Published in El Nuevo Día as part of the collaboration between CienciaPR and that newspaper.
Hot springs have been part of Puerto Rican geography and culture for hundreds of years. Today, the most famous ones are in Coamo. But why do they currently only flow there?
According to hydrogeologist Jobel Villafañe Pagán, a graduate student at the University of Hawaiʻi at Mānoa, the answer lies in the geology of the area.
“The Great Southern Fault Zone crosses through Coamo, branching into smaller-scale faults, such as the Jueyes River fault, associated with and near the hot springs in that area. The thermal spring emerges from a fissure in a hill at the foot of the Coamo River,” he explained.
The origin of hot springs
The interior of planet Earth is hot. Volcanoes are clear evidence of this. However, geologists have discovered that this heat also exists in areas without volcanoes, such as Puerto Rico. For every 1,000 feet (300 meters) of depth, the temperature of the Earth's crust increases by about 15 degrees Fahrenheit (°F).
On the other hand, when it rains, part of the water infiltrates and percolates underground until it reaches the aquifer—that is, layers of subterranean rocks or sediments whose pores and spaces between particles allow for water storage and flow. Sometimes, there are cracks and geological faults that allow water to reach several thousand feet deep and attain temperatures of over 150 °F.
Eventually, these fault and crack systems allow the hot water to return to the surface. On its way, it dissolves minerals and other chemical compounds, such as sulfates and carbonates, to which health benefits are attributed. This is the case with the Coamo hot springs.
Have other hot springs existed in Puerto Rico?
Although the Coamo Baths have been the most famous, hot springs have indeed existed in other areas of the island. In Ponce, the Quintana Baths were available to the public for a long time. In Guayama, the ruins of the Virella Baths and a small pool still remain.
Other hot springs have almost vanished from the island's historical memory. In his 1852 book, “Catechism of Geography of the Island of Puerto Rico,” author Francisco Pastrana mentions the existence of thermal baths in Hato Grande, part of San Lorenzo. Meanwhile, historian Herminio Rodríguez Morales, in “San Lorenzo: Notes for its History,” describes these hot springs as “three excavations (small pools) at their source (...) one league from Caguas.”
In late 1905, “El Boletín Mercantil de Puerto Rico” and newspapers of the time published articles about hot springs in Caguas. The newspaper “La Democracia,” on November 1, 1905, located the “sulfurous spring” in “a delightful plain (…) one kilometer east of the town,” meaning the urban area of Caguas.
These journalistic outlets reported that a group of merchants, led by Juan Boix (who managed the Coamo Baths) and an industrial engineer named Rivero, “formed a syndicate with the aim of exploiting the existing hot springs in this jurisdiction” and visited the Caguas area.
Presently, the exact location of the hot springs in Caguas and San Lorenzo is a mystery. It is very possible that the land was filled in to expand the urban areas of these municipalities.
“The study of hot springs is extremely important in the archipelago of Puerto Rico to explore the viability of geothermal energy. Furthermore, it provides information on the interaction between water and rock in the subsurface and offers opportunities for scientific research in extreme environments,” mentioned Villafañe Pagán.