Application of a reactive transport model for interpreting non-conservative tracer experiments: The Rio Maior case-study

The application of analytical groundwater solute transport models is presented in a case-study in the white-sand aquifer of Rio Maior, Portugal. Two simultaneous experiments were performed using a conservative and non-conservative tracer. The experiments were carried out using an innovative technique developed by Lobo-Ferreira (1988, 1989). A measured quantity of two different tracers one being considered an ideal tracer and the other a non-conservative tracer were injected simultaneously in 1 m³ of water from the aquifer into a piezometer, during a measured period of time. An injection of 1 m³ of fresh water from the aquifer was injected immediately afterwards. Under regional groundwater flow the tracer returned back to the piezometer where the concentration was monitored for both tracers. The purpose of the experiment is to analyze the different behaviour of two tracers with different properties, under the same hydrogeological conditions, using a reactive transport model. The ideal tracer experimental results obtained served to test if the experimental hydrogeological properties of the aquifer, i.e. the velocity and the longitudinal dispersivity, used in the model, fit with the experimental results. As the model performance was shown to be very accurate, the data obtained for the aquifer hydrogeological properties were used in the non-conservative tracer modelling. The results for the non-conservative modelling were adjusted to the field monitoring by changing the initial settings and comparing with the field curve by an inverse problem procedure. This allowed us to calibrate the non-conservative parameters of the tracer. The results obtained in this study illustrate the different behaviour between conservative and non-conservative tracers. The two models, CANALX, which computes solute spatial profiles, and CANALT, which computes the solute breakthrough curve, provide quantitative tools to analyze the behaviour and the data for adsorbing solute transport in groundwater. Given parameters uncertainties and experimental limitations, we consider the model performance to be very encouraging in this application.