1. A computer-implemented method for estimating a fluid flow velocity with a volumetric flow measurement device, comprising: receiving, with a processing device, a plurality of observations from the volumetric flow measurement device corresponding to a concentration of a constituent of a flowing fluid mixture; and computing, with the processing device, a final estimate of an average velocity of the flowing fluid mixture based at least in part on the observations so as to provide an estimate of the volumetric flow rate without necessitating a dedicated flow measurement device, wherein the constituent is undergoing a chemical reaction and the computing implements a reactive transport model comprising the following partial differential equation: ∂ C ⁡ ( x , t ) ∂ t = - u ⁡ ( t ) ⁢ ∂ C ⁡ ( x , t ) ∂ x + K ⁡ ( u ⁡ ( t ) ) ⁢ C ⁡ ( x , t ) , wherein u is the velocity of the flowing fluid mixture, C(x,t) is the concentration of the reacting constituent at a location, x, and time, t, and the concentration decays according to the first order constant, K.