ASSESSMENT OF NATURAL ATTENUATION OF HYDROCARBON CONTAMINATED SITE IN BARUWA, LAGOS STATE

Abstract

Hydrocarbon contamination originating from the activities of man have entered the subsurface environment either on purpose or by accident. The presence and transport of these contaminants constitute a potential threat to human health and ecosystems. Adequate and continuous monitoring of hydrocarbon contaminated sites proves to be the essential key to understanding the behaviour of contaminants once within the subsurface. This research is concerned with characterization and assessment of Natural Attenuation of hydrocarbon contaminated site. The specific study site is Baruwa community in Alimosho Local government area of Lagos state, Nigeria, with a population of over 100,000 people. There are about over 150 wells within the community which have been affected by oil pollution which dates back to 1994, when there was a burst in an N.N.P.C (Nigerian National Petroleum Corporation) Valve pit located at the Federal Housing Estate (Jakande Estate) opposite the study site. This study employed the collection of field data in 2014 from the contaminated site to sufficiently characterize the hydrocarbon contamination within the area. Characterization of the study area was conducted utilizing the methodology prescribed by United States Environmental Protection Agency (USEPA). 25 Hand dug wells within the area were utilized for the study. Water levels and Free Hydrocarbon Thicknesses were measured with the aid of an oil/water interface meter, Total Petroleum Hydrocarbon (TPH) was also measured both in groundwater and soil vapour using a portable Hydrocarbon Analyser. Comparisons were made with data obtained in 2006 from previous studies and Natural Attenuation occurrence established based on Tiers 1 and 2 criteria by USEPA. Decreases in the Light Non-Aqueous Phase Liquids (LNAPL) Thicknesses varying from 86.4% to 99.7% were recorded in comparison with that from previous studies (2006 - 2014). Maximum recorded thickness was measured in 2014 to be 0.49m at BH No. 17 (Shodende). TPH surveys carried out in the groundwater are in agreement with LNAPL flow direction. Decreases in TPH concentrations varying from 33.13% to 57.61% were observed in comparison with earlier studies (2006 - 2014). Maximum TPH reading was obtained in 2014 to be 83.9 mg/l at BH No. 35a (Kamila). Permeability values obtained from slug test carried out onfive wells within the area are in the range of 4.11 x 10-4cm/sec to 5.25 x 10-4 cm/sec which is characterized by Silt, Clay/Silt admixtures or fine Sand/Silt/Clay mixtures. This is also supported by stratigraphic study with the soil profile showing the water bearing aquifer to be within Clayey Sand and Sandy Clay. All the wells investigated within the study area indicated that significant Natural Attenuation has occurred over the period (2006 - 2014). Analysis of all the wells showed that the maximum period for the remediation of the site is 36 years. Advection and dispersion are responsible for the movement and transport of the plume and dissolved contaminants within the subsurface groundwater. Linear velocity (Advection) values calculated ranged from 1.36 – 5.63m/yr, longitudinal dispersion ranged from 37.6 – 156 m2/yr. The results provide further information, adding to the existing database of the level and progress of the contamination and delivers proof of Natural Attenuation (remediation) within the area.