Heavy Metal Geochemistry of Acid Mine Drainage in Onyeama Coal Mine, Enugu, Southeastern Nigeria

Abstract

This paper is aimed at evaluating the chemistry of the Onyeama Coal Mine and Ekulu River with respect to their heavy metal content. Onyeama Coal Mine is one of Nigeria’s oldest coal mines. Production activities stopped there in 1978 due to constant flooding of the mine with very acidic water. pH values range from 2.8 to 4.1. The rainy/dry season cycles affects the chemical composition of the drainage. Major elements characteristics of the AMD include high sulphates (>300mg/l) dissolved sulphides (1.4mg/l)) and high iron (between 8.4mg/l to 73.11mg/l). Chemical analysis for lead (Pb²⁺); iron (Fe²⁺); arsenic (As³⁺) and cadmium (Cd²⁺) showed that the AMD and Ekulu River are heavily polluted with respect to these heavy metals. The average value for lead (Pb²⁺) was 278.59mg/l which is 18570 times greater than the maximum contaminant level suggested by United States Environment Protection Agency. Iron (Fe²⁺) had an average of 25.89mg/l which is 86times greater than the maximum contaminant level. Arsenic (As³⁺)’s average value is 21.63mg/l which exceeds the stipulated limit by 2163 times while cadmium Cd²⁺ (34.14mg/l) exceeds it by 7000 times. These anomalous concentration values for heavy metals is caused or enhanced by the low pH of the waters. The v. of AMD discharged into Ekulu Rivers is much and with these elevated concentration of heavy metals, the Ekulu River will pose great health risks to those who use it for recreation or laundry.

Key words: acid – mine drainage; Onyeama coal mine; heavy metals; maximum contaminant levels.