Estimating the transport parameters of propyphenazone, caffeine and carbamazepine by means of a tracer experiment in a coarse-gravel unsaturated zone

Abstract

Among the emerging contaminants today, pharmaceuticals are some of the most demanding chemical compounds when it comes to understanding their transport within aquifers. The transport of pharmaceuticals in an unsaturated zone is influenced by many factors, including compound sorption and degradation, which is essential in assessing contaminant migration in soil and groundwater. Coarse-gravel aquifers are particularly important for drinking water sources and industrial water supply. Globally, little data on the transport characteristics of coarse-gravel unsaturated zones is available. However, such data is crucial to understanding the transport of pollutants and to implementing the appropriate management strategies to protect the aquifers. In this article, we present tracer experiments employed to determine pharmaceutical transport parameters in the coarse-gravel unsaturated zone. The tracer experiment was performed as a multi-tracer exercise, where deuterated water was infiltrated as a conservative tracer to define the characteristics of the unsaturated zone, together with pharmaceuticals (propyphenazone, caffeine and carbamazepine) as reactive tracers. Based on the breakthrough curves measured at various depths, inverse modelling in combination with analytical and numerical methods (HYDRUS-1D) was performed. Hydraulic parameters for the unsaturated zone were estimated. Linear sorption coefficients (Kd) and degradation half-lives (t_1/2) were evaluated for each pharmaceutical. In the unsaturated zone of the coarse-gravel aquifer caffeine has the lowest sorption capacity (mean Kd = 0.027 Lkg^-1), while the sorption coefficient of propyphenazone is higher (Kd = 0.07 Lkg^-1). Results for the degradation constant of the first order and t_1/2 indicate that caffeine has the fastest decay rate (highest t_1/2 = 69.3 days), followed by propyphenazone (highest t_1/2 = 92.4 days). The parameters for carbamazepine could not be determined using an advection dispersion equation.

Keywords: Coarse-gravel aquifer; Pharmaceuticals; Tracer experiment; Transport parameters; Unsaturated zone; van Genuchten model.