Iloilo City is sinking, and too many wells may be why

ILOILO CITY — A coastal and marine geologist has called on the city government to reduce its dependence on groundwater and explore surface-water sources after a study found that parts of the city are sinking by nearly 9 millimeters per year.

Fernando Siringan, director of the Philippine Reclamation Authority and a research professor at the University of the Philippines Marine Science Institute, said the city should be cautious about its groundwater extraction practices and further development amid documented land subsidence.

A 2024 study by the UP Resilience Institute recorded the city’s highest subsidence rates in the Mandurriao, Molo, and La Paz districts, ranging from 4.0 to 8.8 millimeters per year.

The study, conducted by UP’s NOAH Center with international partners, used satellite radar technology known as Interferometric Synthetic Aperture Radar (InSAR) to track ground movement in major Philippine cities from 2014 to 2020.

The study identified Mandurriao and Molo as focal commercial, industrial, and residential areas.

The deformation zone extending from southeastern Mandurriao to Molo includes one of the city’s most densely populated coastal areas.

In the neighboring towns of Oton, Pavia, and Leganes, subsidence rates are lower than in Iloilo City but still range from 2.6 to 4.7 millimeters per year, the study showed.

Siringan said the relatively faster subsidence in Iloilo City could be linked to the volume of groundwater being extracted, though he emphasized the need to further examine the factors contributing to the phenomenon.

“My guess is that Iloilo City sees faster land subsidence because it has more water extracted from the ground. So Iloilo City should explore, should partner with other municipalities so that you can shift at least partially to surface water,” he said.

Siringan said land subsidence can occur naturally as sediments compact over time, but human activities, including groundwater extraction, can accelerate the process.

“Natural compaction is there, but there are other things that we are doing. We take out water from the ground,” Siringan said.

He said there is nothing inherently wrong with using groundwater, but problems arise when extraction exceeds the natural rate at which groundwater is replenished.

“The problem is if the so-called natural rate of recharge is exceeded by the rate at which we extract water from the ground,” he said.

“If we are already seeing consequences of groundwater extraction that is too much, that means we are already exceeding the natural rate of recharge,” he added.

He also urged the city government to balance its investments and development projects with the city’s water requirements, noting that population growth and new investments would increase demand for water.

“In Manila, we were able to show a data set where population growth, water over-extraction, and subsidence were parallel to each other. They showed the same trend,” he said.

“Iloilo City will have to work hard to look for surface water. I’m not saying there should be no more groundwater. Rather, cut Iloilo City’s dependence on groundwater by at least half, and tap the other half from surface water,” he said.

Separate data presented by Siringan also showed 1,226 open and closed wells across six coastal barangays in the city.

“Look at the density of the water wells, but what can people do if there is no water supplied to them? It is also important to recognize that there is over-extraction of water,” he said.

Siringan warned that excessive groundwater extraction in coastal areas can cause saltwater intrusion, as pumping can push the saltwater-freshwater interface upward and contaminate wells with saline water.

He said salinization can also occur when saline water trapped in clay layers is released.

“In coastal areas, there is an interface between the saltwater underneath and the freshwater. When we extract too much freshwater, the saltwater-freshwater wedge rises. If we over-extract, that interface will rise, and the groundwater eventually intersected by the pipes will already be saline,” he said.