Water quality monitoring for heavy metal contaminants is currently performed using instrumental techniques that rely on random “grab sampling” where samples are collected on-site and then transported to a central laboratory for analysis. Alternatively, there are some portable instruments which are suitable for monitoring heavy metals in the field. However, both approaches require a trained technician and provide only a snapshot of the water quality at a particular time and place. Far preferable would be a measurement approach that might enable 24/7, on-demand monitoring in real time, and in this work we describe progress on a novel electrochemical sensor system that might make such applications feasible. Specifically, we report advances in a new technique termed anodic stripping coulometry (ASC) in which the charge associated with the deposition and stripping of the contents of a fixed-volume, thin-layer sample cell is used to provide, viaFaraday’s law, an absolute determination of metal content. As(III) was chosen as the target analyte, and a double potential step (DPS) procedure for in situ background correction has been developed, which does not require the use of a separate blank solution. With the DPS-ASC approach, response for As(III) is linear between 10 0–1,000 ppb with a detection limit at the 75 ppb level. Accurate results were obtained both for lab samples containing high levels of other metals, such as Cu(II), Cd(II), Pb(II), and Zn(II), and for river water samples spiked with As(III). While further work remains to be done, this represents important progress in the development of ASC-based sensors suitable for calibration-free, on-site heavy metal monitoring networks.