Impacts of mercury contaminated mining waste on soil quality, crops, bivalves, and fish in the Naboc River area, Mindanao, Philippines.

Rice paddy fields in the Naboc area, near Monkayo on the island of Mindanao, Philippines, have been irrigated four times a year over the last decade using Naboc River water contaminated with mercury (Hg) by artisanal gold mining in the Diwalwal area. Silt containing up to at least 90 mg Hg/kg (d.w.) has been deposited in rice paddy fields during the 1990s and this has been repeatedly mixed into the rice root zone through ploughing. Hg in the rice paddy field soils averages 24 mg Hg/kg and generally exceeds the UK and Canadian soil quality thresholds for agricultural soils as well as the proposed Dutch Intervention value and the USEPA soil ingestion Soil Screening Level (SSL) for inorganic Hg. Much lower Hg concentrations (0.05-0.99 mg/kg) within the range expected for uncontaminated soils, characterise soils on which corn and bananas are cultivated, largely because these are not irrigated with Hg-contaminated water from the Naboc River. The estimated total weekly MeHg intake for a person living in the Naboc area related to the weekly consumption of 2.1 kg of rice grown on Hg-contaminated soils (15 microg MeHg) in conjunction with 1 kg of fish (220 microg MeHg) and 100 g of mussels (50 microg MeHg) from the Naboc River, would total 285 microg MeHg (equivalent to 4.75 microg/kg bw for a 60 kg adult), which is nearly three times the JECFA PTWI of 1.6 microg/kg bw. This will significantly contribute to the population mercury exposure and might explain why 38% of the local inhabitants were classified as Hg intoxicated during a mercury toxicity assessment [Drasch GS, Böse, O'Reilly S, Beinhoff C, Roider G, Maydl S. The Mt. Diwata study on the Philippines 1999-assessing mercury intoxication of the population by small scale gold mining. Sci Total Environ 2001; 267(1-3): 151-168.].