An 18th-century account of European first contact with the Pacific Islanders detailed natives being able to drink seawater without harm – a feat we all know is impossible today (the human body cannot process high concentrations of seawater, eventually killing a person through dehydration). By 1887, the local population of Easter Island dropped to just 110 largely due to slave trading and disease, decimating any chance of uncovering the people’s oral history. [...]
When it rains, water beneath the ground flows downhill and exits the ground where the rock meets the ocean. When tides are low, this “results in the flow of freshwater directly into the sea.” The fresh and saltwater mix to create a “brackish but potable water along the coastline” that contains low enough levels of salt to be safely consumed by humans, which the researchers think the islanders must have done.
“Although coastal groundwater sources are of poor quality, they were apparently sufficient to support the population and allow them to build magnificent statutes for which Easter Island is famous,” wrote the authors in their study published in Hydrogeology. [...]
However, others argue that small cisterns found on the island could have been used for collecting rainfall. Lipo argues that if collecting rainfall was central to the culture, then the cisterns would have been much larger (they only hold between 2-4 liters of water each). Furthermore, Rapa Nui only receives 1,240 millimeters of rainfall every year. Paired with the rate of water evaporation here, the cisterns would likely have only been a viable source of water for less than one-third of the year.
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