Groundwater is increasingly exploited for energy production in arid regions, which necessitates a deeper insight into the impact of the enhanced human pressure on the groundwater. This study applied an integrated method (statistical analysis, water table fluctuation method, hydrograph analysis and remote sensing) to identify the impact of Energy Base Water Project on the groundwater in the Subei Lake basin. Groundwater levels in eight observation wells at 30 min intervals during the 2013–2014 period were monitored using automatic groundwater monitoring data loggers. Results showed that precipitation infiltration, irrigation return flow, groundwater pumping and evapotranspiration controlled the hydrodynamics of unconfined groundwater. The average evapotranspiration rates in the Quaternary phreatic aquifer and the Cretaceous phreatic aquifer were 6.15 and 12.48 mm/d. The unusual hourly hydrographs fall into three patterns (mutational, irregular and gradual hydrographs). Different recovery times after being influenced by pumping may be related to the presence of the mudstone lenses. The extent of the groundwater depression cone was qualitatively identified by gradual hydrographs, which may spread from the center area to the western boundary. Only some individual wells from Haolebaoji waterworks had conducted the intermittent pumping activities at random times and caused the decline of the lakes.
The coordinated development of water resources and energy infrastructure in arid regions is of great concern to hydrologists (Cai et al. 2014). Although the large-scale exploitation of energy resources has brought huge economic benefits, it has also often caused some negative impacts on groundwater systems and ecosystems (Kahrl & Roland-Holst 2008; Siddiqi & Anadon 2011). Groundwater resources play a very important role in semi-arid and arid regions (Moiwo & Tao 2014). Natural processes and aquifer properties vary at different spatio-temporal scales, and their variations affect the fluctuations of groundwater levels (Li & Zhang 2007). Over the last decades, groundwater has increasingly been extracted for energy production in arid regions around the world due to the fast growth in water demand for industrial use (Kahrl & Roland-Holst 2008; Siddiqi & Anadon 2011). The increasing concerns about potential threats to groundwater-dependent ecosystems in these areas should be taken into consideration in the framework of sustainable water resource management (Wang et al. 2012; Li et al. 2015; Cai et al. 2016).
The Subei Lake basin is a good illustration of an area where natural and anthropogenic factors significantly affect groundwater resources. It is representative of more than 400 lake basins of diverse sizes distributed in the Ordos artesian basin, which contains the second largest coal reserves in China (Dai et al. 2006). In recent years, numerous waterworks have been built in some lake basins (including the Haolebaoji waterworks, built in the Subei Lake basin) for energy production in the Ordos energy base, which is a large-scale regional industrial zone integrated with coal mining, electrical power generation and coal-based chemical industry (Hou et al. 2006). However, due to a lack of adequate hydrogeological knowledge about these specific lake basins and a reasonable groundwater management strategy, groundwater resources in these specific lake basins are currently subject to increasing pressure from altered hydrodynamic conditions associated with water abstraction. As a result, the intensive groundwater pumping has significantly affected the hydrodynamics and hydrochemistry of the groundwater system. If this continues, it may cause some negative impacts on the groundwater-dependent ecosystem around these lakes (Figure 1) (Liu et al. 2015a). The China Geological Survey Bureau has conducted some regional-scale research on the groundwater resources of Ordos Basin since the 1980s (Zhang et al. 1986; Hou et al. 2008). This research illustrated geology, hydrogeology and a comprehensive knowledge of the quantity and quality of groundwater in this region, which laid a solid foundation for the present study. However, regional-scale groundwater investigations may not provide much precise information on the hydrogeological settings in small lake basins (Toth 1963), as Winter (1999) proposed that lakes in different part of groundwater flow systems had different flow characteristics. Hence, it is crucial to implement local groundwater investigations and distinguish between natural and anthropogenic causes of changes to the groundwater system in order to create sustainable groundwater management in these lake basins.
There are various techniques for distinguishing the impact of anthropogenic and natural factors on groundwater systems with complicated hydrogeological settings. The techniques of hydrochemistry and stable isotopes have been widely used to qualitatively identify the impact of anthropogenic and natural factors on the groundwater (Baudron et al. 2014; Nyenje et al. 2014; Murgulet et al. 2015; Liu et al. 2015a; Liu et al. 2015b). However, groundwater monitoring is necessary because it is a direct measurement for quantitatively identifying the role of anthropogenic and natural factors in the dynamic behaviors of groundwater system (Rajmohan et al. 2007; Ebrahim et al. 2013; McCallum et al. 2013). Differences between groundwater hydrographs at different sites must be related to the distance from the source of influence and reflect the hydrogeological settings near the monitoring wells to a certain extent. Groundwater evapotranspiration (ETGW) is an important component of the groundwater balance in arid regions. The White method estimates daily ETGW using diurnal groundwater level fluctuations from a monitoring well. Moreover, the White method provides the basis for all subsequent ETGW estimation algorithms from the hourly hydrographs (White 1932; Healy & Cook 2002; Loheide et al. 2005; Yin et al. 2013; Zhang et al. 2015). A principal application of the boxplots is to identify possible outliers, which are a minority of observations in a dataset that have different patterns from that of the majority of observations in the dataset (Hadi et al. 2009; Dovoedo & Chakraborti 2014). In addition, the technique of remote sensing was applied to understand the relative impacts of anthropogenic activities on the temporal variation of lake area (Seyoum et al. 2015). Although these methods individually are not new, their combined application provides a novel methodology to identify the impacts of anthropogenic and natural factors on the groundwater, which can be applied to catchments worldwide.
The main objective of the paper is to identify the impact of the Energy Base Water Project on the groundwater by utilizing the high-frequency groundwater level data in this ecologically sensitive area. The related findings will provide an important reference for the establishment of the hydrogeological conceptual model and groundwater flow model. In addition, this research can contribute to the ongoing discussion about the coordinated development between water and energy infrastructure in arid regions, and it will advance necessary knowledge for the sustainable groundwater management in such arid areas.
The study area is located in the northern part of the Ordos Basin, extending between 39°13′30′′ and 39°25′40′′N latitude and between 108°51′24′′ and 109°08′40′′E longitude (Figure 2). The total area of the Subei Lake basin is almost 400 km2, which is controlled by a continental semi-arid to arid climate that is very hot in summer and extremely cold in winter. According to meteorological data from 1985 to 2008 collected at the Wushenzhao weather station, the mean annual precipitation is 324.3 mm/year, and the mean annual pan evaporation is 2,349.1 mm/year. Vegetation cover in Ordos Basin usually begins to grow quickly from April, gets to its peak in July or August and withers after September (Zhang 2010).
The topography of the west, east and north sides in the study area is relatively higher with elevations 1,370–1,415 m above sea level, while the south side is slightly lower with altitudes 1,290–1,300 m above sea level. The main water bodies are the Subei Lake and the Kuisheng Lake (Figure 2). Groundwater is the major recharge source for the two inland lakes (Hou et al. 2006; Wang et al. 2010). The Quaternary sediments and Cretaceous formation outcrop in the study area. The Quaternary sediments are mostly distributed around the Subei Lake with relatively smaller thickness ranging from 0 to 20 m (Figure 3(a)). The Cretaceous strata are mainly composed of sedimentary sandstones and the maximum thickness can be approximately 1,000 m in the Ordos Basin (Yin et al. 2009). Subsurface heterogeneity is very widespread in the Cretaceous strata and the fissures in the Cretaceous rocks may be well developed or poorly developed in different places (Wang et al. 2010).
The Subei Lake basin is a relatively closed hydrogeological unit, given that a small quantity of lateral outflow occurs in a small part of the southern boundary (Wang et al. 2010). Unconfined and confined aquifers can be observed in the basin. The phreatic aquifer is composed of the Quaternary sediments and Cretaceous sandstones (Figure 3(a)), with its thickness ranging from 10.52 to 63.54 m. In terms of borehole data, the similar groundwater levels in the Quaternary and Cretaceous phreatic aquifers indicate a very close hydraulic connection between the Quaternary layer and Cretaceous phreatic aquifer, which can be viewed as an integrated unconfined aquifer in the area (Wang et al. 2010). The precipitation infiltration and lateral inflow are the major recharge sources for the unconfined aquifer. The locally upward leakage from the underlying confined aquifer and irrigation return flow can also provide a small percentage of groundwater recharge. Groundwater evapotranspiration is the main discharge means of the unconfined groundwater. In addition, lateral outflow, artificial exploitation and downward leakage are included in the main discharge patterns (Wang et al. 2010). Groundwater levels were contoured to display the groundwater flow direction in Figure 3(a), which were measured in the regional survey conducted by the Inner Mongolia Second Hydrogeology Engineering Geological Prospecting Institute during September 2003. The groundwater flowed predominantly from the surrounding highlands to lowlands, which is under the control of topography. Overall, groundwater in the phreatic aquifer flowed toward the Subei Lake (Figure 3(a)).
The two adjacent aquifers are separated by a discontinuous aquitard. The aquitard is composed of the mudstone layer, and a discontinuous mudstone lens can be observed in Cretaceous strata (Figure 4). The phreatic aquifer is underlain by a confined aquifer composed of Cretaceous rocks. The hydraulic properties of the confined aquifer are variable in space. The hydraulic conductivity varies between 0.14 and 27.04 m/d, the hydraulic gradient varies from 0.0010 to 0.0045 and the storage coefficient ranges from 2.17 × 10−5 to 1.98 × 10−3 (Wang et al. 2010). The confined aquifer primarily receives downward leakage from the unconfined groundwater and lateral inflow from groundwater outside the study area. The flow direction of the confined groundwater was similar to that of unconfined groundwater (Figure 3(b)). Artificial exploitation is the major drainage of confined groundwater.
In the basin, groundwater was exploited for agricultural, industrial and domestic purposes. The proportions of agricultural, industrial and domestic water demand were almost 30%, 69% and 1%, respectively, in 2009, according to the unpublished hydrogeological report from the Inner Mongolia Second Hydrogeology Engineering Geological Prospecting Institute. The flood irrigation entirely relied on groundwater pumping from scattered irrigation wells due to the scarcity of surface water. Haolebaoji waterworks has 22 production wells stretching northeast to southwest, which have become operational for water supply in energy production since 2006. The production wells were designed in the shape of a rectangular grid at intervals of 1.5 km to each other. The production rate of individual wells was unknown throughout the monitoring period. However, according to the data obtained by this study, the total production rate climbed from 12,932 m3/d in 2006 to 24,746 m3/d in 2012 with an average annual growth rate of 1,969 m3/d, and all the production wells were screened within the confined aquifer due to the huge storage of confined groundwater.
MATERIAL AND METHODS
Data collection of groundwater level
The entire basin was covered by the monitoring network, composed of the eight monitoring wells (with well depths ranging from 6 to 300 m), which were well designed for the monitoring project in 2013 by the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences (Figure 3 and Table 1). The length of screen pipes in all monitoring wells varies from 1 to 10 m, and every monitoring well has only one screen pipe rather than multiple screens. The distance between the bottom of the screen pipe and the total well depth ranges from 0 to 3 m in the study area. Special attention was paid to monitoring the dynamic variation of groundwater level at different aquifer units and identifying the potential observation points affected by large-scale groundwater exploitation. All sites were equipped with a data logger (KADEC-MIZU II, Japan) and the groundwater levels were recorded at 30 min intervals from December 2013 to December 2014. In addition, a rain gauge was installed in the basin for the collection of precipitation data.
|No.||Longitude (°)||Latitude (°)||Type of aquifer||Elevation (m)||Well depth (m)|
|No.||Longitude (°)||Latitude (°)||Type of aquifer||Elevation (m)||Well depth (m)|
Given that the confined aquifer was intensively exploited by 22 production wells for industrial use, except for wells JC01 and JC06 which were used to monitor the unconfined aquifer, other wells were used to monitor the confined aquifer (Figure 3). Well JC01 is located in the highland and was screened within the Quaternary sediments. Well JC06 is located in the lowland and was screened within the Cretaceous rocks. The two monitoring wells were used to quantify the daily ETGW in the Quaternary and Cretaceous phreatic aquifers, respectively (Figure 3(a) and Table 1). The other six wells (wells JC02, JC03, JC04, JC05, JC07, and JC08) were designed to monitor the variation of the confining groundwater level so as to assess the impact of groundwater pumping from Haolebaoji waterworks on the confined aquifer. The six monitoring points were selected in consideration of various factors including the position in the groundwater flow system, the convenience of management and data collection, hydrogeological information, and the distances from 22 production wells (Figure 3(b) and Table 1). The missing values due to data logger malfunctions have been excluded.
In order to analyze the impact of groundwater pumping from Haolebaoji waterworks on the Subei Lake, the manually recorded water level of the Subei Lake and the rate of groundwater pumping from Haolebaoji waterworks from July to November in 2009 have also been collected from previous hydrogeological surveys conducted by the Inner Mongolia Second Hydrogeology Engineering Prospecting Institute.
Water table fluctuation method
The boxplot method for outlier detection
A simple way widely used to identify outliers is based on the concept of the boxplot (Tukey 1977). This graphically-based method for outlier detection is especially attractive not only for its simplicity but, more importantly, because it does not employ the extreme potential outliers that can distort the computing of a measure of spread and lessen the sensitivity to outliers.
Satellite-based lake surface observation
In this study, the temporal variation of lake areas was mapped using LANDSAT Thematic Mapper (TM) images obtained from the United States Geological Survey (http://glovis.usgs.gov/). The satellite images were pre-processed by the standard techniques such as atmospheric correction, radiometric calibration, and so on. NDWI was calculated from the green and near-infrared bands of LANDSAT TM by using processed satellite images. Finally, the images were processed in ArcGIS and were used to calculate the surface area of the lakes.
Temporal variations of the water table
Due to the equipment installed at well JC01 being damaged by anthropogenic factors, only the data from 2 December 2013 to 14 May 2014 were collected in well JC01. The data from 10 February 2014 to 12 May 2014 in well JC06 were missing due to battery failure. The water table in well JC01 ranged from 1,331.30 to 1,331.69 m, with an average value of 1,331.64 m. However, the water table in well JC06 varied from 1,299.27 to 1,299.92 m with a mean value of 1,299.71 m. The ranges of groundwater levels for wells JC01 and JC06 were only 0.39 and 0.65 m (Table 2). Groundwater hydrographs for the two monitoring wells are shown in Figure 5. There existed significant differences between the water table dynamics in well JC01 and those in well JC06 during the monitoring period.
|Well no.||Observation time (d)||Range (m)||Min. (m)||Max. (m)||Mean (m)||Variance|
|Well no.||Observation time (d)||Range (m)||Min. (m)||Max. (m)||Mean (m)||Variance|
|Well no.||Date||ETGW (mm/d)|
|JC01||26 April 2014||5.40|
|27 April 2014||6.30|
|28 April 2014||6.75|
|JC06||24 July 2014||9.90|
|25 July 2014||14.94|
|26 July 2014||12.60|
|Well no.||Date||ETGW (mm/d)|
|JC01||26 April 2014||5.40|
|27 April 2014||6.30|
|28 April 2014||6.75|
|JC06||24 July 2014||9.90|
|25 July 2014||14.94|
|26 July 2014||12.60|
The variation of groundwater level in well JC01 can be classified into two different phases (Figure 5(a)). A stable trend with a groundwater level variation of −0.025 ∼ 0.028 m/d was observed from 2 December 2013 to 28 March 2014, indicating that groundwater inflow and outflow were almost equal. A fluctuating trend with a daily fluctuation variation of −0.24 ∼ 0.26 m was presented from 29 March 2014 to 14 May 2014, when the rainfall increased gradually and irrigation activities became frequent in the Subei Lake basin. However, the low rainfall from individual events may be subject to evapotranspiration before reaching the water table given that most of the rainfall amounts during this period were below 10 mm/d. The obvious decline and recovery of groundwater level was observed within a few hours, which was controlled by the combined effect of short-term groundwater exploitation from the nearby irrigation well and the rapid recharge from irrigation return flow due to the high permeability of the vadose zone, which was composed of the Quaternary unconsolidated sediments. Therefore, flood irrigation was mainly responsible for the groundwater regime during the fluctuating period.
Well JC06 exhibited a different pattern from the previous one (well JC01). The variation of groundwater level in well JC06 can be divided into two different phases (Figure 5(b)). The critical point for the two phases was 22 June 2014, when the groundwater level reached the its minimum value. Overall, the water table declined slowly before 22 June, indicating that a net groundwater outflow controlled the variation of groundwater level. Subsequently, in response to frequent rainfall events, the groundwater level rose quickly within a few days after 22 June. During the second period, precipitation infiltration predominated the hydrological processes and exerted a long-term effect on the groundwater regime. A fluctuation with a larger amplitude took place from 8 July to 17 July due to groundwater pumping from well JC06 itself. In the following days, the water table fluctuated with a relatively smaller amplitude but remained at a higher groundwater level.
It should be noted that the different dynamic patterns of the water table in these two wells may be explained by the local hydrogeological settings (such as lithology, hydraulic properties, the depth to water table, etc.), precipitation and anthropogenic activities (such as groundwater pumping and irrigation return flow caused by the traditional flood irrigation). In addition, the time series length of groundwater monitoring data in well JC01 was shorter than that in well JC06, so it is necessary to continue groundwater level monitoring in order to assess the temporal variations of groundwater level precisely in the next step.
Temporal variations of the confining groundwater level
Figure 6 shows groundwater hydrographs of the six observation wells and presents a wide variety of dynamic patterns during the monitoring period. The observed groundwater hydrographs that seem unrelated to each other are in fact responses to only a few processes or factors of influence. In reality, the local hydrogeological settings of these monitoring wells are different from each other within the small basin due to the aquifer heterogeneity. These hydrographs would be uncommon in a groundwater system controlled only by natural factors, which implied that the confined aquifer was subject to strong interference from anthropogenic activities during this period.
Wells JC02 and JC04 were distributed in the upstream of the production field. However, there existed some differences between the groundwater regime in well JC02 and that in well JC04 during the monitoring period. The highest and lowest groundwater levels were 1,352.86 and 1,351.80 m with an average value of 1,352.59 m for well JC02, which is adjacent to the western boundary (Table 2). Before April, the groundwater level in well JC02 varied from 1,352.48 to 1,352.86 m and maintained a relatively equilibrium state, indicating that the groundwater regime was completely controlled by lateral flow and leakage without any human interference. However, after April, an obvious trough occurred in the daily hydrograph for well JC02, which was related to the disturbance from anthropogenic factors such as irrigation activities or groundwater pumping from the Haolebaoji waterworks (Figure 6(a)). A mean groundwater level of 1,334.10 m was observed in well JC04 close to the northern boundary, with a maximum value of 1,337.26 m and minimum value of 1,332.30 m (Table 2). As is shown in the daily hydrograph for well JC04, a general downward trend in groundwater level can be observed before April, showing that the groundwater inflow was less than the total outflow. Subsequently, the groundwater level exhibited a significant rising trend, indicating the aquifer in that area received more lateral inflow from the groundwater outside the study area or downward leakage from the overlying unconfined aquifer due to the advent of the monsoon season (Figure 6(c)).
The highest and lowest groundwater levels were 1,315.67 and 1,309.81 m for well JC03, the mean value was 1,314.34 m (Table 2). The daily hydrograph for well JC03 can be divided into three stages in the whole hydrologic year. Before April, the groundwater level showed a slow downward trend and varied from 1,314.18 to 1,315.67 m during the first stage. The groundwater level was subject to intense interference from human activities during the second stage from April to September. The groundwater level fluctuated with large amplitudes due to the rapidly increasing water demand with the advent of the irrigation season. The daily hydrograph for the second stage was composed of a series of sharp declines and rises. The sudden drop occurred when well JC03 was pumped for irrigation, while the groundwater level returned to the initial level quickly within a few days after the pumping stopped. After September, the impact of anthropogenic activities on the groundwater level can be neglected with the completion of agricultural activities. The groundwater level began to ascend slowly and reached a relatively steady state until December 2014. During the third stage, the groundwater regime was almost controlled by natural factors (Figure 6(b)).
The maximum and minimum groundwater levels for well JC05 were 1,302.96 and 1,296.00 m, averaging 1,301.22 m (Table 2). The daily fluctuation of groundwater level in well JC05 also showed three stages similar to those of well JC03. The major differences between wells JC05 and JC03 were the groundwater regime in the first and third stages. Unlike the general downward trend in the first stage for well JC03, the groundwater level oscillated with relatively larger amplitude before April. A series of sharp declines and rises occurred in well JC05 from April to September, when human activities were the most frequent of the whole year. It should be noted that after September, the groundwater level rose slowly from the trough until the peak in the middle of October, then decreased gradually until December 2014 (Figure 6(d)).
The groundwater level in well JC07 fluctuated from 1,295.34 to 1,300.47 m with an average value of 1,298.16 m (Table 2). The distinctive characteristic of pulse signals was observed in the daily hydrograph of well JC07. Well JC07 is ideal for monitoring the impact of groundwater pumping from the waterworks on the confined aquifer given that there is not any pumping from that well itself. In a natural state, the groundwater regime should be relatively stable and the annual amplitude would be relatively small. However, the maximum difference between the peak and the trough reached nearly 4 m, by the analysis of the daily hydrograph, indicating that well JC07 must be subject to strong interference from the waterworks. In addition, it is clear that the production wells close to well JC07 had been engaged in intermittent groundwater pumping from December 2013 to the end of October 2014 according to the hydrograph. The daily hydrograph during this period may be explained by the following statements. The on-going groundwater pumping from the waterworks was mainly responsible for the process from the peak to the trough. The original groundwater regime was disrupted by groundwater pumping from the waterworks, and the groundwater level declined continuously during the period from the peak to the trough. The trough in the hydrograph was formed when the production wells stopped pumping, then the groundwater level ascended gradually and formed the process from the trough to the peak. As a result, the cycle was repeated in the daily hydrograph of well JC07 before the end of October. After October, the production wells were not engaged in any pumping activities and groundwater level rose quickly during this period (Figure 6(e)).
Diurnal WTFs and groundwater evapotranspiration loss
In the present study, the monitoring data in unconfined aquifers were used to identify the hydrological processes controlling the WTFs and estimate groundwater evapotranspiration. Given that the missing values and the influence of groundwater pumping have to be excluded, the selected periods for the calculation of ETGW in the two wells were the onset of the growing season and the peak of the growing season, respectively. During these specific periods, the fluctuation of the water table was only affected by recharge, plant transpiration and evaporation, so the results of the ETGW calculation were reliable.
The hourly hydrographs were used to calculate the daily ETGW values for wells JC01 and JC06 using the White method, as shown in Equation (2). In this study, the specific yield and the extinction depth were determined based on the results of groundwater flow modelling in the Subei Lake basin (Wang et al. 2010) and the hydrogeological survey conducted by the Inner Mongolia Second Hydrogeology Engineering Geological Prospecting Institute. The values of specific yield for wells JC01 and JC06 were 0.15 and 0.18, respectively. The extinction depth of ETGW in the study area is 4 m, so the groundwater in wells JC01 and JC06 was subjected to evapotranspiration in different degrees given that the depth to water table was within 4 m. Table 3 shows the calculated ETGW for wells JC01 and JC06. At the beginning of the rapid growing season for vegetation (April), the calculated ETGW for well JC01 ranged from 5.40 to 6.75 mm/d, with an average value of 6.15 mm/d. However, during the summer season (July or August), the calculated ETGW for well JC06 varied from 9.90 to 14.94 mm/d, with an average value of 12.48 mm/d. The results were similar to those obtained by Yin et al. (2011), in which the diurnal ETGW values ranged from 10 to 22 mm/d. Hydrometeorological factors were mainly responsible for the fact that the daily ETGW for well JC06 was clearly higher than that for well JC01. The groundwater level in well JC06 was shallower than that in well JC01, so the former evaporated more easily than the latter. In addition, air temperature played a very important role in groundwater evapotranspiration, and the mean temperature in July was obviously higher than that in April, which contributed to the higher ETGW for well JC06.
Given that there was no groundwater withdrawal occurring at selected periods, by the comprehensive analysis of groundwater hydrographs, the calculated results of ETGW were reliable. However, there are some uncertainties associated with the use of the White method. It is noteworthy that the White method is based on the assumption that diurnal WTFs are only caused by the evapotranspiration of plants (White 1932). However, lateral outflow and leakage discharge can also cause declines in the water table, which is a key source of error in the estimation of ETGW. Another main uncertainty is the limited accuracy of the specific yield (Loheide et al. 2005), which depends on the sediment texture, the initial depth to water table and elapsed time of drainage (Nachabe 2002). In addition, a constant rate of groundwater inflow determined from the rate of change in the water table during a short time interval between 00:00 and 04:00 can result in some uncertainties in the calculation of ETGW (Soylu et al. 2012).
Examples of the diurnal WTFs during the typical growing season in 2014 without any precipitation or overland flow are shown in Figure 7. It was very clear that the diurnal WTFs in the Quaternary phreatic aquifer (well JC01) and the Cretaceous phreatic aquifer (well JC06) were completely different from each other. The amplitudes of diurnal WTFs for well JC01 were approximately 2 cm, and the water table fluctuated in the daytime, reaching a minimum at approximately 16:00–17:30 h. It should be noted that, subsequently, the water table did not recover obviously but fluctuated slightly and continued to decrease until 22:00–23:00 h, when evapotranspiration had almost ceased. The hourly hydrograph for the period from 26 April 2014 to 28 April 2014 indicated the absence of groundwater recharge and the predominance of groundwater outflow from early in the evening until midnight. Subsequently, the groundwater level rose due to a net groundwater inflow from midnight to dawn. However, the amplitudes of the groundwater fluctuations in well JC06 were approximately 8 cm, and the water level reached the minimum at nearly 13:30–15:00 h when evapotranspiration was the strongest in one day. Subsequently, the groundwater level displayed a steady rising trend and reached a peak at about 13:00–14:00 h the next day. The water table decline in well JC06 only lasted for less than an hour when evapotranspiration predominated the hydrological processes, and the average decline rate was astonishingly 0.13 m h−1. Conversely, an average recovery rate was about 3 mm h−1 but lasted for a relatively long time.
The impact of groundwater pumping from the waterworks on the groundwater
The confined aquifer is sensitive to a change in aquifer stress such as earthquakes, changes of atmospheric pressure, groundwater withdrawal, etc. (Bras & Rodríguez 1985). Groundwater levels are inclined to display outliers with the impact of human factors, which can be explained by the micro-dynamics features reflected by the high-frequency groundwater level data. Therefore, it is necessary to identify outliers of groundwater monitoring data in the confined aquifer. A dataset of the rate of daily variation in groundwater level in these monitoring wells can be constructed in order to capture the unusual fluctuations of groundwater levels caused by groundwater pumping from the Haolebaoji waterworks.
In this study, the boxplot method was used to identify the outliers, and only the anomalous dynamics of groundwater level decline are discussed in this paper. The statistical results of exploratory data analysis are shown in Figure 8. As is shown in Figure 8, the outliers of water level decline are almost symmetric with those of water level rise, which indicates that the anomalous decline and rise in water level are concomitant with each other.
In order to explore the possible causes of outliers, the hourly hydrographs were analyzed. The hourly hydrographs in the six monitoring wells can be classified into three different patterns: mutational, irregular and gradual hydrographs (Figure 9). The mutational change in groundwater level (Pattern 1) was observed in all the monitoring wells except well JC07. Sharp decline or rise in groundwater level was closely related to groundwater withdrawals in the monitoring wells themselves. Pattern 1 has two subtypes, which are controlled by a variety of external factors such as the intensity and duration of groundwater pumping, local hydrogeological conditions, etc. Pattern 1(a) was characterized by sharp declines and rises instantaneously as a result of short-term groundwater pumping from the monitoring wells. Moreover, the rapid recovery of the groundwater level occurred within an hour when pumping activities stopped, indicating that the rate of groundwater extraction was less than that of groundwater inflow. The rapid recovery of the water level may be related to the absence of mudstone lenses.
However, Pattern 1(b) can only be observed in well JC08 and exhibited distinctly different dynamics for groundwater level, which can be explained by the impact of intensive groundwater withdrawal from the nearby production wells No. 21 and 22 at a distance of 763 m and 680 m from well JC08, respectively. Groundwater level declined instantaneously with an amplitude of 0.676 m at 13:00, but subsequently the rate of groundwater decline reduced slowly from 0.176 to 0.013 m/h until 07:30 the next day, when groundwater inflow was almost equal to groundwater outflow. It showed that the groundwater level regained a relatively steady state at 07:30 h on 2 April and the obvious recovery, with an amplitude of 0.726 m, was observed within just half an hour when groundwater pumping from the adjacent two production wells ceased. The groundwater level almost returned to the initial water level within a few hours, indicating groundwater pumping for industrial use can be satisfied due to the sufficient groundwater recharge in the vicinity of well JC08. However, it should be noted that the groundwater level in well JC08 took a longer time to regain its original water level than that in well JC03, which implied that well JC08 may be at least partially connected to a mudstone lens.
Pattern 2 (irregular) was characterized by erratic variation and can only be observed in well JC04. Given that the confined aquifer in the northern recharge area was only composed of the fissured Cretaceous rocks, the strong irregular variation in the groundwater level was likely to be caused by the heterogeneity of the confined aquifer (the density and orientation of the fissures, and the connectivity of the groundwater flow path). The strong irregularity was a distinctive characteristic of well JC04, which was located in the northern recharge area.
Groundwater pumping from the Haolebaoji waterworks was primarily responsible for the gradual hydrograph (Pattern 3), which was characterized by a slow continuous decline within a few hours or some successive days in groundwater level. The monitoring wells (JC02, JC05 and JC07) were disturbed by the pumping behaviors from the nearby production wells, which was evidenced by the unusual hydrographs (Figure 9). The three monitoring wells exhibited slightly different groundwater dynamics, due to the significant differences in the pumping patterns and the distance from the adjacent production wells. Well JC02 is close to the western boundary and is a long distance from the nearby production wells. It seemed that well JC02 would not be affected by the production wells, but surprisingly groundwater level gradually declined with an amplitude of 0.395 m from 08:00 to 20:30 h on 3 April, and the daily amplitude was unusual during the monitoring period (Pattern 3(a)). The impacts of natural factors and other human factors (i.e. pumping from well JC02 itself) were excluded, given the abnormally large drawdown and gradual changes in water level. Well JC02 was likely to be close to the margin of the groundwater depression cone. Given that well JC05 is surrounded by the production wells, the groundwater regime must be disturbed as long as the nearby production wells conducted groundwater pumping. It has been validated that the adjacent production wells indeed did carry out groundwater pumping by an analysis of the hourly hydrograph (Pattern 3(b)). For example, the unusual decline in groundwater level, with an amplitude of 1.175 m, was detected from 09:30 to 19:00 h on 9 March as a result of short-term interference from the nearby production wells. Pattern 3(c) was characterized by a steady decline within 24 hours. Well JC07 is the most appropriate monitoring well, given that there is no groundwater withdrawal in the well itself and it is surrounded by the production wells. The groundwater regime in well JC07 will be entirely controlled by natural factors if the nearby production wells do not carry out groundwater exploitation. So it is ideal for monitoring the pumping behaviors of the Haolebaoji waterworks. The hydrograph of well JC07 exhibited distinctly different dynamics (Figure 6) and showed the outliers during the monitoring period. The surprising result that the groundwater level showed a steady, continuous decline within 24 hours has been summarized. As shown in Pattern 3(c), the continuous decline in groundwater level, with an amplitude of 2.644 m, was observed from 3 to 6 March as a result of continuous groundwater extraction from the nearby production wells during this period.
By a comprehensive analysis of hourly hydrographs, the characteristic of groundwater pumping from the Haolebaoji waterworks is that only some individual wells had been engaged in the intermittent groundwater pumping at random times, which can be validated in the hydrogeological survey report from the Inner Mongolia Second Hydrogeology Engineering Geological Prospecting Institute.
The impact of groundwater pumping from the waterworks on the lakes
Satellite-based temporal mapping of the lake surface area was calculated using Landsat TM imagery from 2000 to 2011 for the Subei Lake basin. Given that the Haolebaoji waterworks became operational for industrial use in 2006, the temporal variation of the two lake areas can be divided into two phases: before the operation of the Haolebaoji waterworks (2000–2006) and after the operation of the Haolebaoji waterworks (2006–2011).
By the comparison between the lake area before and after the operation of the Haolebaoji waterworks, it is clear that the lake areas were in decline during the period from 2006 to 2011 (Figure 10). The areas of the Subei Lake and the Kuisheng Lake decreased at a rate of 0.23 km2 and 0.07 km2 per year, respectively. It should be noted that the area of the Subei Lake decreased at a faster rate than that of the Kuisheng Lake in response to groundwater pumping from the waterworks, which may be ascribed to the differences in hydrogeological settings between the Subei Lake and the Kuisheng Lake. Unconfined groundwater is the main recharge source for these two lakes. The production wells of the Haolebaoji waterworks are all distributed in the upstream of the Subei Lake. The overexploitation of the confined aquifer has caused a general piezometric decline and further induced a large downward leakage from the unconfined aquifer. Therefore, the unconfined groundwater that originally flowed toward the Subei Lake was captured by the production wells for industrial use, which resulted in significant decreases in groundwater discharge to the Subei Lake. However, the Kuisheng Lake is located in the northeastern highland, which mainly receives lateral recharge from groundwater outside the study area. Moreover, all the production wells are distributed in the downstream of the Kuisheng Lake. Therefore, the impact of groundwater pumping from the waterworks on the Kuisheng Lake was smaller in comparison.
The temporal variation of the water level in the Subei Lake from July to November in 2009 is shown in Figure 11. It is clear that the mean monthly water level of the Subei Lake decreased continuously at an average rate of 0.059 m during this period. It should be noted that the rate of groundwater pumping climbed to more than 25,000 m3/d in October; as a result, the mean monthly water level in October declined by 0.08 m compared with that in September. Therefore, the evidence of lake area and water level indicate that groundwater pumping from the Haolebaoji waterworks has caused the decline of the lakes, which is consistent with the results obtained by Wang et al. (2010).
The present study identified the impact of the Energy Base Water Project on the groundwater in the Subei Lake basin, which is subjected to strong interference from human activities, with various methods such as statistical analysis, the WTF method, hydrograph analysis and remote sensing. The combination of both the daily and hourly hydrographs has provided a comprehensive understanding of the natural processes and anthropogenic activities that control the groundwater regime of the whole water system. For natural factors, precipitation infiltration and groundwater evapotranspiration have relatively short response times, but may exert a long-term effect on the groundwater regime. However, the impact of human activities (such as groundwater pumping and flood irrigation) on the groundwater is relatively instantaneous. The groundwater regime was subject to intense interference from human activities from April to September. Human activities (groundwater pumping for energy production and irrigation) are the major anthropogenic stresses affecting the groundwater regime. The mean evapotranspiration rates in the Quaternary phreatic aquifer and the Cretaceous phreatic aquifer were 6.15 mm/d and 12.48 mm/d, which can be explained by the significant differences in the hydrometeorological factors and local hydrogeologic conditions (aqueous medium, soil texture, depth to water table).
The unusual hourly hydrographs in the confined aquifers exhibited three different patterns, namely mutational, irregular and gradual hydrographs. Different recovery times after being influenced by pumping may be related to the presence of mudstone lenses. The extent of the groundwater depression cone was qualitatively identified by gradual hydrographs, which may spread from the center area (wells JC05 and JC07) to the western boundary (well JC02). It can be validated that only some individual wells had been engaged in the intermittent pumping activities at random times. Groundwater pumping from the Haolebaoji waterworks has caused the Subei Lake to shrink more seriously than the Kuisheng Lake.
Further field investigation and high-precision groundwater flow models will be required to determine the extent of the aquitard and the exchange capacity of groundwater between the different source/sink terms in order to fully quantify the hydrological and anthropogenic impacts on the groundwater regime. Similar integrated approaches can be applied to understand the impact of anthropogenic activities on the groundwater in other data sparse regions. To achieve the objectives of protecting the vital groundwater resources and vulnerable ecosystems, government agencies should adopt the following strategies: restricting heavy groundwater pumping from the production wells close to the lakes; monitoring water levels in the groundwater depression cone periodically; and developing a long-term monitoring network for assessment of the viability of the groundwater-dependent ecosystem.
This research was supported by the State Basic Research Development Program (973 Program) of China (grant no. 2010CB428805). The authors are grateful to our colleagues for their assistance in data collection and field investigation. Special thanks go to the editor and the anonymous reviewers for their critical reviews and valuable suggestions.