Evaluation of Permeability and Rate of Contaminant Migration within the Subsurface and Heavy Metal Pollution in Ikpeshi, Edo North, Nigeria

Abstract

The uncontrolled expansion of quarrying and marble processing in Ikpeshi, Akoko-Edo LGA, Edo State, Nigeria, raises concerns about degradation of subsurface soil and groundwater quality. This study evaluated subsurface permeability, quantified heavy metal pollution, and examined the relationship between permeability and contaminant migration. The study employed a combined field and laboratory approach. Soil and rock samples were collected from trial pits at 0.5, 1.0, and 1.5 m depths, and water samples were collected from hand-dug wells within 500 m of active pits. Hydraulic conductivity (K) was determined using falling-head and constant-head permeameters per ASTM D4525-90, and porosity per ASTM C97-83. Heavy metals (Pb, Cd, Cr, Fe, Mn, Ni, Zn) were quantified via atomic absorption spectrophotometry. Pollution levels were evaluated using contamination factor (CF), geo-accumulation index (Igeo), pollution load index (PLI), and heavy metal pollution index (HPI), while the influence of permeability on migration was examined using Pearson's correlation and linear regression at p<0.05. The subsurface comprises weathered, fractured Basement Complex materials with low to moderate, anisotropic permeability. K ranged from 8.5 × 10⁻⁷ cm/s at the control (1.5 km away) to 6.8 × 10⁻⁵ cm/s near active pits; porosity declined from 34.2% to 21.7%, and seepage velocity increased from 0.004 to 0.058 m/day. Soil Pb (48.6 mg/kg), Cr (62.4 mg/kg), and Cd (3.1 mg/kg), and groundwater Pb (0.052 mg/L) and Cd (0.006 mg/L) exceeded WHO/NESREA limits. PLI declined from 2.31 at S1 to 0.58 at the control, Igeo from 1.42 to -0.85, and CF from 2.85 to 0.62. HPI at S1 was 118.4 (>100). K correlated positively with seepage velocity (r=0.94, p=0.003), CF (r=0.81, p=0.019) and HPI (r=0.77, p=0.024), and negatively with distance (r=-0.88, p=0.010). Permeability critically governs contaminant migration patterns in Ikpeshi. Therefore, enforcing buffer zones, lining quarry waste, and conducting routine monitoring is imperative. Keywords: Heavy metal pollution, hydraulic conductivity, seepage velocity, contaminant migration, pollution indices, Ikpeshi Edo State, Nigeria.