Weathering Of Carbonate Minerals Research Articles

Hydrochemical Characterization of Groundwater Quality in Nkalagu District, Southeastern Nigeria

Hydrochemical characterization of groundwater quality in Nkalagu District, southeastern Nigeria was carried to determine the main factors controlling the chemistry of groundwater and its suitability for drinking and irrigation purposes. Sixty (60) groundwater samples collected from boreholes and hand-dug wells in different parts of the area were analyzed for a range of physiochemical parameters and heavy metal constituents. The results show that concentration of the major ions were in the order Cl->HCO3->SO42->NO3- and Na>Ca2+>Mg2+>K+. The groundwater samples are slightly acidic with pH of 5.28 to 8.04; moderately hard with TH of 112.88 to 467.78 mg/l. The district is mainly controlled by carbonate and silicate mineral weathering based on the available result. Three main flow regimes were identified with Q-mode cluster analysis. Based on the WQIanalysis results, the groundwater quality in the district was classified, generally as ‘poor’ to ‘excellent’ for drinking purpose. Groundwater quality for drinking purpose were noted to deteriorates as one move from west towards the east of the district, while the north and south part pf the study area indicated the best quality in the district. Groundwater quality for irrigation purpose showed excellent quality based on the United States Salinity Laboratory and Wilcox diagrams. For future use of groundwater resource in the district we recommend implementation rules and guidelines in the area to enhance health and preserve groundwater sources in the district.

Coupled nitrogen transformation and carbon sink in the karst aquatic system: a review

Abstract Carbonate bedrock regions represent that 14% of Earth's continental surface and carbon (C) sink in karst water plays an important role in the global C cycle due to the CO2 consumption during carbonate mineral weathering. Intensive agriculture and urbanization have led to the excessive input of nitrogen (N) into aquatic systems, while the high concentrations of inorganic C in the karst water might affect the N cycle. This paper summarized the characteristics of water in karst regions and discussed the N transformation coupled with the C cycle in the condition of high Ca2+ content, high pH, and high C/N ratios. Carbonates can consume more atmospheric and pedologic CO2 than non-carbonates because of their high solubility and high rate of dissolution, resulting in the higher average CO2 sink in karst basins worldwide than that in non-karst basins. Therefore, carbonate mineral weathering and aquatic photosynthesis are the two dominant ways of CO2 absorption, which are termed as coupled carbonate weathering. As the alkalinity and high C/N content of karst water inhibit the denitrification and mineralization processes, the karst aquatic environment is also served as the N sink.

Dynamics in riverine inorganic and organic carbon based on carbonate weathering coupled with aquatic photosynthesis in a karst catchment, Southwest China

Carbonate mineral weathering coupled with aquatic photosynthesis, herein termed ‘coupled carbonate weathering’ (CCW), represents a significant carbon sink which is determined by riverine hydrochemical variations. The magnitudes, variations and mechanisms responsible for the carbon sink produced by CCW are still unclear. In this study, major ions, TOC and discharge data at the Darongjiang, Lingqu, Guilin and Yangshuo hydrologic stations in Li River basin, a karst catchment typical of this geographic region, were analysed from January 2012 to December 2015 to elucidate the temporal variations in riverine inorganic and organic carbon and their controlling mechanisms. The results show that (1) HCO3− was sourced from carbonate weathering and silicate weathering, carbonate weathering by carbonic acid being predominant; (2) TOC was created chiefly by the transformation of bicarbonate to organic carbon by aquatic phototrophs during the non-flood period; (3) The carbon sink produced by coupled carbonate weathering in the Li River basin was calculated to be 14.41 tC·km−2·yr−1, comprised of the sink attributable to carbonate weathering (12.17 tC·km−2·yr−1) and sink due to the “biological carbon pump” (SBCP) (2.24 tC·km−2·yr−1). The SBCP thus accounted for approximately 15.54% of the total carbon sink, indicating that the proportion of riverine TOC sourced by the transformation from bicarbonate to organic carbon by aquatic phototrophs may be high and must be considered in the estimation of carbonate weathering-related carbon sinks elsewhere.

Prospect of abandoned metal mining sites from a hydrogeochemical perspective.

Land exploitation for mining sector may leave a series of environmental impacts on our ecosystem if not appropriately managed. Therefore, the present study attempts to evaluate the various environmental aspects due to abandoned metal mining including former iron ore, bauxite, and tin mining lands in view of its hydrogeochemical behavior. Mine-impacted waters and sediments were ascertained from former mining ponds, mine tailings, and impacted streams for interpretation of aqueous and sediment geochemistry, major and trace elements, hydrochemical facies, chemical weathering rate and CO2 consumption, and water quality classification. Results indicated that the environmental impact of the long-abandoned iron ore mine was still evident with some high concentration of metals and acidic pH. Higher concentrations of Fe and Mn in water were noticeable in some areas while other trace elements (Pb, Zn, As, Cd, Cr, and Cu) were found below the recommended guideline values. Sediment quality reflected the trend of water quality variables mainly associated with metal(loid) elements, resulting in potential ecological risk, classified as having low to moderate risk. There were variations in terms of hydrochemical facies of the waters suggesting the influence of minerals in water. The chemical weathering rate suggests that contribution of carbonate mineral weathering was more important (up to 60%) than silicate weathering. The resulting CO2 consumption by mineral weathering was estimated to be in the range of 1.7-9.8 × 107 mol/year (former bauxite and tin mining areas can act as temporary sinks for CO2). Water quality classifications according to several chemical indices (Kelly's ratio, sodium absorption ratio, soluble sodium percentage, residual sodium carbonate, magnesium absorption ratio, and permeability index) were also discussed in regards to mine water reuse for irrigation purpose. The findings suggest that a holistic approach that integrates all important hydrogeochemical aspects is essential for a thorough evaluation of the implication of medium- to long-term mining exploitation on its surrounding ecosystems. This would be beneficial in light of restoration potential of degraded mining land so as for future mitigation strategies in the mining sector.

Monsoon impact on groundwater chemistry and geochemical processes in the shallow hard rock aquifer

Impact of monsoon recharge on groundwater chemistry and geochemical processes was evaluated using geochemical methods in the shallow hard-rock aquifer, southern India. Groundwater samples were collected from Salem city and its outskirts during pre-monsoon (PRM) and post-monsoon (PM) seasons and analysed for major and minor ions. Results suggest that during PRM, the average concentration of TDS, major ions and electrical conductivity (EC) in groundwater are higher than the PM season except for Na+ and HCO3–. The major water types also varied with seasons (PRM: NaCl > CaMgCl > CaHCO3; PM: CaHCO3 = NaHCO3 = NaCl). Pearson correlation analysis indicates that Cl− and SO42− show a strong positive correlation with EC, TDS, TH and major cations during PRM season whereas HCO3– and NO3– have a strong positive correlation with most of the ions during PM season. Before monsoon, the water chemistry is governed by the weathering of carbonate and silicate minerals, reverse ion exchange and evaporation processes, which resulted in high EC, TDS, TH and major ions in the groundwater. After the monsoon, the water chemistry is regulated by the mineral weathering, cation exchange and dilution processes. Geochemical modelling studies indicate that groundwater is saturated and oversaturated with carbonate minerals and undersaturated with sulphate and chloride minerals. During monsoon, the recharging water flushed the weathered layer and dissolved the soil CO2 in the vadose zone, which subsequently enhanced the concentration of Na+ and HCO3– in the aquifer. Further, the high concentrations of Cl−, SO42- and NO3– imply that the aquifer is polluted by surface contamination sources. After the monsoon, NO3– implies that monsoon recharge facilitated the pollutant movement from surface to the aquifer. Hence, a proper aquifer management plan is required to protect this aquifer and to provide clean and safe drinking water to the local community.

Assessment of groundwater geochemistry and human health risk of an intensively cropped alluvial plain, NW Italy

Groundwater chemistry data play an essential role to identify the water quality status and assess the exposure to human health. Therefore, in the present research, combined approaches used to assess the geochemistry, sources of ions and human health risk in the groundwater of the Tronzano Vercellese. Moreover, a geographic information system (GIS) technique was applied to identify locations have the most risk to human health. The results of the present study show that the groundwater samples had sequence of Ca2+>Mg2+>Na+>K+ in cationic, HCO3 −>SO4 2−>NO3 −>Cl−>F− in anionic and Zn > Fe > Ba > Ni > Mn > Cu > Cr > As in metals abundance. The hydrogeochemical approaches indicate that the major ions chemistry was mainly controlled by the weathering of carbonate and silicate minerals, and ion-exchange reactions. Furthermore, statistical analysis reveals that the non-lithogenic origin was the primary sources of some elements (Mn, Zn, Ba, NO3 − and K+) in the water samples. The hazard quotient (HQ) and hazard index (HI) were estimated to assess the risk to human health using the United States Environmental Protection Agency (USEPA) methods. Estimated HQ and HI values were higher for the child as compared to the adult and suggesting that the elements posed little hazard individually in the case of the child.

Characterization and formation of the pristine rhizoliths around Artemisia roots in dune soils of Tengeri Desert, NW China

Rhizoliths are the products of mineralization, petrification, or fossilization around and/or within plant roots. Among them, carbonate rhizoliths are the most common. Pristine carbonate rhizoliths with co-existing plant root relicts in the Tengeri Desert, NW China were studied, with a combination of intensive field observations and laboratory methods such as microscopy, scanning electronic microscopy, energy dispersive X-ray spectra, radiocarbon dating, and isotope mass spectrometer. The field observations revealed that the pristine rhizoliths are only present at the sites where Artemisia sphaerocephala Krasch are growing i.e. in swales among sand dunes. Soil moisture of the swales is the main controlling factor of rhizoliths formation. It is in turn affected by the soil physical properties, landscape position, and climate variability, consistent with the locations of sampled rhizoliths in the swales where Artemisia plants are mostly distributed. The 14C AMS dating indicated that the rhizoliths are much older (4000–5000 years) than their co-existing modern plant root relicts in agreement with field observations. Morphological, mineralogical and isotopic analyses revealed that carbon sources used for the rhizoliths formation were partially derived from decomposing plant roots but with significant contribution from dissolution of lithogenic carbonates. The calcium sources were suggested to be the in situ weathering of minerals (mostly lithogenic carbonates) and the pressure-dissolution of carbonates. Enough CO2 from the root decomposition have triggered carbonate accumulation around the root to form rhizoliths. Other minor chemical components of the root are S, N, P, which produce acidic water with the negative ions of SO42−, NO3–, PO43-, have also favored acidic soil environment and enhanced carbonate dissolution and mineral weathering. Redox environment around Artemisia roots were also observed to be a key factor for the pristine rhizolith formation. The pristine rhizoliths were preferentially formed in semi-closed redox condition with water nearly always available at intermediate depths. In addition, they were formed through carbonate epidiagenesis in shallow soils of the desert. Altogether, our results showed that the formation of the pristine rhizoliths was affected by the combination of several environmental factors. This led us to propose a conceptual model of rhizolith formation in desert soils.

Dynamics of major and trace elements during seawater intrusion in a coastal sedimentary aquifer impacted by anthropogenic activities

This study analyzed the dynamics of major ions and trace elements along the groundwater flow path of the coastal sedimentary Todos Santos aquifer in Baja California Sur, Mexico, moderately impacted by anthropogenic activities. The results indicate that the elements Ca2+, Mg2+, Ba2+, Sr2+ and Li+ are mobilized from the aquifer matrix during seawater intrusion, whereas the alkali-elements Na+, K+ and Rb+ are removed from solution, possibly due to cationic exchange process. The anions HCO3– and SO42− and the elements I and B are mobilized due to carbonate mineral weathering, whereas dissolved silica and the halides Br− and F− behave conservatively during salinization. Groundwater NO3− is provided by sewage infiltration. Regarding trace elements behavior, we identify three groups: i) elements that are mobilized during saline intrusion (Fe, Co, V, Se, Re), ii) elements revealing low or no mobilization (Mo, Ni, Cr, Ta, W) and iii) elements that show an undefined tendency (U, As, Ge, Sb, Cu, Mn). The U and NO3− levels in groundwater should be considered carefully because several wells have concentrations close to the permissible levels. This study may be useful as reference for knowing the possible effect of salinization in coastal aquifers under sea level rise scenarios driven by climate change.

Assessment of Groundwater Quality in the Talensi District, Northern Ghana.

A comprehensive chemical quality assessment of groundwater resources in the Talensi District has been conducted using conventional graphical methods and multivariate statistical techniques. The study sought to determine the main controls of groundwater chemistry and its suitability for domestic and irrigation purposes in the district. Silicate and carbonate mineral weathering were identified as the main controls on groundwater chemistry in the district, with reverse ion exchange also playing a role. High nitrate and lead levels observed have been associated with agrochemicals and wastewater from farms and homes. Three main flow regimes have been identified with Q-mode cluster analysis, in which mixed cation water types have been revealed, where areas designated as recharge zones are dominated by Na+ + K+–Mg2+–HCO3− fresh water types characterised by low mineralisation and pH, which evolve into Mg2+– Na+ + K+– HCO3− fresh water type with corresponding increased mineralisation of the groundwater. Based on the water quality index (WQI) technique modified for the district and an interpolation technique using ordinary kriging developed from a well-fitted exponential semivariogram for the estimated WQIs, the groundwater quality has been spatially classified as generally ‘good' to ‘excellent' for domestic purposes. Generally, the quality of groundwater for domestic usage deteriorates as one moves towards the north of the district, whereas waters in the east and west present the best quality. Classifications based on the United States Salinity Laboratory (USSL), Wilcox, and Doneen diagrams suggest that groundwater from the unconfined aquifers of the district is of excellent quality for irrigation purposes.

Using geochemical and isotopic tracers to characterize groundwater dynamics and salinity sources in the Wadi Guenniche coastal plain in northern Tunisia

Northeastern Tunisia is a sub-humid region with annual rainfall ranging from 600 to 650 mm. Unconfined groundwater in this area is easily accessible and represents an important perennial water source for agricultural activities. The present study deals with a multi-parameter investigation of one of the most important basins in northeastern Tunisia: the Wadi Guenniche plain (130 km2). Hydrochemical (major elements and Br−) and isotopic (18O, 2H and 3H) investigations were carried out on 32 shallow wells to gain insights into groundwater recharge and salinity. The dominant water types were Ca–Mg/Cl–SO4 and Na/Cl with electric conductivity values varying between 0.86 and 6.6 mS/cm. Based on ion ratios and saturation indexes, groundwater salinity originated from weathering of carbonate and evaporite minerals and cation-exchange with clays. Additional influence of anthropogenic activities shown by high NO3− concentrations exceeding 100 mg/L was identified in different parts of the plain resulting from irrigation return flow. A six-year record of 18O and 2H signatures of daily rainwater (n = 293) enabled to define a Local Meteoric Water Line for the Bizerte area (δ2H = 7.02 δ18O + 8.27). The isotopic signature of groundwater was very similar to that of the mean annual rainfall in Bizerte suggesting direct recharge of the aquifer. Tritium levels in sampled wells were relatively high and indicated recent recharge and short residence time. Annual recharge of 100 mm was estimated using a mixing model based on 3H contents in precipitation and groundwater.

Hydrogeochemical characteristics of arsenic rich groundwater in Greater Giyani Municipality, Limpopo Province, South Africa

This study evaluated the hydrogeochemical characteristics of groundwater in the Greater Giyani Municipality, Limpopo Province, South Africa. A total of ten (10) groundwater samples were collected from the existing boreholes and analysed for their hydrogeochemical properties using standard laboratory techniques. The data were interpreted using Piper diagram, Gibbs diagram and the scatter plots. The results showed that pH of the samples ranges from neutral to weakly alkaline. The abundance of major anions and cations are in the following order: HCO3− > Cl− > SO42− > NO3− and Na+>Mg2+>Ca2+>K+, respectively. The Piper diagram revealed that the hydrogeochemical facies identified in the study area are Ca–HCO3 and mixed Ca–Na–HCO3 type indicating the predominance of water-rock interaction. The water-rock interaction was further confirmed by the Gibbs diagram. The ionic ratio showed that the major ions originate from weathering of carbonate and silicate minerals. The concentration of arsenic was found to be ranging between 0.1 and 172.53 μg/L with the average of 32.21 μg/L. A total 6 out of 10 samples had arsenic concentration above the WHO recommended limit of 10 μg/L. The study calls for action from public health officials towards evaluating the correlation between arsenic concentration and arsenic related diseases and further development of arsenic remediation techniques.

Current chemical denudation, silicate mineral weathering and erosion in Irish catchments: reflections on the tortoise and the hare

The denudation of any landscape occurs by both physical means (i.e. mechanical weathering and erosion) and also chemical means (i.e. dissolution of minerals). The rates of these different processes can vary greatly in both time and lithology. Herein we present the rates of physical weathering and also data on aluminosilicate and carbonate mineral weathering in a dozen Irish catchments using newly collected river geochemical data and previously published riverine suspended sediment information. Our carbonate denudation yields range from approximately 30 to 145 tons km-2 yr-1, while physical erosion yields are at least an order of magnitude less in most cases and silicate weathering yields are mostly <1 ton km-2 yr-1. Our analysis clearly demonstrates the importance of carbonate mineral dissolution in landscape lowering, supporting the notion previously put forward by Simms (2004a,b) of the analogy of the tortoise and the hare in geomorphologically shaping the surface of Ireland.

Current chemical denudation, silicate mineral weathering and erosion in Irish catchments: reflections on the tortoise and the hare

:The denudation of any landscape occurs by both physical means (i.e. mechanical weathering and erosion) and also chemical means (i.e. dissolution of minerals). The rates of these different processes can vary greatly in both time and lithology. Herein we present the rates of physical weathering and also data on aluminosilicate and carbonate mineral weathering in a dozen Irish catchments using newly collected river geochemical data and previously published riverine suspended sediment information. Our carbonate denudation yields range from approximately 30 to 145 tons km-2 yr-1, while physical erosion yields are at least an order of magnitude less in most cases and silicate weathering yields are mostly <1 ton km-2 yr-1. Our analysis clearly demonstrates the importance of carbonate mineral dissolution in landscape lowering, supporting the notion previously put forward by Simms (2004a,b) of the analogy of the tortoise and the hare in geomorphologically shaping the surface of Ireland.

Hydrochemical processes and groundwater quality assessment in North eastern region of Jordan valley, Jordan

Hydrochemical studies carried out on the groundwater in north eastern part of Jordan valley, Jordan, revealed the confinement of groundwater to two partly overlapping Upper Cretaceous and Tertiary-Quaternary aquifers. Values of Base Exchange Index (r1) and Meteoric Genesis Index (r2) indicate that the groundwater is essentially Na-SO4 type and belongs mainly to the category of Deep Meteoric Percolation type. Major processes responsible for the hydrochemistry of groundwater are: weathering of carbonate and silicate minerals aided by H2CO3 and H2SO4, oxidation of sulphide minerals, domestic waste water, irrigation return flow, reverse and direct ion exchange reactions as well as connate saline water. Values of Water Quality Index indicate the occurrence of excellent and good quality potable water at majority of bore well locations. The groundwater belongs essentially to (C3S1) and (C4S2) irrigation water classes of Richards (1954) and is suitable for cultivation of semi-salt tolerant and salt tolerant crops.

Multivariate analysis and geochemical signatures of groundwater in the agricultural dominated taluks of Jalandhar district, Punjab, India

The present study is focused on the agriculturally dominated taluks of Nakodar, Shahkot and Phillaur in Jalandhar district, Punjab. Integrated multivariate technique along with geochemical signatures and vertical depth profiles of major ions were studied to assess the hydrogeochemical characteristics, processes influencing the aquifer chemistry and suitability appraisal for different uses. The analytical results revealed that the groundwater is hard to very hard in nature and their relative abundance of major ions are Na+ > Mg2+ > Ca2+ > K+ and HCO3– > SO42− > CO32– > Cl− > NO3− > F− > PO42− respectively. Elevated levels of NO3− (2 samples) in Phillaur taluk and F− (1 sample in every three taluks) were observed in shallow aquifer of the study area. The overall results of the water quality index show that a major part of the study area possesses good water quality. The taluk wise WQI results depict that certain pockets fall in poor (3 samples in Phillaur, 1 in Shahkot) to very poor (1 sample of Nakodar) water classes. The chemical compositions of the study area were predominantly Ca2+–Mg2+–HCO3– followed by Ca2+–Mg2+–Cl−–SO42−facies. Both Nakodar and Shahkot taluk shows Na+-Mg2+–HCO3– water type, while Phillaur taluk shows Mg2+–HCO3– water type. The water type of the shallow aquifer is complex in nature showing Na+-Mg2+–HCO3– type, further it changed to Na+–HCO3– in the intermediate zone. Interpretation of the geochemical signatures of the groundwater suggested that natural weathering and dissolution of silicate and carbonate minerals and ion exchange process are largely controlling the aquifer chemistry. Principal component analysis (PCA) extracts five principal components (PC's), where PC1, PC2 and PC4 are influenced by mixed factors i.e., lithogenic and anthropogenic inputs. PC3 is exclusively affected by anthropogenic factors like domestic sewage, fertilizer and irrigation return flow respectively and PC5 is mainly controlled by natural factors.

Effects of aquatic phototrophs on seasonal hydrochemical, inorganic, and organic carbon variations in a typical karst basin, Southwest China.

Karst processes play an important role in the global carbon cycle. Aquatic phototrophs can transform bicarbonate, which is mainly derived from the weathering of carbonates, into organic carbon. Carbonate mineral weathering coupled with aquatic photosynthesis can be considered a stable and durable carbon sink process. In this study, we addressed seasonal variations in water chemistry in the Lijiang River Basin, which is a typical karst basin, through a comprehensive geochemical study of the river water in four seasons. The parameters were measured in situ, including major ions and isotopes of inorganic and organic carbon. The results showed that (1) DIC was mainly derived from the weathering of carbonates; (2) the transformation from bicarbonate to organic carbon by aquatic phototrophs was evident, and the water chemistry changed, especially in spring and autumn and in the mainstream from Guilin to Yangshuo, which benefited the growth of aquatic phototrophs; and (3) the organic carbon derived from bicarbonates by aquatic phototrophs was nearly half the total organic carbon and 8% of the dissolved inorganic carbon. These results imply that aquatic phototrophs in karst basins can significantly stabilize carbon originating from carbonate rock weathering processes in karst areas.

Impact of anthropogenic sulfate deposition via precipitation on carbonate weathering in a typical industrial city in a karst basin of southwest China: A case study in Liuzhou

An analysis was performed of the hydrochemical characteristics of precipitation and river water samples collected from the Liujiang River Basin from June 2014 to May 2015, SW China to determine the carbon source and sink of carbonate weathering by anthropogenic sulfate deposition via precipitation in a typical Karst industrial city. Our findings revealed that (1) the pH of the precipitation samples has a volume-weighted mean (VWM) of 5.78 and that the acid rain rate is 12%. The enrichment factor (EF), sea salt fraction (SSF), crustal fraction (CF) and anthropogenic fraction (AF) were calculated, and the results indicated that precipitation was characterized by high concentrations of SO4, NH4, Ca and NO3. Na and Cl, Ca and NH4, and SO4 (98.2%) and NO3 (99.8%) were mainly derived from sea salt, earth crust, and anthropogenic activities, respectively. Combined with the air mass trajectory (HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory)) and the concentration of non-sea salt (NSS) SO4 and δ34S–SO4nss, the δ34S–SO4sam ranged from −2.6–12.6‰ and atmospheric sulfate was mainly associated with local coal combustion, fossil fuels and the transmission distance of pollution sources from the northwest and southwest in the interior of the continents. (2) An analysis of the river hydrochemical characteristics showed that the cations are mainly Ca and Mg, and they accounted for approximately 90% of the total cations. The anions were mainly HCO3 and SO4, and the hydrochemical composition of the river was mainly controlled by the weathering of carbonate minerals in the river basin. An analysis of the correlation between the changes in the chemical equivalence concentration of [Ca + Mg]/[HCO3+SO4] showed that the ratio of most river water samples was approximately 1, indicating that Ca and Mg in the river need HCO3 and SO4 for chemical equilibrium and that SO4 in the river may be involved in the dissolution of carbonate minerals. (3) Corrections were performed based on the chloride-normalized ratios and the contribution of each terminal element was calculated to estimate the major ions of precipitation input; the results showed that the average contribution ratio of SO4 transported to the river by anthropogenic sulfuric acid in precipitation reached 85.57%. (4) According to the proportional relationships, carbonate weathering accounts for 31.73% of the anthropogenic sulfuric acid via precipitation and would cause an approximate 6.27% increase in carbonate weathering rates but a 19.48% decrease in CO2 consumption rates. Taken together, our results revealed that even in high-carbonate areas of Southwest China, chemical weathering by sulfuric acid can reduce the carbon sink effect caused by carbonate weathering.

Identifying the groundwater flow systems in a condensed river-network interfluve between the Han River and Yangtze River (China) using hydrogeochemical indicators

Hydrogeochemistry and environmental isotopes were used to gain insight into the recharge processes, water–rock interactions, and groundwater residence time, and to identify groundwater flow systems (GFSs) in an interfluve between the Han River and Yangtze River in the eastern Jianghan Plain (China), an alluvial-lacustrine plain in the middle reaches of the Yangtze River. Because of carbonate mineral weathering, groundwater in the plain is predominantly HCO3-Ca or HCO3-Ca-Mg type. The decrease in typical ions and isotopic depletion with increasing depth indicates that the GFSs were divided into local and regional GFSs with an approximate depth limitation of 20 m. The consistent variations are attributable to complex anthropogenic activities, water–rock interactions and groundwater flow patterns. The multiple independent local GFSs exhibited a pattern in which groundwater was discharged into surface waters during the non-flood season. Groundwater age of local GFSs is modern according to the 3H concentrations, so the hydrodynamic circulation is active. Furthermore, the regional GFS pattern is controlled by slow lateral flow from west or northwest to east, eventually discharging into the Yangtze and Han rivers. The distribution of δ18O indicated three zones in regional GFSs that are likely dominated by the altitude effect of recharge areas. The groundwater age of regional GFSs varied from hundreds of years to 5000 years, estimated by 14C isotope data, revealing that the hydrodynamic circulation of regional GFSs is slow to relatively stagnant. The hydrodynamic characteristics and hydrochemical distributions corroborated the mixing zones of differently hierarchical GFSs in the discharge area of the Jianghan Plain.

Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin

The exposure of organic carbon in rocks to oxidative weathering can release carbon dioxide (CO2) to the atmosphere and consume atmospheric oxygen. Alongside volcanism, metamorphism, and the weathering of carbonate minerals by sulfuric acid, this is a major source of atmospheric CO2 over million year timescales. The balance between CO2 release and CO2 drawdown by silicate weathering and organic carbon burial sets the net geochemical carbon budget during weathering and erosion. However, the rates of rock-derived organic carbon (petrogenic organic carbon, OCpetro) oxidation remain poorly constrained. Here, we use rhenium as a proxy to trace and quantify CO2 release by OCpetro oxidation in the Mackenzie River Basin, Canada, where the other carbon fluxes have been well constrained previously. River water and sediment samples were collected between 2009 and 2013 at gauging stations along the Mackenzie River and its main tributaries (Liard, Peel and Arctic Red). To assess rhenium inputs from silicate, sulfide and OCpetro mineral phases we normalize dissolved rhenium concentrations, [Re]diss, to sodium and sulfate ion concentrations. This approach suggests that >85 percent of [Re]diss is derived from OCpetro in the main river channels. [Re]diss and water discharge measurements are used to quantify dissolved Re yields. River sediments provide a measure of the Re to OCpetro ratio of materials undergoing weathering in the basin, and agree well with published rock samples. Dissolved Re yields are combined with river sediment [Re]/[OCpetro] ratios to estimate the CO2 emissions by OCpetro weathering. These are 0.45 +0.19/−0.11 metric tonnes of carbon, tC km−2 yr−1for the Mackenzie River at Tsiigehtchic (3.8 +1.5/−0.9 × 104 moles km−2 yr−1), and 0.94 +0.41/−0.26 tC km−2 yr−1, 0.78 +0.35/−0.21 tC km−2 yr−1 and 1.01 +0.42/−0.25 tC km−2 yr−1 for the Peel, Arctic Red and Liard catchments, respectively. When considered alongside published silicate and carbonate weathering rates and the sedimentary burial of biospheric organic carbon, these data suggest that the upper part of the Mackenzie River Basin presently acts as an atmospheric CO2 sink of ∼1 tC km−2 yr−1 (∼8 × 104 moles km−2 yr−1) as a result of the carbon transfers by weathering and erosion. During the Last Glacial Maximum, it is possible that the net geochemical carbon balance may have been very different: potential increases in CO2 emissions from oxidative weathering of OCpetro and carbonate minerals, coupled with reduced biospheric carbon burial, may have tipped the balance to a net source of CO2.

Hydrochemical and isotopic characterization of groundwater in the southeastern part of the Plateaux Region, Togo

ABSTRACTHydrochemical, inverse geochemical modelling and isotopic approaches are used to assess the hydrogeochemical evolution of groundwater from the basement aquifers in the southeastern part of the Plateaux Region, Togo. Groundwater originates from present-day rainwater infiltration and is mostly fresh and slightly acidic to neutral. Hydrochemical facies are predominantly mixed cations-HCO3 associated with Ca/Mg-Cl, Na-HCO3 and Na-Cl water types in equilibrium with kaolinite and Ca/Mg-smectites. They are related to silicates hydrolysis, anthropogenic contamination, nitrification/denitrification, mixing along flowpaths and dissolution/precipitation of secondary minerals. The pattern of flow paths is in accordance with an increasing trend in total dissolved solids (TDS) toward the potentiometric depression located in the central and southern parts of the aquifer system. Inverse geochemical modelling using the NETPATH-WIN model showed the relative importance of biotite, plagioclase and amphibole weathering and dissolution of secondary carbonate minerals along the flowpath, suggesting that an abundance of minerals is not necessarily the main factor controlling the groundwater chemistry evolution.