An electrochlorination and electrochemical system for the on-site generation and treatment of municipal water supplies and other reservoirs of water, by using a custom mixed oxidant and mixed reductant generating system for the enhanced destruction of water borne contaminants by creating custom oxidation-reduction-reactant chemistries with real time monitoring. A range of chemical precursors are provided that when acted upon in an electrochemical cell either create an enhanced oxidation, or reduction environment for the destruction or control of contaminants. Chemical agents that can be used to control standard water quality parameters such as total hardness, total alkalinity, pH, total dissolved solids, and the like are introduced via the chemical precursor injection subsystem infrequently or in real time based on sensor inputs and controller set points.