Kinetics controlling the removal of pharmaceuticals in aquifers: estimates from laboratory and field scales

Presence of pharmaceuticals in freshwater resources is a burning issue in Europe as it is important to understand their impact on human health and living organisms. For this purpose, the mechanisms of their transport, transformation and accumulation in aquatic systems have to be clarified. Up to now there are a number of studies focusing on the estimation of the attenuation of different groups of pharmaceurticals under different physico-chemical conditions at the laboratory scale. Nevertheless, such studies often lack the geochemical modelling part, which allows determining accurately the kinetic properties of pharmaceuticals. In addition, it is of paramount importance to transfer and verify the magnitudes of kinetic parameters obtained from the lab experiments to/at the in-situ field scale. In this study, reactive modelling (PHREEQC) was used to assess the impact of redox conditions on the fate of pharmaceuticals namely, lamotrigine, carbamazepine, irbesartan, metoprolol and atenolol in the shallow sub-oxic part (< 0.5 mg/L) of the redox stratified aquifer of the Besòs River Delta (Barcelona, Spain). A previous multivariate data analysis and mixing modelling study has identified that the presence of irbesartan, metoprolol and atenolol in the studied groundwater is governed by the river inflow. At the same time, lamotrigine and carbamazepine revealed the tendency towards the accumulation in the aquifer but the modalities of the distributions of their concentrations showed that the concentration of carbamazepine represented better all factors that control the hydrochemistry of the studied aquifer system. To clarify and verify the obtained evidence about the dynamics of the above mentioned pharmaceuticals, batch experiments, which mimicked temperature and sub-oxic conditions of the aquifer, were carried out to measure their degradation. The obtained results were used to run the PHREEQC model to define their kinetic parameters (i.e. maximum rate constants, half-saturation concentrations and inhibition constants). The computed values from the lab studies were used as an input for simulating the behavior of lamotrigine, carbamazepine, irbesartan, metoprolol and atenolol at the field scale.  The results of the study helped to obtain a better insight into the attenuation properties of lamotrigine, carbamazepine, irbesartan, metoprolol and atenolol under the sub-oxic conditions of the aquifer. This information assists in the assessment of the potential of those pharmaceuticals to be used as quantitative or semi-quantitative tracers.