Groundwater recharge environments and hydrogeochemical evolution in the Jiuquan Basin, Northwest China

Applied Geochemistry 27 (2012) 866–878

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Groundwater recharge environments and hydrogeochemical evolution in the Jiuquan Basin, Northwest China Jianhua He a, Jinzhu Ma a,⇑, Peng Zhang a, Liming Tian a, Gaofeng Zhu a, W. Mike Edmunds b, Qinghuan Zhang a a b

Key Laboratory of Western China’s Environmental System, Ministry of Education, Lanzhou University, 222 South Tianshui Road, Lanzhou 730000, China Oxford Centre for Water Research, Oxford University, South Parks Road, Oxford OX1 3QY, UK

a r t i c l e

i n f o

Article history: Received 11 November 2011 Accepted 17 January 2012 Available online 25 January 2012 Editorial handling by R. Fuge

a b s t r a c t The groundwater recharge environments and hydrogeochemical characteristics in the Quaternary aquifer of Jiuquan Basin was investigated using a combination of chemical indicators, stable isotopes, and radiocarbon dating. The d-excess values of winter precipitation and surface water revealed that the meltwater from snow and ice played the dominant role in the basin’s surface water supply. The unconfined groundwater showed gradual enrichment of heavy isotopes along the flow path, but d18O and d2H values were similar to those of surface water, suggesting recent recharge as a result of rapid seepage along rivers combined with the effects of high evaporation. The 14C (pmc) values of unconfined groundwater was between 71.5% and 90.9%, and since 80% modern carbon probably represents the upper limit of initial 14C activity, this suggests that the groundwater is relatively young. The confined groundwater was depleted in heavy isotopes; coupled with low 14C values (20–53%), indicating that the groundwater was mainly recharged as palaeowater during the late Pleistocene and Holocene epochs under a cold climate. The surface water and most groundwater samples were fresh rather than saline, with TDS <490 and <1000 mg L1, respectively. The chemistry of unconfined groundwater changed from HCO 3 -dominated to no dominant ions and then to SO2 4 -dominated moving along the flow path from the Jiuquan-Jiayuguan Basin to the Jinta Basin, and the confined water was SO2 4 -dominated. The results have important implications for groundwater management in the Basin, where a high proportion of the water being used is in effect being mined (i.e., extracted faster than its replacement rate); thus, significant changes are urgently needed in the regional water-use strategy. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The water resources of the arid to semi-arid Hexi Corridor in northern China are fed by melting glaciers and by precipitation in the Qilian Mountains, and are extracted for human use from both surface water and from boreholes created near the region’s oases (Fezer and Halfar, 1990). The region is unique because economic development in the corridor has continued for thousands of years, most famously as part of the Silk Road, and an eco-environmental balance has generally been maintained in the region during this period. However, this situation changed in the 1950s as a result of natural and human factors. Natural factors have been dominated by climate change associated with global warming, and human factors have been dominated by rapid urbanization, the expansion of agriculture, and economic development. Together, these changes have compounded the pressure on the region’s natural water resources and environment.

⇑ Corresponding author. Tel.: +86 (0)931 891 2436; fax: +86 (0)931 891 2330. E-mail address: [email protected] (J. Ma). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2012.01.014

Since ca. 1950, the glaciers of the Qilian Mountains have exhibited large absolute losses and the altitude of the equilibrium line has generally increased because the ablation rate has exceeded the mass accumulation rate as a result of global warming (Ding et al., 2006; Wang et al., 2010, 2011). Although the glacial runoff from mountains has gradually increased (Kang et al., 1999; Chen et al., 2003), it has increased more slowly than the demand for water in this region, and this supply of water will run out when the glaciers disappear. To sustain the region’s rapid population growth and socio-economic development, thousands of wells have been drilled in oasis areas, and excessive removal of groundwater has caused groundwater levels to fall at alarming rates (Feng et al., 2001; Ma et al., 2005; Su et al., 2007). This has led to many problems, such as degeneration of grasslands, desertification, frequent sandstorms, and ecological degradation (Liu and Feng, 2004; Kang et al., 2007; Liu et al., 2010). It is clear that if the current environmental trends and rates of groundwater withdrawal continue, a future water crisis is inevitable. From a hydrogeology perspective, the Hexi Corridor comprises a series of parallel north-south tectonic basins, including the WuweiMinqin Basin, the Zhangye Basin, and the Jiuquan Basin (Fig. 1).

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Fig. 1. Hydrogeological map of the Jiuquan Basin, showing the locations of the sampling sites and of the geological cross-sections shown in Figs. 2 and 3.

During the last two decades, hydrogeochemical indicators and environmental isotope techniques have been used to determine the hydrogeochemical characteristics of the surface water and groundwater and their evolution in the Hexi Corridor. For example, Edmunds et al. (2006) studied the recharge and residence times of groundwater in the Minqin Basin using environmental isotopes and revealed that the deep groundwater was very old and had negligible modern recharge. Based on surveys and chemical analysis, the quality and evolution of surface water and groundwater in the Wuwei Basin has also been determined. The results showed that the surface water has become polluted along the flow paths, and that NO 3 pollution has occurred in groundwater in the northern part of the basin (Ma et al., 2009). Ma and Edmunds (2006) investigated the amount of aquifer recharge using a Cl mass-balance technique and found that the mean recharge rates were only 0.95– 1.33 mm a1 in the adjacent Badain Jaran Desert (Ma and Edmunds, 2006). Wen et al. (2008) and Su et al. (2009) found that the shallow groundwater in the Zhangye and Ejina Basins was generally young and was recharged by seepage from rivers and by irrigation return. These results have provided valuable guidance for regional governments to support more rational use and management of the region’s limited water resources (Cheng, 2002; Gao et al., 2008; Wu et al., 2011). However, these studies were mainly conducted in the Wuwei-Minqin Basin and the Zhangye Basin. Recently, increasing attention has been paid to the Jiuquan Basin, which is located in the middle of the Hexi Corridor depression. It is known in

popular legend as the place where rhubarb was first grown and is also the town where the Portuguese Jesuit missionary and explorer Bento de Góis (1562–1607) was robbed and died destitute. It is also world famous because of the Jiuquan Satellite Launch center and Jiayuguan (It has also been called ‘‘Jiayuguan Pass’’, the first pass at the west end of the Great Wall of China.) With more than 500,000 inhabitants, 750 km2 of cultivated land, and many factories, the region’s water resources face a tremendous challenge. In particular, intensive agriculture and industrial development have placed a heavy demand on the region’s water, and a lack of sufficient enforcement of water use restrictions and pollution control has caused the water quality to deteriorate and the groundwater table to decrease significantly. These changes have attracted the government’s attention and become a high-priority concern. However, there have been no systemic studies of the basin’s water resources. Through a systemic field survey and sample collection, the hydrogeochemical characteristics of the surface and subsurface water and the recharge rates in the basin were determined. The specific goals were (1) to use chemical indicators and environmental isotopes to determine the quality of the surface water and groundwater and to understand the dominant geochemical processes that have occurred in the basin’s aquifers, (2) to identify the sources of recharge for surface water and groundwater, and (3) to define the recharge environment and the residence time of the groundwater. The results will provide a better understanding of the water resources in the basin and help the

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government develop suitable utilization strategies for the water resources. 2. Study area 2.1. General setting The Jiuquan Basin is bounded by the Qilian Paleozoic geosynclinal fold zone to the south, by a fault-block uplift (the Beishan Mountains) to the north, by the Zhangye Basin in the east, and by the Jiuxi depression in the west (Fig. 1). The area contains three different climatological regions: the Qilian Mountains, the JiuquanJiayuguan sub-basin, and the Jinta sub-basin. The southern Qilian Mountains, with an elevation of 3000–5400 m and land above 4600 m covered with snow and glaciers throughout the year, contain the headwaters of the Beidahe River systems as the western tributary streams of Heihe River. The total surface runoff from the mountains reaches 13.27  108 m3 a1, and enters the basin via six tributaries: the Beidahe, Hongshuiba, Hongshan, Guanshan, Fengle and Maying Rivers (Fig. 1). The average annual temperature in the Qilian Mountains decreases from approximately 3.6 °C in the east to 3.1 °C in the west, and the precipitation declines along this temperature gradient, from approximately 500 mm in the east to 200 mm in the west. The Jiuquan-Jiayuguan sub-basin is located in the middle reaches of the Beidahe River, at elevations of 1400–2000 m. The average annual precipitation is 117.5 mm, with an evaporation rate of 2148.8 mm. The average annual temperature ranges from 5.8 to 10.0 °C. This area is an important provisions base in the Hexi Corridor, with 3346 km2 of arable land. The Jinta sub-basin lies in the river’s lower reaches, and is a flatter region with an elevation of around 1200 m. The climate is very arid, with a total annual rainfall of 67.5 mm and an evaporation of 1909.0 mm (http://cdc.cma.gov.cn/index.jsp). The region’s economy depends mainly on agriculture, with a limited industrial base. The Jiuquan-Jiayuguan and Jinta sub-basins are separated by the Beijia Mountains (Fig. 1). 2.2. Geology and hydrogeology The study area transects three geological units: the southern Qilian Palaeozoic geosynclinal fold zone, the Hexi Corridor depression (the Jiuquan-Jiayuguan Basin) and its associated structures, and the northern Jinta Basin, which is bounded to the north by the Beishan tectonic belt (Fig. 1). The uplift of the Qilian Mountains in the south has occurred since the end of the Palaeozoic, and has formed an active fold and thrust-belt that extends along the northeastern margin of the Tibetan Plateau (Tapponnier et al., 1990; Meyer et al., 1998). During the end of the Palaeozoic and throughout the Mesozoic era, the embryonic form of the Hexi Corridor was created. This was followed by a complex tectonic stage when the Heishan-Beijia structural zone formed in the middle of the Hexi Corridor, cutting the region into two parts: the southern and the northern sub-basins. These basins were bounded to the north by the Beishan Mountains. From the late Tertiary, and especially from the end of the Pliocene to the beginning of the early Pleistocene, the surrounding Tibetan mountains began to rise rapidly (Li et al., 1979) and the basin subsided further. The Jinta Basin located in the intersection of Tarim plate and the North China plate structurally, connected with Jiuquan fold basin along the Altun fault in a ENE direction. The Jinta Basin is a long-term area of subsidence, which began in the Upper Paleozoic, had an orogenic process in the Tertiary Period and accumulated thousands of meters of molasse. From the late Tertiary to the Pliocene epoch, the Indian and and Qinghai-Tibet plates crashed and formed a WNW contracted stress field. The Altun fault was also activated during

this period (Cao, 2004). At the same time, intensive erosion of the mountains led to large transfers of clastic material into the basin. As a result, the main aquifer is composed of thick Pleistocene and Holocene diluvial and alluvial sediments (Fan, 1991). The groundwater aquifer system ranges from tens to hundreds of meters in thickness. The Gobi zone of the northeastern Jiuquan-Jiayuguan sub-basin is made up of cobble and gravel deposits with a specific capacity of 100–400 m day1. The Middle and Upper Pleistocene aquifer in this area has an average thickness of 80–200 m, reaching a thickness of 400 m in the piedmont of the Qilian Mountains (Fig. 2). A large amount of surface water seeps down and recharges the groundwater in this aquifer. The water level ranges from 50 to 100 m below the surface. In the fine-soil plains of the JiuquanJiayuguan sub-basin, the groundwater system changes from univocal phreatic to a double-deck unconfined-confined groundwater system with the groundwater less than 5 m below the surface. The aquifer is composed of gravel sands, sands and fine sands with a thickness of 50–100 m. The top deck of the confined aquifer consists of a clay layer that is approximately 10 m thick. The permeability coefficient of the underlying aquifer is 10–100 m day1 and its specific capacity is 3.0–30.0 L s1 m1. To the north and east of the fine-soil plains, the aquifer is composed of interbedded gravels, coarse sands, fine sands, and clay loams that form a multilayer aquifer. The thickness of this aquifer is between 100 and 300 m. The phreatic water level is less than 5 m from the surface, and the top layer of confined water is 10–15 m below the surface. The thickness of each aquifer varies across the basin. The thicker parts have layers ranging from 5 to 20 m in thickness, and the permeability varies. The thinner parts are distributed east of Huangnipu and north of Qiantan, with a single layer of 1–5 m thick. The water yield is poor and the specific capacity is lower than 1.0 L s1 m1. In many places, the groundwater emerges from the ground as springs and forms streams. The groundwater flow direction is mainly from SW to NE (Fig. 2). The Jinta sub-basin is the diluvial and alluvial plain of the Beidahe River. The groundwater system is similar to that of the Jiuquan-Jiayuguan sub-basin, although the grain size in the aquifer is finer. The southern region of Jinta country has unconfined groundwater and is characterized by Middle and Upper Pleistocene pluvial–alluvial coarse sands and gravels interbedded with lenticular bodies of clay and silt. The water level is 5–10 m below the surface. In the northern part, the aquifer system transforms from a monolayer aquifer to a bilayer system interbedded with discontinuous impermeable thin clay and silt layers. The water level of the confined water is about 1.0 m, and the burial depth of the unconfined water averages 1.3 m (Fig. 3). As a result, groundwater is lost to vertical evaporation at the surface and to transpiration by desert vegetation. The groundwater flow direction is from SW to NE. The Jiuquan-Jiayuguan and Jinta sub-basins are two separate groundwater systems, but the overflow of groundwater and surface water in the upstream Jiuquan-Jiayuguan Basin flows along the Beidahe River, is collected in the Yuanyangchi reservoir, and flows out of this reservoir into the lower reaches of the Jinta subbasin (Fig. 1).

3. Materials and methods To obtain sufficient data to cover the study area, a series of field studies were carried out from July 2009 to June 2010. Forty-seven representative samples were obtained, including two meltwater samples from the southern Qilian Mountains, six river samples from the main channels of the Beidahe, Hongshuiba, Hongshan, Fengle, and Maying Rivers, one sample from the Yuanyangchi

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Fig. 2. The geological cross-section (A–A0 in Fig. 1) in the Jiuquan-Jiayuguan sub-basin (modified from the Gansu Geology Survey, 1978).

Fig. 3. The geological cross-section (B–B0 in Fig. 1) in the Jinta sub-basin (modified from Gansu Geology Survey, 1978).

reservoir, two samples from irrigation canals in the Jinta sub-basin, and 36 groundwater samples from water supply and irrigation boreholes that penetrated different lithological units. All of the wells had been pumping continuously for at least 2 months at the time of the measurements. Temperature, specific electrical conductivity (SEC), pH, and total dissolved solids (TDS, a measure of salinity) were measured on-site using a sensION156 portable multiparameter meter (Hach, Loveland, CO), and total alkalinity (as HCO 3 ) was determined using a Model 16900 digital titrator (Hach) using bromocresol green–methyl red indicator, with a precision of ±1% (SD) for values >100 digits. Each sample was filtered into three acid-washed, well-rinsed, low-density-polyethylene bottles. One sample was used to determine the major cations, and was acidified to 1% HNO 3 , producing a pH of around 1.5, which was sufficient to stabilize the cations. The other two filtered, unacidified samples were prepared for anion and stable isotope analysis. A 1.5-L unfiltered sample for radiocarbon analysis was also collected in the field.  + + 2+ Concentrations of major ions (Cl, SO2 4 , NO3 , Na , K , Ca , 2+ 18 2 Mg ) and stable isotope ratios (d O, d H) were determined at the Key Laboratory of Western China’s Environmental System (Ministry of Education), Lanzhou University. Major ions were

determined by ion chromatography using an ICS-2500 ion chromatograph (Dionex, Sunnyvale, CA), and the analytical precision was 3% of the concentration based on reproducibility of repeated analysis of the samples, and the detection limit was 0.01 mg L1. The stable isotope analysis was carried out using a DLT-100 liquid water isotope analyzer (Los Gatos Research, Mountain View, CA). In this analysis, each sample was measured six times and the first two values were discarded; based on the remaining four measurements, the precision was 0.2‰ for 18O and 0.6‰ for D. Hydrogen and O isotope ratios were reported using delta (d) notation as ‰ values relative to Vienna Standard Mean Ocean Water (VSMOW). The chemical data was validated using the ionic balance method, and all samples had a precision better than 5%. Some shallow well samples were chosen to test the stable isotope composition of N (d15N) and O (d18O) in NO 3 using bacterial conversion of NO 3 into N2O and subsequent measurement using a continuous-flow isotope-ratio mass spectrometer at the Laboratory of Applied Physical Chemistry-ISOFYS, Faculty of Bioscience Engineering, Ghent University, Belgium. Nitrogen and O isotope ratios are reported using delta notation as ‰ values relative to the value for N2 in air and the VSMOW reference water, respectively. Radiocarbon samples were measured at the CSIRO Land and Water

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Laboratory, Adelaide, South Australia. Dissolved inorganic C (DIC) was reacted with 85% phosphoric acid to produce CO2, which was then graphitized, and 14C was determined by means of accelerator mass spectrometry (AMS) at the Australian National University, Canberra. d13C was estimated from the single-stage radiocarbon AMS values with an analytical error of ±0.15‰. The 14 C concentration is given as the percent modern carbon (pmc) value and the conventional radiocarbon age, with analytical errors of <0.22% and <55 a, respectively.

4. Results and discussion 4.1. Evolution of water geochemistry The plot of TDS as a function of location (Fig. 4) and the chemical data (Table 1) show the quite different mineralization characteristics of the surface water in different zones. The TDS ranged from 31 to 490 mg L1, with a mean value of 60 mg L1 in the high Qilian Mountains meltwater (Table 1). The medial content of the salinity is found in the Jiuquan-Jiayuguan sub-basin, with a concentration of 168 mg L1. This increased to 302 mg L1 in the Yuanyangchi reservoir, and then increased sharply, especially downstream in the Jinta sub-basin, with an average value of 489 mg L1. Overall, the surface water was considered to be freshwater. This is very different from the Heihe River Basin, where the TDS of the surface water reaches 2157.7 mg L1 in the terminal lakes (Zhu et al., 2008). This may result from the relatively small area of the Jiuquan Basin, in which surface water can quickly reach downstream areas without undergoing intensive evaporation. The pH of the surface water ranged from 6.73 to 8.96 (Table 1), and most of these waters were, therefore, neutral to alkaline. The results of the chemical analyses shown in the Piper diagrams (Fig. 5a) show that both the meltwater and the surface water in  the Jiuquan-Jiayuguan sub-basin were of the CO2 3 + HCO3 type, whereas the surface water of the Jinta sub-basin belonged to the no-dominant-anion class. The cation facies were more complex. The two meltwater samples were Ca2+ and Mg2+ types, most samples in the Jiuquan-Jiayuguan group were Ca2+ types, and all the Jinta samples were in the no-dominant-cation class, with increased Na+ and Cl compared to the other samples (Fig. 5a). In terms of the groundwater’s hydrogeological condition, the samples were divided into four groups: unconfined and confined groundwater in the Jiuquan-Jiayuguan sub-basin, unconfined groundwater in the Jinta sub-basin (widespread from the southern to the northern parts of the sub-basin), and confined water in the northern part of the Jinta sub-basin. These groups are representative of the overall range of groundwater conditions in the Jiuquan Basin. The groundwater in the study area was generally freshwater, but TDS ranged from 225 to 3290 mg L1 (Table 2). The pH value

Fig. 4. TDS of surface water in the region from the Qilian Mountains, through the Jiuquan-Jiayuguan sub-basin to the Jinta sub-basin.

ranged between 5.6 and 8.50, indicating a wide range of characteristics, from acidic to alkaline. The unconfined groundwater in the Jiuquan-Jiayuguan sub -basin was mainly found in oasis areas, as well as in the fine-soil  plains area. Most samples were the CO2 3 + HCO3 type (Fig 5b), but changed into the no-dominant-anions type moving from the SW to the NE. The Cl concentration was very low, with values in most of the samples below 46.3 mg L1 (Table 3). For the cations, the unconfined groundwater generally had more Mg2+ and Ca2+ than Na+. This area is covered by cultivated land, so surface water channels are widespread, and the groundwater can be recharged by rapid seepage from river flow. Based on these results, the groundwater generally has good quality, with a low average salin ity of 386 mg L1 (Table 3) and the same CO2 3 + HCO3 type as the surface water. The confined groundwater of the Jiuquan-Jiayuguan sub-basin, which is found in the fine-soil plains area, showed a wide range of chemical characteristics. The TDS ranged between 237 and 1279 mg L1, and pH ranged between 6.34 and 8.48 (Table 3). The groundwater from the deep wells was mostly in the no-dominant-anion and SO2 4 -dominated categories (Fig. 5b), but changed to the Cl–Na+ type in the shallow boreholes. In the Jinta sub-basin, most of the unconfined water was the SO2 4 -dominated type, with TDS increasing from the south to the north. In the southern part of the sub-basin, the groundwater was generally freshwater, with a TDS less than 1111 mg L1, but the salinity increased to 3290 mg L1 in a shallow well located at the northeastern edge of the basin (well 34; Table 2). In addition, SO2 was much higher in this group than in the previous two 4 groups, reaching a maximum of 2930 mg L1 in the shallow well (well 34), indicating intense evaporation and probably some return of irrigation water. The confined waters in the Jinta sub-basin had lower TDS than in the unconfined group, and were generally in the no-dominant-cation group, and most samples belonged to the SO2 4 -dominated type in the anion facies (Fig. 5b). Moving from upstream to downstream areas, the overall chemistry of the anion facies in the water types changed from HCO 3dominated to no-dominant anions, and then to SO2 -dominated. 4 For the cation facies, most samples belonged to the no-dominant-cation category. The chemical species dissolved in the groundwater are not independent, and the relationships between different ions and TDS can be used to study the characteristics and mineralization sources of the groundwater (Schoeller, 1977). In addition, the Cl ion tends to have a stable concentration because it undergoes few chemical and biological reactions in a natural environment, and has, therefore, been widely used as a conservative reference element to study water–rock interactions. Chloride mainly originates from the dissolution of halite in evaporite deposits and the precipitation of dry fallout from the atmosphere. Statistical analysis between the dissolved species shows a good correlation. The Cl concentration increased with increasing TDS in all of the groundwater samples (Fig. 6a). In arid regions, the build-up of dissolved species through evaporation is a major control on groundwater salinity (Richer and Kreitler, 1993; Drever, 1997), especially for the concentration of Cl, which is highly soluble in groundwater. In most groundwater systems, Cl and TDS can be used to represent the intensity of evaporation. The Na+ and Cl concentrations were also strongly correlated (Fig. 6b): Na+ = 1.730Cl + 0.938 (R2 = 0.692). This suggests that the dissolution of halite is a considerable control on the Na+ and Cl concentrations. The groundwater was below the halite saturation level and the saturation index for all samples was far less than 0, indicating that the halite in fine-grained sediments dissolved into the groundwater (Table 4). A significant feature of the groundwater was the high mNa+/Cl (molar) ratio, with a molar ratio greater than 1 for most samples, especially at low TDS, for which most of the values

871

21.08 18.63 – – 51.88 52.57 – – 9.12 8.9 – –

18.55 20.77 20.32 20.51 17.96 56.09 53.71 54.08 53.25 55.72 9.33 9.31 9.3 9.22 9.21

– – –

– – –

d18O (‰)

d2H (‰)

d-Excess (‰)

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were 2 or higher. However, this effect was masked to some extent in groundwater with high TDS (Fig. 6c), indicating that different geochemical processes occurred in freshwater and saline groundwater. This may result from the weathering of Glauber’s salt in freshwater and cation exchange reactions that release Na+ at the expense of other cations (Ca2+ or Mg2+) in saline groundwater. Various indices of base exchange can be used to determine the effect of this cation exchange. Schoeller (1965) defined two chloroalkaline indices:

3.08 2.77 18.4 15.9 46.7 127.8 193 188 5.47 21.0 83.5 76.9 150 166 400 358 1.95 5.58 22.9 24.0 9.26 35.1 106 87.5 16.7 33.7 61.5 59.0 40.2 39.0 84.1 82.0 173 302 490 488 358 622 990 979 8.96 6.73 8.38 8.17 17.8 27.3 24.1 22.5

3.84 3.89 3.3 3.27 2.88 65.6 91.9 46.7 45.2 47.6 3.58 4.3 4.13 4.01 3.78 137 194 124 113 109 2.62 2.51 2.27 2.69 2.09 7.48 9.72 8.29 8.24 7.89 17.3 23.9 12.8 13.1 13.0 41.8 47.8 32.0 33.5 32.9 180 219 147 145 144 374 451 305 302 298 8.69 8.47 8.41 7.55 8.78 13 13.7 15.2 23 10.6

0.36 2.4 1.26 18 0.21 0.94 1.07 0.21 31 60 8.73 –

2.18 31.4 2.06 56 0.88 1.23 6.81 20.5 90 134 8.78 – Meltwater

Meltwater

River River River River River

River Reservoir Ditch Ditch

M1

M2

S1 S2 S3 S4 S5

S6 S7 S8 S9

Qilian mountain Qilian mountain Maying River Fengle River Fengle River Linshui River Hongshuiba River Beidahe River Yuanyangchi Xibachenguang Yuanyangqu

Ca2+ (mg L1) TDS (mg L1) SEC (lS cm1) PH Temp. (°C) Location Type Sample no

Table 1 Basic physical and chemical data for surface water in the Jiuquan Basin. d-Excess = d2H–8d18O.

Mg2+ (mg L1)

Na+ (mg L1)

K+ (mg L1)

HCO 3 (mg L1)

Cl (mg L1)

SO2 4 (mg L1)

NO 3 (mg L1)





CAI 1 ¼ Cl  ½ðNaþ þ Kþ Þ=Cl  

 2  CAI 2 ¼ Cl  ½ðNaþ þ Kþ Þ=SO2 4  þ HCO3 þ CO3 þ NO3

If ion exchange between Na+ in the groundwater and Ca2+ or Mg2+ occurs in the alluvium or in weathered materials, both indices are positive. CAI 2 was positive in all the groundwater samples (Fig. 6d). However, CAI 1 was negative in low-salinity groundwater and positive in high-salinity groundwater (Fig. 6d), indicating that cation exchange mainly took place in the high-salinity groundwater that was found in clay aquifers enriched in silicate minerals. The mNa+/Ca2+ values (molar ratios) ranged between 0.27 and 4.69 (Fig. 6e), indicating that some reaction of silicate minerals occurred. Increasing groundwater salinity was accompanied by a slow increase in reverse ionic exchange, which represents cation exchange that increases the hardness (Ca2+ content) of the water. The Na+ and SO2 concentrations were strongly and signifi4 cantly correlated (R2 = 0.909; Fig. 6g), suggesting that weathering of Glauber’s salt (Na2SO4) occurred. Field work suggests that Glauber’s salt is common in sediment cores from the study area. However, the plots with values that fall below the 1:1 line in Fig. 6g contradict the suggestion that simple dissolution of Glauber’s salt is responsible for the observed results. Thus, another potential source of excess SO2 4 must be affecting the groundwater. The groundwater contained abundant Mg2+, so the Mg2+/Ca2+ ratio ranged from 0.92 to 3.25 (Fig. 6f), with most of the values greater than 1. The main source of Mg in the groundwater is dolomite in the sedimentary rock, which is common in the Jiuquan Basin. More than 370 mg L1 Mg2+ was observed in one sample (well 26; Table 2) collected from a shallow well in the Jinta sub-basin.  The plot of Mg2+ + Ca2+ versus SO2 4 + HCO3 shows a strong and significant linear relationship that followed the expected 1:1 stoichiometry ratio (Fig. 6h), although a few plots were slightly below the  1:1 line, indicating that the Ca2+, Mg2+, SO2 4 and HCO3 were derived from a mixture of the dissolution of calcite, dolomite and gypsum. Most groundwater samples were at or slightly above saturation with respect to dolomite and calcite (Table 4), but this may be an effect of the wellhead pH measurements not reflecting an equilibrium condition, since a pH decrease of 0.5 units due to the loss of CO2 would bring these values close to saturation. Some saline groundwater samples showed a deficiency in (Mg2+ + Ca2+) rel + ative to (SO2 4 + HCO3 ), and this must be balanced by Na that originates from the dissolution of Glauber’s salt. The plot of Ca2+ versus SO2 shows a strong and significant positive correlation 4 (Fig. 6i), although the slope was lower than the 1:1 line, possibly because the widespread gypsum in the study area had a negative value for the saturation index (Table 4), indicating that the weathering of gypsum is common in this area. The dissolution of gypsum in the groundwater may tend to maintain oversaturation of the carbonate minerals. An additional significant characteristic of the groundwater was + the very low NO 3 and K concentrations; approximately 90% of the groundwater had concentrations of less than 20 mg L1 for both species (Table 2), and all of the samples had concentrations below 50 mg L1, which is the recommended drinking water quality standard for NO 3 (WHO, 2011). This indicates that the quality of the groundwater is very good and that anthropogenic inputs to the

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Fig. 5. Piper trilinear plots for the chemical compositions of (a) surface water and (b) groundwater samples in the study area.

groundwater system have been negligible. Several groundwater samples, e.g. groundwater samples 2, 3, 5, 9, 23, 27, 34, may have been affected by human activities since the NO 3 baseline in the arid areas is less than 10 mg L1 (as NO value). Denitrification is 3 a process performed by heterotrophic denitrifiers under anoxic conditions, with NO 3 as the substrate, leading to the production of N2O. This process can be examined by determining how the O 18 isotope effect covaries with the N isotope effect in NO 3 ; the d O and d15N values of NO change in parallel, thereby maintaining a 3 consistent ratio (e18O/e15N) of about 1 (Granger et al., 2006, 2008) or 0.5 (Lehmann et al., 2003; Granger et al., 2008) during denitrification. For the samples collected from irrigation wells in the unconfined aquifer where the water table was shallow, generally at a depth of 1–3 m, the d18O and d15N of NO 3 ranged between 0.6‰ and 11.9‰ VSMOW and between 4.0‰ and 20.2‰ AIR, respectively, and the slope for the regression line between these two parameters was 0.764 (Fig. 6j), indicating the occurrence of denitrification in the groundwater. 4.2. Recharge environment Because of the geographic characteristics of the Hexi Corridor, the local meteoric water lines may differ between the Qilian Mountains and the plains zone. To test this hypothesis and help to identify the sources of water recharge, stations were examined that were representative of both environments. The Yeniugou station is located at an elevation of 3320 m in the Qilian Mountains SE of the study area (99°380 E, 38°420 N). At this site, the mean annual

precipitation and temperature were 401.4 mm and 3.1 °C, respectively. Zhao et al. (2011) studied the isotopes in precipitation at this station and defined the local meteoric water line (LMWL) as d2H = 7.647d18O + 12.396 (R2 = 0.989, n = 99); they then used this to represent the isotopic characteristics of precipitation at high altitudes in the Qilian Mountains. The Zhangye station, which lies 200 km SE of Jiuquan (100°260 E, 38°550 N), is one of the Chinese meteorological stations in the IAEA Global Network for Isotopes in Precipitation (GNIP) database (www.iaea.or.at:80/program/ri/ gnip). According to the IAEA database, the LMWL for Zhangye is d2H = 7.5d18O + 2.7 (IAEA and WMO, 2004). This line represents the relationship between d18O and d2H in the plains region of the study area. Both lines have lower slopes than the world meteoric water line (WMWL; literature citation) as a result of the secondary evaporation effect that occurs during precipitation. This phenomenon has been reported in many arid regions (Gat, 1980). The slope and intercept of the Yeniugou LMWL were both slightly higher than those of the Zhangye LMWL, indicating more moisture and weaker evaporation conditions in the mountains. Fig. 7 shows the d18O and d2H values in the surface water and groundwater from the study area in relation to the meteoric water lines. The groundwater values form a cluster close to the Yeniugou LMWL, and all of the groundwater samples were above the Zhangye LMWL, with a distinct distribution centered along the Yeniugou LWML. This suggests that the precipitation in the plains is not the main source of recharge of the groundwater. This hypothesis is supported by the fact that the weighted average isotopic values of rainfall at the Zhangye station were 6.3‰ d18O and

Table 2 Basic physical and chemical data for groundwater samples in the Jiuquan Basin. Well depth (m)

Temp. (°C)

pH

TDS (mg L1)

Ca2+ (mg L1)

Mg2+ (mg L1)

Na+ (mg L1)

K+ (mg L1)

HCO 3 (mg L1)

Cl (mg L1)

SO2 4 (mg L1)

NO 3 (mg L1)

d18O (‰)

d2H (‰)

d13C (‰)

14 C (pmc)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

100 80 60 90 60 70 95 60 120 80 120 120 96 120 200 70 60 60 100 80 120 60 60 51 60 15 70 60 60 60 30 60 60 7 60 60

11.1 11.5 11.0 10.4 10.8 10.9 11.0 12.5 12.9 12.9 13.2 12.6 14.4 13.1 13.4 12.9 9.9 11.4 12.7 10.8 15.5 15.4 11.5 12.4 11.4 12.4 10.9 11.8 10.7 12.5 11.2 12.5 12.2 12.4 12.4 12.4

6.15 7.67 7.85 7.90 7.83 8.06 8.18 8.39 7.76 8.04 8.41 8.37 8.21 8.48 8.40 8.26 7.08 8.05 8.37 8.13 6.79 6.34 7.90 8.03 6.99 5.60 5.64 7.83 8.19 8.37 7.98 8.02 8.37 7.74 8.17 8.50

225 283 320 310 398 370 266 277 1030 1279 237 264 337 348 360 295 424 500 454 789 351 801 958 776 1837 2920 1111 663 748 450 2710 2060 724 3290 736 367

36.6 47.3 53.1 50.7 51.4 59.6 41.5 31.0 181 111 30.4 30.9 34.5 39.9 36.3 34.4 55.8 68.8 69.3 77.2 20.0 26.7 97.3 86.6 192 228 132 80.1 60.7 42.0 190 139 57.3 265 83.3 37.3

27.6 34.4 40.8 39.1 55.0 46.3 33.5 33.4 125 172 33.8 33.6 40.5 31.6 41.2 38.5 60.4 69.5 38.3 101 20.9 19.3 113 101 253 373 148 81.8 105 63.2 371 251 87.6 324 84.6 40.1

13.6 16.3 24.9 15.9 26.8 26.5 17.7 34.1 95.8 200 20.9 27.3 31.0 24.1 56.0 33.5 33.0 45.8 57.7 124 108 324 141 91.6 293 675 163 82.1 117 78.1 662 490 133 1040 127 60.1

2.41 3.05 4.70 2.86 5.19 3.46 2.69 4.26 8.55 13.3 4.84 4.02 4.71 3.46 4.96 4.78 4.41 6.23 5.43 12.4 3.55 5.34 9.92 7.69 16.8 24.1 10.7 8.35 10.6 7.24 21.1 16.2 8.80 28.9 7.05 4.38

190 230 292 233 308 210 187 158 214 250 162 151 160 100 198 249 273 393 166 400 173 259 336 249 325 246 349 357 320 235 270 305 251 686 251 200

8.69 11.4 13.7 11.4 20.0 33.2 16.8 17.8 46.3 323 14.1 16.0 41.0 44.2 35.8 17.3 25.2 37.2 29.2 64.5 36.6 284 120 107 266 750 136 56.2 86.0 52.4 545 408 116 303 91.5 37.2

51.3 68.9 72.5 83.9 99.7 139 72.5 105 817 585 85.1 95.1 133 166 123 86.5 165 154 256 412 151 170 517 413 1460 2030 701 283 371 219 2180 1320 350 2930 405 146

7.23 17.9 24.2 11.1 20.0 7.13 6.76 4.73 41.8 13.6 5.29 7.50 12.4 9.40 10.0 2.88 10.0 7.59 8.45 5.97 1.69 2.59 19.9 9.00 16.8 7.64 30.4 14.7 2.94 3.00 5.63 3.58 2.56 40.5 4.52 1.60

8.80 8.90 8.92 9.33 9.30 9.02 9.19 9.98 9.33 9.31 9.91 9.64 9.39 9.49 9.41 8.92 8.21 7.85 9.98 10.28 12.00 11.41 8.44 8.99 8.90 8.76 8.21 8.51 8.74 9.19 9.08 10.18 9.51 7.40 9.95 9.90

53.83 53.77 53.52 54.92 55.35 55.47 54.88 60.23 57.86 60.76 62.29 61.24 59.19 58.34 59.78 57.25 55.6 53.85 66.31 65.07 74.49 74.24 54.81 57.68 55.60 58.83 52.16 53.57 55.29 58.63 59.60 60.63 61.14 49.42 62.41 66.96

– – – – – – 1.86 – 7.77 – 3.82 – – – 2.83 – – – 4.98 – – – – – – – – 9.40 – – – 5.94 – – 8.90 –

– – – – – – 53.06 – 71.51 – 30.83 – – – 20.86 – – – 29.17 – – – – – – – – 90.94 – – – 37.73 – – 24.79 –

Corrected (year)

14

C age

– – – – – 3394 – 927 – 7783 – – – 11,111 – – – 7299 – – – – – – – – Modern – – – 6213 – – 9685 –

J. He et al. / Applied Geochemistry 27 (2012) 866–878

Sample no.

873

3.05 9.75 61.95 4.52 9.19 58.63 1.6 10.18 66.96 16.4 8.56 55.22 40.5 7.4 49.42 2.94 9.08 59.6 6.86 10.2 64.61 13.6 9.31 58.34 1.69 12 74.49 41.8 7.85 53.52

1 SO2 4 (mg L ) 1 NO 3 (mg L ) d18O (‰) d2H (‰)

2.88 9.64 61.24

13.6 8.98 55.9

12.5 8.5 2060 139 251 491 16.2 305 408 1320 12.2 8.02 367 37.3 40.1 60.1 4.38 200 37.2 146 11.63 7.32 1668 148 208 363 15.4 349 263 1210 12.4 8.19 3290 265 373 1040 28.9 686 750 2930 10.7 5.6 663 60.7 81.8 82.1 7.69 246 56.2 283 13.28 7.93 544 49.1 54.6 106 6.39 207 94.4 228 15.5 8.48 1280 111 172 324 13.3 400 324 585 10.8 6.34 237 20.0 19.3 20.9 3.46 100 14.1 85.1 11.6 7.8 386 57.4 49.6 31.4 4.39 238 22.9 157 14.4 8.37 1030 181 125 95.8 8.55 393 46.3 817

Mean Max.

9.9 6.15 225 30.9 27.6 13.6 2.41 151 8.69 51.3 Temp. (°C) pH TDS (mg L1) Ca2+ (mg L1) Mg2+ (mg L1) Na+ (mg L1) K+ (mg L1) 1 HCO 3 (mg L ) Cl (mg L1)

Max.

Jinta confined water (n = 5)

Min. Mean Max.

Jinta unconfined water (n = 9)

Min. Mean Min. Min.

Max.

Jiuquan-Jiayuguan confined water (n = 9) Jiuquan-Jiayuguan unconfined water (n = 13) Parameter

Table 3 Mean and range of the parameter values for the groundwater samples. n represents the sample size.

12.4 8.29 867 71.7 105 178 8.72 248 141 487

J. He et al. / Applied Geochemistry 27 (2012) 866–878

Mean

874

43.2‰ d2H (IAEA and WMO, 2004). These values are more positive than the values for all of the groundwater samples, and it is unlikely that heavy isotope depletion occurred during the groundwater recharge in arid areas. In addition, the average direct recharge rate calculated based on Cl mass-balance calculations ranged between 0.95 and 3.0 mm a1 in regions with an annual rainfall of 89–120 mm a1 in the adjacent Minqin Basin and Badain Jaran Desert (Edmunds et al., 2006; Ma and Edmunds, 2006). This also suggests that precipitation in the plains area had a negligible impact on the groundwater. Thus, the recharge of groundwater from precipitation in the plains seems to be negligible in the study area. As a result, the runoff from the Qilian Mountains exerts the most significant effect on the water systems of the study area. The weighted isotope values from summer and winter rainfall at Yeniugou differ greatly, with values of 4.6‰ and 18.1‰, respectively, for d18O, and 24.6‰ and 120.9‰, respectively, for d2H (Zhao et al., 2011). The winter isotopic compositions, which are depleted in heavy isotopes, are evidence of the cold temperatures and limited evaporation that occurred during the rainfall. The influence of moisture recycling on the isotopic compositions of precipitation events can be seen in the deuterium-excess (d-excess) parameter defined by Dansgaard (1964): d-excess = d2H  8d18O. This parameter reflects changes in the sources of moisture (Merlivat and Jouzel, 1979; Jouzel et al., 1982, 1997). The d-excess of summer rainfall was about 13.5‰ and that of winter snow was 23.9‰ at the Yeniugou station. In this region, precipitation is dominated by westerly air masses during the summer and by the integrated effects of westerly and polar air masses during the winter (Zhao et al., 2011). The river samples from the study area were characterized by high d-excess values of 18.55–21.08‰ (Table 1). These values are most similar to the winter rainfall values at Yeniugou, indicating that meltwater from winter snow may account for most of the surface water in the study area. This differs from results obtained from the neighboring Heihe River Basin, where surface runoff entering the headwaters of the Heihe River comes mainly from summer precipitation (Zhao et al., 2011). The isotopic ratios of the surface water were above all three meteoric water lines (Fig. 7). The d18O values ranged between 9.33‰ and 8.90‰, with a mean value of 9.20‰, and the d2H values ranged between 56.09‰ and 51.88‰, with a mean value of 53.90‰. These values are considerably lower than values from an adjacent area, the Minqin drainage basin, which lies in the area at the margins of the Asian summer monsoon, with maximum values of 3.1‰ for d18O and 27.9‰ for d2H (Edmunds et al., 2006), suggesting that evaporation was limited owing to a short flow path and suggesting a different origin for the surface water. The groundwater data showed different isotope characteristics in different aquifers, although the d18O and d2H values tended to become slightly enriched moving along the flow path from the Jiuquan-Jiayuguan sub-basin to the Jinta sub-basin. The unconfined waters in the Jiuquan-Jiayuguan sub-basin were similar to the surface waters in their isotope characteristics (Fig. 7), ranging from 7.85‰ to 9.64‰ for d18O and from 53.52‰ to 61.24‰ for d2H (Table 3). Most samples had d-excess values greater than 15‰, indicating rapid recharge due to seepage from rivers and canals. Two samples (wells 17 and 18) had d-excess values far below the Yeniugou LWML, indicating strong evaporation from the shallow water table at the edges of the Gobi Desert regions in the study area. The confined groundwater from the fine-soil plains of the Jiuquan-Jiayuguan sub-basin were significantly depleted in heavy isotopes. The d18O values ranged from 12.00 to 9.31‰ and the d2H values ranged from 58.34‰ to 74.49‰ (Table 3). In particular, groundwater in wells 21 and 22 was the most depleted in heavy isotopes. The mean values of d18O and d2H in the confined water were 1.22‰ and 8.71‰ lower, respectively, than the corresponding values in the unconfined water. The recharge temperature of the

J. He et al. / Applied Geochemistry 27 (2012) 866–878

875

Fig. 6. Plots of the relationships between the various groundwater chemical and isotopic properties. Parameters with an ‘‘m’’ prefix, such as m(Na+/Cl), represent milligramequivalent ratios. CAI 1 and CAI 2 represent indices of base exchange (see Section 4.1 for details). The dashed lines represent the theoretical dissolution curves for (b) halite, (g) Glauber’s salt, and (h) dolomite and calcite. Solid lines represent significant regression results.

most depleted groundwater calculated according to the local relationship between the temperature and d18O of rainfall was about 6.5 °C, which is comparable to the value reported for Jinchang city, in northwestern China (Ma et al., 2010). In contrast, unconfined groundwater samples from the Jinta sub-basin were enriched in heavy isotopes, and most samples had values below the Yeniugou LMWL (Fig. 7). The d-excess values

were smaller than those in the Jiuquan-Jiayuguan sub-basin, with an average of 14.23‰. These values could be plotted along a regression line with a slope of 5.702 (d2H = 5.702d18O  6.418, R2 = 0.820), revealing that some evaporation and return of irrigation water had occurred, changing the characteristics of the original recharge. This is a typical phenomenon in arid areas where irrigation is common because water can be lost by evaporation

876

J. He et al. / Applied Geochemistry 27 (2012) 866–878

Table 4 Saturation indices for commonly occurring minerals. Values were calculated using the PHREEQC software for groundwater in the study area. Sample no.

Calcite

Dolomite

Gypsum

Halite

Anhydrite

Aragonite

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

1.67 0.02 0.32 0.25 0.29 0.41 0.37 0.39 0.44 0.58 0.43 0.35 0.27 0.43 0.55 0.5 0.51 0.7 0.66 0.74 1.32 1.54 0.53 0.52 0.25 1.74 1.62 0.48 0.61 0.59 0.56 0.62 0.69 0.82 0.65 0.63

3.32 0.05 0.67 0.52 0.74 0.86 0.79 0.98 0.89 1.53 1.08 0.92 0.81 1 1.33 1.23 0.86 1.56 1.24 1.75 2.41 3 1.28 1.29 0.22 3.09 3.05 1.12 1.61 1.53 1.56 1.67 1.73 1.92 1.48 1.47

2.11 1.92 1.88 1.82 1.79 1.57 1.94 1.91 0.64 1 2 1.95 1.81 1.65 1.82 1.97 1.56 1.54 1.29 1.18 1.96 1.9 1.03 1.13 0.53 0.44 0.83 1.27 1.31 1.59 0.48 0.71 1.35 0.28 1.14 1.75

8.46 8.28 8.02 8.29 7.83 7.61 8.07 7.76 6.97 5.81 8.08 7.91 7.45 7.54 7.26 7.79 7.64 7.34 7.34 6.68 6.96 5.62 6.37 6.6 5.76 4.98 6.27 6.92 6.58 6.96 5.12 5.35 6.4 5.2 6.52 7.21

2.36 2.18 2.14 2.07 2.05 1.83 2.19 2.17 0.89 1.25 2.26 2.2 2.06 1.89 2.08 2.23 1.81 1.8 1.55 1.44 2.21 2.15 1.28 1.38 0.78 0.69 1.09 1.53 1.57 1.85 0.73 0.96 1.6 0.54 1.39 2

1.82 0.14 0.17 0.1 0.13 0.26 0.22 0.23 0.28 0.43 0.28 0.2 0.12 0.28 0.39 0.35 0.66 0.55 0.51 0.59 1.47 1.69 0.37 0.37 0.41 1.89 1.78 0.32 0.46 0.44 0.4 0.47 0.53 0.67 0.5 0.48

from the unsaturated zone or from the water table (Wen et al., 2005). The d18O values of the confined waters ranged from 9.19‰ to 10.18‰ and the d2H values ranged from 58.63‰ to 66.96‰, which is much more depleted in heavy isotopes than the unconfined waters in the same area. The latter had average values of 8.56‰ for d18O and 55.22‰ for d2H, showing a strong influence of evaporation and agriculture on the shallow groundwater. One sample from the northeastern corner of the Jinta sub-basin

(well 32) had a depleted isotopic value of 10.18‰ for d18O and a high d-excess value (20.84‰). This indicated that flash floods that originated in the northern hilly region, with a chilly environment, may also recharge the groundwater. In summary, surface water in the Jiuquan Basin recharges mainly from meltwater formed from winter snows in the Qilian Mountains, and is enriched in d2H, and the unconfined water had characteristics similar to those of the surface water, and was, therefore, mainly recharged by seepage of surface water. In contrast, the confined waters were significantly depleted in heavy isotopes, indicating that their origins were as palaeowater or that they had some minor mixture with modern water. 4.3. Recharge age 14

C, with a half-life of approximately 5730 a (Godwin, 1962), has been widely used to estimate the residence time of pre-modern groundwater. Derivation of an absolute age for groundwater requires a knowledge of the sources and the initial activity of CO2 in the vadose zone during the recharge. However, because the activity of 14C in carbonate minerals and the stoichiometry of congruent and incongruent reactions along the flow path interact to produce the total dissolved inorganic C (as HCO 3 ) in the groundwater, it is difficult to estimate the initial 14C activity. A rough estimate of about 80 pmc for the upper limit on the initial 14C activity of the recharging water has been broadly used in northwestern China (Edmunds et al., 2006; Zhu et al., 2008; Ma et al., 2010). Using this value, the corrected age of the groundwater was calculated (Table 2). The residence time in the Jiuquan Basin ranges from modern to 11.11 ka, which is equivalent to dates ranging from modern to the late Pleistocene. The unconfined groundwater in the Jiuquan-Jiayuguan and Jinta sub-basins was generally young. Two representative samples gave values of 71.5 and 90.9 pmc, which is greater than the 50 pmc value of modern water. Chen et al. (2006) suggested a reference value of about 50 pmc for the initial 14C in the Heihe River Basin groundwater based on the Pearson and Tamers models. In this case, the groundwater was very young. The mean residence time of shallow groundwater in the alluvial deposits and the fine-soil plains in the adjacent Zhangye and Ejina Basins was between 12 and 60 a. The confined waters in the two sub-basins had ages ranging from 6.213 to 9.685 ka, with d13C values ranging from 3.82‰ to 8.90‰. Groundwater of this age was also found in the Heihe River Basin and the Minqin Basin (Zhu et al., 2008; Edmunds et al., 2006). The deep groundwater

Fig. 7. Plot of d18O and d2H for surface water and groundwater in the study area. LMWL, local meteoric water line. WMWL, world meteoric water line.

J. He et al. / Applied Geochemistry 27 (2012) 866–878

in the piedmont of the Qilian Mountains is very old, with one sample from a 200-m-deep well having an age of 11.11 ka and a d13C of 2.83‰. The pmc was 20.86, corresponding to the boundary between the late Pleistocene and the Holocene.

5. Conclusions In the present study, an attempt was made to understand the geochemical evolution and recharge mechanisms of surface water and groundwater in the Jiuquan Basin using hydrogeochemical and isotopic data from sample sites throughout the basin. The surface water and groundwater showed progressive mineralization from the upstream to the lower reaches, indicating that high temperatures and low humidity caused evaporation of water along this flow path. This has been observed in many other arid areas around the world, such as in the Mayo Tsanaga River Basin in western Africa (Fantong et al., 2009) and in the northern Sahara sedimentary basin of Algeria (Guendouz et al., 2003), as well as in other inland river basins in northwestern China, such as the Ejina Basin (Su et al., 2007). However, the water quality in the Jiuquan Basin is high compared to that in other regions of northwestern China such as the Shiyanghe River Basin and the Heihe River Basin, since the TDS of the surface water was lower than 490 mg L1 and most of the groundwater had TDS lower than 1000 mg L1. The chemistry types of the unconfined groundwater changed from HCO 3 -dominated to no-dominant-anions, and then to SO2 4 -dominated, moving from the Jiuquan-Jiayuguan sub-basin to the Jinta sub-basin, and the confined water was dominated by SO2 4 ions. The correlations between the major ions and the saturation indices for several common minerals indicated that the evolution of groundwater characteristics was mainly influenced by geological factors and the region’s climate. The Ca2+, Mg2+, SO2 and 4 HCO 3 were mainly derived from the dissolution of calcite, dolomite and gypsum. In addition, halite provided equivalent amounts of Na+ and Cl, but weathering of Glauber’s salt produced excess Na+ in the groundwater, which undergoes exchanges with Ca2+ or Mg2+ from the aquifer materials in areas with saline groundwater, forming a Mg-enriched groundwater system. Another significant characteristic of the groundwater was the very low NO 3 concentrations, and the consistent e18O/e15N ratio in the NO 3 (0.764) suggests that denitrification has taken place in the aquifer and that anthropogenic N inputs into the region’s water systems has not yet become severe. Natural denitrification occurs in a variety of aquifers where there is a sufficient source of oxidizable organic C, and is associated with a shallow water table, but natural processes cannot be expected to remove the large quantity of NO 3 that is introduced into many aquifers by agricultural activities (Gillham et al., 1990; Hiscock et al., 1991). The combination of stable isotope and radiocarbon data provided information on the recharge sources and residence times of the groundwater in different hydrogeological aquifers. Given the large difference in meteorological conditions between the Qilian Mountains and the lowland plains, two LMWLs were used to examine the recharge sources for the groundwater, and it was found that the LMWL from the Qilian Mountains was most similar to the values of the samples. The surface water had low d18O values and a high d-excess, which suggests that melting snow and ice provided the majority of the inputs into the basin’s streams, since the summer rainfall was enriched in heavy isotopes and had a lower d-excess (d18O was around 4.65‰ and d-excess was around 13.5‰ at the Yeniugou station from June to the middle 10 days of September in 2008 and 2009). Using a two-runoff-component model, Zhang et al. (2008) calculated the contribution of melting ice and snow to streamflow from several stations at the outlet of the Beidahe, Fengle and Hongshuiba rivers; this ranged from 24.2% to 33.4%.

877

Compared to the mean d18O (6.3‰) and d2H (43.2‰) of precipitation in the plains part of the study area, all of the groundwater was depleted, indicating that there was no modern direct recharge. The isotopic signature of the unconfined water from the Jiuquan-Jiayuguan and Jinta sub-basins clearly revealed the recharge source and the evaporation effect on the groundwater. The unconfined groundwater had similar isotope characteristics to those of the surface water, indicating recent recharge as a result of rapid seepage along the rivers, as well as enrichment of d18O from upstream to downstream areas as a result of evaporation. The 14C ages showed that the unconfined groundwater was generally young, with pmc values of 71.5 in the Jiuquan-Jiayuguan subbasin and 90.9 in the Jinta sub-basin. Similarly young groundwater has been observed in other parts of the Hexi Corridor, such as the Dingxin and Ejina oases (Chen et al., 2006). However, the confined groundwater was quite different from the unconfined water, with remarkably depleted heavy isotopes and low pmc values; thus, the confined groundwater was principally maintained by palaeowater that originated under a cold environment, with ages between 6.213 and 11.111 ka, which is equivalent to the late Pleistocene and early Holocene, with an estimated recharge temperature of about 6.5 °C for the most depleted sample. Similar results were obtained from the neighboring Ejina Basin, which had confined groundwater of Holocene origin (Wu et al., 2001). Further evidence for a Holocene origin as well as for recent recharge in the Heihe River Basin was provided by Chen et al. (2006) and for Jinchang City by Ma et al. (2010). The results of the present study have important implications for the future of water resources in this region. Although the unconfined water is recharged by surface water, the recharge rate is far below the exploitation rate, and increased meltwater provided by glacial ablation represents a long-term risk to the region’s water supply, since the glaciers are receding rapidly and will eventually disappear, eliminating this source of recharge. In addition, the confined groundwater is non-renewable on a human time scale, so overexploitation of this water resource due to a lack of rational planning and effective water conservation will inevitably lead to depletion of the resource. Although the groundwater is currently unpolluted, rigorous enforcement of pollution control regulations, particularly for agricultural areas, will be required to avoid pollution in the future. Proper management of the region’s water resources will require substantial changes in the region’s industrial and agricultural structures to avoid depletion of the water, but these changes will be necessary to sustain the region’s socioeconomic development and to protect its fragile ecology. Acknowledgments The research is supported by the National Basic Research Program of China (973 Program) (No. 2009CB421306), the National Science Foundation of China (No. 40872161) and the Keygrant Project of Chinese Ministry of Education (No. 310005). This work also forms part of 111 project (No. B06026) and the wider UK–China collaboration. We thank Mrs. Jingfang Wang and Mr. Xiangyang Zhou in Lanzhou University and Dr. Fred Leaney in the CSIRO Land and Water Laboratory for assistance in the field work and laboratory analysis. We also thank Prof. Bernd Wünnemann and Dr. Tianming Huang for their comments that have led to improvements in the paper. References Cao, D., 2004. Structural analysis of Jinta Coalfield. Gansu. Coal Geol. China 16 (5), 5–11. Chen, R.S., Kang, E.S., Yang, J.P., Zhang, J.S., 2003. A distributed runoff model for inland mountainous river basin of Northwest China. J. Geogr. Sci. 13, 363–372.

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