by Abbey Nastan Wednesday, July 8, 2015
Supertyphoon Haiyan, which battered the Philippines in November 2013, was the most powerful hurricane to make landfall ever recorded. One recent study suggested behemoths like Haiyan, which had maximum sustained winds of about 310 kilometers per hour, might warrant the creation of a new hurricane classification, “Category 6.” In addition to the thousands of deaths and casualties, flattened homes and widespread debris, Haiyan left other, less visible signs of damage in its wake, including to the fragile coastal aquifers that many residents rely on as their sole source of drinking water. Now, a new study has quantified damage done to these environmental resources, and how long it might take for them to recover.
Often, freshwater aquifers near shorelines are naturally sensitive to contamination from infiltrating seawater, yet they are vital sources of potable water for hundreds of millions of people. While seawater infiltration of groundwater due to gradual sea-level rise has been a concern in climate science for more than a decade, aquifer contamination due to short-lived storm surges has not been studied as much.
“It’s rare to focus on groundwater [soon after a storm event] because the surface devastation is so visible,” says M. Bayani Cardenas, a geoscientist at the University of Texas at Austin and lead author of the study, published in Geophysical Research Letters. But with the collaboration of several Filipino scientists, Cardenas and his colleagues were able to begin a field study just two months after Supertyphoon Haiyan.
The study focused on the town of San Antonio on Samar Island, where the storm surge reached 7 meters above sea level, and which has no piped water or sewage system. Residents rely exclusively on household or community wells tapping the two aquifers underlying the town, and on septic systems. Most of the wells draw from a lower limestone aquifer about 10 meters down that’s thought to be sealed, or confined, above by impermeable rock, while some draw from a shallower sandy aquifer 3 to 5 meters underground that is unconfined, meaning surface water can penetrate it freely. Within a few hours of the storm surge receding, residents discovered that the wells were contaminated with seawater. Survivors were forced to subsist on other, scant sources of water, such as coconuts, for three days until aid arrived, Cardenas says.
When Cardenas and his colleagues arrived in San Antonio in January 2014, they first determined the extent and depth of the storm-surge flooding, plotting inundation maps of the area. Next, they analyzed the salinity of 34 of the community’s 300-plus wells, finding more than 90 percent undrinkable based on U.S. Environmental Protection Agency standards, which allow a maximum of 1.3 percent seawater content for potable water. The median seawater contamination in the wells tapping the deeper, confined aquifer was 4.5 percent, reaching a maximum of 17.6 percent.
To study the unconfined aquifer, the authors dug test wells in a line inland from the shore, finding seawater contents ranging from 2 to 8 percent. Using measurements of electrical resistivity — which gauges how conductive, and in this case how salty, materials are — Cardenas and his colleagues also mapped the seawater contamination in the upper aquifer. This showed a saltwater layer 2 meters thick just below the groundwater table. Modeling the flow of this salty layer led them to predict that the seawater plume would likely sink and gradually be flushed toward the sea.
When the team returned to San Antonio six months later, a second electrical resistivity survey showed that the saltwater layer in the unconfined aquifer had already descended 1 to 2 meters and traveled toward the ocean. From the findings, the researchers predicted that the upper part of the unconfined aquifer should be drinkable within one to two years, but that the lower portions will become more saline over the same time and could take five to 10 years to recover.
Cardenas' team also found that San Antonio’s deeper aquifer had freshened considerably over the six months between measurements, with the median saltwater content in the wellwater dropping to just 0.8 percent. The rapid freshening was probably due to the seaward flow of groundwater through the lower aquifer, as well as residents pumping nonpotable water from the wells, the researchers noted.
The freshening also suggests that the lower aquifer was contaminated during the storm surge via the wells themselves, rather than through sustained infiltration of seawater as in the case of the upper aquifer. However, as the denser seawater plume sinks to the base of the unconfined aquifer over the coming years, the deeper aquifer might be at risk of recontamination, Cardenas says, because the extent and permeability of the rock sealing it from above is not well known.
“This study provided rare field measurements … of a major ocean overtopping event,” says Adrian Werner, a hydrogeologist at Flinders University in Australia. As a case study, it should help aid agencies respond to the growing threat from powerful hurricanes, he says. Jie Yang, a hydraulic engineer at Leibniz University in Germany, agrees, adding that it’s also rare for groundwater studies to combine geophysical field work and numerical modeling.
As for mitigating the potential for seawater contamination during storm surges and tsunamis, Cardenas says that raising well mouths off the ground and using better seals could help. However, this won’t prevent another problem associated with storm events — one with which San Antonio residents are still coping after Haiyan. “I’ve been emphasizing that the wells are fresher, not cleaner,” Cardenas says. Most of the wells may still be undrinkable due to bacterial contamination caused when the storm surge carried sewage from septic tanks into the aquifers. The ultimate solution, Cardenas says, would be providing communities like San Antonio with reliable piped water.
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