Mexico's booming tourist industry is polluting the world's largest underwater cave system and harming the world's second largest coral reef.
Pharmaceuticals, cocaine residues, shampoo, toothpaste, pesticides, chemical run-off from roads and many other pollutants have been found in the immense system of underground rivers and aquifers south of the resort city of Canc˙n, located on the Caribbean coast of Quintana Roo state.
"There is little question the pollutants we detected have come from human activity along the coastal region," said Chris Metcalfe, a researcher with the United Nations University's Institute for Water, Environment and Health.
The flow of groundwater takes much of this pollution into the coastal zone and the region's famous Mesoamerican Barrier reef, the second largest in the world, after the Great Barrier Reef off Australia.
Divers have noticed the decline in the region's reefs, says David Placencia, coordinator of reef monitoring for the Centro EcolÛgico Akumal (CEA), a non-governmental organisation in Akumal, 100 kilometres south of CancË™n.
Studies by CEA found that a key reef-building species, Montastraea annularis, has suffered a dramatic decline. Twelve years ago it comprised 45 percent of the reefs around Akumal but in 2010 it had fallen to less than nine percent, he said.
Nutrients and contaminants from sewage promoted the growth of algae, which smother the corals, said Placencia.
There is no sewage treatment in the area, so the polluted run-off goes directly into the groundwater. "There definitely has been a big change in the reefs here," he added.
The state of Quintana Roo is unique in that there are almost no streams or rivers on the surface because the region is comprised of very porous limestone. Instead, the world's largest network of underground rivers, caves and sinkholes known as "cenotes", and aquifers underlies the region. Much of this freshwater flows towards the coast and into the Caribbean Sea.
With a projected tenfold increase in population through 2030, all of these problems are likely to worsen, according to the study by Metcalf and his colleagues.
Metcalf's study sampled groundwater at five inland locations between Playa del Carmen and Tulum, well away from the biggest tourism developments.
"The levels of contaminants found are below those that are a human health hazard," he said. "One of the reasons for the low levels is the large volume and high flow rate of the groundwater.
"However, most of the water flows toward the coastal zone and into the reef areas, and those low concentrations of contaminants accumulate there.
"By looking for caffeine residues, metabolites of nicotine and the ingredients of personal care products, the study proves that human sewage is getting into the region's underground water supply.
"These findings clearly underline the need for monitoring systems to pinpoint where these aquifer pollutants are coming from."