Science in China Series D: Earth Sciences 2006 Vol.49 No.12 1299—1310

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DOI: 10.1007/s11430-006-1299-z

A study of interaction between surface water and groundwater using environmental isotope in Huaisha River basin SONG Xianfang1, LIU Xiangchao1, 2, XIA Jun1, YU Jingjie1 & TANG Changyuan1, 3 1. Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; 2. College of River and Ocean, Chongqing Jiaotong University, Chongqing 400074, China; 3. Chiba University, Chiba 263-8522, Japan Correspondence should be addressed to Song Xianfang (email: [email protected])

Received September 6, 2005; accepted March 7, 2006

Abstract The surface water and groundwater are important components of water cycle, and the interaction between surface water and groundwater is the important part in water cycle research. As the effective tracers in water cycle research, environmental isotope and hydrochemistry can reveal the interrelationships between surface water and groundwater effectively. The study area is the Huaisha River basin, which is located in Huairou district, Beijing. The field surveying and sampling for spring, river and well water were finished in 2002 and 2003. The hydrogen and oxygen isotopes and water quality were measured at the laboratory. The spatial characteristics in isotope and evolution of water quality along river lines at the different area were analyzed. The altitude effect of oxygen isotope in springs was revealed, and then using this equation, theory foundation for deducing recharge source of spring was estimated. By applying the mass balance method, the annual mean groundwater recharge rate at the catchment was estimated. Based on the groundwater recharge analysis, combining the hydrogeological condition analysis, and comparing the rainfall-runoff coefficients from the 1960s to 1990s in the Huaisha River basin and those in the Chaobai River basin, part of the runoff in the Huaisha River basin is recharged outside of this basin, in other words, this basin is an un-enclosed basin. On the basis of synthetically analyses, combining the compositions of hydrogen and oxygen isotopes and hydrochemistry, geomorphology, geology, and watershed systems characteristics, the relative contributions between surface water and groundwater flow at the different areas at the catchments were evaluated, and the interaction between surface water and groundwater was revealed lastly. Keywords: environmental isotope, hydrochemistry, surface water, groundwater, interaction.

Since the 1980s, a drought climate has been continuing, which aroused serious water resource shortages and serious water resources crisis in North China. While contemporarily, under the effects of human acwww.scichina.com

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tivities, runoff generation and recharge conditions in basins changed greatly[1], therefore, the river channels became wadi and water table decreased distinctly. Because of the change on hydrological processes and

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water resources quantity caused by natural variation and human activity effects, serious influences have been imposed on industrial and agricultural productions as well as human lives, and fearful deterioration of environment has been induced[2], at the same time, water supply to the capital of our country has been influenced seriously. Sixty percent of water supply to Beijing was supported by Jingmi Aqueduct from the Miyun Reservoir[3], while the water collecting regions to the Miyun Reservoir were from the Chaobai River basins. In these years, the rainfall-runoff characteristics in the Chaobai River basin have varied greatly, according to the hydrologic statistical data at the hydrological stations of Xiahui in the Chaohe River basin and Zhangjiafen in the Baihe River basin, the rainfall-runoff coefficients in the Chaobai River basin varied from 11% in the 1970s to 7% in the 1980s and 1990s, thus the depression of runoff imposed severe effects on water resources security for Beijing City. The Huairou Reservoir in Huairou District is a transferring reservoir on Jingmi Aqueduct, which take important regulating and complementary functions for water transportation from the Miyun Reservoir. Because the water-and-soil conservation projects implemented in the basin since the 1980s, the rainfall-runoff relationship in the Huaisha River basin has changed seriously, which imposed tremendous influences on runoff flow to the reservoir, and simultaneously weakened the regulating and complementary functions of Huairou Reservoir on Jingmi Aqueduct. The close interrelationships exist between the depression of water resources and transformation of surface water and groundwater in basins, therefore, it is necessary to investigate the depression of basin water resources to evaluate the interaction between surface water and groundwater in basins, and further to reveal the water circulation mechanisms under high intense human activities. Environmental isotope is a new technology with the development of nucleus science since the 1950s and 1960s, and lots of successful case studies have been carried out in water cycle research. The main applications of environmental isotope technology in water cycle research are to trace water cycle, determine sources and components of surface water and groundwater flow and separate water sources, such as,

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Fritz et al.[4] studied the variation of runoff and δ 18O along with time during precipitation by applying two components mixing relationship, and estimated the ratio of rainwater and groundwater flow during the sharply rising limb of hydrograph. By applying the variation of stable isotope compositions in spring waters, Payne et al.[5] described the recharge area of Kalamos spring, and confirmed the recharge source of the spring. Recently, spatial and temporal variation of isotopes and interaction between surface water and groundwater in basins arouse much recognition. For example, Aravena et al.[6] investigated the isotope evolution in river waters in Loa and Tarpaca Rivers in North Chile, illuminating the recharge areas related to groundwater. Katz et al.[7] used the obvious distinction of isotope and hydrochemistry components between groundwater and surface water, and offered quantitative means for investigating the surface water and groundwater systems. Harrington et al.[8] studied the spatial and temporal distribution of groundwater recharge in the middle of Australia, and ascertained the evolvement processes of groundwater. Weyhenmeyer et al.[9] applied the altitude effect of isotope compositions in rainfall under stable climate conditions, and demonstrated the main recharge area and groundwater flow paths, then evaluated the groundwater recharge rates from the different altitudes. In China, the isotope technology is only used for some groundwater researches at some regions and experimental sites. Since 1987, on the foundation of Chuzhou Experimental Station of Institute of Nanjing Hydrology and Water Resources of Department of Water Conservancy, by measuring the different runoff components in discharge hydrograph in the experimental and representative basins, and testing the fundamental presuppositions for two different runoff components by environmental isotope methods, Gu et al.[10] figured out the rainfall-runoff relationship in the experimental basin, and carried out rainfall-runoff relationship, hydrograph separation[11], runoff generation processes[12], and revealed the complexity of isotope conditions in hydrology system in basins[13]. In respect of application studies on water cycle, Tian et al.[14] investigated the oxygen isotope in rainfall and river flow in the Naqu River basin in the Tibetan Plateau, by comparing the daily fluctuation of oxygen stable isotopes in the

A study of interaction between surface water and groundwater using environmental isotope in Huaisha River basin

river measured in the summer of 1998 with those of rainfall observed simultaneously, then analyzed the fluctuation characteristics of δ 18O in the river, and primarily illustrated the water cycle processes. With the characteristics of stable isotopes in groundwater in Alashan Plateau, Gu et al.[15] analyzed the construction influences on groundwater resources after the implementation of Heihe River basin replenishment planning. Liu et al.[16] investigated the amount effect, altitude effect of rainfall in a typical experimental basin in North China by combining environmental isotope technology, and then ascertained the vapor source in the precipitation period. Nowadays, collaboration in respect of isotopic hydrology between China and international becomes increasingly more compact, a plan of “Isotope Tracing of Hydrological Processes in Large River Basins” sponsored by the International Atomic Energy Agency (IAEA) has arouse attention from lots of countries all over the world, hydrological processes in the Yangtze River basin in China by isotope tracers have been included in it. To accompany the project presided by IAEA, China has expanded some corresponding investigations, which are[17]: CHNIR: Chinese Network of Isotopes in River; CHNIP: Chinese Network of Isotopes in Precipitation and CHLeafNet: Chinese Leafnet. The objectives of this study were to delineate water sources and illustrate flow paths of surface water and groundwater in the Huaisha River basin, by sampling for the surface water and groundwater, measuring hydrogen and oxygen stable isotopes and hydrochemistry

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compositions, and then to reveal the interactions between surface water and groundwater affected by the land cover change and human activities. 1

Description of study area

The Huaisha River basin is located in the northwest of Huairou District of Beijing City (Fig. 1), with an area of about 158 km2. The topographical characteristic is higher in the northwest and lower in the southeast with serious altitude difference. The highest point is 1290 m, while the lowest point is located at the catchment outlet to the Huairou Reservoir with an altitude of 62 m. Most of the basin is covered by widely distributed Jurassic granite mainly located in the upstream basins (Fig. 2), central watersheds and lower basin areas; secondly is dolomitic limestone, strip-shaped limestone and gneiss, which is mainly located in the upper-central watershed; while in the central watershed is Quaternary strip-shaped alluvial and diluvial plains along the stream lines. The study basin falls into semi-humid and semiarid climate regions, according to the annually observational hydrologic data (Fig. 3) at the Koutou hydrological station, the annual precipitation in the Huaisha River basin is 645 mm, annual water surface evaporation is 1005 mm, annual runoff depth is 243 mm, and annual rainfall-runoff coefficient is 37.67%. Whereas the seasonal variation of annual precipitation, water surface evaporation and runoff is very obvious, about 82% of precipitation occurred in summer (from June

Fig. 1. Location, topography and stream systems in the Huaisha River basin.

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Fig. 2. Geologic schematic diagram in Huaisha River basin.

to September). However, the rainfall-runoff relationship in the Huaisha River basin changed greatly recently, annual precipitation varied from 723.4 mm in the 1960s and 668.2 mm in the 1970s to 509.2 mm in the 1980s and 1990s, annual runoff depth varied from 287.2 mm in the 1960s and 306.3 mm in the 1970s to 166.4 mm in the 1980s and 1990s, annual rainfall-runoff coefficient varied from 39.70% in the 1960s and 45.84% in the 1970’s to 28.19% in the 1980s and 1990s. 2

dled electrical conductivity meter at the field sites. Surface water samples are river water, and groundwater samples include spring water and well water. Water samples on August 14―21, 2002 were selected particularly for measuring in Hydrological Science Laboratory of Science Department of Chiba University. For hydrogen and oxygen isotope compositions measuring, zinc reaction method and standard carbon dioxide-water equilibrium method were adopted, respectively. The isotope compositions analytical apparatus is Delta S Thermoqest gas source stable isotope mass spectrograph, the results were reported in per mil deviation relative to VSMOW standard, and the analytical precisions are ±2.0‰ and ±0.1‰ for δD and δ 18O, respectively. The water quality was measured in Key Laboratory of Water Cycle and Related Land Surface Processes, CAS by Shimadzu CTO-10ACvp ions chromatograph with the analytical precision of 1%, −

and HCO 3 were measured by diluted vitriol-methylic titration method. 3 Characteristics of hydrochemistry, hydrogen and oxygen isotope in water samples 3.1 Characteristics of electricity conductivity (EC) in water samples

Methods

For illuminating the interaction between surface water and groundwater in Huaisha River basin affected by the natural condition changes and human activities, three times field surveys in June 2002, August 2002 and October 2003, were carried out respectively, and the surface water and groundwater were sampled along the stream lines (Fig. 4), while simultaneously, the pH value, electrical conductivity and water temperature were measured by WM-22EP han-

Electricity conductivity (EC) is the whole reflection of total dissolved ion concentrations in water bodies, and to certain extent, EC value reflects the length of flow paths and residence times in water cycle. During the water moving processes, EC value of water body increases gradually with the extension of flow path and residence time while continually dissolving the dissolved minerals and generating ions exchange with adjoining rocks and soils, except for mixing with

Fig. 3. Monthly rainfall and water surface evaporation and daily runoff in the study basin.

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Fig. 5. Variation of EC value along the main river channel.

Fig. 4. The distribution of water sample sites in Huaisha River basin.

lower EC value water body or separating out of gas or subsidence of dissolved solids. Therefore, on the foundation of spatial distribution tendency of EC value in water bodies in basins, the flow paths could be determined approximately and further the rechargedischarge relationships between surface water and groundwater in basins could be deduced. The lower electrical conductivity values persists in the study basin (Fig. 5), while EC values of surface waters range from 204 to 424 μs·cm−1, and those of groundwater range from 229 to 773 μs·cm−1. According to the spatial distribution differences of EC value of surface waters, ground waters and geographical characteristics in the basin, we can figure out that a similar trend exists between the EC value of surface waters and those of ground waters in the upland catchment, on the whole, however, the EC value of ground waters is a little less than those of surface waters, which indicates that the interaction between surface water and groundwater is that the surface water recharges the groundwater at the headwater area. Downwards from the headwater sources along the main river channel, river bed incised downward seriously because of steep channel slope, while the EC value of river water and runoff flow increases, indicating that surface water also receives that groundwater leakage from two stream sides besides spring recharge. Main river channels at central area lie in intermoun- tainous small plains with gentle channel slope. The EC value of surface water at the central area is higher than that in the upland area, whereas the EC value of surface water in the central area is similar

to that of the groundwater with uncertain variations, which indicates that the transformation between surface water and groundwater is in higher frequency. It was found from field survey that there were several segments of wadi channels in the central area. Surface water infiltrated underground gradually in the upper ends of the wadi channels, and runoff flow became smaller and at last disappeared, therefore came into being the phenomenon of surface water recharging groundwater. While in the lower ends of the wadi channels, groundwater under the wadi channel exfiltrated gradually or pumped out by human activities for sand-quarrying, and came into being the phenomenon of groundwater recharging surface water. Therefore, in the central area, transformation between surface water and groundwater is very frequent. In the middle-lower area, the EC value of surface water and groundwater is higher than that of the central watershed, indicating that the surface water and groundwater have transported longer paths. Meanwhile EC values of surface water and groundwater are evidently different, indicating that surface water system and groundwater system are relatively independent flow systems, and recharge amount from surface water to groundwater is very less or nonexists, suggesting that groundwater is likely to be recharged by groundwater flow from a long distance away. Additionally, as shown in Fig.5, there are two groundwater samples dotted on the left side of the figure, even both of them located near the estuary, but the EC values of them differentiate greatly. The reason is that the recharge source and flow paths of them are quite different, the groundwater sample with lower EC value is spring exposed directly from mountain fissures located to the left side of the river channel with shorter flow path, while the other is well

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water located under the alluvial and diluvial plain to the right side of the river channel, which are groundwater from a long distance away with higher EC value because of the longer flow path. 3.2 Analyses of anion and cation ions in water samples Anion and cation ions in water offered qualitative and quantitative study methods for investigating the generation and involvement of water bodies. Hydrochemistry of water was under the influence of lithology and soil characteristics where water passed by, and evolvement of components was strictly controlled by various chemistry functions. Thus, on combination with the topographical and geological conditions, water circulation paths in study basins can be investigated expressly. As shown in Fig. 6, the hydrochemistry pattern in the study basin is mainly Ca-Mg- HCO3− type belonging to lower degree mineralization water, and further authenticated that the ultimate source of water is mainly meteoric water source. Some differences exist among various water types, which mainly manifests that Ca2+ and HCO3− concentration in river water is higher than that in spring water, whereas Mg2+ and Cl− concentration is in opposite condition. Variation of Ca2+ and Mg2+ concentration in well water and river water is unobvious, while Na+, K+ and Cl− concentrations in well water are higher than those in spring water. The analyses above illuminated that in the re-

charge processes from spring water to river water, Ca2+ and HCO3− that dissolved redundantly in river water generated undissolved CaCO3 sedimentation since environmental condition change; while for well water, relatively high Na+ and Cl− concentrations were observed, indicating that well water had higher degree of mineralization. Well water samples were located in small plain with tender slope generated by alluvium and diluvium layers with lower groundwater hydrological gradient and lower transport speed, therefore well water has longer residence time than spring water and river water. 3.3 Analyses of hydrogen and oxygen stable isotopes in water samples Meteoric water line was acquired by regression calculating on hydrogen and oxygen isotope in precipitation from the 27 Global Networks of International Precipitation Stations constructed in China by IAEA. The calculated Local Meteoric Water Line (LMWL) is δD =7.82·δ18O+8.48, (1) 18 where δD is hydrogen isotope composition, and δ O is oxygen isotope composition. Sources of spring water, river water and well water could be distinguished by comparing of the δD and δ 18Ο compositions of water samples with the LMWL (Fig. 7), and the transformation relationship among them could be illuminated.

Fig. 7. Diagram of hydrogen and oxygen isotope compositions in water samples.

Fig. 6. Piper’s diagram.

Hydrogen and oxygen isotope compositions in spring waters approximately scatter on one line and lie deviously down-right to the LMWL, indicating that certain degree of evaporation processes has occurred

A study of interaction between surface water and groundwater using environmental isotope in Huaisha River basin

before recharging river water and has induced the variation in isotope [18]; while isotope compositions in river waters cluster together, and isotope compositions in well water present obvious difference, which shows that certain degree of difference existed for ground water sources and circulation processes in various regions. Moreover, certain degree of relationship presents between the hydrogen and oxygen isotope compositions in surface water and groundwater and the spatial positions, which to certain degree reflects the interrelationships between surface water and groundwater. Prominent linear relationship exits between the δ 18Ο value in spring waters and the altitudes where the spring exposed, which indicates that prominent altitude effect lies in oxygen isotope in spring water (except for the ascending spring of HS17). The oxygen isotope altitude effect of spring water is the representation of oxygen isotope altitude effect of precipitation, which behaves obviously in highly altitude different regions, such as Bartarya et al.[19] demonstrated the stable isotope altitude effect of spring waters in Himalayas in Kumaun, India, and provided reliable evidence for studying the groundwater sources in that region. An obvious oxygen isotope altitude effect of −0.24‰ per 100 m (eq. (2), Fig. 8) was revealed for spring waters sampled in various altitudes (from 78 m to 720 m). δ18O(‰)=−0.0024 h−8.0154, (2) where h is the altitude the spring is exposed to.

Fig. 8. Relationship between oxygen isotope in spring water and altitude of springs.

Case studies on ascertaining main recharge areas and flow paths of groundwater by altitude effect of oxygen isotope have been carried out successfully overseas[9]. Based on calculation on the equation acquired above, the exposing altitude of spring water

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sample of HS17 should be in a height of 864 m or higher, but the actual exposing altitude is only 305 m, indicating that spring water of HS17 is from confined aquifer. While the recharge boundary of confined aquifer is not yet the topographical catchment boundary, and recharge range of confined aquifer is bigger and further than the surface boundary range, indicating that water in the Huaisha River basin is not only from the runoff within the topographical catchment boundary. Additionally, EC value in spring water of HS17 is higher (324 μs·cm−1) than that in the nearby spring water of HS22 (242 μs·cm−1), and further indicates that spring water has transported a longer flow path and the source should be derived from outside of the basin. Spring water is a kind of discharge patterns of groundwater, and the discharge characteristic of spring water is under the control of geological conditions. From the geological map we can figure out that the exposing sites of spring waters are mainly located in the interface belts of Paleozoic limestone and strip-shaped shale, gneiss and Jurassic granite, while layers of shale and gneiss are aquifers. When the phreatic or confined groundwater in the aquifer was blocked off by the underlain granite aquifuge, groundwater would expose and generate springs in the interface belts between aquifer and aquifuge. Additionally, in comparison of the rainfall-runoff coefficient in the Huaisha River basin and that in the Chaobai River basin in the corresponding period we can find out that the annual rainfall-runoff coefficients in the Huaisha River basin are far bigger than those in the Chaobai River basin in corresponding periods. Annual rainfall-runoff coefficients in the Huaisha River basin are 43% for the 1960s and 1970s, and 28% for the 1980s and 1990s (according to the hydrological data at Koutou hydrological station in the Huaisha River basin, mean annual rainfall in the Huaisha River basin are 695 mm for the 1960s and 1970s, mean annual runoff depth is 296 mm, mean annual rainfall are 590 mm for the 1980s and 1990s, mean annual runoff depth is 166 m), according to investigation by Wang[20], annual rainfall-runoff coefficients in the Chaobai River basin above the Miyun Reservoir are 15% for the 1960s and 1970s, and 8% for the 1980s and 1990s. The climate and vegetation characteristics

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of the Huaisha River basin and the Chaobai River basin are very similar, however, the rainfall-runoff coefficients are quite different, suggesting that the Huaisha River basin receives water recharge from other river basins. By integrating the analyzing results of environmental isotopes, hydrochemistry, geology and rainfall-runoff coefficients, we can speculate that the Huaisha River basin is an unenclosed basin. 4 Estimation on precipitation recharge to groundwater by the chloride concentration It is a successful method to estimate the precipitation recharge to groundwater by the chloride mass-balance approach, since chloride is dissolvable ion with lower concentration in meteoric water. Chloride concentrations in water bodies might increase after evapotranspiration processes, however, the total amount in the residual water body keeps invariable, so the groundwater recharge rate could be estimated by applying the chloride mass-balance approach[21]. This method supposes that: (1) chloride behaves conservatively, and it is not involved in any chloride exchange with soils and adjoining rocks; (2) surface flow from the catchment is negligible, and evapotranspiration is the main water loss in basins; (3) the main reason arose the difference of chloride concentration in precipitation and groundwater is evapotranspiration; and (4) precipitation deposition is the exclusive source of chloride in groundwater. The equation is as follows: R(%)=100·(CClP/CClG), (3) where R is mean annual groundwater recharge rate, CClP is amount weighted-average chloride concentration in precipitation, CClG is mean chloride concentration in groundwater. The lithology in the study basin is mainly carbonates and sulphates, and the chloride in water bodies are mainly from precipitation. According to the precipitation samples sampled in the Huaisha River basin from 2002 to 2003, the amount weighted-average chloride concentration in precipitation in the Huaisha River basin is 0.66 mg·L−1, while the mean chloride concentration in groundwater is 3.25 mg·L−1. Thus the groundwater recharge rate in the study basin is 20.26%.

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5 Spatial transform relationship between surface water and groundwater It is an important aspect to carry out water source separation on various flow paths in basin studies. Isotope and hydrochemistry have provided effective technical methods to implement hydrograph separations, especially in ascertaining such questions as river runoff separation from two sources. The usual applied methods are isotope and hydrochemistry mass-balance approach (eqs. (4), (5) and (6)), such as by applying environmental isotopes D and 18O, chloride concentration and EC value [22, 23]. The equations are as follows: Qt=Qu+Qv, (4) Qt·Ct=Qu·Cu+Qv·Cv, (5) Qu/Qv=(Cv−Ct)/(Ct−Cu), (6) where Q is amount of flow, C is concentration of tracers, t is water body after mixing, u is water source one, and v is water source two. Application of isotope and hydrochemistry massbalance approach to carry out hydrograph separation are based on the assumptions that summation of amount of flow from two sources is equal to that of after mixing, and the summation of tracer flow amount from two sources is equal to that of after mixing (Fig. 9). Amount of flow from source one, source two and amount of flow after mixing are Qu, Qv and Qt, respectively, concentration of tracers from source one, source two and concentration of tracers after mixing are Cu, Cv and Ct, respectively. Then source one, source two and amount of flow after mixing satisfy eqs. (4), (5) and (6).

Fig. 9. Hydrograph separation by the tracer mass-balance approach.

According to the spatial distribution characteristics of stable isotopes and hydrochemistry compositions in water samples and the field survey results, the spatial

A study of interaction between surface water and groundwater using environmental isotope in Huaisha River basin

transformation relationships between surface water and groundwater were made clearly, as well as recharge ratios of flow from main channels and tributaries. The river channels in headwater areas incise downward deeply with strong encroaching forces because of steep slope gradient, and descending springs or ascending spring formed by obvious exposing of groundwater directly recharge river runoff to turn into the sources of main channel and tributaries (Fig. 10). Moreover, groundwater exfiltrates from mountains on both sides of channels and recharges surface water, making the river runoff increase gradually. Spring site of HS18 is located at the granite region with less flow, lower EC value and more depleted isotopes compositions. Hydrogen and oxygen isotopes in spring waters in headwater areas are located near the LMWL, while those in river waters deviate down-right to the LMWL and lay up-right to those in the spring waters. δD and δ18O values in spring HS18 are −66.8‰ and −9.22‰, respectively, while δD value in river waters ranges from −64.5‰ to −61.9‰, δ 18O from −8.40‰ to −8.06‰, respectively. EC value in river waters is higher than those in spring waters, while anion and cation compositions vary in a finite range, indicating that river water has undergone certain degree of evaporation during the runoff process. Hydrochemistry components in river waters from different distributaries present certain degree of difference because of the influence of geological conditions, especially for surface water sample HS20, which is mainly recharged by groundwater flow with longer flow path originating from limestone regions in the headwater drainage basins, therefore the EC value, concentration of Cl− and SO 24− at HS20 are higher. Down to the confluence of tributaries from the headwater drainage basins, the amount of flow in the main channel increases, and the amount of flow from the main tributary is 81% (Fig. 11) of that from the main channel in the upstream areas. About 2 km down to the confluence, surface water was blocked by concrete barrage constructed by human activities and surface flow disappeared behind the barrage, while groundwater flow generated under the channel and wadi channel formed behind the barrage. Spring water samples of HS16 and

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Fig. 10. Transform relationships between surface water and groundwater in the study basin. G, Groundwater; S, surface water; M, meteoric water; Sp, spring water; arrow, flow direction of ground water or surface water (recharge direction).

Fig. 11. Relative contribution ratio at different flow paths to the main channel. Qq Flow direction of surface water and representative runoff; JJG

Q JJGe flow direction of ground water and representative runoff.

HS17 were situated at the limestone and gneiss widely distributed areas with aquifer of abundant water carrying content and large amount of flow, and furthermore the recharge altitude is higher. According to computation the recharge altitude should be in height of 864 m or higher, so it is estimated that it should be from groundwater recharge outside of the basin. δD of spring water samples ranges from −70.1‰ to 69.6‰, δ18O from −9.80‰ to −10.09‰. Isotopic compositions in spring water samples which are located near to the LMWL are more depleted due to the altitude effect, indicating that springs were recharged directly by precipitation.

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River channels in central areas are situated at the intermountainous small plains with mild channel slope gradients formed by thick alluvial and diluvial layers carrying a large quantity of groundwater, where groundwater stored under the plain at the foot of the mountain exposed gradually to recharge river water, thus surface water appeared gradually in the wadi channel (Fig. 10). According to calculation, groundwater flow c from upstream basin and surface flow from tributary d account for 47% and 53% of channel flow in the central area, respectively (Fig. 11). Surface water recharge groundwater by side-filtration and infiltration because of the manual step-dams, and river flow diminish gradually along the channel, finally disappeared 2 km downward the sampling site of HS5. Hydrogen and oxygen isotopes and hydrochemistry of river waters and well waters are very similar, suggesting that frequent transformation present between surface water and groundwater with stronger hydraulic relationships. Heavy isotope enrichment in the water samples means that the water bodies have undergone intense evaporation processes, especially in the wadi channels where residual water accumulated with stagnant flow, where surface water and groundwater transformed into atmospheric water by intense evaporation. δD and δ 18O of surface water HS5 are −74.1‰ and −9.15‰, respectively, and groundwater samples are HS4-2, HS4-1 and HS5-1, δD of them range from −72.7‰ to −72.3‰, δ 18O from −8.62‰ to −8.40‰, respectively. Owing to the decrease in runoff and higher infiltration in this region, some segments of channel became wadi especially in the main channel with sand-dam constructed at the down end of the wadi channel by pumping for sand-mining, thus making the surface water turned into underground with intense recharge-discharge relationships between surface water and groundwater. Groundwater flows from the wadi channel in central areas and that from tributary f occupy 72% and 28% of the surface flow in central watershed channel, respectively (Fig. 11). Channels in central-lower areas gradually enter the narrow inter-valley flat area from the wide plain in central catchment, meanwhile the main channel with gradually increasing amount of flow mainly takes in surface flow from the main tributaries and groundwater flow pumped from underground (Fig. 10). Surface

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flow from channel in central areas and that of conflux from tributary h in the central-lower area occupy 52% and 48% of main channel flow (Fig.11), respectively. River water samples include HS3, HS13, HS14 and HS15 with isotope compositions of δD ranging from −71.5‰ to −64.6‰, and δ 18O ranges from −9.15‰ to −8.47‰, respectively. Isotopes and EC value in water samples imply that surface water has undergone intense mixing and evaporation processes. Well waters in the inter-valley flat area have higher degree of mineralization, but hydrogen and oxygen are close to the LMWL, therefore, it is estimated that groundwater in this region should have transported a long distance in aquifer with meteoric source in higher altitude, and indicate that groundwater transportation should be independent of surface water systems. The groundwater samples include HS9-1 and HS10-1, and δD varies from −72.7‰ to −72.3‰, δ 18O from −8.62‰ to −8.40‰, respectively. Downstream area of the basin are situated at alluvial and diluvial plains in the lowest region. Hydrogen and oxygen in surface waters along the main channel vary greatly, especially from HS9 to HS2 and HS1, isotopic compositions of them are far from the LMWL and enriched gradually, the reason is that the surface water was recharged by groundwater with more enriched isotopic compositions from lower altitude areas, and also, evaporation function of water bodies in channel is a factor that should be paid more attention to. In this region, spring water discharges directly to the main channel (Fig. 10). According to computation, river flow from the downstream area and groundwater flow occupy 94% and 6% of the total flow to the basin outlet (Fig. 11). Spring sample site in the downstream area is HS1-1 with an actual altitude of 78 m and more enriched isotopic compositions, hence more enriched isotopic compositions and higher EC value indicate the most distinct altitude effect and higher evaporation ratio in the lowest altitude. River water samples in this region include HS1, HS2 and HS10-1, and isotopic compositions of them range from −64.7‰ to −62.0‰ for δD, from −7.84‰ to −7.77‰ for δ18O, respectively. Isotopic compositions in groundwater and surface water appear similar, whereas EC value of groundwater is higher than that of the surface water, indicating that groundwater is mainly recharged by groundwater and

A study of interaction between surface water and groundwater using environmental isotope in Huaisha River basin

surface water from upland basin areas. River water samples include HS1, HS2 and HS10-1, with δD of them ranging from −64.7‰ to −62.0‰, δ 18O from −7.84‰ to −7.77‰, respectively. Well water sample is HS1-2, and δD is −72.3‰, δ18O is −8.40‰, respectively. Therefore, based on the obvious spatial distribution characteristics of isotopes, hydrochemistry and EC value in water in the Huaisha River basin, the spatial difference and pattern of recharge-discharge relationships between surface water and groundwater in the study basin were demonstrated. 6

Conclusions

The Hydrochemistry pattern of water quality in the study basin is mainly Ca-Mg-HCO3 type with lower EC values belonging to lower degree of mineralization water, which authenticated that the ultimate source of water in the basin is mainly of meteoric water. Oxygen isotope altitude effect displayed by spring water from various altitudes (from 78 m to 720 m) is −0.24‰/100 m. Using this effect, the main recharge altitude, recharge area and flow paths of groundwater in the upland river basins could be determined by the observed oxygen isotope altitude effect. By applying the correlativity between the oxygen isotope compositions and the exposing altitude of the springs, it is speculated that the exposing altitude of spring HS17 should be in a height of 864 m or higher. On the basis of exposing altitude of spring, source and flow paths of groundwater, the EC value in spring water and the annual rainfall-runoff coefficients of the Huaisha River basin and the Chaobai River basin above the Miyun Reservoir at the corresponding periods, and combining with the geological conditions and field survey results, we can ascertain that the Huaisha River basin is an unenclosed basin. Various water bodies in the basin have undergone different degrees evaporation processes, and the calculated annual mean groundwater recharge rate by chloride concentration is 20.26%. Runoff patterns in the study basin varied greatly under the intense influences of human activities, which change the water circulation model of the basin, then further influence the sustainable use of water re-

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sources. The change of water cycle induced the variation of hydrogen and oxygen isotope and hydrochemistry characteristics, and present obvious difference in spatial distribution. Based on the variation of characteristics of hydrogen and oxygen isotopes and hydrochemistry, the surface water and groundwater circulation conceptual model in the basin was speculated. Acknowledgements The authors would like to thank Sun Tianxiang, Liu Hedi, Zhan Chesheng, Yang Cong, Huang Youbo and Wang Hongping for their assistance in the field work and water sampling. This work was supported by the Knowledge Innovation Project of the Chinese Academy of Sciences (Grant No. CX10G-E01-08-02), the National Natural Science Foundation of China (Grant No. 40371025) and the National Natural Science Foundation of China (Grant No. 50279049).

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