Arsenic and metal levels in snake tissues from Lagoa Santa Karst, Brazil.

Concentrations of one metalloid (As) and eight metals (Cd, Cr, Cu, Hg, Mn, Ni, Pb, and Zn) were determined in tissues (muscle, liver, and kidney) of eight snake species (Bothrops neuwiedi, Crotalus durissus, Dipsas mikanii, Epicrates crassus, Helicops modestus, Micrurus carvalhoi, Oxyrhopus guibei, and Oxyrhopus trigeminus) from Lagoa Santa Karst. Except for Cu and Zn, all other analyzed elements were detected in concentrations within the ranges previously reported for snakes inhabiting polluted areas, emphasizing Hg (specific Hg mean concentrations varied from 0.87 to 9.76 µg g-1 d.w). The highest mean concentrations of all elements except Zn were found in muscle samples of the false corals O. guibei (means ranged from 2.01 [Pb] to 9.76 [Hg]). The highest Zn mean concentration (13.77 µg g-1 d.w) was detected in the kidney of the water snake H. modestus. No significant correlation was found between element concentrations and body size for all species. Significant interorgan differences were observed for As, Cr, Cu, Hg, Mn, Pb, and Zn concentrations in the three tissues in H. modestus. Significant interspecific differences were found in at least one organ for all elements. Significant pairwise differences were found between diet specialist species and between these species and broader diet species, while no significant difference was found between the broader diet species. The bioaccumulation of As and metals in snakes from Lagoa Santa Karst could be associated with natural rock dissolution and erosion processes but also with the wide-scale mining in the region and the increased agriculture and urbanization.

Radionuclide transport in fractured chalk under abrupt changes in salinity.

Internationally, it has been agreed that geologic repositories for spent fuel and radioactive waste are considered the internationally agreed upon solution for intermediate and long-term disposal. In countries where traditional nuclear waste repository host rocks (e.g., clay, salt, granite) are not available, other low permeability lithologies must be studied. Here, chalk is considered to determine its viability for disposal. Despite chalk's low bulk permeability, it may contain fracture networks that can facilitate radionuclide transport. In arid areas, groundwater salinity may change seasonally due to the mixing between brackish groundwater and fresh meteoric water. Such salinity changes may impact the radionuclides' mobility. In this study, radioactive U(VI) and radionuclide simulant tracers (Sr, Ce and Re) were injected into a naturally fractured chalk core. The mobility of tracers was investigated under abrupt salinity variations. Two solutions were used: a low ionic strength (IS) artificial rainwater (ARW; IS ∼0.002) and a high IS artificial groundwater (AGW; IS ∼0.2). During the experiments, the tracers were added to ARW, then the carrier was changed to AGW, and vice versa. Ce was mobile only in colloidal form, while Re was transported as a conservative tracer. Both Re and Ce demonstrated no change in mobility due to salinity changes. In contrast, U and Sr showed increased mobility when AGW was introduced and decreased mobility when ARW was introduced into the core. These experimental results, supported by reactive transport modeling, suggest that saline groundwater solutions promote U and Sr release via ion-exchange and enhance their migration in fractured chalk. The study emphasizes the impact of salinity variations near spent fuel repositories and their possible impact on radionuclide mobility.

Inputs and transport of acid mine drainage-derived heavy metals in karst areas of Southwestern China.

Heavy metal pollution caused by acid mine drainage (AMD) is a global environmental concern. The processes of migration and transformation of heavy metals carried by AMD are more complicated in karst areas where carbonate rocks are widely distributed. Water, suspended particulate matter (SPM), and sediments are the crucial media in which heavy metals migrate and it is important to elucidate the geochemical behavior of AMD heavy metals in these environments. This study tracked AMD heavy metals from release to migration and transformation in a natural river system in a karst mining area. AMD directly impacted the hydrochemical composition of the karst water environment, but the carbonate rock naturally neutralized the acidity of the AMD. AMD heavy metal concentrations decreased gradually after the tributaries from the mining area entered the main river, with the metals tending to accumulate in SPM and sediments. The forms in which heavy metals were present were influenced by pH and their relative concentrations. Raman spectroscopy and transmission electron microscopy of sediments from the mining area suggested that the presence of an iron phase plays an important role in the fate of AMD-derived heavy metals. It is, therefore, necessary to elucidate the mechanisms of iron phase precipitation from sediments in order to control AMD-derived heavy metals in karst mining areas. This study improves our understanding of the geochemical behavior of heavy metals in karst environments and provides direction for the prevention and control of AMD in affected areas.

Effects of Initial pH and Carbonate Rock Dosage on Bio-Oxidation and Secondary Iron Mineral Synthesis.

The effect of pH is a key factor in biomineralization mediated by Acidithiobacillus ferrooxidans to promote the transformation of Fe into secondary iron minerals. This study aimed to investigate the effects of initial pH and carbonate rock dosage on bio-oxidation and secondary iron mineral synthesis. Variations in pH and the concentrations of Ca2+, Fe2+, and total Fe (TFe) in the growth medium of A. ferrooxidans were examined in the laboratory to determine how they affect the bio-oxidation process and secondary iron mineral synthesis. The results showed that in systems with an initial pH of 1.8, 2.3, and 2.8, the optimum dosages of carbonate rock were 30, 10, and 10 g, respectively, which significantly improved the removal rate of TFe and the amount of sediments. At an initial pH of 1.8 and a carbonate rock dosage of 30 g, the final removal rate of TFe reached 67.37%, which was 28.03% higher than that of the system without the addition of carbonate rock, and 36.9 g·L-1 of sediments were generated, which was higher than that of the system without the addition of carbonate rock (6.6 g·L-1). Meanwhile, the number of sediments generated by adding carbonate rock were significantly higher than those without the addition of carbonate rock. The secondary minerals were characterized by a progressive transition from low crystalline assemblages composed of calcium sulfate and subordinated jarosite, to well crystal-line assemblages composed of jarosite, calcium sulfate, and goethite. These results have important implications for comprehensively understanding the dosage of carbonate rock in mineral formation under different pH conditions. The findings help reveal the growth of secondary minerals during the treatment of AMD using carbonate rocks under low-pH conditions, which offers valuable information for combining the carbonate rocks with secondary minerals to treat AMD.

Identification of high ecological risk areas with naturally high background value of soil Cd related to carbonate rocks.

The characteristics of high concentrations or high activity levels of heavy metals, especially Cd, in soils caused by the pedogenesis of rocks are attracting increased attention. Carbonate rocks and black shales often coexist during geological deposition, but the risk characteristics of heavy metals are different after their weathering into the soil. The purpose of this study was to investigate the element concentrations of a naturally high background value area, to identify patterns of different risk areas, and to make recommendations for the safe usage of farmland. The results showed that, compared with the soil in the carbonate rock area, the soil in the black shale area was more acidified and most of the heavy metal elements were leached. Based on the soil pH value and the heavy metal concentrations, an identification method for land risk areas within naturally high background values was established, and land planning was carried out using this method. The exceeding rates of Cd in rice for the preferential protection area and strict control area were 0.0 and 50.0%, respectively. Therefore, in naturally high background area, the identified lithology can apply to maximize the use of farmland resources. This method provides a basis for preliminary ecological risk screening in naturally high background value areas using the results of the soil survey. A suggestion for the prevention and control of soil pollution in areas with naturally high background values was put forward. In carbonate rock areas, the soil should be closely monitored to prevent soil acidification.

Multi-scale approach to assess total porosity and pore size in four different kinds of carbonate rocks.

Petroleum is, at present, still the main energy source in the world. Most of it is stored in carbonate rock reservoirs with complex inner structures and pores ranging from nanometers to dozens of meters. Knowing the rock's entire pore network is indispensable to perform an effective petroleum extraction. X-ray microtomography, a technique that generates images from samples' inner structure and a powerful tool to evaluate the 3D pore network, was employed in this scientific research to scan four kinds of carbonate rocks (Dolomite, Desert Pink, Indiana, and Winterset) in five different pore scales (90 µm, 13 µm, 5.5 µm, 1.0 µm, and 65 nm). A multi-scale approach based on the number of pores was applied to integrate different pore scale data and assess the total porosity as well as each sample pore size distribution. The results were compared to classical Mercury Injection Capillary Pressure (MICP) results, demonstrating a fair agreement in total porosity in the two samples. Multi-scale porosity of the Dolomite sample was 17.7% against (18.9 ± 2.1)% of the MICP porosity. The Winterset sample had multi-scale porosity of 26.2%, while MICP porosity was (31.2 ± 0.6)%. Pore size distribution results were rather satisfactory, especially when overlapping regions in different scales were compared. In general, the multi-scale approach showed good potential. It still needs further evaluation to fine-tune some procedures and fluid flow simulation tests but might become a useful tool to study reservoir rocks with a wide range of pore sizes, such as carbonate rocks.

Simulated Investigation in Wormhole Expansion Law of Gelling Acid Etching and Its Influencing Factors in Deep Carbonate Reservoirs.

Acidizing with gelling acid is the key technology in developing a carbonate reservoir successfully. It is difficult for the laboratory to carry out the radial displacement experiment with a large rock core. It is necessary to establish the gelling acid wormhole expansion model under the radial conditions, simulate the gelling acid wormhole expansion law under the radial conditions, optimize the construction parameters, and provide the basis for the optimal design of carbonate reservoir matrix acidizing. The research objective is to simulate the gelling acid etching wormhole expansion in a deep carbonate reservoir and make clear its influencing factors, which are helpful for reservoir stimulation. The mathematical model of gelling acid wormhole expansion was established, considering the influence of pore microscopic characteristics on acid flow and acid rock reaction. The simulation results indicated that viscosity, surface reaction rate, and hydrogen ion diffusion coefficient have different effects on gelling acid etching wormhole. The spatial distribution of pores determines the trend of gelling acid solution and thus the shape of the armhole. Perforation completion has a significant impact on the expansion of gelling acid etching wormhole. The wormhole extends forward along the perforation hole, and perforation increases the length of the wormhole. This wormhole expansion law is very suitable in situations where a deep carbonate reservoir is needed for gelling acid fracturing.

Predicting the electrokinetic properties on an outcrop and reservoir composite carbonate surfaces in modified salinity brines using extended surface complexation models.

Surface complexation models (SCM), based mainly on the diffuse double layer (DDL) theory, have been used to predict zeta potential at the crude oil-brine-rock (COBR) interface with limited success. However, DDL is inherently limited in accurately predicting zeta potential by the assumptions that all the brine ions interact with the rock surface at the same plane and by the double layer collapse at higher brine ionic strength (>1M). In this work, a TLM-based SCM captured zeta potential trends at the calcite-brine interface with ionic strength up to 3 M. An extended DDL and TLM-based SCMs were used to predict the electrokinetic properties of a composite carbonate rock showing a different mineralogical composition. The extended TLM-based SCM captured the zeta potential prediction trends and magnitude, highlighting the contribution of the inorganic minerals and organic impurities on the composite carbonate surface. In contrast, the extended DDL-based SCM captured the zeta potential trends but failed to capture the magnitude of the measured zeta potential. Interestingly, the TLM-based SCM predicted a positive SP for the rock-brine interface, which could explain the oil-wet nature of composite carbonate rocks due to electrostatic adsorption of negatively charged carboxylic acids. Conversely, the DDL-based SCM predicted a negative SP, leading to an inaccurate interpretation of the electrokinetic properties at the rock-brine interface. Thus, the use of extended TLM-based SCM was required to accurately predict the zeta potential and account for the adsorption of carboxylic acids on the reservoir composite carbonate surface.

High-resolution X-ray diffraction datasets: Carbonates.

X-ray diffraction (XRD) analysis is a versatile and reliable method used in the identification of minerals in solid samples. It is one of the primary techniques geoscientists, mineralogist, solid-state chemists depend on to characterize the composition of unknown samples. In recent years there has been a growing interest among researchers to have readily accessible and large dataset to use to calibrate their experiment or to simply build various statistical models. Sadly, this is difficult to come by. Most well-curated datasets are propriety in nature and often too expensive for the average researcher. Additionally, when these datasets are available, they might not be suitable for purpose due to lack of proper coverage for certain a mineral of interest. For these reasons, we have carefully selected and curated samples rich in calcium carbonate that will be useful for various applications. Our dataset includes 1680 X-ray diffraction scans of samples collected from carbonate rich rock formations outcrops in Spain, Italy, and Saudi Arabia. They represent materials with total carbonate concentration range between 30-99%. The spectra were acquired on a Malvern PANalytical EMPYREAN Diffractometer system at two theta range 2- 70 and 0.01 step size. This dataset will be valuable to geoscientists, mineralogist, solid-state chemists, data scientists alike looking to design experiments, build mineralogical reference databases or statistical models with sufficient data points. We currently use the dataset in our own projects to develop comprehensive carbonate library and felt compelled to share.

[Migration, Transformation Characteristics, and Ecological Risk Evaluation of Heavy Metal Fractions in Cultivated Soil Profiles in a Typical Carbonate-Covered Area].

In China, high heavy metal concentrations in cultivated soil are mainly distributed in carbonate-covered areas. The migration and transformation of heavy metals in such soils are influenced by interactions between natural processes and human activities. This study examined the profiles of nine paddy soils, derived from carbonate rocks in Guangxi. The Cd, As, Zn, Cr, Cu, Hg, Ni, and Pb contents we determined, and soil properties such as pH, Corg content, and fractions of Cd, As, Zn, and Cr were tested. Based on the above data, we assessed the vertical distribution of heavy metal fractions, as well as the ecological risks and factors affecting the migration ability of heavy metals, under the influence of human activities and natural soil formation. The results show that compared with the carbonate rocks in Guangxi, the soil profile of the study area is significantly enriched with all eight heavy metals. Among them, Cd, As, Zn, and Cr exceeded China's agricultural land (paddy field) pollution risk screening values, and Cd and As partially exceed the risk intervention values. All fractions of Cd in the soil profiles are widely distributed. The proportion of water-soluble and ion-exchange Cd fractions with high ecological risk decreased significantly from the surface to greater depths in the soil profile. As, Zn, and Cr in the soil profile were mainly in residual states, and the proportion of water-soluble and ion-exchange fractions did not change considerably with increasing depth. The evaluation results of RAC and RSP show that Cd pollution risk in the study area is relatively high, whereas As, Zn, and Cr are generally pollution-free or risk-free. In naturally developed soil in the study area, the clay mineral content and degree of soil development have significant effects on Cd migration, whereas in the cultivated layer affected by human activities, the soil pH and organic matter content are the main controlling factors. The migration ability of soil As is mainly related to soil organic matter, Fe2O3 content, and soil development degree, but organic matter has an obviously enhanced effect in the tillage layer. The main controlling factor of Zn and Cr migration in soil is pH, and the effects are more intense under the disturbance of human activities.

Interaction analysis of hydrochemical factors and dissolved heavy metals in the karst Caohai Wetland based on PHREEQC, cooccurrence network and redundancy analyses.

In this study, to clarify the interaction between dissolved heavy metals and the coexisting chemical factors in karst wetland waters, surface water samples were collected from the Caohai Wetland during a water year, and the hydrochemistry and heavy metal pollution characteristics of the samples were analyzed. The main influencing factors of heavy metals in different water periods were identified through a cooccurrence network analysis. To further analyze the influence mechanism of these main influencing factors, the forms of heavy metals in the water were simulated with PHREEQC software, and the effects of these main influencing factors on the forms were analyzed by redundancy analysis. The results show that Ca2+ was the main cation in the wetland water, while the main anion was HCO3-. The hydrochemical facies of the Caohai Wetland in the wet and dry seasons were Ca-Mg-SO4-HCO3 and Ca-HCO3, respectively. Cd was the main pollutant in the Caohai Wetland, with Cd levels seriously exceeding the standards. The characteristics of the karst water in the Caohai Wetland are apparent. The cooccurrence network analysis shows that pH, dissolved oxygen (DO), electrical conductivity (EC), SO42- and HCO3- are the main factors regulating heavy metals. The results of morphological simulation and analysis were used to explore the mechanism of action of these factors. These data provide geochemical information useful for water quality assessment and management plans on heavy metal pollution.

Colloid-facilitated transport of 238Pu, 233U and 137Cs through fractured chalk: Laboratory experiments, modelling, and implications for nuclear waste disposal.

The influence of montmorillonite colloids on the mobility of 238Pu, 233U and 137Cs through a chalk fracture was investigated to assess the transport potential for radioactive waste. Radioisotopes of each element, along with the conservative tracer tritium, were injected in the presence and absence of montmorillonite colloids into a naturally fractured chalk core. In parallel, batch experiments were conducted to obtain experimental sorption coefficients (Kd, mL/g) for both montmorillonite colloids and the chalk fracture material. Breakthrough curves were modelled to determine diffusivity and sorption of each radionuclide to the chalk and the colloids under advective conditions. Uranium sorbed sparingly to chalk (log Kd = 0.7 ± 0.2) in batch sorption experiments. 233U(VI) breakthrough was controlled primarily by the matrix diffusion and sorption to chalk (15 and 25% recovery with and without colloids, respectively). Cesium, in contrast, sorbed strongly to both the montmorillonite colloids and chalk (batch log Kd = 3.2 ± 0.01 and 3.9 ± 0.01, respectively). The high affinity to chalk and low colloid concentrations overwhelmed any colloidal Cs transport, resulting in very low 137Cs breakthrough (1.1-5.5% mass recovery). Batch and fracture transport results, and the associated modelling revealed that Pu migrates both as Pu (IV) sorbed to montmorillonite colloids and as dissolved Pu(V) (7% recovery). Transport experiments revealed differences in Pu(IV) and Pu(V) transport behavior that could not be quantified in simple batch experiments but are critical to effectively predict transport behavior of redox-sensitive radionuclides. Finally, a brackish groundwater solution was injected after completion of the fracture flow experiments and resulted in remobilization and recovery of 2.2% of the total sorbed radionuclides which remained in the core from previous experiments. In general, our study demonstrates consistency in sorption behavior between batch and advective fracture transport. The results suggest that colloid-facilitated radionuclide transport will enhance radionuclide migration in fractured chalk for those radionuclides with exceedingly high affinity for colloids.

Leaching Methods for Ba Isotope Studies of Carbonates.

Barium (Ba) stable isotopes in carbonate rock have great potential to provide valuable information on environmental change and the biogeochemical cycles of oceans in the past. Ba in carbonate rock can exist in various phases, such as adsorbable and silicate-bound Ba. However, only the carbonate-bound phase is considered to record the Ba isotopic compositions of ambient seaweater. Here, we designed a two-step leaching experiment to obtain the carbonate-bound Ba in two typical carbonate rocks: limestone and cap dolostone. The results showed that after leaching by 1 mol L-1 ammonium acetate, the carbonate-bound Ba extracted by mixed solution of 1.5 mol L-1 acetic acid and 1 mol L-1 ammonium acetate in each studied sample have indistinguishable isotope ratios in leaching time conditions between 12 and 72 h. More importantly, the carbonate-bound Ba isotope ratios were quite different from those of the residue (up to 10 times of analytic uncertainty, 2SD ≤ ±0.04‰) after leaching in three out of four leaching experiments, indicating that noncarbonated fraction could overprint a primary seawater signal. Our sequential leaching techniques could improve targeting of carbonate-bound Ba isotope signatures in various carbonate rocks to trace the Ba cycling in the oceans.

An enigmatic Maastrichtian small benthic Foraminifera of the Tarbur Formation (Iran, Zagros zone).

The Maastrichtian carbonates of the Tarbur Formation from the Zagros zone (SW Iran) are very rich in larger Foraminifera. Additionally, smaller benthic Foraminifera occur as well, one of them presented here as Zagrosaria pinnata gen. et sp. nov. It occurs abundantly at the Mandegan section within two levels corresponding to the lower photic zone. Based on the existence of a central pile along with an umbilical plate, and the lacks of true interiomarginal apertures, likely represented by a perforate surface, the suprageneric placing of Zagrosaria gen. nov. remains controversial. The ultimate aperture is represented by a central elongate double-bifid slot that arguably lies on a finely perforated and heavy feathered apertural face. Based on the reduced dimensions and lack of dimorphism the new taxon could be considered an epifaunal or shallow-infaunal r-strategist.

[Bioavailability, Translocation, and Accumulation Characteristic of Heavy Metals in a Soil-Crop System from a Typical Carbonate Rock Area in Guangxi, China].

This study uses 68 sets of paddy soil and rice grain samples collected from an area of carbonate rocks in Guangxi Province, China, to explore the ecological risks of heavy metals (As, Cd, Cr, Cu, Hg, Pb, Ni, and Zn) in soils from a high background area. We analyzed the concentrations of these heavy metals in soil and grain samples, and their chemical speciation in soil, and use these data to assess the associated ecological risks by means of statistics, a geo-accumulation index, bioconcentration factors (BCF), and correlation analysis. The arithmetic mean values of heavy metals concentrations in soil samples from the study area were (75.8±50.1), (1.91±1.02), (467.0±253.1), (48.5±9.8), (0.21±0.08), (76.2±28.1), (84.2±25.0), and (258.0±122.6) mg·kg-1 for As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn, respectively, which were remarkably higher than of those from other regions within Guangxi Province and China. In comparison to China's soil environmental quality standard risk screening values (GB 15618-2018), the over-standard rates of Cd, As, and Cr were 95.6%, 86.8%, and 69.1%, respectively. In comparison to risk intervention values, the over-standard rates of Cd, As, and Cr were 27.9%, 17.6%, and 5.9%, respectively. Speciation analysis on heavy metals indicated that As, Cr, Cu, Ni, Pb, and Zn were mainly found in a residual form, and accounted for>80% of the total concentrations, and had a low bioavailability. The bioactive components (F1+F2+F3) of Cd accounted for 21%, and the bioactivity of Cd was higher than other elements. The potential bioavailable components (F4+F5+F6) of Hg accounted for 44%, with low total concentrations, which are understood to have little potential ecological harm for crops. However, the over-standard rates of Pb, Cd, and Cr in rice grains were only 23.5%, 8.8%, and 2.9%, respectively. Correlation analysis showed that there was no significant correlation between the concentrations of heavy metals in soils and the corresponding rice grains. The mean BCFs of each heavy metal were <0.1, and the BCFs of Hg, Pb, As, Cr, and Ni were <0.05. Overall, we found relatively high concentrations, low activity, and low ecological risks for heavy metals in the study area. For high geological background materials such as carbonate rocks, factors such as metal speciation, biological activity, and crop over-standard rates should be taken into account along with the traditional use of the total amount of heavy metals in a soil as the evaluation standard when formulating pollution control policies.

Arsenic release from Floridan Aquifer rock during incubations simulating aquifer storage and recovery operations.

While aquifer storage and recovery (ASR) is becoming widely accepted as a way to address water supply shortages, there are concerns that it may lead to release of harmful trace elements such as arsenic (As). Thus, mechanisms of As release from limestone during ASR operations were investigated using 110-day laboratory incubations of core material collected from the Floridan Aquifer, with treatment additions of labile or refractory dissolved organic matter (DOM) or microbes. During the first experimental phase, core materials were equilibrated with native groundwater lacking in DO to simulate initial non-perturbed anaerobic aquifer conditions. Then, ASR was simulated by replacing the native groundwater in the incubations vessels with DO-rich ASR source water, with DOM or microbes added to some treatments. Finally, the vessels were opened to the atmosphere to mimic oxidizing conditions during later stages of ASR. Arsenic was released from aquifer materials, mainly during transitional periods at the beginning of each incubation stage. Most As released was during the initial anaerobic experimental phase via reductive dissolution of Fe oxides in the core materials, some or all of which may have formed during the core storage or sample preparation period. Oxidation of As-bearing Fe sulfides released smaller amounts of As during the start of later aerobic experimental phases. Additions of labile DOM fueled microbially-mediated reactions that mobilized As, while the addition of refractory DOM did not, probably due to mineral sorption of DOM that made it unavailable for microbial utilization or metal chelation. The results suggest that oscillations of groundwater redox conditions, such as might be expected to occur during an ASR operation, are the underlying cause of enhanced As release in these systems. Further, ASR operations using DOM-rich surface waters may not necessarily lead to additional As releases.

Natural radioactivity in rocks of the Modane-Aussois region (SE France).

The activity concentrations of 40K, 232Th, and 238U in the characteristic rocks of the Modane-Aussois region (Western Alps, France) were determined using an HPGe gamma-ray spectrometry system. The activity concentrations of 40K varied from 18 Bqkg-1 (limestone dolomite) to 392 Bqkg-1 (calcschist), while those of 232Th varied from 0.7 Bqkg-1 (limestone dolomite) to 18 Bqkg-1 (calcschist). The activities associated with 238U ranged from 9 (quartzite) to 29 Bqkg-1 (dolomite). In the investigated rock samples, concentrations of 238U (ppm) and 40K (%) had a strong negative correlation.