An example system includes a separator that accepts and separates a multiphase flowback effluent stream into a plurality of secondary streams. A first sensor assembly monitors the multiphase flowback effluent stream and generates a first signal corresponding to at least one characteristic of the multiphase flowback effluent stream. A second sensor assembly monitors one of the plurality of secondary streams and generates a second signal corresponding to at least one characteristic of the one of the plurality of secondary streams. A signal processor receives the first and second signals and determines a mass flow rate of a substance present in both the multiphase flowback effluent stream and the one of the plurality of secondary streams.