•A physically-based distributed Karst-Liuxihe model (KL) was improved and simplified effectively.•The rainfall-runoff generation and confluence algorithms were improved.•Better flood simulation ...results showed that the simplified and improved model is successful.•The simplified KL model requires less hydrogeological data when modeling in karst basins.
The karst trough valley basin in Zhongliang of Chongqing is one of the most developed karst areas in Southwest China, with unique topographic features such as karst troughs and valleys that are prone to flash floods. Accurate simulation of flood processes in these areas using hydrological models can provide references for predicting future local runoff evolution trends. The main challenge for hydrological modelling in karst areas lies in building a model with limited hydrogeological data. In this study, the Karst-Liuxihe (KL) model was simplified and improved to simulate karst flood processes in the Zhongliang karst trough valley. The KL model consists of multiple, complex structures; a large amount of data is needed to model karst areas. To overcome the modelling data requirements, we modified the model structure and its parameters to propose a simplified KL (SKL) model. Additionally, we improved the runoff generation and confluence algorithms. Compared with the simulation results for 22 flood processes obtained by using the KL model, the results for this SKL model were better with less modelling data. For example, the mean value of the Akaike information criterion (AIC) index decreasing rate was 35%, and the process relative error and the peak flow relative error decreased by 10%, indicating that the technique of simplifying and improving the KL model is effective and that the simplified SKL model can quantitatively depict karst floods in karst trough valley areas.
Karst terrains are generally regarded as a fragile and vulnerable environment. China’s karst is mainly developed in pre-Triassic, old-phase, hard, compact carbonate rock. The lack of soil cover in ...bare karst land can enhance desertification. Its underground drainage system can aggravate both drought and flooding problems; the interconnected surface and subsurface conditions allow for easy contamination by pollutants. Moreover, karst terrains quite often suffer from a series of engineering problems, such as water infiltrating into mines or transportation tunnels, leakage from reservoirs, and failure of building foundations. As resources and energy exploitation intensified in recent years, karst areas in Southwest China faced some severe geo environmental problems. In order to find out how the problems action and evolution in recent years, field and laboratory investigations were conducted in Guangxi autonomous region, Guizhou, Yunnan, Guangdong, Hunan, Hubei provinces, and Chongqing municipality. About 100 experts from the Provincial Geological Survey and graduate students took part in this project. Several symposiums were held during those 3 years. Besides the in situ survey, data were collected from hydrogeological survey results since 2000 when the new round of land resources investigation began. After analyzing these data, rock desertification, drought and flooding, and contamination and engineering geological environmental problems were considered the most prevalent problems in karst areas in Southwestern China. The status of each problem is elaborated upon in this paper and should be considered for future sustainable development.
•The algorithms of rainfall-runoff and parameter optimization were improved.•Water exchange between surface river and karst groundwater was evaluated.•Flood forecasting in Xijiang karst watersheds ...can ignore this water exchange.
The complex subsurface conditions and spatial anisotropy of water-bearing media in karst basins result in much more variable runoff generation and confluence processes and water circulation than those in nonkarst basins. Therefore, accurate flood forecasting is more difficult in karst areas, and it is challenging to accurately quantify the water exchange relationship between surface rivers and karst aquifers. To provide key technical support for accurate flood forecasts in karst basins, this study improved a distributed physical hydrologic model, the Karst-Liuxihe model, by improving the runoff generation and confluence algorithms and the parameter optimization algorithm to simulate surface floods in three different karst watersheds in southern China. Additionally, in the model calculations, this study focused on the influence of water exchange on surface flood simulations in changing watersheds. The results showed that for the largest basin, Xijiang Basin (353,100 km2), considering or not considering the water exchange between the surface river and karst aquifer had little effect on the surface flood simulation results. However, this water exchange had a great effect on the surface simulations for the smallest basin, the Beijiang Basin (1,700 km2). For instance, the simulated average peak flow showed a 29 % difference, and the total amount of flooding showed a 25 % difference between dry years and normal years. Therefore, when forecasting surface floods in small karst areas, the water exchange relationship between surface rivers and karst aquifers must be considered. The results showed that the impact of groundwater on surface rivers can be disregarded when forecasting surface floods in the large Xijiang karst basin and can be treated directly as surface flood forecasting in nonkarst areas. The application of this strategy will greatly simplify the cost of hydrologic modelling in the Xijiang basin. By improving the Karst-Liuxihe model, the accuracy of its flood simulation and general applicability in karst basins were greatly enhanced. The improvement in the karst hydrological model in this study provides a good tool for flood simulation and forecasting in the karst regions of Southwest China.
The Nandong Underground River System (NURS) is located in a typical karst agriculture dominated area in the southeast Yunnan Province, China. Groundwater plays an important role for social and ...economical development in the area. However, with the rapid increase in population and expansion of farm land, groundwater quality has degraded. 42 groundwater samples collected from springs in the NURS showed great variation of chemical compositions across the study basin. With increased anthropogenic contamination in the area, the groundwater chemistry has changed from the typical Ca–HCO
3 or Ca (Mg)–HCO
3 type in karst groundwater to the Ca–Cl (+
NO
3) or Ca (Mg)–Cl (+
NO
3), and Ca–Cl (+
NO
3
+
SO
4) or Ca (Mg)–Cl (+
NO
3
+
SO
4) type, indicating increases in NO
3
−, Cl
− and SO
4
2− concentrations that were caused most likely by human activities in the region. This study implemented the R-mode factor analysis to investigate the chemical characteristics of groundwater and to distinguish the natural and anthropogenic processes affecting groundwater quality in the system. The R-mode factor analysis together with geology and land uses revealed that: (a) contamination from human activities such as sewage effluents and agricultural fertilizers; (b) water–rock interaction in the limestone-dominated system; and (c) water–rock interaction in the dolomite-dominated system were the three major factors contributing to groundwater quality. Natural dissolution of carbonate rock (water–rock interaction) was the primary source of Ca
2+ and HCO
3
− in groundwater, water–rock interaction in dolomite-dominated system resulted in higher Mg
2+ in the groundwater, and human activities were likely others sources. Sewage effluents and fertilizers could be the main contributor of Cl
−, NO
3
−, SO
4
2−, Na
+ and K
+ to the groundwater system in the area. This study suggested that both natural and anthropogenic processes contributed to chemical composition of groundwater in the NURS, human activities played the most important role, however.
•The nitrogen concentration in karst groundwater has spatial heterogeneity.•The redox characteristics of different karst aquifer media are heterogeneous.•Different karst aquifer media have different ...N transformation processes.
Karst aquifers, a significant source of drinking water worldwide, are vulnerable to anthropogenic pollutants because of their special hydrogeological conditions. However, distinguishing the nitrogen transformation process is difficult owing to the heterogeneity of karst media. Therefore, the process of nitrogen transformation in different karst aquifer media remains poorly understood. In this study, multiple stable isotopic and physicochemical indicators are combined, the sources of water and nitrogen are analysed, and the degree of openness and redox conditions of different media are discussed to elucidate the nitrate sources and N transformation in groundwater with different degrees of openness. Water isotopes indicate local meteoric groundwater origin. However, different karst aquifer media have different degrees of openness, resulting in the highest deuterium–oxygen isotope values in surface water, followed by conduit water, fissure water, and isolated cave water. Based on the evidence from isotope and physicochemical indicators, different karst aquifer media have different redox processes. Isolated cave water environments were under anoxic conditions dominated by sulfate reduction reactions. Fissure water was under temporary anoxic or sub-anoxic conditions dominated by denitrification reactions. Conduit water and karst surface water were between micro-oxygen and oxidising conditions, and nitrification was the key process. This phenomenon was observed because the diversity of karst media produces different conservation conditions and openness. The nitrate isotope and Bayesian mixing model indicate that the main sources of nitrate in karst groundwater in the study area are mostly associated with manure and sewage wastes, and a few are from soil organic nitrogen, atmospheric deposition, and chemical fertilizer. This study helps identify the migration and transformation patterns and processes of nitrogen in different karst aquifer media, providing a scientific basis for the restoration and remediation of contaminated karst groundwater.
•A new distributed Karst Tunnel Hydrological Model (KTHM) was successfully developed.•Runoff simulated effects by the KTHM were satisfactory in karst areas.•Tunnels have the greatest influence on dry ...season runoff and the least influence on the flood.
Tunnel excavation in karst areas could affect natural karst hydrological processes and runoff, resulting in groundwater drainage and even underground river cut-off. The hydrological model is a good tool with which to study the hydrological effect of tunnels. The application of current hydrological models in karst areas usually requires a large amount of modelling data, especially hydrogeological data that are much more difficult to obtain than those in non-karst areas. To overcome the difficulty of modelling in karst areas, this study presents a new fully physically based distributed karst tunnel hydrological model (KTHM) with a simple structure and simple parameters. The underground confluence module is divided into only two layers: confluence in the epikarst zone and the underground river. Such a simple structure makes it possible to build a model in karst basins with only a small amount of hydrogeological data. In the structure of this KTHM, the tunnel is incorporated in a specially designed submodule to quantify the influence of the tunnel hydrological effect on the water volume. If the tunnel submodule is turned off, then this model becomes a karst hydrological model (KHM) and can be used in other karst basins without tunnels. The simulation results of 20 flow processes and 2 annual runoffs modelled by the KHM and KTHM are compared; the KTHM is more accurate in the study area, which indicates that the tunnel submodule in the model is necessary and that the KTHM is feasible for performing runoff simulations in the study area. The sensitivity sequence of hydrological effects of tunnels on runoff in karst areas is as follows: dry season runoff volume > normal runoff volume > interannual runoff volume > flood peak flow > flood volume.
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•PAHs were dominated by dissolved PAHs during the dry season.•PAHs were dominated by SPM-associated PAHs during the rainy season.•Highly hydrophobic PAHs strongly bind to the ...particulate phase.•PAHs transferred through the sinkholes and soil seepage water during the rainy season.•PAHs mainly transferred through the soil seepage water during the dry season.
Water, colloid, and suspended particulate matter (SPM) samples were collected during one given year at two sites from the Laolongdong (LLD) underground river in Southwest China and analyzed for 16 priority polycyclic aromatic hydrocarbons (PAHs), dissolved organic carbon (DOC), and particulate organic carbon (POC). The concentrations of the SPM-associated, colloid-associated, freely dissolved, and dissolved organic matter (DOM)-associated PAHs varied from 202 to 1731 ng/L, 142 to 956 ng/L, 228 to 1280 ng/L, and 7.3 to 173 ng/L, respectively. The mean percentages of 4- and 5–6 ring PAHs showed increasing trends along the order of the dissolved phase, colloidal phase and particulate phase. The behavior and fate of PAHs influence by DOC and POC were examined. Overall, organic carbon was the primary factor controlling the environmental behaviors and fates of the 16 PAH species in the LLD underground river. POC in underground river has more lipophilicity to PAHs in this study. The correlation analysis suggested that the migration pathways of PAHs from the surface soil to the underground river through the sinkholes and soil seepage water both had important contribution during the rainy season. However, PAHs mainly transferred through soil seepage water in the form of dissolved PAHs during the dry season.
The unique hydrogeology of karst makes the associated groundwater respond quickly to rainfall events and vulnerable to anthropogenic pollutions. In this study, high-frequency monitoring of spring ...discharge, temperature, electrical conductivity (EC) and pH, along with monthly hydrochemical and microbial analyses, was undertaken at the outlet of Laolondong karst underground river in Nanshan, southwestern China. The aim was to explore the environmental effects of the catchment’s urban area on the karst groundwater resources. The monitoring data of a tracer test and the response of discharge to rainfall events demonstrate that conduits and narrow fissures coexist in the Laolongdong karst aquifer. The EC, Na
+
, Cl
−
and SO
4
2−
values (840 μS/cm, 33.7, 38.6 and 137.2 μg/L, respectively), along with high concentrations of fecal coliform bacteria, at the outlet indicate considerable urban pollution in this area. The contaminants sulfate and nitrate showed different relationships with discharge and EC in different stages of a rainfall event. This behavior provided information about aquifer structure and the influence of transport properties. Meanwhile, the hydrological processes of groundwater flow could be modified by urbanization and result in increasing magnitude of urban floods in the underground river. In addition, sulfuric and nitric acids introduced by urbanization not only impact the karst groundwater quality, but also result in a significant perturbation to the carbon cycling system in the karst area.
Chongqing, located in the southwestern China, was named as “A spa city of the world”. It's most geothermal reservoirs are developed in carbonate rock aquifers, and those thermal waters are ...characterized by high concentrations of sulfate and low HCO3−. However, little was known about the origin and genesis of those thermal waters, which is necessary for their protection and determination of regime of exploitation. Therefore, thermal and surface waters were sampled seasonally and analyzed for hydrogeochemical and isotopic compositions (δ18O/δ2H; δ34S–SO4; δ18O–SO4) to decipher the origin of the thermal waters in the Triassic carbonate aquifers in Chongqing area. Key questions include the relative significance of hydrothermal processes and solute supply. The results showed that: (1) the major chemical composition of analyzed the thermal waters from Chongqing area was characterized by Ca-SO4; (2) the δ2H and δ18O values of the thermal waters ranged from −48.6‰ to −63.1‰ with an average value of −54.2‰, and from −6.5‰ to −9.2‰ with an average value of −8.0‰, respectively, indicating that the thermal waters originated from the local rain water with an elevation of 415 m to 1453 m above mean sea level; (3) the estimated geothermal temperatures varied from 63.8 °C to 78.3 °C (Quartz), indicating that the depth of the geothermal reservoir varied from 411 m to 1728 m, which is located in the Lower Triassic Jialingjiang formation; (4) the δ34S-SO42- and δ18O-SO42- in the geothermal waters ranged from 29.7‰ to 34.1‰ with a mean value of 32.1‰, and from 12.5‰ to 16.5‰ with a mean value of 15.2‰, respectively, suggesting that the high SO42− concentrations resulted mainly from the dissolution of gypsum in the second part of the Lower Triassic Jialingjiang Formation; (5) three processes of water-rock interactions (gypsum dissolution, carbonate dissolution by carbonic acid and carbonate dissolution by sulfuric acid) in the thermal system were unveiled by the hydrogeochemical and isotopic models. Among these three processes, gypsum dissolution seems to be primarily responsible for solute composition of the thermal water. This study also indicates the integration of hydrogeochemical and isotopic data is very useful tool to decipher the origin and genesis of the carbonate thermal waters.
•Origin of thermal waters in Chongqing was studied by the integration use of hydrogeochemical and stable isotopic data.•Three water-rock interactions in thermal system were unveiled by hydrogeochemical and isotopic models.•Anomalously high SO42− resulted mainly from gypsum dissolution rather than rather than sulfide oxidation.•A conceptual model of formation and evolution of thermal water in Chongqing was established.
•Temperature was the main contributor of the variability of precipitation isotopes.•Seasonal contributions of moisture sources and microphysical processes explained the d-ET0 relationship.•Lowest ...δ18O value and precipitation amount related by convection activity.
Stable hydrogen and oxygen isotopes in precipitation, which are sensitive to climate change and the water cycle, are widely used as natural tracers to investigate hydrological processes and paleoclimate reconstruction. High resolution event-based precipitation sampling was conducted in Guilin, a subtropical monsoon climate region in Southwest China, from 2010 to 2016 to identify the temporal variability of stable isotopes of precipitation and its dominant meteorological factors. The results showed that the stable isotopes of precipitation exhibited significant seasonal variability, with lower values in the wet season and higher values in the dry season, reflecting the changes in the character of precipitation in response to seasonal variability of the moisture sources and atmospheric dynamics. δ18O values were significantly negatively correlated with temperature and potential evapotranspiration (ET0) on the daily and monthly timescales; however, there was no significant correlation between monthly δ18O and the amount of rainfall. Multivariate regression analysis showed that when temperature, precipitation amount and ET0 were considered, 39% of the variability of the monthly oxygen isotope composition of precipitation was explained. Monthly mean temperature had a higher covariation with precipitation δ18O, indicating that the variations of precipitation isotope were more dependent on temperature, but not substantially dependent on the amount effect of rainfall. Howerver, the main influence of meteorological factors on the isotopic variations of precipitation varied seasonally. Temperature strongly influenced the isotopic composition of precipitation in summer, while the amount effect was prominent due to the enhanced convective activity characterized by more depleted δ18O values. Although evapotranspiration is a minor contributor of precipitation isotope variability, the impact of evapotranspiration on the enrichment of precipitation isotopes in autumn is a substantial control and should be considered, which makes a major contribution to compensate for the depletion in heavy isotopes of rainfall. The non-linear relationship between d-excess and ET0 was attributed to the variable contribution of large-scale convective moisture and local evaporation, as well as water vapor transfer processes. The response of precipitation stable isotopes to convective events was characterized by distinct stage patterns, associated with the water vapor sources and physical processes during rainout. The significant depletion of precipitation δ18O values during convective events was not due to the ‘amount effect’, but due to large-scale convection activity. These findings provide further insight into intraevent-scale isotopic variations associated with organized convection and their key influencing factors, which ultimately improves our interpretation of the paleoclimate records in subtropical monsoon regions.
