Qichen Hao, Jingli Shao, Yali Cui, Zhenhua Xie. Applicability of Artificial Recharge of Groundwater in the Yongding River Alluvial Fan in Beijing through Numerical Simulation. Journal of Earth Science, 2014, 25(3): 575-586. doi: 10.1007/s12583-014-0442-6
Citation:
Qichen Hao, Jingli Shao, Yali Cui, Zhenhua Xie. Applicability of Artificial Recharge of Groundwater in the Yongding River Alluvial Fan in Beijing through Numerical Simulation. Journal of Earth Science, 2014, 25(3): 575-586. doi: 10.1007/s12583-014-0442-6
Qichen Hao, Jingli Shao, Yali Cui, Zhenhua Xie. Applicability of Artificial Recharge of Groundwater in the Yongding River Alluvial Fan in Beijing through Numerical Simulation. Journal of Earth Science, 2014, 25(3): 575-586. doi: 10.1007/s12583-014-0442-6
Citation:
Qichen Hao, Jingli Shao, Yali Cui, Zhenhua Xie. Applicability of Artificial Recharge of Groundwater in the Yongding River Alluvial Fan in Beijing through Numerical Simulation. Journal of Earth Science, 2014, 25(3): 575-586. doi: 10.1007/s12583-014-0442-6
A groundwater transient flow model was developed to evaluate the applicability and effectiveness of artificial recharge scenarios in the middle-upper part of the Yongding River alluvial fan in Beijing. These scenarios were designed by taking into account different types of recharge facilities and their infiltration rate with the Middle Route Project for South-to-North Water Transfer (MRP) as the recharge water source. The simulation results suggest that: (1) the maximum amount of artificial recharge water, for scenario Ⅰ, would be 127.42×106 m3 with surface infiltration facilities; and would be 243.48×106 m3 for scenario Ⅱ with surface infiltration and recharge wells under the constraint of the upper limit of groundwater; (2) with preferred pattern of recharge facilities, groundwater levels in both optimized recharge scenarios would not exceed the upper limit within the given recharge period; and (3) implementation of the recharge scenarios would efficiently increase the aquifer replenishment and the groundwater budget will change from −54.11×106 to 70.89×104 and 183.36×104 m3, respectively. In addition, under these two scenarios groundwater level would rise up to 30 and 34 m, respectively, without increasing the amount of evaporation. The simulation results indicate that the proposed recharge scenarios are practically feasible, and artificial recharge can also contribute to an efficient recovery of groundwater storage in Beijing.
Generally groundwater is a major water supply source for Northern China, Beijing in particular, where two thirds of the total water supply for municipality comes from groundwater. During the decade-long dry period (1999-2009), groundwater was extremely vital in providing a secure water supply to Beijing. However, a number of large regional groundwater depression cones have developed due to the overexploitation of groundwater over the past decades. This situation has put water supply security, the geological environment, and the eco-environment of Beijing at imminent risk (Pang et al., 2013).
To relieve water shortage, Chinese government initiated the Middle Route Project for South-to-North Water Transfer (MRP) (CCSNWD, 2008). The completion of the MRP project is expected not only to alleviate water shortage issues, but also provide opportunities for the groundwater recovery and efficient management of water resources. One of the major approaches to achieve these goals would be artificial recharge of groundwater.
Artificial recharge of groundwater has gradually become an effective approach for the groundwater management to increase groundwater storage (Bouwer, 2002; Ma and Spalding, 1997). This is practically necessary for the areas like Beijing where water demand continuously increases with growing populations and groundwater is over-exploited for decades. The artificial recharge of groundwater plays an essential role in the sustainable management of groundwater resources (Kimrey, 1989). Replenishing the groundwater using artificial recharge has been carried out in various locations of the world for the last few decades, especially, in some arid or semi-arid areas (Al-Assa'd and Abdulla, 2010; Peters and Ji, 2010; Wright and Toit, 1996; Babcock and Cusing, 1942). The artificial recharge of groundwater can be achieved by some surface facilities with permeable soils, such as basins, furrows, and ditches, or through injection wells at the locations where permeable soil and/or sufficient land area for surface infiltration are not available (Bouwer, 2002). In addition, numerous papers have highlighted the efficiency and benefits of different artificial recharge systems with different water sources as recharge water (Gontia and Patil, 2012; Thomas and Vogel, 2012; O'Shea and Sage, 1999).
Groundwater modeling has been an effective tool to improve the understanding and estimate groundwater flow in natural conditions or under the impact of human activities on groundwater. Quantitative studies of modeling the artificial recharge of groundwater have been conducted by numerous researchers worldwide. Vandenbohede et al. (2008) developed a detailed solute transport model to calculate the capture zone, residence time and volume of recharged water in the aquifer. Thomas and Vogel (2012) built a regional multivariate regression model to determine the potential effects of recharge best management practices on groundwater elevations. Based on a novel representation of geologic heterogeneity, Tompson et al. (1999) initiated the application of a detailed numerical model of groundwater to understand flow path, migration rate, and residence time of recharged groundwater.
Artificial recharge of groundwater in the Yongding River alluvial fan (YRAF) in Beijing has also been the focus of a number of studies. Liu et al. (1988), and Wang et al. (2005), carried out different artificial recharge experiments at various hydrogeological units and concluded the significant recovery of the ground water levels. Their results also indicated that aquifers could be good natural filters and cleaners. Cui et al. (2009) analyzed the constraints on groundwater level and present groundwater level, and determined the recoverable space that was available for storing recharge water. Li et al. (2010) proposed three engineering models for the management of the artificial recharge water by using different recharge water sources, regulation patterns, and utilization styles. Consequently, YRAF was considered to be an ideal site for the artificial recharge of groundwater (Xu et al., 2009).
Some modeling studies on the artificial recharge of groundwater have also been conducted in this area. Liu et al. (1988) used a numerical model to estimate ground water level variation, infiltration, and the potential regulation capabilities of aquifers for artificial recharge of groundwater. By using a numerical model, Zheng et al. (2009) predicted groundwater level variations in the west suburbs of Beijing under the control of groundwater exploitation by considering the impacts of MRP after it is completed.
Using the water from MRP as a recharge water source to replenish aquifers may not only efficiently increases the groundwater storage, but also provides a solution for disposed residual water from MRP during a period of low water consumption. Particularly, it could secure the supply to Beijing throughout dry years or periods of water contamination.
In view of the previous studies, this work was initiated to investigate the facilities available for artificial recharge through hydrologic surveys. And two detailed artificial recharge scenarios were designed by considering different facility types with water from MRP as recharge water. Finally, a groundwater flow model was developed and well calibrated for simulating the response of groundwater on proposed scenarios. It was aimed at checking the applicability of proposed recharge scenarios and predicting the efficiency of these scenarios in the alluvial aquifers, by using MODFLOW code. In this way, it was planned to clarify the possibility and reliability to supplement the aquifers by artificial recharge. The simulation result could provide useful information for local water resource management.
2.
STUDY AREA
2.1
Description of the Study Area
YRAF is a part of the Beijing Plain (Fig. 1) and falls into a temperate continental climate zone, which consists cold and dry winters, hot and humid in summers, and moderate temperature during springs and autumns. The long-term (1959-2006) average annual precipitation is approximately 590 mm and most of the rainfall occurs from June to September. The long-term (1979-2006) average annual evaporation is 1 725 mm, which is approximately three times the precipitation.
Figure
1.
Location and lithology of the study area. Ⅰ. Single layer of pebbles and gravels, K>200 m/d; Ⅱ. multiple layers of pebbles and gravels, 200 m/d>K>100 m/d; Ⅲ. multiple gravels and few sand layers, K < 100 m/d; Ⅳ. multiple sand layers and few gravel layers; Ⅴ. multiple layers of sand; 1#-7#. water supply plants; FT. Fengtai water supply plant. Diameters of the water plants indicated herein are related to the space of the plants. Observation well 22 is located inside Beijing Normal University as a groundwater level observation well, approximately in the center of the depression cone.
The Yongding River is the main river in this area. However, due to reservoir construction at upstream of the Yongding River, where flow rate declines significantly, especially in dry years there is hardly any inflow.
2.2
Geological and Hydrogeological Setting
YRAF essentially consists of Quaternary deposits varying from tens meters to more than 500 meters, and is underlain by a series of Paleozoic to Mesozoic geological formations forming impermeable bottom of aquifers (Fig. 2). The groundwater depth at the top of the alluvial fan is deep and gradually lessens towards the mid-lower part. From west to east, the aquifers change from one layer to multiple layers. The single-layer areas range from 10 to 50 m thickness while the multi-layer areas are more than hundreds of meters thickness.
Figure
2.
Hydrogeological cross-section of YRAF. Ⅰ-Ⅴ. correspond to five lithology subareas in Fig. 1. ① are phreatic aquifers and ②, ③ are confined aquifers. This figure is adapted from the report "Groundwater Flow Modeling and Groundwater Resource Potential in the Plain Area of Beijing".
The western and northwestern aquifers boundaries are impermeable boundaries composed of sandy shale and lava. The eastern aquifers boundary consists of a natural aquifer boundary with respect to groundwater system, and is composed of relative less permeable sediments, including multiple layers of sand and clay. These boundary conditions make the aquifers become relative independent hydrogeological units as well as an ideal groundwater reservoir for storing water resource covering approximately 333 km2 (Fig. 2).
From northwest to southeast, the sediment thickness increases with an decrease in grain size, and aquifer systems change from a single gravel aquifer to multiple aquifer system of sand layers separated by silt and clay layers with land surface topography gradually sloping southeastward (Fig. 1).
Under natural conditions, groundwater obtains recharge from precipitation infiltration, lateral inflow, and river leakage. Groundwater flows downstream and discharges as seepage to rivers and as underground outflow to downstream areas, as well as evaporation from phreatic aquifers.
Extensive human activities have deteriorated groundwater recharge and discharge conditions within the past few decades. At present, recharge of groundwater not only includes precipitation infiltration and lateral inflow from surrounding mountains, but also includes irrigation infiltration. However, river leakage is substantially reduced since rivers are completely dry most of time in the year. Due to the extensive extraction from aquifers, human exploitation has become the main way of discharging groundwater. The exploitation rate is 364.51×106 m3/a, which accounts for 88.49% of the total groundwater discharge. As the groundwater level is quite deep, there is hardly any evaporation.
Overexploitation during the past decades already resulted in significant groundwater level decline and reduction of groundwater storage in the study area. Consequently, aquifers under balance have a deficit of about 119.75×106 m3/a. The groundwater level has dropped by an accumulated average of 10-30 m during the years of 1970-2010 (Fig. 2). Figure 3 shows the typical water level time series trend for observation well 22 at the middle-upper part of YRAF which has an accumulative groundwater level decline of 30 m during 1965-2010. Despite of the absence of the observation data of couple years, a falling trend is still clearly presented, especially in the past 15 years. Obviously, this decline has created large spaces in aquifers for artificial groundwater recharge. Based on present water levels and the upper limit of groundwater levels, the spaces available for storing recharge water was calculated to be approximately 870×106 m3 (Cui et al., 2009).
Figure
3.
Groundwater level time series for observation well 22.
Artificial recharge of groundwater can be achieved in three ways: surface infiltration, vadose zone infiltration and direct well injection (Bouwer, 2002). According to the hydrogeologic survey, the promising surface infiltration facilities in the study area include: one dry river named Nanhan River (NH River) and four gravel pits (XHC, LGZ, BW and LS). Additionally, a number of recharge wells could be constructed at the sites relatively close to the rivers where no buildings exist. Figure 4 shows all facilities and sites suitable here for artificial recharge of groundwater. The recharge wells can be constructed along NH River and its branches as needed. Kunming Lake is used for regulation of water from MRP, through which the recharge water can be delivered into every recharge facility.
Figure
4.
Distribution of the facilities and sites suitable for artificial recharge in the study area. There are four gravel pits: Xihuangcun (XHC) gravel pit, Liaogongzhuang gravel pit (LGZ), Beiwu gravel pit (BW), Laoshan gravel pit and a dry river: Nanhan River (NH River).
In this study, the analog method is used to determine the recharge rates on recharge facilities. Liu et al. (1988) once conducted an experimental study on artificial recharge in this area and recharge facilities that they used included: (1) an average 5.5 m wide, about 1 150 m long dry river (R0) which has an steady recharge rate of 0.31 m3/s (26 784 m3/d); (2) the XHC gravel pit with an area of 26 000 m2, which had a steady recharge rate of approximately 1 m3/s (86 400 m3/d); (3) a 25-meter-deep large-diameter hole (8 m) whose steady recharge rate was 0.3 m3/s; (4) and a 42-meter-deep well with a diameter of 0.5 m and with a recharge rate of 0.09 m3/s.
The recharge facilities used in the test above sit in the same hydrogeological unit as our recharge facilities. Since these recharge facilities are quite close, they are assumed to have the similar hydrogeological setting. Therefore, it is reasonable to determine the recharge rate herein by analogue analysis. As the artificial recharge process normally lasts for a long time, therefore, the vertical hydraulic gradient could be assumed closely to be one. We thus approximately determined the recharge rates of the surface infiltration facilities (the gravel pits and the river) linearly correlate to their infiltration areas and the hydraulic conductivity of the aquifers. On these locations, the recharge rates of the surface infiltration facilities are then calculated as.
(1)
where αi is the correction coefficient taking into account adverse factors, e.g., plugging; Ai, area of the surface infiltration facility; Ki, hydraulic conductivity at the locality of the infiltration facility; A0, bottom area of XHC gravel pit or riverbed floor area of R0; K0, hydraulic conductivity at the locality of XHC gravel pit or R0; Q0, volumetric recharge rate of XHC or R0. Using this formula, we can calculate the artificial recharge rates of the four gravel pits and NH River as presented in Table 1. The total recharge rates of these surface facilities theoretically equal to 838 080 m3/d.
Table
1.
Calculated results of recharge rate of the surface infiltration facilities. The results are calculated theoretically when the vertical hydraulic gradient constantly is 1, i.e., the effect of water level depth on infiltration is not taken into account. XHC and RO herein are references for analogue analysis whose infiltration rates have been derived from previous field test. Considering plugging and the potential time used to dredge up the recharge facilities, the correction coefficient is conservatively determined as 0.6 or 0.8
It is impractical to construct large-diameter wells (e.g., 8 m) in the study area because urbanization has occupied most of the land. However, a certain number of recharge wells with relatively smaller diameter could be constructed along rivers when more facilities are in need for artificial recharge of groundwater. Liu et al. (1988) investigated the infiltration of the recharge wells and considered the difference in the hydraulic conductivity of aquifers and the time needed for backwash, 0.05 m3/s (4 320 m3/d) was conservatively reported as the infiltration rate of a 60-meter-deep recharge well with a diameter of 0.5 m.
3.2
Formulation of Artificial Recharge Scenarios
It has become apparent that water shortage is serious and it's impossible to induce natural recharge or reduce natural discharge due to almost all rivers being dry and an increasing population. Therefore, artificially enhanced groundwater recharge methods are the only processes that can be applied to increase groundwater recharge.
Considering low water consumption periods and water abandonment during the rainy season, as well as the influence of temperature on the groundwater viscosity (Cheng et al., 2011), the best recharge time in Beijing should be limited to six months (184 days) lasting from May to October when it has relatively high temperature and large amount of rainfall water.
As expected, when MRP is put into service, it will deliver 1.40×109 m3 of water to Beijing every year. When both the north and south part of China are in wet year or low water consumption periods, the surplus water will be discharged as residual water. If there is 10 percent water surplus, approximately 0.14×109 m3 of residual water will be produced. And this amount of water can be directly used as a recharge water resource without treatment. This is because the water source for MRP comes from the Danjiangkou Reservoir, where the water quality is excellent and has met the national standard of drinking water for many consecutive years (Li and Zhang, 2006).
As calculated above, total recharge rate of all surface infiltration facilities available is 838 080 m3/d. If artificial recharge process could last for six months, the recharge capacity of all surface recharge facilities would be up to 0.15×109 m3. Hence, it is possible to put 0.14×109 m3 of residual water into aquifers only using surface infiltration facilities.
If there is more than 10 percent water surplus from MRP or water source from upstream surface reservoir is considered as recharge water, a number of recharge wells would need to be constructed to directly inject the water into aquifers. The upstream surface reservoir refers to Guanting Reservoir, where the annual average runoff is 0.70×109 m3 (1955-2010) and the annual average abandoned water can be as much as 0.28×109 m3 (Cheng, 2012). Although the reservoir has a record of water contamination in the past, the water quality is currently improving and can be used as a recharge water source.
According to the amount of water source available for artificial recharge, two scenarios were formulated. For scenario Ⅰ, only surface infiltration facilities (rivers and gravel pits) were considered for use with a recharge capacity of 154.21×106 m3. For scenario Ⅱ, besides surface infiltration facilities, a number of recharge wells were further considered to be constructed along the NH River. Based on field survey, approximately 200 recharge wells with a diameter of 0.5 m can be constructed. As preliminarily calculated, the recharge capacity of scenario Ⅱ is up to 313.18×106 m3 (Table 2).
Table
2.
Proposed artificial recharge scenarios of groundwater
Rises in groundwater levels for aquifers with infiltration facilities constructed would resultantly occur. In order to prevent waterlogging or undue water level rises of the recharge area and adjacent areas, transient groundwater flow modeling was used to predict water level rises and to determine the suitable recharge rate of facilities so that groundwater levels in each recharge scenario would not exceed the upper limit.
The upper limit of groundwater levels refers to the highest elevation of groundwater level that does not have the negative effects on the environment, such as water percolating through landfills. In this study, the elevation of the upper limit of groundwater levels was calculated according to the suitable groundwater level required by environmental geology and engineering geology (Cui et al., 2009).
4.
NUMERICAL SIMULATION OF ARTIFICIAL RECHARGE OF GROUNDWATER
4.1
Development of a Simulation Model
4.1.1
Mathematical model
In order to estimate the applicability and predict the potential impact of artificial recharge on the groundwater systems, a transient regional groundwater flow model was developed. The model consists of three aquifers: the top aquifer being phreatic and the remaining aquifers being confined (Fig. 2). The elevations of surface and bottom of each aquifer were delineated based on DEM data and the 49 boreholes data. They were generated by interpolating using Kriging method. As the aquifers of study area are porous medium, the governing equations of groundwater flow can be expressed as.
(2)
where Kxx, Kyy, and Kzz are the values of hydraulic conductivity along the x, y and z coordinate axes, which are assumed to be parallel to the major axes of hydraulic conductivity; h is the potentiometric head; W is the volumetric flux per unit volume representing sources and/or sinks of water; S is the specific storage of the porous material; h0 is the initial heads; μ is the specific yield; p is the precipitation rate; q is the flux rate through boundary; hr is the reference head in the boundary; σ is the resistance coefficient; Γ0 is the phreatic surface boundary; Γ1 represent the flux boundary; Γ2 represent the general head boundary condition; and t is time. The governing equations were solved by the finite differential program MODFLOW (McDonald and Harbaugh, 1988).
4.1.2
Spatial and temporal diescretization
The model was discretized vertically into 3 layers and consists of 131 rows, 119 columns with 34 732 active regular cells. Each cell is 200 m×200 m in the horizontal plane. The calibration time was from Jun. 16, 2000 to Jun. 15, 2011 with stress period of a month, which resulted in 132 stress periods. For each stress period, the average hydrologic conditions for that period were assured to remain constrain. Temporal discretization was introduced in the form of time steps within each stress period. The length of the transport time step was assigned to three days.
4.1.3
Specification of parameters
The aquifer parameter values that pertained to groundwater flow were zoned and assigned based on the sediment features and the results from previous investigation. The values of horizontal hydraulic conductivity of aquifers ranged from 30 m/d to 300 m/d and would be manually calibrated based on the new observation data of recent years. Taking into account the existence of some relative aquitards, the values of vertical hydraulic conductivity were one thousandth to one tenth of the values of horizontal hydraulic conductivity. The data on recharge and discharge of aquifers were collected from previous studies and investigations (Zhai and Wang, 2012; Zheng et al., 2009; Zhang et al., 2008). All parameters were adjusted during the model calibration process until the model adequately reproduced the observed water level distribution throughout the aquifers.
4.1.4
Boundary conditions
The mountain front in the west side of the study area was defined as inflow boundary and the flux rate from the western boundary was determined according to a previous study (Zhang et al., 2008). To represent the lateral flow from the west, injection wells were assigned to the west boundary of the model with a specified flow rate of 67× 106 m/a. The northern and southern boundaries were defined as no-flow boundaries because the flow lines were generally parallel to them under present conditions. But the northern and the southern boundaries were changed to general head boundaries (McDonald and Harbaugh, 1988) in the prediction model to cope with the influence of artificial recharge on the flux rate out of the boundaries (Fig. 1). The eastern boundary was also defined as general head boundary (GHB) and the groundwater heads were assigned into the GHB package according to measured data. The amount of outflow from the model area can also be automatically calculated by the GHB package.
4.2
Model Calibration and Model Results
The model was manually calibrated by trial and error (Chenini and Mammou, 2010; Wilsnack et al., 2001). At the very beginning, the sensitivity of different model parameters should be identified as a prior process. Preliminary analysis indicated rainfall and artificial exploitation posed the greatest influence on the groundwater level fluctuations of the model. Therefore, the rainfall recharge and artificial exploitation rates had to be adjusted to minimize the difference between the simulated and the measured heads. Then, the hydraulic conductivity values of different subareas were adjusted so as to further reduce the difference between the simulated and the measured heads. Finally, the storage coefficient was adjusted so that the simulated water levels were more close to the measured levels. These steps were repeated until the difference between simulated and measured heads of each observation well were within 1 m. The average difference of all stress periods is referred to as the mean residual. Table 3 shows the mean residual of the observation wells. In general, the mean residual values range from -0.71 to 0.97 m.
To further assess the accuracy and reliability of the calibrated model, the Nash-Sutcliffe efficiency (NSE) is used as shown in equation (3).
(3)
where Yiobs is the ith observed head of one observation well, Yisim is the ith simulated head of this observation well, Yisim is the mean of observed head of the observation well, and n is the total number of observations.
Table 3 shows the value of NSE for all of 15 observation wells. All of the value except one are higher than 0.9. The simulated results get a well agreement with the observed heads, indicating that the calibrated model can be judged as satisfactory.
Figure 5 shows the examples of simulated groundwater levels fitting to the observed levels. The simulated changing patterns of groundwater levels have the same trend as those of the observed groundwater levels and the simulation results could reproduce seasonal and annul water level fluctuations in most stress periods. The simulated groundwater level contours at the end of the calibration period match the observed groundwater level contours as shown in Fig. 6. These simulation results indicate that the model is able to take into account of hydrogeological properties of the aquifers in the study area, and can be used to simulate the scenarios of artificial groundwater recharge. The hydrogeological parameter partitions and the calibrated values for the phreatic aquifer are shown in Fig. 7 and in Table 4.
Figure
5.
Calculated heads versus observed heads of observation wells.
Groundwater budget is another important metric for the model that reflects the distribution of groundwater source-sink and groundwater storage variations in aquifers. The budget of the study area was calculated using the Water Budget module in MODFLOW.
Table 5 shows the mean groundwater budget of the study area from 2000 to 2011. Under natural conditions, the areal recharge from precipitation infiltration is the main recharge of the aquifers, which accounts for 39.26% of the total recharge, follows by lateral inflow and valley subsurface flows. The exploitation is the main discharge of the aquifers, which accounts for 88.49% of the total discharge resulting in a quite small quantity of lateral outflow. As the groundwater level is quite deep, phreatic evaporation value is nearly zero. The simulation result shows that the total discharge exceeds the natural recharge and there is a net deficit of -119.75×106 m3 in aquifers water balance, suggesting groundwater is being fast depleted during the past decade.
Table
5.
Mean groundwater budget of study area during simulation period (2010-2011)
The proposed artificial recharge scenarios were simulated using the calibrated numerical model to evaluate its applicability and to predict future effects of these scenarios on groundwater levels and water budget. The groundwater levels measured in May 2011 were used as the initial groundwater level for the prediction model. The prediction time lasted ten years from 2011 to 2021. In the prediction period, groundwater recharge/discharge values were adjusted according to changes in water distribution conditions in Beijing. When MRP is put into service, groundwater exploitation in the study area will reduce by about 20%. As urbanization increases, the agricultural withdraws will gradually decrease. However, urban pipelines leakage will accordingly increase. For other natural recharges, including precipitation, lateral inflow, and valley subsurface flow, their long term annual average data was used for the prediction model, while the lateral outflow was calculated by the prediction model.
Preliminarily proposed artificial recharge scenarios may result in groundwater levels exceeding its upper limit and consequently negatively effecting the environment and groundwater quality. In order to prevent undue rise of water levels, both recharge scenarios were optimized through the method of trial and error via numerical modeling. When the simulated groundwater level under the recharge facilities or adjacent areas is higher than the upper limit of the groundwater level, the recharge period or the number of recharge facilities in the scenarios would be adjusted until the simulated groundwater levels of both scenarios are lower than the upper limit.
5.
SIMULATION RESULTS AND DISCUSSION
5.1
Optimized Scenarios
Table 6 shows the optimized artificial recharge scenarios under constraints of the upper limit of groundwater level. Through optimization, the practically feasible total recharge rates of Scenario Ⅰ and Scenario Ⅱ, under the constraint of the upper limit of groundwater level, are 127.42×106 and 243.48×106 m3, respectively, which are a little less than those of initially proposed scenarios in Table 2.
Table
6.
Optimized artificial recharge scenarios of groundwater
Because the LS gravel pit has a relative large recharge rate, water levels under the LS gravel pit and the adjacent area will exceed the upper limit after continuous recharge of six months. In the optimized scenarios, the recharge pattern was changed from continuous to discontinuous: recharged in May and June, suspended in July and August, and then continued to recharge in September and October when the mound had declined. All the other recharge facilities could last continuously for six months and water level would not exceed the upper limit.
As mentioned above, 200 recharge wells might be constructed along rivers when it is in need. But, through simulation, it is not acceptable to construct so many recharge wells because the constraint of the upper limit of groundwater levels. Ultimately, the number of recharge wells was reduced to 146 in scenario Ⅱ. Even though nearly a 25% reduction in the number of recharge wells compared with the initially proposed in scenario, there was still a large increase in the quantity of recharge water in scenario Ⅱ compared to that of scenario Ⅰ.
5.2
Groundwater Level Changes
Figure 8 shows the changes in groundwater levels when artificial recharge stops for the two scenarios. The maximum water levels rises, compared with no artificial recharge, are 30 and 34 m for scenario Ⅰ and scenario Ⅱ, respectively. For the purpose of presentation, it is considered that the coverage of water level rise is represented by the 0.5 m isoline of water rise. When the process of artificial recharge has been finished, the water level rise area will cover 119.64 and 149.32 km2, respectively, including the 3rd water supply plant. Subsequently, coverage will gradually increase and more groundwater supply well fields could be maintained due to groundwater level rise.
Figure
8.
Groundwater level rise contour under scenario Ⅰ (a) and groundwater level rise contour under scenario Ⅱ (b) VS no artificial recharge.
Figure 9 shows that groundwater levels under recharge facilities (LS gravel pit) fluctuate abruptly and the highest rise of water level reach the upper limit of groundwater levels for scenario Ⅱ. Even for scenario Ⅰ, there is also not much space for recharging more water into aquifers under LS gravel pit. On the other hand, groundwater levels will immediately decline as soon as recharge activity stops, which means the aquifers are sufficiently transmissive to accommodate lateral flow of the infiltrated water away from the recharge area.
Figure
9.
Responses of groundwater levels to artificial recharge scenarios.
The response of groundwater at the third water supply plant, which has a certain distance from the recharge area, is relatively moderate and there is obvious hysteresis phenomenon on its response to artificial recharge scenarios. The maximum rise of groundwater in the third water plant is about 2.5 and 5.0 m for scenario Ⅰ and scenario Ⅱ, respectively, at least 10 m below the upper limit of groundwater. It indicates that there is still much space for artificial replenishment of groundwater and it might be acceptable to consider artificially recharging of groundwater for consecutive years rather than one year, so that more surface water could be recharged into aquifers and a sustainable management of groundwater might be achieved.
5.3
Changes in Groundwater Budget
Table 7 shows the changes in groundwater budget under the two recharge scenarios and under the condition of no artificial recharge. Based on the calculation results, the deficit in groundwater balance would be changed under both scenarios with the balance increasing from -54.11×106 to 70.89×106 and 183.36×106 m3, respectively. It indicates that the proposed artificial groundwater recharge scenarios can effectively improve the groundwater storage of the aquifers.
Table
7.
Changes in groundwater budget under artificial recharge scenarios in the first hydrological year
Under scenario Ⅰ, precipitation infiltration will still be the main recharge source, which accounts for 29.27% of the total recharge. But artificial recharge water, under Scenario Ⅱ, will become the majority of recharge, which accounts for 43.83% of the total recharge, followed by rainfall infiltration (i.e., 33.08% of the total recharge). These results indicate that artificial recharge can be an efficient way to store water into the aquifers.
For both scenarios, exploitation is the main way of discharge, accounting for 82.13% and 81.33% of the total discharge, respectively. Even though a large amount of water is artificially recharged into aquifers, lateral outflow will increase marginally at the first hydrological year increasing from 63.47×106 to 65.89×106 and 69.47×106 m3, respectively. Neither of scenarios will result in increased evaporation and evaporation remains to be zero. This is because the groundwater level is quite deep and will not be higher than the extreme evaporation depth even after artificial recharge. Therefore, within a short time, the artificially recharged water will be almost stored in the aquifers so that it could efficiently increase the storage of groundwater in this area.
6.
CONCLUSIONS
Groundwater in YRAF plays an important role in the water supply for Beijing urban area, which has resulted in a consecutive decline of groundwater levels in the past decades. Due to over-exploitation, there is a mean deficit of -119.75×106 m3 per year in groundwater balance from years 2000 to 2011 in the study area.
When MRP is put into service, there will be more surface water with high water quality available to be used as recharge water, and it is necessary to consider recovering groundwater storage by artificial groundwater recharge in Beijing. It can be achieved by surface infiltration or direct well injection in the study area. With these facilities, different recharge scenarios can be designed according to the amount of water needed to be recharged. And recharge rate of these facilities can be determined using the analog method.
A numerical model was applied to evaluate the applicability and the effect of proposed artificial recharge scenarios of the aquifers. As the LS gravel pit has a large recharge rate, discontinuous recharge should be designed to prevent water levels from exceeding the upper limit of the groundwater level.
The simulation results show that groundwater levels will rise under artificial recharge condition, and the water level recovery area would eventually cover some important well fields, especially the 3rd water supply plant, which would have a rise in water level of 2.5 and 5.0 m for scenario Ⅰ and scenario Ⅱ, respectively. It is also expected to efficiently augment the groundwater storage without inducing much discharge, like lateral outflow or phreatic water evaporation, from aquifers in a short time. This indicates that it is practically feasible and effective to artificially recharge groundwater in YRAF with MRP as the water source.
Two recharge scenarios were proposed based on the assumption that groundwater exploitation will reduce when MRP is put into service. In fact, groundwater resources will increase as a result of artificial recharge and accordingly it is reasonable to consider pumping more water from aquifers so as to meet the increasing demand of water. If not, the recharged groundwater might eventually flow out of the study area years later instead of being used as supply water source for urban area of Beijing. What's worse is that when the recharged water flows into the downstream area of the alluvial fan, it may even give rise to an excessive rise in water level of phreatic aquifers and thereby secondary environmental geological problems would occur. So, artificial recharge under the conditions of increasing exploitation would be more efficient for the management of the groundwater resources. It is necessary to carry out further study on designing artificial recharge scenarios which will take into consideration water supply planning in Beijing.
When the recharge scenarios are implemented, the water level will not exceed the upper limit of groundwater level except at the recharge site. This indicates there is still a large space for artificial groundwater recharge in the aquifers. The storage of the aquifers may be further increased by multi-year artificial recharge scenarios. Ultimately, an optimization model should be developed to optimize artificial recharge scenarios, e.g. recharge period, recharge rate and infiltration facilities pattern, so that more efficient groundwater management could be achieved.
ACKNOWLEDGMENTS:
This work was supported by the National Basic Research Program of China (No. 2010CB428804) and the Public Welfare Industry Special Funds for Scientific Research from Ministry of Land and Resources of P. R. China (No. 201211079-4). The authors are grateful to the Beijing Institute of Hydrogeology and Engineering Geology and the Institute of Hydrogeology and Environmental Geology for their great help during our research.
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Table
1.
Calculated results of recharge rate of the surface infiltration facilities. The results are calculated theoretically when the vertical hydraulic gradient constantly is 1, i.e., the effect of water level depth on infiltration is not taken into account. XHC and RO herein are references for analogue analysis whose infiltration rates have been derived from previous field test. Considering plugging and the potential time used to dredge up the recharge facilities, the correction coefficient is conservatively determined as 0.6 or 0.8
Figure 1. Location and lithology of the study area. Ⅰ. Single layer of pebbles and gravels, K>200 m/d; Ⅱ. multiple layers of pebbles and gravels, 200 m/d>K>100 m/d; Ⅲ. multiple gravels and few sand layers, K < 100 m/d; Ⅳ. multiple sand layers and few gravel layers; Ⅴ. multiple layers of sand; 1#-7#. water supply plants; FT. Fengtai water supply plant. Diameters of the water plants indicated herein are related to the space of the plants. Observation well 22 is located inside Beijing Normal University as a groundwater level observation well, approximately in the center of the depression cone.
Figure 2. Hydrogeological cross-section of YRAF. Ⅰ-Ⅴ. correspond to five lithology subareas in Fig. 1. ① are phreatic aquifers and ②, ③ are confined aquifers. This figure is adapted from the report "Groundwater Flow Modeling and Groundwater Resource Potential in the Plain Area of Beijing".
Figure 3. Groundwater level time series for observation well 22.
Figure 4. Distribution of the facilities and sites suitable for artificial recharge in the study area. There are four gravel pits: Xihuangcun (XHC) gravel pit, Liaogongzhuang gravel pit (LGZ), Beiwu gravel pit (BW), Laoshan gravel pit and a dry river: Nanhan River (NH River).
Figure 5. Calculated heads versus observed heads of observation wells.
Figure 6. Fitting curve of the observed groundwater level contours VS calculated groundwater level contours at the end of the calibration period.
Figure 7. Partitions for the hydrogeological parameters of the phreatic aquifer. 1-16. Number of partition.
Figure 8. Groundwater level rise contour under scenario Ⅰ (a) and groundwater level rise contour under scenario Ⅱ (b) VS no artificial recharge.
Figure 9. Responses of groundwater levels to artificial recharge scenarios.
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