Ancient inhabitants of Easter Island (Rapa Nui) survived periods of drought due to their utilization of brackish groundwater discharge that surfaces buoyantly at coastlines, according to new research led by Binghamton University’s Professor Carl Lipo.
The population of Easter Island in pre-historic time is believed to have numbered in the thousands although typical perennial sources of drinking water are nearly absent from the island. Image credit: Thomason Jiang.
European accounts of first encounters with Easter Island in the 18th century include passages where the natives appear to simply drink seawater.
In order to determine whether or not the water close to the shores had a salt concentration low enough for humans to safely drink, Professor Lipo and co-authors measured the salinity of coastal water around the island.
Finding it safe for human consumption, and by eliminating other options as primary sources of drinking water, they concluded that groundwater discharge was a critical factor in the sustenance of the large population the island is thought to have harbored.
“The porous volcanic soils quickly absorb rain, resulting in a lack of streams and rivers,” Professor Lipo said.
“Fortunately, water beneath the ground flows downhill and ultimately exits the ground directly at the point at which the porous subterranean rock meets the ocean.”
“When tides are low, this results in the flow of freshwater directly into the sea.”
“Humans can thus take advantage of these sources of freshwater by capturing the water at these points.”
The freshwater mixes with the saltwater slightly, creating what’s called brackish water, but not enough for the water to contain harmful levels of salt to human consumers.
It does, however, mean that the islanders rarely used salt on their foods, because the water they drank contributed so drastically to their daily salt intake.
“Researchers have often wondered why the island’s famous statues (moai) are only located in certain places around the island, with a high concentration of them near the coasts,” Professor Lipo and colleagues said.
“Now that we know more about the location of freshwater, however, the location of these monuments and other features makes tremendous sense: they are positioned where freshwater is immediately available.”
“There are very few sources of freshwater on the island, including two lakes that are perilously difficult to access, no streams, and one spring that is often reduced to a wetland bog.”
Much of the opposition to this research is the presence of taheta on the island, which are small, carved-out cisterns used for collecting rainfall.
To refute this argument, the scientists explained that if collecting rainwater was extremely necessary to island survival, the cisterns would be much larger, instead of being able to hold only between two and four liters of water each.
The study shows that the little amount of rainfall that Easter Island receives (1,240 mm/yr), coupled with the basic evaporation rate of water in a climate such as the island’s, means that on average, taheta could not be used as viable sources of drinking water 317 days out of the year.
This led the team to conclude that there must be a different source of drinking water, in order for a population numbering in the thousands to sustain itself.
The study was published in the Hydrogeology Journal.
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Tanya Brosnan et al. Coastal groundwater discharge and the ancient inhabitants of Rapa Nui (Easter Island), Chile. Hydrogeology Journal, published online October 4, 2018; doi: 10.1007/s10040-018-1870-7
