A sensor system and method for measuring the concentration, or indicating the presence or presence at a predetermined level of, a target contaminant species in an aqueous medium are described. The system is based upon a sensor element having a sample receiving area for receiving a sample of aqueous medium to be sampled and which comprises at least three electrodes each comprising a layer of conductor deposited upon an insulating substrate. The system also includes a power source to apply a pre-determined potential difference across two of the electrodes determined by the potential associated with an electrochemical reaction characteristic of the target species, and output means to output data corresponding to the current generated thereby when a sample is in place on the sampling area.