Environmental, physico-chemical and hydrogeophysical investigation of a wetland dumpsite in Lagos State, Nigeria.
Engineering geological, physicochemical and hydrogeophysical investigations were carried out on a wetland dumpsite in Epe, Lagos State, Nigeria. This was with a view to investigating the impact of the dumpsite leachate on the physicochemical properties of surface and groundwater in the typical wetland environment. 2-D Dipole-Dipole profiling and 1-D Vertical Electrical Sounding (VES) techniques were carried out along five traverses established within the study area. The 2-D imaging adopted 10 m inter-electrode spacing and expansion factor (n) varying from 1-5. The data acquired were inverted into 2-D resistivity structures using the DIPRO for window V. 4.0 software. The VES were conducted with the Schlumberger array. The depth sounding curves were interpreted quantitatively using the partial curve matching technique and computer assisted 1-D forward modeling with the RESIST software. Geoelectric sections were generated from the VES interpretation results. Eighteen (18) soil profile samples were collected from four (4) boreholes. These samples were analyzed for hydraulic/geotechnical properties using standard methods. Forty (40) water samples were collected from 10 stations (including 2 controls) comprising 5 each of surface and groundwater sources during the rainy and dry seasons of 2014 and 2015. The samples were analysed for physicochemical and bacteriological parameters using relevant reagents and equipments including colorimeter, turbidity meter, UV-Visible Spectrophotometer, and Atomic absorption Spectrophotometer. The lithological log revealed that the study area was underlain by intercalations of sand and clayey sand. The subsoil was characterised by low bulk density (mean of 1.89±0.162 g/cm3); moderate porosity and permeability (mean of 37.84±8.42% and 3.0 x 10-4± 1.1 x 10-4 cm/sec); low moisture content (mean of 17.57±15.18%); and fine to medium grained texture. The coefficient of uniformity value of < 6 was recorded for these samples. AK and KQ type curves with increasing layer resistivities at shallow depth were observed within virgin areas while the H, QH and HA with decreasing layer resistivities within the upper two to three geoeletric layers were observed in the premises of the dumpsite. VES interpretation results delineated 3 – 4 subsurface layers. 2-D images also delineated two geologic layers with resistivity values 212 to 2165 Ωm. These values generally decrease towards the waste dump and unreclaimed wetland portion. South - southwestward groundwater flow direction from the waste dump was established. The overall mean and standard deviation of physical parameters were Temperature 27.6±1.4oC; Turbidity 21.0±17.0 NTU; Conductivity 905.8±1038.6 μS/cm; TDS 463.6±523.5 mg/L; and TSS 30.2±39.4 mg/L. Chemical parameters were pH 6.2±0.6; Acidity 169.0±181.4 mg/L; Hardness 645.2±782.5 mg/L; DO 2.1±0.8 mg/L; and BOD 212.0±83.8 mg/L. Mean values for major ions in mg/L were Na+62.1±32.2; K+ 11.7±4.6; Ca2+ 279.3±352.9; Mg2+ 103.1±127.1; NO3- 42.3±29.8; SO42+129.8±107.9; PO42- 29.8±26.2; and Cl- 285.5±279.2. Mean heavy metals concentrations in mg/L were Cr 0.8±0.8; Fe 1.8±2.0; Pb 0.3±0.3; Ni 0.1±0.1; Mn 3.7±6.3; Cd 0.0±0.0; Co1.0±0.8; Cu 0.3±0.3 and Zn 8.8±5.7. Bacteriological parameters in cfu/ml were THB 2664500.0±1811930.0 and TC 4477.4±7081.0. The study concluded that shallow water table and thick column of unconsolidated sandy formation typical of wetland condition influenced waste dissolution and migration of leachate thereby seriously polluting water resources around the investigated dumpsite.