By Ellalyn De Vera-Ruiz
Land subsidence due to excessive groundwater utilization has contributed to the worsening Metro Manila flooding, experts said.
According to Kelvin Rodolfo, professor of Earth and Environmental Sciences at the University of Illinois in Chicago, “it is true that global warming is causing the sea level to rise, but only about 3 millimeters per year. That amounts to only a little more than one inch over 10 years.”
Residents board a boat as rescue personnel of Brgy Tatalon in Quezon City wade through flood waters along Araneta Ave after continuous heavy rains caused chest deep flood in the area, August 3, 2018. (Mark Balmores / MANILA BULLETIN)
He said the overutilization of groundwater has led to subsidence due to loss of support below ground, which eventually aggravated flooding in some parts of Metro Manila.
“The big cause of worsening flooding is land subsidence because people are withdrawing too much water from wells. That causes sinking by as much as six inches every year,” he pointed out.
Many areas in Metro Manila are still reeling from floods as torrential monsoon rain swept through the region over the weekend.
Rodolfo explained that population growth has exerted high pressure on groundwater resources causing the city to “sink.”
“Metro Manila’s groundwater demand is still increasing. Consequently, subsidence will continue and may even accelerate,” he said.
Geologist Narod Eco of the University of the Philippines-National Institute of Geological Sciences cited a research conducted from 2003 to 2011, which found that some areas in Metro Manila have been sinking by two to four centimeters every year.
Ground subsidence have been evident in Caloocan, Malabon, Navotas, Valenzuela, Manila and Las Piñas.
Outside Metro Manila, ground subsidence has also been observed in parts of Bulacan and Pampanga; and San Pedro, Biñan, Dasmariñas and Rosario in Cavite during the same period.
Eco said subsidence is occurring fast in Caloocan, Malabon, Navotas and Valenzuela.
Rodolfo pointed out that rapid subsidence of coastal lands is enhancing the risk of flooding and high tides. He added that storm surges, which is an ever-worsening threat is due in part to climate change but also from subsidence.
In East Asia, he cited Tokyo and Osaka in Japan; Shanghai, China; Yun-lin, Taiwan; Hanoi, Vietnam; Bangkok, Thailand; and Jakarta, Indonesia as also ‘sinking’ due to excessive groundwater utilization.
“We believe the subsidence was caused by over-extraction of groundwater, likely from industries, agriculture and aquaculture,” Eco said.
“To address the problem, first step is to ban deepwell pumping of groundwater in affected areas. Where it's already banned, authorities should closely monitor compliance,” Eco added.
He pointed out that it is likely that “even if groundwater levels return to past levels, subsidence could be permanent.”