Hydrogeology Journal (2018) 26:1401–1415 https://doi.org/10.1007/s10040-018-1761-y
PAPER
Assessing groundwater availability and the response of the groundwater system to intensive exploitation in the North China Plain by analysis of long-term isotopic tracer data Chen Su 1 & Zhongshuang Cheng 1 & Wen Wei 1 & Zongyu Chen 1 Received: 28 June 2017 / Accepted: 11 March 2018 / Published online: 28 March 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract The use of isotope tracers as a tool for assessing aquifer responses to intensive exploitation is demonstrated and used to attain a better understanding of the sustainability of intensively exploited aquifers in the North China Plain. Eleven well sites were selected that have long-term (years 1985–2014) analysis data of isotopic tracers. The stable isotopes δ18O and δ2H and hydrochemistry were used to understand the hydrodynamic responses of the aquifer system, including unconfined and confined aquifers, to groundwater abstraction. The time series data of 14C activity were also used to assess groundwater age, thereby contributing to an understanding of groundwater sustainability and aquifer depletion. Enrichment of the heavy oxygen isotope (18O) and elevated concentrations of chloride, sulfate, and nitrate were found in groundwater abstracted from the unconfined aquifer, which suggests that intensive exploitation might induce the potential for aquifer contamination. The time series data of 14 C activity showed an increase of groundwater age with exploitation of the confined parts of the aquifer system, which indicates that a larger fraction of old water has been exploited over time, and that the groundwater from the deep aquifer has been mined. The current water demand exceeds the sustainable production capabilities of the aquifer system in the North China Plain. Some measures must be taken to ensure major cuts in groundwater withdrawals from the aquifers after a long period of depletion. Keywords Groundwater age . Quaternary aquifers . Hydrochemistry . Aquifer depletion . China
Introduction Intensive exploitation of groundwater to accommodate increasing water demand has resulted in depletion of aquifers as well as deterioration of water quality in many aquifers, especially in arid and semi-arid areas. In these areas, groundwater resources are often the most important sources for water supply. For sustainable development and management of these water resources, there needs to be an understanding of the response of groundwater systems to exploitation and an assessment of groundwater availability. Groundwater Published in the special issue BGroundwater sustainability in fastdeveloping China^ * Zongyu Chen
[email protected] 1
Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Zhonghua Bei Dajie 268, Shijiazhuang, Hebei, China
monitoring data provide valuable information for clarifying and analyzing the current state of and trends in groundwater systems due to exploitation. Unfortunately, very often, regular information on groundwater levels and water quality in most groundwater monitoring programmes is inadequate and insufficient to assess the long-term evolution of these aquifers. A multiple isotope approach has proven to be important in understanding the hydrodynamic responses and sustainability of intensively exploited aquifers (Chen et al. 2005; IAEA 2006; Grassi et al. 2011; Madioune et al. 2014; Su et al. 2014a, b; Hssaisoune et al. 2017). The North China Plain (NCP) is the centre of agriculture and industry in China. Groundwater is one of the major water resources, which provides ~75% of the water supply for more than 100 million people with a total production rate of about 21 × 109 m3/year. The intensive exploitation of the Quaternary aquifers for water supply has greatly altered the natural groundwater flow systems during the past 40 years. The high rate of water withdrawal has resulted in the decline of water levels and development of a regional depression cone with a
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total area of about 70,000 km2 in the central plain. It is vital to understand sustainability of the intensively exploited aquifer system in this densely populated and intensively cultivated area. Though the basic hydrogeologic parameters of the aquifers have been determined by previous investigations (Zhang et al. 2009; Foster et al. 2004), little is known about the changes of flow pattern or hydrodynamic response to groundwater abstraction due to the complex geology. Previous studies about the isotopes and hydrochemistry in the NCP have confirmed that the flow field has changed and the overall water resource has been depleted because of the overexploitation of groundwater (Zhang et al. 1997; Chen et al. 2005). The groundwater quality has been found to have changed as a result of excessive groundwater abstraction and irrigation return in the piedmont plain, based on the analysis of hydrochemistry (Lu et al. 2008; Xing et al. 2013). The groundwater had been extensively pumped for more than 40 years; however, the response of the groundwater system to intensive exploitation by long-term monitoring of isotopic and hydrochemistry tracers has been seldom reported. The long time series of monitoring data could give more information about the changes in the groundwater flow system. A study of the long-term hydrochemistry data has revealed that deep groundwater has mixed with shallow groundwater in a horizontal transition zone located between the piedmont plain and the central plain (Zhan et al. 2014). Recently, effort had been made to understand the chemical and isotopic responses to intensive groundwater abstraction in the piedmont plain by analysis of the long term data (Cheng et al. 2017); however, there has been very little systematic comparative study on the response of isotopic tracers to intensive exploitation of groundwater in the piedmont plain and the littoral plain. Hydrochemistry and isotopes analysis can be used to characterize intensive exploitation, and it can provide valuable information for assessing the current state of the groundwater system and for forecasting trends in groundwater quality, thus helping to clarify the extent of human impacts on the groundwater system in time and space. The purpose of the present work is to understand the groundwater system responses to intensive exploitation from the tracer data. The key focus is on a groundwater isotope technique, used to assess aquifer sustainability in the NCP. Three main questions are addressed: (1) How have the chemical composition, stable isotopes and groundwater age distribution responded to the exploitation of the aquifer? (2) What changes in aquifer conditions can be learned from the responses of those environmental tracers? (3) Is the aquifer depleting now?
Study area The NCP, covering about 150,000 km2, is one of the most densely populated areas of the world with great agricultural
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importance for China (Fig. 1). The flat plain slopes generally west to east from an elevation of ~100 to 1–2 m above sea level. The slope is 2–1‰ in the piedmont plain, and 0.3–0.1‰ in the eastern coastal plain. The plain geographically consists of the piedmont pluvial plain, the central alluvial plain and the littoral delta plain. The climate is continental semiarid, with a mean annual temperature of ~13 °C. The annual precipitation ranges from 500 to 600 mm. Precipitation is dominated by the summer monsoon from June to August. The Plain is drained by the Haihe River, the Luanhe River, and their tributaries. Most surface water flow occurs in the rainy season. The NCP is a huge subsidence basin of Cenozoic age. The crystalline basement consists of Archean gneiss and Proterozoic carbonate rocks overlain by thick Neogene and Quaternary formations. The thickness of the Quaternary deposits ranges from 150 to 600 m. The aquifer system is a large Quaternary unconfined and confined aquifer system, and consists of four aquifers (Fig. 2). Aquifer 1 is an unconfined aquifer, about 60 m thick, consisting of coarse-grained sand in the piedmont plain to fine-grained sand in the littoral plain. Groundwater is fresh in the piedmont plain but saline in the central and littoral plains (TDS > 2 g/l). Aquifer 2 is a shallow confined aquifer, 60 m thick, consisting of sandy gravel, medium to fine sand. As in aquifer 1, groundwater is saline, with total dissolved solids (TDS) > 2 g/l, from the central to the littoral plains. Aquifer 3, more than 90 m thick, is a confined aquifer consisting of sandy gravel in the piedmont plain to medium to fine sand in the central and the littoral plains. In contrast to aquifers 1 and 2, groundwater is fresh, with TDS of 0.3–0.5 g/l. Aquifer 4 is 50–60 m thick and > 350 m deep and consists of cemented sandy gravel and a thin layer of weathered sand 20–40 m thick. The groundwater in aquifer 4 is fresh, with TDS <1 g/l. Both aquifers 3 and 4 are the targets of exploitation, and they are generally regarded as the deep confined aquifers. The aquifer system had been intensively exploited, mainly for irrigation, since late 1970s, when the mean production rate was 15.7 × 109 m3/year; however, mostly since the 1980s, pumping of groundwater dramatically increased and reached up to the current level of about 21 × 109 m3/ year, which resulted in a continuous water-table drawdown in the unconfined aquifer and the decline of the potentiometric surface in the deep confined aquifers (3 and 4). The unconfined aquifer in the piedmont plain had experienced a water-table decline of more than 30 m, and developed tens of depression cones with individual area more than 100 km2. The biggest one, the Ningbolong depression, extended over about 2,252 km2 with the depth to the water table about 50 m (Zhang et al. 2009). The rate of the watertable drawdown in the unconfined aquifer in Shijiazhuang was about 0.9 m/year in the period of years 1975–1985 and 0.6 m/year in the period of years 1990–2000, while the potentiometric surface drawdown in the aquifer 3 at
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Fig. 1 Study area and sampled sites (revised from Chen et al. 2005). I: Piedmont plain, II: Central plain, III: Littoral plain
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Fig. 2 Hydrogeologic cross section A–A′ shown in Fig. 1 (revised from Zhang et al. 1987)
Cangzhou has been 3 m/year in the period of years 1975– 1985 and 2 m/year in the period of years 1990–2000. After 2004, the extraction of groundwater had been reduced and the drawdown rate was decreasing. Pumping of groundwater from aquifer 3 in the central-littoral plain had resulted in depletion of groundwater reserves with a decline of the potentiometric surface of about 100 m. The evolution of water level and potentiometric surface drawdown in the water depressions is shown in Fig. 3 for aquifer 1 and aquifer 3 respectively. The depth to the potentiometric surface for aquifer 3 increased to 109 m in Cangzhou in 2005, resulting in ground fractures and land subsidence in those areas (Zhang et al. 2009).
Materials and methods The chemical, isotopic and groundwater age data generated during the past studies were synthesized with the data obtained during the current study. Eleven well sites (Fig. 1) were sampled in order to understand the changes in isotope tracers. A total of 62 groundwater samples for isotope analyses were collected at these sites from 1985 to 2014, 25 of which were collected during the present work. All samples were taken from municipal or agricultural wells that were equipped with pumps. Prior to sampling, the wells were pumped until the electrical conductance remained constant. Samples for the measurement of δ18O and δ2H were collected in 50-ml glass bottles with gas-tight caps. The samples for 14C measurement of dissolved inorganic carbon (DIC) were extracted as the precipitate of BaCO3, by adding excess BaCl2 to about 120 L of water previously brought to pH ≥12 with CO2-free NaOH. Samples for 14 C, δ 18 O and δ2 H were measured at the Institute of Hydrogeology and Environmental Geology, Chinese
Academy of Geological Sciences. The δ18O and δ2H analyses were performed on a Finnigan MAT 253 mass spectrometer, and reported in standard δ notation represented as per mil deviation from the V-SMOW (Vienna Standard Mean Ocean Water) standard. The precision of the δ2H and δ18O values were ± 1.0 and ± 0.2‰, respectively. 14C of DIC was determined radiometrically by liquid scintillation counting (1220 Quantulus) after conversion to benzene. The specific 14C activity was reported in the unit of percent modern carbon (pMC), and the precision of 14C activity was ±0.3%. The mean annual values for chemical components were taken directly from Zhang et al. (2009). In their studies, major cations (K+, Na+, Ca2+, Mg2+) analyses were performed by inductively coupled plasma– atomic emission spectrometry (ICP-AES) on a Thermo Scientific ICAP 6300 ICP spectrometer. Analyses for the concentrations of Cl−, SO42−, and NO3− were carried out by the molybdenum blue spectrophotometric method. HCO3− and TDS were measured by acid-base titration and gravimetric analysis, respectively. The error was always less than 3%. The sample sites were selected as representatives of spatial distribution within the aquifer system. Sites S1, S2 and S3 were located in the piedmont plain where the exploited aquifer is the shallow unconfined aquifer (aquifer 1). S4 and S5 were located in the transition zone between the piedmont plain and central plain, where the exploited aquifer was the semi-confined aquifer (aquifer 3). S6–S11 were located in the central plain where the exploited aquifer was the deep confined aquifers (3 and 4). The time series data of isotopes and hydrochemistry were constructed using the data of previous studies in years 1982–1999 (Chen et al. 2005; Guo et al. 1996; Zhang et al. 1987) and the data which were collected in years 2000–2014 for this study (Tables 1 and 2). Data from Zhang et al. (1987), Cheng
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Fig. 3 Evolution of water-level and potentiometric surface drawdown in the water depressions area: a Shiazhuang cross section B–B′ shown in Fig. 1 for aquifer 1; b Cangzhou cross section C–C′ shown in Fig. 1 for aquifer 3
(1988), Chen et al. (2005) and the present study were acquired through analyses undertaken at the Laboratory of the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences (IHEG-CAGS), and data from Kreuzer et al. (2009) were acquired through analyzes undertaken at both the IHEGCAGS and the Institute of Environmental Physics (IUP), H e i d e l b e rg , G er m a n y. D a t a v a l i d a t i o n in v o l v e d crosschecks and provided assurance that the data were adequate for the study. There was no significant problem found with respect to the data coming from different sources. The data set of groundwater-level depth was from the databases in Hebei Municipal General Station of GeoEnvironment Monitoring (HMGGEM). Stable isotope ratios (δ18O, δ2H) and hydrochemistry were used to understand the responses of hydrodynamics to groundwater abstraction. The 14C activity was used to assess groundwater sustainability and aquifer depletion.
Results and discussion Responses of hydrochemistry and isotopes to aquifer exploitation Changes in the piedmont plain In the piedmont plain, significant changes in hydrochemistry had been found in aquifer 1, which were characterized by increase in concentrations of TDS, major ions and pollutants (Cheng et al. 2017). The data of chemical composition in groundwater monitored in Shijiazhuang are plotted in Fig. 4 and show an increasing trend in the concentrations of chloride, sulphate, and nitrate with groundwater exploitation. The elevated concentrations of Ca2++Mg2+ and TDS were 244 and 340 mg/l pre-exploitation in 1959, respectively. At the early stage of groundwater exploitation, the concentration of Ca2++ Mg 2 + was about 255 mg/l in 1977. However, the
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Hydrogeol J (2018) 26:1401–1415 Time series data of stable isotopes and 14C for selected well sites in the NCP
Site No.
Date (month-year)
Sample no.
Well depth (m)
δ2H (‰)
δ18O (‰)
14
C (pMC)
Sources
Aquifers of exploitation
S1
04-1985 05-1987
NC-4 NO87-1
45 40
−61 −58
−8.3 −8.7
93.59 –
Zhang et al. (1987) Cheng (1988)
Aquifer 1
S2
S3
S4
S5
S6
S7
S8
08-1999
NO30
50
−64
−7.7
–
Chen et al. (2005)
05-2001
3
40
−56
−7.4
–
MP
06-2002
C34
40
-53
−7.8
–
MP
03-2004
1
40
−59
−7.8
–
Kreuzer et al. (2009)
08-2012 10-2014
LQ120 SJZ004
40 40
−55 −55
−7 −7.3
– –
MP MP
04-1985 05-1987 08-1999
NC2 NO87-2 NO31
45 35 40
−61 −62 −63
−8.3 −9 −8.3
– – 111.07
Zhang et al. (1987) Cheng (1988) Chen et al.(2005)
05-2001 06-2002 03-2004
4 C35 2
35 30 40
−57 −60 −59
−7.5 −7.8 −7.8
– – 89.36
MP MP Kreuzer et al. (2009)
08-2012 10-2014 04-1985 05-1987
LQ144 SJZ010 NC1 NO87-3
50 40 60 45
−53 −56 −61 −58
−7.3 −7.3 −8.4 −8.7
– – – –
MP MP Zhang et al. (1987) Cheng (1988)
05-2001 03-2004
G26 9
40 80
−53 −60
−8.2 −8.1
– –
MP Kreuzer et al. (2009)
08-2012 10-2014
SJZ42 SJZ014
70 60
−56 −58
−7.3 −7.5
– –
MP MP
04-1985 05-1987 05-1994
NC18 No87-16 Shi
180 80 180
−65.6 −59 −65.9
−9 −7.9 −8.84
67.5 – 62.4
Zhang et al. (1987) Cheng (1988) Guo et al. (1996)
08-1999
No27
180
−69
−8.9
76.92
Chen et al. (2005)
05-2001 04-2005
oh-3 41
180 180
−63 −65-
−8.5 −8.9
61.1 48.11
MP Kreuzer et al. (2009)
08-2012 04-1985 05-1994
DGC8 nc60 Shi 2
180 260 260
−68 −68 −70
−9.3 −9.7 −9.8
49.9 16.1 10.99
MP Zhang et al. (1987) Guo et al. (1996)
08-1999
No25
260
−70
−8.9
56.3
Chen et al. (2005)
04-2005
36chi6-2005
260
−71
−9.9
21.22
Kreuzer et al. 2009
11-2013 08-1999 05-2001 03-2004
HS-08 No21 OH-5 12
260 385 385 385
−68 −82 −84 −82
−9.3 −10.6 −10.7 −10.9
32.4 10.22 18.04 2.3
MP Chen et al. (2005) MP Kreuzer et al. (2009)
04-2013 04-1985 05-1994
HS-04 NC50 Heng 1
385 472 472
−81 −76 –
−10.8 −10.4 –
3.66 4.5 4.37
MP Zhang et al. (1987) Guo et al. (1996)
08-1999
22
340
−85
−10.8
–
Chen et al. (2005)
05-2001 03-2004
C9 17
472 472
−79 −81
−11.1 −10.8
2.707 0.89
MP Kreuzer et al. (2009)
04-2013 05-1994 03-2004
HS-02 Heng2 18
300 350 350
−83 −78 −78
−11 −10.6 −10.6
1.17 4.89 0.93
MP Guo et al. (1996) Kreuzer et al. (2009)
05-2008 11-2013
CZ16 CZ-08
350 350
−81 −85
−10.8 −11.1
1.449 0.98
MP MP
Aquifer 1
Aquifer 1
Aquifer 3
Aquifer 3
Aquifers 3 and 4
Aquifers 3 and 4
Aquifers 3 and 4
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Table 1 (continued) Site No.
Date (month-year)
Sample no.
Well depth (m)
δ2H (‰)
δ18O (‰)
14
C (pMC)
Sources
Aquifers of exploitation
S9
05-1994 08-1999
HB-58 No15
370 370
−78 −80
−10.28 −10.7
7.16 6.29
Guo et al. (1996) Chen et al. (2005)
Aquifers 3 and 4
03-2004
19
370
−81
−10.9
1.25
Kreuzer et al. (2009)
04-2013
CZ-05
400
−80
−10.6
0.51
MP
08-1999 03-2004
No11 20
310 300
−82 −78
−10.5 −10.5
9.94 1
Chen et al. (2005) Kreuzer et al. (2009)
S10
S11
05-2008
05-CERZ
300
−69
−10.3
0.79
MP
04-2013 04-1985 05-1994
CZ-04 NC38 HB46
300 350 350
−77 −74.7 −79
−10.2 −9.98 −10.2
0.54 6.4 4.71
MP Zhang et al. (1987) Guo et al. 1996
08-1999
No9
310
−79
−10.2
7.12
Chen et al. (2005)
03-2004
21
350
−78
−10.5
0.49
Kreuzer et al. (2009)
05-2008 04-2013
BLZ CZ-01
350 350
−76 −76
−10.2 −9.9
0.43 0.44
MP MP
Aquifers 3 and 4
Aquifers 3 and 4
MP measurement in present study
concentration of Ca2++Mg2+ rose to 457 mg/l in 2005 after three decades of intensive pumping. These high values were recognized to be attribution from water–rock interaction—e.g. Table 2 Time series data of chloride and total dissolve solids (TDS) for selected well sites in the NCP Site No.
Date (year)
Sample No.
Well depth (m)
Cl (mg/l)
TDS (mg/l)
S4
1985 1999
NC18 No27
180 180
21 18
– –
2001 2005
OH-3 41
180 180
15.57 21.4
– –
2012 1985 1993 1999 2005 2013 1975 1985 1999 2013
DGC8 NC60 Shi2 No25 36 HS-08 Heng 30 Heng 31 No21 HS-04
180 260 260 260 260 260 346 346 385 385
12.25 7.4 20 66.7 59.6 44.7 112 150 168.3 204.8
– 312 505 611.6 607.4 647.8 569 646 699.58 849.8
1975 1985 1995 2013 1975 1985 1995 2013
Xian 4-3 Xian 4-4 Xian 4-5 CZ-05 Cang8-3.2 Cang8-3.3 Cang8-3.4 CZ-01
395.6 395.6 395.6 400 346 346 346 350
86.9 75 85.08 112 203.9 196.7 209.1 215.3
463.9 456.4 465.33 465.6 1,089 1,065 1,134 1,054
S5
S6
S9
S11
dissolution of calcite (CaCO3) and dolomite [CaMg(CO3)2]— due to the enhancement of active groundwater flushing by the intensive abstraction (Cheng et al. 2017). PHREEQC was used to calculate saturation indexes (SI) of calcite and dolomite in groundwater at site S1 (Table 3). The SI value of calcite and dolomite ranged mostly from 0 to 0.5, indicating that the groundwater was saturated with respect to calcite and dolomite. However, intensive groundwater abstraction had promoted groundwater circulation and therefore groundwater took up more CO2 in the recharge zones, and the hydrolysis and dissolution reactions of limestone and dolomite were to form dissolved bicarbonate, calcium and magnesium ions. The elevated hardness and concentration of HCO3− supported this inference. The molar Mg2+/Ca2+ ratios in groundwater at S1 were lower than 0.8, which suggested that dissolution of limestone was faster than dissolution of magnesium, and no calcite precipitation was available (Table 3). All these factors indirectly reflected that the groundwater in the piedmont plain was young and took an active part in the modern water cycle. Elevated concentrations of Cl − , SO 42− and NO 3 − in groundwater were another significant response of the hydrochemistry to intensive exploitation in the piedmont plain (Fig. 4). The concentration of Cl− was 73 mg/l in 1981 and increased to 136 mg/l in 2000, while the concentration of SO42− increased from 82.25 mg/l in 1978 to 139 mg/l in 2000. These increases possibly resulted from the leaching of chloride-bearing and sulphate-bearing fertilizers by irrigation since there was no evidence on the occurrence of chloride and sulphate-bearing evaporates in the piedmont plain. The concentration of nitrate in groundwater was about 2.35 mg/L preexploitation in 1959. It dramatically increased after the 1970s and was up to 56.2 mg/l in 2005. Some studies confirmed that
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Hydrogeol J (2018) 26:1401–1415 Table 3 The hydrochemical results of groundwater abstracted from the unconfined aquifer at site S1 Date (month-year)
Molar Mg2+/Ca2+
Log(SI) Calcite
Dolomite
04-1985
0.31
0.13
08-1999
0.13
−0.16
0.33 0.43
05-2001 03-2004
0.27 0.24
0.35 0.19
0.50 0.52
10-2014
0.41
0.36
0.37
groundwater abstraction since 1985 at S1, S2 and S3 in the piedmont plain (Fig. 5). The δ18O value for the three sites S1, S2, S3 varied from −9.0 to −8.2‰ with a mean value of −8.5 ± 0.3‰ in the mid-1980s (years 1984–1987), −8.2 to −7.8‰
Fig. 4 Hydrochemical changes with aquifer exploitation over time in the Shijiazhuang piedmont plain (from Cheng et al. 2017): a depth to the water table in the monitoring well for aquifer 1 near site 1 and pumping rate associated with urban aquifer exploitation; b TDS and hardness changes with aquifer exploitation; c Ion concentration changes with aquifer exploitation; d SO42− and Cl− concentration changes with the NO3− concentration
the sources of nitrate were fertilizer and manure (Chen et al. 2006; Liu and Chen 2009). These elevated concentrations of nitrate resulted from repeated irrigation and use of fertilizers. The drawdown of water level due to intensive exploitation allowed the oxidation of ammonia to nitrate, which dissolved into the groundwater. It is seen from Fig. 4 that the concentrations of Cl− and SO42− increased with the concentration of NO3−, which supported the inference mentioned earlier. The isotopic responses to groundwater exploitation were characterized by increasing δ18O and δ2H concentrations with
Fig. 5 Changes in δ18O with exploitation of the unconfined aquifer of the piedmont plain for: a site S1; b site S2; c site S3
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with a mean value of −7.8 ± 0.27‰ in the early 2000s (years 2001–2004), and − 7.5 to −7.0‰ with a mean value of −7.3 ± 0.16‰ in the early 2010s (years 2012–2014). These samples had measurable tritium (>10 TU) indicating modern recharge. All the samples are plotted as δ18O vs. δ2H in Fig. 6. The local meteoric water line (LMWL), δ2H = 7.32 × δ18O + 2.99, was derived from data at the Shijiazhuang station of the Global Network of Isotopes in Precipitation (GNIP) for China. Samples fell in a fitted evaporation line with a slope of 4 (δ2H = 4 × δ18O – 30). The isotopic composition of these waters prior to evaporation was estimated from the intercept of the evaporation line with the LMWL and it was about −8.8‰ for δ18O and − 61‰ for δ2H, which was close to the weighted mean value of precipitation in Shijiazhuang, indicating that the recharge source is the local precipitation. With the aquifer exploitation and decline of water level, groundwater was gradually enriched in 18O from years 1985–1999 to years 2001– 2004 and 2012–2014. However, the value of δ18O and δ2H for precipitation fluctuated up and down around their mean value from 1985 to 2003, which showed a different trend to that of the groundwater in the piedmont plain. It was suggested in a previous study (Cheng et al. 2017) that the enriched heavy stable isotope (18O) in groundwater was contributed by enhanced local recharge and irrigation return due to intensive groundwater exploitation. Pumping groundwater to irrigate crops during the dry season had led to wet soils and aquifer recharge by irrigation water. Further, the continuous recycling of groundwater used for irrigation had resulted in increasing δ18O values over time; groundwater circulation has been enhanced due to intensive exploitation of the unconfined aquifer in the piedmont. This inference had been substantiated by the
Fig. 6 Plot of δ18O vs. δ2H for unconfined groundwater in the piedmont plain
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hydrochemical responses discussed in the preceding. The content of tritium ranged from 3 TU to more than 100 TU for samples collected in 1985, and from 1 to 35 TU for those in 2000. The depth of the tritium-null-line had been found to move downward about 50 m from 1985 to 2000 (Cheng et al. 2017). These factors indicate that the shallow aquifer had been overexploited. Changes in the transition zone Sites S4 and S5 abstract groundwater from aquifer 3 and are located in the transition zone between the piedmont plain and the central plain. The chemical data from S4 and S5 presented different responses to the aquifer exploitation. The S4 well is close to the front of the piedmont plain and the well depth is 150–180 m, where the aquifer is semi-confined. The concentrations of chloride and TDS have shown no significant changes since the 1980s. The chloride concentration was 21 mg/l in 1985 and 12.25 mg/l in 2012, and varied within a range of about 10 mg/l; however, both δ18O and 14C activity has exhibited a decreasing trend with groundwater exploitation since the 1980s. The δ18O value varied from −7.9‰ in 1987 to −8.9‰ in 1999 and −9.3‰ in 2012, and the 14C activity slightly decreased from 67.5 pMC in 1985, to 48.11 and 49.9 pMC in 2004 and 2012, respectively (Fig. 7). The corresponding apparent age of groundwater increased from 3.25 ka in the 1980s to 5.85 ka in the 2000s. These ages indicate that a much larger fraction of old age groundwater (with depleted δ18O) had been pumped in the 2000s than that in the 1980s, and the aquifer had been depleting with the decline of water level. The S5 well is located on the edge of the confined portion of the aquifer which is overlaid with a layer containing brackish water. The sampling well draws fresh groundwater and is at a depth of 260 m. Hydrochemical data showed the concentrations of chloride and TDS significantly increased with groundwater exploitation since the 1980s. The chloride concentration was 7.4 mg/l in 1999, and increased to 59.6 mg/l in 2005. Accordingly, TDS elevated from 312 mg/l in 1985 to 607 mg/l in 2005. These data might imply mixing with the brackish water from the shallow aquifer. Before 1993, both the δ18O value and 14C activity slightly decreased with aquifer exploitation, and this indicated that the pumping water was from the aquifer storage and associated with water-level decline at the early stage of aquifer exploitation. The δ18O value showed no significant changes after 1993, except in 1999. The 14C activity increased from 10.99 pMC in 1993 to 32.4 pMC in 2013, which was equivalent to a water age change from 18.25 ka in 1993 to 9.32 ka in 2013. It is of interest to note that the maximum content of Cl−, TDS, δ18O and 14C occurred in 1999 when groundwater pumping reached its peak, before the cuts in production rate in 2005. This finding indicates that the influx of young brackish water
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Fig. 7 Changes in isotopes and chloride concentration with exploitation from aquifer 3 in the transition zone for: a−c site S4; d −f site S5
with high δ18O to aquifer 3 was induced by the large-scale abstraction of deep confined groundwater. Changes in the central plain In the central and littoral plain where the deep confined aquifers (3 and 4) had been intensively exploited, most of the wells were not found to have significant variation in water quality. The concentrations of Cl− and TDS for S6, S9 and S11 are shown in Fig. 8. The S6 well is located in the western part of the confined aquifer with well depth of ~350 m. As for well S6, the concentrations of chloride and TDS presented a general increase (569 mg/l in 1975 and 850 mg/l in 2013) with aquifer exploitation. Unfortunately, the δ18O data pre-1999 were absent, and the existing data did not show an enrichment of the heavy isotope (18O) in pumped water after 1999; however, the 14C activity revealed an increase of groundwater age after 2001. This indicated that more older-age water with a relatively high TDS had been pumped from the deep confined aquifers (3 and 4), which was displaying water-level drawndown, after 2001. Unlike S6, the wells at both sites S9 and S11, which draw from the highly confined aquifers 3 and 4, did not exhibit an increase in concentrations of chloride and TDS. The previous
studies demonstrated that the quality of groundwater in the deep confined aquifers (3 and 4) in the central plain had not substantially changed for the last 40 years, especially for those wells of >400-m depth (Zhang et al. 2009). However, an elevated concentration of fluoride emerged in groundwater with the exploitation at Cangzhou (S11), which is in the centre of a regional depression cone (Wang et al. 2011). High fluoride concentrations were found in pore water within the aquitard between aquifer 2 and 3 (Zhang 1991). Thus, the elevated concentration of fluoride in the groundwater was caused by aquitard compaction and leakage to the groundwater at the Cangzhou location. The changes in fluoride concentration and cumulative land subsidence are shown in Fig. 9, in which the cubic curve-fitting method was applied to determine the trend line of fluoride concentration. The process of land subsidence showed three stages, with the first being a slow subsidence process which happened before 1982 when the depth of the potentiometric surface was less than 50 m. The second was a rapid subsidence process during 1983–1996 when the depth of the potentiometric surface was 50–70 m. The last stage was another slow subsidence process which emerged after 2000. The fluoride concentration trends were in approximate agreement with the slow subsidence processes, which indicated that leakage was dominated by water released from
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curve-fitting method in Fig. 11. Groundwater samples from the well at site S7, which abstracts from aquifers 3 and 4 in the central plain, had 14C content of 4.50 pMC in 1985, 4.36 pMC in 1994, 2.27 pMC in 2001 and 1.17 pMC in 2013, and the apparent ages of the groundwaters were equivalent to 25.6, 25.9, 29.8 and 36.7 ka, respectively. Groundwater samples at S11, which is close to the littoral plain, had 14C content of 6.40 pMC in 1985, 4.71 pMC in 1994, 0.49 pMC in 2004 and 0.44 pMC in 2013. The apparent ages of the groundwaters were equivalent to 22.7, 25.2, 43.9, and 43.9 ka, respectively. This increase of groundwater age over time was possibly caused by the exploitation of the old water, associated with dramatic groundwater-level decline in the depression area, and indicated that the deep confined aquifer had been substantially depleted.
Implications for groundwater availability Changes in hydrodynamics and water quantity
Fig. 8 Changes in a Cl− and b TDS concentration with deep confined aquifer exploitation in the central plain
aquitard compaction. This suggests that the deep aquifer had been depleted and groundwater at this site has been mined. The δ18O values for groundwater samples at S7–S11 were in the range of −10.0 to −11.1‰. They did not reveal a significant systematic variation with groundwater exploitation (Fig. 10). This was probably because these groundwaters were recharged during the glacial period and had a narrow range of depleted δ18O values; however, the 14C activity decreased with aquifer exploitation, shown by the 14C trend line with the quadric
Fig. 9 Changes in fluoride (F) concentration with deep confined aquifer exploitation and land subsidence in the central plain (revised from Wang et al. 2011)
The isotopic tracers can delineate not only the natural groundwater flow system but also the disturbance caused by the groundwater exploitation. The spatial distribution of 14 C age reveals that the regional groundwater flow direction is from the piedmont plain to the coastal area. Modern age water with detectable tritium is associated with the local flow system and short flow paths in the piedmont plain, and paleowater in the deep confined aquifers (3 and 4) is associated with the regional flow system and long flow paths to the coastal plain. The water in the unconfined part of the piedmont is naturally drained by springs in the distal fan, with a dominant presence of modern waters, whereas the confined part is drained by upward leakage to the overlying aquifers in the central to coastal plain (Chen et al. 2010). This flow regime, however, has been disturbed by exploitation during the past 40 years. The flow between the natural recharge and discharge areas has been intercepted, and the original flow directions have been changed due to the depression. In the piedmont plain, both the enrichment of heavy stable isotopes (18O) and downward displacement of the tritiumnull-line suggest that intensive groundwater abstraction had promoted the downward movement of young inflow of the local groundwater system, as well as an acceleration of groundwater flow velocities. However, the continuous decline of the water level, with a mean rate of about 1–3 m/year, indicated more outflow than inflow for the intensively exploited aquifer, leading to significant depletion of the aquifer storage. In the central plain, the intensive exploitation of the deep confined aquifers (3 and 4) had altered hydraulic gradients and developed a regional cone of depression. Analysis of groundwater in the confined parts of the central plain suggested that 14 C ages might date to more than 40 ka BP and became
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progressively older with distance and depth. A study of noble gas temperatures (NGT) suggested that recharge took place during the last glacial period (Kreuze et al., 2009). This conclusion agreed with the results of a pollen study (Kung and Tu 1980) and records of paleohydrology (Shi et al. 1998). These suggested a slow natural replenishment rate to the highly confined portions. The significant increase in 14C age of groundwater in the central plain (S7–S11) after 1994 indicated that a high production rate seriously affected the total reserve, including groundwater storage, and resulted in lowering of hydraulic heads. This increasing groundwater age in samples was linked with a gradual decline of water level, and suggested that more old waters were exploited from the upward-flowing deep regional groundwater system, or from the mixing with old water released from clay layers that were squeezed by land subsidence. Numerical modeling of the NCP showed a similar age distribution in the deep aquifers, and the older water flowed upward due to groundwater pumping (Cao 2013). With exploitation of the deep confined aquifers (3 and 4), the groundwaterlevel displayed drawdown and groundwater reserves were progressively depleted, resulting in some negative effects such as land subsidence. This situation indicates that Fig. 10 Changes in δ18O with deep confined aquifer exploitation in the central plain for: a site S6; b site S7; c site S8; d site S9; e site S10; and f site S11
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groundwater in the deep confined aquifers (3 and 4) is being mined. Changes in water chemical composition and quality The intensive exploitation of groundwater has resulted in groundwater quality degradation in the NCP, particularly in the piedmont plain and the transition zone. These changes are characterized by increases of TDS and pollutants. Significant increases in TDS, total hardness (Ca2++Mg2+), bicarbonate, chloride, sulphate, and nitrate had been found in the piedmont plain (Zhang et al. 2009). The response of stable isotopes suggested that this aquifer degradation could be attributed to the dissolution of carbonate minerals, irrigation recharge (with a water of different chemical composition or polluted water), and the downward leaching of fertilizers, which were induced by the lowering of the water table (Zhan et al. 2014). The increase in chloride and TDS concentrations in the transition zone were the result of downward influx of brackish water from superposed aquifers into the exploited aquifers owing to the decline of the water level. The increasing trend of fluoride concentration in Cangzhou implied not only depletion of the groundwater resource but also the potential risk of leakage
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Fig. 11 Changes in 14C content with aquifer exploitation in the central plain for: a site S6; b site S7; c site S8; d site S9; e site S10; and f site S11
from the shallow saline aquifer due to deep water abstractions, even though the water quality degradation was not yet evident.
Implication for aquifer sustainability Groundwater age data showed that modern-age water was only found in the wells located in the piedmont plain, and paleowater, which might date to Pleistocene times, was found in the wells located in the central-littoral plain (Chen et al. 2003). The very negative δ18O and δD values of groundwater in the deep confined aquifers (3 and 4) reflect the water being recharged during the past glacial period when recharge was greater because of the lack of vegetation in colder winter periods. These factors imply that the paleowater in the deep confined aquifers (3 and 4) constitute an inherently finite and limited resource. In the piedmont plain, the chemical and isotopic responses indicate more outflow than inflow for the intensively exploited aquifer, leading to significant depletion of aquifer storage; additionally, the enrichment of the heavy oxygen isotope and elevated concentration of nitrate suggested that intensive exploitation might increase the potential risk of aquifer contamination. The decline of the water table and development of a
water depression cone indicated unsustainable groundwater exploitation. In the central plain, lowing 14C activity with aquifer exploitation suggested a greater fraction of older water was being exploited. The results from the numerical modeling, to analyze the impacts by pumping, were consistent with the results of the long-term isotopic tracer analyses. The numerical modeling revealed that the groundwater age distribution in the NCP was mainly affected by the change in the dynamic conditions of groundwater pumping, and that groundwater storage was depleted (Shao et al. 2009; Cao et al. 2013). The primary inflow to the deep aquifer was downward leakage from the shallow aquifer, which was enhanced by the extensive development of the deep aquifer (Cao 2013). The water released from compaction of the overlying aquitard (the result of intensive overexploitation of the deep aquifer) accounted for about 15–30% of the groundwater recharge to the deep confined aquifers in the NCP (Li et al. 2012; Su et al. 2014a, b). The land subsidence indicated that groundwater in the deep confined aquifer was being mined. The current demand for water exceeds the sustainable production capabilities of the aquifer system in the NCP. Some measures must be taken to ensure major cuts in groundwater withdrawals after the recorded long period of depletion.
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Conclusions This study shows that isotopic and hydrochemical tracer data contribute to the understanding of the various responses of aquifers to intensive groundwater exploitation, and answers the interrelated questions on groundwater sustainability of intensively exploited aquifer systems. Some helpful information on aquifer sustainability can be acquired from use of multiple environmental tracers. The stable isotopes, groundwater age and chemical compositions had been used as indicators for variations in recharge processes and hydrodynamics of intensively exploited aquifer systems. The present work suggests that the use of stable isotopes and chemical compositions of water was more effective in understanding the variations of recharge and hydrodynamics in the unconfined aquifer than those in the confined aquifers in the NCP, and groundwater age was a valuable indicator for assessment of groundwater sustainability. The time series data of 14C activity was confirmed to be valid for assessing the sustainability of aquifers, and could be routinely used for early warning of aquifer depletion caused by groundwater abstractions. The response of isotopic and chemical tracers indicated that the whole aquifer system in the NCP is currently being depleted. A comprehensive strategy should be developed for effective management that ensures long-term stable and flexible water supplies to meet water demand in the NCP. Funding Information This study was financially supported by the National Natural Science Foundation of China (NSFC grant No. 41602268 and No. 41702283) and the coordinated research project of the International Atomic Energy Agency (IAEA-CRPF33019, Research Contract No.17314).
References Cao G (2013) Evaluation of the groundwater system in the North China Plain using groundwater modelling. PhD Thesis, China University of Geosciences, Beijing Cao G, Zheng C, Scanlon BR, Liu J, Li W (2013) Use of flow modelling to assess sustainability of groundwater resources in the North China Plain. Water Resour Res 49:159–175. https://doi.org/10.1029/ 2012WR011899 Chen Z, Qi J, Xu J, Ye H, Nan Y (2003) Paleoclimatic interpretation of the past 30 ka from isotopic studies of the deep confined aquifer of the North China Plain. Appl Geochem 18(7):997–1009. https://doi. org/10.1016/S0883-2927(02)00206-8 Chen Z, Nie Z, Zhang Z, Qi J, Nan Y (2005) Isotopes and sustainability of ground water resources, North China Plain. Ground Water 43(4): 485–493. https://doi.org/10.1111/j.1745-6584.2005.0038.x Chen J, Tang C, Yu J (2006) Use of 18O, 2H and 15N to identify nitrate contamination of groundwater in a wastewater irrigated field near the city of Shijiazhuang, China. J Hydrol 326(1–4):367–378. https:// doi.org/10.1016/j.jhydrol.2005.11.007 Chen Z, Qi J, Zhang Z (2010) Application of an isotopic hydrogeology method in a typical basin in northern China. Science Press, Beijing Cheng R (1988) Groundwater replenishment analysis through isotope composition of water in nature. In: Liu C, Ren H (eds) Air, surface,
Hydrogeol J (2018) 26:1401–1415 soil and groundwater interaction (in Chinese). Science Press, Beijing, pp 275–286 Cheng Z, Zhang Y, Su C, Chen Z (2017) Chemical and isotopic response to intensive groundwater abstraction and its implications on aquifer sustainability in Shijiazhuang, China. J Earth Sci-China 28(3):523– 534. https://doi.org/10.1007/s12583-017-0729-5 Foster S, Garduno H, Evans R, Olson D, Tian Y, Zhang W, Han Z (2004) Quaternary aquifer of the North China Plain: assessing and achieving groundwater resource sustainability. Hydrogeol J 12(1):81–93. https://doi.org/10.1007/s10040-003-0300-6 Grassi S, Doveri M, Cortecci G, Amadori M (2011) Hydrogeological study of the intensely exploited aquifer of the Santa Croce leatherproducing district, Tuscany (central Italy). Hydrogeol J 19(3):671– 684. https://doi.org/10.1007/s10040-011-0710-9 Guo Y, Liu S, Zhang Z, Shen Z, Zhong Z (1996) The formation law of deep-lying groundwater resources in areas south of Jingjin in the Hebei Plain (in Chinese). Geol Rev 42(5):410–415 Hssaisoune M, Bouchaou L, N’Da B, Malki M, Abahous H, Fryar AE (2017) Isotopes to assess sustainability of overexploited groundwater in the Souss–Massa system (Morocco). Isot Environ Health Stud 53(3):298–312. https://doi.org/10.1080/10256016.2016.1254208 IAEA (2006) Isotopic assessment of long-term groundwater exploitation. IAEA-TECDOC-1507, IAEA, Vienna Kreuzer AM, Von Rohden CV, Friefrich R, Chen Z, Shi J, Hajdas I, Kipfer R, Werner A (2009) A record of temperature and monsoon intensity over the past 40 kyr from groundwater in the North China Plain. Chem Geol 259(3–4):168–180. https://doi.org/10.1016/j. chemgeo.2008.11.001 Kung C, Tu N (1980) Vegetational and climatic changes in the past 30, 000–10,000 years in Beijing. Acta Bot Sin 22:330–338 Li W, Cui Y, Su C, Zhang W, Shao J (2012) A study on an integrated numerical groundwater and land subsidence model of Tianjin (in Chinese). J Jilin Univ (Earth Sci Ed) 42(3):805–813 Liu J, Chen Z (2009) Using stable isotope to trace the sources of nitrate in groundwater in Shijiazhuang (in Chinese). Environ Sci 30(6):42–48 Lu Y, Tang C, Chen J, Song X, Li F, Sakura Y (2008) Spatial characteristics of water quality, stable isotopes and tritium associated with groundwater flow in the Hutuo River alluvial fan plain of the North China Plain. Hydrogeol J 16(5):1003–1015. https://doi.org/ 10.1007/s10040-008-0292-3 Madioune DH, Faye S, Orban P, Brouyère S, Dassargues A, Mudry J, Stumpp C, Maloszewski P (2014) Application of isotopic tracers as a tool for understanding hydrodynamic behavior of the highly exploited Diass aquifer system (Senegal). J Hydrol 511(16):443– 459. https://doi.org/10.1016/j.jhydrol.2014.01.037 Shao J, Zhao Z, Cui Y, Wang R, Li C, Yang Q (2009) Application of groundwater modelling system to the evaluation of groundwater resources in North China Plain (in Chinese). Resour Sci 31(3):361–367 Shi D, Yin X, Sun J, Yin Z (1998) A simulation study on the evolution of groundwater circulation systems in Cenozoic basins of northern China. Acta Geol Sin 72:100–107. https://doi.org/10.1111/j.17556724.1998.tb00737.x Su A, Chen Z, Liu J, Wei W (2014a) Sustainability of intensively exploited aquifer Systems in the North China Plain: insights from multiple environmental tracers. J Earth Sci-China 25(3):605–611. https://doi.org/10.1007/s12583-014-0441-7 Su C, Chen Z, Chen J, Chen J, Fei Y, Chen J, Duan B (2014b) Mechanics of aquitard drainage by aquifer-system compaction and its applications for water-management in the North China Plain. J Earth SciChina 25(3):598–604. https://doi.org/10.1007/s12583-014-0440-8 Wang Y, Chen Z, Fei Y, Liu J, Wei W (2011) Estimation of water released from aquitard compaction indicated by fluorine in Cangzhou (in Chinese). J Jilin Univ (Earth Sci Ed) 41(sup.1):298–302 Xing L, Guo H, Zhan Y (2013) Groundwater hydrochemical characteristics and processes along flow paths in the North China Plain. J Asian Earth Sci 70-71:250–264. https://doi.org/10.1016/j.jseaes.2013.03.017
Hydrogeol J (2018) 26:1401–1415 Zhan Y, Guo H, Wang Y, Li R, Hou C, Shao J, Cui Y (2014) Evolution of groundwater major components in the Hebei Plain: evidences from 30-year monitoring data. J Earth Sci-China 25(3):563–574. https:// doi.org/10.1007/s12583-014-0445-3 Zhang R (1991) A mechanism on the increase of fluorine in the deep confined aquifer in eastern Hebei Plain (in Chinese). Hydrogeol Eng Geol 18:56–58 Zhang Z, Zhang H, Sun J, Yang L, Cui Y, Wu L, Qi J, Xu J (1987) Environmental isotope study related to ground water age, flow
1415 system and saline water origin in quaternary aquifer of Hebei Plain (in Chinese). Hydrogeol Eng Geol 96(1):1–6 Zhang Z, Shi D, Ren F, Yin Z, Sun J, Zhang C (1997) Evolution of Quaternary groundwater system in North China Plain. Sci China Ser D 40(3):276–283 Zhang Z, Fei Y, Chen Z (2009) Investigation and assessment of sustainable utilization of groundwater resources in the North China Plain (in Chinese). Geological Publishing House, Beijing
