Alarming rates of land subsidence—the gradual sinking or settling of the earth’s surface—have been found in several cities in metropolitan areas of the Philippines from the Greater Metro Manila Area down to Metro Davao. The study, which spanned from 2014 to 2020, highlighted Bulacan as the most affected area in the Philippines, sinking at a staggering rate of 109 millimeters per year.
This paper corroborates earlier findings from a 2019 study by Climate Central that indicates that Bulacan and other coastal regions are threatened to be underwater by 2050.
Local government officials have taken an interest in these findings, expressing either concern and commitment to address these issues or skepticism about its nature. Iloilo City Mayor Jerry Treñas, while acknowledging the troubling implications of the study, has called for more scientific investigation into the geological threats facing the city. As local leaders grapple with the ramifications of studies like these, the call for proactive measures becomes increasingly urgent.
While the cumulative displacement rates of cities in Cebu are nowhere near matching that of Bulacan, cities in Cebu are still under threat of sinking. The latest study reveals that cities like Naga are experiencing measurable subsidence rates of 10.6 millimeters per year.
These are projected to only escalate if more pressing issues such as careless groundwater extraction and reckless urban development continue unchecked.
Tracing the path of least resistance
“[Land subsidence] is not a single problem; it’s a myriad of problems coming together at the same time,” explains Bill Granert, the Executive Director of the Soil and Water Conservation Foundation, Inc. (SCWFI) in Cebu City. This foundation has been a driving force in environmental preservation in Cebu, Bohol, and other regions in the Philippines for 46 years. Working in protected areas, the foundation promotes hands-on environmental education, establishing biodiversity reserves in high schools and communities.
Granert was alarmed at the findings of the recent study, but he and his team have been at work to try to combat it for a while now. He cites a major roadblock to their success: rapid urbanization.
“When you urbanize, you essentially waterproof lands with roads and parking areas,” Granert says. When land is urbanized, large portions of the natural landscape are replaced with concrete, asphalt, and other impermeable materials used in roads, parking lots, and buildings. These surfaces, unlike soil or vegetation, do not absorb water. Instead, they create “waterproof” areas that prevent rainwater from soaking into the ground.
Granert uses the chain of shopping malls SM as an example. In addition to the building itself, these malls often have expansive parking lots that are made of concrete.These hard surfaces block rainwater from seeping into the earth, causing water to pool or flow rapidly toward drainage systems. As a result, less water infiltrates the ground to replenish aquifers, and more surface runoff is created, which can contribute to flooding and exacerbate land subsidence.
“The threshold level of Cebu City is 20 millimeters of rain. If you get more of that, it’s continuous rain. Then, you flood Colon and some others,” Granert illustrates.
Worse, Granert notes, is when there are buildings on flood plains—natural, low-lying areas next to rivers or bodies of water that are designed by nature to absorb and spread excess water during heavy rains or floods. When people build structures on flood plains, they disrupt this natural process. These structures block the water’s path, preventing it from spreading out and being absorbed by the land. Since the water now has nowhere to go, it accumulates and leads to severe flooding in the surrounding areas, including the very places that have been built upon.
“The more waterproofing you have, the faster [the water] comes down,” Granert notes. While individual businesses may have the means to implement their own flood control measures, communities often do not. He uses the situation in A.S. Fortuna Street in Mandaue City as an example. The natural water flow of a nearby creek in the area has been significantly narrowed to make room for businesses, reducing its capacity to handle large volumes of water.
This narrowing has exacerbated flooding concerns, prompting action from local officials. Last April, a P90-million road concreting and drainage project to address severe flooding issues on A.S. Fortuna was bid by Rep. Emmarie Ouano-Dizon.
Getting water out of stone
Granert describes water as “the great equalizer.” “It doesn’t matter whether you are rich or poor. If you don’t have enough water, you’re in trouble,” he says.
Granert invites us to imagine how much water one 40-story building will take up for all its uses: plumbing, irrigation, sanitation, etc. All these processes require substantial amounts of water to be drawn from local sources. This leads to groundwater extraction as the building relies on underground aquifers to meet its needs.
As multiplebuildings in an urban area, particularly in a densely populated city, draw from the same groundwater sources, the cumulative effect can lead to significant depletion of these aquifers. Over time, excessive extraction can lower the water table, reduce the availability of freshwater, and contribute to land subsidence as the ground settles in response to the loss of water pressure in the aquifers.
“There’s only so much groundwater,” Granert informs. “Even during the rainy season, they tank water in the mountain barangays because the springs are gone.”
Granert goes back to the question: Where do you get your water? A decrease in rainfall forces communities to rely more on available water sources, starting with surface water and eventually turning to underground aquifers. As groundwater extraction increases due to dwindling surface water, a critical dilemma emerges: will saltwater intrude into these aquifers, or will the land sink from the loss of water pressure?
Fish out of water
Saltwater intrusion occurs because freshwater is less dense than saltwater; thus, when the freshwater level declines, the denser saltwater can move inland to replace the lost volume. When the water table drops significantly due to excessive extraction, it creates a pressure imbalance that allows saltwater from the ocean to intrude into the freshwater aquifers.
“If you look at the saltwater intrusion in Metro Cebu, it is at about four kilometers. If it gets to eight—the magic number—[the effects are] irreversible,” Granert explains.
Some salination companies are tapping into brackish sources, making it cheaper and easier to purify this water since the salt content is lower. However, the process of desalination accumulates in large mounds, leading to disposal challenges. More often than not this excess salt finds its way into the oceans, disrupting the natural salinity balance. This poses a threat to the marine ecosystem, killing organisms and forcing them to migrate in search for more favorable conditions.
With the increase in salinity and the increasing effects of climate change, the ocean levels rise. Granert advises anyone to peer into Pier 1 at high tide and notice that the water is almost at the level of the pier itself, a situation that “didn’t used to happen.”
Rising sea levels exacerbate saltwater intrusion, pushing even saltier water further inland, complicating freshwater availability. It complicates the situation of coastal communities, especially those who rely on springs right on the shoreline that provide fresh water during low tide and become salty during high tide. If the tide continues to rise, these springs may be wiped out, leaving the local population without a reliable source of fresh water and forcing them to travel inland to find it.
This cycle creates a critical feedback loop: as populations grow and demand for fresh water rises, the need for desalination intensifies, leading to further salt extraction and environmental pressures that could ultimately threaten both freshwater resources and marine ecosystems.
Muddying the waters
Besides salt contaminating freshwater, waste does, too. Granert complains that Cebu is one of the few cities in the world of its size that doesn’t have a wastewater recycling system.
In 2018, the Metropolitan Cebu Water District had to shut down eight wells due to nitrate contamination. This comes from human sewage.
“If you have a septic tank and [you] think it’s cleaning up [your water], it’s not. What it is doing is removing the solids; the liquids go out the top of the tank,” Granert explains. “Where do [these liquids] go? They go into the canals, creeks, then oceans.”
During the 2024 Cebu water crisis, a deep well that was closed five years ago due to nitrate contamination was reopened to accommodate the water needs of the population.
While Granert believes that businesses and corporations have better waste disposal measures, they may also be among the top contaminators. Granert shares his personal experience when he used to live in Mandaue City. An electroplating company was improperly disposing of its chromium waste in an open pond near a residential subdivision, exposing many residents to the toxic substance and leading to skin issues such as itching and other health complications.
“They had to condemn the whole system because of the contamination,” Granert mentions. An international body had to rescue them from the electroplating company’s recklessness, disposing of their chromium waste safely and without further implications.
On shaky ground
Underlying all this is the material that literally underlies all of Cebu: limestone.
“Limestone supplies most of the freshwater underground,” Granert notes. “But it is also one of the most sensitive geological formations.”
A place made of limestone is susceptible to sinking because limestone is a porous and soluble rock. Over time, water, particularly acidic groundwater, can dissolve the limestone, creating underground voids or caverns. As these spaces grow larger, the ground above can weaken and collapse, leading to land subsidence or sinkholes.
In 2022, the Department of Environment and Natural Resources (DENR) discovered 815 sinkholes in Boracay Island, warning the popular tourist destination that it is in danger of collapsing. Boracay’s land surface is made entirely of limestone.
Similarly, most of Metro Cebu is made up of limestone. Using a ground-penetrating radar, Granert observed that some schools and even a senior citizen facility in Cebu was basically “sitting on air” with the amount of voids underneath them.
Limestone also makes areas above it susceptible to landslide. State geologist Liza Manzano reveals that the devastating 2018 Naga landslide, which killed 78 people and caused a state of calamity around Naga City, was caused by gradual karst subsidence. Karst is a specific terrain that forms in areas abundant with limestone.
In the latest UPRI study, Naga is the city in Metro Cebu most in danger of collapsing.
Rising out of a sinking ship
Back in 2018, Granert and his team at the SCWFI actually recommended to Cebuano policymakers to send engineers overseas—Canada, US, or European countries—to give them at least short courses on how to engineer limestone.
“A lot of the engineers [here] do not know how to engineer on limestone,” Granert laments. Since the Philippines is tropical and often visited by rain, the limestone dissolves overtime.
He recommends then to urban planners to think before they build. “People are more interested in how much they can sell the land per square meter than how much they are going to spend to fix the problem of subsidence 10 years down the line,” Granert says.
He notes that the greatest hindrance to this is the 3-year term of politicians.
“If you don’t have a continuous administration, then a new one comes in [with] different ideas, then they forget what happened to the last one,” Granert bemoans. “That’s the political side of it. But the thing is: the technical side keeps going.”
This is where he and his team at SCWFI, along with government entities like DENR and scholarly bodies like UPRI, come into play. By providing accurate information and getting the news out to the people in a form in which they can understand, they activate the community to take action in delaying the subsidence of their areas.
Granert’s wife calls it “laymanizing” material. “It’s hard to talk to somebody [about] what’s going to happen five years from now. You have to do it in more immediate terms,” he says. “You have to put up an information system that can be understood at a barangay level, so people will go: ‘Oh, this is where I live in Metro Cebu, and this is what’s going to be affected.’”
As urbanization continues to shape Cebu’s landscape, the need for comprehensive environmental planning becomes even more urgent. Experts and officials alike are calling for stronger regulations, better infrastructure, and a unified effort to prevent further subsidence and flooding. The challenge now lies in translating these warnings into actionable policies that protect both the land and the communities that depend on it. With the groundwork laid by studies and initiatives, the hope is that Cebu can strike a balance between development and sustainability before it’s too late.
