Arsenic contamination of reduced shallow aquifers, as in deltas around the world, including Bangladesh and India has been found to frequently co-occur with dissolved ferrous iron. In this study the deployment of smart phone Fe test kit for identification of arsenic-affected villages was found to be a powerful, quick screening instrument for problem mapping in areas dependent for their water supply on tubewell groundwater from arsenic-contaminated, reduced aquifers. The research consisted of laboratory investigation of smart phone test kit accuracy under different conditions (light, exposure time, smart phone brand) and a proof-of-concept field study by screening 365 tube-wells in a potentially arsenic-affected area in Bhojpur, Bihar (India).