In underground low-pressure gas pipelines, a leak may result in a complication more dangerous and difficult to detect, known as the water ingress problem. Groundwater enters the gas pipeline through a crack, and eventually blocks the gas flow. Vibration sensors are used to detect the water ingress problem. The sensor output indicates a marked increase in the occurrence of spikes from the vibration sensor once the water ingress starts. Since the signal is not much greater in magnitude than the baseline signal it is difficult to definitively detect. The estimation error from a Kalman filter is used to detect the occurrence of water ingress and with multiple sensors even the location of the water ingress can be detected.