Construction and performance of infiltration galleries (skimming wells): A review of applications to Pacific atoll islands

The method of extracting fresh groundwater in many Pacific islands, particularly atolls, often involves the installation of horizontal or inclined perforated pipes, connected to an extraction well/sump containing a pump. These “infiltration galleries” (or skimming wells) skim fresh groundwater from shallow depths while minimizing saltwater upconing beneath thin freshwater lenses, which are typically 3–21 m thick. This study summarizes the state of knowledge of infiltration galleries in the Pacific, including their design and performance, to share the decades-long experience of Pacific hydrogeologists with the global groundwater community. Our review compiles information on infiltration gallery layout, pipe characteristics and pumping rates, as well as the performance of infiltration galleries (pumping rates and salinities), among other aspects. Notably, we found a lack of investigations optimizing gallery design in terms of the aforementioned characteristics, leaving this as a critical area for future research effort. While many studies have examined horizontal wells in other contexts, the specific conditions of small islands, especially atolls, places unique demands on infiltration galleries. For example, infiltration gallery assessments should account for freshwater lost as discharge to the sea and through mixing with saltwater – key components of the water budget often overlooked in atoll island water-balance analyses. The insights gained from applying infiltration galleries to Pacific atolls offers opportunities for more widespread applications within continental aquifers, in particular to capture submarine fresh groundwater discharge that is otherwise lost through mixing with seawater and to mitigate seawater intrusion, thereby contributing to global groundwater management strategies.