Groundwater contamination near automobile workshops is a growing environmental concern due to the leaching of heavy metals and other pollutants into the soil and water sources. This study aims to assess the physicochemical and heavy metal characteristics of groundwater in the Central Business District of Oxbow Lake, Swali, Bayelsa State, before and after treatment. Physicochemical parameters, including pH, total dissolved solids (TDS), electrical conductivity, temperature, chemical oxygen demand (COD), biological oxygen demand (BOD), and heavy metal concentrations (Fe, Cu, Pb, Ni, and Mn), were analyzed before and after treatment. The results show that pH increased from 6.80 ± 0.02 to 7.70 ± 0.02, remaining within the permissible limits of 6.5–8.5 (WHO) and 6.6–9.0 (NSDWQ). Total dissolved solids (TDS) ranged from 122 ± 1.5 to 141 ± 1.5 mg/L, and conductivity reduced from 238 ± 2.0 to 221 ± 2.0 µS/cm, both well below the regulatory limits. Temperature varied marginally from 27.4 ± 0.1°C to 27.8 ± 0.1°C. Chemical oxygen demand (COD) and biological oxygen demand (BOD) increased from 0.58 to 1.06 mg/L and 0.45 ± 0.01 to 0.71 ± 0.01 mg/L, respectively. Heavy metal analysis revealed that Iron concentrations were significantly reduced after treatment, from 0.36 ± 0.01 mg/L to below the detection limit (<0.001 mg/L) slightly exceeded the WHO and NSDWQ limit of 0.3 mg/L, while manganese (0.14–0.42 mg/L) exceeding the WHO guideline. Copper concentrations dropped below the detection limit (<0.001 mg/L) after treatment, from an initial 0.50 ± 0.01 mg/L. Both nickel and lead were below the detection limit (<0.001 mg/L) before and after treatment, indicating zero contamination from these metals. Despite the low concentration observed for the various parameters, continuous monitoring and control measures is recommended.