Silicate Dissolution Research Articles

Establishing a water baseline for the unconventional gas industry: A multiple environmental isotopes assessment (18O, 2H, 3H, 13C, and 14C) of surface and groundwater in the São Francisco Basin, Brazil

Unconventional hydrocarbon production has become the target of an intensive environmental debate due to the risks it poses to water resources. Fracking, while enabling the extraction of oil and gas from ultra-low permeability reservoirs, also possesses the risk of polluting water systems through failures from hydraulic fracturing and its associated procedures. The need to foster national industrial development with a transitional energy matrix has led Brazil to discuss the environmental suitability before producing its large unconventional reserves. Many studies have highlighted the need for a robust environmental characterization before the development of the unconventional industry. In this sense, multiple environmental isotopes may work as a proxy for identifying water contamination right from the early stages. Environmental isotopes may also be applied to enhance the understanding of the natural geochemical processes intrinsic to a given area. This study presents an environmental isotopes baseline for the groundwater and riverine water systems within the São Francisco Basin, a proven tight gas reservoir in Brazil, in a pre-operational context. δ18O, δ2H, 3H, δ13C, and Δ14C were evaluated in three different seasons in groundwater and surface water samples, along with other auxiliary parameters such as physical-chemical parameters (in situ), major ions, and d-excess. The δ2H and δ18O in surface water shows an upstream → downstream enrichment trend, with some variations suggesting baseflow interactions in the surface water systems. An evaporation line for the study area was defined as δ2H = 4.6903 δ18O + 10.362. δ13C indicates a mutual dissolution of silicates and carbonates in the groundwater system and suggests a group of samples highly related to the recharge areas. Groundwater dating denotes the Serra da Saudade Formation as a modern fractured aquifer with a strong recharge capacity. These findings support stakeholders in environmental monitoring and management of the unconventional gas industry.

Hydrochemical analysis and groundwater suitability for drinking and irrigation in an arid agricultural area of the Northwest China

Groundwater is the foremost water source in the arid and semiarid regions of Northwest China. Assessing groundwater's drinking and irrigation quality is essential for protecting these valuable groundwater resources. In this study, a total of 24 confined groundwater samples and 54 phreatic groundwater samples were collected in the southern and central Ningxia area for hydrochemical analysis and quality assessment. The hydrochemical results revealed that hydrochemical types of phreatic and confined groundwater consistently belonged to Na-SO4-Cl and Na-Mg-SO4-Cl types. The driving forces of groundwater chemistry were determined by gypsum dissolution, silicate dissolution, and positive cation exchange for phreatic and confined aquifers. The entropy-weighted water quality index (EWQI) and irrigation water quality index (IWQI) showed that the drinking water quality and irrigation quality were better in phreatic groundwater than in confined groundwater due to the Neogene-Paleogene groundwater system recharge and strong evaporation. Measures such as controlling groundwater extraction and optimizing well placement need to be implemented. The achievements would be helpful for groundwater management and protection in agricultural areas under semi-arid and arid climates.

Contrasting riverine K and Li isotope signatures during silicate weathering in the southeastern Tibetan Plateau

Mountain uplift produces steep slopes and rapid erosion rates, and hence exposes more silicate rocks to weathering and accelerates the drawdown of atmospheric CO2, driving global cooling over the Cenozoic. Riverine potassium (K) and lithium (Li) isotopes are two promising proxies for silicate weathering because the isotopes fractionate during secondary mineral formation associated with weathering and these two elements are highly enriched in silicate rocks. This study explores similarity and difference between K and Li isotope behaviors during weathering across the Lancang River in the southeastern Tibetan Plateau, in order to evaluate the role of these two isotopes in tracing silicate weathering in tectonically active mountains. Dissolved K and Li in the Lancang River mainstream are mainly derived from the dissolution of silicates. River waters are enriched in the heavy isotope of K (−0.35‰ to +0.05‰) and Li (+7.1‰ to +20.4‰) relative to suspended sediments (−0.56‰ to −0.42‰ and −2.3‰ to 0‰, respectively), and the element partitioning between dissolved and suspended loads controls dissolved δ7Li and δ41K values. Interesting, dissolved δ7Li and δ41K show the opposite evolution trends toward downstream. As the weathering intensity increases downstream (higher Si/(Na + K)⁎ ratios), the dissolved δ7Li values increase from 7.4‰ to 13.6‰, whereas dissolved δ41K values decrease from −0.04‰ to −0.26‰. Compared to its headwater with limited weathering, more secondary minerals (e.g., clays) are formed in the lower reaches (e.g., floodplains) with low physical erosion rates and high annual precipitation, driving more 6Li incorporated into clays, thus high dissolved δ7Li values downstream. While dissolved δ41K values and the fraction of 39K incorporated into secondary minerals are low in floodplains, this is consistent with observations that illite is a main K-bearing phase among clay minerals, and its content in the clay fraction in river sediment decreases as weathering intensity increases. Our study further supported that the increase in Cenozoic seawater δ7Li is related to weathering in floodplains rather than weathering of mountains, whereas K isotope fractionation mainly occurs in mountain regions. If Cenozoic seawater δ41K rise, as with δ7Li change, weathering at higher elevation may exerts more effect on high riverine and seawater δ41K values. This study improves our knowledge of K-Li isotope behaviors during present-day silicate weathering, and the response of K-Li isotopes in sedimentary archives to silicate weathering over geological time.

Refining Boron Isotopic Measurements of Silicate Samples by Multi‐Collector‐Inductively Coupled Plasma‐Mass Spectrometry (MC‐ICP‐MS)

Solution MC‐ICP‐MS is an established technique for high precision boron isotope measurement results (δ11BSRM 951) in carbonates, yet its application to silicate rocks has been limited. Impediments include volatilisation during silicate dissolution and contamination during chemical purification. To address this, we present a low‐blank sample preparation procedure that couples hydrofluoric acid‐digestion and low‐temperature evaporation (mannitol‐free), to an established MC‐ICP‐MS measurement procedure following chemical purification using B‐specific Amberlite IRA 743 resin. We obtain accurate δ11BSRM 951 values (intermediate precision ±0.2‰) for boric acid (BAM ERM‐AE121 19.65 ± 0.14‰) and carbonate (NIST RM 8301 (Coral) 24.24 ± 0.11‰) reference materials. For silicate reference materials covering mafic to felsic compositions we obtain δ11BSRM 951 with intermediate precision < ±0.6‰ (2s), namely JB‐2 6.9 ± 0.4‰; IAEA‐B‐5 ‐6.0 ± 0.6‰; IAEA‐B‐6 ‐3.9 ± 0.5‰ (2s). Furthermore, splits of these same reference materials were processed by an alternative fusion and purification procedure. We find excellent agreement between δ11BSRM 951 measurement results by MC‐ICP‐MS of the reference materials using both sample processing techniques. These measurement results show that our sample processing and MC‐ICP‐MS methods provide consistent δ11BSRM 951 values for low B‐mass fraction samples. We present new data from Mid Ocean Ridge Basalt (MORB) glass, documenting a range in δ11BSRM 951 from ‐5.6 ± 0.3‰ to ‐8.8 ± 0.5‰ (2s), implying some upper mantle δ11BSRM 951 heterogeneity.

Microbial Catalysis for CO2 Sequestration: A Geobiological Approach.

One of the greatest threats facing the planet is the continued increase in excess greenhouse gasses, with CO2 being the primary driver due to its rapid increase in only a century. Excess CO2 is exacerbating known climate tipping points that will have cascading local and global effects including loss of biodiversity, global warming, and climate migration. However, global reduction of CO2 emissions is not enough. Carbon dioxide removal (CDR) will also be needed to avoid the catastrophic effects of global warming. Although the drawdown and storage of CO2 occur naturally via the coupling of the silicate and carbonate cycles, they operate over geological timescales (thousands of years). Here, we suggest that microbes can be used to accelerate this process, perhaps by orders of magnitude, while simultaneously producing potentially valuable by-products. This could provide both a sustainable pathway for global drawdown of CO2 and an environmentally benign biosynthesis of materials. We discuss several different approaches, all of which involve enhancing the rate of silicate weathering. We use the silicate mineral olivine as a case study because of its favorable weathering properties, global abundance, and growing interest in CDR applications. Extensive research is needed to determine both the upper limit of the rate of silicate dissolution and its potential to economically scale to draw down significant amounts (Mt/Gt) of CO2 Other industrial processes have successfully cultivated microbial consortia to provide valuable services at scale (e.g., wastewater treatment, anaerobic digestion, fermentation), and we argue that similar economies of scale could be achieved from this research.

Physico-chemical quality of tap water in an urban environment: The case of the Massengo district, arrondissement n°9 Djiri, Brazzaville

The drinking water supply in the Massengo Soprogi district in Brazzaville's Djiri 9 arrondissement is provided by the Djiri river, whose mineralization dictates that of the water in the distribution network. Tap water in this district was analyzed for various parameters in order to understand the acquisition of mineralization. The physico-chemical analysis of these water samples showed that the water has very little mineralization, and is not very hard due to the geological nature of the Djiri riverbed (sandy soil). The pH values in the wet and dry seasons give the water a weakly acidic nature and, taking into account the concentration of bicarbonate ions, a low level of complete alkalinity. The geochemical processes responsible for this mineralization are mainly: water-rock interaction and ion exchange, and silicate dissolution. The geochemical methods used were the Gibbs diagram and the calculation of a few characteristic ionic ratios. Tap water has a strong aggressive and corrosive tendency, as revealed by Langelier's saturation index and Ryznar's stability index.

Geochemical Investigations of the Geothermal Waters in the Kangding Area, SW China: Constraints from Hydrochemistry and D-O-T Isotopy

Geothermal resources, as a representative of clean energy, has been paid significant attention in the world. Due to active neotectonics and widespread magmatic rocks, the abundant geothermal waters in the Kangding area have been investigated. Hydrochemistry and D–O–T isotopy studies were carried out to clarify the genetic mechanism of geothermal waters. The hydrochemical types of geothermal waters are mainly Ca2+–Na+–HCO3− type, Na+–Cl−–HCO3− type, and Na+–HCO3− type. Silicate dissolution and the cation exchange process are the water–rock interactions determining hydrochemical compositions. The recharge elevation of geothermal water was calculated to be 3034–3845 m, with an average of 3416 m. The reservoir temperatures of shallow and deep geothermal reservoirs vary from 50 to 115 °C and from 114 to 219 °C, respectively, and the mixing ratio of cold water is 0.56–0.89. These findings help to reveal the genetic mechanism of geothermal waters in the Kangding area.

Hydrochemical Characteristics, Water Quality, and Evolution of Groundwater in Northeast China

Groundwater is vital to local human life and agricultural irrigation, and the quality of the water is critical to human health. As a result, it is critical to investigate the hydrochemical evolution and water quality of groundwater in the Sanjiang Plain. There were 259 samples obtained. Furthermore, hydrogeochemical simulation was performed to highlight groundwater’s hydrochemical features, evolution process, and water quality. The analytical results show that the groundwater in the study area is somewhat alkaline with a mean TDS of 285.94 mgL−1 and the primary contributing ions being Ca2+ and HCO3−. The closer the concentration of TDS and NO3− is to the city, the higher the concentration, indicating that the chemical composition of the water body has been affected by certain human activities. The Piper diagram, Gibbs diagram, and correlation analysis results demonstrate that the chemical type of groundwater is mostly HCO3-Ca and the hydrochemistry is primarily regulated by weathering and carbonate and silicate dissolution. According to the entropy-weighted water quality index, the groundwater quality in this location is pretty acceptable. This study could help strengthen groundwater quality monitoring based on local conditions, identify the source of nitrate, provide data support for the safe use of local water resources, and serve as a reference for global water chemical evolution and water quality evaluation in cold regions.

Groundwater–Surface Water Exchange and Spatial Distribution of Arsenic in Arid and Semi-Arid Regions: The Case of Aksu River in Xinjiang, Northwestern China

The Aksu River, a quintessential inland river, exhibits elevated arsenic (As) concentrations in certain sections of its natural waters. Further investigation is necessary to determine the role of surface water and groundwater (SW-GW) exchanges in contributing to these high As concentrations. Both surface water and groundwater constitute crucial components of the basin water cycle, and the interaction between the two has been a central focus in basin water cycle research. In this study, a total of 59 groundwater samples and 41 surface water samples were collected along the river’s course within the basin. Among the groundwater samples, 18.64% exceeded the permissible drinking limit for As concentrations (10 μg/L), while 39.02% of the surface water samples exceeded this threshold. The water bodies in the Aksu River Basin are mildly alkaline, with total dissolved solids (TDSs) in surface water significantly surpassing those in groundwater. The chemical compositions of surface water and groundwater are strikingly similar, with the predominant anions being chloride (Cl−) and sulfate (SO42−) and the principal cations being sodium (Na+). The dissolution of silicate and carbonate minerals primarily influences the water chemistry characteristics of surface water and groundwater in the Aksu River Basin, followed by the dissolution of salt rocks. Human activities also play a major role in affecting the river’s water quality. The distribution of groundwater with elevated As content is entirely encompassed within the spatial distribution of surface water. Groundwater–surface water exchange plays a vital role in As enrichment in surface water.

The integration of geochemical and isotopic approaches for thermo-mineral water characterization: the case of Tebessa (North Eastern Algeria)

This research aims to assess the hydrogeochemical evolution and the assessment for drinking and irrigation use of the spring water from the alluvial aquifer and major karst aquifer systems in Tebessa (the northeastern part of Algeria). For achieving this goal, 25 groundwater samples from several springs, including thermo-mineral springs, were examined and subjected to multivariate statistical analysis (principal component analysis), isotopic approaches, and geochemical modelling. However, it was revealed that the hot waters interact at depth with Triassic evaporates located in the hydrothermal conduit (fault), giving rise to the Na+-Cl- water type. Furthermore, the freshwater characterized the karst aquifer marked by the Ca2+ HCO3 - water type with low salinity concentrations. On the other hand, the majority of cations and anions and electrical conductivity, which characterize the chemical composition of the overall water springs, were below the limits allowed for drinking water according to the standards. In terms of hydrochemical facies, it was discovered that throughout the study area, two chemical facies were predominant (Ca2+- HCO3 - and Na+- Cl-). Water-rock interaction, characterized by the dissolution of carbonates and silicates, plays a primordial role in the chemical composition of the groundwater. Stable isotopic analyses of the δ18O and δ2H compositions of the waters suggest that the cold waters of the study area are of meteoric origin. Anyway, it was concluded that the meteoric recharge was precipitation, which recharged from a higher altitude (600–1700 m) and infiltrated through deep faults and fractures in the carbonate formations of the Tebessa Mount.

Distal fluvial to lacustrine/palustrine carbonate facies of the El-Ris Formation (Late Miocene?) at the Bahariya Depression, Western Desert, Egypt: Depositional model and controls

This article aims to discuss the distal fluvial to lacustrine/palustrine carbonate facies of the El-Ris Formation (Late Miocene?) in the Bahariya Depression (Western Desert, Egypt), in terms of their depositional evolution and factors (base-level, climate, tectonics) controlling deposition. The El-Ris Formation disconformably overlies the Lower Cenomanian Bahariya Formation. It is up to 11 m thick, and includes siliciclastic facies, followed upward by carbonate facies. The siliciclastic facies consist of greenish-white, fine-grained calcareous sandstones interbedded with subordinate yellowish-brown mudrocks having insects ichnofossils reflecting deposition in a distal fluvial distributary system. The carbonate facies include the fossiliferous (mostly ostracods and a few dispersed gastropods, Lanistes carinatus,) fine crystalline lacustrine/palustrine carbonates, and calcretes-dolocretes. The lacustrine/palustrine beds are grey to pale-yellow colored massive dolomitic beds, with exposure features, reflecting deposition in a shallow, low-gradient (marginal) lake with frequent base-level changes. Calcretes-dolocretes are nodular to massive orange beds, developed at the expense of siliciclastic facies, that formed a fringe between the distal alluvial and central lacustrine-palustrine environments. Palustrine carbonate and duricrust (calcrete/dolocrete) facies increase progressively toward the top of the studied rock unit. The development of silica nodules in the palustrine facies is a result of intense evaporation just above the water table. Silica could have been released by silicate dissolution from the host alluvial deposits into the groundwater. δ13C and δ18O data indicate a hydrologically closed palustrine/lacustrine environment fed by meteoric waters, that originally precipitated calcite. Progressive evaporation of freshwater in this closed system led to dolomitization. The upward increase of carbonates in the El-Ris Formation reflects a progressive decrease of a terrigenous clastic supply in the source area and a dominance of carbonate development in the study area. During the deposition of the El-Ris Formation, the climate had marked more arid seasons with fewer intermittent humid periods. Also, the tectonic influence cannot be ruled out, since tectonic instability led to the creation of the basin depression and subsequent accommodation space that made the deposition of the El-Ris Formation.

Hydrochemical investigation of groundwater in a trans-Himalayan region of Ladakh, India, using geochemical modelling and entropy technique.

Evaluating the hydrogeochemistry and groundwater quality status is vital to understand the sources and extent of groundwater contamination. Chemometric analysis, geochemical modelling and entropy technique were explored to delineate the hydrogeochemistry of groundwater in the trans-Himalayan region. Analysis of hydrochemical facies revealed that 57.14, 39.29, and 3.57% of samples were Ca-Mg-HCO3-, Ca-Mg-Cl- and Mg-HCO3- water types, respectively. Gibbs diagrams illustrate the effects of the dissolution of carbonates and silicates during weathering on groundwater hydrogeochemistry. The PHREEQC modelling depicted that most of the secondary minerals are supersaturated except for halite, sylvite, and magnetite which are undersaturated and in equilibrium with nature. Multivariate statistical techniques, including principal component analysis, were applied for source apportionment indicating that the hydrochemistry of the groundwater was mainly controlled by geogenic sources (rock-water interaction) along with secondary pollution through increased anthropogenic sources. Heavy metal accumulation in groundwater depicted the order of Cd > Cr > Mn > Fe > Cu > Ni > Zn. EWQI analysis revealed that none of the samples fell into excellent and good categories. In total, 92.86% of groundwater samples were in an average category while the rest of the samples (7.14%) were unfit for drinking. This study will provide baseline data and a scientific framework which can be used in source apportionment studies, predictive modelling and efficient management of water resources.

Understanding the Impact of KNO3 on the Carbonation Behavior of γ-C2S

γ-dicalcium silicate (γ-C2S) has received little attention in engineering applications due to its low hydraulicity, but it exhibits high carbonation reactivity and rapid strength gain with the added benefit of CO2 absorption, which demonstrates a vast potential for application in building materials. Included herein is an investigation of the promotion impact concerning a potassium nitrate (KNO3) solution on the carbonation behavior of γ-C2S, including the degree of carbonation (DOC), microstructure, and mechanical properties. Results show DOC increases when KNO3 is introduced into the system, and as the concentration of KNO3 solution rises, DOC can reach as high as 60%, compared with 49% without KNO3; The porosity of the system is reduced by 37%; The compressive strength after 24 h of accelerated carbonation, with the KNO3 concentration of 2 mol/L, reaches 90 MPa—34% higher than that of the blank group. The presence of KNO3 solution contributes to retaining water—the dominant position for carbonation—in the sample and reducing evaporation, thus prolonging the carbonation time. The increased volume of carbonation products densifies the whole system as the reaction occurs. A barrier to γ-C2S dissolution, the amorphous silica layer wrapping on the surface of grains, is destroyed due to the introduction of K+, facilitating the continuous dissolution of silicate. The incorporation of KNO3 results in a larger crystal in size and more regularity in shape. Prolongation of the reaction duration allows for more adequate crystal growth, resulting in compacts with superior mechanical properties.

Hydrochemical, D–O–Sr isotopic and electromagnetic characteristics of geothermal waters from the Erdaoqiao area, SW China: Insights into genetic mechanism and scaling potential

The Erdaoqiao area has been the hot area of geothermal resource prospecting in southwestern China. Significance difference of scaling potential has been found between left and right riverbanks of the Erdaoqiao area. However, there still has no interpretation for the scaling difference since the genetic mechanism of geothermal waters remains unclear. In this study, hydrochemical, D–O–Sr isotopic and electromagnetic (EM) approaches were integrated to clarify the genetic mechanism and quantitively evaluate scaling potential of geothermal waters. All geothermal waters belonged to Ca–Na–HCO3 type and were determined by the weathering and/or dissolution of carbonates (66.87%) and silicates (31.53%) with cation exchange based on hydrochemical and Sr isotopic analyses. Na–K–Mg triangle diagram indicated they were immature water mixed by shallow cold water. Quartz geothermometer indicated the shallow reservoir temperatures (after cold–water mixing) of left riverbank (88.46 °C–90.51 °C) were higher than those of right riverbank (61.20 °C–87.89 °C). Silica – Enthalpy methods revealed the initial geothermal reservoir temperatures and cold-water mixing ratio were 102.03 °C–138.93 °C and 65.24%–85.87% in the right riverbank, 144.11 °C–146.76 °C and 73.08%–74.79% in the left riverbank of the Erdaoqiao area. D-O isotopes proposed the geothermal waters were recharged by atmospheric precipitation in the elevation of 4957 m to 5118 m. Therefore, the genetic model of geothermal waters in the Erdaoqiao area can be constructed. Stronger calcite scaling in left riverbank was attributed to deeper circulation identified by higher reservoir temperature and electromagnetic exploration. Quantitively evaluation demonstrated that the weight of scaling during 48 h in left riverbank (2968 kg) was four times higher than that in right riverbank (748 kg). The achievements of this study contribute to geothermal exploration in the Erdaoqiao and other similar areas worldwide.

Both natural and anthropogenic factors control surface water and groundwater chemistry and quality in the Ningtiaota coalfield of Ordos Basin, Northwestern China.

An understanding of the vertical variations in hydrogeochemical processes in various aquifers and quality suitability assessment is crucial for the utilization of groundwater in the Ningtiaota coalfield of Ordos Basin, Northwestern China. Based on 39 water samples collected from surface water (SW), Quaternary pore water (QW), weathered fissure water (WW), and mine water (MW), we conducted self-organizing maps (SOM) algorithm, multivariate statistical analysis (MSA), and classical graphical methods to elucidate the mechanisms controlling the vertical spatial variations in SW and groundwater chemistry and conducted a health risk assessment. The findings indicated that the hydrogeochemical type showed a transition from the HCO3--Na+ type in SW to the HCO3--Ca2+ type in QW, then to the SO42--Mg2+ type in WW, and back to HCO3--Na+ type in MW. Water-rock interaction, silicate dissolution, and cation exchange were the main hydrogeochemical processes in the study area. Additionally, groundwater residence time and mining operations were critical external factors that affect water chemistry. Contrary to phreatic aquifers, confined aquifers featured greater circulation depth, water-rock interactions, and external interventions leading to worse quality and higher health risks. Water quality surrounding the coalfield was poor, causing it to be undrinkable, with excessive SO42-, arsenic (As), and F-, etc. Approximately 61.54% of SW, all of QW, 75% of WW, and 35.71% of MW can be used for irrigation.

Hydrochemical Characteristics and Water Quality of Shallow Groundwater in Desert Area of Kunyu City, Southern Margin of Tarim Basin, China

The Tarim Basin in Xinjiang is located in the northwest inland arid region of China, but research shows that the region is rich in groundwater resources. To understand the hydrochemical characteristics and water quality of shallow groundwater in the desert area of the southern margin of the Tarim Basin, the groundwater was systematically sampled and tested. The ion characteristics and evolution mechanism of groundwater were analyzed by mathematical statistics, Schukarev classification, Piper three-line diagram, Gibbs model and ion ratio. Water quality was evaluated by the water quality index method (WQI) and irrigation water suitability-related parameters. The results indicated that the dominant cation in the study area is Na+, and the main dominant anions are SO42− and Cl−. According to total dissolved solids (TDS), the groundwater mainly belongs to brackish water and semi-saline water. The hydrochemical chemistry types are mainly Cl·SO4-Na·Mg type, followed by Cl-Na type, and the ion source is mainly the weathering and dissolution of evaporation rock, silicate and sulphate. The hydrochemical process is primarily controlled by evaporation concentration and rock weathering, and the cation exchange is weak. Furthermore, the WQI spatial distribution map shows that the groundwater in the middle of the study area is unsuitable for drinking and there are two areas with high WQI values greater than 500. In contrast, the good-excellent groundwater is scattered in the East. The groundwater generally has high to very high salinity, with significant changes in alkalinity. In addition, 54% of the water samples exceed the magnesium hazard (MH) limit. Therefore, certain measures should be taken before irrigation. This study has important implications for the rational development and reasonable utilization of local groundwater.

Sr Isotope, Major, and Trace Element Signatures in Karst Groundwaters

Natural and anthropogenic factors highly influence the concentration of major (Na, Mg, K, Ca) and trace (Sr, Ba, Mn, Li) elements, anions (HCO3−, NO3−, SO42−, Cl−), and Sr isotopic signatures. The current study identified the Sr isotopic signature in groundwaters from the Southern Carpathians and Apuseni Mountains karst areas of Romania and its relation to the water’s chemistry. The Sr concentration ranged between 16.5 and 658 µg/L, but in most groundwaters, it was below 200 µg/L. A considerable spatial variation and a low temporal variation, with a slightly lower Sr concentration in the winter than in spring, were observed. The strong positive correlation of the Sr with Ca, Mg, K, and Na indicated the common source of these elements. The main source of the Sr in groundwaters was the dissolution of carbonates, especially calcite, and dolomite to a lesser extent. The 87Sr/86Sr isotopic ratio ranged between 0.7038 and 0.7158. Generally, waters with a high Sr concentration and moderate 87Sr/86Sr ratios indicated carbonate dissolution, whereas samples with low Sr concentrations and high 87Sr/86Sr ratios suggested the dissolution of silicates.

Silicate Dissolution Mechanism from Metakaolinite Using Density Functional Theory.

Metakaolin (MK) is a high-quality, reactive nanomaterial that holds promising potential for large-scale use in improving the sustainability of cement and concrete production. It can replace cement due to its pozzolanic reaction with calcium hydroxide and water to form cementitious compounds. Therefore, understanding the dissolution mechanism is crucial to fully comprehending its pozzolanic reactivity. In this study, we present an approach for computing the activation energies required for the dissolution of metakaolin (MK) silicate units at far-from-equilibrium conditions using the improved dimer method (IDM) and the transition-state theory (TST) within density functional theory (DFT). Four different models were prepared to calculate the activation energies required for breaking oxo-bridging bonds between silicate or aluminate units. Our results showed that the activation energy for breaking the oxo-bridging bond to a silicate neighbor is higher than that to an aluminate neighbor due to the ionic interaction. However, for complete silicate tetrahedra dissolution, a higher activation energy is required for breaking the oxo-bridging bond to the aluminate neighbor compared to the silicate neighbor. The findings provide methodology for missing input data to predict the mesoscopic dissolution rate, e.g., by the atomistic kinetic Monte Carlo (KMC) upscaling approach.

Spatial distribution and driving factors of groundwater chemistry and pollution in an oil production region in the Northwest China

Concerns have been raised on the deterioration of groundwater quality associated with anthropogenic impacts such as oil extraction and overuse of fertilizers. However, it is still difficult to identify groundwater chemistry/pollution and driving forces in regional scale since both natural and anthropogenic factors are spatially complex. This study, combining self-organizing map (SOM, combined with K-means algorithm) and principal component analysis (PCA), attempted to characterize the spatial variability and driving factors of shallow groundwater hydrochemistry in Yan'an area of Northwest China where diverse land use types (e.g., various oil production sites and agriculture lands) coexist. Based on the major and trace elements (e.g., Ba, Sr, Br, Li) and total petroleum hydrocarbons (TPH), groundwater samples were classified into four clusters with obvious geographical and hydrochemical characteristics by using SOM - K-means clustering: heavily oil-contaminated groundwater (Cluster 1), slightly oil-contaminated groundwater (Cluster 2), least-polluted groundwater (Cluster 3) and NO3− contaminated groundwater (Cluster 4). Noteworthily, Cluster 1, located in a river valley with long-term oil exploitation, had the highest levels of TPH and potentially toxic elements (Ba, Sr). Multivariate analysis combined with ion ratios analysis were used to determine the causes of these clusters. The results revealed that the hydrochemical compositions in Cluster 1 were mainly caused by the oil-related produced water intrusion into the upper aquifer. The elevated NO3− concentrations in Cluster 4 were induced by agricultural activities. Water-rock interactions (e.g., carbonate as well as silicate dissolution and precipitation) also shaped the chemical constituents of groundwater in clusters 2, 3, and 4. In addition, SO42−-related processes (redox, precipitation of sulfate minerals) also affected groundwater chemical compositions in Cluster 1. This work provides the insight into the driving factors of groundwater chemistry and pollution which could contribute to groundwater sustainable management and protection in this area and other oil extraction areas.

Hydrochemical variations and driving mechanisms in a large linked river-irrigation-lake system

A linked river-irrigation-lake system exhibits intricate and dynamic hydrochemical variations, closely related to changes in natural conditions and anthropogenic activities. However, little is known about the sources, migration and transformation of hydrochemical composition, and the driving mechanisms, in such systems. In this study, the hydrochemical characteristics and processes in the linked Yellow River-Hetao Irrigation District-Lake Ulansuhai system were studied, based on a comprehensive hydrochemical and stable isotope analysis of water samples collected during spring, summer, and autumn. The results showed that the water bodies in the system were weakly alkaline with a pH range of 8.05–8.49. The concentrations of hydrochemical ions showed an increasing trend in the water flow direction. Total dissolved solids (TDS) were less than 1000 mg/L (freshwater) in the Yellow River and the irrigation canals, and increased to more than 1800 mg/L (saltwater) in the drainage ditches and Lake Ulansuhai. The dominant hydrochemical types varied from SO4•Cl–Ca•Mg and HCO3–Ca•Mg types in the Yellow River and the irrigation canals to Cl–Na type in the drainage ditches and Lake Ulansuhai. The ion concentrations in the Yellow River, the irrigation canals, and the drainage ditches were highest during summer, while ion concentrations in Lake Ulansuhai were highest during spring. The hydrochemistry of the Yellow River and the irrigation canals was mainly affected by rock weathering, while evaporation was the principal controlling factor in the drainage ditches and Lake Ulansuhai. Water-rock interactions including the dissolution of evaporites and silicates, the precipitation of carbonates, and cation exchange were the main sources of hydrochemical compositions in this system. Anthropogenic inputs had a low impact on the hydrochemistry. Therefore, greater attention should be paid in future to hydrochemical variations, especially salt ions, in the management of linked river-irrigation-lake system water resources.