Water ingress is a common and costly problem in tunnel construction, and understanding the hydrogeological characteristics of a site is crucial for mitigating its effects. This study presents a detailed analysis of the water ingress problems experienced during the construction and operation stages of the Lowari Tunnel in Khyber Pakhtunkhwa, Pakistan, and evaluates the suitability of the water for various purposes. The study found that the water quality varied significantly, depending on the geological conditions and water quantity, with the south portal of the tunnel dominated by Mg cations and bicarbonate anions. The water was found to be suitable for tunnel support systems, including concrete and steel installations, with negligible corrosion observed over the study period. However, the water coming out of the tunnel at the south portal was found to be unsuitable for drinking due to its low pH value. The Wilcox plot classified the water samples as excellent for irrigation, which could benefit local agriculture in the area. The findings of this study can provide valuable insights into the hydrogeological characteristics of the Lowari Tunnel, contributing to the design and construction of similar infrastructure projects in the region. Furthermore, these insights can help improve access to safe and reliable water sources for local communities.