Determination of migration rates of contaminants in a hydrocarbon-polluted site using non-reactive tracer test in the Niger Delta, Nigeria

Abstract

A natural gradient tracer test with sodium chloride as the injectate was conducted using one injection well and eleven observation wells in an oil-contaminated site in the study area. The aim was to characterize the site and to determine migration rates of the contaminants to provide information on their fate in groundwater resources in the area. The migration of the tracer was constantly monitored by sampling all the observation wells spaced 5 m from each other at 15–30 min intervals for a period of 6 days. The test soil layer is a clayey silty sand consisting of 50 % sand, 30 % silt and 20 % clay. The average static water level at the site is about 2.5 m. Gas chromatography fingerprinting showed that kerosene and diesel are the dominant fractions within the soil matrix. The arrival times of the breakthrough moments of the migrating tracer plume from two observation piezometers situated 2.0 and 7.22 m away from the injection well showed that the mean velocity in the longitudinal direction in the aquifer is 1.403 × 10−3 ms−1 (121 m/day). The longitudinal and transverse dispersion coefficients of 2.45 × 10−3 m2s−1 and 2.3 × 10−3 m2s−1 were used to obtain longitudinal and transverse dispersivities of 1.78 and 1.64 m, respectively. High hydraulic conductivity denoted by the high migration rate of the tracer implies that contaminant at the site will migrate through the aquifer with relative ease. However, the significant clay fraction within the soil provides the medium for adsorption and partitioning of contaminants to the soil.