IN-SITU PILOT SCALE REMEDIATION OF HYDROCARBON CONTAMINATED GROUNDWATER (A CASE STUDY OF BARUWA COMMUNITY, LAGOS STATE, NIGERIA)

Abstract

This research is concerned with geo-environmental impact assessment and remediation of contaminated soil and groundwater. The specific study site is Baruwa community in Alimosho Local Government Area of Lagos State, Nigeria. Free hydrocarbon product thickness up to 0.72m was found on top of the groundwater. The researches focuses on one hectare (100m x 100m) of the most contaminated area and remediate the groundwater to permissible level using the state of the art equipment and technology. Detailed hydraulic and geochemical characterization of the field site subsurface soil was determined. The Hand dug wells are between 0.75m and 2.00m in diameter, groundwater is about 25m below ground surface in the wells and the floating petroleum product varies in thickness from sheen to 0.7m. Slug test was carried out on the well system to determine the hydraulic conductivity of the Wells and three models were used to analyze the data from the slug test to determine the hydraulic conductivity of the well system. These models are Hvorslev, Ferris-Knowles and Earth model. The hydraulic conductivity of the aquifer obtained from each of the six wells at the site was 10-4 cm/sec. The results of the hydraulic conductivity show the concentration of the oxidant adopted for the cleanup is moderate and does not have adverse effect on the hydraulic conductivity of the well system that can impede the treat ability of the Contaminant of Concerns (COCs). Skimming Technology was used to extract the light non-aqueous liquid (LNAPL) from the subsurface. Also, Total Petroleum Hydrocarbon (TPH) of the collected samples was measured on site with the PHA-100 Hydrocarbon Analyzer. In-Situ Chemical Oxidation (ISCO) commenced with the injection of 50gm/litre potassium permanganate (KMnO4) as the oxidizing agent into the six wells. Key parameters such as pH, Dissolved Oxygen (DO), Biological Oxygen Demand (BOD), Total Organic Carbon (TOC), Total Dissolved Solid (TDS), Nitrate, Nitrite and sulphate in the groundwater. The pH values ranged within 6.5-8.0 standard units in the pilot test area, the DO increased significantly after injection of oxidant and reduced per time as remediation progresses. Conductivity increased shortly after injection of oxidant and later reduced significantly with values that are less or equal to 1000 μS/cm (≤1000 μS/cm ) which is the Maximum contaminant level for United States of Environmental Protection Agency (USEPA). Significant decline was observed in TDS values during the first 2 months of ISCO process for all the monitoring wells. Although, the TDS values for all the monitoring wells are not within the mandatory limit value of 500 mg/l as specified by USEPA. The TOC values reduced significantly and fall below the permissible limit (2 mg/l) as specified by World Health Organization (WHO) in all the monitoring wells in the pilot test area. There were drastic reductions in the values of TPH from a maximum of 500 ppm to 20, 0, 25, 6, 6, 0.7 ppm for the six boreholes. Multiple Linear Regression analysis using the Ordinary least Square (OLS) estimate was used to determine the physico-chemical factors affecting the Total Petroleum Hydrocarbon (TPH) in groundwater samples during ISCO remediation. It was further revealed that while DO, Conductivity, TDS, TOC and Nitrite had a positive significant effect on the TPH of the monitoring well groundwater sample after ISCO remediation, BOD and Nitrate had a negative effect on TPH of the well water sample during the same ISCO remediation, the coefficient of multiple determinations which explain the goodness of fit of the regression model (R2) was 0.924, which indicate that 92.4% of the changes in TPH of the groundwater during ISCO remediation were accounted by the physicochemical parameters included in the model. Biostimulation process was carried out at Baale’s well (W20) and Pa Oyewole’s well (W41) with the injection of NPK Fertilizer amendment (Nitrogen, Phosphorous and Potassium) at Ratio of 1:1:1 with a concentration of 50gm/litre. Monitoring and sampling was carried out at the two wells periodically for 3 months. Evidence of the occurrence of bioremediation in the monitoring wells included depletion of dissolved oxygen (DO) from 2.3 mg/l to 1.5 mg/l, and 3.4 mg/l to 1.2 mg/l for Baale’s well (W20) and Pa Oyewole’s well (W41) respectively, also decrease in the value of nitrate from 1.4 mg/l to 0.99 mg/l, 1.78 mg/l to 1.32 mg/l for Baale’s well (W20) and Pa Oyewole’s well (W41) respectively. The depletion of DO and Nitrate value indicate that the microbes involved in the bioremediation of petroleum hydrocarbon can adapt to aerobic and anaerobic condition