Increased fresh groundwater extraction from wells in coastal aquifers by simultaneous extraction of brackish groundwater: a numerical modeling study

In coastal areas, freshwater availability is often limited to fresh groundwater lenses that are fed by natural recharge. Overexploitation causes these freshwater reserves to shrink, resulting in increased seawater intrusion and salinization of groundwater wells. This will only intensify by the increasing freshwater demand in coastal urban areas resulting from rapid population growth and economic development. Simultaneously, the vulnerability of coastal regions to saltwater intrusion increases by ongoing climate change through sea level rise and changes in natural recharge patterns. Hence, proper management is required to extract fresh groundwater sustainably in coastal areas and to prevent saltwater intrusion.

Targeted extraction of brackish groundwater underneath freshwater lenses may be an effective measure to prevent salinization of fresh groundwater extraction wells in coastal aquifers. Moreover, it can increase the potential for freshwater infiltration, minimize freshwater losses by lateral outflow, and subsequently cause the volume of the freshwater lens to increase. But there is also a drawback, as extraction of brackish groundwater underneath freshwater lenses may result in the loss of a portion of the fresh groundwater. On the other hand,  the extracted brackish groundwater may provide an attractive alternative to seawater for desalination.

In this generic numerical modeling study, the dynamics of fresh, brackish and saline groundwater were studied for the case that fresh and brackish groundwater are extracted simultaneously. The wells are placed in an unconsolidated island aquifer that hosts a freshwater lens that is fed by recharge. A radial symmetric variable-density groundwater flow and transport model was constructed with SEAWAT. The performance of the brackish groundwater extraction well was assessed by investigating its effect on the potential fresh groundwater extraction and the associated freshwater losses. A sensitivity analysis was carried out to determine how hydrogeological characteristics and operational parameters affect the performance of the brackish groundwater extraction.

Results so far indicate that the extraction of brackish groundwater increases the volume of fresh groundwater that can be extracted by the freshwater well without salinization, due to the mitigation of upconing of brackish groundwater and the reduction of freshwater losses towards the coast. Placement of the brackish groundwater extraction well right below the freshwater well results in a more effective protection of the freshwater well, as reflected by a lower required brackish groundwater extraction rate. On the other hand, the volume of the freshwater lens may increase when the brackish groundwater extraction well is placed deeper below the fresh groundwater extraction, which may be beneficial for freshwater availability on a regional scale.