Calcite Saturation Index Research Articles

Complex hydrology and variability of nitrogen sources in a karst watershed.

Streams draining karst areas with rapid groundwater transit times may respond relatively quickly to nitrogen reduction strategies, but the complex hydrologic network of interconnected sinkholes and springs is challenging for determining the placement and effectiveness of management practices. This study aims to inform nitrogen reduction strategies in a representative agricultural karst setting of the Chesapeake Bay watershed (Fishing Creek watershed, Pennsylvania) with known elevated nitrate contamination and a previous documented groundwater residence time of less than a decade. During baseflow conditions, streamflow did not increase with drainage area. Headwaters and the main stem lost substantial flow to sinkholes until eventually discharging along large springs downstream. Seasonal hydrologic conditions shift the flow and nitrogen load spatially among losing and gaining stream sections. A compilation of nitrogen source inputs with the geochemistry and the pattern of enrichment of δ15N and δ18O suggest that the nitrogen in streams and springs during baseflow represents a mixture of manure, fertilizer, and wastewater sources with low potential for denitrification. The pH and calcite saturation index increased along generalized flow paths from headwaters to springs and indicate shorter groundwater residence times in baseflow during the spring versus summer. Given the substantial investment in management practices, fixed monitoring sites could incorporate synoptic water sampling to properly monitor long-term progress and help inform management actions in karst watersheds. Although karst watersheds have the potential to respond to nitrogen reduction strategies due to shorter groundwater residence times, high nitrogen inputs, effectiveness of conservation practices, and release of legacy nutrients within the karst cavities could confound progress of water quality goals.

Abiotic processes control carbon dioxide dynamics in temperate karst lakes.

Inland waters are crucial in the carbon cycle, contributing significantly to the global CO2 fluxes. Carbonate lakes may act as both sources and sinks of CO2 depending on the interactions between the amount of dissolved inorganic carbon (DIC) inputs, lake metabolisms, and geochemical processes. It is often difficult to distinguish the dominant mechanisms driving CO2 dynamics and their effects on CO2 emissions. This study was undertaken in three groundwater-fed carbonate-rich lakes in central Spain (Ruidera Lakes), severely polluted with nitrates from agricultural overfertilization. Diel and seasonal (summer and winter) changes in CO2 concentration (CCO2) DIC, and CO2 emissions-(FCO2)-, as well as physical and chemical variables, including primary production and phytoplanktonic chlorophyll-a were measured. In addition, δ13C-DIC, δ13C-CO2 in lake waters, and δ13C of the sedimentary organic matter were measured seasonally to identify the primary CO2 sources and processes. While the lakes were consistently CCO2 supersaturated and FCO2 was released to the atmosphere during both seasons, the highest CCO2 and DIC were in summer (0.36-2.26 µmol L-1). Our results support a strong phosphorus limitation for primary production in these lakes, which impinges on CO2 dynamics. External DIC inputs to the lake waters primarily drive the CCO2 and, therefore, the FCO2. The δ13C-DIC signatures below -12‰ confirmed the primary geogenic influence on DIC. As also suggested by the high values on the calcite saturation index, the Miller-Tans plot revealed that the CO2 source in the lakes was close to the signature provided by the fractionation of δ13C-CO2 from calcite precipitation. Therefore, the main contribution behind the CCO2 values found in these karst lakes should be attributed to the calcite precipitation process, which is temperature-dependent according to the seasonal change observed in δ13C-DIC values. Finally, co-precipitation of phosphate with calcite could partly explain the observed low phytoplankton production in these lakes and the impact on the contribution to increasing greenhouse gas emissions. However, as eutrophication increases and the soluble reactive phosphorus (SRP) content increases, the co-precipitation of phosphate is expected to be progressively inhibited. These thresholds must be assessed to understand how the CO32- ions drive lake co-precipitation dynamics. Carbonate regions extend over 15% of the Earth's surface but seem essential in the CO2 dynamics at a global scale.

Study on the Hydrochemical Characteristics and Evolution Law of Taiyuan Formation Limestone Water under the Influence of Grouting with Fly Ash Cement: A Case Study in Gubei Coal Mine of Huainan, China

The hydrogeological conditions of Huainan Coalfield are complex. The Taiyuan formation limestone water (Taihui water) in this area is a direct threat to the water source of the 1# coal mining floor. In order to prevent and control water disasters, Gubei Coal Mine adopted ground high-pressure grouting with fly ash cement to block the hydraulic connection between the Taiyuan formation limestone aquifer and the Ordovician limestone aquifer. However, the injected slurry will destroy the original hydrochemical balance of Taihui water and change its hydrochemical characteristics. Taking the influence area of the 2# karst collapse column in the Beiyi 1# coal mining area of Gubei Coal Mine as an example, a total of 25 Taihui water samples were collected. The hydrochemical characteristics and evolution law of Taihui water before and after grouting are studied via the multivariate statistical method. The research methods include constant index statistics, Piper diagram, correlation analysis, ion combination ratio, and saturation index analysis. The results show that after grouting, the concentrations of Na+ + K+, Ca2+, Mg2+, and Cl− in Taihui water decrease, while the concentrations of SO42− and HCO3− increase. The average values of PH and TDS become larger. The hydrochemical types of Taihui water are more concentrated, mainly HCO3-Na and Cl-Na. The correlations between conventional indicators decrease. According to the analysis of ion combination ratio, dissolution, cation exchange, and pyrite oxidation mainly occur in Taihui water, and these effects are enhanced after grouting. The saturation index results show that after grouting, the saturation index of dolomite, calcite, and gypsum is significantly reduced, and the saturation index of rock salt is slightly increased. The conclusion of this study is that the hydrochemical characteristics of Taihui water are greatly affected by fly ash cement. Moreover, because fly ash cement contains a lower calcium oxide content than ordinary Portland cement, the effect of fly ash cement on the ion concentration of Taihui water and the resulting hydrogeochemical effect are significantly different. Therefore, in the treatment of mine water disasters, the hydrogeochemical evolution law affected by fly ash cement grouting should be identified.

Responses of underground air and drip water geochemistry to meteorological factors: A multi-parameter approach in the Rull Cave (Spain)

Our research aims to assess the complex interactions between the elements that constitute and influence a cave system through the analysis of an extensive dataset of climatic and environmental parameters (222Rn, CO2, drip rates, chemical composition, and environmental isotopes) measured in air, water, and solid in the Rull Cave (southeastern Spain). Of particular importance is understanding the effect of rainfall and temperature on water and gas transport through the epikarst and the involved processes. Our results show that the cave gaseous concentration patterns do not only depend on the temperature-caused movement of air masses, but they can also be affected by abundant rainfall. The δ18O and δD composition of cave water also relies on such precipitations for the effective transfer of the rainfall signal into the cave, which can take between 3 and 7 days. The elemental ratios (Sr/Ca and Mg/Ca) show high responsiveness to the water drip rate, hinting that enhanced prior calcite precipitation (PCP) occurs at slower drip rates. Despite this, and regardless of drip rates, calcite saturation indices follow a seasonal variation pattern inversely proportional to the cave air CO2 concentration, while δ13C-DIC is proportional. Our results show how the interlinkage between these multiple components defines the dynamics of the atmosphere-soil-cave system. Cave monitoring is then essential to understand the karstic vadose zone, which is highly sensitive to climatic influence and its changes.

Hydrochemical Characterisation of Groundwaters in a Crystalline Basement Environment: The Case of Fractured Aquifers in the Poni Watershed in the South-West Region of Burkina Faso, West Africa

The Poni watershed is located in the south-west of Burkina Faso. Its geology is made up of fractured crystalline and crystallophyllian basement, where fractured aquifers are the most exploited for water needs. The aim of this study is to determine the physico-chemical characteristics of the groundwater in the fractured aquifers of the Poni watershed in order to gain a better understanding of the mineralisation processes and hydrochemical properties of the groundwater in these aquifers. To do this, approaches based on the determination of Calcite Saturation Indices (CSI) and Dolomite Saturation Indices (DSI), Principal Component Analysis (PCA) and hydrochemical properties show that the majority of groundwater in the basin is undersaturated with respect to carbonates (79.37%) and divided into three families in relation to circulation speed: very slow circulation water (20.63%), slow circulation water (55.56%) and fast circulation water (23.81%). The approaches also show that the mineralisation of groundwater in the basin is governed by the acid hydrolysis of minerals in the surrounding rocks, residence time and surface inputs. They are characterised by calcic and magnesian bicarbonate facies (85.7%), calcic bicarbonate facies (6.35%), sodium and potassium bicarbonate facies (1.59%) and calcic and magnesian sulphate chlorides (6.35%).

Recycling and deposition of inorganic carbon from calcium carbonate encrustations of charophytes

AbstractMany aquatic primary producers can use bicarbonates as a carbon source for photosynthesis. Charophytes of the two genera: Chara and Nitellopsis are quite efficient in this process. Some species of these macroalgae produce carbonate encrustations, mainly calcium carbonate, constituting up to 86% of the summer maximum dry weight of the standing crop. In this study, we analyzed the fate of inorganic carbon accumulated this way in Chara spp. and Nitellopsis obtusa from six Polish lakes located in two regions (warmer W Poland and cooler NE Poland). Our study distinguished two groups of charophyte species that differed in the way of CaCO3 release from their summer standing crops. On average, the corticate Chara rudis and C. tomentosa belonging to the first group were less efficient in depositing CaCO3 from summer to autumn than the less corticate C. contraria and ecorticate N. obtusa of the second group. The latter two species were more efficient in inorganic carbon burial in sediments. On the contrary, dissolution of encrustation was more typical of the first species group and was facilitated by decreasing the pH and saturation index of calcite in lake water. The final output of CaCO3 loss mainly resulted from combined species‐specific features, lake water properties and overwintering patterns. Our study revealed that inorganic carbon cycling through charophytes involves burial and dissolution and is more complex than previously thought.

Impact of anthropogenic and land use pattern change on spatio-temporal variations of groundwater quality in Odisha, India.

The groundwater physicochemical parameters were studied to understand their spatiotemporal variations and groundwater quality, using the statistical and entropy weights methods. Out of ten parameters that were considered, F-, TDS, and Cl- appear to be the major contributors influencing the quality of groundwater. The Principal Component Analysis (PC1, PC2) indicates that the majority of ions are derived from both natural and anthropogenic sources. Studies of saturation indices of gypsum, Halite, dolomite, and calcite indicate that dissolution of these salts also affects groundwater salinization. In coastal areas, a few of the water samples also appear to be contaminated by the mixing of seawater. The entropy weights, which are free from subjective biases were used to estimate the water quality index. The entropy-based water quality index (EWQI) varies from excellent to good quality for the year 2012-13, and appears to degrade after 2015 onwards. For the year 2018-19 and 2021-22, 71.42% and 68.42% of the study areas show excellent water quality, followed by 25.33% and 24.33% (good), 2.54% and 3.54% (average), 0.7% and 3.7% study area shows as poor quality respectively. The groundwater quality, particularly in the western, central, northern, and eastern parts of the region, appears to be average, poor, and very poor in several small patches, respectively. Industrial developments, mining activities, irrigation, changing land use patterns, agricultural activities, and increased anthropogenic activities may be contributing to the degradation of the water quality.

Large seasonal and spatial variability in δ44/40Ca values in the Godavari River, India: Implications for basalt weathering, groundwater interaction, and carbonate precipitation

We present stable calcium isotopic compositions (δ44/40Ca) of the Godavari River, the largest river in peninsular India, from the upper 750 km of its course, where it primarily drains the Deccan basalts. In addition to river water samples collected during the pre-monsoon and monsoon seasons, we also present δ44/40Ca values of groundwater, bedrock Deccan basalts, suspended and bedload sediments, and carbonate nodules from the Godavari Basin as well as zeolites and hydrothermal calcite from several locations within the Deccan Traps. The δ44/40Ca values (reported relative to NIST SRM 915a) of the dissolved load of the Godavari River main channel display large variability during both the pre-monsoon (0.73–1.64 ‰) and the monsoon (0.56–1.28 ‰) seasons. In the upper reaches (within 160 km of the source), the river water samples during both seasons show δ44/40Ca values (0.56–0.89 ‰) and Sr/Ca (mmol/mol) ratios (1.47–2.86), which are in the same range as that of the bedrock Deccan basalts (0.53–0.78 ‰ and 1.1–5.19 mmol/mol, respectively), suggesting control of water chemistry by basalt weathering. In contrast, in the middle reaches of the river (beyond 160 km from the source), the riverine δ44/40Ca values (0.99–1.64 ‰) are much higher than δ44/40Ca of bedrock basalt, but similar to that of groundwater in the middle reaches. We conclude that groundwater discharge to the river during the low-flow period has a significant control on the observed high δ44/40Ca values of the Godavari River. Additionally, the positive correlation between δ44/40Ca values versus Sr/Ca ratios and weak positive correlation of δ44/40Ca values versus saturation index of calcite suggests that carbonate precipitation from the river water impacts the δ44/40Ca values of the Godavari River in the middle reaches; this conclusion is consistent with the low δ44/40Ca of acetic-acid leached carbonate nodules (0.52–0.56‰) collected from the Godavari riverbed.The δ44/40Ca values of zeolites exhibit a large range from −0.72‰ (heulandite) to 1.67‰ (apophyllite) while the δ44/40Ca values of leached hydrothermal calcite samples range from 0.75‰ to 0.92‰. These values indicate that weathering of these Ca-bearing minerals has limited contribution to the calcium isotopic composition of the Godavari River, which contrasts with observations from relatively smaller rivers draining basalt in Iceland. The δ44/40Ca values of the dissolved load of the basalt-draining Godavari River is significantly higher than the estimated global riverine input to the oceans (0.88‰); given the high weatherability of basalts, the high δ44/40Ca values of basalt-draining rivers suggests that the estimate of the δ44/40Ca value of the average continental flux to the oceans may have been underestimated. The results of this study also demonstrate distinct differences in geochemical and Ca isotopic compositions of basalt-draining rivers from Iceland and India, which reflects fundamental differences in the weathering process in very different climatic regions.

Reactive transport modelling of potential near-well mineralogical changes during seasonal heat storage (HT-ATES) in Danish geothermal reservoirs

Potential near-well mineralogical changes caused by seasonal high temperature aquifer thermal storage were investigated by numerical reactive transport modelling for two mineralogically different Danish geothermal reservoirs; the mineralogical mature and calcite free Gassum Fm. and the calcite containing Bunter Sandstone Fm. Modelling scenarios simulated injection of heated formation water with temperatures up to 150 °C into the reservoirs for a period of 6 months. For the Bunter Sandstone Fm., the formation water was modified to a saturation index for calcite of −0.1 prior to injection to replicate a situation where precautions are made to avoid loss of injectivity due to calcium carbonate scaling at elevated temperatures. Based on mineral dissolution and precipitation calculated by the numerical model, the changes in porosity as a result of heat storage were estimated. For the Gassum Fm., no significant changes in porosity were calculated when injecting formation water with temperatures up to 100 °C. At temperatures ≥120 °C, the content of albite and siderite was significantly reduced due to dissolution, but these mineralogical changes were not expected to significantly affect the reservoir porosity due to the low content of these minerals in the reservoir. However, significant precipitation of calcite near the injection well at temperatures ≥120 °C may reduce injectivity at these temperatures. For the calcite containing Bunter Sandstone Fm., injection of the slightly modified formation water led to significant calcite dissolution in the reservoir at all investigated temperatures and to dolomitisation at 150 °C. In the case of dolomitisation, our results suggest a risk for clogging of the near well environment by freshly precipitated dolomite. For the remaining temperatures, the calcite dissolution causes an increased porosity and a potential risk for compromising the rock integrity which should be further investigated. For both reservoirs, only very small changes were predicted in the aqueous composition of the formation water. The mineralogical changes in the reservoir are therefore not easily detected by monitoring changes in the aqueous composition of the formation water.Our results imply that high temperature aquifer thermal energy storage in reservoirs with a high calcium carbonate content such as the Bunter Sandstone Fm. is challenging. If precautions are taken to avoid calcium carbonate scaling by reducing the calcium content of the formation water, injection of the Ca-depleted formation water into the reservoir will cause dissolution of calcium carbonates. On the other hand, if precautions are not taken, it is well-known that precipitation of calcium carbonates will cause scaling and clogging problems. In contrast, our results suggest that storage of excess heat in mineralogical mature and calcite free sandstones such as the Gassum Fm. may be feasible. This is especially the case at temperatures <100 °C.

Desalination by batch reverse osmosis (RO) of brackish groundwater containing sparingly soluble salts

Batch RO desalination is a new approach to high-recovery, energy-efficient desalination. So far, however, batch RO has been tested mostly with pure sodium chloride solutions. An important application of batch RO is desalination of brackish groundwater which, besides sodium chloride, contains sparingly soluble salts. In this experimental study of a batch RO system, we used simulated groundwater (with total dissolved solids ranging from 1180 to 3637 mg/L) following compositions of samples taken from a location in Egypt and a location in India. The groundwater contained high levels of salts which could be expected to cause scaling on the RO membrane surface. For example, the Langelier Saturation Index (LSI) for calcite reached 2.6 in the brine. Nonetheless, the system resisted scaling throughout >100 h of operation. Membrane permeability remained almost unchanged, as demonstrated by tests conducted before and after the experiments. Induction time calculations showed that salinity cycling did not fully explain the scaling inhibition. Other anti-scaling mechanisms – such as periodic flushing, osmotic backwash, and feed flow reversal – were also likely contributors. At recovery of 0.8, hydraulic specific energy consumption (SEC) was <0.5 kWh/m3 and close to that obtained with sodium chloride solution at equivalent osmotic pressure.

Cross-Hole and Vadose-Zone Infiltration Tracer Test Analyses to Determine Aquifer Reactive Transport Parameters at a Former Uranium Mill Site (Grand Junction, Colorado)

The U.S. Department of Energy Office of Legacy Management is responsible for the long-term care and maintenance of former uranium mill sites in the United States. Prior predictions of site flushing times (monitored natural attenuation) are not being met due to the presence of secondary contaminant sources associated with uranium-rich sediments in the vadose zone and organic-rich sediments near the water table below and near former mill tailings (tailings have been moved to a separate disposal site). Updated sitewide modeling for future releases of contaminants (including uranium) from these secondary sources to the groundwater need appropriate input parameters. To test field techniques, two cross-hole tracer tests and one infiltration tracer test were completed at a former uranium mill site in Grand Junction, Colorado. Reactive transport modeling was completed to derive physical and geochemical parameters. The observed data from saturated zone cross-hole tracer testing was adequately simulated using PHT-USG (reactive transport model) and PEST++ (calibration routine) with reasonable estimates of hydraulic conductivity, dispersion, effective porosity, cation exchange, calcite saturation index, and uranium sorption potential. The use of multiple layering in one cross-hole model was able to capture hydraulic conductivity variations with depth, which produced a double hump in the tracer concentrations. Estimated parameter values were very similar to prior estimates from column testing and single-well push–pull testing, except for a lower uranium sorption potential in one cross-hole test. This difference is likely due to the larger scale of the cross-hole testing including pathways with a lower uranium sorption potential. The infiltration testing released constituents from the vadose zone that can contribute to ongoing groundwater contamination. Modeling simulated the immediate release of these constituents to the water table similar to downward displacement of the existing residual porewater. Delayed drainage of the infiltration water was more difficult to simulate. However, the overall contaminant release concentrations from the vadose-zone secondary sources and ongoing groundwater contamination are adequately simulated for current site purposes. Additional details on vadose-zone processes may be needed if various remedial fluids are evaluated.

Experimental precipitation of cryogenic carbonate

Cryogenic cave carbonates (CCC) represent a specific type of speleothem precipitating from freezing water in caves. Over the past decade, considerable progress has been made concerning cryogenic calcite petrography, crystallography and geochemistry. Uncertainties remain, however, as the cave waters from which ancient cryogenic calcites form are not preserved. The present study provides an experimental approach to re-calculate the isotopic composition of the precipitating solution using CCC data. Calcium-rich bicarbonate water prepared by bubbling CO2 gas with a very low δ13C value (∼ −35 ‰) into the water was cooled under controlled laboratory conditions to account for the water chemistry changes during freezing. The experiments yielded cryogenic calcite and vaterite precipitating at temperatures of +1, +0, −0.5, −0.7, −1 and − 2 °C, with complete freezing occurring at −0.7, −1 and − 2 °C and partial freezing at −0.5 °C. The δ18Owater and δ13CDIC values, pH, electrical conductivity and alkalinity were recorded before and after the experiments. Our work documents an increase in pH, suggesting CO2 degassing eventually reaches supersaturation with calcite, leading to CaCO3 precipitation. Calcite precipitation rates are lower at the longer experiment (> 45 days). This is further confirmed by the decreasing calcite saturation index (SIcc) with time. Carbon isotope analyses of the water and carbonates revealed kinetic effects during rapid freezing. A large increase in Δ13CDIC values of the water (up to about 30 ‰) was found between the start and end of the experiments. Reasons may include CO2 degassing with increasing time leading to a markedly 13C-enriched solution. Based on our experimental data, the cryogenic crystal morphotypes are related to the precipitation sequence. Sharp-edged rhombohedra (calcite) precipitate first. Subsequently, spherulitic (vaterite) morphotypes precipitate from almost completely frozen water characterised by a high SIcc. This first experimental work dealing with cryogenic carbonate precipitates and their stable isotope composition sheds light on the origin of these peculiar speleothems and provides constraints to interpret morphologies and isotopic compositions of ancient CCCs.

Corrosion of carbonate speleothems by bat guano

Interactions between bat guano and speleothems were studied in caves of Slovakia (Domica, Drienovská, and Jasovská caves) and Poland (Nietoperzowa Cave). This contribution focuses on the estimation of the rate of guano-induced corrosion of carbonates and on the transformations of speleothems caused by the corrosion. The intense interaction between bat guano accumulations and cave carbonates is mainly controlled by the chemical composition of guano. Corrosion of carbonates by guano leachates, suggested by negative calcite saturation index, was confirmed by weight loss of experimental carbonate tablets exposed to interaction with guano in situ in caves, and under laboratory conditions. The calculated corrosion rate of the tablets was up to 0.111 mm/y. However, measurements of selected corrosion forms in the studied caves, compared to the guano 14C age, suggest rates of the corrosion up to 0.65 mm/y. Transport of aggressive guano leachates is possible in the presence of water; increased drip water supply is considered as contributing to the higher dissolution rates. Crystallization of phosphates as byproducts of the interaction of guano with carbonates can also alter the dissolution process. Phosphates, represented by ardealite, brushite, and hydroxylapatite, were observed on the surfaces of experimental tablets and speleothems alike. Interestingly, hydroxylapatite was the only mineral associated with guano-related hiatuses within speleothem sections. Phosphates are usually accompanied by various dissolution features, such as corrosion pits and considerably dissolved calcite crystals. Dissolution features currently forming on speleothem surfaces are analogous to those identified in the fossil record. U-series dating of guano corrosion episodes within stalagmites from Domica and Drienovská caves indicates their Pleistocene age, while within a flowstone from Nietoperzowa Cave, a Holocene age. Considerable guano accumulations are common in low-latitude caves, whereas in temperate climatic zones guano accumulates irregularly in space and time. The guano accumulations are usually restricted to cave sections distant no more than few hundred meters from the nearest entrance. Considering the widespread distribution of bats across the globe, this process is expected to be common in caves. Nevertheless, the random distribution of guano in caves and its temporal deposition, but also the climate-dependent availability of water, suggest that guano-induced corrosion can be regarded as a site specific process.

Hydrogeochemical Characteristics and Human Health Risk Assessment of Fluoride Enrichment in Water in Faulted Basins of Qinghai-Tibet Plateau—A Case Study of Sanhe Plain in Guide Basin

Fluoride (F) is an essential element of drinking water for human health, especially for bone development and enamel creation. However, if the fluoride content in drinking water is higher than 1.5 mg/L or lower than 0.5 mg/L, it will cause endemic diseases, such as dental fluorosis. There are two main hydrogeological characteristics: the properties of the water-bearing rocks and groundwater conditions controlled the groundwater in guide basin. The geothermal water can be divided into fracture convection and sedimentary basin geothermal water according to its geological environment and heat transfer mode. Inductively coupled plasma spectrometry is a significant tool for groundwater quality analysis. The geochemical factors of fluoride enrichment in confined geothermal water mainly include pH, ion exchange, and mineral saturation. Both groundwater samples are slightly alkaline, while the phreatic water and surface water record pH values of 8.5, 7.78, and 7.8, respectively. The salinity of groundwater water is not high, but for confined geothermal water, phreatic water, and surface water measures 706.0, 430.1 and 285.9 mg/L respectively. The higher the pH of groundwater, the more beneficial it is to the enrichment of fluoride. In contrast, the main cations in phreatic water and surface water are calcium ions and magnesium ions. The anions in groundwater and surface water mainly include SO42− and HCO3−, followed by Cl−, indicating that the groundwater and surface water here is mainly leaching. Fluoride was shown to be positively correlated with sodium and bicarbonate. Moreover, the results indicate that F− enrichment is usually associated with high HCO3− and Na+ concentrations in water, while a high Ca2+ concentration tends to lower the F− concentration in water. This means that the ion exchange between calcium ions and sodium ions may lead to fluoride enrichment in natural water. As mentioned above, high-sodium and low-calcium water are favorable for fluoride enrichment. Moreover, saturation indices of fluorite, gypsum, dolomite, and calcite, as well as the saturation index of fluorite, represent a vital method to understand the fluoride enrichment. According to this study, fluoride as a pollutant poses great risks to human health overall, whether lower than or higher than the drinking water limit. Children face higher health risks than adults caused by confined geothermal water drinking intake. This study suggests that groundwater treatment should be conducted to reduce fluoride concentration in drinking water. It is suggested that when confined geothermal water is used as drinking water, it should be mixed with phreatic water and surface water in a certain proportion to make the fluoride in groundwater reach the range of safe drinking water.

Chemical denudation dynamic based on hydrochemical measurements in the area of Drienovecká jaskyňa Cave basin (Slovak karst, Slovakia)

Denudation is a fundamental process for the development of karst relief. It has different intensity in different geological and climatic condition and beside this many other partial factors influence the intensity and dynamic of the process during the year. In karst systems, there is often a positive correlation between water partial pressure of CO2 and denudation. The Drienovecká jaskyňa Cave spring represents typical active fluviokarstic spring and its catchment area is represented by Wetterstein and Waxenec limestones, and Drienovec conglomerates. This catchment study area is located at the contact of the conti-nental and oceanic climate, in terms of altitude at the border between the lowland and moun-tain climate. During three hydrological years we measured main karst water parameters that helped us bring first results on chemical denudation in the catchment area. The water from the spring is of calcium-magnesium-bicarbonate type. Hydrogeochemical analyses in-dicate that it is mostly water from an open hydrogeological structure. Water of the cave spring is oversaturated with respect to calcite, which is documented by the saturation index of calcite, whose values are higher than the solubility product of calcite. During the studied period, the dynamic of the chemical denudation rate is clearly observable with peaks in spring months in year 2014 and 2016. The average value of denudation rate for the entire period is 21.06 mg Ca2+. Factors that influence chemical denudation include, for example, type of carbonate rock, CO2 partial pressure, water and air temperature, chemical compo-sition of the aqueous solution, activity of soil organisms, vegetation and others. Water from melted snow can also play a significant role in changes in water chemistry and thus chemical denudation. URL: https://www.gcass.science.upjs.sk/

Groundwater quality evaluation based on water quality indices (WQI) using GIS: Maadher plain of Hodna, Northern Algeria.

In a semi-arid region of Maadher, central Hodna (Algeria), groundwater is the main source for agricultural and domestic purposes. Anthropogenic activities and the presence of climate change's effects have a significant impact on the region's groundwater quality. This study's goals were to use water quality indices to evaluate the groundwater's quality and its suitability for drinking and irrigation, as well as to identify contaminated wells using a geographic information system (GIS) and the spatial interpolation techniques of ordinary kriging and inverse distance weighting (IDW). The results reveal that all water samples exceeded the World Health Organization's standards for nitrate ions and had alarming concentrations of calcium, chlorine, and sulfate (WHO). According to Piper's diagram, the groundwater hydrochemical facies is composed of the elements sulfate-chloride-nitrate-calcium (SO42--Cl-NO3--Ca2+ water type). The majority of samples fall into the poor water category, slightly more than 10% fall into the very poor water category, and less than 10% fall into the good to the excellent quality category, per the water quality indices, which classify samples in a similar manner. According to irrigation water indices, every sample is suitable for irrigation. Depending on the direction of groundwater flow, the spatial distributions of Ca2+, Na+, Mg2+, SO42-, and Cl- show that their concentrations are high north of the area and relatively low south of Maadher village (Fig.3). Nitrate concentrations are high in the majority of samples, particularly those close to the Bousaada wadi. In most samples, particularly those close to the Bousaada wadi, nitrate levels are high. Various water quality models were described, and GIS spatial distribution maps were created using standard kriging and inverse distance weighting (IDW) techniques through selected semi-variograms predicted against measurements. To determine the origin of mineralization and the chemical processes that take place in the aquifer-which include the precipitation and dissolution of dolomite, calcite, aragonite, gypsum, anhydrite, and halite-the groundwater saturation index was calculated.

Characterization of seasonal groundwater origin and evolution processes in a geologically heterogeneous catchment using geophysical, isotopic and hydro‐chemical techniques (Lough Gur, Ireland)

AbstractLough Gur is a shallow groundwater fed eutrophic lake situated within a small agricultural catchment containing volcanic and karst rock features in mid‐west Ireland. Seasonally active conduits linking two spring discharge locations from the lake under high flow conditions were revealed using dye tracing and a terrestrial geophysical survey, highlighting the architecture of the conduit flow path from Lough Gur to its discharge spring. A radon survey combined with a lake geophysical survey identified the locations of in‐lake discharge springs and thickness of the lakebed sediments. Falling head hydraulic characterization experiments illustrated the heterogenous nature of lakebed sediments and hydrograph analysis coupled with stable isotopes of water (δ18O and δ2H) revealed significant surface water ‐ groundwater interaction during high flow periods. Significantly, δ18O and δ2H signatures plot above the global meteoric water line and local meteoric water line indicating hydration of silicate minerals and direct isotope exchange of δ18O between water and rock minerals. Groundwater δ18O and δ2H signatures during low flow periods indicate that recharge sources are influenced by enriched surface waters and precipitation while a wider range of signatures during high flow periods indicates a greater variation of sources. D‐excess signatures illustrate rapid rainfall infiltration under high flow conditions, thereby demonstrating the vulnerability of the groundwater, while lake water signatures confirm widespread surface water‐groundwater interaction/mixing. Hydrochemical analyses confirm both silicate weathering and carbonate dissolution as primary geochemical processes with Mg/Ca ratios suggesting greater groundwater residence time during low flow periods. Correlations between δ13CDIC and dissolved organic carbon suggest a seasonal switch in the source of DIC to groundwaters between the oxidation of organic matter in summer and dissolution of carbonate minerals in winter. The SI saturation index for calcite (SIC) illustrates calcium carbonate precipitation along with CO2 evasion to be a perennial processes. Finally, the spatial variation for nitrate isotopic signatures (δ18ONO3‐ and δ15NNO3‐) suggests a number of nitrate sources to groundwaters including soil organic nitrogen, manure and/or domestic effluent with indications of denitrification processes under low flow conditions.

From water analysis to scale and corrosion control improvements: A Permian Basin example

Oil and gas production and geothermal energy exploitation generate large volumes of produced water (PW), which is causing various operational challenges including environmental sustainability, scale, corrosion, and other economic and safety issues. Using the Permian Basin as an example, this study traced the PW sources and developed a new procedure for scale and corrosion management improvements based on the PW compositions from U.S. Geological Survey National Produced Waters Geochemical Database (USGS PWGD). Both correlations between the major ions and Br− and between the water isotopes (δD versus δ18O) showed that most PW samples evolved from seawater, and a group samples from foreign sources were detected. The downhole pH, CO2% and calcite saturation index (SI) at surface were predicted by assuming PW is at equilibrium with calcite at downhole conditions. The SI values of barite, halite, and gypsum were also predicted. A dosage of 1 mg/L of NTMP (nitrilotris(methylene phosphonic acid)) was recommended to control such scale formations for >95% Permian wells. Furthermore, this study incorporated a mechanistic corrosion module to calculate the corrosion rate, which correlated well with the measured Mn2+ concentration. Such good correlation suggested that Mn2+ concentration can be utilized as a proxy for the steel corrosion. This recommended procedure has been shown to provide vital information for formation water source evolution analysis and scale and corrosion management improvements based on produced water composition with nonideal data.

Enhanced soil erosion threatens fluvial tufa landscapes after an Ms 7.0 earthquake in the Jiuzhaigou World Heritage Site, southwestern China

Tufa is a porous freshwater deposit comprising primarily calcite (CaCO3) and organic matter. Massive tufa depositions can spread for up to several kilometers, forming tufa landscapes that have been recognized as national parks and World Heritage Sites. Previous studies have suggested that enhanced soil erosion owing to human activities (e.g., deforestation and agriculture) is one of the major causes of fluvial tufa decline in many places worldwide. In 2017, an Ms 7.0 earthquake occurred in Jiuzhaigou, which greatly increased soil erosion in the catchment. We compared the water chemistry and tufa deposition before and after the earthquake to understand the impact of soil erosion on tufa landscapes in Jiuzhaigou. After the earthquake, we found that high turbidity greatly reduced the aesthetic value of the lakes. Enhanced soil erosion increased NO3−, dissolved organic carbon (DOC), and PO43− concentrations in surface water, which may worsen the problems of increased algal biomass and marsh development. Enhanced soil erosion reduced alkalinity, HCO3−, and the saturation index of calcite (SIc), thereby decreasing the potential to generate new calcite. Enhanced soil erosion may also increase the annual tufa deposition rates by increasing the soil and organic materials in the sediment. In addition, the tufa sediment affected by enhanced soil erosion was loose, highly porous, and contained numerous diatoms. This study provides observational data to explain the impact mechanisms of soil erosion on tufa landscapes and assess the necessity and achievements of artificial soil erosion control.

Controls on riverine calcium isotope ratios during basalt weathering in the Skagafjörður watershed, Iceland

This study examines the Ca isotope (δ44/40Ca) geochemistry of Icelandic rivers with the overall aims of improving the δ44/40Ca tracer, constraining solute sources, and addressing the hypothesis that basalt weathering disproportionately regulates Earth’s long-term climate. We report Ca isotope and elemental data for mainstem rivers, tributary streams, thermal and non-thermal springs, basalt, calcite, soil, vegetation, and sediment collected from the Skagafjörður region in North Iceland. Waters and calcite were further analyzed for their carbon isotope (δ13C) composition, and we conducted experiments to characterize δ44/40Ca values of riverine colloids, the clay-size fraction (<2 µm) of soil, and exchangeable leachates obtained from soil and bedload sediment. The dataset includes analyses of travertine and coexisting water samples collected from a CO2-rich spring (Hvanná river) in South Iceland.Samples show a ∼2‰ range, with groundwater, the ultrafiltered fraction of river water, and hydrothermal calcite producing among the highest δ44/40Ca values and travertine, soil exchangeable Ca, and vegetation the lowest. Riverine δ44/40Ca values are on average ∼0.20‰ higher than those for basalt, which shows minimal isotopic variability. Mainstem glacial-fed rivers entering the Skagafjörður valley from the soil- and vegetation-free highlands have higher δ44/40Ca values than direct-runoff tributaries draining catchments to the north where pedogenesis is more pervasive. Riverine δ44/40Ca values correlate with Sr/Ca and Na/Ca ratios, as well as the saturation index of calcite (SIcal). Riverine δ13C values increase from ∼−8‰ (atmospheric) to ∼−3‰ (calcite) as SIcal values increase from highly-undersaturated to near-equilibrium. One watershed (Svartá river) shows unique trends, where riverine δ44/40Ca values and Sr/Ca ratios do not correlate, and soil exchangeable leachates produced anomalously high amounts of Ca. Clay-size fractions isolated from three horizons composing a brown andosol show slightly lower δ44/40Ca values than bulk basalt, but the values overlap with those for primary minerals. Colloids appear to have lower δ44/40Ca values than truly dissolved Ca. The proxy could be developed as a tracer for colloidal contributions to riverine dissolved loads, especially in soil-mantled catchments where the inputs appear most pronounced. Groundwater δ44/40Ca values correlate with pH, temperature, distance from the coast, and Sr/Ca ratios, all consistent with fractionation control yielding varying degrees of geochemical evolution. The combined riverine patterns largely reflect three-component mixing between basalt weathering, calcite weathering, and hydrothermal water inputs. Subsurface Ca isotope fractionation indirectly impacts riverine δ44/40Ca values by elevating the δ44/40Ca values of groundwater and hydrothermal calcite. This study highlights the significance of hydrothermal water inputs of Ca and HCO3 to Icelandic rivers, which have been previously underappreciated. Mixing calculations suggest that a maximum of ∼20% of the Ca in mainstem rivers derives from surficial basalt weathering. Small tributaries draining the flanks of the valley show the clearest signals of basalt weathering by atmospheric CO2, but these waters have much lower solute concentrations than those employed in previous attempts to estimate basalt weathering rates and parameterize the climate sensitivity of the basalt weathering feedback.