In this study, the temporal trends of 14 drinking water quality parameters in groundwater wells across Gilan Province were evaluated over a 10-year period (2013–2022) using the non-parametric Mann–Kendall test and Sen’s slope estimator. Data were obtained from the Statistical Database of Gilan Water and Wastewater Company, and one representative well was selected from each county. The Mann–Kendall test was used to determine trend significance, and Sen’s slope was applied to estimate the annual rate of change. Results indicated that parameters such as nitrate, electrical conductivity (EC), total dissolved solids (TDS), and iron showed both increasing and decreasing trends across different areas. Sodium, sulfate, calcium, total hardness, and manganese exhibited increasing trends, while magnesium, nitrite, and potassium showed decreasing trends. No significant trend was observed for chloride and bicarbonate. Out of 224 examined cases, 34 trends (15%) were statistically significant, including 18 increasing and 16 decreasing trends. Nitrate showed the highest number of significant trends (11 cases). The highest annual rate of change was observed in EC 12 and nitrate 1.2 mg/l in Rezvanshahr and Fuman counties, respectively.