diminished groundwater recharge and circulation …with similar environments, such as the badain...

HYDROLOGICAL PROCESSESHydrol. Process. 24, 147–159 (2010)Published online 15 September 2009 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.7438

Diminished groundwater recharge and circulation relativeto degrading riparian vegetation in the middle Tarim

River, Xinjiang Uygur, Western China

Zhonghe Pang,1* Tianming Huang 1,2 and Yaning Chen3

1 Key Laboratory of Engineering Geomechanics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China2 Graduate University of Chinese Academy of Sciences, Beijing, 100039, China

3 Key Laboratory of Oasis Ecology and Desert Environment, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi,Xinjiang, 830011, China

Abstract:

Riparian vegetation in the middle Tarim River is being degenerated, and groundwater in the riparian system is a key factorcontrolling this process. Vegetation degeneration is related to the reduced inflow from the upstream, reduced recharge toriparian groundwater, lowered water table and increased salinity of the groundwater system. Systematic chemical and isotopicsampling of river water and groundwater transects perpendicular to the river show that riparian groundwater is fed by theriver. River water picks up salts from the river bed and/or from irrigation returns as it flows downstream, and groundwaterfrom riverbank sediments mirror this pattern. Mineral dissolution and evapotranspiration are the main mechanisms of saltaccumulation in groundwater. Reduced recharge from the river has resulted in a distinctive zoning pattern in groundwatersalinity. Groundwater residence times derived from tritium contents indicate that the extent of modern recharge (since the1960s) is limited to approximately 1500 m from the riverbank in the middle reaches of the Tarim River. Vegetation growsbetter in habitats with modern groundwater than in other areas. The embankment built in 2001 along one side of the TarimRiver appears to have reduced groundwater recharge, causing the water table to drop and ecosystem to degenerate. Copyright 2009 John Wiley & Sons, Ltd.

KEY WORDS riparian vegetation; groundwater recharge; isotopes; Tarim River; eco-hydrology; salinity

Received 22 August 2008; Accepted 15 July 2009

INTRODUCTION

In arid inland regions, quantities and patterns of riverflow are often affected by climatic and anthropogenicactivities upstream. Ecosystems in turn are controlledby the spatio-temporal patterns of water resources. Acommon scenario is the degeneration of natural riparianvegetation in middle and lower river reaches due to insuf-ficient ecological flow. Development of rational waterresources management strategies is of high relevance tothe sustainability of these riparian ecosystems. Hence, itis necessary to achieve a reasonable knowledge of thegroundwater system upon which the riparian vegetationdepends.

The Lower Tarim River in western China dried upafter the Daxihaizi Water Reservoir was built in 1972and riparian ecosystems downstream have degeneratedseriously as a consequence. In order to recover thevegetation and stop or slow down desertification, waterhas been diverted from the Kongque (Peacock) Riverthrough the 927-km Ku-Ta canal to the Tarim River atthe Daxihaizi Water Reservoir, where it is stored andperiodically released to the lower reaches. During 2000

* Correspondence to: Zhonghe Pang, Key Laboratory of EngineeringGeomechanics, Institute of Geology and Geophysics, Chinese Academyof Sciences, Beijing, 100029, China. E-mail: [email protected]

and 2006, altogether eight intermittent water diversionshave taken place. Monitoring studies have shown thatthe groundwater depth, water chemistry and vegetationhave all responded positively to the water delivery (Wanget al., 2002; Chen et al., 2006, 2008a,b; Deng, 2007; Houet al., 2007a,b).

In order to ensure the efficiency of this water diversionproject, which was designed for water to reach the termi-nal Lake Taitema, an embankment was built along oneside of the river in 2001 with several outlets, called ‘eco-gates’, built in for intermittent water releases to majorareas of natural vegetation beyond the embankment.

Increasing agricultural activities are also affecting thearea. In the 10-km-wide buffer zone along the middlereaches of Tarim River, where woodlands and wetlandsare naturally found, areas of cropland are increasing andnatural vegetation is decreasing (Zhou et al., 2006), andthese trends are projected to continue (Wang et al., 2002;Hu et al., 2004; Chen, 2007).

Groundwater quantity and quality control the conditionof riparian vegetation. However, little is known about therecharge regime to this riparian groundwater system innatural conditions and how much this has changed as aresult of cross-catchments water diversion. The isotopesof the water molecule (2H, 3H, 18O), together withgroundwater depth and water chemistry data, facilitate

Copyright 2009 John Wiley & Sons, Ltd.

148 Z. PANG, T. HUANG AND Y. CHEN

the formulation of more reliable conceptual models ofgroundwater systems at both local and regional scales,particularly in arid regions that are under stress ofclimate change and intensive human activities (Coplenet al., 2000; Mook, 2000; Glynn and Plummer, 2005;Divine and McDonnell, 2005; Michel, 2005). Thesemodels are essential for assessing the trends of ecosystemdegradation that will serve as scientific basis for rationalwater resources management and sustainable ecosystempreservation in the Tarim River Basin.

The aims of this study are to characterize groundwaterrecharge, estimate groundwater residence time and eluci-date mechanisms of groundwater salinization in the mid-dle reaches of the Tarim River and to establish the rela-tionship between groundwater circulation and the riparianvegetation.

REGIONAL HYDROLOGY AND HYDROGEOLOGY

The Tarim River Basin is located in the south of Xin-jiang Uygur Autonomous Region, western China. It hasan area of 1Ð02 ð 106 km2 and is flanked by the TianshanMountains to the north and by the Kunlun Mountains tothe south (Figure 1). The Taklimakan Desert, the largestdesert in China, is located in the centre of this basin,occupying an area of 3Ð37 ð 105 km2. The Tarim RiverBasin is a Mesozoic–Cenozoic basin with folded periph-eral mountains. Outcrop of the strata from late Paleozoicto Cenozoic is shown in Figure 1, and the Paleozoic iswidely distributed in mountain areas. Archaeozoic andProterozoic schist and gneiss, Palaeozoic and Mesozoicsand stones, conglomerates and magmatic rocks occurin the source area of the Tarim River (XETCAS, 1965;

Figure 1. Schematic geology of the Tarim River Basin modified from Li et al. (2000) and sampling locations

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 24, 147–159 (2010)DOI: 10.1002/hyp

GROUNDWATER RECHARGE IN MIDDLE TARIM 149

Zhu et al., 1981). The large fault between basins andmountains controls the tectonic evolution (Cai et al.,1997). The major depressions in the Tarim River Basinare Kuche Depression and North Depression, SoutheastDepression and Southwest Depression. The major sedi-ment is Tertiary in these depressions. For instance, theKuche Depression with depth of thousands of metrescontains Tertiary sediment with a maximum depth of4500 m. The Quaternary deposit is also extensively foundin these depressions (Figure 1).

The occurrence and distribution of groundwater issimilar between the Southern Tianshan watershed andNorthern Kunlun watershed. In general, the aquifer ofthe desert region is mainly formed by fluvial deposits ofthe southern rivers (Li et al., 2000). The common sinkof the two groundwaters systems is located in the centralpart of the basin, south of the Tarim River.

On the northern side of the Taklimakan Desert, wherethe Tarim River flows through the desert, detailed hydro-geological data and water resources assessment are notavailable because of lack of development in these areas.The diluvial aquifer from the southern Tianshan moun-tains is formed of sand deposits that are 100–300 mthick, constituting one unconfined aquifer layer in whichthe present day water tables range between 20 and 200 mbelow surface (Figure 2). This allows a certain amountof surface runoff in the piedmont fan to infiltrate andrecharge the aquifer. At the southern edge of this diluvialfan, the aquifer comprising alluvial loam becomes con-fined or semi-confined with the thickness of about 200 m.In the Tarim River local riparian groundwater system, theriver water recharges the groundwater through riverbankinfiltration. The regional flow is from north to south (Liet al., 2000).

The headwaters of the Tarim River are the AksuRiver, the Yarkant River and Hotan (Hetian) River innatural conditions. Owing to river regulation, the lattertwo presently recharge the Tarim River only during

large floods. The headwaters are mainly fed by alpineglacier-snow melt and precipitation in the TianshanMountains. The long-term average annual surface runoffis 39Ð8 billion m3. Currently, the Aksu River is themain source for the main channel of the Tarim River(>70%). The Tarim River starts from Aral and endsat Taitema Lake with a total length of 1321 km. Theupper stream is from Aral to Yingbazha (495 km), themiddle reaches are from Yingbazha to Qiala (398 km)and the lower reach is from Qiala to Taitema Lake(428 km) (Figure 1). The Tarim River Basin is far fromthe sea and the presence of the Qinghai–Tibet Plateauis responsible for the arid climate and formation of thedesert (Zhang et al., 1995). According to meteorologicalobservation at Yuli and Luntai in the middle reaches,the precipitation is less than 80 mm/year and potentialevaporation is from 1800 to 2900 mm/year, while annualaverage ambient temperature is 10Ð5°. By comparisonwith similar environments, such as the Badain JaranDesert, direct recharge is expected to be less than1 mm/year (Ma and Edmunds, 2006; Gates et al., 2008).

The region is primarily an alluvial plain, with desertin the middle reaches of the Tarim River. Because oflow riverbed gradients (1/8000 to 1/5000), floods causethe river to change its course frequently, and complexchannel networks of the alluvial plain have been formedup to 100 km in width (Wang et al., 2002). The wetseason occurs from July to September. In August, thedischarge rate is 380 m3/s, while in dry season fromNovember to February it is only about 10 m3/s (Chen,2007). In April, river flow decreases due to abstractionfor irrigation. Since the 1960s, river discharges havebeen generally decreasing due to unbridled water use anddevelopment in the upper stream of the Tarim River andthe headwater catchments (Feng et al., 2005) (Figure 3).Future trends of surface runoff have been studied (Chenet al., 2008c) and major reductions are predicted after2010.

Figure 2. A north–south hydrogeological cross section of the northern Tarim River Basin modified from Li et al. (2000) with the location in Figure 1



Figure 3. The annual average runoff change from 1957 to 2005 atYingbazha hydrological station in the middle Tarim River

The main soil types include Populus euphratica Oliv.soil, meadow soil, bog soil and sandy soil. The regionbelongs to a temperate shrub and semi-arid shrub zone,where the main tree is Populus euphratica Oliv., the mainshrubs are Tamarix ramosissima (branchy tamarisk),Tamarix hispida, Lycium ruthenicum Mur and Halimod-endron halodendron and the main herbs are Phragmitescornmurlis, Poacynum hendersonii, Glycyrrhiza inflate,Alhagi sparsifo and Karelinia caspic (Liu et al., 2008).The young Populus euphratica Oliv. grows mostly nearriver water with low total dissolved solids (TDS). Themagnitude of coverage is strongly influenced by ground-water depth and soil moisture (Zhang et al., 2003).

SAMPLING AND ANALYTICAL METHODS

A sampling campaign was implemented to collect surfacewaters and groundwaters from the middle reaches ofthe Tarim River in August 2007, which was withinthe wet season. Groundwater samples were collectedfrom boreholes at varying distances from the river alongfive groundwater monitoring transects: Shajilike (MA),Shazihe (MB), Wusiman (MC), Aqike (MD) and Tieyizi(ME), listed in order from upstream to downstream(Figure 1). There are six boreholes for each sectionexcept for MD, which has five. Distances betweenadjacent boreholes range from 200 to 500 m. In addition,four river samples were collected from the middle reachesand one was collected from the lower reaches.

Depth of boreholes, groundwater depth, location (byGPS), water temperature, pH, TDS and electrical con-ductivity (EC) were measured at each site. Samples of50 ml were collected for stable isotopes measurements inthe Stable Isotopes Laboratory, Institute of Geology andGeophysics, Chinese Academy of Sciences. 2H/1H and18O/16O were measured by isotope ratio mass spectrom-etry (MAT253) by chrome reduction and equilibrationwith CO2, respectively. Results are reported as υ18O andυ2H �υ D �Rsample/Rstandard –1� ð 1000� with respect to

the Vienna Standard Mean Ocean Water (VSMOW). Pre-cision of measurements for stable isotope is š0Ð02‰ forυ18O, and š0Ð1‰ for υ2H. Water samples of 500 ml werecollected for tritium measurement in the GroundwaterTracing Laboratory, Institute of Geology and Geophysics,Chinese Academy of Sciences. Tritium was prepared byelectrolytic enrichment with a tritium enrichment factorof 20 and measured by liquid scintillation counter (Quan-tulus 1220) with a precision of š0Ð3 TU (tritium unit).Water chemistry was measured using ion chromatography(Dionex-500) at the Beijing Research Institute of Ura-nium Geology. Cation measurements were standardizedagainst the National Analysis Standard DZ/T0064Ð28–93and anions against DZ/T0064Ð51–93. Alkalinity wasmeasured with automatic titrator (785 DMP). The aver-age ionic balance was š5Ð8% except for six samples, inwhich it was larger than š7%.

RESULTS AND DISCUSSION

The field measurements and isotope results are shown inTable I. Water chemistry results are shown in Table II.Groundwater residence times, groundwater recharge andhydrochemical characteristics relative to vegetation in themiddle Tarim River are discussed below on the basis ofthese results.

Groundwater residence time: tritium

Tritium, the radioactive isotope of hydrogen of massthree with a half-life of 12Ð32 years, is an important trac-ing element to determine recent groundwater recharge,movement and residence times for surface and ground-waters in hydrological studies (Cook and Walker, 1996;Solomon and Cook, 2000; Michel, 2005). For such appli-cations, the tritium input sequence is needed for quan-titative interpretation of surface and groundwater resi-dence times from the pattern of tritium concentrationsalong a flow path. However, tritium in precipitation isnot locally available in most regions of the world. Itis necessary in this case to extrapolate data from moredistant locations. In our study area, Urumqi and Hotanstations are involved in the Global Network of Isotopesin Precipitation (GNIP), with monthly precipitation tri-tium contents available from 1986 to 2001. Wei et al.(1980) developed models for reconstructing tritium inprecipitation in China according to the linear relationshipbetween tritium and both increasing latitude and increas-ing distance from Hong Kong with long-term data. Guan(1986) developed a model using tritium records in sev-eral pluviometers. The tritium sequence in Lop Nur (eastXinjiang) from 1952 to 1996 constructed by Jiao et al.(2004) is used in this study, incorporating the Urumqi tri-tium data from 1986 to 2001 (IAEA and WMO, 2006).The data in the overlapped period (1986–1996) is similarfrom both results. Tritium in precipitation has decreasedfrom 2586 TU in 1963 to 25 TU in 2001 (Figure 4).Using an exponential decay equation with a half-life of12Ð32 years, the decayed precipitation tritium for 2007,



Table I. Field measurements and isotope composition of Tarim River and groundwater sections

Samplecode

Waterdepth (m)

Distanceto river (m)

TDS(g/l)

EC(µs/cm)

Temp(°)

pH υ18O(‰)

υ2H(‰)

3H(TU)

Aksu River

A1 0Ð217 0Ð451 30Ð7 7Ð8 �9Ð9 �73Ð7Tarim River

T1 0Ð608 1Ð240 31Ð1 7Ð6 �10Ð0 �74Ð9R8 0Ð666 1Ð345 23Ð0 8Ð8 �8Ð4 �58Ð7 18Ð4R9 0Ð642 1Ð296 24Ð8 8Ð0 �8Ð3 �57Ð9 20Ð5R7 0Ð631 1Ð276 27Ð7 8Ð5 �7Ð8 �54Ð9 20Ð2R6 0Ð617 1Ð243 24Ð8 8Ð1 �8Ð5 �59Ð4 18Ð8R5 0Ð727 1Ð449 26Ð5 8Ð1 �8Ð1 �56Ð7 19Ð9MA: Shajilike groundwater wells

W56 4Ð47 450 0Ð794 1Ð597 18Ð4 7Ð3 �9Ð3 �69Ð8 50Ð0W57 5Ð02 650 0Ð500 1Ð020 16Ð7 7Ð5W58 3Ð54 850 1Ð318 2Ð59 17Ð7 7Ð2 �9Ð5 �72Ð0 1Ð3W59 5Ð62 1150 4Ð45 8Ð30 17Ð2 6Ð7W60 5Ð66 1350 4Ð56 8Ð47 19Ð0 7Ð1 �9Ð7 �72Ð8 <0Ð3W61 6Ð12 1850 2Ð39 4Ð58 17Ð2 7Ð0 �9Ð7 �75Ð9 2Ð9MB: Shazihe groundwater wells

W50 6Ð55 850 7Ð26 13Ð18 15Ð2 6Ð6 �7Ð9 �60Ð2 <0Ð3W51 6Ð86 1050 10Ð47 18Ð48 16Ð8 6Ð9 �8Ð2 �62Ð4 7Ð1W52 7Ð26 1300 8Ð52 15Ð3 6Ð9W53 7Ð92 1600 0Ð790 1Ð416 16Ð4 7Ð2W54 8Ð00 1800 2Ð47 4Ð74 16Ð4 7Ð1 �8Ð2 �62Ð8 <0Ð3W55 7Ð76 2300 2Ð82 5Ð40 13Ð7 7Ð2 �7Ð8 �60Ð1 <0Ð3MC: Wusiman groundwater wells

W44 3Ð20 50 1Ð773 3Ð44 19Ð0 7Ð7 �7Ð6 �58Ð7 23Ð7W45 3Ð09 250 3Ð02 5Ð74 18Ð6 7Ð4 �8Ð3 �62Ð9 110Ð1W46 2Ð46 500 1Ð565 3Ð05 20Ð8 7Ð7W47 3Ð20 750 1Ð322 2Ð60 18Ð9 7Ð5 �8Ð2 �62Ð3 52Ð2W48 3Ð98 950 1Ð699 3Ð30 18Ð7 7Ð6W49 5Ð10 1300 2Ð70 5Ð14 16Ð5 7Ð7 �8Ð2 �60Ð4 94Ð4MD: Aqike groundwater wells

W39 2Ð40 250 1Ð641 3Ð21 16Ð6 7Ð3W40 3Ð12 650 2Ð43 4Ð65 19Ð7 7Ð3 18Ð4W41 5Ð44 950 2Ð86 7Ð3W42 2Ð83 1150 1Ð438 2Ð89 19Ð2 7Ð4W43 4Ð98 1700 4Ð72 8Ð75 17Ð3 7Ð1 0Ð4ME: Tieyizi groundwater wells

W33 2Ð97 310 1Ð485 2Ð91 17Ð3 7Ð5 �8Ð2 �62Ð5 43Ð9W34 4Ð00 500 0Ð825 1Ð655 17Ð6 7Ð8W35 4Ð28 700 1Ð503 3Ð00 17Ð6 7Ð8 �8Ð2 �62Ð5 4Ð7W36 4Ð74 1050 5Ð64 10Ð30 18Ð2 7Ð3 �8Ð6 �64Ð2 10Ð0W37 4Ð26 1200 12Ð46 21Ð70 18Ð4 �8Ð3 �63Ð6 5Ð3W38 3Ð00 1600 10Ð55 18Ð53 19Ð2 7Ð5 �7Ð3 �56Ð1 <0Ð3

which would represent tritium concentrations in ground-water that had infiltrated between 1952 and 2001, iscalculated (Figure 4). The decayed tritium content of pre-cipitation changed from 50 TU in 1962 to 225 TU in1966, and 10 TU in 1967 to 35 TU in 2001. In thisstudy, tritium is mainly used to distinguish the tritiumpeak and pre-modern water.

Tritium contents for the Tarim River samples rangefrom 18Ð4 to 20Ð5 TU with an average of 19Ð6 TU.The tritium contents for surface waters from the adjacentKaidu-Kongque River range from 26Ð9 to 28Ð3 TU,higher than those of the Tarim River. In addition, tritiumcontents for the Tarim River were from 21Ð2 to 55Ð3 in

2001 and 25Ð3 to 52Ð1 TU for the Aksu River in the sameyear (Table III). Tritium in precipitation generally rangesfrom 20 to 40 TU and in some cases up to 60 TU from2001 to present, based on tritium content from rivers.This high tritium content may be related to high latitude,nuclear tests and arid climate (Liu, 2001).

Tritium content was measured for the five transects(Table I, Figure 5). In the Shajilike section (MA), thenearest groundwater sample from the river (sample w56,450 m away from the river) has a tritium content of 50Ð0TU, indicative of recharge by modern river water. Exceptfor sample w57, which was not measured, each of the oth-ers (w58, 850 m; w60, 1350 m; w61, 1850 m) has low



Table II. Chemical composition of Tarim River and groundwater sections (mg/l)

Sample Ca2C Mg2C NaC KC HCO3� Cl� SO4

2� F� NO3�

Tarim River

R8 67Ð9 33Ð3 153 9Ð2 132 287 247 0Ð96 2Ð58R9 67Ð1 33Ð3 160 10Ð1 128 272 233 1Ð08 2Ð46R7 73Ð3 35Ð8 150 13Ð8 115 238 265 0Ð95 0Ð76R6 65Ð0 31Ð2 150 10Ð0 130 264 234 0Ð72 2Ð72R5 67Ð3 37Ð5 185 12Ð4 128 326 254 1Ð12 2Ð55

MA: Shajilike groundwater wells

W56 131 83Ð9 153 14Ð8 203 349 282 1Ð34 1Ð25W58 93Ð3 84Ð8 434 21Ð4 692 474 380 0Ð64 2Ð04W60 130 342 1301 25Ð7 703 2155 1479 10Ð4 5Ð00W61 110 128 713 22Ð2 371 1122 941 1Ð64 0Ð56

MB: Shazihe groundwater wells

W50 433 543 2168 88Ð3 704 4030 2870 2Ð15 non-dectW51 606 836 3417 69Ð8 664 5644 4993 non-dect non-dectW54 173 233 544 34Ð7 420 1266 970 1Ð14 1Ð46W55 172 224 743 34Ð8 577 1271 1169 3Ð14 0Ð92

MC: Wusiman groundwater wells

W44 82Ð0 77Ð2 540 18Ð7 360 741 607 1Ð02 0Ð82W45 224 180 804 29Ð3 365 1216 1316 1Ð80 non-dectW47 132 95Ð6 300 24Ð3 323 559 521 1Ð06 1Ð26W49 181 125 806 26Ð6 432 1219 1325 3Ð28 4Ð34

ME: Tieyizi groundwater wells

W33 155 122 335 30Ð7 233 745 560 0Ð50 0Ð80W35 73Ð4 64Ð6 506 21Ð1 248 701 619 2Ð95 1Ð06W36 310 263 1864 67Ð8 402 2939 2106 2Ð65 2Ð35W38 258 459 3816 114 843 6216 2909 4Ð00 2Ð00

Figure 4. The precipitation tritium input from 1952 to 2001 and thedecayed tritium content of precipitation to 2007 (1952 to 1996 fromJiao et al. (2004) and 1986 to 2001 from IAEA and WMO (2006) with

similar data in share period (1986–1996))

tritium content (1Ð3 TU, <0Ð3 TU, 2Ð9 TU, respectively),indicative of pre-modern water. The groundwater depthfor these pre-modern groundwaters ranges from 3Ð54 to6Ð12 m.

In the Shazihe section (MB), the nearest groundwatersample to the river (w50, distance 850 m) is tritium-free.Tritium content for sample w51 is 7Ð1 TU, while w54 andw55 were both tritium-free. All of these groundwaters

Figure 5. Post map of tritium content (TU) in groundwater with a solidline showing the scope of modern recharge

were apparently recharged before the 1950s, and sam-ple w51 may have included a small component of directrecharge. The boundary for groundwater receiving mod-ern recharge is approximate 600 m from the riverbank inthe MB section. Beyond 600 m, the vegetation is main-tained by pre-modern water and thus is not sustainable.The groundwater depths range from 6Ð55 to 8Ð00 m andthe ecosystem has degenerated seriously.

In the Wusiman section (MC), the groundwater depthabout 1000 m from the river is relatively shallow



Table III. Stable isotopes and tritium of the Tarim River and its headwaters (Aksu River)

Basin Sampling location υ18O (‰) 3H (TU) Sampling time Source

Aksu River Tailan River �10Ð5 35Ð4 2001 Li et al. (2006)Aksu river in Weisu �10Ð9 38Ð2 2001 Li et al. (2006)

Tuoshigan River �8Ð9 29Ð1 2001 Li et al. (2006)Xidaqiao (A1) �9Ð9 25Ð3 2001 Li et al. (2006)

Lower Aksu River �10Ð0 52Ð1 2001 Li et al. (2006)Tarim River Aral �10Ð5 5/1989 Liu et al. (1997)

Aral �9Ð9 10/1989 Liu et al. (1997)Aral �9Ð4 21Ð2 2001 Li et al. (2006)

Tarim River (R8) �8Ð8 55Ð3 2001 Li et al. (2006)

(2–4 m). At borehole w49, 1300 m from the river, thedepth is 5Ð10 m. Tritium content in the section is high(Figure 5). Sample w44, very near to the river, wasrecharged recently. The tritium content from the lattersamples is extremely high and was recharged in the tri-tium peak period. The main reason for more recharge bymodern water in the MC section is due to the presenceof a river oxbow that has apparently increased aquifertransmissivity.

In the Aqike section (MD), the groundwater depth atabout 1700 m from the river is 4Ð98 m at borehole 43. Itcontains pre-modern recharge (0Ð4TU). At borehole w40,the groundwater depth is 3Ð12 m. Tritium content in thesection is high (18Ð4TU). At a distance of 1200 m tothe river, groundwater was recently recharged. Within thedistance, vegetation cover is relatively dense (Liu et al.,2007).

Within section Tieyizi (ME), only w33 (43Ð9 TU)appears to have been recharged by modern water. Othergroundwater samples in the section are pre-modern, basedupon low tritium content and apparent desertification.Two groundwater samples of Aqike (MD) were not anal-ysed for tritium. Groundwater depths in this section rangefrom 2Ð40 to 5Ð44 m, and vegetation cover is relativelydense (Liu et al., 2007). The groundwaters of this sectionare very likely to be recharged by modern water consid-ering that there are eco-gates distributed in the section.

Summarizing the analysis above, recharge from mod-ern groundwater is restricted to within about 600 m ofthe river in sections MA, MB and ME, about 1500 m inMC and about 1200 m in MD (Figure 5). Groundwaterfurther beyond this is pre-modern.

Groundwater recharge: stable isotopes

Stable isotopes in precipitation. Isotope trends in ter-restrial waters are examined relative to meteoric trendsincluding local (LMWL) and global meteoric water lines(GMWL) (Craig, 1961). The Urumqi and Hotan sta-tions are both located at lower elevations than therunoff-generating mountainous area. The Urumqi station(43Ð78N, 87Ð62E, 918 m above sea level) is located tothe north of the Tianshan Mountains and has monitor-ing data available from January 1986 to December 2001(107 samples in total). The υ18O values from this dataset range from �28Ð0‰ to 1Ð8‰with an average of�10Ð8‰, while υ2H ranges from �204Ð5‰ to �8Ð9‰

with an average of �74Ð4‰ (flux-weighted mean). Theaverage deuterium excess (υ2H-8υ18O, Dansgaard, 1964)is 11Ð6‰. The Urumqi local meteoric water line can bedescribed by the following Equation:

υ2H D 7Ð15υ18O C 1Ð41, R2 D 0Ð95, n D 107 �1�

Hotan station (37Ð08N, 79Ð56E, 1375 m a.s.l.), locatedto the north of the Kunlun Mountains, had a monitoringperiod from January 1988 to December 1992 (47 samplesin total). The υ18O values range from �29Ð8 to 3Ð8‰withan average of �5Ð7‰, while υ2H ranges from �23Ð6to 45Ð5‰with an average of �35Ð5‰. The averagedeuterium excess is 10Ð6‰. The Hotan local meteoricwater line can be described by the following equation:

υ2H D 8Ð40υ18O C 11Ð44, R2 D 0Ð99, n D 47 �2�

The variability in oxygen and hydrogen delta valuesfrom individual rainfall events is very large at bothstations. Generally speaking, the stable isotope values ofprecipitation are mainly controlled by air temperature,moisture source and precipitation history (Dansgaard,1964; Rozanski et al., 1993; Clark and Fritz, 1997).In middle and high latitude regions, stable isotopes inprecipitation have a notable relationship with temperature(Dansgaard, 1964; Rozanski et al., 1993). The heavyoxygen isotope is enriched in the summer and relativelydepleted in winter in both stations. υ18O has a relativelystrong correlation with air temperature (r D 0Ð86) in theUrumuqi station, with a gradient of 0Ð41 ‰/ °C comparedto the global average of 0Ð6 ‰/ °C (Rozanski et al.,1992).

There is a difference in air masses for the two sta-tions. In summer, air masses from the Atlantic Ocean(westerly) control the precipitation in the Tianshan andKunlun Mountains, and the latter receives some addi-tional moisture from the Arabian Sea. In winter, whilethe westerly flow also controls the major part of westernChina. Polar air mass also contributes to the precipita-tion in northern Xinjiang (Tian et al., 2007). As none ofthem is considered more representative for Tarim Basin interms of location, it is difficult to justify the use of eitherone against the other to be the local meteoric water line;we have chosen to use the Global Meteoric Water Lineas a reference line for our discussions below.



Stable isotopes in river and groundwater. The υ18Ovalues of river water samples collected during the sum-mer wet season range from �7Ð8 to �8Ð5‰ with anaverage of �8Ð2‰; υ2H values range from �59Ð4 to�54Ð9‰ with an average of �57Ð5‰. They plot nearthe GMWL (Figure 6). The average deuterium excess is8Ð1‰. The regression line is

υ2H D 7Ð18 ð υ18O C 1Ð41, n D 5, R2 D 0Ð998 �4�

with the slope of 7Ð18, close to that of the precipitationline for Urumqi. This indicates that little evaporation isinvolved due to the relatively fast flow of the river.

To understand seasonal and annual changes, compar-ison is made with river waters from the Aral hydro-logic station (T1) and Luntai (R8) and Aksu river basins(Table III). Overall, a large υ18O isotope variation, from�10Ð5 to �7Ð8‰, is observed. It is probably larger inwinter due to temperature effects and glacier melt modi-fication. From the Aksu River to the Tarim River, isotopesgenerally evolve towards greater enrichment (Figure 7).Because there is no water flowing into those rivers exceptfor mountain runoff, the trend is probably caused byevaporation and mixing with irrigation return. This is

Figure 6. Stable isotope composition of the Tarim River and groundwa-ters from the five sections in the middle reaches

Figure 7. Oxygen isotope and salinity changes from Aksu River to theLower Tarim River compared to those of groundwater

supported by a concurrent trend of increasing TDS (seebelow).

In the five transects with groundwater stable isotopeand tritium analysis, υ18O ranges from �9Ð7 to �7Ð3‰,while υ2H ranges from �75Ð9 to �56Ð1‰ (Figure 6).The isotopes in groundwater are defined by υ2H D7Ð53υ18O–0Ð64 with a regression coefficient of 0Ð98. Deu-terium excess ranges from 1Ð7 to 4Ð9‰ with an averageof 3Ð4‰, which is far less than that of the Tarim River(8Ð1‰). The regression line is not considered to be anevaporation line on the basis that the slope is 7Ð53, whichis slightly greater than the slope of the regression line forriver waters (7Ð18). On the basis of the isotope changesfrom Aksu to the Tarim River, groundwaters appear to beevaporated after river infiltration and mixing with riverrecharge of different isotope compositions from differentseasons. Averaging the oxygen isotope values for eachsection, there is an increasing trend along the directionof river flow (Figure 7). The average υ18O for MA sectionis �9Ð5‰ and �8Ð1‰ for the lower sections (MB, MCand ME). The oxygen isotope trends in groundwater cor-respond to that of the river water, suggesting that theriver recharges groundwater and that the deviation maybe caused by a complex recharge regime.

Chemical characteristics of groundwater

TDS values for the Tarim River samples in the wetperiod range from 0Ð617 to 0Ð727 g/l with an averageof 0Ð657 g/l. This is consistent with recent work byFan et al. (2002) who showed that the average monthlymineralization at all Tarim River sections since 1958was greater than 1 g/l over the entire year except forthe flooding season in August due to runoff drop andsalt drainage of the main canals. The average pH is 8Ð3.The Piper plot for the Tarim River samples (Figure 8)shows that Na C K are the dominant cations (about50% in meq/l) and the dominant anions are Cl and SO4

(more that 25% in meq/l) with a water type of Cl-SO4-Na-(Ca). The TDS for Xidaqiao (A1) in Aksu Basin isas low as 0Ð217 g/l. TDS in river water also shows anincreasing trend from the headwaters to lower reaches(Figure 7) due to soil salt input and evapoconcentration.Nitrate in the Tarim River and groundwater is low,ranging from non-detectable to 4Ð34 mg/l (w49) with anaverage of 1Ð67 mg/l. The fluoride for most groundwateris less 4Ð0 mg/l, except 10Ð4 mg/l for w60. The fluorideconcentration in 6 of 16 samples ranges from 1 to 2 mg/land 6 samples lie within the range of 2 to 4 mg/l.The ratios of F to Cl range from 0Ð0027 to 0Ð0040 forthe Tarim River and ratios for most groundwaters arealso within this range [0Ð0005 (w50) to 0Ð0048 (w60)].Therefore, the high fluoride concentration in groundwateris mainly due to evapoconcentration of river water withhigh fluoride concentration. One reason for the highfluoride is that there is abundant fluoride in metasomaticdeposits in intrusive contact with carbonate, acid igneousand Jurassic coal layers in some cases. Another reason isextensive surface water–groundwater interactions in themountain headwater areas.



Figure 8. Piper plot of groundwater and river water

Figure 9. Tritium versus TDS of groundwater

TDS of groundwaters for all sections range from 0Ð50to 12Ð46 g/l. The major anion is Cl (up to 50% formost groundwaters) and the major cations are NaCKand Mg, forming Cl-SO4-Na-Mg type water. The areanearest the river (within 800 m) has low TDS. The area1000 š 200 m from the river has a relative high TDSand beyond 1400 m there is relatively low TDS exceptfor Transect ME. Low TDS nearest the river results fromrecharge by modern river water with low TDS (area Ain Figure 9). Beyond 1400 m from river, the pre-modernwaters (area B in Figure 9) are low in TDS due to lowcirculation rates and minimal evaporation from the watertable. The intermediate pre-modern groundwater (area Cin Figure 9) is most likely to have been affected by bothevapotranspiration and dissolution because of relativelyshallow water table and long residence time.

Deuterium excess (d) is a valuable indicator of ground-water salinization mechanisms. As deuterium excesswould decrease during evaporation and is less affectedby the isotope composition of initial water, it is poten-tially useful for discriminating contributions of evapo-concentration and mineral dissolution for a given watersample. The relationship between the initial reservoirand deuterium excess can be deduced from the Rayleigh

Figure 10. Deuterium excess versus TDS for groundwaters, river watersand Boston Lake

distillation equation. The contribution of evapoconcen-tration and dissolution in a given water body can thenbe determined by comparing the salinity with the ini-tial water. The relationship between deuterium excessand TDS for groundwaters, the Tarim River, the KaiduRiver and Boston Lake for a relative humidity of 75%and temperature of 25° is shown in Figure 10. Evapo-ration enriches the salinity and heavy isotopes. Owingto incomplete mixing, there is a difference between thelake waters. The evapoconcentration trendline of salinityagainst deuterium excess shows the evaporation effect inthe study area, though some of the salinity for the lakewaters is caused by mineral dissolution. The saliniza-tion of the Tarim River mainly appears to be controlledby dissolution of soluble minerals with little evaporationeffect. As evaporation decreases the d value significantlybut increases salinity little, the relationship suggests con-tributions of both evapo-concentration and mineral disso-lution, with the latter as the dominant process for ground-water.

Most of the salinity in rivers is from precipitationinput and weathering (Zhao et al., 2007). The atmo-spheric input may be significant in some cases (Stallardand Edmond, 1981). Salinity in precipitation can reachseveral tens of milligrams per litre, even up to 200 mg/lin arid inland basins in China (Xu et al., 2000). Atmo-spheric inputs are usually not thought to be significant inhigh-TDS rivers and groundwaters. The relation betweenNa/(NaCCa) or Cl/(ClCHCO3) and TDS can be usedto determine the factors of dissolution evolution (Gibbs,1970). Results for groundwaters and the Tarim Riverwater are indicative of evaporation/fractional crystalliza-tion dominance. The main factor controlling groundwaterand Tarim River water is mineral dissolution, which isconsistent with evidence from deuterium excess versusTDS discussed above.



Abundant salt in the unsaturated zone and low per-meability of the aquifer causes high salinity of ground-water in the middle Tarim River. It is also the terminusof water and salt of the fluvial and lacustrine depositsfrom the Tianshan Mountains. The soluble salts in thesoils/aquifers in the middle Tarim River are mainly evap-orites (NaCl, CaXMg�1–X�SO4) and carbonate minerals(CaXMg�1–X�CO3) due to long-term concentration andaccumulation. Halite and gypsum are the most commonevaporites in the region (Zhang et al., 1995), though somesulfates of Ca, Mg, Na and K are also present (Okadaet al., 1992). Groundwater salinity in the lower reachesfurther increases as salt accumulates downstream (Cheng,1993; Li et al., 2010).

The compositional relations among dissolved speciescan reveal the origin of solutes (Fisher and Mullican,1997; Herczeg and Edmunds, 2000). The processes ofdissolution, precipitation and cation exchange are activelytaking place within the groundwater system (Adamset al., 2001). Chloride, plotted against Na in Figure 11a,is used as an indicator of evaporation source. Thesedata show an approximately 1 : 1 trend line. Na and Clare mainly from halite, suggesting a mainly evaporiticsource for groundwaters. A plot of Ca C Mg and SO4

(Figure 11b) also shows an approximately 1 : 1 trend line.SO4 is probably from sulfates of Ca (as gypsum) and

Mg, while Ca and Mg are also partly from carbonates,consistent with Figure 11c showing that Ca C Mg islarger than HCO3. Saturation indices (SI) are used todetermine mineral dissolution or precipitation. The SI ofthe Tarim River water and groundwaters is calculatedwith PHREEQC software (Parkhurst and Appelo, 1999).All waters are undersaturated with respect to gypsumand supersaturated with respect to calcite and dolomite(except for sample w61). Saturation of calcite anddolomite controls the concentration of HCO3. The ratio ofCa to Mg is inversely correlated with TDS (Figure 11d).This is indicative of incongruent dissolution, with pro-gressive enrichment of Mg over Ca consistent withCa/Mg’s ratio being less than 1 for groundwaters:

CaMg�CO3�2 C Ca2C ��! Mg2C C 2CaCO3 �1�

Silicate weathering is also implied but this is moredifficult to model (Sarin et al., 1989; Zhang et al., 1995).

GROUNDWATER AND ECOSYSTEM

Both natural and human influences affect temporal andspatial characteristics of vegetation changes in the mid-dle reaches of the Tarim River (Wang et al., 2002). Usingremote sensing and GIS, Chen (2007) analysed land cover

Figure 11. Ionic ratio for the Tarim River and groundwaters



changes in the middle reaches of the Tarim River andshowed that the area of vegetation is declining and deser-tification has increased by 14% from 1992 to 2000 inriparian areas. Large amounts of water were consumedand salts were deposited as result of cropland increasefrom 1992 to 2000. The ecosystem is maintained bygroundwater recharged from the Tarim River. Ground-water depth from the five sections illustrates that ground-water is deeper with increasing distance from the river(Table I).

On the basis of data from 12 monitoring sectionswith 58 monitoring wells and 58 vegetation sample plotslocated in the middle and lower reaches of the TarimRiver (Chen et al., 2003; Xu et al., 2004), ¾3Ð5 mis regarded as the lowest groundwater level acceptablefor restoration of the natural meadow vegetation and¾5Ð0 m for Populus euphratica Oliv. Below this, itis difficult for the seeds to germinate and for youngtrees to grow, though this range of groundwater depthis suitable for the larger and older Populus euphraticaOliv. to grow. The vegetation in section MA and MEfurther confirms this. Young Populus euphratica Oliv. andTamarix ramosissima are dying, as well as large Populuseuphratica Oliv., despite dense vegetation in MA. In ME,the area surrounding groundwater sample w33, rechargedby modern water, is covered by large shrubs and herbs,but the surrounding area without modern recharge hassparse vegetation and shows signs of desertification.

Liu et al. (2008) investigated 30 plots of naturalplant communities around the boreholes in the middlereaches of the Tarim River and analysed the communitystructure and species diversity. Table IV lists the averagegroundwater depth, the distance to the river for theboundary of pre-modern versus modern groundwaterand vegetation important value, which is a measure ofsignificance of a species in its community defined as thesum of relative coverage, relative density and relativefrequency by Curtis and McIntosh (1951), and richnessindices of vegetation at the five sections in the middlereaches of the Tarim River. There is a good linear relationamong groundwater depth, scope of modern rechargeand vegetation coverage due to moisture content of theunsaturated zone by sustained capillary water relative tothe groundwater level. For MA and MB with deepergroundwater depth and limited scope of modern recharge

(within 600 m), the species of the largest important valueis Populus euphratica Oliv., while for shallow depths itis Phragmites cornmurlis. The habitat condition of MCand MD is better than the others, with the groundwaterrecharged by modern water. In contrast, those of MB andME are the worst. Hao et al. (2010) also shows that thevegetation coverage exhibits a continued declining trendwith dropping water level.

CONCLUSIONS

Tritium, stable isotopes and water chemistry data obtainedin this study have enabled investigation of ripariangroundwater recharge and evolution in the middle TarimRiver. The following conclusions can be drawn from theresults currently available:

At the catchment scale, TDS and stable isotopes of theTarim River become enriched from the headwaters to thelower reaches. Groundwater is recharged by the river inthe middle reaches, which is evidenced by variable butcorrespondingly enriched isotopes at different sections.There is a discernable boundary between modern andpre-modern water determined by tritium content, whichis between 600 and 1500 m measured from the riverbank.The recharge mechanism has caused heterogeneous dis-tribution of water chemistry. The TDS of groundwateralong the lateral transects starting from the riverbank fluc-tuates from low to high, and back to low, with increasingdistance. This pattern is probably a combined effect ofriver/groundwater circulation regime, mineral dissolutionand evapotranspiration.

Groundwater depth increases with increasing distancefrom the river, corresponding to more vegetation degener-ation. Correlation between tritium content and vegetationdensity has been observed. Hence, deep groundwater lev-els and low tritium contents can be considered good‘warning signs’ for ecosystem deterioration.

As tritium content is still high in the water cycle inthe Tarim Basin, monitoring tritium together with stableisotopes in precipitation and river water in the basinand headwater areas will further contribute to a betterunderstanding of eco-hydrological processes and waterresources management.

Riparian vegetation in the middle Tarim River is alsofragile and under stress. Degrading natural vegetation

Table IV. The average important value and richness indices of five inspective sections and groundwater depth for the middle reachesof Tarim River

Section Averagegroundwater

depth (m)

Scope ofmodern recharge

(m)

Vegetationcoveragea

Specie of thefirst important

valuea

Specie of thesecond important

valuea

Richnessindicesa

MA 5Ð1 600 0Ð383 PeO: 1.91 TR: 0.75 2Ð7MB 7Ð4 600 0Ð259 PeO: 1.19 KC: 0.94 4Ð2MC 3Ð5 1500 0Ð563 PC: 1.37 PeO: 0.79 4Ð7MD 3Ð8 1200 0Ð431 PC: 1.43 PeO: 1.13 5Ð5ME 3Ð9 600 0Ð292 PC: 1.45 TR: 0.76 4Ð1

PeO: Populus euphratica Oliv.; TR: Tamarix ramosissima; KC: Karelinia caspic; PC: Phragmites cornmurlis.a data from Liu et al. (2008).



gives rise to desertification, which has affected not onlythe lower but also the middle reaches of the river.Because the area of vegetation in the middle reaches isin fact much larger than that of the lower reaches, dueattention should be paid to this issue when regulatingthe water resources of the river. Negative impacts of theembankment need further investigation and evaluation inorder to establish suitable mitigation strategies.

ACKNOWLEDGEMENTS

The research is supported by the Knowledge InnovationProgram of the Chinese Academy of Sciences (kzcx2-yw-127) and National Natural Science Foundation ofChina (Grant No. 40672171). The authors wish to thankXiaoming Zhou and Xinhe Jiang for their assistance inthe field work. We thank John Gates for reading earlierversions of the manuscript to improve the English andfor his valuable comments on the content.

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