Groundwater remains a primary source of domestic water supply in Lafia Local Government Area (LGA), yet its quality is increasingly threatened by rapid population growth, inadequate sanitation, and changing hydroclimatic conditions. This study assessed the physicochemical and microbiological characteristics of groundwater during the rainy and dry seasons using 20 representative boreholes and hand-dug wells. Parameters measured included pH, electrical conductivity (EC), total dissolved solids (TDS), major ions, trace metals, and microbial indicators (Escherichia coli and total coliforms). Results were evaluated against the World Health Organization (WHO, 2022) Guidelines for Drinking-water Quality and the Nigerian Standard for Drinking Water Quality (NSDWQ, 2015). Findings show widespread exceedances of permissible limits, notably extremely high concentrations of lead (Pb), cadmium (Cd), chromium (Cr), and arsenic (As), and frequent detection of E. coli. Rainy-season samples exhibited greater microbial contamination and turbidity, whereas dry-season samples recorded higher EC, hardness, and toxic metal concentrations. The combined seasonal trends highlight both anthropogenic contamination—primarily from sanitation sources—and geogenic mobilization of heavy metals. The study concludes that groundwater in Lafia poses substantial acute and chronic health risks, requiring urgent interventions including water safety planning, point-of-use treatment, improved well protection, and strengthened regulatory monitoring.