Small­scale Gold Mining in the Ambalanga Catchment, Philippines: Its Control on Mercury Methylation in Stream Sediments

Elevated mercury (Hg) concentrations have been found in the Ambalanga Catchment, Benguet, where smallscale gold mining (SSM) is the main source of livelihood. To determine the distribution of total mercury (THg) and methylmercury (MeHg), and the possible physical controls on Hg methylation in the watershed, stream sediment sampling in the eight subbasins of the Ambalanga Catchment was conducted on two periods: lowflow (LF) stage in November 2001 and highflow (HF) stage in July 2003. Results revealed high THg concentrations in three subbasins (Acupan, Dalicno, and Sangilo) in the 2001 period and six subbasins (Acupan, Dalicno, Sangilo, Surong, Gold Creek, and Upper Ambalanga) in 2003. The increase in contaminated subbasins is attributed to the increase in river discharge which resulted in increased erosion of contaminated sediments. Significantly high concentrations of MeHg were found in two subbasins (Acupan and Lucbuban) in 2001 and four subbasins (Acupan, Dalicno, Sangilo, and Upper Ambalanga) in 2003. The high MeHg may be driven by the enhancement of the methylation process in areas with large amounts of finegrained sediments that accumulated in lower hydrologic gradient (and ensuing increased organic activity) close to and at some distance downstream from the point sources.