The use of mercury (Hg) by artisanal gold miners in SE Asia, sub-Saharan Africa and Latin America has increased significantly in the last decade, and may now exert an important influence on the total anthropogenic flux of contaminant mercury into the environment. A Technology Development and Research (TDR) programme to address the adverse impacts of mining-related Hg contamination was initiated by the British Geological Survey (BGS) in April 1995 under funding from the UK Government Overseas Development Administration (ODA), Engineering Division. The central aims are:
1) to provide a protocol for monitoring the sources, transport pathways and fates of Hg in artisanal gold mining regions.
2) to assess the human and ecotoxicological impacts of Hg pollution in selected case-study areas.
3) to formulate and promote improved methods of gold recovery within the small-scale mining sector.
4) to make practical recommendations for legislative abatement of mining-related Hg pollution.
In July 1995 a pilot study of selected gold-rush areas in Eastern Mindanao was undertaken by BGS personnel in close liaison with the Philippines Mines and Geosciences Bureau (MGB). Pre-existing data, collated by the MGB suggest that the Agusan River basin, the principal catchment in Eastern Mindanao, has received an influx of at least 140 t of Hg as a consequence of escalating artisanal mining activity during the 1980s and early 1990s. The principal objectives of the pilot survey were, therefore, to assess the general magnitude of Hg enhancement recorded in a range of sample media, to evaluate the justification for further, more detailed geochemical and ecotoxicological surveys, and to test a number of alternative field sampling methods.
Hydrochemical and geochemical sampling was undertaken in three gold-rush areas; the Gango/Guintosan Creek area near Cagayan de Oro, and the Mainit and Diwalwal localities in Davao del Norte. The last-named locality supports the largest artisanal mining community on Mindanao, currently numbering around 50,000 (having peaked at approximately 100,000 during the late 1980s). At Gango and Diwalwal, the use of Hg for amalgamation is probably now in decline, due to the local development of CIP cyanidation facilities, operated on a cooperative basis.
Physico-chemical field measurements and multi-element ICP-ES and data for surface waters from the pilot-survey locations provided no indication of gross perturbation, with maximum conductivity values of 310 µS and aggregate heavy metal abundances generally
XRF analyses of major oxides and a wide range of minor and trace elements (excluding Hg) in sediments from all pilot-survey areas produced geochemical signatures which clearly depict lithogeochemical variations (including base-metal mineralisation), but no widespread anthropogenic overprint.
Data showing the concentration of Hg in water, sediment and heavy mineral concentrates (HMC) at Gango, Mainit and Diwalwal were acquired by cold vapour atomic fluorescence spectrophotometry (CVAFS). Data for water from Gango and Guintosan Creek showed no systematic contamination, with only one anomalous value (2.84 µg/l) recorded in outflow water from a CIP plant engaged in the reprocessing of tailings previously treated by amalgamation. Data for sediments from Gango highlight contaminant influences more clearly, values rising to more than two orders of magnitude above the regional background at a sampling station 200 m downstream of the principal adits and processing plant. Acute Hg contamination in the vicinity of the Diwalwal gold-rush area is indicated by the data acquired for all sample media. Surface water values of up to 2906 µg/l were recorded within the settlement area, declining by more than three orders of magmtude over a c. 10 km downstream distance. The HMC component of the drainage sediment was found to be the most enriched medium at the perceived contaminant source, declining by two orders of magnitude 10 km downstream. Concentrations in sediment decline more gradually with distance. with only a two-fold reduction evident (from 23 mg/kg to 11 mg/kg) 10 km from the source. This sample type may, therefore, provide the best available medium for systematic Hg dispersal mapping.
The maximum Hg values recorded in water and, to a lesser extent, sediment from parts of the Agusan River basin compare unfavourably with published data for large gold-rush areas in Amazonia A more detailed geochemical and ecotoxicological appraisal of the Hg hazard in this part of Mindanao is therefore recommended.
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