Groundwater quality is crucial for rural communities, especially in regions where sanitation practices directly influence aquifer health. This study evaluates the impact of seasonal changes on the physico-chemical characteristics of groundwater in Sheikhpura village, Patna, Bihar, India, with particular attention to the role of onsite sanitation systems. Water samples were systematically collected from hand pumps and shallow wells over three distinct seasons—pre-monsoon, monsoon, and post-monsoon. Each sample was analyzed for pH, temperature, electrical conductivity (EC), total dissolved solids (TDS), total hardness, alkalinity, nitrate, chloride, phosphate phosphorus, and major ions (Na⁺, K⁺, Ca²⁺), adopting standard APHA (1998) protocols. The results reveal pronounced seasonal variations in water quality, with contamination levels peaking during the monsoon, attributed to increased surface runoff and leaching from sanitation sources. Greater fluctuations in parameters were observed in summer and rainy seasons, indicating vulnerability of groundwater sources to contamination due to dynamic recharge and dilution patterns. The study demonstrates the need for regulated distances between drinking water sources and sanitation structures, enhanced sanitation design and routine monitoring to mitigate risks to safe drinking water. The aim of this research is to inform policy decisions and guide local interventions for safe water supply by highlighting seasonal vulnerabilities in sanitation-influenced rural aquifers.