Nitrate pollution is of significant global concern as a potential threat to eco-environmental safety and human health, especially in coastal zones where freshwater resources are limited. However, the information regarding the contribution of seawater intrusion to NO3- in coastal groundwater, as well as NO3- pollution driven by geological conditions remains limited. Here, an integrated method combining hydrochemical parameters, multi-isotope techniques (δ15Nnitra, δ18Onitra, δ34Ssul, and δ18Osul), and source apportionment models (PMF and MixSIAR) was applied to reduce uncertainties in estimating NO3- sources and to clarify biogeochemical mechanisms controlling NO3- pollution in coastal groundwater. The results revealed that manure & domestic sewage and soil nitrogen can account for over 80% of the total NO3- pollution sources (86.3 ± 4.44% from PMF and 83.5 ± 5.42% from MixSIAR), implying the considerable potential of anthropogenic emissions for nitrogen seepage. In contrast to the substantial role of surface inputs in supplying NO3- to groundwater, seawater intrusion contributed negligibly to groundwater NO3- (~5%). Nitrification mainly attributes to the intensification of NO3- pollution. The HNO3 from nitrification is involved in carbonate weathering and may enhance the simultaneous release of NO3-, Ca2+, and HCO3- into coastal groundwater. Whereas denitrification and/or DNRA processes occur to a limited extent only in local areas where fresh groundwater is distributed. Our study highlights the significance of natural factors in NO3- pollution in coastal groundwater, contributing to be a better understanding of nitrogen cycling in global coastal zones.

Agricultural HNO3 can affect mineral weathering, regional carbon cycling, and river NO3– pollution, which are linked to complex biological and hydrogeochemical processes. However, the involvement of HNO3 in carbonate weathering and associated CO2 fluxes is still not fully understood, especially in piedmont areas. Herein, 20 river water samples were collected from a typical piedmont region (Qingshui River basin) in September 2023 and analyzed for hydrochemical ions and multiple isotopes. The hydrogeochemical analyses, coupled with the Bayesian and forward models, and stoichiometric relationships, were employed to elucidate the major mechanisms underlying the release of HNO3 from agricultural NH4+ inputs and its quantitative effects on carbonate weathering and CO2 sink flux. The hydrochemical evolution was driven by the piedmont strong runoff condition, resulting in the dominance of the HCO3-Ca facies with less-soluble minerals. On the other hand, the riverine NO3– concentrations in the piedmont agricultural region were primarily derived from sewage and manure, with a mean contribution rate of 92.0 ± 2.07

Water quality and hydrogeochemical mechanisms of river water in recharge areas are of great significance for environmental safety and ecological stability. Here, water quality and associated mechanisms in river water of the Qingshui River were assessed using hydrochemical and isotopic data (δ2HH2O and δ18OH2O), combined with the Entropy-weighted Water Quality Index (E-WQI) and positive matrix factorization (PMF) model. The results showed that water quality of the agricultural reach is dominated by an "excellent level," while ~ 43% of samples from the urban reach showed a "good level." Water quality is mainly driven by mineral dissolution (36.8% and 57.0% for the agricultural and urban reaches) and agricultural inputs (37.7% and 21.7% for the agricultural and urban reaches), while positive cation exchange contributes less to hydrochemical components. Furthermore, the construction of artificial channels enhances the effect of evaporation on hydrochemistry and alters the water-rock interaction processes. Those results provide new insights into evaluating hydrogeochemical mechanisms in the river system of piedmont regions.

The geological environment of the karst mountain area is fragile. After heavy rainfall, the internal hydrodynamic conditions of the karst mountain change significantly, threatening the stability of the karst landslide. This study take Guanling landslide as a case study to clarify the influence of rapid karst conduit flow formed by heavy rainfall on karst landslide. Combined with field investigation and analysis of hydrogeological conditions, the seepage influence mode of karst conduit flow was obtained: (1) In the dry season, karst conduit flow seizes the bedrock debris flow head on both sides, erodes it downward, and expands the depth of the karst conduit system; (2) When heavy rainfall occurs, karst pipes flow to both sides and recharge, forming a watershed. The fluid-structure coupling numerical simulation experiment was carried out using two-dimensional discrete element coding UDEC software. The results showed that the transmission of high-pressure water head from the top recharge area to the foot of the slope had lagged for one hour. The drainage capacity of the clastic rock area at the foot of the slope was limited, which made the high-pressure water head conducted by the overlying karst conduit flow unable to be discharged in time. The retained high-pressure water head caused shear failure at the foot of the slope. It caused the overlying height difference of nearly 200 m sliding body and the landslide disaster. The instability mechanism proposed in this paper can provide a reference for other landslides induced by karst conduit flow and provide theoretical support for predicting and preventing such landslides.

The karst groundwater resource is the key water supply for native residents and industrial and agricultural production in karst regions of southwest China and is even the only water supply for rural regions. Understanding the formation mechanism of hydrochemical compositions is practically imperative to the sustainable development and utilization of karst groundwater resources. The hydrochemical ions of 34 karst spring samples and 12 underground river water samples from the Qingjiang River Basin were measured and analyzed in this study. On the basis of investigating the distribution characteristics of hydrochemical components, methods such as hierarchical clustering analysis, Gibbs model, and relationships between hydrochemical components were used to comprehensively reveal the chemical genesis mechanism of karst groundwater from the perspectives of water-rock interaction and anthropogenic activities, especially in the disturbance mechanism of acidic pollutants on the natural dissolution of carbonate rocks. The results showed that the karst groundwater in the Qingjiang River Basin was weakly alkaline （7.12 ≤pH ≤ 8.69）, and the overall water quality was good. Only the maximum concentration of NO3- in the karst spring （80.2 mg·L-1） exceeded the maximum acceptable limit for drinking purposes （10 mg·L-1 as N）. The Ca2+ and HCO3- were identified as the dominant ions, associated with the main water phrase of HCO3-Ca·Mg in approximately 97.1% of karst spring samples and all underground river water samples. The dominant ions were derived from the strong dissolution of calcite, whereas Mg2+、SO42-, and F- were controlled by the weak dissolution of dolomite, gypsum, and fluorite, respectively. Karst groundwater NO3- was derived from the agricultural fertilizers and rainwater input, whereas the H2SO4 acid rain input was identified as another important source of karst groundwater SO42-. In addition, the carbonate rocks in the study area were mainly naturally dissolved； however, the input of exogenous acid by humans has changed the original karst hydrogeochemical evolution process and further decreased the carbonate sink flux of karst aquifers, accelerated the decrease rate of karst groundwater table, and increased the pollution risks of karst groundwater.

This study focuses on the karst region of South China,centered in Guizhou Province and encompassing Yunnan,Guangxi,Hunan,Hubei,Sichuan,Chongqing,and Guangdong.The study area covers 780,000 km2 with survey data indicating 520,000 km2 of exposed karst and 260,000 km2 of covered or buried karst.Karst water resources in the region amount to 169.536 billion m3·a-1,representing 83.12％of China's total karst water resources Characterized by a tropical-subtropical monsoon climate-with annual rainfall ranging from 1,000 to 2,200 mm,60％to 80％of which occurs between May and September-the region exhibits complex hydrogeological structures.These include highly heterogeneous aquifers,frequent surface water-groundwater(SW-GW)transformations(with more than 13 types),rapid flow dynamics(conduit flow velocities exceeding 80 m·h-1 during wet seasons),and marked ecological vulnerability. Since the 1970s,with collaborative efforts,the China Geological Survey(CGS)and provincial departments have completed 1∶250,000-scale hydrogeological surveys covering 780,000 km2 and 1∶50,000-scale hydrogeological surveys covering 350,000 km2.Despite the accumulation of extensive data,no integrated technical standards have been established.The 2023 Notice on Carrying out Water Resource Baseline Surveys,issued by the Ministry of Natural Resources,mandates unified investigations of all water bodies.Against this backdrop,this study aims to:(1)systematically define the unique characteristics of water resources and hydrological cycles of the karst areas in Southern China;(2)construct an integrated SW-GW investigation and evaluation framework;(3)identify key technical challenges and propose solutions. Based on four decades of survey data and relevant literature,we systematically synthesized the characteristics of water resources and hydrological cycle.By integrating current standards(e.g.,DD 2019-01,DZ/T 0469-2024)with karst-specific practices,we developed a comprehensive technical workflow.The key results are as follows:(1)Characteristics and Patterns:Seven water resource traits were identified:systemic structural complexity,spatiotemporal heterogeneity of resources,frequent SW-GW exchange,occurrence heterogeneity,hydrochemical erosivity,geomorphological control,and ecological vulnerability.Six hydrological patterns were observed:diverse circulation pathways;close interconnection among atmospheric precipitation,surface water,and groundwater;the high-frequency exchange;rapid flow velocity;multi-layer aquifer connectivity;and coexistence of fast and slow flows.(2)Technical Framework:A six-phase workflow was established:pre-survey planning → field investigation →monitoring → evaluation → database development → reporting.This approach replaces traditional map-based surveys with karst watershed units.(3)Critical Technical Solutions:Unit delineation-Level V karst water systems were designated as core assessment units.Boundary identification-discriminant boundaries were determined based on hydrodynamic fields and geological structures.SW-GW flux quantification-13 transformation subtypes were defined,each with corresponding monitoring strategies.Hydrological process characterization-differentiation was based on the intensity of karst development.Monitoring network optimization-stratified deployment was implemented according to aquifer structure;at least one surface water or groundwater monitoring site per system,with denser coverage at critical exchange nodes(e.g.,sinkholes,spring clusters).Evaluation methods-scale-adapted models,such as Modflow-CFP,were applied for small basins. The integrated framework addresses technical gaps in holistic SW-GW assessment for karst regions in South China,effectively resolving challenges of spatial heterogeneity and dynamic transformations.The six technical pillars-unit delineation,boundary identification,transformation quantification,process characterization,monitoring optimization,and scale-adapted evaluation-provide standardized support for water resource surveys and sustainable management.

As a result of their tendency to enter various ecosystems, bioavailable heavy metals (HMs) have become major human public health concerns. The diverse characteristics and variations in the bioavailable forms of various HMs can cause a range of specific health risks. To verify this, sources of HMs in the sediments of the Yixun river basin were identified, with a focus on studying the aggregation and risks of HMs in various speciation forms. The results showed that the HMs in the sediments were, in order of concentration: Ti > Mn > Zn > Ni > Cu > Pb. The major sources of pollution in the sediments were mining (39.86%), industrial activities (18.24%), and agricultural activities (12.32%). For adults and children, the non-carcinogenic hazard indexes were 0.37 and 1.88, respectively-thus indicating significant non-carcinogenic health impacts on children. These impacts were mainly attributed to the enrichment of Pb bound to Fe-Mn oxides in sediments from agricultural pollution, leading to a hazard quotient of > 1 for children through dermal exposure pathways. Given that industrial pollution contributed the most to the total carcinogenic risk index value, the carcinogenic risks for both adults and children were relatively significant. However, although mining represents the primary source of HMs in river sediments, it is not the main source of health risks in the Yixun river basin, owing to the low bioavailability of the HMs forms in the region. This study confirmed that mining activities can lead to HMs enrichment in river basin sediments, but that the health risks arising from agricultural and industrial sources also merit enhanced monitoring.

Water resources, as critical ecological and environmental assets, are essential to the social and economic development of countries and regions worldwide. The Jinzi River valley functions as a discharge area for industrial and domestic sewage from mining operations and residential communities along both banks. However, due to human activities, both groundwater and surface water in the region have been contaminated to varying extents. Multiple gold mines are located in the research area. Due to years of unregulated mining, this has had a serious impact on the local ecological environment and water quality. However, research on groundwater and surface water in this region, which are crucial components of the ecological environment, remains limited. This study integrates local socio-economic and hydrogeological conditions, employing methods such as multivariate statistical analysis, the Piper trilinear diagram, the Gibbs model, and ionic ratio relationships to analyze the characteristics and origins of major ions in the region. (1) The primary hydrochemical type of surface water was HCO3-Ca·Na, while groundwater was predominantly of the HCO3-Ca·Mg type. This hydrochemical pattern was consistent across the region, with ion concentrations significantly higher in areas dominated by carbonate rock compared to those with silicate rock. (2) Using principal component analysis, water–rock interaction modeling, and ion source analysis, it was determined that groundwater chemistry was primarily influenced by the weathering of diorite and carbonate rock, along with inputs from domestic and agricultural wastewater. In contrast, surface water chemistry was largely controlled by the weathering of carbonate rocks and the discharge of industrial wastewater. (3) Components such as SO42−, NO3−, Cl−, and total dissolved solids in surface water exhibited a pronounced sensitivity to human activities, with their concentrations significantly exceeding those in groundwater. This indicates that surface water is more heavily impacted by human activities, particularly from industrial, agricultural, and domestic wastewater sources.

Accurately quantifying the carbon sink effect resulting from chemical weathering caused by anthropogenic H2SO4 is imperative to improve the assessment of the global carbon budget. Nevertheless, there is still a lack of precise understanding regarding the impact of anthropogenic H2SO4 on CO2 consumption during chemical weathering. Here, spring water samples were collected monthly from three catchments with distinct bedrock lithologies affected by severe acid precipitation in Southwest China for analyses of hydrogeochemistry and delta 13CDIC to quantitatively estimate the effect of anthropogenic H2SO4 on the weathering carbon sink budget. The results show that carbonates contribute 97.4 %, 95.0 % and 88.8 % of the total cationic load using a straightforward method in the Beidiping carbonate catchment, as well as in the Shegengyan and Bianyan silicate catchments, respectively. The [Ca2++Mg2+]/[HCO3- ] (0.98-1.19) and [Ca2++Mg2+]/[HCO3- +SO42- ] (approximately 1) equivalent ratios, and delta 13CDIC values (-16.8 to -8.0 parts per thousand) of the samples suggest that besides H2CO3, H2SO4 is involved in carbonate weathering. The stoichiometry of the chemical compositions of spring water indicates that the presence of H2SO4 enhances carbonate weathering rates by 14.8 %, 8.1 % and 7.5 % while decreasing the CO2 consumption by 8.2 %, 4.3 % and 4.0 % in Beidiping, Shegengyan and Bianyan, respectively. Thus the reduced proportion of karst carbon sink in the carbonate catchment is approximately 2 times that in the silicate catchment, suggesting that carbonate weathering in the karst catchment is more sensitive to acid precipitation. The impact of acid precipitation on rock weathering in the silicate catchment is constrained by the soil buffering effect. Our study highlights the important role of anthropogenic H2SO4 in carbonate weathering, which should be critically evaluated in regional and global carbon cycles in future studies.

Rivers in agricultural countries widely suffer from diffuse nitrate (NO3-) pollution. Although pollution sources and fates of riverine NO3- have been reported worldwide, the driving mechanisms of riverine NO3- pollution associated with mineral dissolution in piedmont zones remain unclear. This study combined hydrogeochemical compositions, stable isotopes (δ18O-NO3-, δ15N-NO3-, δ18O-H2O, and δ2H-H2O), and molecular bioinformation to determine the pollution sources, biogeochemical evolution, and natural attenuation of riverine NO3- in a typical piedmont zone (Qingshui River). High NO3- concentration (37.5 ± 9.44 mg/L) was mainly observed in the agricultural reaches of the river, with ~15.38 % of the samples exceeding the acceptable limit for drinking purpose (44 mg/L as NO3-) set by the World Health Organization. Ammonium inputs, microbial nitrification, and HNO3-induced calcite dissolution were the dominant driving factors that control riverine NO3- contamination in the piedmont zone. Approximately 99.4 % of riverine NO3- contents were derived from NH4+-containing pollutants, consisted of manure & domestic sewage (74.0 % ± 13.0 %), NH4+-synthetic fertilizer (16.1 % ± 8.99 %), and soil organic nitrogen (9.35 % ± 4.49 %). These NH4+-containing pollutants were converted to HNO3 (37.2 ± 9.38 mg/L) by nitrifying bacteria, and then the produced HNO3 preferentially participated in the carbonate (mainly calcite) dissolution, which accounted for 40.0 % ± 12.1 % of the total riverine Ca2+ + Mg2+, also resulting in the rapid release of NO3- into the river water. Thus, microbial nitrification could be a new and non-negligible contributor of riverine NO3- pollution, whereas the involvement of HNO3 in calcite dissolution acted as an accelerator of riverine NO3- pollution. However, denitrification had lesser contribution to natural attenuation for high NO3- pollution. The obtained results indicated that the mitigation of riverine NO3- pollution should focus on the management of ammonium discharges, and the HNO3-induced carbonate dissolution needs to be considered in comprehensively understanding riverine NO3- pollution in piedmont zones.

Qingshui River is a vital source for human life and industrial production in Zhangjiakou City. Determination of the formation mechanism of the main hydrochemical ions is important for the sustainable development and utilization of surface water resources in the Qingshui River. In view of this, 20 surface water samples were collected from the agricultural and urban reaches of the Qingshui River in July 2022. Based on the detection of hydrochemical ions and stable isotopes （δ2H-H2O and δ18O-H2O）, the hydrogeochemical methods, multivariate statistical analysis, and positive matrix factorization model （PMF） were used to comprehensively understand the chemical formation mechanism of surface water from qualitative to quantitative perspectives. The results showed that the average pH value in the Qingshui River was 8.66, indicating weakly alkaline water. The average concentrations of cations and anions decreased in the orders of Ca2+ > Na+ > Mg2+ > K+ > NH4+ and HCO3- > SO42- > NO3- > Cl- > F- > NO2- in the river water, respectively. The main hydrochemical type was identified as HCO3-Ca·Mg water. For the natural background, surface water was mainly controlled by rock weathering and evaporation crystallization and rock weathering was identified as the primary driving factor. The main hydrochemical compositions in the Qingshui River were derived from the dissolution of carbonate rocks and silicate rocks. Regarding anthropogenic activities, the portions of river water in the agricultural reaches were mainly affected by the excessive application of chemical fertilizers. Furthermore, the concentration of NO3- （1.88-47.4 mg·L-1） exceeded the standard for drinking purposes （10 mg·L-1）. The concentration of F- （0.38-1.92 mg·L-1） was above the acceptable limit for drinking purposes （1.0 mg·L-1） in the southern urban reaches portion of the river water, which could have been attributed to the industrial sewage discharge. The obtained results from the PMF model showed that the hydrochemical compositions in the river water of agricultural reaches were mainly controlled by four factors, namely synthetic fertilizers （29.6%）, livestock manure （16.4%）, rock weathering （17.4%）, and natural geology （13.7%）, while hydrochemical compositions in the river water of urban reaches were mainly affected by the domestic pollutants （30.5%）, industrial discharges （20.1%）, natural geology （18.4%）, and rock weathering （16.7%）.

In South China, karst groundwater is an important water resource for industrial, agricultural, and drinking purposes. However, karst aquifers are highly vulnerable to pollution, leading to deteriorating karst groundwater quality and posing potential health risks to local residents. In this study, 22 groundwater samples were collected from a karst aquifer in the southwestern part of Hubei Province. The hydrogeochemical characteristics and their controlling factors were examined, and the potential health risks associated with groundwater pollutant concentrations in karst groundwater were assessed. The results showed that the groundwater is slightly alkaline with low chemical oxygen demand values, indicating good water quality. The groundwater facies type was identified as HCO3‐Ca at most sample spots, showing low total dissolved solids concentrations. Substantial spatial variations in Na+, CO32−, and NO2− concentrations were found, whereas spatial variations in the K+, Ca2+, Cl−, HCO3−, and F− concentrations were small. In addition, the dissolution of gypsum deposits and magnesium carbonate sedimentary rocks at sampling sites resulted in groundwater facies types of HCO3•SO4‐Ca and HCO3‐Ca•Mg, with low total dissolved solids concentrations. The karst groundwater chemistry in the study area was mainly controlled by water–rock interactions, as well as by the dissolution of gypsum deposits and magnesium carbonate sedimentary rocks at specific groundwater sampling sites. The groundwater Cl− concentrations were mainly affected by atmospheric precipitation. NO3− was mainly derived from atmospheric precipitation, domestic sewage, septic tanks, and industrial activities, whereas SO42− was derived from atmospheric precipitation, sulfate rock dissolution, and sulfide mineral oxidation. These results highlight the absence of potential human health risks of NO3− and F− to infants, children, and adults, as their concentrations are below the corresponding regional background values. In contrast, the potential health risks of Cl− cannot be ignored, particularly for infants. This study offers scientific guidelines for protecting and allocating local groundwater resources.

To investigate the presence of metal elements and assess their health risk for the populace in the Nandong Underground River Basin (NURB), we conducted an analysis of eleven common heavy metals in the water body. A Health risk assessment (HRA) model was employed to analyze 84 water samples from the NURB. The detection results revealed the following order of heavy metals concentrations: Fe > Al > Mn > Zn > As > Cd > Pb > Cr > Ni > Cu > Hg. Correlation analysis indicated a certain similarity in material source and migration transformation among these eleven metal elements. Our study identified that the health risks for local residents exposed to metal elements in the water of NURB primarily stem from carcinogenic risk (10−6–10−4 a−1) through the drinking water pathway. Moreover, the health risk of heavy metal exposure for children through drinking water was notably higher than for adults. The maximum health risks of Cr in both underground and surface water exceeded the recommendation standard (5.0×10−5 a−1) from ICRP, surpassing the values recommended by the Swedish Environmental Protection Agency, the Dutch Ministry of Construction and Environment and the British Royal Society (5.0×10−6 a−1). The results of the health risk assessment indicate that Cr in the water of NURB is the primary source of carcinogenic risk for local residents, followed by Cd and As. Consequently, it is imperative to control these three carcinogenic metals when the water was used as drinking water resource.

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During the process of cleaning aquaculture ponds, the drainage contributes significantly to antibiotic pollution in the surrounding water environment. Therefore, we conducted a study on the distribution of 26 antibiotics in 57 ponds within the Taihu Lake basin. The results revealed that the detection frequency of antibiotics ranged from 1.75 % to 80.7 %, with the overall detection concentrations ranging from 3.27 to 708.72 ng/L. Among them, the detection rate of 8 antibiotics exceeded 50 %. Regarding the spatial distribution, the concentration of antibiotics was relatively high in aquaculture ponds located in the Changzhou area, with the highest concentration reaching 708.72 ng/L. This observation is likely due to the large size and intensive breeding practices in Changzhou. Fish ponds exhibited a significantly higher total antibiotic concentration of 3.27 to 445.57 ng/L compared to crab ponds (13.01 to 206.30 ng/L) and shrimp ponds (23.17 to 107.40 ng/L). Quinolones and sulfonamides were the predominant antibiotic classes found in fish ponds, accounting for 51.49 % of the total antibiotic concentration. Notably, sulfamethoxazole (SMX) and enrofloxacin (ENR) exhibited the highest antibiotic concentrations. Risk assessments demonstrated that SMX, ENR, and ofloxacin (OFX) contributed significantly to ecological risks. Furthermore, the study found that the tertiary constructed wetland treatment process achieved a remarkable removal rate of 92.44 % for antibiotics in aquaculture wastewater, while other treatment processes displayed limited effectiveness in removing antibiotics. This study addresses the knowledge gap concerning antibiotic pollution during the cleaning process of aquaculture ponds within the Taihu Lake basin.

Carbonate minerals are the largest carbon (C) reservoir on earth. Quantitative assessment of carbonate weathering and CO2 sink flux caused by anthropogenic acids in karst wetland regions is imperative to improve understanding of the hydrogeochemical evolution and global C cycle in karst ecosystem. In this study, a typical field observation was conducted in the Huixian karst wetland (HKW) in South China. The hydrogeochemical and isotopic (δ15N-NO3–, δ18O-NO3–, and δ13CDIC) analyses combined with the MixSIAR model and stoichiometric coefficients were used to quantitatively estimate the influence of H2CO3, HNO3 and H2SO4 on carbonate weathering and CO2 sink flux. The results obtained showed that anthropogenic HNO3 and H2SO4 affect carbonate weathering in the HKW, evidenced by the significant correlations between hydrogeochemical parameters and δ13CDIC value. The quantitative results showed high contributions of HNO3 and H2SO4 to carbonate weathering in karst groundwater, with contribution ratio values of 20.91 ± 19.29 and 21.69 ± 26.88%, respectively. Besides karst water over-exploitation, this finding suggested that these anthropogenic acids were also an important cause of a decrease in ecological water levels, resulting in the shortage of karst water resource and degradation of wetland ecosystem. Moreover, the involvement of anthropogenic acids in the natural chemical weathering has significantly decreased the CO2 sink flux by 11.23 ± 7.94 and 41.60 ± 30.24% for surface water and groundwater, respectively, thereby potentially affecting the karst C cycle and global climate change. This present study provided an integrated quantitative approach for estimating the influence of H2CO3, HNO3, and H2SO4 on carbonate weathering and CO2 sink in karst wetland regions. Our study highlighted that the effects of anthropogenic acids on carbonate weathering need to be considered in future studies on the restoration of wetland ecosystems, particularly in intensive agricultural study areas.

Accurate estimate of carbonate weathering and the related carbon sink flux induced by anthropogenic H2SO4 is of great significance for improving understanding of the hydrogeochemical evolution and the global carbon cycle. Here, to quantitatively evaluate the influence of anthropogenic H2SO4 on different lithological carbonate weathering and the related carbon sink budget, karst spring water in the typical limestone and mixed limestone–dolomite catchments in Yaji and Beidiping affected by acid precipitation in southwest China were sampled monthly for the analysis of hydrochemical and δ13CDIC characteristics. Results show for the period of sampling (August 2013 to December 2014) that the average contribution rates of atmospheric inputs and carbonate weathering to total dissolved cations are 2.24% and 97.8%, and 3.09% and 96.9% in Yaji and Beidiping, respectively. The δ13CDIC values (−17.0% to −14.7‰) and the [Ca2+ + Mg2+]/[HCO3−] (0.98 to 1.25) and [Ca2+ + Mg2+]/[HCO3− + SO42−] (approximately 1) equivalent ratios of samples prove that H2CO3 and H2SO4 simultaneously participate in carbonate weathering. The contribution rates of H2SO4 to [Ca2+ + Mg2+] and [HCO3−] produced by carbonate weathering in Yaji and Beidiping are 0–30% and 0–18%, and 0–37% and 0–23%, with average values of 14% and 7%, and 19% and 11%, respectively, suggesting that the influence of H2SO4 on different lithological carbonate weathering is different. H2SO4 precipitation participating in carbonate weathering increases the weathering rate by 14–19%, whereas it decreases the flux of karst carbon sink by 7–11% in Southwest China. Therefore, anthropogenic acids have influenced the global carbon cycle and climate change by carbonate weathering due to the large karst areas in the world, and their influences on different lithological carbonate weathering should not be ignored in the regional and global carbon cycles in future studies.

Wastes from social economic activities had great impact on water quality thereby limiting water usability for domestic purposes. Sewages discharge from people activities, usually consist of undesirable concentrations of soluble chemicals that infiltrate into the surrounding surface and underground water, and then constitute health risk to the populace. In order to investigate the concentration characteristics and health risk for the local residents in Nandong Underground River Watershed (NURW), eleven common heavy metals in the water body analysis were conducted. Health risk assessment (HRA) was taken to analyze eleven heavy metals of 84 water samples from surface and underground waters in NURW: 36 samples underground water and 48 samples surface water. Our results showed that the heavy metals concentration order is that of Fe > Al > Mn > Zn > As > Cd > Pb > Cr > Ni > Cu > Hg. Correlation analysis indicates that these eleven metal elements have certain similarity on material source and migration transformation. The health risks for local residents exposed to metal elements in the water of NURW mainly from carcinogenic risk (10−6～10−4 a−1) through drinking way, and the health risk of heavy metals exposed to children through drinking way was much higher than adults. The maximum exposing health risks of Cr in both underground and surface water were higher than the recommendation standard (5.0×10-5 a-1) from ICRP, and all the values over the standard (5.0×10-6 a-1) recommended by the Swedish Environmental Protection Agency and the Dutch Ministry of Construction and Environment and the British Royal Society. The results of health risk assessment shows that Cr in the water of NURW was the mainly source of carcinogenic risk for the local residents, following by Cd and As. Consequently, it is necessary to control the three carcinogenic metals when the water was used as drinking water source.

The identification of fluoride (F-) sources and enrichment mechanisms is imperative to understand the multiple fluorine (F) pathways, and further, to control regional diffuse F- contamination in groundwater. However, the factors that control high-F- groundwater are not fully understood in desert climate regions. Hence, a sampling campaign was conducted from 71 desert groundwater sites and six river water sites in the Cherchen River Basin (CRB), northwestern China. This study combined hydrochemical compositions with an optimized forward model, with the aim of determining the potential sources and enrichment mechanisms in F--contaminated desert groundwater. Approximately 58.46% of the samples had F- concentrations over the national standard of 1.0 mg/ L. More severe F- contamination was found in the multi-layered structured confined aquifer (MCA) of the alluvial plain (1.42 +/- 1.11 mg/L). The primary contributors of desert groundwater F- were the dissolution of Fbearing minerals containing evaporite (-58.80%), silicate (-15.89%), and carbonate (-12.94%), followed by the river water input (-12.08%). In contrast, anthropogenic activities (-0.16%) and precipitation contributed less to desert groundwater F- . The dissolution equilibrium of CaF2 was important for F- enrichment in desert groundwater. Compared with the piedmont plain, intensive evaporation and salinization were more conducive to F- enrichment in the alluvial plain. Under alkaline condition, the dissolutions of evaporite and fluorite allowed extra F- to release into desert groundwater when Ca2+ and Mg2+ were up to oversaturation. Moreover, the desorption of F- was promoted by competitive adsorption of OH- and HCO3 -, and the adsorption capacity of Fwas weakened by cation exchange of K++Na+ with Ca2++Mg2+. As a result, desert groundwater had a higher concentration of F- in the alluvial plain. Our study provided a comprehensive understanding of multiple F pathways in desert groundwater. This study also highlights the effect of hydrogeochemical background on highF- desert groundwater.

Nitrate (NO3-) pollution in karst water is an important environmental issue in intensive agricultural regions worldwide. The integrated understanding of the spatiotemporal variability and control factors of NO3- pollution in karst water is imperative for controlling the diffuse pollution caused by agricultural activities. In this study, 49 water samples were collected from surface water (SW) and groundwater (GW) in the Huixian karst wetland (HKW) and analyzed using hydrogeochemical and isotopic data (delta O-18-NO3-, delta N-15-NO3- - and delta C-13(DIC)) in combination with a Bayesian mixing model to investigate the spatiotemporal distribution and control factors in NO3- -polluted karst water. The results showed that approximately 40.82% of the karst water samples exceeded the natural threshold value of 3 mg/L for NO3 -N, and 32.14% of the GW samples exceeded the permissible limit for drinking water established by WHO (10 mg/L as NO3- -N), indicating that high levels of NO3- were mainly found in GW samples from the agricultural core area, especially in the dry season. The NH4+-synthetic fertilizer (NHF) and soil organic nitrogen (SON) were the dominant factors controlling pollution sources in the HKW, accounting for 36.13% +/- 4.66% and 28.68% +/- 4.75% of the karst GW NO3 concentration, respectively. However, the seasonal differences in NO3 pollution sources were not significant in GW. Microbial nitrification was the main process affecting the NO3- levels in GW, whereas the occurrence of denitrification did not significantly affect NO3 concentration in the HKW due to the relatively low rate. Moreover, the HNO3 produced from NH4+ via microbial nitrification facilitated carbonate weathering, thereby controlling NO3- enrichment in karst GW. Our results suggest that NHF should be controlled to prevent further GW pollution in the HKW. Our study also provides a scientific basis for understanding the factors controlling the NO3- concentrations in karst water systems.