Geoenvironmental assessment of abandoned mines and quarries in South-western Nigeria

Abstract

Environmental degradation due to abandoned mines and quarries in South-western Nigeria was assessed on the basis of measured physical parameters namely acidity and conductivity and concentrations of heavy metals and other trace elements in collected water and soil samples vis-à-vis minerals mined and site by site basis. The toxicology of trace elements which concentrations were enriched compared to their upper continental crustal abundance was reviewed and each site ranked on the basis of computed Integrated Pollution Index. Trace element abundance in soil samples in relation to geochemical affinities and minerals mined were also investigated. The physical impacts were based on slope failures, subsidence, mine openings as well as change in vegetation.Overlying strata have collapsed into mine voids in the form of sinkholes in some locations while there were mine openings and cavities of significant physical hazard in the form of threat to the general population. Natural vegetation has been replaced by secondary forest with emergent trees in some areas while vegetation in some other locations was sparse and scattered. There were elevated concentrations of Fe and Mg in surface and groundwater in abandoned quarries and mines respectively; and Al, Fe, Mn and Mg in surface water samples in abandoned mines. The concentration of some trace elements in soil samples was considerably enriched compared to their upper continental crustal abundance indicating intensive past mining activities. However at the tourmaline abandoned mines, all trace elements except uranium have concentrations below their upper continental abundance. The computed Integrated Pollution Index indicated that mining has contributed more to environmental degradation than quarrying and strong acidity due to mining has resulted to groundwater impairment in one of the locations.