A sensor including a substrate and a sensing element disposed on a substrate is provided. The sensing element includes a sensing matrix in contact with a flow of water, an indicator for one or more chemical species in the flow of water, and a selectivity component that reacts reversibly with the one or more chemical species. The sensor also includes a light source configured to direct light through the substrate and the sensing element. The sensor further includes a light detector configured to receive transmitted light from the substrate and the sensing matrix and to generate a signal representative of selective wavelengths of the light indicative of the one or more chemical species in the flow of water.