GEOENVIRONMENTAL EVALUATION OF ABANDONED MINE SITES IN JOS AREA, PLATEAU STATE, NIGERIA

Abstract

The geoenvironmental evaluation of mine sites in Jos area of Plateau State, Nigeria was carried out to assess the extent of environmental degradation as a result of mining activities in the study area. The terrain mapping was carried out using the Shuttle Radar Topographic Mission (SRTM) to determine the impact of mining activities on landscape of the study area. The derived impact of mining on land use/cover of the study area between 1975 and 2014 were also determined to assess the extent of degradation in the study area. A time series analysis of Landsat MSS satellite image November 1975, Landsat TM satellite imagery acquired in 1988, Landsat ETM+ acquired October 2001, and Landsat OLI acquired in 2014 of the study area were used. Landuse/ landcover were classified from the images using a hybrid manual and spectral based approach. The geochemical and radiological investigations of mine wastes as well as soil and water samples of the study area were carried out. These samples were analysed at the Geochemistry Laboratory of the Nigerian Geological Survey Agency (NGSA) in Kaduna, Nigeria. The physico-chemical properties as well as heavy metal concentration of water were determined using methods stated in APHA (2005). The results were compared with World Health Organisation (WHO) standard to evaluate the portability of the water samples. The degree of acidity as well as heavy metal concentration in soil and mine waste samples were evaluated in accordance with the provisions of United States Environmental Protection Agency, (USEPA). The geo-accumulation index (Igeo) of heavy metal in soil were thereafter determined. Samples of soil and mine wastes were analysed for 40K, 226Ra and 232Th using a sodium iodide (NaI) radiation detector while water samples were analysed for gross alpha and beta activity using MPC 2000 radiation counter in accordance with the provisions of International Atomic Energy Agency, IAEA (1996) at the Centre for Energy Research and Training (CERT) Zaria. The digital terrain map revealed that mining activity was concentrated around the central part of most of the mine locations in the study area. The edges of the study area had high land-terrain with steepness down their respective centres. The result shows that out of the 1,523 sq.km total size of the study area, the degraded area/land, built-up area, water bodies and exposed rock outcrop increased by 238.2 sq.km (15.68%), 71.4 sq.km (4.68%), 0.9 sq.km (0.06%) and 220.9 sq.km (14.5 %) respectively while arable farmland and forest reserve decreased by 430.8 sq.km (28.29%) and 100.6 sq.km (6.63%) respectively. It was discovered that the water bodies in the study area were not suitable for consumption due to low pH of 5.2 - 6.6 in addition to the undesirable concentration of heavy metals above the threshold limits approved by WHO as a most of the water sources had been affected by the mining activities. The Igeo in the soil of the study area are of the order Fe > Mn > Cu > Cd > Cr > Pb > As. The soil and the water bodies in the area contained toxic contaminants like Cd, Cr, Pb and As. The high Igeo of 1.3 - 2.2 and 1.4 - 2.1 for arsenic and lead respectively in soil could lead to their accumulation in food crops due to water intake by plants from the soil. This could have serious health effects, including mental retardation in children. These may cause diseases not only to humans but also to the basic links in the ecosystem foodweb as well as the environmental niches. The alpha and beta activity recorded in most locations were above the control value as well as the maximum permissible activity of 0.1 Bq/l and 1.0 Bq/l set by the WHO. The annual effective doses of radiation recorded in all the soil and mine waste samples were above the control value, the 1mS/yr limit for general public set by the International Commission for Radiological Protection (ICRP), the average world annual effective dose of 0.07mSvy-1 and the average Nigeria annual effective dose of 0.098 mSvy-1. The source of livelihood of the locals is mostly farming. Mining of tin and associated minerals in the study area has resulted in a high degree of degradation of arable land, vegetation and landscape. It is recommended that the mine wastes in the study area be properly controlled and confined away from the public so as not to expose those living and working in the area to chemical and radiological incidents. The present use of water from the mine ponds for irrigation and other domestic and industrial purposes and of mine tailings for baking and frying as well as for plastering of houses should be discontinued as these could result in exposure to radiation from the Naturally Occurring Radioactive Materials (NORMs). People in close proximity to the mines should also be dissuaded as this expose them to high concentrations of heavy metals and radiations. Further research endeavour on the reclamation of the mined out pits and the remediation of the study area is also recommended.