Mercury pollution in sediments of Hijo River draining the gold processing plants in Tagum, Davao del Norte [Philippines]

The uncontrolled use of mercury in the small scale gold mining activities in Eastern Mindanao [Philippines], resulted to mercury pollution in the bodies of water draining the mining area. This study was focused on the lower reaches of the Hijo River draining the gold processing plants in Apokon, Tagum, Davao del Norte, and its estuary in the Davao Gulf. Thirty (30) sediment samples from the river and estuary were collected, and subjected to grain size fractionation and analyzed for their mercury content and mineralogical composition. The grain size analysis showed that the sediments from the estuary contains higher percentage of fine materials (clay) compared to that of the sediments from Hijo River. This is due to the higher transport energy of the river water compared to that in the estuary allowing mostly the coarser sediments (sand, coarse silt) to be deposited in river beds while most of the finer materials (medium silt, fine silt, clay) were deposited in the estuary. The results of the chemical analysis revealed that mercury is not homogeneously distributed in the different sediment grain sizes. Mercury concentration increases as particles size decreases. In general, mercury concentration is highest in clay and lowest in sand. The sediment samples from the estuary contain higher mercury concentrations than the sediments from the river. This is due to the fact that the estuarine sediments contain more clay materials and organic matter than that of the river sediments. Mercury occurs in the sediments as adsorbed particle in clays and organic matter. This was also confirmed by the mineralogical analysis using XRD. Mercury pollution assessment using I sup geo and comparison with the Sediment Quality Criteria for the Protection of Aquatic Life indicates that the study area is moderately to excessively polluted, and exceed the minimal effects Hg threshold value, and the toxic effects threshold value in some areas