Environ Geol (2008) 55:1109–1122 DOI 10.1007/s00254-007-1059-1

ORIGINAL ARTICLE

Recharge source and hydrogeochemical evolution of shallow groundwater in a complex alluvial fan system, southwest of North China Plain Fadong Li Æ Guoying Pan Æ Changyuan Tang Æ Qiuying Zhang Æ Jingjie Yu

Received: 9 March 2007 / Accepted: 1 October 2007 / Published online: 18 October 2007 Ó Springer-Verlag 2007

Abstract Many cities around the world are developed at alluvial fans. With economic and industrial development and increase in population, quality and quantity of groundwater are often damaged by over-exploitation in these areas. In order to realistically assess these groundwater resources and their sustainability, it is vital to understand the recharge sources and hydrogeochemical evolution of groundwater in alluvial fans. In March 2006, groundwater and surface water were sampled for major element analysis and stable isotope (oxygen-18 and deuterium) compositions in Xinxiang, which is located at a complex alluvial fan system composed of a mountainous area, Taihang Mt. alluvial fan and Yellow River alluvial fan. In the Taihang mountainous area, the groundwater was F. Li Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences (CAS), Shijiazhuang 050021, China F. Li (&) Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan e-mail: [email protected]; [email protected]

recharged by precipitation and was characterized by Ca– HCO3 type water with depleted d18O and dD (mean value of –8.8% d18O). Along the flow path from the mountainous area to Taihang Mt. alluvial fan, the groundwater became geochemically complex (Ca–Na–Mg–HCO3–Cl–SO4 type), and heavier d18O and dD were observed (around –8% d18O). Before the surface water with mean d18O of –8.7% recharged to groundwater, it underwent isotopic enrichment in Taihang Mt. alluvial fan. Chemical mixture and ion exchange are expected to be responsible for the chemical evolution of groundwater in Yellow River alluvial fan. Transferred water from the Yellow River is the main source of the groundwater in the Yellow River alluvial fan in the south of the study area, and stable isotopic compositions of the groundwater (mean value of –8.8% d18O) were similar to those of transferred water (–8.9%), increasing from the southern boundary of the study area to the distal end of the fan. The groundwater underwent chemical evolution from Ca–HCO3, Na–HCO3, to Na–SO4. A conceptual model, integrating stiff diagrams, is used to describe the spatial variation of recharge sources, chemical evolution, and groundwater flow paths in the complex alluvial fan aquifer system.

G. Pan Institute of Resources & Environment, Henan Polytechnic University, Jiaozuo 454000, China

Keywords Groundwater recharge
Hydrogeochemistry evolution
Stable isotope oxygen-18
Deuterium
Complex alluvial fan

C. Tang
Q. Zhang Faculty of Horticulture, Chiba University, Chiba 271-8510, Japan

Introduction

J. Yu Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographical Sciences and Natural Resources Research, CAS, Beijing 100101, China

Alluvial fans form vital aquifer systems supporting local development in agriculture, industry and socio-economy. Groundwaters in alluvial fans represent important water resource in semi-arid and semi-humid areas in China

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(Foster et al. 2004; Zhang et al. 2000, 2004) and worldwide (Bull 1977; Robertson 1991; Stimson et al. 2001). The Quaternary Aquifer in the North China Plain (NCP) (Fig. 1) is one of the largest aquifer systems in the world (Foster et al. 2004; Zhang et al. 2000). Owing to the abundance of groundwater resources, the alluvial fans associated with this formation are ideal sites for developing cities including Beijing, Tianjin, and Shijiazhuang municipalities in NCP (Fig. 2). Research on palaeochannels shows that the alluvial fans in NCP were formed by depositional processes of the Yellow River, the Haihe River, and the Luanhe River since the late Pleistocene (Wu 1996; Wu et al. 1996a, b; Xu et al. 1996). Before B.C. 602, the Yellow River flowed toward the northeast along the east of Taihang Mt. across the south of Xinxiang, east of Anyang, west of Hengshui, and then converged with Haihe River. Impacted by anthropogenic and natural factors, the route of the Yellow River has been changed up to 1,600 times since B.C. 602 (Yao 1987). The seven vital channel changes were described by Yao (1987), Sun (1987), and Tang and Xiong (1998). During this period, sediment originating from Taihang Mt. gradually increased, and the Taihang Mt. alluvial fans were developed quickly, pushing the palaeochannel of the Yellow River eastward (Tang and Xiong 1998; Wu et al. 1996b). As a result, the flow direction of the Yellow River was changed from northward before A.D. 70, towards the east (A.D. 70–1048), south (A.D. 1049 to 1854), and then the east again from A.D. 1855 to present. The sediment of the Yellow River formed its alluvial fan on the palaeochannels (Wu et al. 1996b). Consequently, the complex alluvial fan system including the mountainous area, the Taihang Mt. alluvial fan, and the Yellow River alluvial fan were formed along the Taihang Mt. (Fig. 2). As one of Chinese most important social, economic, and agricultural regions, the NCP is facing an increase in water demands associated with rapid population increase, industrial development, and expansion of irrigated land which has led to overexploitation of surface and ground water resources with resultant water-shortage and poor water quality (Liu et al. 2001). Consequently, the shallow groundwater table declined significantly, at a mean rate of approximately 1 m/year in this area over the past several decades, and the area of the groundwater cone of depression has expanded more than 14,000 km2 (Chen 1999; Yang et al. 2002; Zhang et al. 2002). This trend can be also found in most developing small- to medium-sized cities such as Xinxiang in the NCP. From 1974 to 2005, the water table in Xinxiang declined by 6 to 18 m with a depression cone area of 40 km2, associated with severe groundwater pollution (Pan 2006). In Xinxiang, the hydrogeochemical characteristics of groundwater around the alluvial fan have been a subject of several recent surveys

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Fig. 1 Location of the study area and North China Plain (NCP)

(HGMRPDB and XWSO 1988; Pan 2006; XWSO 2000), but few detailed studies have been done to identify the recharge sources and the hydrogeochemical characteristics of groundwater. Hydrogeochemical and isotopic methods have been popular tools for groundwater research all over the world (Guendouz et al. 2003). Allison (1988) reviewed the physical, chemical, and isotopic techniques available to evaluate groundwater recharge in arid environments. Recently, Glynn and Plummer (2005) gave a comprehensive review of the significant contributions of geochemistry to the understanding of groundwater systems over the past 50 years. Furthermore, hydrogeochemical and isotopic methods have been successful as economical ways to study local and regional groundwater in alluvial fans, for example to determine groundwater types in the arid and semiarid environments (Schu¨rch and Vuataz 2000); to identify sources of groundwater; (Abd El Samie and Sadek 2001; Matter et al. 2006; Plummer et al. 2004; Stimson et al. 2001); to evaluate the quantity of groundwater recharge (Abu-Jaber and Wafa 1996; Ne´grel et al. 2003; Wood and Sanford 1995); and to research the interaction of different waters such as deep and shallow groundwater (Dassi et al. 2005), surface and groundwater (Ne´grel et al. 2003), and the replenishment of groundwater (Zhang et al. 2005). These methods have provided insights into characteristics of recharge which are difficult and expensive to obtain with physical-based methods. For example, in arid UAE and Oman, Tang et al. (2001) illustrated the spatial distribution of recharge to groundwater using hydrogeochemical and isotopic (Deuterium and oxygen-18) methods in five

Environ Geol (2008) 55:1109–1122

1111

Fig. 2 Alluvial fans at North China Plain (modified after Zhang et al. 2004 and Wu et al. 1996a). The study area is designated by square

geomorphologic settings (mountain area, wadis, sand dunes, inland sabkhas, and coastal sabkhas). In NCP, hydrogeochemical and isotopic methods have been used to determine the chemical evolution of groundwater, to evaluate its past, current status and future, and to provide understanding for groundwater management (Chen 1999; Chen et al. 2004, 2005; Liu et al. 1997; Zhang et al. 1987, 2000, 2004). However, these works paid more attention to the spatial evolution, interconnection of individual aquifers, and vulnerability of groundwater from the piedmont, central plains, and littoral plain on a regional scale. Moreover, most of them were carried out in the central or north NCP rather than the southwest. In this paper, the authors attempt to acquire insight into groundwater characteristics in a complex alluvial fan system under semi-arid and semi-humid climates at Xinxiang, NCP, China. Based on the analyses of hydrogeochemistry and stable isotopes (oxygen-18 and deuterium) in both groundwater and surface water, the principle objectives of this study are to determine the recharge sources and to evaluate the spatial distributions of groundwater chemistry as well as its evolution in the complex alluvial fan system.

Site description Located at complex alluvial fans consisting of the Taihang Mt. alluvial fan and the Yellow River alluvial fan in the southeast of Taihang Mt., Xinxiang belongs to an area of warm and monsoonal climate. Characterized by large temporal and spatial variations, 70% of rainfall occurs from July to September. Annual rainfall ranges from 335 to 1,168 mm with an average of 573 mm (1951–2003). Potential evaporation is highest from May to June, with a total amount of 1,928 mm/year. Annual mean relative humidity is 65%. Annual mean temperature is 14°C, with the highest in June (27°C) and the lowest in January (2.1°C). The study area (34°57–35°270 N, 113°46–113°580 E) comprises the south-slope mountainous area with elevations from 100 to 290 m and a gently-sloping plain varying from 69 to 73 m above sea level (Fig. 3). Divided by the Middle Route of South to North Water Transfer and the Weihe River, there are three geomorphological units from north to south in the study area, i.e. a mountainous area, the Taihang Mt. alluvial fan and the Yellow River alluvial fan (Figs. 2, 3). In the Yellow River alluvial fan, the Yellow

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River has been transferred by the East Mengjiangnu¨ River for agricultural irrigation for more than 40 years. The Weihe River originates from Taihang Mt. and flows across the center of Xinxiang. To meet the demands of developments in agriculture and industry, artificial canals, such as Communistic Canal and People’s Victory Canal (Fig. 3), have been constructed since the 1960s. Mainly supplied by groundwater, the water resources of the study area amount to 315 m3 per capita (Pan 2006),

Fig. 3 Location of sample sites and schematic geology map of the study area. Section A–A’ is described in detail in Fig. 4. SNWT-MR designates the Middle Route of South to North Water Transfer projects. filled circle in dashed line and open circle in solid line designate the division of aquifers among mountainous area, Taihang Mt. alluvial fan, and Yellow River alluvial fan

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which is far below the aggregate for China (2,220 m3 per capita in 1997) (Liu and Chen 2001) and the international average of 8,549 m3 per capita (WRI 2005). Groundwater consists of 65% of local water resources for residential and industry use. Recently, sharp declination of the water table and expansion of the cone of depression have been caused by overexploitation and poor protection, with the increase in population and the radical developments in agriculture and industry. As a consequence, groundwater resources

Environ Geol (2008) 55:1109–1122

have become a limiting factor for social and economic growth.

Geological and hydrogeological setting The general geological setting of the study area (Figs. 3, 4) is described briefly as follows:

Ordovician stratum (O) Later Ordovician (O1): This comprises dolomite facies sediment containing flint belts or blocks and two to four thin layers of yellowish green shale. Its depth, including the underlying early Cambrian, varies from 60 to 140 m from surface. Middle Ordovician (O2): Lark marlite and shale underlie the middle Ordovician. Its upper layer consists of dolomite and breccia. The thickness of the middle Ordovician ranges from 100 to 300 m.

Neozoic stratum layer (N) The Neozoic stratum, with depths of 10 to 280 m, outcrops only at Luwangfen, contacting unconformably with underlying Ordovician limestone. In the south of Luwangfen, the Neozoic underlies the Quaternary layer and expands southward with the depth. Based on the borehole data, Neozoic lithology in underlying layers are composed of mudstone, argillaceous sandstone, and solid middle-fine sandstone. Karst fissures are present in the upper layer, composed of sandwich limestone of gray to white color, brecciform marlite, and marlite.

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Quaternary stratum layer (Q) Covering 94% of the study region, the Quaternary stratum is characterized by significant spatial variations in lithofacies and lithology. Its thickness increases from north to south. Later Pleistocene (Q1): In the northern part of the study area, lithology, comprising red-brown clay, loam, and boulder-clay, formed from glacial drifty and fluvio-glacial sedimentation. In the wide southern Yellow River alluvial fan, lacustrine facies sediments, lying 150 m below surface, comprise a lithology of thick red-brown clay and clayey soil interspersed with manganese nodules. Middle Pleistocene (Q2): These are gradient pluvial sediments composed of loess-like clayey soil, with two to four layers of interbeded paleosols in the north. It lies 20 to 80 m below surface in the south and consists of clayey soil in red-brown or brownish yellow color, interspersed with middle-fine and middle-coarse sand of alluvial origin. Epipleistocene (Q3): In the north, it comprises pluvial– alluvial facies sediment which are composed of loess-like clayey soil and clayey loam, interbedded with sandy gravel layers in parts. As alluvial facies sediment in the south, it lies 10 to 20 cm below surface and is composed of lark loam and silt in upper and lower layer, respectively. Holocene (Q4): As alluvial facies sediment, it is widely distributed over the study area with the upper layer of lark or gray clayey and loam, and the lower layer of fine sand and silt. The hydrogeology along the section A–A’ in the study area is shown in Fig. 4. The mountainous area and piedmont are underlain by partial outcrop limestone of Ordovician age. Abundant groundwater develops in karstic fractures along fracture zones which are impacted by tectonism. Underlaid Tertiary marlite, with thickness of

Fig. 4 Schematic cross-section A–A’ through the study area from mountainous area to Taihang Mt. alluvial fan, and Yellow River alluvial fan (see Fig. 3 for location)

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17–65 m at the center of the formation at Lishitun, is to the east of Qingyangkou fault. The Taihang Mt. alluvial fan is composed of several sub-alluvial or proluvial fans consisting primarily of loess-likely sandy loam soil, clayey soil, sand, and lenticle gravel, with poor water yield. For the Yellow River alluvial fan in the southern study area, the aquifer lithology is primarily made of fluvial facies of medium-fine sand, with great thickness and high porosity. Based on these background conditions, the groundwater types can be divided into three groups: karstic fractured carbonate groundwater, clastic-rock fracture groundwater, and loose-rock pore water which can be further partitioned as shallow, medium, and deep groundwater in terms of their depth and exploitation setting.

Methods To analyze the concentrations of major ions and stable isotopes, 21 samples, 19 in the shallow aquifer and 2 in the deep aquifer in terms of the classification defined by Chen (1999), were collected from private, factory, and observation wells in March 2006 (Fig. 3). Water tables and well depths were measured in situ using calibrated plastic tape prior to sampling. Also, six surface water samples were taken from Communist Canal, Weihe River, and an irrigation canal transferring from the Yellow River (Table 1; Fig. 3). Information about water use was obtained by interviews with well users. All samples were analyzed in Chiba University laboratories. EC and pH were measured by portable pH and EC meter (Compact meter, Horiba, Japan) in situ and verified in the laboratory using higher precision meter (D23, Horiba, Japan). No significant errors were found between them. HCO–3 was measured by titration using 0.01 N H2SO4. All samples were filtered through 0.45 lm cellulose acetate filter membrane before using ion chromatograph (Shimadzu, Japan) to analyze major ions (Na+, K+, Ca2+, Mg2+, Cl–, SO2– 4 ). The chemical results were only accepted when the charge balance error was within ±5%. Total dissolved solid (TDS) was calculated as summation of all major ions. To determine the sources of water and the effects of evaporation, the isotopic compositions of oxygen (18O) and deuterium (2H or D) in groundwater and surface water were examined by a modified H2O–CO2 equilibration method (Epstein and Mayeda 1953) and zinc-reduction method (Coleman et al. 1982), respectively. All samples were measured by mass spectrometer (Delta-S, Theomoqt) at Chiba University, and the results of 18O and D were expressed in per mil unit as(d-notation relative to Vienna Standard Mean Ocean Water (VSMOW) standard, as shown in Eq. 1. The precisions for d18O and dD were 0.1 and 1%, respectively.

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 d¼

 Rsample  1  1000 Rstandard

ð1Þ

where, Rsample and Rstandard were the measured isotopic ratio for sample and for standard, respectively.

Results General groundwater chemistry characteristics All samples were relatively similar in pH values (7.0–8.1) with slightly higher values observed in the Yellow River alluvial fan (Table 1). The average EC values in groundwater increased from 608 lS/cm in mountainous area to 1,748 lS/cm in the Taihang Mt. alluvial fan and 2,542 lS/ cm in the Yellow River alluvial fan. A low EC value was found in the water transferred from the Yellow River (mean value 892 lS/cm), whereas the other surface waters had higher EC (mean 1,257 lS/cm). The average EC value of samples from the deep aquifer was 1,290 lS/cm. Three chemical types of groundwater are apparent from the Piper diagram (Fig. 5): Ca–HCO3 type in the mountainous area, Na–SO4–Cl in the Yellow River alluvial fan and deep aquifer at site 37, and Ca–Na–Mg–HCO3–Cl– SO4 type in the Taihang Mt. alluvial fan. Figure 5 also shows that the groundwater in the whole complex alluvial fan system is characterized by two end members, one in the mountainous area of Ca–HCO3 type and another in the Yellow River alluvial fan of Na–SO4 type.

Composition of oxygen-18 and deuterium Stable isotopes (oxygen-18 and deuterium) can provide useful information on the sources of water. Figure 6 shows the relation between d18O and dD for both surface water and groundwater in the study area. d18O and dD values in groundwater varied from –11.8 to –7.0% with mean value of –8.7%, and –83 to –54% with mean value of –66%, respectively. The groundwater at Site 2 was extremely enriched in both d18O and dD, and the isotopic composition deviated from the Local Meteoric Water Line (LMWL) in a direction indicative of fractionation due to possible strong evaporation. The data cited from the Global Network for Isotopes in Precipitation dataset (IAEA/WMO 2004) were used as LMWL. These data are based on monthly accumulated precipitation from Zhengzhou, China, which is around 80 km east of the study area (Figs. 1, 2), from September 1985 to December 1992. Similar isotopically enriched water was found at Site 70 located within the Yellow River alluvial fan. However, there was an isotopically depleted zone (mean of –8.8% d18O) in the

170

20

25

20

22

20

50

35

25

20

25 10

25

40

100

150

15

12

15

25

10

0

0

0

0

0

0

81

82

98

99

86

2

5

13

28

45

43 58

40

10

12

37

54

66

68

70

78

93

104

113

114

116

117

71.0

72.0

71.5

72.0

72.0

76.0

72.6

74.0

73.2

73.5

74.0

71.0

70.6

69.6

70.9

72.6 72.0

71.8

73.0

70.5

72.5

80.4

75.0

106.5

90.0

115.0

162.7

Altitude (m)

–

–

–

–

–

–

Shallow

Shallow

Shallow

Shallow

Shallow

Deep

Deep

Shallow

Shallow

Shallow Shallow

Shallow

Shallow

Shallow

Shallow

Shallow

Shallow

Shallow

Shallow

Shallow

Shallow

Aquifer

SW

SW

SW

SW

SW*

SW*

Well

Well

Well

Well

Well

Well

Well

Well

Well

Well Well

Well

Well

Well

Well

Well

Well

Well

Well

Well

Well

Site type

R

R

R

R

R

R

YRAF

YRAF

YRAF

YRAF

YRAF

TMAF

TMAF

TMAF

TMAF

TMAF TMAF

TMAF

TMAF

TMAF

TMAF

TMAF

TMAF

M

M

M

M

Description

6.8

7.1

6.9

6.8

6.9

7.9

8.0

8.1

7.9

7.4

7.9

7.9

7.6

7.6

7.6

7.7 7.7

7.0

7.8

7.6

7.5

7.4

7.6

7.8

8.1

7.7

7.7

pH

808

1422

1454

1345

893

890

1610

2200

2400

3500

3000

1440

1140

2400

2600

1790 2600

1360

1550

1540

1060

1200

1380

660

530

580

660

EC (lS/cm)

–8.5

–8.7

–8.8

–8.6

–8.1

–9.7

–9.6

–7.5

–9.6

–9.1

–8.3

–9.9

–11.8

–8.7

–7.7

–7.9 –7.9

–8.4

–7.7

–9.0

–8.7

–7.0

–8.8

–8.5

–10.0

–8.3

–8.6

d18O (per mil)

–64

–65

–64

–71

–57

–74

–71

–71

–72

–66

–54

–83

–79

–67

–58

–56 –62

–60

–59

–66

–65

–64

–67

–67

–74

–61

–63

dD (per mil)

0.76

1.32

1.37

1.16

0.85

0.84

1.71

2.38

2.54

4.42

3.92

1.43

0.99

2.89

2.70

1.88 2.61

1.25

1.55

1.39

1.26

1.06

1.43

0.64

0.52

0.62

0.65

TDS (g/L)

0.51

0.83

0.89

0.71

0.90

0.95

3.60

5.91

4.41

2.78

2.31

2.52

1.01

1.18

1.27

1.94 1.18

1.09

0.98

0.79

0.78

0.34

0.79

0.22

0.25

0.23

0.23

Na/Ca

0.84

0.69

0.70

0.67

0.81

0.66

1.98

2.43

1.91

1.55

1.68

1.05

0.69

1.07

1.53

1.99 1.08

1.27

1.26

1.01

0.98

0.49

0.77

0.63

0.62

0.60

0.78

Mg/Ca

1.40

1.46

1.45

1.34

1.74

1.38

2.01

2.28

2.07

1.66

1.77

5.10

4.33

1.14

0.90

1.32 1.08

1.26

0.96

1.00

1.47

0.74

0.66

0.58

1.50

0.65

0.84

Na/Cl

2.62

1.96

1.96

2.26

2.20

1.12

1.01

1.26

1.33

1.73

2.29

5.73

6.76

1.00

0.98

1.09 1.24

1.30

0.96

1.13

1.59

1.01

0.47

1.17

1.68

1.39

1.83

SO4/Cl

SW Surface water; *: transferred from Yellow River; M mountainous area, TMAF Taihang Mt. alluvial fan, YRAF Yellow River alluvial fan, R river, Na/Ca, Mg/Ca, Na/Cl, and SO4/Cl are molar ratio

Well depth (m)

Site ID

Table 1 Well constructions, hydrogeochemistry and stable isotope results of groundwater sampling at a complex alluvial fan system, March 2006

Environ Geol (2008) 55:1109–1122 1115

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Fig. 5 Piper diagram of ion percentages in groundwater and surface water sampled at a complex alluvial fan system, March 2006. Open triangle: well in mountainous area; open circle: Taihang Mt. alluvial fan; filled circle: Yellow River alluvial fan; open square: River; star: water transferred from Yellow River; filled inverted triangle: deep aquifer

mountainous area (including Sites 81, 82, 98, and 99) where elevations were more than 90 m. Another isotopically depleted zone, with average of –8.8% d18O, was found at the Yellow River alluvial fan. This d18O value in groundwater was similar to that of water transferred from the Yellow River (–8.9% d18O). As a result, the groundwater in the Taihang Mt. alluvial fan generally shows greater isotopic enrichment than those in the mountainous area and the Yellow River alluvial fan. The lightest isotopic compositions were observed at deep aquifer Sites 12 and 37, with values of –11.8 and –9.9% for d18O, –79 and –83% for dD, respectively.

Discussion Recharge sources of groundwater in a complex alluvial fan system Stable isotopic compositions of d18O and dD in groundwater and surface water in the complex alluvial fan system of the study area correspond closely with the LMWL, indicating a predominantly meteoric source (Fig. 6). However, the

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precipitation has undergone different evaporative effects before becoming surface water or recharging into groundwater.

Mountainous area The wide range of stable isotopic compositions of groundwater in the mountainous area indicates that this groundwater was subject to more variable evaporation and isotopic fractionation than groundwater in the alluvial fans. When the water table is shallow, stronger evaporation may lead to enrichment of stable isotopes. The d18O values of groundwater in the mountainous area were similar to that of precipitation measured in the middle area of Taihang Mt. (Li et al. 2007). Lower EC, TDS, molar Na/Ca ratios (Table 1), and major ion concentrations of groundwaters in the mountainous area suggest that they have undergone shorter residence times, and that the area is a possible recharge source for the surrounding region. In general, the results indicate that groundwater in the mountainous area is mainly recharged by infiltrating precipitation.

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1117

18

-12

-10

δ O (per mil) -8

-6

-4 -20

-40

-80

δ D (per mil)

-100

)))

-60 12 37

LMWL Mountainous area River Taihang Mt. alluvial fan Deep aquifer Yellow River alluvial fan

-120

Fig. 6 d18O-dD plot of all groundwater and surface water samples

Taihang Mt. alluvial fan Heavier d18O values were observed in the Taihang Mt. alluvial fan (mean value of –8.2%). In the mountainadjacent portion of this fan (Site 2), the groundwater had the heaviest d18O of all samples. Strong evaporation caused the enrichment of isotopes in areas where the water table was very shallow. Furthermore, heavier stable isotopic compositions suggest the groundwater in this area has a fast local water cycle process. Figure 6 and the groundwater flow direction in Fig. 3 indicate that the groundwater in the Taihang Mt. alluvial fan is recharged by a lateral flow from mountainous area, as well as by a vertical infiltration from precipitation and irrigation channels. Figure 5 also shows that surface water and groundwater have similar hydrogeochemical compositions. The values of deuterium excess defined by Dansgaard (1964) varied from +7.2 to –7.9%, indicating that the waters have undergone strong evaporation during recharge into the local groundwater system.

Deep groundwater Water samples at sites 12 and 37 were collected from the deep aquifer located in the groundwater depression cone in the north of the study area (Fig. 3). The results show that the groundwater in the deep aquifer has different characteristics in stable isotope as well as in hydrogeochemistry. They were found to be extremely isotopically depleted, which cannot be explained by recharge from local precipitation because the groundwater was sampled during the spring without heavy rain in the whole winter season, which is characterized by high evaporation (Zhang 1999). The overexploitation of groundwater for local agricultural

irrigation and urban water supply caused recharge into the cone from a deeper aquifer and/or the northern mountainous area. The water recharged from precipitation during a colder climate period with depleted isotope compositions. One might assume an upper age limit of 14 ka B.P. (Before Present) for these two sites because glacial-aged water would be expected to be isotopically lighter than presentday water. The investigation of groundwater in NCP, reported by Chen et al. (2003b), supports the presented assumption to some extent. They reported that the deeper groundwater was possibly recharged by water formed at least 12 ka B.P. with ranges of –9.4 to –11.7% d18O. Moreover, the faults shown in Fig. 4 provide a channel for deeper groundwater recharge. The groundwater in mountainous areas, such as site 98, with very light stable isotopic composition (–10.0% d18O) may be connected to the deep groundwater. However, the high molar ratios of Na/Cl and SO4/Cl, with values of 4.33 and 6.76 at site 12, and 5.10, and 5.73 at site 37, respectively, indicate that the recharge source of deep groundwater is different from that of shallow groundwater and has undergone very strong water– rock interaction during the long residence time. The low Cl concentration suggests recharge under cooler, wetter conditions (Bekele et al. 2006). The very low Cl concentration and the depleted stable isotopic composition observed in the deep aquifer indicate that the deep aquifer was recharged from an ‘‘old’’ source. However, water samples collected from sites completed in the deep aquifer, (Sites 12 and 37) plot in different portions of Fig. 5. Site 12 was of Ca–Na–SO4 water type, whereas site 37 was Na–SO4. The sodium and solute concentrations at Site 37 were approximately twice those of Site 12. This indicates that the chemical evolution of water from Site 37 was different from water from Site 12. The Cl concentrations in the deep aquifer were distinctly lower than those around them, and the values of TDS were very low (0.99 and 1.43 g/L, respectively). This can also likely be explained by different sources to the deeper aquifer recharging them by the fault shown in Figs. 3 and 4. The molar Na/Ca ratios were over three times that of other samples in the mountainous area. It suggests that they undergo longer evolution time, that is, the groundwater has longer residence time in deep aquifer.

Yellow River alluvial fan Previous investigation showed that d18O in the Yellow River at Huayuankou (Fig. 2), which is near to the entrance of People’s Victory Canal (Fig. 3), was –9.1% in August 1986 during abnormal drought conditions (Zhang et al. 1995). Su et al. (2004) observed the d18O varied from –8.0% in the rainy season (August to September in 2000)

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to –9.3% in the dry season (March to April in 2001) at the same site. This suggests that the local groundwater aquifer in the Yellow River alluvial fan is affected by the transferred water by recharge from irrigation return and infiltration along the canal bed. The data indicate that the length of Yellow River recharge influence is approximately 6 to 8 km, calculated from the southern boundary up to Site 66 at the distal end of the Yellow River alluvial fan, where the stable isotopic compositions were –9.1% for d18O and –66% for dD. The recharge source to the Yellow River alluvial fan is obviously different from the mountainous area and the Taihang Mt. alluvial fan. The isotopic compositions fell near the LMWL and had values similar to the water transferred from the Yellow River (at sites 104 and 93), except Site 70 at which the well depth was 25 m with shallow water table. Furthermore, Fig. 5 shows that the transferred surface waters were enriched in Mg, Na, and Ca. Along the flow direction B to B’, TDS and all concentrations of ions in shallow groundwater increased toward the Weihe River, whereas the Mg/cation ratio and Cl/anion ratio kept approximately constant. The Na/SO4 ratios also increased with decreasing Ca/HCO3. The highest ion concentrations were found along the southern Weihe River (sites 54 and 66). Therefore, this implies that the groundwater in the Yellow River alluvial fan has historically been recharged by lateral flow from the Taihang Mt. fan, and has been affected greatly since the irrigation channels were made available in the 1960s. As a result, transferred Yellow River water is washing the ions from the shallow aquifer in the fan to reduce ion concentrations and TDS. The molar Na/Ca ratio was lower than 0.3 in the mountainous area and lower than 1.0 in the Taihang Mt. alluvial fan; however, it was more than 2.3 in the Yellow River alluvial fan (Table 1). This suggests that the groundwater in the outskirts of this region has had longer residence time for ion exchange or chemical mixture which results in the enrichment of Na compared to other areas.

Geochemical evolution of groundwater Mountainous area The groundwater in the mountainous area had mean TDS of 0.60 g/L (Table 1). It was characterized as Ca–HCO3 type with slight variations in cation ratios. The water at Site 98 had a distinctly higher HCO3 concentration. The chemical evolution in the mountainous area can be explained by direct infiltration of precipitation with subsequent uptake of CO2 in soil, resulting in the dissolution of underlying limestone and marlite which interbeds the

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dolomite (Fig. 4). The hydrolysis and dissolution reactions of limestone and dolomite to form dissolved bicarbonate, calcium and magnesium ions can be described as (Appelo and Postma 2005):  CaCO3 þ H2 O ! Ca2þ þ 2HCO 3 þ OH

ð2Þ

þ CO2 þ H2 O ! HCO 3 þH ;

ð3Þ

 CaMg(CO3 Þ2 þ 2H2 O ! Ca2þ þ Mg2þ þ 2HCO 3 þ 2OH

ð4Þ

þ 2CO2 þ 2H2 O ! 2HCO 3 þ 2H

ð5Þ

The molar Mg/Ca ratios in groundwater samples in mountainous area were lower than 0.8 which suggests that dissolution of limestone is faster than magnesium, and no calcite precipitation is available. Taihang Mt. alluvial fan With the groundwater flowing into the Taihang Mt. alluvial fan, the TDS amounted to 1.25 g/L, and water type changed to Ca–Na–Mg–HCO3–Cl-SO4, with concentrations Ca[Na[Mg and HCO3[Cl[SO4. The water infiltrated through the karstic formation and, at this stage, attained a relatively stable chemical composition. The highest concentration of Cl was found at Site 86. However, it is difficult to estimate the origin of Cl by current data. Anthropic origin was the most possible response for the extreme values (Panno et al. 2006). Generally, as it enters into the Taihang Mt. alluvial fan, groundwater continues to dissolve more underlying marlite which extends from piedmont to the alluvial aquifer with depths varying from 0 to 180 m below surface. In summary, the chemical evolution of groundwater in the complex fan has two directions. One is from the mountainous area to the Taihang Mt. alluvial fan where the molar ratios of ions in groundwater were characterized by increases in Na, Cl, and SO4, decreases in Ca and HCO3, and a slight change in Mg. The water chemistry varied from Ca–HCO3 in the piedmont to Ca–Cl in the front of the fan. The other is along the Communistic Canal and the Weihe River where groundwater chemistry evolved from Mg–Ca–HCO3 in the upper reaches to Na–SO4 in the center of the fan as well as the lower reaches of the canal and the river. Similarly, the TDS of groundwater along these two directions increased significantly. In the Taihang Mt. alluvial fan, cations were controlled by Mg, Ca, and Na in similar concentrations (Fig. 5), and the molar ratio of

Environ Geol (2008) 55:1109–1122

1119

anions was primary characterized by SO4 with over 30% concentration of anion for most sites.

Yellow River alluvial fan In the Yellow River alluvial fan, the groundwater chemistry was of Ca–Na–HCO3–SO4 type at the south part of the study area. With the transferred water at Site 93 flowing into the distal end of Yellow River alluvial fan at Site 66, a linear chemical evolution with a slightly various ratio of Cl occurred, and the Ca–Na in the transferred water was replaced by Na–Mg in cation and HCO3–SO4 by SO4–Cl in anion. The original aquifer has lacustrine facies sediment at 150 m depth, over which the Yellow River alluvial fan formed a 150 m thick aquifer with alluvial facies sediment. Since the 1960s, the Yellow River has been transferred to the study area for agricultural irrigation. Before transferring, the aquifer contained very high Na and SO4 with Na– SO4 type water originating from the lacustrine facies sediments. The surface water in the Yellow River from Huayuankou was Ca–HCO3 type with mean TDS of 0.52 g/L (data from the period 1960–2000) (Chen et al. 2003a, 2006). Hence, the chemical evolution of groundwater can be described by the processes of mixture and ion exchange. When transferred river water of Ca–HCO3 type infiltrates, it may mix with local groundwater of the alluvial aquifer which was originally Na–SO4 type. The chemical mixture formed Na–HCO3 type water associated with precipitation of gypsum according to the following equation (Wang 1986):

By mapping hydrogeochemical facies in different areas, the groundwater flow patterns in the complex alluvial fan can be described. The hydrogeochemical facies reflected the response of chemical processes occurring within the lithologic framework and also the pattern of groundwater flow (Back 1960, 1966). The Stiff plotting technique (Stiff 1951) was used to describe the chemical evolution of groundwater along water flow path. The area of the diagram is an approximate indication of total dissolved salts (Hem 1989; Stiff 1951). Integrated with a conceptual model of the origin and chemical evolution of groundwater hydrogeochemistry, the Stiff diagrams of samples are shown in Fig. 8 along the

ð6Þ

In fact, transferred water was dominated by Ca and HCO3 with low TDS (mean value of 0.84 g/L). In the Yellow River alluvial fan, Na and SO4 were the dominant ions with high TDS (mean value of 2.99 g/L), which increased gradually toward the distal end of the fan (Table 1). There is an apparent refreshening effect along the groundwater flow path. It can be described as ion exchange (Appelo and Postma 2005): 1=2Ca2þ þ Na-X ! 1=2Ca-X2 þ Naþ

Conceptual model of complex alluvial fan

ð7Þ

where X indicates the soil exchanger. Ca was taken up from transferred water from the Yellow River. As a result, the transferred water evolved from Ca–HCO3 to Na–HCO3 when discharged into theYellow River alluvial fan. Consequently, along the transferring route, molar ratios of Ca/Mg in recharge water diminished significantly due to chemical mixture and ion exchange in the aquifer, and the

40 66

y = 1.4217x + 5.9863

35

2

R = 0.9607

30 Na (meq/L)

þ 2 Ca2þ + 2HCO 3 + 2Na + SO4

CaSO4 þ 2Naþ + 2HCO 3

water type was converted into Na–HCO3 in the area between the transferred water (Sites 104 and 93, for example) and the distal end of the Yellow River alluvial fan such as Sites 54, 66, and 68. Notably, molar ratios of Cl remained constant during the refreshening (Fig. 5). Figure 7 shows the relationship between Na and Cl in groundwater. It indicates that the groundwater in the mountainous area and the Taihang Mt. alluvial fan undergo similar evolution with a molar Na/Cl ratio of 1:1 similar to river water. However, the regression equation shows that the relationship between Na and Cl for the samples in the Yellow River alluvial fan and deep aquifer has a gradient of 1.4 and intercept of 6.0, respectively. It can be explained by ion exchange between Ca and Na leading to the increase in Na.

25

70

68

54

20 15 10

37

78

12

y = 1.0155x + 0.0053

5 0 0

2

R = 0.9112 5

10 15 Cl (meq/L)

20

25

Fig. 7 Relationship of Na and Cl for water collected from canal and river (open square), mountainous area and Taihang Mt. alluvial fan (open circle) and Yellow River alluvial fan and deep aquifer (filled circle)

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Fig. 8 Conceptual model of a complex alluvial fan system showing hydrogeochemistry, d18O, groundwater types, and groundwater flow direction along section A–B’ and B’–B (Their positions are shown in Fig. 3 )

groundwater flow direction A–B’ from the mountainous area to the Taihang Mt. alluvial fan, and B–B’ from southwest to northeast in the Yellow River alluvial fan. The variations of d18O are also indicated in the figure. Along the section A–B’, TDS in groundwater increased significantly from the mountainous area to the Taihang Mt. alluvial fan with slight isotopic variation. Groundwater evolved from Ca–HCO3 type in the mountainous area, to Ca–Cl type in the mountain-front, and Na–SO4–Cl type in the margin of the northern Taihang Mt. alluvial fan. Carbonates dissolution and ion exchange occurred and led to the variation of water types. Along the flow path (B–B’), the water transferred from the Yellow River characterized as Ca–HCO3 type (Chen et al. 2003a, 2006) recharged into the southern Yellow River alluvial fan and resulted in the decline of TDS and refreshed the groundwater in the study area. The water chemistry varied from Ca–HCO3, Ca–Na–Mg–HCO3–Cl– SO4 type, to Na–Ca–SO4–Cl type, and to Na–SO4 type. However, the distal end of the Yellow River alluvial fan (south of Weihe River) was still controlled by the original lacustrine facies sediments. As the water flows into the Taihang Mt. alluvial fan, the chemistry of groundwater appeared Ca–Na–Mg–HCO3–Cl–SO4 type with slight variation in TDS. The fault may cause upward vertical

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groundwater flow as presented by Plummer et al. (2004) and consequently results in the isotopic depletion and TDS decline at Site 13. The deep aquifer, which is not illustrated in Fig. 8, appears markedly lower in TDS than other groundwater in the alluvial fan, and its water chemistry was Na–SO4 type rather than mixing type as in the upper layer. The d18O composition and water chemistry suggest that the deep aquifer is separated from the mountainous area, the Taihang Mt. fan, and Yellow River alluvial fan.

Conclusion Based on the hydrogeochemical and isotopic data, the recharge sources of a complex alluvial fan aquifer system in the study area can be recognized as follows: (1) In the mountainous area, groundwater is fed by direct recharge of infiltrating precipitation; (2) In the Taihang Mt. alluvial fan, groundwater is recharged by lateral infiltration from the adjacent mountainous areas, precipitation, and surface water from canals and rivers; (3) In the Yellow River alluvial fan, transferred water from the Yellow River is believed to be washing the groundwater from the southern study area to the distal end of this fan.

Environ Geol (2008) 55:1109–1122

The hydrogeochemical evolution of groundwater in the mountainous area was controlled by hydrolysis and dissolution reactions of limestone and dolomite. Thus, the water chemistry was dominated by Ca–HCO3 with slight variations of major ions. In the Taihang Mt. alluvial fan, the groundwater was enriched in Ca, Na, Mg and SO4 and Cl, and evolves from Ca–Cl in the front of the fan and Mg–Ca– HCO3 in the upper reaches of Communistic Canal and the Weihe River to Na–SO4 in the center of the fan and the lower reaches of the canal and river. Groundwater in the deep aquifer of the Taihang Mt. alluvial fan had lower Cl concentrations, markedly high Na/Cl and SO4/Cl and low TDS. Its isotopic depletion hints at a difference in recharge age and hydrogeochemical evolution processes. The Yellow River alluvial fan is dominated by Na–SO4 and has been influenced by water transferred from the Yellow River after irrigation canals were built in the 1960s. Hence, water chemistry evolves from Ca–HCO3 and Na–HCO3, to Na–SO4 in the distal end of this fan. The conceptual model along the two cross-sections of groundwater flow exhibits clear and consistent variations of stable isotope and hydrogeochemistry. Acknowledgments This work was supported by National ‘‘863’’ Project (No.2006AA100204), the Knowledge Innovation Programs of Chinese Academy of Sciences (KSCX2-YW-N-017–001), Grant-inAid for Scientific Research of Japan Society for the Promotion of Science (No. 19310004), and Xinxiang Government Financial Foundation. We would like to thank an anonymous reviewer for helpful comments. The authors are grateful to John Gates at Oxford University Centre for the Environment for useful suggestions and detailed edits in the final English version.

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