Design Blueprint: Well Rain-Ingress Witness Logger — Build Specification Derived from the Well Ingress Mixing and Alarm Paper

DESIGN BLUEPRINT — SIMULATED, NOT MEASURED — NOT BUILT. A complete build specification for a well rain-ingress warning instrument: a submersible low-cost conductivity and temperature logger sampling every 10 min (EC resolution 1 µS/cm, temperature resolution 0.05 °C), with an alarm rule derived from the mixing physics of the source paper — an EC drop of at least 4 µS/cm against a trailing 24 h rolling median, sustained for 2 h (12 consecutive samples) — firing at an ingress of 0.81% of the well volume: 19.0 L in a 2,356 L hand-dug well (1.0 m diameter, 3.0 m water column) and 2.85 L in a 353 L borehole (0.15 m diameter, 20.0 m water column). Derived from the source paper: https://doi.org/10.5281/zenodo.24108770. The single-sample EC detection floor is 0.20% of well volume (4.72 L hand-dug; 0.71 L borehole); the alarm produced zero modelled false alarms over an 89-day scored baseline (largest modelled drop 2.80 µS/cm against the 4 µS/cm threshold) 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 25 L every 4 h pattern). The blueprint gives the design basis quoted from the source paper, instrument specification and sensor placement for hand-dug well and borehole variants (probe 0.5 m below the surface, near the intake, depth fixed), a bill of materials in generic specification classes, an installation sequence including the 24 h baseline warm-up, the operating envelope with derived alarm and floor volumes for stored volumes from 200 L to 5,000 L and for six well geometries, critical fits tied to the source figures, failure modes and mitigations, a condition-based maintenance schedule, and the verifications a first deployment must make. Stated at full size throughout: the instrument witnesses ingress only — it never detects pathogens — and ingress below the 0.81% alarm fraction (including a complete 0.5% event) passes unwitnessed at alarm level; read as a trend instrument in coastal wells it detects saline intrusion at drift-limited floors of 0.79 µS/cm per week over 30 days and 0.48 µS/cm per week over 90 days, provided drift is managed by reference checks. The source paper states no power consumption, battery capacity or cost, and this blueprint states none: power is builder's choice within the envelope of continuous 10 min sampling for the full adopted logging duration (90 days reference; 180 days for the longest trend use). Nothing has been built or measured; every derived quantity is reproducible from the accompanying compute.py.