO₄) and hashemite (BaCrO₄) release Cr(VI) very slowly. Thus, pH and mineral solubility have been identified as key factors in the retention and the release of the hexavalent CrO₄ ²⁻ and Cr₂O₇ ⁻ from the COPR-rich pond sediments.     

Geochemistry of shallow ground water in coastal plain environments in the southeastern United States: implications for aquifer susceptibility

Ground-water chemistry data from coastal plain environments have been examined to determine the geochemical conditions and processes that occur in these areas and assess their implications for aquifer susceptibility. Two distinct geochemical environments were studied to represent a range of conditions: an inner coastal plain setting having more well-drained soils and lower organic carbon (C) content and an outer coastal plain environment that has more poorly drained soils and high organic C content. Higher concentrations of most major ions and dissolved inorganic and organic C in the outer coastal plain setting indicate a greater degree of mineral dissolution and organic matter oxidation. Accordingly, outer coastal plain waters are more reducing than inner coastal plain waters. Low dissolved oxygen (O₂) and nitrate (NO₃⁻) concentrations and high iron (Fe) concentrations indicate that ferric iron (Fe (III)) is an important electron acceptor in this setting, while dissolved O₂ is the most common terminal electron acceptor in the inner coastal plain setting.The presence of a wide range of redox conditions in the shallow aquifer system examined here underscores the importance of providing a detailed geochemical characterization of ground water when assessing the intrinsic susceptibility of coastal plain settings. The greater prevalence of aerobic conditions in the inner coastal plain setting makes this region more susceptible to contamination by constituents that are more stable under these conditions and is consistent with the significantly (p<0.05) higher concentrations of NO₃⁻ found in this setting. Herbicides and their transformation products were frequently detected (36% of wells sampled), however concentrations were typically low (<0.1 μg/L). Shallow water table depths often found in coastal plain settings may result in an increased risk of the detection of pesticides (e.g., alachlor) that degrade rapidly in the unsaturated zone.     

Geochemistry of an acidic chromium sulfate plume

The historical disposal of acidic chromium sulfate solutions into unlined lagoons between 1953 and 1970 at an industrial site resulted in formation of a dense aqueous phase liquid (DAPL) plume [specific gravity 1.11 g/cm³, pH 3, up to 4700 mg/L Cr(III), and up to 90,000 mg/L SO₄]. The DAPL sank through the shallow glacial till aquifer to an underlying impermeable gneissic bedrock from where it migrated downgradient along buried channels incised in the bedrock. Because of its high density, the plume chemistry is sharply stratified vertically. Chromium(III) predominates in the DAPL because excess Cr(VI) not reduced in the original process has been reduced by Fe(II) derived from silicates, while Cr(OH)₃(am) occurs as surface coatings on silicate minerals and as discrete particles mixed with Fe(OH)₃(am) and Al(OH)₃(am). The solubility of Cr(OH)₃(am) accurately describes Cr(III) concentrations in the plume and nearby groundwater, while Al and Fe in solution are also consistent with solubility-controlling oxyhydroxides. Because of these solubility controls, metal cations are attenuated relative to more mobile Cl and SO₄, resulting in a chromatographic separation of solutes downgradient from the plume origin. The good agreement between predicted and observed solution concentrations illustrates the utility of equilibrium modeling when interpreting metal transport characteristics and in determining the efficacy of natural attenuation in subsurface systems.     

Geochemistry and Cr stable isotopes of Cr-contaminated groundwater in León valley,Guanajuato, México

León valley, located in the Mexican state of Guanajuato, has a long history (35+ a) of Cr contamination of groundwater and surface water. Here data are presented for Cr, major ion and trace element concentrations and Cr stable isotope measurements of groundwater in a heavily contaminated aquifer in Buenavista, where Cr ore processing residue piles (COPRPs) located in a chromate production factory are the main source of Cr. The aquifer directly beneath the factory still retains very high Cr(VI) concentrations (∼121 mg/L). Ongoing pump and treat remediation is keeping the high concentration plume confined to the factory area and immediate vicinity, though Cr is also detected at some distance away. Chromium isotope data of the aquifer directly under the factory show only a small increase in δ⁵³Cr (+0.33‰ to +0.81‰) and indicates minimal reduction of Cr(VI) to Cr(III). Very high Cr(VI) concentrations have possibly overwhelmed natural reductants and furthermore fresh Cr(VI) was being leached into the groundwater from the COPRP. From just one year of Cr isotope data it is clear that more aggressive remediation techniques will be necessary to reduce or eliminate the contamination. The fringes of the Cr plume have substantially lower concentrations and can be partially explained by transport of the main plume or mixing between waters from the nearby landfill and highly contaminated waters from QC. While the source of Cr at the fringes of the plume could be attributed to a source from the nearby landfill instead of the main plume from the factory, the Cr stable isotope data show enrichment in the heavier isotopes and point to varying amounts of reduction. Isotopic enrichment seen between 2007 and 2008 along the fringe may indicate either reduction or less unreacted Cr(VI) is being transported to the fringes. In either case some potential for natural attenuation of Cr(VI) exists at the western margin of the plume with the landfill playing a role.