Punjab scavenger wells for sustainable additional groundwater irrigation

Abstract We aim to solve the longstanding problem of sustainable groundwater extraction in the Pakistani Punjab (Indus Basin). Since public scavenging wells were abandoned in the 1990s, farmers have largely embraced the practice of skimming wells to prevent or reduce salinity in their abstracted water. Although this may be an effective short-term solution, salinity levels will continue to rise over time, rendering groundwater unsuitable for irrigation. We investigate the creation of balanced scavenging wells to reduce salinization so that sustainable groundwater use is possible as an additional source for irrigation. To achieve such sustainability, a certain amount of brackish water must be discharged, and this amount is determined by assessing the combined salt and water budgets. MATLAB-based SEAWAT models were used to show that (1) recirculation wells can substantially delay (but not prevent) salinization as a mid-term solution and (2) scavenging is the only long-term option to solve the salinization problem. Final (i.e., long-term) salinity does not depend on hydraulic parameters or initial groundwater salinity, which can only delay or speed up the process of salinization. A sensitivity analysis showed that vertical anisotropy (ratio of horizontal to vertical hydraulic conductivity) is the most important hydraulic variable for reducing the depth reached by streamlines, which reduces the time required to reach the equilibrium salinity. We conclude: (1) the extraction of freshwater can be determined from leakage, return flow and saltwater extraction; (2) the required saltwater extraction does not depend on freshwater extraction, although for recirculation, saltwater extraction does depend on saltwater injection; and (3) the final salinity in the saturated zone only depends on the salinity of the irrigation water, saltwater extraction, leakage from the irrigation canal system and, in the case of recirculation, the saltwater injection.