Hydrocarbon contamination and public health risks: A comparative groundwater quality and microbial load analysis in Abeokuta and the Niger Delta, Nigeria

Ensuring safe water for present and future generations requires constant monitoring of quality. This study evaluated groundwater from 19 oil pollution-prone communities in the basement complex terrain of Abeokuta and the sedimentary terrain of the Niger Delta, Nigeria. A total of 87 groundwater sources were analysed for temperature, electrical conductivity (EC), total alkalinity, total hydrocarbon content (THC), biological oxygen demand, hardness (MH, CH, TH), bacterial load (BL), faecal coliforms, pathogenic bacteria and protozoans using standard methods. Most parameters complied with regulatory standards, except THC (0.79 ± 0.84–895.00 ± 91.92 mg/L), TH (262.74 ± 32.75–632.79 ± 3.71 mg/L), and BL (nil - 590,000 cfu/mL). The THC levels in Abeokuta (223.50 ± 21.92–895 mg/L) greatly surpassed those of the Niger Delta (0.79 ± 0.84–3.53 ± 1.21 mg/L) indicating point-source pollution. Principal Component Analysis identified THC and EC as the dominant factor (PC1: 61.80% variance) influencing water quality. Microbiological contamination was widespread: 93.1% exceeded Standard Organization of Nigeria microbial limits while faecal coliforms were detected in 38% of groundwater samples. Fourteen bacterial species were isolated, with Klebsiella pneumoniae (52.9%), Bacillus subtilis (39.1%), and Escherichia coli (36.8%) most prevalent. The protozoan parasite Entamoeba histolytica was present in 26.44% of samples. The Water Quality Index classified 81.67–100% of sources as unsuitable for direct use. Overall, populations in Abeokuta face a dual threat from extreme hydrocarbon and microbial contamination, while the Niger Delta faces risks from high hardness and pervasive pathogens. These findings underscore the urgent need for regulatory enforcement, pollution control, improved sanitation, and public health education to safeguard groundwater resources.