**8. Groundwater recharging through MWPS under constant head conditions**

It is already mentioned in Section 7 that farmers of the region have adopted the MWPS in big way and their dependency on canal water has declined considerably. It is also true that these freshwater lenses are very limited and are now under stress. For long‐term sustainability of these limited freshwater lenses, enhancement in freshwater recharging rate is required. Therefore, we have conceived an idea to recharge the freshwater zones with surplus canal water when available during the monsoon season through the existing MWPSs in the region [37]. Aquifer storage recovery (ASR) is worldwide recognized technique for managing brackish/saline aquifers [38] in which freshwater when available is stored in the aquifer and later on recovered for use. However, our proposal was little different from this basic technique. In the ASR technique, recharging of the brackish/saline aquifer is generally performed with the fully penetrating well of high capacity in which the injected freshwater displaces the saline water. As a result, considerable portion of the stored freshwater gets salinized before its recovery. The native groundwater movement and the buoyancy effect are the two factors which are responsible for the irrecoverable portion of stored freshwater. With the use of multiple partially penetrating wells in a single bore hole, the freshwater losses can be reduced considerably [39]. In this modified ASR technique, freshwater is injected in the deeper portion of the aquifer whereas its recovery is made from the shallow well. On the other hand, as the MWPS operates under limited head conditions and it penetrates only the upper portion of the freshwater zone of the aquifer through smaller capacity wells, it causes minimum disturbance to the denser native brackish water which is flowing in the lower portion of the aquifer [8]. The recharged water gets an opportunity to store in the wide‐spread portion of the aquifer. Hydraulic head distribution in the aquifer while recharging through MWPS is shown in **Figure 15**. While evaluating this, we have considered the recharging rates through individual laterals equal to the pumping rates as mentioned in Section 6. Perusal of **Figure 15** indicates that the freshwater got an opportunity to store in the upper portion of the aquifer and it causes minimum disturbance to deeper brackish water zone.

**Figure 15.** Hydraulic head distribution in the aquifer while recharging through MWPS.

Other than the theoretical investigations, field study was also conducted to monitor the change in quality of aquifer while recharging with freshwater. In the field study, canal water was

allowed to recharge the aquifer through MWPS under constant head conditions. Radial flow filter was used for removing turbidity from the canal water (**Figure 16**).

water when available during the monsoon season through the existing MWPSs in the region [37]. Aquifer storage recovery (ASR) is worldwide recognized technique for managing brackish/saline aquifers [38] in which freshwater when available is stored in the aquifer and later on recovered for use. However, our proposal was little different from this basic technique. In the ASR technique, recharging of the brackish/saline aquifer is generally performed with the fully penetrating well of high capacity in which the injected freshwater displaces the saline water. As a result, considerable portion of the stored freshwater gets salinized before its recovery. The native groundwater movement and the buoyancy effect are the two factors which are responsible for the irrecoverable portion of stored freshwater. With the use of multiple partially penetrating wells in a single bore hole, the freshwater losses can be reduced considerably [39]. In this modified ASR technique, freshwater is injected in the deeper portion of the aquifer whereas its recovery is made from the shallow well. On the other hand, as the MWPS operates under limited head conditions and it penetrates only the upper portion of the freshwater zone of the aquifer through smaller capacity wells, it causes minimum disturbance to the denser native brackish water which is flowing in the lower portion of the aquifer [8]. The recharged water gets an opportunity to store in the wide‐spread portion of the aquifer. Hydraulic head distribution in the aquifer while recharging through MWPS is shown in **Figure 15**. While evaluating this, we have considered the recharging rates through individual laterals equal to the pumping rates as mentioned in Section 6. Perusal of **Figure 15** indicates that the freshwater got an opportunity to store in the upper portion of the aquifer and it causes

minimum disturbance to deeper brackish water zone.

62 Groundwater - Contaminant and Resource Management

**Figure 15.** Hydraulic head distribution in the aquifer while recharging through MWPS.

Other than the theoretical investigations, field study was also conducted to monitor the change in quality of aquifer while recharging with freshwater. In the field study, canal water was

**Figure 16.** Radial flow filter for removing turbidity of canal water before recharging through MWPS.

For monitoring the change in quality of groundwater due to recharging, a water sampling well of length 9.144 m having diameter 10.16 cm was installed at a radial distance of 3.27 m from the well point W24 (**Figure 10**). The perforated length of the observation well was 6 m. For critically analyzing the effect of recharging on the quality of brackish water, it was necessary to collect water samples depth‐wise with minimum disturbance. For this purpose, a specialized water sampler was fabricated. In spite of this, for achieving accuracy, considerable time was given between consecutive samplings. Before starting the recharging operation, groundwater samples from various depths were collected and analyzed for EC and pH. The recharging operation was continued for net 65 days and on its completion, water samples were collected from the same depths and were again analyzed for EC and pH. Comparison of the change in groundwater quality due to freshwater recharging is shown in **Figures 17** and **18.** Perusal of both the figures indicates that the recharging of freshwater in fresh–brackish aquifer through MWPS is a viable technique for reviving freshwater lenses in the region. However, for planning any recharging project through MWPS, theoretical investigations are required *a priori*. Realizing this need, Vashisht and Shakya [8] have proposed a semianalytical solution for evaluating hydraulic head distribution and recharging rate in a single leaky aquifer while recharging freshwater under constant head conditions through MWPS. The proposed solution was based on the introduction of a new relation termed as "position‐oriented *opportunistic proportion.*" The most important advantage of recharging through MWPS is that it covers the wide‐spread region of the aquifer without disturbing much the native brackish water. The developed hydraulics for MWPS may be considered as the backbone for planning and execution of policies for managing brackish groundwater aquifers in the region.

**Figure 17.** Change in EC of groundwater depth‐wise after recharging canal water through MWPS.

**Figure 18.** Change in pH of groundwater depth‐wise after recharging canal water through MWPS.

#### **9. Conclusion**

Long‐term field investigations were made on the MWPS for checking its performance as a drainage system, a skimming system, and a recharging system. On the basis of the field results and theoretical investigations, it is concluded that MWPS is a viable technique for managing brackish aquifers of a region.
