Archaeologists are piecing together more details about how the Rapanui people once erected the formerly enigmatic stone statues, or moai. But one of the island’s lingering mysteries is how the Rapanui found enough water to sustain thousands of people on a small island. Easter Island, or Rapa Nui, has no permanent streams, and its three lakes are hard to reach and far from archaeological evidence of settlement. But when European colonists arrived in the late 1700s, thousands of people already lived on the island, and they had to be getting their drinking water somewhere.



According to geoscientist Tanya Brosnan of California State University, the Rapanui probably got at least some of their drinking water from places along the coast where fresh groundwater seeped out of the island’s bedrock and into the sea. The resulting mixture would have been brackish but safe to drink, and it could have sustained populations of thousands on an island with few other reliable sources of fresh water. That’s common knowledge among the modern Rapanui people, but it hasn’t been clear that pre-contact people got their water the same way.

“Our work was certainly not ‘discovering’ anything that people didn’t already know about. Rather, we worked to put together an overall picture of groundwater and its accessibility for past populations,” Binghamton University archaeologist Carl Lipo, a coauthor on the study, told Ars.

Sometimes you’ve got to be a little salty

Rapa Nui is the product of overlapping lava flows from a group of volcanoes that began erupting deep beneath the surface of the Pacific about three million years ago. When rain falls, the water soaks quickly into the soil and porous bedrock (it doesn’t tend to pool in lakes or meander across the surface in permanent rivers or streams). Once there, it flows through the island’s pores until it reaches the sea.

But if people could catch the groundwater where it seeps out into the sea, they’d have fresh water—more or less. Water is safe to drink if it contains less than about 9,000mg of salt per liter; that threshold exists because we can’t safely drink water saltier than our own bodily fluids, and blood plasma clocks in at 9,000mg/L. For comparison, sea water contains a whopping 35 grams of salt per liter, while anything less than 1,000mg/L is considered properly fresh. Everything in between is called brackish water, and it’s apparently an acquired taste.

“If you have no other choice, that is simply what one gets used to,” Lipo told Ars. “We certainly tried the water. One could get used to the taste, but you certainly learn quickly to access the spots where the best seeps are.”

Water, water everywhere

To map the island's water resources, Brosnan and her colleagues trekked across the eastern side of Rapa Nui to map freshwater sources, including small carved cisterns called taheta, which they say would have held a little water but not enough to sustain the island’s whole population. And then they hiked along the eastern coast, measuring the salinity of the water to look for seeps of fresh groundwater. Rapa Nui, it turned out, is continually leaking fresh water into the sea all over the place. The survey team found groundwater seeps along most of the coast, ranging from areas just slightly fresher than seawater to some pools of almost completely fresh water.

Near Tongariki, the team found an area where the water contained just 3,690mg of salt per liter. Not far from there, they found a structure that probably helped pre-contact Rapanui collect fresh water from the seep: a rectangular trench lined with stones, called a puna. Brackish groundwater would seep out of the walls to fill the trench, which also provided a handy way to catch rainwater. And the water in the puna when Brosnan and her colleagues stopped by contained just 5,920 grams of salt per liter. Another puna near La Perouse Bay held slightly fresher water at 4,920mg/L.

Brosnan et al. 2018

Brosnan et al. 2018

Brosnan et al. 2018

Brosnan et al. 2018

Historical accounts also describe people just skimming water from the surface of the sea, much to the consternation of European observers. And the Rapanui may once have built rock walls in the tidal zone to hold the relatively fresh water close to shore, where they could scoop it up whenever they wanted. No archaeological evidence of those structures remains, however.

Bones don’t lie

But there is direct evidence that the indigenous people of Rapa Nui were drinking brackish coastal water long before Europeans stumbled onto the scene. Any water that’s exposed to sunlight becomes home to a thriving population of single-celled algae called diatoms, which build silica cell walls that tend to preserve pretty well in the fossil record. In this case, they’re preserved in the dental calculus (the technical term for really, really old plaque) of people who lived and died centuries ago on Rapa Nui.

Because different diatom species prefer different environments, looking at the diatoms in a person’s dental calculus can reveal something about where they got their drinking water. A 2014 study did just that, and it turned out that many of the diatoms still stuck in long-dead individuals’ teeth were from species that preferred brackish water. That means the pre-contact Rapanui must have gotten at least some of their water from the coastal seeps.

That could go a long way toward explaining why most of the archaeological remains of settlement on Easter Island are closer to the coast than to either of the freshwater lakes—including the iconic moai. It also explains the observations that so thoroughly flabbergasted early European visitors.

“Some of our people really saw them drink of the sea water when they were thirsty,” wrote a physician on James Cook’s 1774 voyage. Cook himself described the water as “brackish and stinking” but “rendered acceptable by the extremity of their thirst.” Acquired taste, indeed.

A story of survival

It’s a taste that later residents of Rapa Nui seem to have picked up out of necessity. Brosnan and her colleagues found stone wells and water catches below seeps, dating to relatively modern times, and most of the modern Rapanui they spoke with knew several sources of fresh water on the island, including coastal seeps. Without that knowledge, the Rapanui might not have survived the five generations of drought that struck the island from about 1570 to 1720.

The drought is now recorded in layers of sediment at the bottom of Raruku Lake; thin layers without much sediment accumulation point to less rainfall to wash soil into the lake, and changing pollen species mixed into the sediment reflect the drought’s impact on the island ecosystem. The long, deep drought played a role in killing off the island’s palm forests—a loss for which scientists once blamed the Rapanui themselves. But the people survived.

Hydrology Journal, 2018. DOI: 10.1007/s10040-018-1870-7 (About DOIs).