ILOILO CITY — A recent study on land subsidence in metropolitan cities in the country showed that Iloilo City has recorded significant ground subsidence with rates reaching 9 millimeters (mm) per year.

The University of the Philippines study of "Ground subsidence in major Philippine metropolitan cities from 2014 to 2020" said that land subsidence or the sinking of the ground surface "has become a significant global issue caused by excessive groundwater extraction, rapid urbanization, and natural sediment compaction, exacerbated by climate change through rising sea levels."

In Iloilo City, the most significant subsidence was observed in Mandurriao and its southeastern portion going to the Molo district, and the La Paz district, with rates ranging from 4.0 to 8.8 mm per year using the interferometric synthetic aperture radar.

The study said the areas are key commercial, industrial, and residential hubs, while the southeastern areas are particularly the most densely-populated coastal district in the city.

The study also identified instances of saline water intrusion in wells within the Molo district, likely due to the encroachment of saltwater from the nearby Guimaras Strait. Such intrusion can lead to groundwater contamination, which is difficult to rectify.

"Subsidence in these coastal areas can also worsen tidal flooding within the City Proper district, wherein some areas already experienced gutter to ankle-deep flooding during sunny days due to higher tides," the study added.

In neighboring towns of Oton, Pavia, and Leganes, the subsidence rates, although lower than in Iloilo City, range between 2.6 to 4.7 mm per year.

The subsidence in the less urbanized town of Oton is linked to clustered built-up developments along the coastal region, driven by the rapid population growth and urbanization.

The study also underscored the critical issue of groundwater extraction in Iloilo City, which could further accelerate subsidence rates.

The National Water Resources Board, the agency that regulates water wells through the Water Code, faces a noticeable gap in the frequency and comprehensiveness of groundwater monitoring.

Currently, there are only 34 monitoring wells across the Philippines, with just four located in Iloilo City — a figure deemed as "insufficient," given the numerous registered and unregistered water wells in the country.

The study suggested the use of interferometric synthetic aperture radar for targeted, long-term monitoring, which would allow for more precise tracking of subsidence over time.

Geophysical surveys such as seismic reflection and ground-penetrating radar could also aid in understanding subsurface deformation and the types of aquifers present, helping to assess subsidence permanence.

"These measures aim to guide groundwater management, land-use planning, and mitigation efforts to prevent excessive land subsidence, especially in faulted areas prone to hazards associated with ground sinking," the study added.

The study also determined the maximum subsidence rates in other major cities in the Philippines such as Metro Cebu at 11 mm per year, Legazpi City at 29 mm per year, Metro Davao at 38 mm per year, and Greater Manila Area at 109 mm per year.