A Rain-Ingress Witness for Hand-Dug Wells and Boreholes: Mixing-Derived Detection Floors, Alarm Behaviour and Lead Time from a Low-Cost Conductivity and Temperature Logger

Design study (simulated, not measured) of a conductivity/temperature logger that witnesses storm-driven surface-water ingress into wells. A well-mixed model of dilute ingress (100 uS/cm, 25 C) into well water (600 uS/cm, 17 C) gives a single-sample EC detection floor of 0.20% of well volume (4.72 L in a 2,356 L hand-dug well; 0.71 L in a 353 L borehole). An alarm rule (EC drop of 4 uS/cm vs a trailing 24 h median, sustained 2 h) fires at 0.81% ingress, with no false alarms in a seeded 89-day scored baseline simulation, and gives 2.61 h of warning before the end of a 5%-in-6-h design storm (0.61 h before the next draw in the stated pattern). Drift-limited saline-trend detection is 0.48 uS/cm per week over 90 days. The device witnesses ingress; it does not detect pathogens.