Hydrochemistry and Dissolved Inorganic Carbon (DIC) Cycling in a Tropical Agricultural River, Mun River Basin, Northeast Thailand

Staged SWAT calibration with bias-corrected precipitation product for enhancing flow data continuity in tributaries of the Mekong River

Accurate and continuous flow data are crucial for effective water management in large river basins worldwide. However, these catchments often face challenges regarding data continuity in the mainstream and their tributaries. This study proposes a methodological framework for enhancing flow data continuity that uses a staged calibration scheme of Soil and Water Assessment Tool (SWAT) modeling with an appropriate satellite precipitation product (SPP) for each watershed. This framework is successfully applied in the Mun-Chi River Basin, the largest tributary of the Lower Mekong River, over a 20-year period at 34 stations. The staged calibration involves partitioning the catchment into smaller sub-catchments, guided by the flow monitoring stations and flow discontinuity structures such as dams. The sequential calibration from upstream to downstream partitions the overall model calibration challenge into smaller problems and contributes to a more efficient and accurate calibration and validation. Alternative SPPs were considered to overcome monitoring gaps. Their bias was corrected using quantile mapping, and their performance was evaluated with flow simulations using the SWAT model. The assessment indicates that the CMORPH-CRT product, with a spatial resolution of 0.25°, demonstrates good suitability for hydrological modeling of the Mun-Chi River Basin. The proposed methodological framework provides a continuous time series of flow discharge at multiple stations within the watershed, offering valuable insights for sustainable water resource management strategies in river systems under changing climate and land use conditions, and supporting future studies on environmental issues.

Effect of damming on hydrogeochemical characteristics and potential environmental risks in a large reservoir: Insights from different vertical layer sampling

The water environment of large reservoirs is fragility due to effects from hydrological regulation of damming and anthropogenic inputs. As a critical path to quantify the natural chemical weathering and assess environmental risks, solute chemistry of river has been widely focused on. However, the complexed hydrological conditions of large reservoir affect the chemical compositions, and the significance of solute vertical geochemistry as an indicator of chemical weathering and water quality health remains explore. Therefore, the Three Gorges Reservoir (TGR) was selected as a typical study area, which is the world's largest hydropower project and subject to frequent water quality problems. Then, the chemical compositions in stratified water were determined. Ca2+ (52.8 ± 4.3 mg/L) and HCO3- (180.9 ± 8.9 mg/L) were the most abundant ions among cations and anions, respectively. Incremental mean concentration of total major ions followed with the increase of riverine depth and flow direction. An improved inversion model was used to quantify the source contribution, which weathering of dolomite (34%) and calcite (38%) contributed the most to total cations, and the influences of agriculture and sewage discharge were limited. Additional contributions of evaporite and pyrite oxidation were found in analysis of deeper water samples, which also results in 2%-67% difference in estimated CO2 release flux using data from different depth, indicating additional information about sulfuric acid driven weathering was contained. Finally, the water quality of the reservoir was assessed for irrigation and non-carcinogenic risks. Results showed the stratified water of TGR can be used as a good water source of irrigation. However, NO3- (5.1 ± 1.1 mg/L) may have a potential non-carcinogenic risk to children, especially in surface water. To sum up, this study provided an indispensable supplement to the water chemistry archives in the TGR basin, serving as theoretical references for environmental management of large reservoirs.

Combining hydrochemistry and 13C analysis to reveal the sources and contributions of dissolved inorganic carbon in the groundwater of coal mining areas, in East China

East China is a highly aggregated coal-grain composite area where coal mining and agricultural production activities are both flourishing. At present, the geochemical characteristics of dissolved inorganic carbon (DIC) in groundwater in coal mining areas are still unclear. This study combined hydrochemical and carbon isotope methods to explore the sources and factors influencing DIC in the groundwater of different active areas in coal mining areas. Moreover, the 13C isotope method was used to calculate the contribution rates of various sources to DIC in groundwater. The results showed that the hydrochemical types of groundwater were HCO3-Ca·Na and HCO3-Na. The main water‒rock interactions were silicate and carbonate rock weathering. Agricultural areas were mainly affected by the participation of HNO3 produced by chemical fertilizer in the weathering of carbonate rocks. Soil CO2 and carbonate rock weathering were the major sources of DIC in the groundwater. Groundwater in residential areas was primarily affected by CO2 from the degradation of organic matter from anthropogenic inputs. Sulfate produced by gypsum dissolution, coal gangue accumulation leaching and mine drainage participated in carbonate weathering under acidic conditions, which was an important factor controlling the DIC and isotopic composition of groundwater in coal production areas. The contribution rates of groundwater carbonate weathering to groundwater DIC in agricultural areas and coal production areas ranged from 57.46 to 66.18% and from 54.29 to 62.16%, respectively. In residential areas, the contribution rates of soil CO2 to groundwater DIC ranged from 51.48 to 61.84%. The results will help clarify the sources and circulation of DIC in groundwater under the influence of anthropogenic activities and provide a theoretical reference for water resource management.

Carbon Biogeochemistry of the Estuaries Adjoining the Indian Sundarbans Mangrove Ecosystem: A Review

The present study reviewed the carbon-biogeochemistry-related observations concerning CO₂ and CH₄ dynamics in the estuaries adjoining the Indian Sundarbans mangrove ecosystem. The review focused on the partial pressure of CO₂ and CH₄ [pCO_2(water) and pCH_4(water)] and air-water CO₂ and CH₄ fluxes and their physical, biogeochemical, and hydrological drivers. The riverine-freshwater-rich Hooghly estuary has always exhibited higher CO₂ emissions than the marine-water-dominated Sundarbans estuaries. The mangrove sediment porewater and recirculated groundwater were rich in pCO_2(water) and pCH_4(water), enhancing their load in the adjacent estuaries. Freshwater-seawater admixing, photosynthetically active radiation, primary productivity, and porewater/groundwater input were the principal factors that regulated pCO_2(water) and pCH_4(water) and their fluxes. Higher chlorophyll-a concentrations, indicating higher primary production, led to the furnishing of more organic substrates that underwent anaerobic degradation to produce CH₄ in the water column. The northern Bay of Bengal seawater had a high carbonate buffering capacity that reduced the pCO_2(water) and water-to-air CO₂ fluxes in the Sundarbans estuaries. Several authors traced the degradation of organic matter to DIC, mainly following the denitrification pathway (and pathways between aerobic respiration and carbonate dissolution). Overall, this review collated the significant findings on the carbon biogeochemistry of Sundarbans estuaries and discussed the areas that require attention in the future.

Geochemical and Seasonal Characteristics of Dissolved Iron Isotopes in the Mun River, Northeast Thailand

Dissolved iron (Fe) isotopes in river water have a pivotal role in understanding the Fe cycle in the surficial environment. A total of 13 samples of river water were collected from the Mun River to analyze the Fe isotopes and their controlling factors in river water, such as dissolved organic carbon (DOC) and different supply sources. The results showed that dissolved Fe (DFe) concentrations ranged from 21.49 μg/L to 232.34 μg/L in the dry season and ranged from 10.48 μg/L to 135.27 μg/L in the wet season, which might be ascribed to the dilution effect. The δ56Fe of the dry season (−0.34 to 0.57‰, with an average 0.09‰) was lower than that of the wet season (−0.15 to 0.48‰, with an average 0.14‰). Combined with the δ56Fe and DFe/DAl ratios, the end-members of DFe were identified, including rock weathering (high δ56Fe and low DFe/DAl ratio), anthropogenic inputs (high δ56Fe and high DFe/DAl ratio) and groundwater inputs (low δ56Fe and low DFe/DAl ratio). The relationship between δ56Fe and DOC concentrations suggested that the chelation of organic matter with heavy Fe isotopes was one of the important sources of heavy Fe isotopes in river water.

Seasonal and Spatial Variations of δ13CDIC Values in the Mun River, Northeast Thailand

As an important part of the global carbon cycle, dissolved inorganic carbon (DIC) concentration and its stable carbon isotopic composition (δ13CDIC) have been used to constrain the sources of DIC in rivers. In this study, we systematically investigated the water chemistry, DIC contents, and δ13CDIC values in a tropical agricultural river in northeast Thailand. The water temperature ranged from 20.3 to 31.3 °C, and water pH values ranged from 6.4 to 8.4, with seasonal variations. Based on the major ion compositions, the hydro-chemical type of the Mun River water was a unique Na–Ca–Cl–HCO3 type, controlled by evaporite and silicate weathering. Seasonal variation of DIC concentrations and its carbon isotopic composition was obvious; DIC and δ13CDIC were significantly lower in the wet season (135 to 3146 μmol/L and −31.0‰ to −7.0‰) compared to the dry season (185 to 5897 μmol/L and −19.6‰ to −2.7‰). A high level of 12C-enriched DIC/CO2 from soil respiration and organic matter oxidation may cause the low pH values, δ13CDIC values, and high partial pressure of CO2 (pCO2) in the middle and lower reaches during the wet/rainy season compared to the dry season. This may be responsible for the seasonal and spatial variations of DIC concentrations and δ13CDIC values in the Mun River. According to the relationship between pCO2 and δ13CDIC values, CO2 outgassing may be more significant in the dry season, due to the greater influx of groundwater with higher pCO2 levels; and the rapid CO2 diffusion into the atmosphere will continuously increase the δ13CDIC values and decrease pCO2 levels. These results show that riverine biologic effects and CO2 outgassing play important roles in the DIC and δ13CDIC evolution of this typical agriculturally-dominated watershed.

Characteristics of and Influencing Factors of Hydrochemistry and Carbon/Nitrogen Variation in the Huangzhouhe River Basin, a World Natural Heritage Site

In karst areas, the characteristics of water chemistry and carbon and nitrogen are of great significance to basic research. The contents of Ca²⁺, Mg²⁺, K⁺, Na⁺, HCO₃^-, SO₄^2-, NO₃^-, Cl^-, dissolved organic carbon (DOC), and total nitrogen (TN) in water samples from 18 rivers and 14 springs in the Huangzhouhe River Basin were determined. The results showed that the water chemistry type in the Huangzhouhe River Basin is HCO₃-Ca-Mg. The chemical composition is mainly affected by dolomite weathering and also by ion exchange and other human activities. The river and spring DIC remain at the same level in the upper and middle reaches and decrease in the lower reaches. The NO₃-N and TN of river water and TN of spring water increase in the middle reaches, while NO₃-N of spring water decreases in the lower reaches. The DOC in the basin increases with the increase of SO₄^2- and Cl^-, mainly due to the human influence of agricultural and domestic sewage. In the basin, the NO₃-N and TN in spring water are larger, and the DOC in river water is larger, mainly because there are more phytoplankton and human activities in the river water. The carbon and nitrogen in the Huangzhouhe River Basin are mainly HCO₃^- and NO₃^- ions. The evaluation of pH, Cl^-, NO₃-N, SO₄^2-, and TDS shows that the water quality is good and the ecological environment is good.

A Strontium and Hydro-Geochemical Perspective on Human Impacted Tributary of the Mekong River Basin: Sources Identification, Fluxes, and CO2 Consumption

As the largest and most representative tributary of the Mekong River, the Mun River Basin (MRB) provides critical understanding of regional hydro-geochemical features and rock weathering processes on a basin scale. The present study measured strontium (Sr) isotopes with hydro-geochemistry data of 56 water samples in detail in the MRB in northeast Thailand. The dissolved Sr contents and 87Sr/86Sr isotopic ratios were reported to be 8.7–344.6 μg/L (average 126.9 μg/L) and 0.7085–0.7281 (average 0.7156), respectively. The concentrations of dissolved Sr in the mainstream slightly decreased from upstream to downstream, while the variation trend of 87Sr/86Sr was on the contrary. Correlation analysis showed that Na+ strongly correlated with Cl− (0.995, p < 0.01), while Ca2+ exhibited weak relationships with SO42− (0.356, p < 0.01). Samples of the MRB exhibited lower Mg2+/Na+, Ca2+/Na+, HCO3−/Na+ and 1000Sr/Na ratios, and gathered around the end-member of evaporite dissolution, with slight shift to silicate weathering end-member, demonstrating the dominant contribution of evaporite dissolution and silicate weathering on dissolved loads. Comparing with data of major world rivers from previous research, our results remained consistency with rivers draining through similar geological conditions. The dissolved Sr flux to the adjacent Mekong River was estimated to be 20.7 tons/year. In accordance with the forward model, silicate weathering rate and CO2 consumption rate during dry season were calculated to be 0.73 tons/km2/year and 1.94 × 104 mol/km2/year, and may get underestimated due to intense water consumption by extensive agricultural activities. The superimposed effect of anthropogenic impacts on the water environment could enhance chemical weathering, and thus should be taken into account in regional ion cycles and carbon budgets. These findings highlight the coupling analysis of Sr isotopes and hydro-geochemistry in Earth surface processes and provide basic investigation for sustainable regional water treatment mechanisms in the pan basin of the Mekong River.

Tracing riverine sulfate source in an agricultural watershed: Constraints from stable isotopes

The sulfate pollution in water environment gains more and more concerns in recent years. The discharge of domestic, municipal, and industrial wastewaters increases the riverine sulfate concentrations, which may cause local health and ecological problems. To better understand the sources of sulfate, this study collected water samples in a typical agricultural watershed in East Thailand. The source apportionment of sulfide was conducted by using stable isotopes and receptor models. The δ³⁴S_SO4 value of river water varied from 1.2‰ to 16.4‰, with a median value of 8.9‰. The hydrochemical data indicated that the chemical compositions of Mun river water were affected by the anthropogenic inputs and natural processes such as halite dissolution, carbonate, and silicate weathering. The positive matrix factorization (PMF) model was not suitable to trace source of riverine sulfate, because the meaning of the extracted factors seems to be vague. Based on the elemental ratio and isotopic composition, the inverse model yielded the relative contribution of sulfide oxidation (approximately 46.5%), anthropogenic input (approximately 41.5%), and gypsum dissolution (approximately 12%) to sulfate in Mun river water. This study indicates that the selection of models for source apportionment should be careful. The large contribution of anthropogenic inputs calls an urgent concern of the Thai government to establish effective management strategies in the Mun River basin.

Dissolved iron and isotopic geochemical characteristics in a typical tropical river across the floodplain: The potential environmental implication

Iron (Fe) is an essential element for bio-physiological functioning terrestrial organisms, in particular of aquatic organisms. It is therefore crucial to understand the aquatic iron cycle and geochemical characteristics, which is also significant to obtain the key information on earth-surface evolution. The stable iron isotopic composition (δ⁵⁶Fe) of the dissolved fraction is determined in the Mun River (main tributary of Mekong River), northeast Thailand to distinguish the human and nature influenced riverine iron geochemical behavior. The results show that dissolved Fe concentration ranges from 8.04 to 135.27 μg/L, and the δ⁵⁶Fe ranges from -1.34‰ to 0.48‰, with an average of 0.23‰, 0.14‰ and -0.15‰ in the upper, middle and lower reaches, respectively. The δ⁵⁶Fe values of river water are close to that of the bulk continental crust and other tropical rivers. The correlations between δ⁵⁶Fe and Fe, Al, and physicochemical parameters show mixing processes of different Fe end-members, including the rock weathering end-member (low Fe/Al ratio and high δ⁵⁶Fe), the urban activities end-member (high Fe/Al ratio and moderate δ⁵⁶Fe), and a third end-member with probable sources from the Chi River and reservoir. For the most river water samples, the primary contribution is attributed to rock weathering, and the second is urban activities (only a few samples are from the upper and middle reaches). Thus, Fe isotopes could be employed as a proxy to identify and quantify the natural and anthropogenic contributions, respectively. These findings also provide data support for the scientific management of water resources in the Mun River catchment and other large tropical rivers.

Identifying the sources of nitrate contamination using a combined dual isotope, chemical and Bayesian model approach in a tropical agricultural river: Case study in the Mun River, Thailand

Environmental issues triggered by increasing nitrate in agricultural river has become global concern. Identifying nitrate sources and transformation are crucial for water sources protection and eliminating nitrate contamination in an agricultural watershed. In this study, chemical and dual isotopic compositions of nitrate were employed to trace the nitrate sources and transformation processes, and proportional contribution of NO₃^- source were estimated by SIAR based on Bayesian model. NH₄⁺ concentrations in middle Mun and lower Mun in wet season were significantly higher than NO₃^-, suggesting enhanced runoff processes by flood promote agricultural fertilized NH₄⁺ leaching into the river. Higher Cl^- concentration and NO₃^-/Cl^- indicated that manure and sewage was the dominate nitrate source in the Lam Takhong River and the upper Mun. The overall values of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- ranged from -3.9‰ to +16.6‰ and from -5.2‰ to +40.0‰, respectively. The results of nitrate isotopes indicated that NO₃^- mainly originated from soil N nitrogen, chemical fertilizer, and manure and sewage wastes. Spatially, soil N and chemical fertilizer contributed the most nitrate in the mainstream of lower Mun, middle Mun, and the Lam Takhong River; whereas over 60% of nitrate was derived from manure and sewage in the upper Mun. The spatial variation of water discharge and rainfall, together with the nitrate concentration and isotopes inferred that the nitrate sources and transformations in rain-fed river in tropical zone were distinguished from other rivers. High water discharge driven by rainfall events accelerated the nitrate export and the contribution of atmospheric deposition in wet season, and enlarged the contribution of manure and sewage in dry season. This study provided an example for further researches and approaches to assess the effects of tropical climate and agriculture on nitrate accumulation in watershed.

Distribution, fractionation and sources of rare earth elements in suspended particulate matter in a tropical agricultural catchment, northeast Thailand

Forty-eight suspended particulate matter (SPM) samples were collected from the Mun River, northeast Thailand and its junction with the Mekong River, to investigate the relationship between the distribution of rare earth elements (REE) in SPM and the soils in the watershed. The total REE contents (∑REE) in SPM in the Mun River ranged from 78.5 to 377.8 mg/kg with the average of 189.3 mg/kg, which was lower than ∑REE of 222.3 mg/kg at the Mekong River (one sample at junction). The Post Archean Australia Shale (PAAS)-normalized ratios of light REE (LREE), middle REE (MREE) and heavy REE (HREE) were averaged to 1.0, 1.3 and 1.0, which showed a clear enrichment in MREE. In short, along the Mun River, the REE contents in SPM were decreasing, and the PAAS-normalized patterns of REE showed gradually flat. The REE content in SPM and soils are highest in the upper catchment, indicating that soil/bedrock is the most important source of REE in SPM. Additionally, the positive Eu anomaly was enhanced by the higher Ca content in SPM (R = 0.45), which may be caused by more feldspars or carbonates with Ca and Eu substituting Ca. The results present the REE behaviors of SPM in the Mun River and relationship between REE in SPM and soil/bedrock, the findings may support the other studies in catchment weathering.

Major Elements in the Upstream of Three Gorges Reservoir: An Investigation of Chemical Weathering and Water Quality during Flood Events

Rivers transport terrestrial matter into the ocean, constituting a fundamental channel between inland and oceanic ecosystem and affect global climate change. To reveal chemical weathering processes and environmental health risks during flood periods, water samples were collected in the upper reaches of Three Gorges Reservoir (TGR) in 2020. HCO3− and Ca2+ were the most abundant anions and cations of the river water, respectively. The range of HCO3− concentration was between 1.81 and 3.02 mmol/L, while the mean content of Ca2+ was 1.03 mmol/L. The results of the Piper diagram and element ratios revealed that the river solutes were mainly contributed by carbonate weathering and gypsum-rich evaporite dissolution. A mass balance model indicated that the contribution order of sources to cations in the main channel (Yibin-Luzhou) was evaporites > carbonates > atmospheric input > silicates. The order in the Chongqing—Three Gorges Dam was carbonates > atmospheric input > evaporites > silicates. These results showed a lithologic control on hydrochemical characteristics. Most sampling sites were suitable for agricultural irrigation according to the water quality assessment. However, indexes sodium adsorption ratio (SAR) and soluble sodium percentage (Na%) were higher than 1.0 in Yibin-Luzhou and 30% in Yibin–Chongqing, respectively, suggesting a potential sodium hazard. In addition, except Tuojiang River and Shennong River, the risk of sodium hazard in tributaries was relatively low. High Na+ concentration in irrigation water can damage soil structure and function and ultimately affect agricultural production. Water quality in the upstream of a Piper diagram should attract enough attention.

Changes in dissolved inorganic carbon in river water due to urbanization revealed by hydrochemistry and carbon isotope in the Pearl River Delta, China

Under natural conditions, the dissolved inorganic carbon (DIC) in river water is dominantly derived from carbonate or silicate dissolution by carbonic acid. However, sulfuric and nitric acids produced by human activities provide additional acidity for chemical weathering, which would affect the DIC flux and change its isotopic composition. To identify the natural and anthropogenic impacts on DIC, the major ion concentrations and stable carbon isotopes of the DIC (δ¹³C-DIC) of river waters were measured in the Pearl River Delta (PRD) region, which is one of the most developed and populated areas in China. The mass balance calculations for DIC-apportionment showed that carbonate dissolution by carbonic acid was the dominant origin of DIC in the Beijiang (BJ) River (67%) and Xijiang (XJ) River (78%) and silicate dissolution by carbonic acid was the dominant origin of DIC in the Guangzhou (GZ) Channel (37%) and Dongjiang (DJ) River (50%), which was related to the lithology of the catchment. The contribution of carbonate dissolution by sulfuric and nitric acids, which represented the contribution of human activities to the total DIC concentrations in river water, showed high proportions in the GZ Channel and DJ River, with averages of 42% and 34%, respectively, which were associated with a high degree of urbanization. Evidence of hydrochemical parameters and δ¹³C-DIC signatures indicated that human activities had impacts on the DIC pool. Carbonate dissolution by sulfuric and nitric acids caused by human activities changed DIC apportionments rather than the DIC flux, and this part of DIC would ultimately become a source of CO₂ to the atmosphere on the geological timescale and affects the CO₂ budget. An increase in nutrient concentration due to increased sewage discharge in the urbanized area could promote phytoplankton photosynthesis, which could change the DIC pool and increase the δ¹³C-DIC value. This study quantitatively highlights the influence of human activities on DIC apportionment in river water, suggesting that anthropogenic impacts should be seriously considered when evaluating the evolution of DIC.

Temporal Variations of Water Chemistry in the Wet Season in a Typical Urban Karst Groundwater System in Southwest China

It is important to investigate temporal variations of water chemistry for the purpose of improving water quality in karst groundwater systems. Groundwater samples were collected daily under various land uses of Guiyang. Major ions and stable carbon isotope composition of dissolved inorganic carbon (δ¹³C_DIC) were analyzed to understand the biogeochemical processes. The water chemistry was dominated by Ca²⁺, Mg²⁺, HCO₃^-, and SO₄^2-, which mainly derived from the dissolution of carbonate rocks (limestone and dolomite) and oxidation of sulfide. The groundwater was defined as of the HCO₃-Ca Mg and HCO₃·SO₄-Ca·Mg type, according to its hydrochemical characteristics. Results suggested that hydrochemical concentrations changed quickly, in response to rainfall events. The fast response revealed that karst groundwater was easily impacted by rainfall and anthropogenic inputs according to temporal variation of water chemistry. The distribution of DIC (dissolved inorganic carbon) and δ¹³C_DIC showed that DIC is mainly sourced from soil CO₂(g) influx and carbonate dissolution. δ¹³C_DIC and major ions ratios suggested that carbonate minerals were dissolved by H₂SO₄ at groundwater in wooded area, contributing an important source for DIC due to the slight enrichment of heavy δ¹³C_DIC. More negative δ¹³C_DIC values were observed after rainfall reflected the fact that soil CO₂(g) and organic carbon oxidation influxes accounted for a large share during DIC formation. Various δ¹³C_DIC and hydrochemical patterns were observed under various land use and human activity conditions. Meanwhile, relative high nitrate loads were found in groundwater after rainfall, suggesting high anthropogenic inputs following rainwater as having side effects on water quality. This study suggests that water chemistry and isotopic proof provide a better understanding of water quality and carbon dynamics responding to rainfall events in the karst groundwater systems.