A portable water treatment system and apparatus is disclosed that can effectively and efficiently treat aqueous fluids by quickly and reliably adjusting and controlling the free residual level of disinfectants, contaminants or additives through the addition of one or more treating agents such as oxidizing chemicals and/or other special-purpose additives, and that can continuously store, log, retrieve and report the related fluid composition data and other operating parameters on a real-time basis at either the use site or a remote location. A preferred use for the subject system and apparatus is managing the chemistry of disinfectant, contaminant and/or additive levels in aqueous fluids used in hydraulic fracturing operations, and controlling the free residual levels of the disinfectant or contaminants within the fluids, including fluids maintained in frac tanks during temporary cessation of a hydraulic fracturing operation.