ABSTRACT

This study was conducted with the aim of determining the vertical movement of metal contaminants in a free drainage lysimeter at Dareta village, Anka Local Government Area of Zamfara State in northern Nigeria. A free drainage lysimeter was designed and constructed for collecting soil leachates from lead polluted soil. Lead movement in the soil was simulated using CHEMFLO-2000 computer simulation programme. Saturated hydraulic conductivity of soil samples were found to be slow (0.19 g cm-3), moderately slow (1.61 g cm-3), to moderate (4.64 g cm-3), with mean values ranging from 0.19-4.64 cm sec-1. Bulk density () of the soil surface (0-20 cm) of the study area were classified as moderate to high across the sampling coordinates. Mean values recorded were moderate to high with a range of 1.40-1.73 g cm-3. The mean total pore spaces ranges from 37.73-47.30% in the upper 0-20 cm depth. The vertical distribution of Pb with the highest concentration was 2025 mg kg-1 in Dareta North at the surface soil located near the gold ore processing site. Farms in Dareta South recorded lower values than Dareta North with values ranging between 1310-1586 mg kg-1. Efficiency of leachates collection in the free drainage lysimeters installed in soil profiles across the sampling direction ranged from 40.03-50.65% while discharge values obtained ranged from 8.01-10.13 li per 3 days interval for a period of fourty days. The results of concentration of leachates collected with field drainage lysimeters compared with the laboratory soil columns shows the quantity collected from the former method were significantly higher than those from the laboratory soil columns. The contamination of soils was a result of processing of weathered lead-rich gold ores (604-2025 mg kg-1) and results from this research showed that the soils of the farmlands are highly contaminated with Pb. Leaching losses of Pb contaminants towards groundwater will increase with time as the drivers involved in solute transport in this study are identified as soil texture, porosity, concentration of contaminant and frequency of irrigation. Results from simulation using CHEMFLO-2000 modelling software indicate that Pb contaminants flux increases with increase in time. A flux density of 9.1 x 10-5 cm hr-1 was obtained at the initial stage of the simulation (0) and increased rapidly when the time of interest increased to 12.1 x 10-5 cm hr-1. While results for estimated values of Pb flux, distance of travel and breakthrough time using convective flux equations in Dareta village suggests that Pb travel through the soil down the groundwater at an increasing flux as this will require immediate measures to curtail this. As the break through time needed for Pb to travel at a distance of 3.46 x 10-3 cm hr-1 with a flux of 3.81 x 10-4 through the soil to the ground water (with at distance of 10 m from the ground) after ten years if all things being equal is estimated to be 289080 hr-1.