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In this paper we describe a combination of paper-based sensors and a novel smart-phone application for on-site quantification of colorimetric readouts as an ultra-low cost solution to monitoring water quality. The system utilizes a paper-based analytical device (μPAD) that produces a colorimetric signal that is dependent on the concentration of a specific target; a cell phone equipped with a camera for capturing images of two μPADs – one tested with a water sample and the other tested with clean water that is used as a control; and an on-site image processing app that uses a novel algorithm for quantifying color intensity and relating this to contaminant concentration. The cell phone app utilizes a pixel counting algorithm that performs with less bias and user subjectivity than the typically used lab-based software, ImageJ. The use of a test and control strip reduces bias from variations in ambient lighting, making it …