In situ estimation of parameters in chemical models describing groundwater pollutant transport and fate is considered. During tracer tests, the observed breakthrough of a conservative species yields information characterizing the hydraulic behavior of a multiple‐well system. A streamline advective solute transport model incorporating the chemical model of interest is scaled so that it applies to all streamlines in the flow system. The hydraulic and chemical data are coupled through a combination integral which calculates the production well effluent concentration of a reactive tracer as a function of time. This mathematical model is combined with the measured effluent data and a nonlinear estimation procedure to estimate the parameters in the chemical models. The procedure is tested in controlled laboratory experiments. A later paper will present a field application.