Environ Earth Sci DOI 10.1007/s12665-013-2989-4

ORIGINAL ARTICLE

Identifying interactions between river water and groundwater in the North China Plain using multiple tracers Yu Dun • Changyuan Tang • Yanjun Shen

Received: 11 May 2013 / Accepted: 5 November 2013 Ó Springer-Verlag Berlin Heidelberg 2013

Abstract Interactions between river water and groundwater have been used to help understand the movement of water and to evaluate water quality in the semi-arid area of the North China Plain (NCP). Stable isotopes, chlorofluorocarbons (CFCs) and hydrochemistry were used to study the influence of surface water from the Xiao River on regional groundwater. Using a mass balance approach based on chloride concentrations, hydrogen and oxygen isotope ratios, the average fraction of surface water recharging to groundwater was 50–60 %. CFC results indicated that the groundwater recharge age varied from 22.5 to 39.5 years. The vertical flow velocity of groundwater was estimated at about 1.8–3.5 m year-1. Nitrate concentrations in groundwater varied from 9.42 to 156.62 mg L-1, and exceeded 50 mg L-1 in most aquifers shallower than 80 m bordering the Xiao River. The d15NNO3 data indicate that the major sources of nitrogen in groundwater are human sewage and animal excreta. Because groundwater is the main source of drinking water, there should be concern about public health related to the elevated nitrate concentrations in the NCP. Keywords River water  Groundwater  Stable isotope  Nitrate  CFCs  Hydrochemistry

Y. Dun  C. Tang (&) Graduate School of Horticulture, Chiba University, Matsudo 648, Chiba 271-8510, Japan e-mail: [email protected]; [email protected] Y. Shen Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Shijiazhuang 050021, China

Introduction The North China Plain (NCP) is the largest alluvial plain in eastern Asia, and is one of China’s most important social, economic and agricultural regions (Liu et al. 2001). The decreased precipitation and the increased demands on water for agricultural, industrial and domestic purposes have caused a water crisis in the NCP (Chen 2010). Water extraction from thousands of pumping wells has caused the water table in the unconfined aquifer to decrease at a rate of approximately 1 m year-1, and the regional groundwater depression cone has expanded by more than 14,000 km2 (Yang et al. 2004). It is clear that, because of long-term human activities, the regional water cycle in the semi-arid NCP has been severely disrupted (Liu and Xia 2004; Sato et al. 2008). Many dams have been built in the last half-century to regulate the rivers that flow across the NCP (Chen et al. 2012). Runoff and recharge patterns have changed greatly (Zhuang et al. 2011) such that rivers have become the main source of groundwater recharge in the NCP. Worldwide, there is great interest in using hydrochemical and stable isotope (dD and d18O) methods to help understand relationships between surface water and groundwater (Bennetts et al. 2006; Cook et al. 2003; Darling et al. 2003; Dor et al. 2011; Stellato et al. 2008; Wang et al. 2012; Yuan et al. 2011). Song et al. (2006, 2011) analyzed stable isotope ratios and major hydrochemical components in the Huaisha River basin and the Taihang mountain region to examine the interactions between surface water and groundwater, and to determine the hydrochemical characteristics. River systems in the semi-arid region of the NCP are important not only for providing recharge water, but also for transporting nutrients. Owing to severe water shortages in the NCP, much of the surface water that is used for

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Environ Earth Sci

Fig. 1 Study area and sampling locations

irrigation is polluted by wastewater. Using this polluted surface water for irrigation has caused groundwater pollution problems (Chen et al. 2006; Yang et al. 2012; Li et al. 2008a, c). Worldwide, nitrate concentrations have increased in both surface water and groundwater over recent decades (McIsaac et al. 2001; Yang and Liu 2010; Orban et al. 2010). This should give rise to concern, as it is known that elevated groundwater nitrate concentrations can have detrimental ecological effects and can cause public health problems (Weiskel and Howes 1992; Zhang et al. 1996; Agrawal et al. 1999; Nolan and Hitt 2006; Kumazawa 2002; Li et al. 2008b). Nitrogen isotope ratios (d15N-NO3), based on the principle that the nitrogen component of nitrate may have different isotopic signatures (Panno et al. 2001; Katz et al. 2004), have been used since the early 1970s to identify nitrate sources and indicate denitrification processes (Kohl et al. 1971). The key to understanding the movement of water in the watershed is the residence time. Dating techniques based on transient tracers such as chlorofluorocarbons (CFCs) have been used to investigate groundwater travel time

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(Oster et al. 1996; Gooddy et al. 2006; Han et al. 2012). Groundwater recharge rates and residence times can also help us to understand chemical transport in aquifers (Busenberg and Plummer 2008). Many new hydrological issues associated with disrupted water cycles need to be addressed in response to increasing water demands in arid and semi-arid areas. However, there is limited knowledge of the processes involved. The aim of this study was to use a combination of multiple tracers, such as stable isotope ratios (dD, d18O, d15N) and CFCs, to (1) assess the hydrochemistry of river water and groundwater; (2) identify the effect of river water on regional groundwater; (3) estimate groundwater residence times, flow patterns, and recharge rates and (4) evaluate the transport and fate of nitrate in aquifers.

Hydrological setting The study area is located on the alluvial fans of the Taihang Mountains near the city of Shijiazhuang in the NCP (Fig. 1). It has a continental, semi-arid climate with a mean

Environ Earth Sci

annual temperature of 14 °C, and an annual mean relative humidity of 65 %. Precipitation is dominated by the Asian monsoon. Annual precipitation ranges from 335 to 1,168 mm, with a mean annual precipitation of 573 mm (based on data for the period 1951–2003). The rainy season, which is characterized by large temporal and spatial variations, occurs from July to September and accounts for 70 % of the annual precipitation received (Yang et al. 2006). Potential evaporation is highest from May to June and totals 1,928 mm year-1 (Li et al. 2008a). The NCP can be divided into the piedmont fluvial plain, the central alluvial flood plain, and the littoral delta plain. This study focuses on the piedmont fluvial plain and the central alluvial plain. The elevation of the piedmont fluvial plain varies from 40 to 100 m, while the elevation of the central alluvial plain is less than 40 m. The aquifer in this area is mainly composed of Quaternary cobbles, gravel, sand and laminated or lensed clay to a depth of 400–500 m. The aquifer in our study area is separated into four strata: phreatic aquifer (Q1, 10–50 m depth), semi-confined aquifer (Q2, 120–210 m depth), confined aquifer (Q3, 250–310 m depth) and a deep confined aquifer (Q4, no depth data available) (Zhang et al. 2009). Q2 is the main water supply stratum for agriculture and industry in the area. In the absence of a stable aquiclude, there is direct exchange between Q1 and Q2. The boundary of the confined aquifer lies somewhat east of Shijiazhuang and is at a depth of about 110 m (Kreuzer et al. 2009). Hydrochemical and isotopic composition clearly separate younger and older waters, which have a boundary at about 100 m deep (Lu et al. 2008; Chen et al. 2003). The Xiao River is 110 km long, and is one of a few rivers that still flow in the NCP. It originates in the Taihang Mountains, and flows through Shijiazhuang, Lunancheng, Zhaoxian and Ningjin, before entering the Fuyang River. Shijiazhuang, the capital of Hebei province, has developed from a small village to one of the biggest cities in the NCP in the past 70 years. The population of the city is about 2.31 million, and its urban area has expanded to 155 km2. The Huangbizhuang Reservoir, one of the biggest reservoirs in the Taihang Mountains, was built in the upstream portion of the Xiao River in 1958 to manage the water supply for both agriculture and the city of Shijiazhuang. To facilitate discussion in this study, the Xiao River has been divided into three parts: the upper reaches (S1), middle reaches (city zone; S2–S4) and lower reaches (S5–S12). The study area is a major grain-producing region, and follows the typical Chinese winter wheat–summer maize crop rotation (Mo et al. 2009). Annual water consumption for agriculture is about 800–900 mm. Evaporation is higher than available rainfall, so a reasonable amount of river water and groundwater is used for irrigation to maintain high grain yields (Liu et al. 2002). With increasing

urbanization, more and more wastewater is being released to the Xiao River. At present, 80 % of the water used to irrigate farmland on both sides of the river is urban wastewater from cities and counties along the river (Tang et al. 2004). The farmland at a distance of 2 km from the river and beyond is irrigated with pumped groundwater. As a result, the irrigation practices in the study area have changed the structure of hydrological cycle so that the river has become the recharge source for groundwater. Also, wastewater that leaches from the riverbed or infiltrates to farmland is threatening the drinking water source aquifers.

Methods Field surveys were conducted to collect surface water and groundwater samples in March and October 2010 (Fig. 1). Surface water samples were collected at 12 sites along the Xiao River, of which 11 were collected in March and 8 were collected in October. A total of 16 groundwater samples were collected, nine in March and seven in October, to help understand the effects of irrigation practices on groundwater. All of the sampled wells were used for domestic, industrial or agricultural purposes, and were purged before sampling. Basic information, such as groundwater usage, well depth, screen position and land use, was acquired from well managers. Electrical conductivity (EC), dissolved oxygen (DO) and pH were measured on-site using portable meters (Horiba, Japan). Measuring precisions of EC, DO and pH were ±1 lS cm-1, ±0.01 mg L-1 and ±0.1 pH, respectively. Samples were brought back to a laboratory at Chiba University and stored at 4 °C until analysis. Each sample was filtered through a 0.45-lm cellulose acetate filter membrane before determining major ion concentrations (Na?, K?, Ca2?, Mg2?, Cl-, NO3-, SO42-) with ion chromatography (Shimadzu, Japan). The limit of detection (LOD) was 0.01 mg L-1 and the coefficient of variation of repeated measurements for the mixed standard was less than ±1 %. Concentrations of HCO3- were measured by titration using 0.01 N H2SO4. Results of chemical analyses were accepted only when the chargebalance error was within ±5 %. Total dissolved solids (TDS) were calculated as the total of all the major ions. Deuterium (dD) and oxygen (d18O) isotope ratios were determined with laser spectroscopy (DLT-100; Los Gatos Research Inc.), as described by Lis et al. (2008). The results were reported as the per mil deviation relative to standard mean ocean water (SMOW) with precisions of ±2.0 and ±0.1 % for dD and d18O, respectively. The sampling and analytical procedures for d15N-NO3 determinations were conducted using the methods of Silva et al. (2000). In brief, a column filled with Cl- anion-exchange

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resin (DOWEX 1-X8 200–400 mesh, Sigma-Aldrich, USA) was used to extract nitrate from a 2-L water sample that flowed through the column. Then, 10 mL of 3 mol L-1 HCl was added to the column to wash out the nitrate. The effluent was converted to AgNO3 by adding silver oxide and then precipitated by freeze drying in a vacuum system. The dried AgNO3 was analyzed for d15N ratios using mass spectrometry (PDZ-Europa Integra-CN system). A standard was measured once every five samples as a quality-control measure. The results were reported as the per mil deviation relative to AIR; analytical precisions were ±0.2 %. The isotopic results were given in d units, defined as d18 OðdD; d15 NÞ ¼

Rsample  Rstandard  103 &; Rstandard

ð1Þ

where R is the ratio of the heavy isotope to the light isotope (18O/16O, D/H or 15N/14N). To understand how river water influenced groundwater, the age of the groundwater in the study area was determined using CFCs, based on the methods of Oster et al. (1996). Groundwater samples for CFC analysis were collected in October 2010. Samples were conserved in brown glass bottles sealed with aluminium-lined caps. CFCs in water samples were measured by gas chromatography equipped with an electron capture detector (Shimadzu GC14B, Japan).

Results and discussion General hydrochemical characteristics Table 1 shows the values of the main hydrochemical parameters along the Xiao River in spring (March) and autumn (October). The pH values of most samples ranged from 6.7 to 8.7, except for one sample at S8 in autumn (pH = 9.7). DO concentrations in the upper and middle reaches ranged from 4.20 to 5.05 mg L-1, which indicated that surface water was oxic before entering Shijiazhuang city. After sewage inputs, DO in the lower reaches ranged from 0.10 to 2.30 mg L-1, with an average of 0.60 ± 0.70 mg L-1 (average ±r). From the Huangbizhuang Reservoir to the downstream portion of the Xiao River, EC values in surface water increased from 241 to 2,690 lS cm-1 in spring, and from 603 to 2,260 lS cm-1 in autumn. EC values in the upper reaches were higher in autumn than in spring. Large wastewater discharges are responsible for the increase in EC values and fast decline in DO values after the river passes through the urban zone. In the upper reaches, Ca2? and SO42- were the main ions in surface water. Ca2? concentrations were 30.40 mg L-1 in spring and 54.37 mg L-1 in autumn. SO42- concentrations

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were 61.22 mg L-1 in spring and 165.82 mg L-1 in autumn. In spring and autumn, ion-exchange reactions involving Ca2? and Na? occurred as water flowed from site S1 through sites S5–S12. The mole equivalence ratios of Ca2? decreased from 60 to 20 %, and those of Na? increased from 15 to 70 %. As a result, the Xiao River changed from a Ca-SO4 type in the upper reaches, to a Na-Cl type in the urban reaches, and finally to a Na-HCO3 type in the lower reaches of the study area. Table 1 also shows the values of the main hydrochemical parameters for groundwater. DO concentrations in groundwater ranged from 2.74 to 7.35 mg L-1, and EC values varied from 399 to 1,636 lS cm-1. Concentrations of Cl- in groundwater (133.16 ± 82.08 mg L-1) were lower than those of surface water (234.15 ± 133.04 mg L-1). Cl- in surface water is the result of inputs of untreated or inadequately treated sewage. Groundwater in the study area is classified as the Ca-HCO3 type, although the mole equivalence ratios of HCO3- decreased, and SO42- and Clincreased, in the lower reaches of the Xiao River. Oxygen and deuterium isotopes in surface water and groundwater Stable isotopes (oxygen-18 and deuterium) can provide useful information on the sources of water. In this study, isotopic data from surface water and groundwater have been interpreted in the context of the NCP’s regional isotope hydrology. The local meteoric water line (LMWL) was determined from the ‘Global Network Isotope in Precipitation’ data for Shijiazhuang for the period from 1985 to 2003 (IAEA/WMO 2004). The slope of the LMWL was less than that of the global meteoric water line (GMWL) because of evaporative fractionation (Fig. 2). Both the groundwater and surface water isotopic composition deviated from the LMWL in a direction indicative of fractionation due to strong evaporation. Sources of surface water and groundwater can be distinguished by comparing the d18O and dD compositions of water samples with the LMWL. In surface water, d18O values ranged from -7.2 to -8.4 % with an average value of -7.9 ± 0.3 %, while dD values ranged from -54.1 to -60.5 %, with an average value of -57.9 ± 2.1 %. In groundwater, d18O values ranged from -7.5 to -9.8 %, with an average value of -8.7 ± 0.7 %, and dD values ranged from -54.3 to -67.9 %, with an average value of -62.7 ± 4.1 %. The heaviest values for oxygen (-7.2 %) and hydrogen isotopes (-54.1 %) were found at the reservoir site (S1), indicating the increased evaporation relative to other sites. There was isotope enrichment at site G1, as it was recharged by reservoir evaporation-influenced water (S1). The lightest isotopic compositions in groundwater were found at site G10, with values of -9.8 % for d18O and -67.9 % for dD.

Environ Earth Sci Table 1 Hydrochemical parameters, major ions and stable isotopes in surface water and groundwater ID

Latitude

Longitude

EC (lS cm-1)

DO (mg L-1)

pH

Na? (mg L-1)

K? (mg L-1)

Ca2? (mg L-1)

Mg2? (mg L-1)

Spring (March 2010) S1

N38°140 58.600

E114°180 30.700

241

4.20

8.8

11.60

0.86

30.40

6.30

S2

0

N38°6 17.4

E114°250 47.300

397

4.50

7.9

27.40

1.84

74.04

22.81

S3

N38°20 0.900

E114°320 4.100

372

4.34

7.9

27.64

1.79

70.54

22.26

E114°270 20.500

582

5.05

7.5

33.39

1.61

113.48

31.08

E114°310 24.000

0

00

00

S4

N38°2 14.9

S5

N37°590 30.600 0

00

0

1,372

0.60

7.9 122.10

9.40

133.97

37.62

00

S6

N37°57 28.3

E114°31 26.8

2,690

0.51

7.8 310.50

16.25

131.77

37.75

S7

N37°560 51.100

E114°310 27.900

2,510

0.36

8.7 307.52

19.40

126.93

36.74

S8

N37°530 51.000

E114°320 4.400

2,420

0.44

7.5 255.16

14.60

134.45

36.89

S10

N37°450 21.400

E114°420 42.600

2,270

0.10

8.4 281.61

15.22

136.58

39.67

S11 S12

N37°390 56.400 N37°330 32.200

E114°490 31.100 E114°560 30.100

2,630 1,940

2.30 0.21

8.1 348.17 8.2 220.78

16.24 12.25

139.12 134.54

38.03 38.94

G2

N37°560 37.400

E114°310 5.500

1,283

7.35

8.0 107.23

1.37

186.99

74.08

G3

00

N37°57 52.7

E114°310 45.800

784

3.51

8.2

33.67

0.81

121.47

51.31

G4

N37°570 52.800

E114°330 30.400

654

3.33

8.3

30.92

1.00

115.14

48.70

G5

N37°570 52.500

E114°340 0.200

715

n.d.

8.0

54.17

0.84

92.57

32.05

G6

N37°540 31.600

E114°380 23.600

731

2.74

8.4

28.09

0.11

126.19

39.07

G7

N37°540 11.400

E114°330 51.000

1,101

3.30

8.3

68.85

0.16

216.69

73.05

E114°320 46.500

976

3.15

8.3

64.33

0.69

167.69

70.39

1,636

3.85

8.1 128.85

0.57

234.44

85.35

469

4.04

8.5

19.10

0.92

78.81

35.36 21.37

0

0

00

G11

N37°54 6.3

G12

N37°530 59.100

E114°310 11.100

G13

N37°550 54.400

E114°300 8.600

Autumn (October 2010) E114°180 30.700

603

n.d.

7.1

32.70

1.92

54.37

S5

00

N37°59 30.6

E114°310 24.000

1,356

n.d.

6.9 141.28

36.32

89.74

29.37

S6

N37°570 28.300

E114°310 26.800

2,050

n.d.

6.7 291.16

34.29

102.92

29.80

S8 S9

N37°530 51.000 N38°500 3.000

E114°320 4.400 E114°360 56.700

1,950 2,260

n.d. n.d.

9.7 244.61 7.1 329.34

37.13 51.21

104.29 111.54

30.94 31.39

S10

N37°450 21.400

S1

N38°140 58.600 0

E114°420 42.600

2,130

n.d.

7.1 294.38

45.78

100.80

30.42

S11

00

N37°39 56.4

E114°490 31.100

2,260

n.d.

7.0 359.10

53.88

108.43

31.99

S12

N37°330 32.200

E114°560 30.100

2,050

n.d.

7.1 287.43

44.79

99.08

30.33

G1

N38°130 51.600

E114°190 7.800

846

n.d.

7.1

32.26

1.03

110.85

26.62

G8

N37°530 45.600

E114°320 14.700

1,252

n.d.

7.0

28.65

0.80

176.60

38.44

G9

N37°530 44.100

E114°320 26.600

992

n.d.

7.0

51.15

0.75

109.65

33.88

0

0

00

0

00

399

n.d.

6.9

27.54

0.49

44.69

11.37

1,156

n.d.

7.2

42.30

1.42

137.04

56.59

G10

N37°53 41.2

E114°32 50.0

G14

N37°500 2.500

E114°360 51.700

G15

N37°450 58.800

G16 ID

E114°390 39.400

531

n.d.

6.9

17.73

0.53

63.16

21.51

00

E114°420 42.200

1,132

n.d.

7.1

36.64

0.74

126.11

47.60

NH4? (mg L-1)

Cl(mg L-1)

0

N37°45 27.0

NO3(mg L-1)

SO42? (mg L-1)

HCO3(mg L-1)

Water type

dD (%)

d18O (%)

d15N (%)

Spring (March 2010) S1

2.98

16.14

7.74

61.22

63.00

Ca-SO4

n.d.

n.d.

11.3

S2

0.00

25.46

14.75

140.41

198.45

Ca-HCO3

n.d.

n.d.

10.4

S3

0.00

27.34

14.13

136.52

195.30

Ca-SO4

n.d.

n.d.

12.3

S4

0.00

59.99

30.81

241.05

182.70

Ca-HCO3

n.d.

n.d.

11.1

S5

63.45

229.46

0.00

124.73

680.40

Ca-HCO3

n.d.

n.d.

n.d.

S6 S7

93.78 145.01

412.32 372.63

13.16 0.00

370.26 313.42

648.90 891.45

Na-Cl Na-HCO3

n.d. n.d.

n.d. n.d.

21.9 n.d.

123

Environ Earth Sci Table 1 continued NH4? (mg L-1)

Cl(mg L-1)

SO42? (mg L-1)

HCO3(mg L-1)

Water type

dD (%)

d18O (%)

d15N (%)

S8

119.01

349.57

S10

102.83

371.57

0.00

214.67

913.50

Na-HCO3

n.d.

n.d.

n.d.

0.00

179.22

878.85

Na-HCO3

n.d.

n.d.

S11

89.74

n.d.

368.73

0.00

752.26

478.80

Na-SO4

n.d.

n.d.

S12

n.d.

83.65

346.35

0.00

201.83

787.50

Na-HCO3

n.d.

n.d.

n.d.

G2

0.00

239.07

108.81

205.80

491.40

Ca-HCO3

n.d.

n.d.

6.1

G3

0.00

128.76

37.13

81.85

374.85

Ca-HCO3

n.d.

n.d.

10.5

G4

0.00

122.92

40.33

72.90

355.95

Ca-HCO3

n.d.

n.d.

7.9

G5

0.00

50.29

19.25

54.48

447.30

Ca-HCO3

n.d.

n.d.

11.5

G6 G7

0.00 0.00

117.32 238.80

32.75 89.22

80.98 167.69

346.50 535.50

Ca-HCO3 Ca-HCO3

n.d. n.d.

n.d. n.d.

14.7 9.0

G11

0.00

203.19

67.65

164.63

450.45

Ca-HCO3

n.d.

n.d.

9.2

G12

0.00

310.57

156.62

234.01

570.15

Ca-HCO3

n.d.

n.d.

9.1

G13

0.00

54.88

28.32

33.53

318.15

Ca-HCO3

n.d.

n.d.

9.4

ID

NO3(mg L-1)

Autumn (October 2010) S1

n.d.

52.57

6.69

165.82

167.75

Ca-SO4

-54.13

-7.23

12.2

S5

n.d.

134.64

0.00

182.40

689.30

Na-HCO3

-60.47

-8.10

n.d. 19.2

S6

n.d.

274.50

46.00

266.70

460.55

Na-Cl

-60.51

-7.83

S8

n.d.

250.18

20.84

264.19

509.35

Na-HCO3

-56.70

-7.87

6.5

S9

n.d.

313.55

5.67

338.15

518.50

Na-Cl

-56.45

-7.67

2.8

S10

n.d.

276.39

0.00

272.12

732.00

Na-HCO3

-57.08

-8.01

n.d.

S11

n.d.

299.29

0.00

414.11

610.00

Na-SO4

-57.98

-8.00

n.d.

S12

n.d.

268.26

0.00

327.05

594.75

Na-HCO3

-59.97

-8.36

n.d.

G1

n.d.

47.83

30.94

170.38

311.10

Ca-HCO3

-54.26

-7.53

7.8

G8 G9

n.d. n.d.

178.62 104.21

75.80 39.75

98.93 108.34

375.15 366.00

Ca-HCO3 Ca-HCO3

-61.70 -64.92

-8.46 -9.32

7.3 11.4

G10

n.d.

8.88

4.03

11.08

274.50

Ca-HCO3

-67.90

-9.85

n.d.

G14

n.d.

163.66

59.87

80.72

411.75

Ca-HCO3

-61.60

-8.43

7.5

G15

n.d.

38.19

9.42

12.29

338.55

Ca-HCO3

-66.01

-9.08

8.0

G16

n.d.

123.43

63.02

96.33

387.35

Ca-HCO3

-62.29

-8.40

n.d.

n.d. no data

The stable isotopic composition of surface water is below the LMWL, indicating evaporation of rainfall inputs from the river channel. The Huangbizhuang Reservoir and regional groundwater are the main sources of water used for domestic and industrial purposes. The isotopic compositions suggest that river water mixes with reservoir water (S1) with high isotopic values and groundwater (G10) with low values (Fig. 2). The surface water at S5 and S6 was affected greatly by sewage effluent discharge. When compared with the LMWL, the stable isotopic compositions of d18O and dD in groundwater in the study area indicate a predominantly meteoric source. Groundwater data were scattered, and were to the right of the meteoric water line, indicating that precipitation underwent various evaporative effects before recharging the groundwater. However, the comparatively wide range of stable isotopic compositions in groundwater indicates that it

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underwent more variable evaporation and isotopic fractionation than surface water. Human activities, such as irrigation and mixing of groundwater, soil water and rainfall, have strongly influenced the near-surface groundwater layer. In the NCP, deep groundwater receives lateral recharge from the mountains with d18O values lower than -9 % at depths deeper than 100–150 m (Zhang et al. 2000). Results from this study are in agreement with this previous study, as the hydrochemical characteristics and stable isotopic compositions indicate that deep groundwater (G10) at a depth 150 m receives lateral recharge from the mountain area in the western part of the study area. Interaction between surface water and groundwater Exchanges between groundwater and surface water occurred in the study area before the 1960s. Since the

Environ Earth Sci Fig. 2 d18O and dD values of groundwater and surface water samples collected in autumn

Fig. 3 Relationship between Cl- and Na? concentrations and distance from the Huangbizhuang Reservoir Dam on the Xiao River, and interactions between Ca2?, Na? and Mg2?

1970s, water demand has doubled because of the rapid development of cities and industries, and the expansion of farmland irrigation. The overuse of surface water resources and excessive exploitation of groundwater has drastically altered the regional hydrological cycle in the study area. Between 1980 and 1997, about 2,878 km3 of shallow groundwater was over-extracted annually (Liu et al. 2001). The Xiao River, a unique river in the study area, has become an important resource for groundwater recharge. The contribution of river water is a key for assessing the quality and quantity of regional groundwater along the Xiao River.

The average surface water concentration of Cl- was 234.50 ± 158.98 mg L-1 in spring and 233.67 ± 85.33 mg L-1 in autumn, while the average Na? concentration was 176.90 ± 127.50 mg L-1 in spring, and 247.50 ± 101.41 mg L-1 in autumn. Cl- and Na? concentrations reflect domestic wastewater enrichment (Panno et al. 2006), so concentration changes highlight the effects of human activity on the chemical status of the Xiao River (Fig. 3). Na? and Cl- concentrations were low in the upper reaches near Shijiazhuang but rose rapidly as urban wastewater was released into the Xiao River. There are very few natural waterbodies in the study area to dilute the

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Fig. 4 Variations in Cl- concentrations in groundwater with distance from the river channel

wastewater. Na? and Cl- concentrations declined slowly and remained relatively high compared with those in the upper reaches, indicating the important role of wastewater in the lower reaches. Cl- concentrations in groundwater decreased rapidly in the Xiao River basin, from more than 230 mg L-1 to about 50 mg L-1 (average concentration of 133.16 ± 82.08 mg L-1), as distance from the river increased (Fig. 4). This indicates that regional groundwater in the Xiao River basin is strongly influenced by river infiltration, except in certain deep wells such as G10. G10 is protected by an aquiclude with a low Cl- concentration (8.8 mg L-1), and no significant pollutants were observed at this site. Cl- and stable isotopic (dD and d18O) data suggest that the sites near the Xiao River may have a mixture of old water and modern recharge water. When these water sources are considered as groundwater end-members, it is possible to estimate mixing proportions between the waters using a mass-balance approach (Christophersen and Hooper 1992; Clark and Fritz 1997; Kendall and Mcdonnell 1998). Assuming that groundwater is the result of mixing old groundwater and river water, the following equation can be used to assess the contributing fraction (cf) of river water to groundwater: cf ¼

Ci  COG ; CR  COG

ð2Þ

where Ci is the Cl-, dD or d18O (mg L-1 or %) of the mixed groundwater sample, CR is the Cl-, dD or d18O of river water at the closest river site, and COG is the Cl-, dD or d18O of the old groundwater that has not been influenced by river recharge. In the NCP, groundwater with d18O values lower than -9 % is considered to have formed before 10 ka, while groundwater with values higher than -9 % is considered to have formed after 10 ka (Zhang et al. 2000). The former is found at site G10 at a depth of

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150 m, while the latter is found at the other sites that are closer to the Xiao River. Compared with the shallow groundwater, the d18O value at G10 is also different (Fig. 2), so G10 was selected as the old groundwater endmember in this study. Using Cl-, dD and d18O, the average contributing percentages are 50, 47 and 60 %, respectively. Mixing calculation results show that groundwater in study area is made up of 50–60 % river water. This suggests that more than half the volume of local groundwater is recharged by the Xiao River, showing that recharge from the Xiao River is the main source of pollution to the regional groundwater. Furthermore, concentrations of CFC-12 and CFC-11 in groundwater were analyzed at six sites to estimate the residence time of shallow groundwater in the aquifer. The apparent ages estimated by CFC-12 and CFC-11 data gave similar ages at five sites (G1, G9, G14, G15 and G16). However, high concentrations of CFC-12 and CFC-11 showed that site G8 was a modern groundwater site (Table 2). The contributing fraction estimated from Cl-, dD or d18O was used to calculate the CFC-12 apparent age of young groundwater expected to have mixed with older groundwater in the aquifer. The corresponding apparent groundwater residence times are shown in Table 2. Results indicate that groundwater at sites G1, G9, G14, G15 and G16 was recharged during the 1970s and 1980s. From the sites for which CFC ages were estimated, the youngest groundwater was at G8, close to the river, while the oldest groundwater was at G15, far from the river, indicating that groundwater age increases as distance from the river increases. Based on well depth and age data obtained using CFCs, the vertical flow velocity of groundwater was estimated at about 1.8–3.5 m year-1. Rohden et al. (2010) employed several environmental tracers (3H–3He, noble gases, and the stable isotopes 18O and 2H) to estimate groundwater recharge and residence times in the Shijiazhuang area. They showed that groundwater in the unconfined parts of the piedmont plain contained tritium to depths of about 100 m and exhibited 3H–3He ages of less than 40 years. The overall vertical velocity of about 2.5 m year-1 is close to the rate estimated in this study using CFCs. Therefore, it is likely that approximately half of the volume of local groundwater has been recharged by the Xiao River in recent decades. Fate and transport of nitrate in surface water and groundwater Concentrations of NO3- in the Xiao River from the reservoir to the urban zone ranged from 7.74 to 30.8 mg L-1, but NO3- was not detected after the river flowed through the city, except at site S6, which was near the sewage treatment plant (STP) outlet. Instead, ammonium (NH4?)

±5.8 1,976

Fig. 5 Variations in NO3- concentrations with well depth and distance from the river

Contributing fractions were calculated by Cl-, dD or d18O

C. CFC concentration is greater than possible values in equilibrium with modern air, NA not applicable

a

75 G16

0.113

1.184

2.497

57.8 ± 15.2

2.050

4.324

1,987.5

±4.8

±4.5

1,961 1,970.5 80 G15

2.492

0.148

0.163

23.4 ± 13.2

0.631

0.698

1,985

±0.3 1,975

1,976 3.710 1.156

1.060 31.0 ± 6.0

57.0 ± 6.0 2.114

0.627 0.328

0.659 60 G14

0.244 120 G9

0.086

2.024

1,975.5

±1

NA

1,974

NA

1,972

NA

1.785

C. C.

0.752 92.3 ± 4.8

65.3 ± 7.0 4.436

1.647 0.694

14.448 58 G8

1.15 112 G1

0.005

Recharged year (by CFC11) Measured CFC-12 (pmol kg-1) Distance away from river (km) Well depth (m) ID

Table 2 Groundwater well data and recharge year

Measured CFC-11 (pmol kg-1)

Average contributing fraction ± standard deviation (%)

Calculated CFC-12 (pmol kg-1)

Calculated CFC-11 (pmol kg-1)

Recharged year (by CFC12)

Standard deviation of recharged year

Environ Earth Sci

concentrations were high in the downstream part of the Xiao River, indicating the potential contribution of mineralization of organic matter and fertilizer to the enhanced NH4? concentrations (Table 1). NO3- was detected in all groundwater samples, and concentrations ranged from 0.10 to 2.30 mg L-1 with an average value of 104.86 ± 38.37 mg L-1. NH4? however was not detected in groundwater samples in the study area (Table 1). The data therefore suggest that nitrification occurred in the unsaturated zone under farmland irrigated by sewage and also during the sewage treatment process. The vertical and horizontal distributions of NO3- are shown in Fig. 5. Elevated NO3- values ([45 mg L-1) mainly occurred in groundwater from wells shallower than 80 m, while in a horizontal direction, elevated values were detected at distances of up to 2 km. The source of the high nitrate values in groundwater along both sides of the Xiao River may reflect leaching from the riverbed or from the sewage-irrigated farmland, highlighting the severity of the nitrate pollution in the study area. Water quality deterioration in the aquifers coincides with recharge by sewagecontaminated surface water. The contribution of surface water to groundwater is strongly correlated with NO3- concentrations (R2 = 0.851) in the lower reaches (Fig. 6). NO3- concentrations increased as the contributing fraction of surface water increased. This trend indicates that the increased NO3- concentrations in groundwater are the result of recharge from polluted surface water. Site G1 is located in the upper reaches near the Huangbizhuang Reservoir, which was not affected by sewage. Hence, even though most of the groundwater at G1 (cf = 89.2 %) is recharged by surface water infiltration, NO3- concentrations were lower. Cl- is a conservative ion in aquifer environments and can be used in most settings to determine surface water and

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Fig. 6 Calculated fraction (cf) of surface water to groundwater recharge and its relationship to NO3- concentrations. Contributing fractions were calculated by Cl-, dD or d18O

Fig. 7 The relationship between NO3- and Cl- concentrations in groundwater

groundwater movement (Panno et al. 2006). Generally, Cloriginates from natural sources and discharges of industrial, agricultural and domestic wastewaters (Tang et al. 2003). There are no natural sources of Cl- in the aquifer, which implies that the sources of Cl- in the Xiao River are anthropogenic. The significant linearity (R2 = 0.908) between NO3- and Cl- concentrations indicates a common source of nitrate in groundwater (Fig. 7), which suggests that the elevated NO3- concentrations in groundwater are related to anthropogenic activities. The d15N-NO3 can be used to help identify NO3sources, in that NO3- from different sources has characteristic d15N values (Fig. 8). The relationship between d15N-NO3 and residual nitrate can be used to identify N transformations by denitrification, volatilization and fractionation (Robinson 2001; Cao et al. 2012). Surface water with high NH4? concentrations is usually used to irrigate farmland, where NH4? and organic nitrogen in the aerobic unsaturated zone are transformed to NO3- that can move

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Fig. 8 The relationship between d15N-NO3- and NO3-. The blue, green and red boxes show the range of typical d15N-NO3- values for manure and sewage, soil N and fertilizer, respectively. The boxes show the typical d15N-NO3- ranges for manure and sewage, soil nitrogen and N fertilizer. The height of each box is important, but the box width has no specific meaning

into groundwater. Analyses of d15N-NO3 are based on the principle that the three main sources of nitrate pollution have their own characteristic ranges of nitrogen isotope ratios: N fertilizer, -6 to ?6 %; natural soil-N (organic nitrogen), 0 to ?8 %; and manure and sewage, ?4 to ?25 % (Xue et al. 2009). d15N-NO3 values in surface water ranged from 2.8 to 21.9 %. The d15N-NO3 value from the Huangbizhuang Reservoir (S1), at 12.4 %, was characteristic of manure/ septic effluent. However, the NO3- concentration in reservoir water was low (6.72 mg L-1), showing that it contributed little to NO3- in the study area. Concentrations of NO3- and NH4? in industrial and domestic wastewater effluent were 55.28 and 99.18 mg L-1, respectively. The maximum values for d15N (19.2 % in autumn, 21.9 % in spring) were found at the sampling point near the STP drainage outlet, indicating that discharges from the city cause nitrate pollution. The groundwater had higher NO3- concentrations and a narrower d15N-NO3 range (6.1–14.8 %, with an average value of 19.0 ± 2.1 %) than surface water. The d15N-NO3 values of all groundwater samples fall within the typical range of manure/septic effluent and soil organic nitrogen, rather than that of synthetic fertilizer. This indicates that human sewage and animal excreta are the main sources of groundwater nitrate pollution.

Conclusions In this study, hydrogeochemistry, stable isotopes and CFCs were used to determine the relationships between surface

Environ Earth Sci

water and groundwater and anthropogenic influences in a typical NCP watershed. The main findings are as follows: 1.

2.

3.

4.

5.

Concentrations of Cl- and Na? were low in the upper reaches but increased rapidly owing to large discharges of untreated or slightly treated sewage. Therefore, the Xiao River changed from being a CaSO4 type in the upper reaches, to a Na-Cl type in the urban reaches, and finally to a Na-HCO3 type in the lower reaches of the study area. Groundwater in the Xiao River basin belongs to the Ca-HCO3 type. Surface water is the most important source of recharge for groundwater in the NCP. Water from the Xiao River contributed 50–60 % to groundwater, as calculated by Cl-, dD and d18O. CFC results indicated that groundwater age varied from 22.5 to 39.5 years. The vertical flow velocity of groundwater was about 1.8–3.5 m year-1. Most of the surface water and 56.2 % of the groundwater samples were not suitable for drinking. Nitrate enters the groundwater via sewage-contaminated irrigation water and infiltration from the Xiao River channel. Nitrate concentrations in groundwater up to 80 m deep and within a 2 km boundary on both sides of the river exceed the World Health Organization drinking water standard. The significant linearity between the conservative Cl- and NO3ions indicated that human activities are the main source of elevated NO3- concentrations. The d15N-NO3 values highlighted several NO3sources in the Xiao River basin. The surface water showed a wide d15N-NO3 range, indicating multiple sources of NO3- pollution. There were fewer sources of NO3- pollution to groundwater than surface water in the Xiao River, with results showing that human sewage and animal excreta were the main NO3pollution sources in groundwater.

Acknowledgments The authors thank Assistant Prof. Dr. Miwa Matsushima of Chiba University for d15N analyses and also thank Dr. Zhang Yucui of Agricultural Resources Research Center for sample collections. Especially thank reviewers and Dr. Cao yingjie for improving our manuscript.

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