Journal of Hydrology 508 (2014) 12–27
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Journal of Hydrology
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Chemical and isotopic constraints on evolution of groundwater salinization in the coastal plain aquifer of Laizhou Bay, China ⇑ D.M. Han a, , X.F. Song a, Matthew J. Currell b, J.L. Yang c, G.Q. Xiao c a Key Laboratory of Water Cycle & Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China b School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne 3001, Australia c Tianjin Institute of Geology and Mineral Resources, Tianjin 300170, China article info summary
Article history: A hydrochemical-isotopic investigation of the Laizhou Bay Quaternary aquifer in north China provides Received 19 June 2013 new insights into the hydrodynamic and geochemical relationships between freshwater, seawater and Received in revised form 28 September 2013 brine at different depths in coastal sediments. Saltwater intrusion mainly occurs due to two cones of Accepted 26 October 2013 depression caused by concentrated exploitation of fresh groundwater in the south, and brine water for Available online 1 November 2013 salt production in the north. Groundwater is characterized by hydrochemical zonation of water types This manuscript was handled by Corrado Corradini, Editor-in-Chief, with the (ranging from Ca–HCO3 to Na–Cl) from south to north, controlled by migration and mixing of saline water assistance of Michel Bakalowicz, Associate bodies with the regional groundwater. The strong adherence of the majority of ion/Cl ratios to mixing Editor lines between freshwater and saline water end-members (brine or seawater) indicates the importance of mixing under natural and/or anthropogenic influences. Examination of the groundwater stable isotope 18 2 Keywords: d O and d H values (between 9.5‰ and 3.0‰ and 75‰ and 40‰, respectively) and chloride con- Laizhou Bay tents ( 2 to 1000 meq/L) of the groundwater indicate that the saline end-member is brine rather than Coastal aquifers seawater, and most groundwater samples plot on mixing trajectories between fresh groundwater Groundwater hydrochemistry (d18O of between 6.0‰ and 9.0‰; Cl < 5 meq/L) and sampled brines (d18O of approximately 3.0‰ Stable isotopes and Cl > 1000 meq/L). Locally elevated Na/Cl ratios likely result from ion exchange in areas of long-term Saltwater intrusion freshening. The brines, with radiocarbon activities of 30 to 60 pMC likely formed during the Holocene as a result of the sequence of transgression-regression and evaporation; while deep, fresh groundwater with depleted stable isotopic values (d18O= 9.7‰ and d2H= 71‰) and low radiocarbon activity (<20 pMC) was probably recharged during a cooler period in the late Pleistocene, as is common throughout northern China. An increase in the salinity and tritium concentration in some shallow groundwater sampled in the 1990s and re-sampled here indicates that intensive brine extraction has locally resulted in rapid mixing of young, fresh groundwater and saline brine. The d18O and d2H values of brines ( 3.0‰ and 35‰) are much lower than that of modern seawater, which could be explained by 1) mixing of original (d18O enriched) brine that was more saline than presently observed, with fresh groundwater recharged by pre- cipitation and/or 2) dilution of the palaeo-seawater with continental runoff prior to and/or during brine formation. The first mechanism is supported by relatively high Br/Cl molar ratios (1.7 10 3–2.5 10 3) in brine water compared with 1.5 10 3 in seawater, which could indicate that the brines originally reached halite saturation and were subsequently diluted with fresher groundwater over the long-term. Decreasing 14C activities with increasing sampling depth and increasing proximity to the coastline indi- cate that the south coastal aquifer in Laizhou Bay is dominated by regional lateral flow, on millennial timescales. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction between 580 and 1945 per square kilometer, and of China’s 1.3 bil- lion people, over 60% live in 14 coastal provinces (Shi, 2012). The Currently about 44 percent of the world’s population lives with- rate of population growth in coastal areas is accelerating and in 150 km of the coast (UN Atlas, 2010). Along much of China’s increasing water resources demand and economic development 18,000 km of continental coastline, population densities average adds to pressure on the environment. Seawater intrusion is a widespread geologic hazard in many
⇑ Corresponding author. Present address: Jia11#, Datun Road, Chaoyang District, coastal regions around the world. It generally occurs when with- 100101 Beijing, China. Tel./fax: +86 10 64889849. drawal of fresh groundwater from coastal aquifers results in E-mail address: [email protected] (D.M. Han). declining groundwater levels, facilitating lateral and/or vertical
0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.10.040 D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27 13 migration of saline water, causing deterioration of groundwater plain of Laizhou Bay in relation to recharge, salinity sources, quality (Barker et al., 1998; Barlow and Reichard, 2010). Saliniza- mixing behavior and palaeo-evolution of groundwater by con- tion of coastal aquifers can occur due to simple, direct seawater sidering a wide selection of geochemical indicators. The data intrusion, but it can also involve a range of complex geochemical examined includes field observations of water table fluctuations processes which control water quality in different ways; e.g., in- and analysis of hydrochemical and stable isotopic compositions ter-aquifer mixing, mobilization of brines, water–rock interaction of groundwater. It is likely that much of the water being and anthropogenic contamination (Vengosh et al., 2005). Rising exploited was recharged under different climatic conditions to sea levels, and increasing climate variability (e.g. longer droughts) the present day (e.g. Chen et al., 2003; Kreuzer et al., 2009) are likely to place increasing pressure on coastal aquifers both in and that the distribution of salinity has resulted from a combi- northern China and globally in coming years (Werner and nation of past and recent processes, including a number of Simmons, 2009; Green et al., 2011). In this context, a rigorous phases of regression, transgression, brine formation and mixing. understanding of the physical and chemical processes involved in The results have significant implications for the management of salinization of coastal aquifers is vital for the future management the exploitation and recharge of coastal aquifers that are inten- of these vulnerable water resources. sively exploited globally, and demonstrate the value of using The Bohai Sea Coast is the region most seriously affected by sea- isotopic indicators in conjunction with other hydrogeological water intrusion in China, particularly Laizhou Bay in northern information. Shandong province (Fig. 1). The problem has resulted due to inten- sive groundwater extraction during rapid development in the re- gion since the 1970s, and has been recognized by local 2. Site description authorities since 1976 (Ji, 1991). The coastal aquifers of Laizhou Bay have recently been the focus of attention due to increasing 2.1. Background of study area stress on fresh water resources and environmental degradation. Management of fresh groundwater resources in coastal aquifers re- The study area is the southern coastal area of Laizhou Bay, quires an understanding of the processes controlling groundwater including the 43 km-long coastline and adjacent area of approxi- geochemical evolution and flow system dynamics. By examining mately 1400 km2 (Fig. 1). Elevation of the coastal plain ranges from hydrochemical and isotopic data together with geology, topogra- 30 m a.s.l. in the south to 1–2 m a.s.l. in the north (Chen et al.,1997) phy and hydrogeological data, seawater intrusion can be compre- with an average slope of 0.5‰ towards the sea. The Wei River, Yu hensively assessed as a geologic hazard (e.g. Vengosh et al., River and Jiaolai River flow through the study area into Laizhou 2005; Andersen et al., 2005; Jorgensen et al., 2008). The approach Bay. The area belongs to a sub-humid monsoon climate, with mean of using multiple sources of data can improve understanding of air temperature of 11.9 °C. The annual mean precipitation is hydrogeologic processes and aid the reliability of groundwater 660 mm, generally concentrated in June, July and August. Average flow models (e.g. Vengosh et al., 2002; Carrera et al., 2005). How- annual potential evapotranspiration is approximately 1400 mm. ever, comprehensive approaches to groundwater problems involv- Past work in this region has been carried out examining the mech- ing use of chemistry and isotopic indicators are few in coastal anism, development, and preventative countermeasures for sea- aquifer research in China. water intrusion (e.g. Zhang and Peng, 1998; Xue et al., 2000), and The objectives of the present paper are to provide an under- the formation of preferential channels of salt water intrusion standing of the evolution of groundwater in the south coastal (e.g. Zhang et al., 1996; Li et al., 2000).
Fig. 1. Location map of the study area, modified after TJR (2006). TDS concentrations were measured in November 2005. Drawdown cone refers to area enclosed by 0 m water table height contour. P–P0 line is the location of the cross-section in Fig. 2. Right-upper map for showing the sampling wells in the study area. 14 D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27
2.2. Hydrogeological conditions conditions (e.g. rainfall and runoff amounts) and particularly in re- cent years, the volumes of groundwater exploitation. Tidal fluctu- Depositional facies of the aquifer sediments change from south ations in water level and inversion of water density with dense to north, from alluvium in the south to diluvium, to marine sedi- saltwater overlying less-dense freshwater can also drive down- ments on the coastal plain. The coastal plain consists of silty-mud- ward leakage of saline water into coastal aquifers (Landmeyer dy coastal sediments that are typical in China, while the seafloor of and Belval, 1996), however these influences are considered to be Laizhou Bay is composed of an epi-continental sediment plain with small in comparison to the hydraulic effects of groundwater a gently sloping gradient of 0.3–0.8‰ (Yang, 2005). The primary extraction, and have not been investigated in detail in this study. aquifers in the study area are composed of Quaternary sediments, with a thickness of 30–50 m in the south, up to 360 m in the north (Peng et al., 1992). Close to the piedmont area, shallow and deep 2.3. Development of groundwater depression groundwater is unconfined, while towards the coastline, deep groundwater becomes confined due to increasing occurrence of Since the 1990s, over exploitation of fresh groundwater and upper sandy clay. The strata in the upper proluvial fan in the south brine resources has resulted in variation of the natural groundwa- are mainly gravel and grit, grading to fine sand, silt, sandy clay, and ter flow field, forming groundwater depression cones (Fig. 1). The silty clay towards the coast. The aquifers are characterized by a depression cone near Changyi city initially formed in the shallow complex multi-layered framework (Fig. 2), often trending from unconfined aquifer in 1984, and is currently characterized by an coarse grain size in the lower part to fine-grained in the upper part. oval-shaped depression with northeast axis length of 24.3 km Coarse sand layers are highly permeable, with hydraulic conductiv- and west-east direction 14.3 km (Fig. 1). The variation in the distri- ities of 35–150 m/d (TJR, 2006). Bedrock, which outcrops in the bution of the shallow groundwater table and salinity (represented southern mountainous regions of Laizhou Bay (TJR, 2006), is by the 2 g/L TDS contour) over recent years is shown in Fig. 2. With mainly composed of Archaeozoic and Proterozoic metamorphic the increasing demand for fresh groundwater, the water table has rock and Cretaceous and Neogene basalt, andesite, trachyte and descended and the 2 g/L TDS contour has moved inland towards pyroclastic rock that consist of anorthite, albite, K-feldspar and bio- the south. In 1985, the water table in the center of the depression tite (Ning, 2004). The minerals in the aquifer include quartz, anor- cone was at 2.4 m a.s.l. (water table depth of 9.8 m); in 1995 it thite, albite, plagioclase feldspar, picrite, biotite, aragonite, was at 9.3 m a.s.l. (water table depth of 17.5 m) (Zhang et al., dolomite, calcite, kaolinite, gibbsite and Ca-montmorillonite, and 1997), and in 2007 it had declined to 14.5 m a.s.l. (water table minor amounts of evaporites (e.g., gypsum, anhydrite, halite, poly- depth of 22.7 m). The area encompassed by the cone of depression halite, bischofite, epsomite) (Zhang et al., 1996; Han et al., 1996; – defined as that enclosed by the 0 m a.s.l water table contour – in- Xue et al., 2000). Gypsum deposits are common in the coastal salt- creased from 170.0 km2 in 1984–248.6 km2 in 2000, to 353.1 km2 water zone (Zhang and Peng, 1998). The clay layers are composed in 2007. The development of groundwater depression has allowed of illite, kaolinite, chlorite, and calcite-rich nodules (Zhao, 1996; saltwater (brine and/or seawater) to intrude into the fresh ground- Han et al., 1996). water aquifers in a southward direction. The frontal interface of Under natural conditions, fresh groundwater flows from south salt-water intrusion, north of Changyi, was pushed 2.9 km south- to north through the alluvial and marine sediments toward the ward from 1980 to 2000 with a rate of 145 m/y, and a further coastline. The unconfined aquifer is recharged by precipitation, 3.2 km southward from 2000 to 2005 with a rate of 533.3 m/y horizontal flow of water from bedrock in the south, vertical infil- (TJR, 2006). The interface between brackish and fresh groundwater tration along the stream channel of Wei River and irrigation return (defined as 1 g/L TDS) in August 2007 was located at a distance of flows, and it is mainly discharged by groundwater exploitation and 25.4 km from the coastline. Additionally, in the north of the study evaporation in shallow aquifers. Horizontal hydraulic gradients area, exploitation of brine water for the purpose of harvesting salt vary from 2.64% in the upstream area to 1.64% along the coastline. (by means of pumping and evaporation) have resulted in formation Groundwater dynamics in the shallow unconfined and deep of a strip of decreased groundwater levels parallels the shoreline confined aquifers are influenced by climatic and hydrological (Fig. 1), which began to appear in the 1990s. Currently, the width
Fig. 2. Change in groundwater levels of shallow aquifer and TDS concentrations along the hydrogeological cross-section (P–P0 in Fig. 1) of study area. Modified after TJR (2006). Water table in April 1992 obtained from Zhang et al. (1997). Legend: 1. Pre-Quaternary Bedrock; 2. Gravel and sand; 3. Medium and fine sand; 4. Clayey Sand; 5. Sandy clay; and 6. Boundary of bedrock. Table 1 Hydrochemical data from water samples in the south coastal aquifer of Laizhou Bay.
Water Well Distance from Sampling Well EC pH T (°C) Cl (mg/L) NO 2 HCO (mg/L) Ca 2+ (mg/L) Na + K + Mg 2+ B Sr TDS (g/L) 3 SO 4 3 type label seashore (km) time depth (m) (ms/m) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) Fresh SG03 45.4 August 2009 14 102 8.0 15.3 62.5 89.2 72.3 162.6 161.5 41.0 0.6 19.5 0.03 0.88 0.6 Fresh SG04 41.2 August 2009 14 139 7.7 20.1 148.5 115.0 116.2 158.4 206.6 73.0 1.9 30.0 0.04 0.94 0.8 Fresh SG06 36.8 August 2009 12 154 7.9 14.6 120.0 213.6 158.9 167.9 258.6 52.3 0.2 29.6 0.03 1.05 1.0 Fresh SG07 33.6 August 2009 25 119 8.4 15.7 88.9 245.6 61.6 120.7 179.1 43.4 2.0 28.4 0.03 0.99 0.8 Fresh SG08 31.7 August 2009 28 122 8.5 22.7 105.6 176.3 103.6 132.7 188.3 33.1 0.6 36.2 0.03 1.45 0.8 Fresh SG11 23.7 August 2009 23 166 8.1 16.5 134.6 7.0 117.3 304.3 16.4 387.0 14.0 8.7 1.19 0.15 1.0 Fresh SG29 22.1 August 2009 36 155 7.6 15.1 144.4 118.7 120.3 218.2 250.9 58.8 1.2 35.1 0.04 1.21 0.9 Fresh SG53 32.2 August 2009 25 142 8.3 15.7 95.2 346.5 65.4 136.9 201.2 52.1 1.2 36.1 0.03 1.27 0.9 Fresh SG42 40.8 August 2009 30 99 8.2 18.9 119.5 31.1 95.0 113.8 99.5 54.3 3.5 16.4 0.03 0.43 0.5 Fresh SG57 31.6 August 2009 22 115 8.1 16.2 140.4 90.2 82.7 119.6 159.1 69.3 1.1 20.2 0.04 0.64 0.7 Fresh SG58 32.2 August 2009 40 92 8.3 21.2 90.2 54.9 77.4 111.2 131.2 50.3 0.7 18.5 0.03 0.63 0.5 Fresh SG59 29.4 August 2009 33 93 8.7 24.2 116.8 37.3 26.6 132.7 86.5 57.8 0.4 38.6 0.05 1.52 0.5 Fresh SG03 45.4 June 2008 17 102 7.4 15.7 73.1 81.3 77.8 159.5 108.3 43.0 1.1 20.2 0.03 0.89 0.6 Fresh SG04 41.2 June 2008 14 170 7.2 17.8 176.5 218.4 152.4 192.2 171.6 89.3 2.0 33.7 0.04 1.01 1.0 Fresh SG06 36.8 June 2008 12 111 7.3 15.3 91.0 130.1 110.2 126.8 140.8 41.9 0.5 22.4 0.03 0.72 0.7 Fresh SG07 33.6 June 2008 25 105 7.7 16.6 83.4 222.1 54.6 134.5 141.5 43.0 1.9 27.0 0.05 0.96 0.7 12–27 (2014) 508 Hydrology of Journal / al. et Han D.M. Fresh SG30 30.4 June 2008 30 154 7.6 14.9 118.9 228.2 115.7 199.8 144.6 65.5 2.0 44.9 0.06 1.29 0.9 Fresh SG40 14.8 June 2008 14 214 8.2 15.4 301.1 101.2 107.6 30.5 413.9 19.3 24.0 2.13 0.38 1.0 Fresh SG34 23.7 June 2008 40 128 8.6 15.6 141.6 95.0 220.6 2.3 298.0 7.7 3.1 0.71 0.04 0.8 Fresh SG33 24.1 June 2008 38 112 7.6 15.8 100.4 20.8 88.6 287.7 57.9 101.7 14.4 43.5 0.29 0.77 0.7 Fresh SG29 22.1 June 2008 36 122 7.6 15.5 117.7 83.4 89.1 256.7 124.6 51.8 1.4 26.9 0.04 0.94 0.8 Fresh SG43 25.8 June 2008 45 202 8.1 16.3 90.7 59.6 489.7 6.0 270.8 11.4 6.4 0.77 0.09 0.9 Fresh SG42 40.8 June 2008 30 85 7.2 16.8 113.0 12.6 96.9 90.3 106.2 61.0 5.1 16.5 0.04 0.43 0.5 Fresh DG14 23.3 August 2009 210 95 8.2 112.3 23.9 165.2 80.3 102.1 1.4 21.9 0.06 0.79 0.5 Brackish SG10 25.4 August 2009 36 334 8.2 15.9 288.4 645.0 381.6 31.7 680.0 28.4 35.7 1.35 0.45 2.1 Brackish SG12 22.0 August 2009 50 236 8.2 16.2 256.5 2.9 276.9 329.2 10.0 511.8 16.2 9.9 0.84 0.12 1.4 Brackish SG19 19.0 August 2009 25 558 8.0 16.2 1288.2 204.3 300.3 43.9 1103.0 29.2 60.4 1.50 0.81 3.0 Brackish SG21 18.0 August 2009 6 388 7.9 15.8 646.6 164.5 448.7 359.4 336.9 410.6 22.8 96.9 0.26 2.02 2.5 Brackish SG04’ 23.7 August 2009 43 216 7.7 27.2 322.8 26.6 151.5 254.4 27.9 421.1 11.8 22.1 0.67 0.37 1.2 Brackish SG40 14.8 August 2009 14 227 8.4 21.2 338.0 185.8 275.4 122.2 273.3 11.6 76.7 0.48 1.08 1.3 Brackish SG39 18.2 August 2009 40 289 8.0 15.2 619.3 22.7 171.5 228.2 260.2 206.3 7.5 74.7 0.13 1.65 1.6 Brackish SG33 24.1 August 2009 38 462 8.4 15.7 1145.4 286.4 267.0 245.3 213.3 715.5 1.5 77.2 0.23 2.03 3.0 Brackish SG44 25.4 August 2009 26 235 8.0 16 220.1 130.7 143.3 359.4 49.4 390.1 20.8 47.9 0.65 0.58 1.4 Brackish SG47 21.1 August 2009 35 430 7.8 15.4 747.5 417.9 331.0 39.7 954.9 32.2 10.0 1.15 0.45 2.5 Brackish SG50 19.0 August 2009 50 544 8.0 17.2 1220.2 296.2 275.4 17.0 1129.0 21.2 27.8 1.69 0.26 3.0 Brackish SG51 16.5 August 2009 40 448 8.0 15.1 841.2 16.1 395.2 288.5 42.2 891.8 33.3 45.0 1.05 0.42 2.6 Brackish SG52 16.7 August 2009 35 327 7.5 23.1 498.3 465.6 314.8 220.1 420.3 18.2 85.0 0.43 1.40 2.0 Brackish SG54 22.4 August 2009 37 200 7.4 17.5 236.2 74.5 146.9 301.6 200.9 186.1 10.0 52.3 0.26 1.14 1.2 Brackish SG02 51.3 June 2008 14 202 7.0 15.2 392.7 221.5 123.7 209.4 225.8 121.5 0.7 51.3 0.06 1.95 1.3 Brackish SG10 25.4 June 2008 40 318 8.0 16 276.1 649.8 406.2 11.6 681.5 29.1 36.9 1.47 0.35 2.1 Brackish SG12 22.0 June 2008 35 190 8.2 14.6 259.2 12.7 190.7 248.5 24.3 373.4 21.6 24.1 0.86 0.29 1.2 Brackish SG36 19.2 June 2008 29 265 8.3 16.8 516.2 91.6 294.4 8.5 549.2 16.4 15.0 1.36 0.17 1.5 Brackish SG39 18.2 June 2008 40 242 7.6 17.8 637.4 11.6 111.2 235.2 186.0 156.3 7.3 75.1 0.17 1.52 1.4 Brackish SG35 22.4 June 2008 37 279 8.0 15 371.3 148.2 199.8 372.0 28.8 583.8 25.7 30.0 1.04 0.40 1.8 Brackish SG44 25.4 June 2008 26 246 8.0 15.2 280.9 129.3 149.5 775.2 31.7 402.3 31.1 69.4 0.74 0.64 1.9 Brackish SG47 21.1 June 2008 35 494 8.2 16.5 878.3 438.1 663.2 20.4 1006.0 37.3 16.3 1.24 0.26 3.1 Brackish SG48 15.6 June 2008 30 554 7.6 13.6 1320.6 23.6 242.9 551.2 282.6 810.6 5.2 120.3 0.60 1.85 3.4 Brackish DG04 24.1 August 2009 65 807 7.7 17.3 2120.3 25.6 354.6 215.1 268.4 1501.0 27.9 189.4 0.65 2.58 4.7 Brackish DG07 11.8 August 2009 270 472 8.0 20.6 1026.3 8.0 200.1 174.4 25.3 743.9 6.8 72.8 0.09 0.44 2.3 Brackish DG09 18.7 August 2009 210 204 8.1 19.6 464.3 71.8 130.6 162.3 201.1 3.2 48.2 0.05 1.52 1.1 Brackish DG10 5.7 August 2009 180 1487 7.6 19.5 5049.2 605.5 135.1 554.4 2420.0 15.4 369.4 0.32 5.09 9.1 Brackish DG04 24.1 June 2008 38 304 7.7 18.3 641.2 26.8 170.2 275.2 36.6 590.5 18.9 50.3 0.86 0.66 1.8 Brackish DG08 9.4 June 2008 180 188 7.8 22.9 334.3 94.7 344.8 12.6 388.9 1.9 13.6 0.07 0.15 1.2 15 Saline SG55 21.4 August 2009 18 2860 7.0 16.4 9713.2 1132.1 194.1 1228.0 4359.0 58.5 988.0 0.65(continued 13.16 on 17.7 next page) 16 D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27
of the depression varies from 2.9 km in the east to 7.1 km in the west. Groundwater salinity in the study area varies both in the verti-
TDS (g/L) cal and lateral directions. Subsurface brines with an average TDS of 110 g/L occur in the northern part of the study area. Brine water in Quaternary aquifers is mainly distributed within 10 km of the
Sr (mg/L) shoreline, (Zhang and Peng, 1998) extending to a depth of up to 60 m (Jiang, 1992). The brines have previously been proposed to have originated from palaeo-seawater intrusion, following three B (mg/L) occurrences of marine incursion and regression along the coast of Laizhou Bay since the Upper Pleistocene (Xue et al., 2000). There 2+ are three major layers containing brine in the sediments, in the Mg (mg/L) depth ranges 0–14.8 m, 33.2–42.3 m, and 58.6–74.1 m, with thick- nesses of 7.3 m, 12.7 m, and 9.9 m, respectively. There are a num-
+ ber of clay inter-bed layers between different layers of marine (mg/L) K deposits, which locally form confining layers, and probably inhib- ited vertical mixing between these individual layers of brine under
+ natural conditions. In the downstream region of the Wei River, a (mg/L) thick paleochannel containing coarse grained sediments occurs, deposited in the Late Pleistocene (Fig. 1). Paleochannels may pro- vide further potential conduits for brine and/or seawater intrusion (mg/L) Na
2+ (Li et al., 2000). The cones of depression have become the dominant influence on groundwater flow directions, resulting in hydraulic gradients directed from the northern saltwater areas towards the fresher
(mg/L) Ca south (with local influence of the cones also important). Although 3 there may be shallow groundwater discharge into sea, it is impos- HCO sible that groundwater within our investigated depths currently discharges to the sea due to these gradients. 2 4 (mg/L) SO 2.4. Environmental issues 3 Environmental problems caused by salt water intrusion, such as (mg/L) degeneration of groundwater quality, soil salinization and ecolog- ical deterioration, have become a major obstacle in the future eco-
(mg/L) NO nomic development of this coastal area. Due to the lack of significant recharge to the deep confined aquifer and intrusion of saltwater due to pumping, there is a significant risk that this water C) Cl ° resource may be irreversibly compromised. That rapid changes to the distribution of groundwater levels and quality have occurred over a timescale of years indicates that the flow field is highly pH T ( responsive to changes in the water balance. Climate change is also predicted to reduce overall rainfall in the study area (Liu et al., 2004). In the 1980s when rainfall was the lowest over the last five EC (ms/m) decades, saltwater intrusion occurred rapidly in the study area, while the velocity of intrusion decreased as rainfall briefly in- creased after 1990 (Feng et al., 2006). This is probably attributable to the responsiveness of coastal water users to water shortage – Well depth (m) e.g. they turn to groundwater in the absence of rainfall. Future pro- tection and management of the groundwater resources in the coastal region of Laizhou Bay and other areas must take into ac- count the likely impacts on the flow field driven by water demand, which is in turn responsible for the extent of saltwater intrusion,
Sampling time whether seawater or brine (e.g. Ferguson and Gleeson, 2012).
3. Materials and methods
3.1. Sampling Distance from seashore (km)
) Water sampling (a total of 65 samples) was conducted at differ-
Well label ent aquifer depths over 2 campaigns from June 2008 to August 2009 in the Changyi-Liutuan area of Laizhou Bay (Fig. 1). Groundwater continued ( samples can be divided into shallow groundwater (<60 m depth, hereafter denoted as SG) and deep groundwater (>60 m depth, here- SalineSaline SG56Saline SG38Saline 4.8 DG05Saline 16.9 DG11Saline 8.9 DG06Brine 5.1 DG05Brine 8.9Brine DG08’ August 8.9 2009Brine June DG12 2008 9.4Brine DG13 August 2009 9.1Brine DG07 August 23 2009 4.7Saline SG41 June 11.8 2008 125 45 SG27 June 2008 180 SW 8.9 August 2009 8.5 5730 August 2009 86 August 7.1 125 2009 5400 2470 August 2007 58 5720 16.6 7.4 7.5 65 July 2007 7.3 16.4 15.5 20734.5 75 November 2005 60 4540 18.5 14,860 2240 19470.5 7727.0 7.4 40 21010.9 14,250 6.5 7.4 August 2009 17 30 15,120 16.6 17.6 6.5 13,750 6.4 16.2 64201.0 6.6 16101.3 15,260 2532.2 8709.6 16.3 67284.6 255.7 6.8 14,100 2276.3 66464.6 228.5 1618.5 2437.7 6.5 146.9 273.6 149.5 71088.0 734.8 7750 8526.5 162.2 608.1 87674.0 1882.6 7.4 190.2 9099.1 152.2 60756.3 905.5 217.4 28.2 8152.1 204.6 619.3 13700.0 148.2 31761.1 1182.0 11070.0 8576.6 469.5 1046.0 5213.0 10670.9 12220.0 463.0 620.4 1590.0 191.3 1052.0 558.0 8.40 137.7 267.8 1824.0 6932.6 1532.0 3601.0 39280.0 1709.0 0.73 865.2 511.4 9126.0 1014.0 36860.0 8.29 1.05 37.9 1006.7 5563.4 2.94 40510.0 5997.0 11.24 177.1 963.2 975.8 920.9 14.4 39.9 1137.0 3.40 1429.0 8.97 5548.0 0.33 3.45 1076.3 35.9 31495.5 7153.0 0.79 4.06 23.47 2.19 37513.5 38.4 950.5 11.50 15.7 1453.2 120.3 19.77 4767.6 33140.0 7.78 860.9 23.97 5518.4 15.6 121.0 699.0 125.1 29.5 4600.1 19410.0 669.5 4011.0 2.464 118.9 13.1 144.6 108.4 62.3 Water type
Table 1 after denoted as DG), with shallow groundwater corresponding to D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27 17
Table 2 Isotope composition for water samples in the south coastal area of Laizhou Bay.
Label Water type Sampling time d18O(‰) d2H(‰) Label Water type Sampling time d18O(‰) d2H(‰)
SG03 Fresh August 2009 55 7.5 SG47 Brackish August 2009 49 5.4 SG04 Fresh August 2009 50 6.6 SG50 Brackish August 2009 57 7.5 SG06 Fresh August 2009 46 6.0 SG51 Brackish August 2009 56 7.5 SG07 Fresh August 2009 51 6.6 SG52 Brackish August 2009 54 7.4 SG08 Fresh August 2009 48 6.0 SG54 Brackish August 2009 53 7.3 SG11 Fresh August 2009 58 8.0 SG02 Brackish June 2008 58 7.0 SG29 Fresh August 2009 56 7.0 SG10 Brackish June 2008 53 6.5 SG53 Fresh August 2009 57 7.4 SG12 Brackish June 2008 57 8.0 SG42 Fresh August 2009 40 4.4 SG36 Brackish June 2008 52 8.3 SG57 Fresh August 2009 36 4.1 SG39 Brackish June 2008 54 6.7 SG58 Fresh August 2009 41 5.0 SG35 Brackish June 2008 59 7.2 SG59 Fresh August 2009 54 8.1 SG44 Brackish June 2008 58 7.5 SG03 Fresh June 2008 53 6.9 SG47 Brackish June 2008 51 5.8 SG04 Fresh June 2008 49 6.0 SG48 Brackish June 2008 51 7.0 SG06 Fresh June 2008 45 5.2 DG04 Brackish June 2008 58 7.2 SG07 Fresh June 2008 53 6.9 DG04 Brackish August 2009 57 7.8 SG30 Fresh June 2008 56 7.3 DG07 Brackish August 2009 70 9.3 SG40 Fresh June 2008 45 4.9 DG09 Brackish August 2009 71 9.6 SG34 Fresh June 2008 57 7.4 DG10 Brackish August 2009 62 8.2 SG33 Fresh June 2008 61 8.0 DG08 Brackish June 2008 74 9.6 SG29 Fresh June 2008 55 6.8 SG55 Saline August 2009 50 7.0 SG43 Fresh June 2008 58 7.6 SG56 Saline August 2009 35 3.6 SG42 Fresh June 2008 35 3.6 SG38 Saline June 2008 40 3.7 DG14 Fresh August 2009 71 9.7 DG05 Saline August 2009 60 7.5 SG10 Brackish August 2009 54 6.8 DG11 Saline August 2009 53 6.4 SG12 Brackish August 2009 52 6.7 DG06 Saline June 2008 61 7.2 SG19 Brackish August 2009 55 7.4 DG05 Saline June 2008 64 7.7 SG21 Brackish August 2009 47 6.9 DG08 Brine August 2009 41 3.9 SG40 Brackish August 2009 48 6.0 DG12 Brine August 2009 36 3.5 SG39 Brackish August 2009 52 6.3 DG13 Brine August 2009 39 4.0 SG33 Brackish August 2009 56 7.2 SG41 Brine July 2006 43 6.2 SG44 Brackish August 2009 56 7.0 SG27 Brine November 2005 39 4.7
pore water in the shallow unconfined aquifer and deep groundwa- Surface Processes, Institute of Geographic Sciences and Natural Re- ter to water from the confined aquifer layer(s). Most groundwater sources Research, China Academy of Sciences, using a Finigan MAT samples were collected along a monitoring profile oriented N-S 253 mass spectrometer. The d18O and d2H values were measured from the coast of Laizhou Bay (Fig. 2). Surface water samples relative to internal standards that were calibrated using IAEA stan- 18 2 (labeled in SU) were also collected. Groundwater samples have been dards, using equilibration with CO2–He for O and H–He for H. classified into four types on the basis of salinity; fresh, brackish, sal- The d18O and d2H values are given hereafter in d-units calculated ine and brine water, with Total Dissolved Solids (TDS) concentra- with respect to VSMOW (Vienna Standard Mean Ocean Water) ex- tions of <1 g/L, 1–10 g/L, 10–100 g/L, and >100 g/L, respectively. pressed in ‰. The analytical precision of long-term standard mea- surements for d2His±2‰ and for d18O is ±0.5‰. Results are shown 3.2. Analytical methods in Table 2. Some discrepancies in the measurement of the d2H and d18O concentrations and activities from saline water, due to the ef- Physical and chemical parameters, including depth to water ta- fect of hydration of ions, have been pointed out (Horita and Gat, ble, temperature, pH and electrical conductivity (EC) were directly 1988; Cartwright et al., 2009). While this effect may have some im- measured in the field using portable meters, allowing for temper- pact on our results – particularly for d2H, where high salinity in ature compensation and calibration with appropriate standards. brines may cause discrepancies of up to 10‰; this will not be sub- Samples for major anion analysis were collected in polyethylene stantial for the majority of the sample set. In order to reduce the bottles, tightly capped and stored at 4 °C until analysis. All water effect of this discrepancy in our interpretations, we have generally samples were filtered promptly after collection for analysis of hyd- used relationships between d18O (where the maximum expected rochemical composition using 0.45 lm membrane filters. Samples difference is <0.5‰) and Cl to discuss different groundwater mix- for cation analysis (Na, K, Mg and Ca) were preserved in acid- ing processes. The analytical methods used for determining radio- washed polyethylene bottles, and acidified to pH 2 with 6 N carbon and tritium activities in water samples have been described
HNO3. The concentrations of cations, together with B and Sr, were in Han et al.(2011). measured using inductively coupled plasma analysis (ICP-OES) on
filtered samples. Concentrations of Cl, SO4,NO3 and F were deter- mined by a High Performance Ion Chromatograph (SHIMADZU) 4. Results at the Key Laboratory of Water Cycle & Related Land Surface Pro- cesses, Institute of Geographic Sciences and Natural Resources Re- 4.1. Groundwater level and salinity monitoring search, China Academy of Sciences. Alkalinity was determined on
filtered samples in the field, by titration with H2SO4 (0.22N) on There are 5 long-term observation wells (locations ‘‘a–e’’ in the day of sample collection. Charge balance errors in all analyses Fig. 1) monitoring water table dynamics in the shallow Quaternary are less than 8%. The hydrochemical and physical data are shown in aquifers in the study area. Plots of observed water levels in the Table 1. unconfined aquifer are shown in Fig. 3. The monitoring results The analysis of stable isotopes of water (d18O and d2H) were can be summarized as follows: (i) Qingxiang well (‘‘a’’ in Fig. 3) carried out at the Key Laboratory of Water Cycle & Related Land shows the water table increasing by 4.3 m from March 2006 to 18 D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27
4.2. Hydrochemistry
4.2.1. Salinity distribution of regional groundwater Groundwater salinity and hydrochemistry exhibits regular zonation from south to north with different types of groundwater distributed in strips parallel to the coast. The TDS concentrations range from 0.5 g/L in the south to 144.6 g/L in the north, with brackish water between the fresh zone and brine zones (Fig. 4). In areas of intensive exploitation, the zoned pattern of salinities is disrupted (Fig. 4), showing that the pumping is having a major impact on flow fields and salinity distributions. The distribution of the salinities with depth depends on hydrogeological conditions, such as the presence of local paleo-channels and confining strata, as well as the intensity of groundwater exploitation. Cross sections of TDS and Cl distributions with depth (Figs. 2 and 5) provide a depiction of the degree of hydrochemical variability through the strata. In general, the salinity pattern resembles a classic coastal salt-fresh water wedge interface, with high salinities at depth and near the coast, with some local variations near the two pump- ing centers (Figs. 2 and 5).
4.2.2. Major ion trends The anions in most water samples are dominated by Cl (up to 93% of total anion concentrations, where% is proportion to total an- ions in miliequivalents), this is especially true (>85%) in saline and brine water (Cl concentrations up to 88 g/L). The cations of the fresh groundwater are dominated by Ca (67%) while the brackish, saline and brine groundwater are characterized by higher Na con- centrations. Hydrochemical types of groundwater change from
south to north in the series (Fig. 4): Ca–HCO3 Cl SO4, Ca–Cl SO4 and Ca–Cl HCO3(SO4) in the fresh water zone, to Na–Cl HCO3 in the brackish water zone, to Na–Cl in saline and brine water zones.
Na–HCO3 type groundwater (such as SG11 and SG43) is also pres- Fig. 3. Plots of observed water levels in five wells from the unconfined aquifer (see ent sporadically near the center of groundwater depression cone, Fig. 1 for locations) Ground surface elevation is 2.5 m a.s.l. in ‘‘a’’-Qingxiang, and fresh groundwater (such as SG40 and SG48) of Na–Cl type is 4.6 m a.s.l. in ‘‘b’’-Nanfanjiazhuang, 6.2 m a.s.l. in ‘‘c’’-Liujiapu, 7.1 m a.s.l. in ‘‘d’’- locally present in the brackish water zone. Lijiapu, and 14.3 m a.s.l. in ‘‘e’’-Weihe River bridge. The evolution of the major ions in groundwater up to high salin-
ities can be seen from Fig. 6. Towards the coastline, Na, SO4, and Mg August 2007, and decreasing by 4 m to November 2009. The water concentrations increase approximately linearly with chloride con- level is probably controlled by recent adjustment measures centrations (Fig. 6a, b and d). Apart from the brines, Mg contents restricting brine exploitation in the area; a number of wells have are generally well below those of seawater (104.7 meq/L) been abandoned or had extraction limits emplaced since 2006. (Fig. 6d). The saline and brine water are characterized by low ratios
TDS values of the brines in this region range from 108.4 of Na/Cl, SO4/Cl, Ca/SO4, and Mg/Cl compared to the fresh and to144.6 g/L. (ii) The water table at Nanfanjiazhuang (‘‘b’’ in Fig. 3) brackish water (Fig. 7); indicating a contribution of mineral weath- showed a decrease by 3.6 m during the monitoring period from ering to the overall solute load at low salinities, which is over- March 2006 to July 2008, meanwhile the TDS increased from whelmed at high salinity. In the saline waters, Na/Cl ratios are 2.5 g/L in November 2005 to 7.2 g/L in August 2007, indicative of nearly constant and are mostly between the seawater value of saltwater intrusion. (iii) The water table at Lijiapu monitoring well 0.86 and observed brine water value of 0.71 (Fig. 7). The fresh
(‘‘c’’ in Fig. 3) decreased 6.5 m from March 2006 to October 2010, and brackish groundwaters show a wide range of SO4/Cl ratios with a velocity of 1.6 m/a. Groundwater similarly increased in (0.09–1.74) relative to seawater (0.05) and brine (0.08–0.10), and TDS from 0.9 g/L in November 2005, to 1.7 g/L in August 2007. plot above the mixing line between fresh water and seawater/ (iv) The monitoring well ‘‘d’’ in Liujiapu is located at the margin brine, showing some sulfate enrichment relative to chloride of the cone of depression. The water level declined by 6.4 m from (Fig. 7b). March 2006 to October 2010, although seasonal recharge during The Br/Cl ratio may be used to indicate the origin of groundwa- June to August allowed for some recovery of water levels each year. ter salinity (Edmunds et al., 2006). In this study, there is a general Monitoring well ‘‘e’’ near the Wei River Bridge, is located at a depth linear relationship between the bromide concentration and chlo- of 16 m in the piedmont plain, where the aquifer is composed of ride concentration (Fig. 8), with Br slightly enriched relative to coarse sand and gravel (TJR, 2006). The water table in this well des- the expected path for meteoric water or seawater evaporation. cended by 1.9 m from March 2006 to October 2010; although in The Br/Cl ratio maintains a constant value until halite saturation the dry season (outside of the monsoon period), an increase in is reached during the evaporation of meteoric water or seawater water table in this well may have be caused by decreased ground- (Bottomley et al., 1994). In this case, none of the samples were at water exploitation or use of transferred water for irrigation. The halite saturation (see Section 4.2.3). Br/Cl ratios of most groundwa- groundwater had a TDS of 0.8 g/L in November 2005 and 0.4 g/L ters plot above the evaporation line (Fig. 8), and are highest for sev- in August 2007, which is consistent with this mechanism. eral shallow brackish groundwater samples. D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27 19
Fig. 4. Distribution of different hydrochemical types of shallow groundwater in the study area.
Fig. 5. Groundwater flow patterns along the Changyi-Liutuan cross-section (P–P0 location in Fig. 1). 1. Gravel and sand; 2. Medium and fine sand; 3. Clayey Sand; 4. Sandy clay; 5. Pre-Quaternary Bedrock; 6. Boundary of bedrock; 7. Shallow groundwater level in November 2005; 8. Potentiometric surface for the confined aquifer in November 2005; 9. Contour line of Cl concentration (in mg/L) in November 2005; and 10. Concentrated groundwater exploitation zone. Arrows indicate groundwater flow direction.
4.2.3. Saturation indices trend when plotted against the chloride and sulfate contents; all
Saturation indices were calculated by PHREEQC 2 (Parkhurst samples are below saturation (SIhalite <0,Fig. 9c and d). and Appelo, 1999) to better understand the hydrogeochemical pro- cesses that take place in the aquifer. The saturation indices of cal- 4.3. d18O and d2H compositions cite (SIcalcite) and gypsum (SIgypsum) vary between 0.5 and +0.5 and indicate saturation (or equilibrium) or near saturation with re- Isotopic values of groundwater samples from the coastal aquifer spect to these minerals in the saline and brine water (Fig. 9a and b). at Laizhou Bay are related to the recharge sources of the water, mix- The evolution of halite saturation indices shows an increasing ing and modification by evaporation. Groundwater samples in the 20 D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27
Fig. 6. Hydrochemical relationships between selected ions in groundwater and mean seawater composition, as given by Stumm and Morgan (1981). Explanation: Seawater evaporation trajectory from Fontes and Matray (1993): G, H, E, S, C and B stand for point of precipitation of gypsum, halite, epsomite, sylvite, carnallite and bischofite, respectively. The mean values for rainfall are referenced from Liu et al. (1993), Wang et al. (1993), and Huang et al.(1993). Mixing line between seawater and fresh groundwater (SG03) is shown in dashed lines, and mixing line between fresh groundwater (SG03) and brine water (DG13) is shown in solid red lines. Numbers along the mixing lines show the percentage (%) of seawater/brine with 10% increments under the simple mixing behavior. area have d18O values between 9.7‰ and 3.5‰, and d2H values and TDS contents – Fig. 13), the data indicate that variations of between 74‰ and 35‰ (Table 2). The stable isotope values are 14C activities are mainly due to radioactive decay and reflect the shown in Fig. 10, along with the GMWL (Craig, 1961) and a local residence time in aquifer. Age dating estimates made on the basis meteoric water line (LMWL: d2H = 7.4d18O + 1.1), obtained from of geochemical correction (Han et al., 2011) can be seen in Fig. 11c. monthly rainwater data sampled at the Changyi weather station The 3H concentration of most groundwater samples (Fig. 11b), in 2006 (n = 12, R2 = 0.91). The mean stable isotopic value in modern expressed as 3H units (TU), varies from below detection to 11.8, precipitation is 6.9‰ for d18O, and 48‰ for d2H. Most water sam- with a wide range (1.8–15.3 TU) observed in shallow fresh and ples, especially the shallow groundwater, fall along a mixing trend brackish groundwater samples. The last recorded 3H concentration line between these rain-like values and shallow brine and/or in- in precipitation in the North China Plain was 16 TU, monitored at shore seawater from Laizhou Bay (d2H = 5.4 d18O 16.5, n = 63, the Shijiazhuang Station of IAEA in 2002 (IAEA/WMO, 2006). Tri- r2 = 0.88; Fig. 10). The d2H and d18O values in deep fresh and brack- tium concentrations above 5 TU are therefore probably indicative ish groundwater are in places depleted relative to rainfall and shal- of recharge in the post-nuclear testing era. The low salinity low groundwater (Fig. 11a). groundwaters mostly have therefore had relatively short residence times in the flow system and most were recharged within the last 4.4. Carbon-14 and tritium five decades (or at least contain a substantial mixing component from this time period). In deep groundwater, there is a fairly The measured groundwater carbon-14 activities vary between narrow range of tritium values from below detection up to 5.5 and 106.9 pMC. Most shallow fresh and brackish groundwater 3.5 TU – the upper value is an indication that some modern water samples are within the range 60–107 pMC, indicating there is rel- has locally reached the deeper aquifer (probably as a result of in- atively recent recharge from both before and during the atmo- duced leakage from pumping). spheric nuclear testing period of the 1950s and 1960s. The carbon-14 activity in the confined aquifer is less than 6 pMC, which is consistent with the palaeo-water signatures and residence time 5. Discussion estimates previously reported in groundwater from the North Chi- na Plain (Chen et al., 2003; Kreuzer et al., 2009). There is a very 5.1. Origin and recharge of the groundwater obvious decline in 14C activities with increasing sampling depth (Fig. 12a), and towards the coastline from the recharge area. Apart The d18Ovs.d2H values of the fresh, shallow groundwater are from some mixing effect (observed when looking at radiocarbon mostly scattered close to or slightly to the right of the LWML D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27 21
Fig. 7. Evolution of hydrochemical ratios with increasing salinity, and correspondence to seawater and brine compositions. Symbols and mixing lines are same as in Fig. 7.
Fig. 8. Chloride versus bromide (a); and versus corresponding elements to chloride molar ratios (b) of groundwater from the south coastal area of Lazihou Bay. The dashed lines represents seawater Br/Cl ratio ( 1.5 10 3). Br concentrations of groundwater in this region referenced from Han et al.(1996) and Han et al.(2011). Symbols are same as in Fig. 7.
(Fig. 10a), indicating that modern precipitation is their predom- also suggest that the brine water formed during the Holocene, as inant origin. Stream water from Weihe River has d18O values the corrected 14C ages are approximately 2.3–7.0 ka BP (notwith- ranging from 7.6‰ to 4.6‰ with mean value of 5.8‰), standing effects of mixing – discussed below in Section 5.2). The and tritium of 5.1–7.8 TU (mean 6.5 TU) that are also similar substantially lower stable isotopic values (e.g. d18O< 3.0‰)in to some of the shallow groundwater compositions, which may the brine water compared to modern seawater, present an inter- indicate that these are further recharge sources locally. The esting question as to the sources of salinity and water in the trend towards 18Oand2H depletion in the deep fresh groundwa- brine, and a couple of hypotheses are explored further in Section ter (Fig. 10b) can be attributed to a colder climate during past 5.3. recharge (e.g. during the last glacial maximum; cf. Kreuzer et al., 2009). The carbon-14 vs. d18O relationship observed 5.2. Mixing processes (Fig. 14) with modern waters (>60 pMC) having relatively en- riched stable isotopic values (d18O> 6.0‰) and older water 5.2.1. Major ion indications being more isotopically depleted, is characteristic of the Based on a mass-balance model, mixing lines between two recharge history of northern China during the last 30 kyr end-member samples for a range of hydrochemical indicators were (Kreuzer et al., 2009; Currell et al., 2012). The radiocarbon data calculated and plotted based on: 22 D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27
Fig. 9. Saturation indices of groundwater samples with increasing selected anion concentrations.
Cm ¼ Cf X þ Cb=s ð1 XÞð1Þ
where X is the mixing fraction of fresh groundwater, Cm is the ion concentration of mixture sample; Cf for the ion concentration of fresh groundwater; Cb/s for the ion concentration of brine or seawa- ter (separate lines on each plot in Figs. 6 and 7). The relatively close fit between the majority of samples and the mixing lines (Fig. 6a, b, and d) implies predominantly conservative mixing processes be- tween freshwater and a saline end-member. The scatter observed around mixing lines for fresh and brackish groundwater suggests that additional processes, such as water–rock modification (adsorp- tion–desorption, dissolution-precipitation, and oxidation–reduction processes), also control the chemical composition of the groundwa- ter to an extent (e.g. Appelo and Postma, 2005).
5.2.2. Isotopic indications With increasing salinity, both the shallow and deep groundwa- ter evolve towards more enriched d18O and d2H values. However, in general, the groundwater does not show a tendency to evolve to- wards standard isotopic seawater (e.g. VSMOW); rather, samples plot on a mixing trend between fresh groundwater with a meteoric signature, and brine, which has similar d18O and d2H values of be- tween 3.0‰ to 4.0‰ and 30‰ to 40‰ in both shallow and deep aquifers (Figs. 10 and 15). This mixing trajectory between fresh groundwater and the coastal brine is not obvious from exam- ining the mixing lines in the ionic data alone (Figs. 6 and 7), as the ionic ratios of typical seawater and the brine are very similar (whereas the stable isotopic composition is notably different). Accounting for the influence of mixing with brine, many of the fresh and brackish water samples appear to be derived from a com- mon ‘‘source composition’’ along the LMWL, at around 60‰ and 2 18 Fig. 10. Stable isotope compositions of groundwater and rainfall in the study area. 8‰ for d H and d O, respectively. This source composition is Symbols are same as in Fig. 7. depleted relative to the average modern precipitation value, D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27 23
Fig. 11. Different tracers’ compositions of water in the south coastal area of Laizhou Bay. Explanation: SG – shallow groundwater; DG – deep groundwater; F – fresh; BA – brackish; SA – saline; BR – brine; M – modern water. The data of tritium and corrected 14C age in groundwater are referenced from Han et al.(2011) and Xue et al.(2000). The d18O data of rainfall are from the GNIP database (IAEA/WMO, 2006), IAEA network, the average monthly (1986–1990) monitored at Yantai station (37°3202400N, 121°2400000) some 250 km NE of Changyi city.
Fig. 13. Carbon-14 activities vs. TDS concentrations of groundwater. The lines show predicted mixing trends between brines and fresh water in shallow and deep aquifer. Data referenced from Han et al. (2011). Symbols are same as in Fig. 7.
Fig. 12. Carbon-14 activities (pMC) for the groundwater in south coastal aquifer of Laizhou Bay. (a) Variations in percentage modern carbon (pMC) with depth. (b) Variations in pMC with distance from the coastline. Arrows show general trend in aquifer. Symbols are same as in Fig. 7. suggesting either that preferential recharge takes place during the Fig. 14. d18O vs. carbon-14 activities for shallow and deep groundwaters from the wettest season of the year (when isotopic value is depleted relative south coastal aquifers of Laizhou Bay. Symbols are same as in Fig. 7. to the average) and/or that deep groundwater contains palaeo- water with the depletion characteristic of the late Pleistocene aged waters (Fig. 15). The first obvious trend in this plot is that the groundwaters of northern China. Additionally Fig. 13 shows radio- majority of samples fall between fresh groundwater end members carbon activities vs. TDS concentrations for the groundwater in this (with Cl concentrations of 2 meq/L and d18O values of 6.0‰ study area. The lines show two distinctive mixing trends between and 9.0‰ for shallow and deep groundwater, respectively), and the brines and fresh water in both the shallow and deep aquifers. the brines – with d18O values of 3.0‰ and Cl concentrations of 2000 meq/L. The relative contributions of freshwater and salt 5.2.3. Mixing model water to the mixed samples was estimated using two end-mem- Concentrations of Cl together with d18O were further used to bers, based on the mass-balance mixing model (Eq. (1)), for both examine the mixing trajectory and quantities for the different Cl and d18O. In general, the brine mixing proportions are estimated 24 D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27
seawater. This indicates that it either did not form due to simple evaporative concentration of seawater and/or that it was subse- quently modified by mixing. The natural movement of groundwa- ter through the aquifer towards the coast, and the migration of fresh, saline and brine water caused by intensive exploitation has clearly resulted in mixing, which could have over-printed the initial brine isotopic signatures, however, the extent of this over-printing is now unclear. The current isotopic values may have resulted from the mixture of originally more enriched brine and subsequent mixing, or, the original brine may have been depleted relative to standard seawater prior to the mixing (or a combination of the two). One mechanism by which this could have occurred is that the brine itself mixed with stream water (i.e. continental run- off) in Laizhou Bay during its formation (e.g. Dakin et al., 1983). It is Fig. 15. Relationship between chloride content and isotopic signature of surface interesting to note that the stable isotopic values of the seawater and ground-water samples as a means to differentiate mixing processes in the area. collected from Laizhou Bay in this study (Fig. 10 samples marked Symbols are same as in Fig. 7. ‘inshore seawater’) are more isotopically depleted than standard seawater – this may be indicative of a relatively large degree of dilution of water in the bay with continental runoff; and may have been important in the brine formation process. Another explanation is that there may have been an isotope depletion effect occurring during brine formation by evaporation, this is known to occur during late stages of brine formation in hy- per-saline evaporating basins, due to reduction in the humidity contrast between the boundary layer and atmosphere (Clark and Fritz, 1997; Cartwright et al., 2009). Typically this effect occurs only at very high salinities, and so subsequent dilution (e.g. via some mixing and displacement with regional groundwater flow) would have to have taken place to reach the observed brine com- positions. Elevated Br/Cl ratios (1.6 10 3–4.5 10 3, Han et al., 1996 and Han et al., 2011) in most of the saline and brine waters compared to Br/Cl ratio ( 1.5 10 3) in seawater (Davis et al., 1998) are consistent with an original brine that was more saline (e.g. at halite saturation), which subsequently was diluted by fresh groundwater or runoff (McCaffrey et al., 1987). The Br/Cl ratios in brines (equal or greater than typical marine signatures) confirm that evapotranspiration, rather than halite dissolution, was the Fig. 16. Distribution of cationic deltas of the groundwater samples with the change dominant mechanism of brine formation. Based on the relationship in distance from the coastline. between the Br/Cl ratio in residual evaporated seawater and the degree of evaporation (i.e., the ratio of Br dissolved in the residual to be between 5% and 30% of the mixed waters. The brine mixing brine to its concentration in the initial seawater – cf. Vengosh and process has important implications for assessing the extent of sea- Hendry, 2001), the maximum degree of evaporation of the brines water intrusion –a smaller mixing volume produces a much great- in the study area prior to any dilution with fresh water can be esti- er salinisation effect in comparison to mixing with standard mated at about 5.5 times. seawater. The fresh-brine mixing pattern is adhered to strongly in the 5.4. Water–rock interaction deep samples, with increasing proportions of brine observed to- wards the coast, consistent with gradual mixing between the two From the major ion and stable isotope data, it is clear that mix- end-members along the regional S–N groundwater flow direction. ing between fresh groundwater and brine is the dominant control The patterns in shallow groundwater are somewhat more com- on groundwater quality and hydrochemistry. However, the fact plex; this likely reflects the added complexity in the water sources that ion ratios in many cases plot away from strict adherence to and processes in the shallow aquifer, e.g., inputs from modern pre- the mixing lines (Fig. 7) indicate water–rock interaction (e.g. rock cipitation, surface water and leakage of irrigation water, plus evap- weathering, ion exchange) is also important, particularly in con- orative concentration from locally high water tables. Two shallow trolling the shallow and brackish groundwater chemistry. Satura- saline groundwater samples also appear to fit a seawater-fresh tion with respect to halite is not reached in any of the samples, groundwater mixing trajectory better than a brine-groundwater precluding halite precipitation under current conditions – hence mix, suggesting classic seawater intrusion. The high values of Cl is assumed to behave relatively conservatively. An examination 18 d O observed in a number of shallow fresh groundwater samples, of the ion/Cl ratios is therefore the primary means of detecting with relatively low Cl contents (Fig. 15) point to local evaporation water–rock interaction processes. prior to recharge where the water table is shallow, probably due to High Na/Cl ratios (>1.0) of most fresh and brackish groundwater irrigation return flow. samples may be attributed to feldspar weathering (e.g., þ 2NaAlSi3O8 ðAlbiteÞ þ 2CO2 þ 11H2O ! 2Na þ Al2Si2O5ðOHÞ4 ðKaoliniteÞþ 5.3. Brine formation 4H4SiO4 þ 2HCO3 ) and/or release of Na from exchange sites in the aquifer during freshening from palaeo seawater intrusion, which The brine itself is characterized by both d18O and d2H values lo- has not yet exhausted Na+ on the exchanger (Appelo and Postma, cated below the LMWL; significantly lower than that of modern 2005). Mg/Cl ratios in many of the brackish and fresh waters plot D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27 25 below the mixing line between the freshest water and seawater/ variation in tritium in the shallow fresh and brackish groundwater brine, suggesting uptake on an exchanger. Weathering of K-feld- may reflect the complexity of groundwater flowpaths and variation spar, Ca-bearing plagioclase and/or calcite could explain the rela- of travel time in the regional system, including the up-coning deep tively high K/Cl and Ca/Cl ratios of the freshest waters; while yet groundwater caused by strong exploitation. Additionally, the leak- again cation exchange may be important; e.g. an ‘intrusion signa- age of modern high-nitrate water through the shallow aquifer into ture’ occurring due to recent migration of Na-rich water into fresh- the underlying deeper aquifer suggests the presence of intercon- water aquifers could explain the elevated Ca/Cl. nected pathways vertically form the relatively rapid flow.
The wider variations of SO4/Cl ratios in fresh and brackish groundwater than that in seawater and brine indicate both sulfate enrichment and depletion processes occurring. The decreasing SO4/ 6. Conclusions and environmental implications Cl ratios with increasing salinity are likely caused by saline water reaching saturation with respect to gypsum, rather than the This paper has examined the processes involved in saltwater de-sulfation of the aquifer sediments, which would be expected intrusion into the aquifer below the Wei River plain, an example to result in generation of acidity (e.g. Andersen et al., 2005). Some of complex groundwater dynamics and salinization mechanisms locally elevated SO4/Cl ratios could relate to oxidation of sulfidic in the coastal plains of northern China – where groundwater material. High Ca/SO4 ratios in most fresh and brackish groundwa- extraction is intensive. Physical and chemical parameters were ter samples would seem to rule out significant gypsum dissolution. examined as multiple lines of evidence to refine understanding In all cases, the ion/Cl ratios begin to approximate the predicted of the evolution of groundwater salinization in the coastal plain mixing lines more closely as salinity increases, due to other pro- aquifer of Laizhou Bay. The main findings are: cesses being overwhelmed by the total solute load. To further determine the importance of water–rock interaction, 1. The continued decrease in groundwater levels in recent years the concentrations of cations were presented as a function of Cl due to groundwater extraction for water supply and salt pro- concentration using ionic deltas (Pulido-Leboeuf, 2004). Fig. 16 de- duction is having noticeable water quality impacts in the aqui- picts the behavior of major cations with distance from the coast- fer, inducing mixing and salinization. This is most severe in line. The 4Na is usually positive, while the other values (4Ca, areas of intensive extraction, for example, near the Changyi 4Mg and 4K) are largely near zero, with no systematic trend. groundwater depression. The relative excess of Na implies that during freshening of the 2. Mixing between fresh groundwater derived from meteoric aquifer, there is likely a direct cation-exchange of Ca and/or Mg sources and saline brines stored in the aquifer near the coast in groundwater for Na in the clay layers present near the coastline. is supported by ion/chloride ratios but is particularly evident
This process may be responsible for the Na–HCO3 type water ob- when looking at stable isotope compositions, and radiocarbon served near Liutuan (Fig. 4). It is notable that cation exchange ap- data. This has important implications for the assessment of sal- pears to be much more significant near the coast. ine intrusion in coastal aquifers. Firstly, analysis of mixing or the extent of intrusion may not accurately determine salinity 5.5. Anthropogenic influences on groundwater sources unless isotope data are examined in conjunction with traditional hydrochemical analysis. Secondly, brines, which The hydrochemical investigation by Xue et al.(2000) showed can occur in coastal systems due to a history of sea-level rise, that most groundwater in the fresh water zone in 1994 was fall and evaporative enrichment, can pose an equal or greater
Ca–HCO3 type. However, the current study shows that the ground- water quality threat than seawater, and cause more rapid sali- water in the fresh water zone has changed to Ca–HCO3 Cl type, nization due to their higher salinity. Detailed analysis of coastal indicating some influence of mixing with the more saline (and groundwater geochemistry is therefore extremely important to Cl-rich) water. As noted earlier, increasing TDS values were also assessing the risk posed by groundwater extraction in coastal observed in some of the monitoring wells between 2005 and areas, as the traditional conceptual model of a simple fresh/sea- 2007 (Section 4.1). Shallow groundwater has also clearly been con- water interface may not be adequate. taminated by agricultural irrigation and is characterized by high 3. Ion/chloride ratios (e.g., Br/Cl and Na/Cl) in the system suggest
NO3 concentrations (up to 346 mg/L in SG53) including within an original evaporative source of salinity in the brines, which the fresh water zone (i.e. TDS < 1 g/L) south of Changyi city. This was intensive during formation, but was probably followed by compares with much lower concentrations observed before 2000 dilution with meteoric water, resulting in d18O and d2H values
(Xue et al., 2000). Relatively lower NO3 concentrations were found lower than seawater. The variation in chemical composition of in the shallow groundwater near the Wei River (31.1 mg/L in groundwater demonstrates some complex (i.e. non-conserva- SG42), but overall concentrations are high in the shallow ground- tive) hydrogeochemical processes, such as mineral weathering water; this is a regional problem throughout the North China Plain and cation exchange that are more noticeable at low salinities. area (e.g. Chen, 2001), and indicates that irrigation return is now a 4. The different stable and radio-isotopic values of shallow and dominant recharge mechanism to the shallow aquifers (e.g. Currell deep groundwaters provide a distinction between modern and et al., 2012). Locally high nitrate-enriched groundwater in the con- palaeo-recharge. The isotopic signature of groundwater fined aquifer indicates the vertical leakage of modern high-nitrate recharged during the late Pleistocene differs markedly from water through the shallow formation into the underlying confined that of the Holocene, characterized by depleted stable isotopic aquifer. The high NO3 concentrations in groundwater have likely compositions and low carbon-14 activities in the deepest resulted from the extensive use of nitrogen fertilizers in this area. waters, furthest from the recharge areas. The middle layer (33–42 m) of brines were observed to have a 5. Anthropogenic input into shallow groundwater is reflected by tritium concentration less than 1 TU in April 1994 (Xue et al., the distribution of NO3 concentrations, which can be attrib- 2000), while this had reached 5.3 TU in July 2006 (Han et al., uted to the general over-utilization of fertilizers in agricul- 2011). This is compelling evidence that the intensification of brine tural activities. Elevated and increasing levels of tritium, extraction has led to increased mixing between modern, shallow chloride and salinity in areas of intensive pumping support recharge and the brine. The increased hydraulic gradients between the hypothesis that induced mixing and water quality degra- shallow and deep layers may enhance the downward movement of dation have taken place over relatively short time scales (e.g. the shallow groundwater into the deep aquifer. The wide range of decades). 26 D.M. Han et al. / Journal of Hydrology 508 (2014) 12–27
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