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 (1) having a sample receiving area (2) for receiving a sample of aqueous medium to be sampled and which comprises at least three electrodes (5, 6, 7) each comprising a layer of conductor deposited upon an insulating substrate. The system also includes a power source (13) 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, eg. Ammonia NH 3 , and output means (19) to output data corresponding to the current generated thereby when a sample is in place on the sampling area. In a preferred embodiment, differential pulsed square wave voltammetry is used.