This study investigates seasonal variations in groundwater quality and heavy metal contamination in five oil-exposed rural communities—Dema, Ibotirem, Samanga, Ajakajak, and Udungama—within the Isiokwan District of Andoni Local Government Area, Niger Delta, Nigeria. Groundwater samples were collected from hand-dug wells in each community during the dry, wet, and post-rainy seasons and analysed for physicochemical parameters and toxic metals. Parameters assessed included pH, turbidity, electrical conductivity (EC), total dissolved solids (TDS), dissolved oxygen (DO), biochemical oxygen demand (BOD), and chemical oxygen demand (COD), while concentrations of lead (Pb), cadmium (Cd), arsenic (As), and iron (Fe) were measured using Atomic Absorption Spectrophotometry. Results showed that pH remained acidic across all seasons, with values ranging from 5.19 to 5.42. EC and TDS were highest during the dry season, indicating concentration effects due to reduced aquifer recharge, while turbidity and organic pollution indicators increased during the wet and post-rainy seasons, respectively. Pb, Cd, and As concentrations consistently exceeded WHO safety thresholds in all samples and seasons, posing serious public health risks. Iron exceeded aesthetic limits but remained within health-based guidelines. The study concludes that the hand-dug well water in the investigated communities is unsafe for direct human consumption without treatment. The findings reveal critical groundwater vulnerability due to hydrocarbon pollution and seasonal hydrological shifts, highlighting the urgent need for integrated water safety planning, well protection, and strengthened environmental regulation in oil-producing regions of the Niger Delta.