Report Number: 113
Year: 2006

Impact of Metal-Enriched Leachate from Ordot Dump on the Heavy Metal Status of Biotic and Abiotic Components in Pago Bay

Pago Bay was suspected of being the final resting place for heavy metal contaminants discharged into the Lonfit River from the Ordot Dump. To test this hypothesis, surface sediments (top 1-3 cm) were collected from 44 sites within the bay in January 2005. Thirty-two of these sites were located intertidally at ~100-m intervals along the beach. The remaining sites were positioned at ~100-200-m intervals along five transect lines running perpendicular to the shore. Later in the year, (June-September 2005), dominant biotic representatives (algae, seagrass, sea cucumbers and bivalves) were taken for analysis from 48 sites within the bay in order to identify any metal uptake abnormalities that might be occurring in the resident fisheries resources. Biota collection sites were largely confined to the inner moat and outer reef flat. The organisms selected for study were chosen because of their known or suspected bioindicator potential and their ability to reflect biologically available metal fractions in specific segments of the environment. Algae, for example, only respond to the soluble metal fraction in the water column, whereas seagrasses derive their metal load mainly from sediment pore waters via their root systems. Sea cumbers are sediment ingesters, while bivalves obtain their trace metal load from both suspended particulates and the soluble fraction in the surrounding water. Data from such a suite of organisms thus provides the investigator with a means of differentiating between sediment-bound and soluble metal fractions and rating the relative importance of each.

The findings of the study are discussed with reference to heavy metal levels previously determined in water and sediments from the Lonfit River and in leachate from the Ordot Dump. They are also compared with values for clean and polluted coastal sediments and biota from tropical environments elsewhere. It was concluded that metal concentrations in the biotic and abiotic components of Pago Bay are generally low by world standards and largely reflect natural contributions associated with the alluvial discharges from the Pago River (volcanic detrital material), and groundwater intrusion. Localized areas of light enrichment were noted for lead, mercury and zinc in shoreline sediments at the northern end of the bay and are likely associated with the type of wastewater disposal systems currently servicing the Marine Lab and WERI. A highly localized area of moderate lead enrichment was noted at the southern end of the bay away from any obvious signs of illegal dumping or other potential source of lead contamination. Metal levels in biotic representatives from these sites, though marginally elevated in some species, remained well within the ranges typical of relatively clean environments.

The study clearly demonstrates that Pago Bay is not a permanent sink for sediment bound metal contaminants mobilized downstream from the Ordot Dump. We therefore conclude that any contaminated sediments deposited in and around the river mouth, the reef channel and the southern half of the bay during a normal wet season, are re-suspended and flushed from the system by major storms (typhoons) that approach the eastern side of the island. Under such conditions, the reef channel serves as a conduit for their transportation and dispersion into offshore waters beyond the reef margin. Thus the climatic and topographic characteristics of the area combine to provide an effective means of periodically flushing out pockets of contaminated sediments from the entire watershed into the ocean.

Gary R. W. Denton
Walter C. Kelly III
Rick H. Wood
Yuming Wen