Dieldrin Contamination of the Groundwater in a Former US Military Base (Clark Air Base, Philippines)

A comprehensive monitoring program was undertaken for the groundwater wells at Clark Air Base, a former United States military facility in the Philippines that operated until 1991. It was established that the pesticide dieldrin continued to exceed drinking water standards (<0.03 mu g/L) in several wells even after 16 years since the chemical was last used. Historical dieldrin readings exhibit episodic high and low values throughout the year, indicating that values are primarily controlled by seasonal variations in rainfall. June typically registers the highest dieldrin concentration (0.27 mu g/L), which then tapers off during the rest of the rainy season due to continued dilution. Surface sediment sampling at the sites, which in 1997 showed extremely high dieldrin concentrations (11 mg/kg) are not detected in more recent monitoring studies. This is interpreted as being partly due to the degradation of the compound, but more probably due to dieldrin's percolation into the deeper underlying sediments, which is then flushed into the aquifer during, every rainfall event. All wells that have shown contamination have been decommissioned. However, monitoring continues in all water producing wells to ensure that water of suitable quality is supplied to all current end-users. Finally, all previous studies on contamination at Clark Air Base are unpublished, and, therefore, this research is one of the first significant documentations of contamination in a military facility in the country.