4.1 Effect of different recharge and discharge conditions on the groundwater system
The difference of the modeling results in groundwater Cl− concentration under different precipitation scenarios is small than 10 mg/L. The prediction results show that Cl− concentrations in the unconfined groundwater could be rapidly diluted without RW recharge as RW is the only source of high concentrations of Cl− in the model. There is almost no difference in the modeling results in Cl− concentration under different groundwater extraction scenarios. Therefore, only the influence of different scenarios on the groundwater table will be discussed below.
Precipitation infiltration as the main source of recharge for unconfined groundwater affects the variations of groundwater levels. In different precipitation scenarios, the groundwater table in wet scenario > baseline scenario > normal scenario > dry scenario (Fig. S2). The interannual variation of the groundwater table under the baseline scenario is the smallest, approximated as a state of balance. Long-term low rainfall will cause the groundwater table to drop. In Fig. S2d, we can see that the groundwater table in the wet scenario is up to 2 m and 4 m higher than in the normal and dry scenarios, respectively, after 10 years.
The scenarios with baseline precipitation and RW recharge (scenarios 4 and 5) demonstrated the impact of groundwater exploitation. Fig. S3 shows the groundwater table under different groundwater extraction scenarios. The results show that the reduction in groundwater withdrawal can recover the groundwater table. The closer to the well field, the greater the groundwater table changes affected by groundwater extraction. For example, the well G01 closest to the well field has a maximum groundwater table amplitude of ~ 2 m between the increase and reduction scenarios (Fig. S3a).
The scenario of baseline precipitation and groundwater extraction (scenario 6) was designed to demonstrate the influence of RW recharge. Compared with the baseline scenario, the groundwater table dropped when there was no RW recharge and then reached a relatively stable state. The difference in groundwater table with and without RW recharge is about 3m, 5m, 11m, and 5m at G01, G15, G22, and G30, respectively (Fig. S4). It indicates that the continuous RW recharge helps maintain the groundwater table stable. When there is no RW recharge, the groundwater table around the river channel will reduce to a low level.
4.2 Variations of the unconfined groundwater storage
The unconfined groundwater storage can be calculated by the simulated groundwater table each year at the end of December (Fig. 5). The storage decreased year by year from 4.83×108 m3 at the end of 2007 to 3.88×108 m3 at the end of 2011. Although the groundwater table around the river has risen after the RW recharged, the groundwater table in the north Xiangyang sluice has been falling due to the continuous groundwater exploitation. Since 2012, the section between Henan rubber dam and Suzhuang rubber dam has been replenished by the RW. The storage increased to 4.1×108 m3 by the end of 2012, then continued to decline and was up to 3.76×108 m3 at the end of 2014 (Fig. 5).
In December 2014, the South-to-North Water Diversion Project began to transfer water to Beijing. The amount of groundwater extraction in the well field, which is located in the north of the Xiangyang sluice, has been halved. The declining trend of the confined groundwater level has slowed down. Moreover, the annual average leakage of the unconfined aquifer has decreased from 1.04×108 m3/a (2008–2013) to 8.81×107 m3/a (2014–2017). Due to the continuous RW infiltration and the groundwater extraction reduction, storage began to rise slowly since 2014. It rose from 3.76×108 m3 at the end of 2014 to 3.85×108 m3 at the end of 2017 (Fig. 5).
4.3 Distribution of the unconfined groundwater-affected by the RW infiltration
The increase of Cl− concentrations may reflect the impact of RW infiltration on groundwater. The zones where the Cl− concentration is higher than the initial concentration (5~75 mg/L) after RW recharge is defined as being affected by RW infiltration. Fig. 6 shows zones of the unconfined groundwater quality affected by RW for the 1st, 2nd, 5th, and 10th years after RW restoring the river channel. The Cl− transport is controlled by both the groundwater flow rate and the concentration gradient. In the early stage of reclaimed water replenishment, the groundwater table elevated rapidly due to the RW infiltration, resulting in quick Cl− movement with the groundwater flow. Thus, the zones affected by RW are distributed at both sides of the river. Due to the unconfined groundwater flows from southwest to northeast, the affected zone of the left bank is more extensive than that at the right bank (Fig. 6). The affected zones gradually expanded with the increase of time and recharged channels, but the annual increasing rate of the area decreased year by year (Table 4). It indicates that Cl− movement slowed down with the groundwater table stable around the river.
Table 4
Variations of the zones affected by RW infiltration for the unconfined groundwater
Year
|
2008
|
2009
|
|
2010
|
2011
|
2012
|
2013
|
2014
|
2015
|
2016
|
2017
|
Area (km2)
|
11.7
|
14.9
|
|
16.4
|
17.8
|
22.2
|
23.3
|
24.6
|
25.7
|
26.4
|
26.7
|
Increasing rate (%)
|
|
27.4
|
|
9.8
|
8.8
|
24.5
|
5.3
|
5.3
|
4.6
|
2.5
|
1.4
|
4.4 Variations of the Cl- and NO3-N loads
4.4.1 Cl- loads in the unconfined groundwater
The Cl− loads in the model area were decreased annually, dropped from 1.66×104 t at the end of 2007 to 3.85×103 t at the end of 2017 (Fig. 5). In contrast, the Cl− loads in the affected zones rose from 1.8×103 t at the end of 2008 to 3.8×103 t at the end of 2017. There are rising trends both in the Cl− concentrations, and Cl− loads of the unconfined groundwater in the affected zones. The zone can represent where the unconfined groundwater quality is affected by RW in terms of solute concentration and corresponding loads.
4.4.2 NO3-N loads in the affected zones
NO3-N is the typical contaminant in the RW (Pan et al., 2018; Li et al., 2019). Although NO3-N has been partially removed during the RW infiltration process, NO3-N from the RW still enters the groundwater (Li et al., 2019). The observed NO3-N concentrations of some monitoring wells occasionally exceed 10 mg/L (guideline for drinking water recommended by WHO (World Health Organization, 2017)). Furthermore, the change trends of groundwater NO3-N concentrations during monitoring periods were consistent with those of groundwater Cl− concentrations, being affected by seasonal precipitation and temperature changes (Li, 2020).
Figure 5 shows the calculated groundwater NO3-N loads within the affected zones at the end of each year, declining from 29.8 t at the end of 2008 to 11.9 t at the end of 2017. It indicates that the RW infiltration did not increase the NO3-N loads in the unconfined groundwater.
Although the maximum NO3-N concentration in the RW is as high as 20.2 mg/L, the unconfined groundwater has been subjected to denitrification during the RW infiltration, especially in the south of the Xiangyang sluice (Liu et al., 2018). Denitrification hotspots are easily formed during surface water infiltration (Rivett et al., 2008; Ranalli and Macalady, 2010; Trauth et al., 2018). The attenuation rate of NO3-N near the monitoring well G22 can reach 99.6% (Li et al., 2019).
There was a high level (48.8 t) of NO3-N loads at the end of 2011. The NO3-N contents near the Xiangyang sluice are relatively high due to the existing thick gravel layer, which is not conducive to remove NO3-N (Xiong, 2009). The north channel of the earth dam has been replenished only in May and October. Sediments can adsorb NH4-N during the wet period, which can be converted by nitrification reaction into NO3-N during the dry period (Böhlke et al., 2006). Therefore, the unconfined groundwater near the intermittent water-receiving channel appeared high NO3-N concentrations (~ 14.7 mg/L).
4.5 Environmental implications for management of a river channel recharged by RW
RW has been used to restore the Chaobai River, while the unconfined aquifer near the river channel was unintentionally recharged. It can be considered as a riverbank filtration (RBF) system, a way of anthropogenic aquifer recharge (AAR) (Todd, 1959; Morel-Seytoux, 1985; Dillon, 2005; NRMMC et al., 2009; Maliva, 2020a). As a result, the RW infiltration affects the groundwater level and quality around the river channel, especially the unconfined groundwater.
The RW infiltration has quickly replenished the unconfined groundwater, resulting in the groundwater table around the river channel to rose rapidly by 3~4 m in the first two years (2007–2009) (Zheng et al., 2015; He et al., 2021). However, increased recharge via the RW infiltration may not necessarily result in a corresponding increase in the groundwater storage volume (Maliva, 2020b). According to the calculated results of the unconfined groundwater storage in this study, it is necessary to combine the increase RW recharge and reduction extractions to sustain the groundwater storage. Increasing recharge in isolation may not solve the problem of groundwater over-extraction (Gale et al., 2006; Foster and Garduño, 2013).
Water quality issues are usually the most concern in the RW utilization. The infiltration process driven by natural filtration and groundwater pumping improves or degrades the recharged water quality. It depends on the quantity and hydrochemical characteristics of RW and native groundwater, and the geochemical processes occurring during RW passes via riverbed and underlying aquifer (Ray, 2008; Stuyfzand, 2011; Tyagi et al., 2013; Hu et al., 2016). In the study area, the subsurface hydrochemistry might be modified by mixing the infiltrated RW with native groundwater and potential water-rock interactions, especially the mixing process and cation exchange (Yu, 2013; Liu et al., 2018). The hydrochemical type of the unconfined groundwater has changed from HCO3-Ca·Mg into HCO3·Cl-Na·Ca, which is the RW type (Li et al., 2019; Jiang et al., 2020). The hydraulic travel time of RW infiltration into the 30 m depth was about 6.5 months (Li et al., 2019). The average proportion of the RW in the unconfined aquifers is about 53% from 2007 to 2018 (He et al., 2021). NO3-N in the RW was well attenuated with an attenuation rate of 99.6% during infiltration (Li et al., 2019). Some emerging contaminants have been detected in the unconfined groundwater from this area, such as endocrine-disrupting compounds (Li et al., 2013a; Ma et al., 2015) and antibiotics and antibiotic resistance genes (Zhang et al., 2018).
Although the RW recharge to a river channel may inevitably affect the groundwater quality of the underlying aquifers, the affected zones are limited. In the affected zones, the Cl− loads of the unconfined groundwater were increased and NO3-N loads of the unconfined groundwater attenuated in the zones. But outside of these affected zones, the groundwater quality is characterized by the initial concentration of the aquifer.
RW utilization for recharging the river channel and the ambient groundwater system is usually a double-edged sword, which can bring the environment both benefits and harms. The main benefits are restoring the riverine ecosystem, increasing and maintaining the groundwater table and storage, and reducing groundwater pumping. At the same time, the disadvantage lies in the potential deterioration of groundwater quality, which depends on the inputs. attenuation of contaminants, and potential water-rock interaction in river-aquifer systems. Where conditions are hydro-geologically favorable, RW recharge to the river channel can be a valuable way of alleviating water shortages (Maliva, 2020a). It is important and necessary to investigate and evaluate whether the field is suitable for RW recharge (Alam et al., 2021). Based on the numerical modeling, this study shows that apart from the affected zones, the RW has lesser environmental impacts of RW infiltration on groundwater quality. In future work, we should pay more attention to variations of water quality after RW recharge, and take necessary measures to manage RW recharge. It is an essential aspect of ensuring the safe use of RW. Additionally, as the unconfined groundwater table rise, a threshold of water level should be set to prevent soil salinization in the RW receiving area.