Abstract

Groundwater is an important component of the global water cycle, and acts as a receptor and information carrier of global environmental changes. Therefore, it is of great importance to research the chemical evolution of groundwater under a changing environment. Historical data shows that groundwater hydrochemical types are becoming more complicated, groundwater quality is deteriorating and the scope of pollution is expanding. This is attributed to an increasingly dry climate and the gradual deterioration of the original ecological environment, together with the unreasonable groundwater exploitation and intense agricultural activities of the past 30 years. Climate change and human activities are intertwined, and are responsible for changing the original groundwater system and forming a new evolutionary system.

INTRODUCTION

Groundwater is part of the global water cycle system and is the link between the lithosphere, the biosphere and the atmosphere. As an important freshwater resource, groundwater plays an important role in promoting agricultural production, supporting economic development, ensuring residents' lives, and maintaining ecological balance and diversity (Ramakrishnaiah et al. 2009; Prasanna et al. 2010; Kløve et al. 2014). In the context of global climate change and increased human activity, groundwater, as an environmentally sensitive factor, is bound to change (Figura et al. 2011; Taylor et al. 2012). Particularly in arid and semi-arid regions, owing to long-term over-exploitation of groundwater and the arbitrary discharge of pollutants, a continuous decline of the groundwater level and deterioration of water quality have been observed.

The evolution of groundwater systems in changing environments has thus attracted the attention of relevant scholars. Since the variation of groundwater in quantity can be directly demonstrated by visible indicators, such as water level or spring flow, and is directly related to the supply and demand of water resources, relevant studies mainly focus on the ‘quantitative change’ of groundwater (Okkonen & Kløve 2010; Ali et al. 2012; Costa-Cabral et al. 2012; Kløve et al. 2014). However, the ‘qualitative change’ of groundwater is largely invisible, secluded and hysteretic. Therefore, there are few studies on the hydrogeochemical evolution of groundwater in a changing environment (Treidel et al. 2011).

The circulation process of groundwater includes not only the transport of solvents, but also the transport of solutes (Li et al. 2019). Therefore, water quantity and water quality are inseparable for a complete groundwater system (Cao et al. 2009), and the change in water quality should receive more attention. In northern China, for example, 65% of domestic water, 50% of industrial water and 33% of agricultural irrigation water comes from groundwater (Jiao 2015). The change in water quality is directly related to health, industrial production and crop growth. The hydrogeochemical evolution of groundwater arises from the exchange of complex materials and energy between the lithosphere, the biosphere, the atmosphere and the hydrosphere during the circulation process. Hence, groundwater chemical parameters, such as major or trace elements, isotopic compositions and inert gas contents, contain rich information on environmental changes, and can be used as information carriers to indicate environmental changes on different time-scales (Giedraitiene et al. 2002; Wang et al. 2005; Fairchild et al. 2006). The study of groundwater chemical evolution in a changing environment can better reveal the interaction mechanism between groundwater and its changing environment in different regions, and help us obtain a deeper understanding of the impact of climate change and human activity on the groundwater environment.

Taking the typical arid and semi-arid inland basin in the West Jilin Province (WJP) in China as an example, the impact of environmental changes on hydrogeochemical evolution was studied by analyzing water chemistry data and environmental change data from 1980 to 2017. The aim is to determine the response mechanism of groundwater hydrochemistry under a changing environment, promote relevant research on the groundwater environment, and provide a basis for the formulation of future policies on the sustainable utilization of groundwater resources and water environment protection.

MATERIALS AND METHODS

Study area

The West Jilin Province (WJP) is located in China's farming–pastoral transitional zone. The climatic features of low precipitation and strong evaporation make the hydrogeochemical characteristics of this region very sensitive to the changing environment, resulting in a series of water safety problems. WJP, as shown in Figure 1, includes the two administrative districts of Baicheng and Songyuan, with a total area of 47,074.5 km2 and a population of about 4.72 million. The annual average temperature is 3–6 °C, the average annual precipitation is 400–500 mm, and the average annual evaporation is 1,500–2,000 mm. The main rivers in WJP are Nen River, Songhua River and Taoer River. The economy of WJP is mainly based on agriculture and animal husbandry. Irrigation relies heavily on groundwater extraction, and there are two main irrigated areas along Songhua River and Nen River.

Figure 1

Location, topography and geomorphologic partition of WJP.

Figure 1

Location, topography and geomorphologic partition of WJP.

WJP is higher in the east, west and south, and lower in the middle and north (Figure 1). According to the difference in intensity and form (uplift or subsidence) of neotectonic movements, WJP was divided into six geomorphic partitions: mountainous region, valley plain, Songnen low plain, Songla interfluve, Taoer alluvial fan and tableland as shown in Figure 1. Controlled by topography, the groundwater flows from the east, south and west to the middle and north, and flows into the Nen River and Songhua River at the northern boundary. Pore unconfined groundwater and pore-fissure confined water in Quaternary unconsolidated sediments are currently the main aquifers for water supply and the subject of this study. In the Taoer alluvial fan, the unconfined aquifer consists of sand gravel with a thickness of 10–40 m and is mainly recharged by river and precipitation infiltration, while in the low plain, the aquifer is mainly fine sand, followed by loess soil with a thickness of 3–20 m. Due to the low and flat terrain, aquifer recharge comes mainly from precipitation infiltration, followed by lateral recharge of underflow, and discharge is mainly through evaporation and artificial exploitation. The confined aquifer is composed of sand gravel layers, covered with relatively stable silty cohesive soil (aquitard), and the mudstone forming the waterproof floor. It is widely buried under 40–60 m of low plain ground, with a thickness of 10–30 m.

Historical data

The hydrogeochemical data of the Songyuan and Baicheng groundwater monitoring network from 1979 to 2013 was collected and collated. The monitoring items included pH, Mg2+, Ca2+, K+, Na+, Cl, , , , , Fe3+, , , TH, TDS, and F; a total of 3,986 groups of data, including 1,919 groups of unconfined groundwater and 2,067 groups of confined water. The map of groundwater hydrochemical type spatial distribution in WJP in the 1980s was taken from Groundwater and Quaternary Geology in Baicheng Area (1984). Meteorological data from 1958 to 2013, including precipitation, evaporation and temperature, was obtained from China Meteorological Date Service Centre (http://data.cma.cn/). Land-use data is quoted from Wang et al. (2008). The data on groundwater exploitation from 1980 to 2013 was obtained from Songyuan Water Resources Bulletin (1980–2013) and Baicheng Water Resources Bulletin (1980–2013). The data of grain yield and consumption of chemical fertilizers from 1978 to 2013 is available in the Jilin Province Statistical Yearbook (1978–2013).

Sample collection and analysis

Water samples were collected from November 3 to 24, 2017, including 156 unconfined groundwater samples and 133 confined-water samples (Figure 1). The sampling was carried out 5–10 min after the start of the pumping, to drain the stagnant water in the suction pipe. The water samples were filtered through a 0.45 μm membrane and added to a 350 mL polyethylene plastic bottle, which was rinsed three times with deionized water. Three bottles of water were taken from the same well, and 10% HNO3 was added to one of the bottles to make the water sample pH less than 2 for cation analysis. Water temperature and pH were tested on-site using the calibrated HANNA (HI99131) portable pH/temperature analyzer, and the alkalinity was determined on-site by Gran titration.

The water samples were tested at Pony Testing International Group in Changchun within one week. The water quality testing method is based on the Standard Examination Methods for Drinking Water (GB5750–2006). The main cations (K+, Na+, Mg2+, Ca2+, Fe3+) were tested by the HK-8100 inductively coupled plasma atomic emission spectrometer (ICP–AES) with a detection limit of 0.1 mg/L. The main anions (Cl, , , F) were tested by EXPEC–7000 ion chromatography with a detection limit of 0.1 mg/L. and contents were determined by 760–CRT ultraviolet and visible spectrophotometer with a detection limit of 0.005 mg/L.

RESULTS

Hydrogeochemical evolution over time

Evolution of hydrochemical type

In general, the hydrochemical types of unconfined groundwater in 2017 have become more complicated compared with 1980. The hydrochemical types of the alluvial fan, interfluve and tableland remained basically unchanged, that is, HCO3-Ca or HCO3-Ca·Mg. NO3-polluted water is dotted in the alluvial fan and interfluve, while NO3-Cl-polluted and Cl-polluted waters are concentrated in the southern part of the tableland (Figure 2). The main change in the low plain is that the water hydrochemical type in the Zhenlai irrigation district was transformed from HCO3-Ca·Na to HCO3-Ca·Mg, while in the Qianguo irrigation district it was transformed from HCO3-Na·Ca to HCO3-Ca·Na, and there is a slight expansion of HCO3-Na type water. Polluted-type water is scattered in the low and valley plains, and there is a concentrated distribution of NO3-polluted type water in the center of the basin. The variation of hydrochemistry types in the study area is mainly reflected by the decrease of HCO3 types and the increase of polluted-types, but the general evolution law does not change.

Figure 2

Unconfined groundwater hydrochemical types in (a) 1980 and (b) 2017.

Figure 2

Unconfined groundwater hydrochemical types in (a) 1980 and (b) 2017.

Evolution of aggregative indicator: TDS

The concentration of TDS is controlled by the landform, lithology, occurrence conditions and human activities, which, to some extent, reflect the quality of groundwater. Therefore, TDS is selected as an aggregative indicator to reflect the impact of a changing environment on groundwater chemistry. Figure 3(a) shows that the TDS of unconfined groundwater in different regions shows an upward trend to different degrees, and a clear trend is observed in that the scope of TDS and the value of outliers is expanding year by year. The rate of annual increase is 6–15 mg/L in the alluvial fan, valley plain, tableland and low plain, and 0.47 mg /L in the interfluve. Confined water shows a slight upward trend, with an annual increase of 6.13 mg/L. The TDS excessive area (TDS > 1 g/L) is mainly concentrated in low-plain areas, and its range increases year by year (especially after the 1990s) and gradually extends from the southwest to central and north-central, while the range of the low-TDS area (TDS < 0.5 g/L) decreases year by year.

Figure 3

Variation of (a) TDS, (b) F, and (c) NO3-N in unconfined groundwater.

Figure 3

Variation of (a) TDS, (b) F, and (c) NO3-N in unconfined groundwater.

Evolution of characteristic indicator: F

As the content of fluorine in groundwater is mainly related to natural factors, fluorine is selected as the characteristic index to reflect the impact of natural changes on groundwater chemistry. Figure 3(b) shows that the spatial distribution of fluorine does not change significantly between years, and the slight change in some regions may be related to the selection of control points. It can be seen from Figure 4(b) that there is no significant increase or decrease in the scope of F and the value of outliers, and the content remains relatively stable.

Figure 4

Variation of (a) TDS, (b) F and (c) NO3-N in different regions.

Figure 4

Variation of (a) TDS, (b) F and (c) NO3-N in different regions.

Evolution of pollution indicator: NO3

WJP is a typical agricultural area, and nitrogen fertilizer and rural-living wastewater are the main sources of N in the groundwater environment. Therefore, the content of N could be an indicator of the intensity of human activities. As the contents of NH4-N and NO2-N are low and unstable, NO3-N is taken as the pollution indicator reflecting the impact of human activities on groundwater. The hierarchical point diagram (Figure 3(c)) shows that the distribution of NO3-N in groundwater has strong spatial anisotropy, but a clear trend of increasing NO3-N is observed. In the 1980s, the few samples that exceeded the standard of 20 mg/L were mainly distributed in the low plain. By the year 2000, the number of such samples was higher, and more obvious in the south of the tableland, but the content was within 80 mg/L (Figure 4(b)). In 2017, the content of NO3-N in the low plains and tableland continued to increase, with some samples exceeding 100 mg/L, and samples exceeding the standard appeared in each region. Figure 4(b) shows that the content of NO3-N in each region increased to different degrees in both the scope and value of outliers. Unconfined groundwater in the tableland, alluvial fan and low plain increased the most, with an annual increase of 0.4–0.5 mg/L, while a relatively slow increase was observed in the interfluve and valley plain. There was a slight increase of confined water, with an indistinctive annual increase of 0.108 mg/L but a clear increase of outlier values.

Environmental changes

Climate change

Figure 5(a) shows an obvious increase in temperature in the last 30 years, with a rise rate of 0.037 °C/a. Precipitation showed a decreasing trend, with a decrease rate of −0.67 mm/a. The evaporation of the evaporating dish decreased by −5.86 mm/a. Drought degree increased in WJP as a whole. The trend is consistent with the climate change trend in northeast China, which is a regional reflection of global warming.

Figure 5

Changing environment: (a) anomaly percentage of precipitation, evaporation and temperature, (b) variation of land use, (c) variation of groundwater exploitation, (d) variation of fertilizer application and grain yield.

Figure 5

Changing environment: (a) anomaly percentage of precipitation, evaporation and temperature, (b) variation of land use, (c) variation of groundwater exploitation, (d) variation of fertilizer application and grain yield.

Human activities

Land use

Figure 5(b) shows that the land-use change in WJP in recent years presents an increase of cultivated land and saline–alkali land, while there was a decrease of grassland and water area. From 1989 to 2008, the proportion of cultivated land increased from 47.9% to 52.91%; the proportion of saline–alkali land area increased from 10.1% to 13.6%; the proportion of grassland area decreased from 22.4% to 16.57%, and the proportion of water area fell from 6.5% to 3.52%.

Groundwater exploitation

Figure 5(c) shows that the amount of groundwater in WJP increased by 2.4 times from 5.9 × 108m3 in 1980 to 20.1 × 108m3 in 2013. Especially in the late 1990s, the growth rate of exploitation increased to 103.3 million m3/a, and a groundwater depression was formed in some areas due to excessive exploitation. The proportion of confined water increased year by year from 36.6% in 2000 to 55.2% in 2013.

Agricultural development

Figure 5(b) shows that the cultivated area growth rate of WJP is 1.79 × 104hm2/a (1989–2008), and the upland field accounts for 56.01% while the paddy field accounts for 43.99% in growth. At the same time, the growth rate of fertilizer application was 3.01 × 104t·a−1 (1978–2013). The fertilizer used was mainly nitrogen fertilizer and compound fertilizer, accounting for 79% of the total amount (2013), followed by phosphate fertilizer and potash fertilizer. With the increase in cultivation area and fertilizer application, the agricultural grain yield in WJP increased year by year, with a growth rate of 2.75 × 104kg·a−1.

DISCUSSION

Impact of natural changes on hydrogeochemistry in WJP

Precipitation is the main recharge source of groundwater in WJP, and the only source in the center of the basin. Taking WJP as a system, a reduction of precipitation reduces the amount of replenishment of the system from precipitation infiltration. Thus, it is equivalent to the concentration of groundwater, if the effects of other recharge sources are not taken into account. But the quantitative evaluation of its effects is hard, since it requires accurate calculation of the groundwater storage amount; the actual precipitation recharge amount and seasonal desalination (Várallyay 1994) should be considered too. In addition, the reduction of precipitation may indirectly increase the exploitation of groundwater by human activities (mainly agriculture) and amplifies the effect of over-exploitation of groundwater on the hydrochemical field (Ali et al. 2012).

The average annual temperature in the study area rose, while the groundwater evaporation declined (Figure 5(a)), in line with the ‘evaporation paradox’ that has emerged globally over the past 50 years (Yang et al. 2013), especially in the arid zones of agricultural activities. Pan evaporation means free evaporation from a limited water surface, strictly speaking, it represents the index of how much solar energy is accepted by an area, but not the change of water quantity. Actual evapotranspiration from the land surface is the objective variable to measure the change of water quantity. Brutsaert (1998) suggested that observed decreases in pan evaporation can be interpreted as evidence for increasing terrestrial evaporation, but this is only an explanation for wet areas. For arid areas much uncertainty still remains. Pan evaporation and actual evapotranspiration are related models but cannot be substituted for each other. As a dominant factor affecting hydrochemistry in WJP, the variation of evaporation intensity affects the concentration degree of groundwater components. However, the decline of groundwater level caused by over-exploitation can weaken the evaporation to some extent (Chen & Hu 2004).The non-significant variation of F (Figures 3(b) and 4(b)) in WJP may indicate that the present changes in evaporation are not sufficient to alter the hydrochemical process, or it may be obscured by artificial exploitation and agricultural irrigation. Therefore, further studies are needed to identify the impact of groundwater exploitation when evapotranspiration is estimated.

Related research in the central Jilin region just east of WJP shows that unconfined and confined water are subject to varying degrees of temperature rise caused by global warming. The average water temperature increased by 1.3 °C and 0.6 °C respectively during 1979–2001 (Fang et al. 2005; Lin et al. 2009). The strongly temperature-dependent biogeochemical processes in groundwater and the quality of water for drinking-water is likely to be affected by large-scale climatic forcing (Figura et al. 2011).

Impact of human activities on hydrogeochemistry in WJP

The double pressure of the drought and the increasing water demand has forced the continuous increase of groundwater exploitation in WJP (Figure 5(c)). The exploitation has led to major changes in groundwater circulation, causing the destruction of the natural water–salt balance. On the one hand, the decline in the water level makes the unsaturated zone thicker, lengthens the infiltration path of precipitation, and increases the quantity of leached salt into groundwater. On the other hand, it makes the aquifer thinner and weakens the dilution capacity of the aquifer. In addition, strong exploitation may be one of the reasons for the increase of NO3-N contamination, as the over-exploitation will gradually change the groundwater system from a close-reduction to a semi-open weak-oxidation environment (Liao & Lin 2004), which is beneficial to nitrification. Furthermore, organic carbon required for denitrification mainly comes from the surface layer of the soil enriched with organic material (Starr & Gillham 1989), and the excessive depth is not conducive to the entry of organic carbon into the aquifer. Therefore, over-exploitation provides a conducive environment to the enrichment of NO3-N to some extent.

Drilling is another manifestation of exploitation. Since the 1980s, the water quality of unconfined groundwater in WJP has undergone a series of changes (Figure 3(b)), such as the increase of TDS and NO3-N content, which is related to the increase of exploitation of confined water (Figure 5(c)). As a result of mining, the confined-water level keeps falling, and the difference from the unconfined groundwater level gradually expands. The water level difference increases the ability of the leakage, especially the area of the thinner aquitard. The aquitard consists of cohesive soil enriched in organic matter and with strong reduction potential, and is conducive to the enrichment of Fe2+, which is an electron donor of autotrophic denitrification, therefore is easily reduced to N2 (Postma et al. 1991).

Therefore, in theory, it is difficult to induce an increase in NO3-N in confined water, which means that there are other ways for unconfined groundwater to enter the confined aquifer. Considering the large agricultural area in WJP, the grain plan and drought conditions over the years have increased the number of agricultural irrigation wells. In 2000, the well density was 1.32 holes per square kilometre, and the density in the urban and agricultural area could reach more than ten holes per square kilometre (Liao & Lin 2004). Referring to past drilling data, with a lack of protection of confined-water consciousness, confined-water wells were generally not sealed up when the well was formed and filter pipes were set in both unconfined and confined aquifers. This largely penetrated the confined aquifer roof, and artificially increased the hydraulic connection between unconfined groundwater and confined water. Therefore, the drilling allowed the confined water to mix with the unconfined groundwater of poorer quality directly through nonstandard wells to cause mixed pollution. In addition, the field investigation found that there were many abandoned confined wells in rural areas and farmland that were treated improperly. These wells became a shortcut to the infiltration of domestic sewage and precipitation leached with garbage, feces, and fertilizer. Therefore, the increasing exploitation of confined water and the irregularity of the well-forming process are the main reasons for the deterioration of the confined water.

There is a desalination layer of 7–20 cm in the grassland surface which is suitable for stoloniferous plants. However, land reclamation and cultivation blindly turn the alkaline soil layer to the surface (Fan et al. 2002), causing the desalination layer to be removed and forming saline alkali land. Figure 5(b) shows that the cultivated land area in 2008 increased by 3.41 × 105hm2 compared with 1989, the saline–alkali land area increased by 3.01 × 105hm2, and the grassland area decreased by 2.93 × 105hm2. The change in land use indicates that the expansion of the cultivated land area and the destruction of grass are the direct causes of the increase of saline–alkali land. In addition, with the development of animal husbandry, the contradiction between animal husbandry and grassland intensifies, directly resulting in the reduction of grassland area. Secondly, over-grazing leads to cattle repeatedly trampling the grassland, resulting in the destruction of surface vegetation, accelerating the evaporation of soil surface moisture and increasing the rate of soil salt accumulation (Ibrahimi et al. 2014). The change of land use makes secondary salinization more serious and breaks the balance of water and salt migration of the original grassland, and seasonal desalination may be enhanced.

Agricultural land in WJP accounted for 45.65%–52.92% of the total area, and strong agricultural activities greatly influenced the evolution of regional hydrochemistry. For paddy fields, most of the diverted river water infiltrated into the ground and mixed with groundwater, except that which was absorbed by crops or evaporated (Wang et al. 2017). This may explain the transformation from Ca·Na to Ca·Mg in the Zhenlai irrigated area or HCO3-Na·Ca to HCO3-Ca·Na in the Qianguo irrigated area (Figure 3(b)). In addition, irrigation can uplift the water level around the irrigated area, and cause secondary salinization. Nitrogen fertilizer has become the main source of N in the groundwater environment, including ammonium nitrogen fertilizer, nitrate nitrogen fertilizer, amide nitrogen fertilizer and so on. However the utilization ratio of fertilizer in WJP was less than 30% (Liang et al. 2007), while the fertilizer application increased at a rate of 3.01 × 104 t·a−1, resulting in the expansion of the NO3-N pollution range. Organic nitrogen pesticides (insecticides, fungicides and herbicides) are also one of the sources of N (Debrewer et al. 2008). The content of NO3-N can be as high as 298 mg/L in unconfined groundwater, and the average content in the tableland reaches 35.46 mg/L (Figure 4(c)), exceeding the limit of 20 mg/L in China's standard for drinking water. Compared with the hydrochemical types in 1980, 25 groups of water samples in 2017 showed new hydrochemical types, among which 68% of the samples showed as the main (or secondary) anion. These were widely distributed in WJP, especially in the western part of the Qianguo irrigated area and the southern part of the tableland (Figure 2(b)). In major grain production areas, where agriculture is more developed and the region is more inhabited, NO3-N pollution is relatively heavy, while in saline, swamp, and pastoral areas, NO3-N pollution is relatively rare due to less farmland and less fertilizer and pesticide application (Debrewer et al. 2008) (Figure 3(c)).

Through field investigation, a serious lack of domestic sewage collection and treatment facilities, a common phenomenon of direct emission of sewage and open dumping of household garbage and livestock manure, were found in rural areas. The lack of infrastructure and control measures make the pollution more severe (Huang et al. 2013). For a long time, the pollutants mentioned above, under the action of leaching, caused higher content of TDS, NO3-N, Cl, in groundwater. This is especially true for the low plain, where the water flow is relatively sluggish and the cumulative effect (Silverstein et al. 1994) makes the situation worse. The Cl, and become the main (or secondary) anions in groundwater samples in 2017 (26.92% of unconfined groundwater samples and 14.29% of confined water). In recent years, with the improvement of living standards and the implementation of the national toilet policy, the pollution may have been reduced to some extent.

Hydrochemical evolution is not only controlled by natural geological factors, but also intertwined with human activities, which changes the original groundwater flow and chemistry system and has a profound impact on the groundwater environment.

Suggestions for sustainable use and environmental management of groundwater

The sustainable use of groundwater resources not only refers to the rational development and utilization of groundwater resources, but also includes protection of groundwater quality. Therefore, based on the discussion of climate change and human activities in the hydrochemical evolution of WJP above, the strategies that the governmental agencies could adopt are proposed as follows:

  • (1)

    Combine groundwater exploitation and utilization with changes in the hydrogeochemical environment, make rational planning for groundwater resource exploitation, and formulate countermeasures to deal with the negative impact of climate change.

  • (2)

    Actively develop green and water-saving agriculture, strengthen control over the use of pesticides and fertilizers, by adjusting the types and quantities of fertilizers and pesticides.

  • (3)

    Strengthen the supervision of the formation quality of confined water wells, and seal the wells that connected the unconfined and confined water.

  • (4)

    The supply of water to residents, industry and agriculture shall be separated according to the water quality in different aquifers.

  • (5)

    Build drainage and sewage treatment systems in rural areas and strengthen supervision of sewage discharge.

  • (6)

    Make scientific planning of land resources and actively prevent land salinization and grassland degradation.

CONCLUSIONS

  • (1)

    The environmental changes are mainly reflected in the following aspects: the climate is generally getting drier and warmer; the ecological system deteriorates and the area of saline–alkali land increases; the amount of groundwater exploitation continues to increase; the agricultural development is rapid, and the cultivated land area and fertilizer amount increase.

  • The changes in groundwater chemistry characteristics are mainly reflected in the following aspects: the hydrochemistry types are more complicated, and polluted types with , , and Cl as primary or secondary anions have emerged; the levels of TDS and in unconfined and confined water continue to rise; the fluorine content is relatively stable. In general, groundwater quality is deteriorating.

  • (2)

    The potential impact of climate change on the evolution of groundwater chemistry is reflected in the increase of groundwater salinity and temperature, amplifying the impacts of artificial exploitation, and there is much uncertainty regarding the impact of evaporation. The decline in water level caused by over-exploitation may be causing an increase in salinity and nitrate pollution, and at the same time enhancing the down-leakage of inferior unconfined groundwater to confined water. The irregular confined wells provide a shortcut-type pollution channel. The change of land use makes the secondary salinization more serious, breaking the water–salt migration balance of the original grassland. Irrigation of the paddy fields and dry fields has changed the original hydrodynamic field and the hydrochemical field, and pesticide and fertilizer use is the root cause of the increase of content in groundwater. The direct discharge of rural domestic sewage and the lack of collection and treatment facilities have led to an increase in TDS, , Cl and in groundwater. Climate change and human activities are intertwined, changing the original groundwater flow and hydrogeochemical system in WJP and forming a new evolutionary system.

ACKNOWLEDGMENTS

The research was funded by Natural Science Foundation of China (No. 41572216), the China Geological Survey Shenyang Geological Survey Center ‘Hydrogeological investigation in the Songnen Plain’ project ([2019]DD20190340-W09), the Provincial School Co-construction Project Special Leading Technology Guide (SXGJQY2017-6), and the Jilin Province Natural Science Foundation (20140101164JC). This work is also partially funded by the Engineering Research Center of Geothermal Resources Development Technology and Equipment, Ministry of Education, Jilin University, China.

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