GEO-ENVIRONMENTAL ASSESSMENT AND REMEDIATION OF CONTAMINATED SITES: COMMUNITY, LAGOS, NIGERIA

Abstract

The relationship between hydraulic, chemical and geotechnical properties is of crucial importance for the transport and fate of contaminants in the subsurface (porous media). This research presents a methodology for physical laboratory aquifer modeling, field studies and development of a numerical solute transport model, for the simulation of the laboratory and field experiments. Three-dimensional (3D) sand tank:(I.8 m x 0.3 m x 0.8 m) experiments were conducted to study contaminant transport and natural attenuation within the sand tank. The model was used to study the transport of an inorganic tracer (Chloride) and petroleum hydrocarbon in groundwater, within a tropical aquifer (porous media) material. The research has resulted in the designing and testing of the laboratory aquifer model (3-D sand tank) for subsurface contamination assessment and remediation techniques evaluation. This facility is now available at the Geotechnical Engineering Laboratory, Federal University of Technology, Akure. Detailed geotechnical, hydraulic and geochemical characterization of the chosen laboratory experimental porous media material (Owena sand) and the field site subsurface soil were performed. This will provide basic experimental data on porous media materials, which would be widely useful to geotechnical/environmental researchers and engineers who are involved in contaminated site assessment and remediation in Nigeria. The Owena sand is a poorly graded sand (SP) with 88.1% Sand and 11.9% Gravel. Relevant geotechnical properties of the sand are; Coefficient of uniformity C, = 2.53, coefficient of curvature C, = 0.181, Hydraulic conductivity K = 5.76 X 10-4m/s, Bulk density p = 1.9 Mg/m3, Effective Porosity n, = 0.215 and Median grain diameter D50= 0.55mm. Other relevant hydraulic and solute transport parameters, such as dispersion coefficients and dispersivity were also established for the soil. The numerical model SandModel was developed and used to replicate the laboratory sand tank model and to simulate field conditions at the contaminated Baruwa, Nigeria site. The model simulations predicted patterns of groundwater flow, chloride tracer flux and petroleum hydrocarbon plume advancement/dispersion in the groundwater zone. The method of finite difference was used to solve the 3-D Advection-Dispersion equation, while the FORTRAN computer language was used to code the algorithm and develop the SandModel computer code. The results were displayed graphically using the MATLAB application package. The Laboratory and Field-Scale SandModel is applicable to conservative and reactive solute transport modeling at the laboratory and field scale configuration and has been validated for porous media materials. The plots of the tracer breakthrough curves (TBCs) for the twelve (12) multi-level observation points within the model domain show a reasonable match of the simulated and observed tracer breakthrough curves. Calculated t (paired-t-statistic (student t quantile) value for each observation well is less than the critical value of t (tcritical = 2.16 for df=13), and the calculated P value for each well is greater than the chosen significance level (p=0.05), therefore the null (no difference) between observed and simulated data sets hypothesis is accepted. The analysis also reveals high to very high correlation (R2=O.79 to R2=O.91) between the observed and simulated values. A sensitivity analysts showed that the time required for complete source depletion, was most dependent on the source definition, biodegradation rate and the hydraulic conductivity K of the porous medium. Soil particle size (D50) and hydraulic conductivity (K) data have been used to establish a correlation between the hydrodynamic transport parameters (Dispersivity [α]) for the laboratory and field porous media soils. The proposed empirical relation for the determination of the field dispersivity [α] parameter is; D50(L) (KCL)/ D50(F) K(F) = α (F)/ α(L) A Retardation factor, R = 2.5, and biodegradation rate of 0.0004/day (half life of 5 years) were obtained from the model calibration for BTEX. A clean-up time for BTEX of about 45 years was estimated by natural attenuation from model predictions. The results are highly relevant in the light of the increasing awareness of the precarious trend of lack of monitoring and remedial feasibility data for the subsoil environment in the thousands of gasoline fuel stations, and petroleum storage/distribution underground infrastructure in Nigeria.