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Geochemical evolution of groundwater salinity at basin scale: a case study from Datong basin, northern China Ya Wu and Yanxin Wang* A hydrogeochemical investigation using integrated methods of stable isotopes (18O, 2H), 87Sr/86Sr ratios, Cl/ Br ratios, chloride-mass balance, mass balance and hydrogeochemical modeling was conducted to interpret the geochemical evolution of groundwater salinity in Datong basin, northern China. The d2H, d18O ratios in precipitation exhibited a local meteoric water line of d2H ¼ 6.4 d18O 5 (R2 ¼ 0.94), while those in groundwater suggested their meteoric origin in a historically colder climatic regime with a speculated recharge rate of less than 20.5 mm overall per year, in addition to recharge from a component of deep residual ancient lake water enriched with Br. According to the Sr isotope binary mixing model, the mixing of recharges from the Shentou karst springs (24%), the western margins (11%) and the eastern margins (65%) accounts for the groundwater from the deep aquifers of the downgradient parts in the central basin is a possible mixing mechanism. In Datong, hydrolysis of silicate minerals is the most important hydrogeochemical process responsible for groundwater chemistry, in addition to dissolution of carbonate and evaporites. In the recharge areas, silicate chemical weathering is typically at the bisiallitization stage, while that in the central basin is mostly at the monosiallitization stage with limited evidence of being in equilibrium with gibbsite. Na exchange with bound Ca, Mg prevails at basin scale, and intensifies with groundwater salinity, while Ca, Mg exchange with bound Na locally occurs in the east pluvial and alluvial plains. Although groundwater salinity increases with the progress of
Received 7th January 2014 Accepted 19th February 2014
water-rock/sediment interactions along the flow path, as a result of carbonate solubility control and continuous evapotranspiration, Na–HCO3 and Na–Cl–SO4 types of water are usually characterized
DOI: 10.1039/c4em00019f
respectively in the deep and the shallow aquifers of an inland basin with a silicate terrain in an arid
rsc.li/process-impacts
climatic regime.
Environmental impact Groundwater is the most important source of water supplied to domestic, industrial and agricultural activities in arid and semiarid areas of northern China. However, problems due to over-exploitation and geogenic arsenic, uoride and iodine contamination of groundwater are the major environmental and public health concerns in these regions. Increased knowledge of the geochemical processes controlling the evolution of groundwater salinity is urgently needed for effective groundwater resource management and a sustainable safe water supply. In this study, an investigation using integrated hydrogeochemical tools was conducted to interpret geochemical evolution of groundwater salinity at regional scale in Datong basin, northern China. The work provides new insights into the water-rock/sediment interactions in groundwater systems of arid and semiarid regions.
1
Introduction
Water shortage is an increasingly serious issue in the arid and semiarid regions of northern China where groundwater is the most important source of water supplied to domestic, industrial and agricultural activities. Problems such as attenuating regional groundwater tables, land subsidence and groundwater quality deterioration have been induced in these regions by the
State Key Laboratory of Biogeology and Environmental Geology & School of Environmental Studies, China University of Geosciences, 430074 Wuhan, China. E-mail: [email protected]; Fax: +86-27-87481030; Tel: +86-27-67883998
This journal is © The Royal Society of Chemistry 2014
over-exploitation of groundwater due to rapid population growth and economic development in recent decades.1–3 In addition, salinization of groundwater and soil, and geogenic arsenic, uoride and iodine contamination of groundwater in northern China due to geological, hydrogeological and climatic conditions are the major environmental and public health concerns.4 Groundwater has been exploited intensively for more than 30 years in Datong basin, a famous coal industry base in China and an important agricultural base in Shanxi province.2 Many efforts have been made to understand the hydrogeology and hydrochemistry of groundwater containing of high levels of arsenic,
Environ. Sci.: Processes Impacts, 2014, 16, 1469–1479 | 1469
View Article Online
Published on 19 February 2014. Downloaded by University of California - Riverside on 23/10/2014 07:29:16.
Environmental Science: Processes & Impacts
uoride and iodine in this region.2,5–15 Increased knowledge of the geochemical processes controlling the evolution of groundwater salinity can contribute to effective management and sustainable exploitation of groundwater resources, and guide the development of a safe and clean groundwater supply. Determining the origins of solutes in groundwater and the control of hydrochemical variations is vital to understanding these geochemical processes. In this study, integrated methods of stable isotopic ratios (d18O, d2H), 87Sr/86Sr ratios, Cl/Br ratios, chloride-mass balance, mass balance and hydrogeochemical modeling were applied to interpret the geochemical evolution of groundwater salinity in Datong. The work provides new insights into the water-rock/sediment interactions in groundwater systems of arid and semiarid regions.
2 Study area Datong basin, covering an area of 8700 km2 located in the north part of Shanxi province (Fig. 1), is one of the Cenozoic faulted basins of the Shanxi ri system. The basin is surrounded by the Hongtao Mountains to the west and the Hengshan Mountains to the east. Bedrocks to the east and the north are mainly the
Paper
Neo-Archaean Hengshan metamorphic complex, including migmatite, tonalite, granodiorite and granitic gneiss, as well as mac lenses, pegmatites and granites.16 Ordovician carbonate, as well as Cambrian, Carboniferous and Permian sedimentary rocks occur in the west and the south. Cenozoic unconsolidated sediments occur all over the basin with varying thicknesses reaching more than 2000 m in the center of the basin. A common feature of inland basins is that the grain size of unconsolidated sediments gradually decreases from the margins to the central basin. Sediments types in the margins are alluvial–pluvial sand and gravel while in the plain areas and the central parts of the basin mainly consist of alluvial and alluvial–lacustrine interstratied sand, silt, silty clay and clay. Datong has an arid climate with an average annual temperature of 6.8 C, an average annual atmospheric precipitation in the range 370–420 mm, and an average annual evaporation up to 1800 mm. Sanggan River and its biggest tributary, Huangshui River, are the main ephemeral rivers that run through the basin from the southwest to the northeast. Pluvial, alluvial–pluvial and alluvial–lacustrine sediments, especially middle-upper Pleistocene alluvial–pluvial sands and gravels at depths < 150 m constitute the major Quaternary hydrostratigraphic units.7
Fig. 1 Location of the study area and sampling sites. (a) China and the location of Shanxi province, (b) Shanxi province and the location of Datong basin, (c) study area and locations of the sampling sites; the groundwater table altitude was compiled from Han (2008).7
1470 | Environ. Sci.: Processes Impacts, 2014, 16, 1469–1479
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 19 February 2014. Downloaded by University of California - Riverside on 23/10/2014 07:29:16.
Paper
Quaternary aquifers occur in three groups: phreatic aquifers (depths < 10 m), semi-conned aquifers (depths 20–50 m) and conned aquifers (depths > 50 m).10,13 Precipitation, permeating water from bedrocks and inltrating surface water are the recharges of the Quaternary groundwater aquifer systems (Fig. 2).7,13 Evaporation, pumping and leakage into the deeper conned aquifers through aquitards, especially in the areas of groundwater depression cones,10 are the main ways of discharge of the shallow groundwater. Three groundwater ow paths can be delineated from the groundwater table altitude: ow path 1 from the western margins, ow path 2 from the eastern margins to the central parts of the basin, and ow path 3 from the southeast margins mostly from the Shentou karst springs through the basin along the NE-trend (Fig. 1). The groundwater ow system is shown in the hydrological cross-section (A–A0 –A00 , Fig. 2): the mountains and the margins of the basin (pluvial plains) are natural recharge zones, while the alluvial plains and the lower-lying central parts of the basin could be identied respectively as the ow-through zones and the discharge zones. According to the results of XRD analysis, silicate minerals and their secondary minerals account for >80% among the minerals phases, suggesting that hydrolysis of silicate minerals is the most important hydrogeochemical process responsible for groundwater chemistry in Datong.10 There are generally three stages for silicate chemical weathering, which are limited by leaching regimes and are associated with the formation of different types of secondary clay minerals.17–20 During the early stages of weathering, leaching is weak in areas with a precipitation lower than 500 mm. The most soluble cations such as Na, K, Ca and Mg are dissolved and released from the host rocks, while most of the silica is retained in solid phases. As a result, secondary clay minerals of the illite or smectite groups are formed. Since illite and smectite have a 2 : 1 sheet structure
Environmental Science: Processes & Impacts
(Si/Al ¼ 2), this stage is called bisiallitization. As weathering progresses, leaching is moderate in zones with a precipitation of 500 to 1200/1500 mm. The relatively less soluble cations such as Mn, Cu, Co, Ni and Fe are removed but part of the silica is retained, resulting in the formation of the kaolinite group. Owing to the ratio of Si/Al equals unity in kaolinite, the stage is named monosiallitization. In the third stage of chemical weathering, leaching is intense in regions with a precipitation higher than 1500 mm. The basic elements and even silica are completely removed, leaving mostly unushed Al as hydroxides such as gibbsite. Goethite may also form in this stage. Therefore, the stage is usually called allitization, also termed lateritization or ferrallitization.
3 Methods 3.1
Sampling
Three sampling campaigns were conducted in July and November 2011 and September 2012 using sampling scheme of combination of orthogonal grids and locations of villages, and a total of 161 groundwater samples were collected across the study area. The groundwater samples were mostly collected from private boreholes, and a few small central water supplies or irrigation wells. When sampling groundwater from boreholes, samples and measurements were taken aer pumping for 3–10 minutes. One soil solution sample (depth ¼ 2 m, September 2010) was collected using pre-cleaned and pressurevacuum self-made porcelain sampler from the unsaturated zone in a representative agricultural eld in the central basin where insertion at a depth of 2 m remains above the local groundwater table. The Eh, pH, electrical conductivity (EC) and temperature of the samples were determined using portable multi-parameter meters (HACH, HQ40D Field Case) during sampling. The Eh
Fig. 2 Groundwater aquifer system and groundwater flow system, a hydrological cross-section A–A0 –A00 as indicated in Fig. 1, modified from Han (2008).7
This journal is © The Royal Society of Chemistry 2014
Environ. Sci.: Processes Impacts, 2014, 16, 1469–1479 | 1471
View Article Online
Published on 19 February 2014. Downloaded by University of California - Riverside on 23/10/2014 07:29:16.
Environmental Science: Processes & Impacts
values reported in this study have not been corrected relative to the standard hydrogen electrode (SHE) but instead can be used as relative values. The alkalinity of the samples was determined by titration within 24 hours aer sampling. The samples were collected in 50 mL PET bottles aer ltration using a 0.45 mm lter. Samples for the analysis of cations and trace elements were preserved by acidication with GR grade HNO3 (15 mol L1) to pH < 2, while those for the analysis of anions were unpreserved. 3.2
Chemical analyses
Samples were analyzed in State Key Laboratory of Biogeology and Environmental Geology and School of Environmental Studies, China University of Geosciences. Cations (Ca, K, Mg, Na and Si) were measured using inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermo Electron Corporation IRIS Intrepid II XSP). Anions (Cl, SO4, NO3, F and Br) were determined by ion chromatography (IC) (DIONEX ICS1100 and Metrohm 761 Compact). In the ICP-OES and IC runs, spiked standard solutions were used to calibrate the determined values (1 spiked standard solution per 10 samples). Therefore, the spiked standard solutions could be also used to ascertain the precision (relative standard deviation, RSD) of the analyses. 3.3
Compiled data
The analytical data set was augmented by the results of earlier hydrogeochemical investigations,12,14,15 which included 87Sr/86Sr ratios and H, O isotopic compositions. The Water Resources Programme of Global Network of Isotopes in Precipitation (GNIP) conducted by the International Atomic Energy Agency (IAEA) and World Meteorological Organization (WMO) provides H, O isotopic compositions in precipitation worldwide. The H, O isotopic compositions in precipitation from nearby GNIP Taiyuan station (inland basin, 112.55 E, 37.78 N) which has a climate similar to Datong and lies approximately 200 km from the study area were quoted to represent that in Datong. Twenty GNIP (Taiyuan) samples were collected monthly between February 1986 and October 1988.21 3.4
Statistical analyses and hydrogeochemical modeling
Descriptive statistics of hydrochemical parameters and replicated spiked standard solutions were calculated using PASW statistics 18 (SPSS Inc, USA). Hydrogeochemical modeling, including saturation indices (SI) and log activities of Ca, Na, K and H4SiO4 were performed using PHREEQC 2.18 (USGS, 2012). End-member compositions of the silicate stability eld from the equilibrium relationship of Tardy (1971)22 and Drever (1997)23 were quoted.
4 Results and discussion In the ICP-OES and IC runs, the RSDs were in the range 2.00– 5.51% (average 3.29%) and 2.22–3.65% (average 2.80%), respectively. Since the values of samples obtained directly from instruments were calibrated against a spiked standard solution,
1472 | Environ. Sci.: Processes Impacts, 2014, 16, 1469–1479
Paper
the calibrated data used for plotting, statistical analysis and hydrogeochemical modeling were assumed to be accurate. The analytical charge imbalance of 96 samples (60%) was within 5%, while that of 10 samples (6%) was beyond 10%. The latter were not used for analysis. 4.1
Hydrochemistry
Statistics (a ¼ 0.05) of chemical parameters of groundwater samples were listed in Table 1. The average pH of groundwater at different hydrological parts of the study area was 7.8–8.4, while the Shentou karst springs had a lower average pH of 7.4. Although Eh values as low as 170 mV were found in the central basin, especially the deep parts, the aquifers in the recharge and the ow-through zones seemed to be suboxic with an average Eh of 10–80 mV, and higher Eh values (average 70–80 mV) were found in the west alluvial plains. The east and the west pluvial plains, in addition to the east alluvial plains, were characterized by low total dissolved solids (TDS) values which were typically 1000 mg L1, and up to several to tens of thousands mg L1 for the down-gradient parts of ow path 3, while those in their deep aquifers were generally
Published on 19 February 2014. Downloaded by University of California - Riverside on 23/10/2014 07:29:16.
PAPER
Cite this: Environ. Sci.: Processes Impacts, 2014, 16, 1469
View Journal | View Issue
Geochemical evolution of groundwater salinity at basin scale: a case study from Datong basin, northern China Ya Wu and Yanxin Wang* A hydrogeochemical investigation using integrated methods of stable isotopes (18O, 2H), 87Sr/86Sr ratios, Cl/ Br ratios, chloride-mass balance, mass balance and hydrogeochemical modeling was conducted to interpret the geochemical evolution of groundwater salinity in Datong basin, northern China. The d2H, d18O ratios in precipitation exhibited a local meteoric water line of d2H ¼ 6.4 d18O 5 (R2 ¼ 0.94), while those in groundwater suggested their meteoric origin in a historically colder climatic regime with a speculated recharge rate of less than 20.5 mm overall per year, in addition to recharge from a component of deep residual ancient lake water enriched with Br. According to the Sr isotope binary mixing model, the mixing of recharges from the Shentou karst springs (24%), the western margins (11%) and the eastern margins (65%) accounts for the groundwater from the deep aquifers of the downgradient parts in the central basin is a possible mixing mechanism. In Datong, hydrolysis of silicate minerals is the most important hydrogeochemical process responsible for groundwater chemistry, in addition to dissolution of carbonate and evaporites. In the recharge areas, silicate chemical weathering is typically at the bisiallitization stage, while that in the central basin is mostly at the monosiallitization stage with limited evidence of being in equilibrium with gibbsite. Na exchange with bound Ca, Mg prevails at basin scale, and intensifies with groundwater salinity, while Ca, Mg exchange with bound Na locally occurs in the east pluvial and alluvial plains. Although groundwater salinity increases with the progress of
Received 7th January 2014 Accepted 19th February 2014
water-rock/sediment interactions along the flow path, as a result of carbonate solubility control and continuous evapotranspiration, Na–HCO3 and Na–Cl–SO4 types of water are usually characterized
DOI: 10.1039/c4em00019f
respectively in the deep and the shallow aquifers of an inland basin with a silicate terrain in an arid
rsc.li/process-impacts
climatic regime.
Environmental impact Groundwater is the most important source of water supplied to domestic, industrial and agricultural activities in arid and semiarid areas of northern China. However, problems due to over-exploitation and geogenic arsenic, uoride and iodine contamination of groundwater are the major environmental and public health concerns in these regions. Increased knowledge of the geochemical processes controlling the evolution of groundwater salinity is urgently needed for effective groundwater resource management and a sustainable safe water supply. In this study, an investigation using integrated hydrogeochemical tools was conducted to interpret geochemical evolution of groundwater salinity at regional scale in Datong basin, northern China. The work provides new insights into the water-rock/sediment interactions in groundwater systems of arid and semiarid regions.
1
Introduction
Water shortage is an increasingly serious issue in the arid and semiarid regions of northern China where groundwater is the most important source of water supplied to domestic, industrial and agricultural activities. Problems such as attenuating regional groundwater tables, land subsidence and groundwater quality deterioration have been induced in these regions by the
State Key Laboratory of Biogeology and Environmental Geology & School of Environmental Studies, China University of Geosciences, 430074 Wuhan, China. E-mail: [email protected]; Fax: +86-27-87481030; Tel: +86-27-67883998
This journal is © The Royal Society of Chemistry 2014
over-exploitation of groundwater due to rapid population growth and economic development in recent decades.1–3 In addition, salinization of groundwater and soil, and geogenic arsenic, uoride and iodine contamination of groundwater in northern China due to geological, hydrogeological and climatic conditions are the major environmental and public health concerns.4 Groundwater has been exploited intensively for more than 30 years in Datong basin, a famous coal industry base in China and an important agricultural base in Shanxi province.2 Many efforts have been made to understand the hydrogeology and hydrochemistry of groundwater containing of high levels of arsenic,
Environ. Sci.: Processes Impacts, 2014, 16, 1469–1479 | 1469
View Article Online
Published on 19 February 2014. Downloaded by University of California - Riverside on 23/10/2014 07:29:16.
Environmental Science: Processes & Impacts
uoride and iodine in this region.2,5–15 Increased knowledge of the geochemical processes controlling the evolution of groundwater salinity can contribute to effective management and sustainable exploitation of groundwater resources, and guide the development of a safe and clean groundwater supply. Determining the origins of solutes in groundwater and the control of hydrochemical variations is vital to understanding these geochemical processes. In this study, integrated methods of stable isotopic ratios (d18O, d2H), 87Sr/86Sr ratios, Cl/Br ratios, chloride-mass balance, mass balance and hydrogeochemical modeling were applied to interpret the geochemical evolution of groundwater salinity in Datong. The work provides new insights into the water-rock/sediment interactions in groundwater systems of arid and semiarid regions.
2 Study area Datong basin, covering an area of 8700 km2 located in the north part of Shanxi province (Fig. 1), is one of the Cenozoic faulted basins of the Shanxi ri system. The basin is surrounded by the Hongtao Mountains to the west and the Hengshan Mountains to the east. Bedrocks to the east and the north are mainly the
Paper
Neo-Archaean Hengshan metamorphic complex, including migmatite, tonalite, granodiorite and granitic gneiss, as well as mac lenses, pegmatites and granites.16 Ordovician carbonate, as well as Cambrian, Carboniferous and Permian sedimentary rocks occur in the west and the south. Cenozoic unconsolidated sediments occur all over the basin with varying thicknesses reaching more than 2000 m in the center of the basin. A common feature of inland basins is that the grain size of unconsolidated sediments gradually decreases from the margins to the central basin. Sediments types in the margins are alluvial–pluvial sand and gravel while in the plain areas and the central parts of the basin mainly consist of alluvial and alluvial–lacustrine interstratied sand, silt, silty clay and clay. Datong has an arid climate with an average annual temperature of 6.8 C, an average annual atmospheric precipitation in the range 370–420 mm, and an average annual evaporation up to 1800 mm. Sanggan River and its biggest tributary, Huangshui River, are the main ephemeral rivers that run through the basin from the southwest to the northeast. Pluvial, alluvial–pluvial and alluvial–lacustrine sediments, especially middle-upper Pleistocene alluvial–pluvial sands and gravels at depths < 150 m constitute the major Quaternary hydrostratigraphic units.7
Fig. 1 Location of the study area and sampling sites. (a) China and the location of Shanxi province, (b) Shanxi province and the location of Datong basin, (c) study area and locations of the sampling sites; the groundwater table altitude was compiled from Han (2008).7
1470 | Environ. Sci.: Processes Impacts, 2014, 16, 1469–1479
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 19 February 2014. Downloaded by University of California - Riverside on 23/10/2014 07:29:16.
Paper
Quaternary aquifers occur in three groups: phreatic aquifers (depths < 10 m), semi-conned aquifers (depths 20–50 m) and conned aquifers (depths > 50 m).10,13 Precipitation, permeating water from bedrocks and inltrating surface water are the recharges of the Quaternary groundwater aquifer systems (Fig. 2).7,13 Evaporation, pumping and leakage into the deeper conned aquifers through aquitards, especially in the areas of groundwater depression cones,10 are the main ways of discharge of the shallow groundwater. Three groundwater ow paths can be delineated from the groundwater table altitude: ow path 1 from the western margins, ow path 2 from the eastern margins to the central parts of the basin, and ow path 3 from the southeast margins mostly from the Shentou karst springs through the basin along the NE-trend (Fig. 1). The groundwater ow system is shown in the hydrological cross-section (A–A0 –A00 , Fig. 2): the mountains and the margins of the basin (pluvial plains) are natural recharge zones, while the alluvial plains and the lower-lying central parts of the basin could be identied respectively as the ow-through zones and the discharge zones. According to the results of XRD analysis, silicate minerals and their secondary minerals account for >80% among the minerals phases, suggesting that hydrolysis of silicate minerals is the most important hydrogeochemical process responsible for groundwater chemistry in Datong.10 There are generally three stages for silicate chemical weathering, which are limited by leaching regimes and are associated with the formation of different types of secondary clay minerals.17–20 During the early stages of weathering, leaching is weak in areas with a precipitation lower than 500 mm. The most soluble cations such as Na, K, Ca and Mg are dissolved and released from the host rocks, while most of the silica is retained in solid phases. As a result, secondary clay minerals of the illite or smectite groups are formed. Since illite and smectite have a 2 : 1 sheet structure
Environmental Science: Processes & Impacts
(Si/Al ¼ 2), this stage is called bisiallitization. As weathering progresses, leaching is moderate in zones with a precipitation of 500 to 1200/1500 mm. The relatively less soluble cations such as Mn, Cu, Co, Ni and Fe are removed but part of the silica is retained, resulting in the formation of the kaolinite group. Owing to the ratio of Si/Al equals unity in kaolinite, the stage is named monosiallitization. In the third stage of chemical weathering, leaching is intense in regions with a precipitation higher than 1500 mm. The basic elements and even silica are completely removed, leaving mostly unushed Al as hydroxides such as gibbsite. Goethite may also form in this stage. Therefore, the stage is usually called allitization, also termed lateritization or ferrallitization.
3 Methods 3.1
Sampling
Three sampling campaigns were conducted in July and November 2011 and September 2012 using sampling scheme of combination of orthogonal grids and locations of villages, and a total of 161 groundwater samples were collected across the study area. The groundwater samples were mostly collected from private boreholes, and a few small central water supplies or irrigation wells. When sampling groundwater from boreholes, samples and measurements were taken aer pumping for 3–10 minutes. One soil solution sample (depth ¼ 2 m, September 2010) was collected using pre-cleaned and pressurevacuum self-made porcelain sampler from the unsaturated zone in a representative agricultural eld in the central basin where insertion at a depth of 2 m remains above the local groundwater table. The Eh, pH, electrical conductivity (EC) and temperature of the samples were determined using portable multi-parameter meters (HACH, HQ40D Field Case) during sampling. The Eh
Fig. 2 Groundwater aquifer system and groundwater flow system, a hydrological cross-section A–A0 –A00 as indicated in Fig. 1, modified from Han (2008).7
This journal is © The Royal Society of Chemistry 2014
Environ. Sci.: Processes Impacts, 2014, 16, 1469–1479 | 1471
View Article Online
Published on 19 February 2014. Downloaded by University of California - Riverside on 23/10/2014 07:29:16.
Environmental Science: Processes & Impacts
values reported in this study have not been corrected relative to the standard hydrogen electrode (SHE) but instead can be used as relative values. The alkalinity of the samples was determined by titration within 24 hours aer sampling. The samples were collected in 50 mL PET bottles aer ltration using a 0.45 mm lter. Samples for the analysis of cations and trace elements were preserved by acidication with GR grade HNO3 (15 mol L1) to pH < 2, while those for the analysis of anions were unpreserved. 3.2
Chemical analyses
Samples were analyzed in State Key Laboratory of Biogeology and Environmental Geology and School of Environmental Studies, China University of Geosciences. Cations (Ca, K, Mg, Na and Si) were measured using inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermo Electron Corporation IRIS Intrepid II XSP). Anions (Cl, SO4, NO3, F and Br) were determined by ion chromatography (IC) (DIONEX ICS1100 and Metrohm 761 Compact). In the ICP-OES and IC runs, spiked standard solutions were used to calibrate the determined values (1 spiked standard solution per 10 samples). Therefore, the spiked standard solutions could be also used to ascertain the precision (relative standard deviation, RSD) of the analyses. 3.3
Compiled data
The analytical data set was augmented by the results of earlier hydrogeochemical investigations,12,14,15 which included 87Sr/86Sr ratios and H, O isotopic compositions. The Water Resources Programme of Global Network of Isotopes in Precipitation (GNIP) conducted by the International Atomic Energy Agency (IAEA) and World Meteorological Organization (WMO) provides H, O isotopic compositions in precipitation worldwide. The H, O isotopic compositions in precipitation from nearby GNIP Taiyuan station (inland basin, 112.55 E, 37.78 N) which has a climate similar to Datong and lies approximately 200 km from the study area were quoted to represent that in Datong. Twenty GNIP (Taiyuan) samples were collected monthly between February 1986 and October 1988.21 3.4
Statistical analyses and hydrogeochemical modeling
Descriptive statistics of hydrochemical parameters and replicated spiked standard solutions were calculated using PASW statistics 18 (SPSS Inc, USA). Hydrogeochemical modeling, including saturation indices (SI) and log activities of Ca, Na, K and H4SiO4 were performed using PHREEQC 2.18 (USGS, 2012). End-member compositions of the silicate stability eld from the equilibrium relationship of Tardy (1971)22 and Drever (1997)23 were quoted.
4 Results and discussion In the ICP-OES and IC runs, the RSDs were in the range 2.00– 5.51% (average 3.29%) and 2.22–3.65% (average 2.80%), respectively. Since the values of samples obtained directly from instruments were calibrated against a spiked standard solution,
1472 | Environ. Sci.: Processes Impacts, 2014, 16, 1469–1479
Paper
the calibrated data used for plotting, statistical analysis and hydrogeochemical modeling were assumed to be accurate. The analytical charge imbalance of 96 samples (60%) was within 5%, while that of 10 samples (6%) was beyond 10%. The latter were not used for analysis. 4.1
Hydrochemistry
Statistics (a ¼ 0.05) of chemical parameters of groundwater samples were listed in Table 1. The average pH of groundwater at different hydrological parts of the study area was 7.8–8.4, while the Shentou karst springs had a lower average pH of 7.4. Although Eh values as low as 170 mV were found in the central basin, especially the deep parts, the aquifers in the recharge and the ow-through zones seemed to be suboxic with an average Eh of 10–80 mV, and higher Eh values (average 70–80 mV) were found in the west alluvial plains. The east and the west pluvial plains, in addition to the east alluvial plains, were characterized by low total dissolved solids (TDS) values which were typically 1000 mg L1, and up to several to tens of thousands mg L1 for the down-gradient parts of ow path 3, while those in their deep aquifers were generally
