Occurrence and modeling of pharmaceuticals on a sewage-impacted Mediterranean river and their dynamics under different hydrological conditions
Highlights
► The occurrence of pharmacological compounds in a Mediterranean river at variable hydrological conditions was studied. ► The impact of the flow changes on the concentrations was assessed using relative sensitivity coefficients. ► A plug-flow model was developed to explain the observed variations in the load of the most relevant compounds analyzed. ► The model takes into consideration the circulating flow, the average upstream emissions and an overall decay constant.
Introduction
Pharmacologically active compounds (PhACs) constitute an environmentally relevant group of compounds due to their increasing consumption and their intrinsic biological activity. Around 3000 different compounds belonging to different therapeutic classes are used in human medicine in the European Union (EU), covering a broad range of chemical structures and physico-chemical properties (Richardson and Ternes, 2005). The main route of entry of PhACs into the aquatic environment is through waste water treatment plant (WWTP) effluents because their generally polar nature makes their removal from WWTPs challenging (Conley et al., 2008). Despite physico-chemical and biological treatment, many PhACs are able to reach surface and ground waters. As a consequence, PhACs are now recognized to be widespread pollutants in the aquatic environment (Petrovic et al., 2010). More than 150 PhACs have been identified in surface, ground and even drinking waters (Benotti et al., 2009). Levels of PhACs detected in WWTP effluents are in the range of μg/L, whereas they are much lower in river and groundwater, generally in the ng/L range (Gros et al., 2010). Nevertheless, little attention has been paid to the transport behavior of these emerging contaminants in surface waters once they are discharged from WWTP into a river. They are transported by water and may be removed from the dissolved phase through adsorption to suspended particles and may accumulate in sediments. Chemical bounded to sediments can be remobilized by re-suspension (Petrovic et al., 2011). Levels of PhACs can also be reduced by biotic and abiotic (e g. photodegradation) natural degradation processes. However, the efficiency of these processes is highly dependent on seasonally fluctuating environmental factors such as sunlight intensity, water temperature, and stream flow.
The Llobregat River (Catalonia, NE Spain) constitutes a typical example of a Mediterranean behavior (Marcé et al., in press), suffering from low flows during normal conditions (5 m3/s) and extraordinary peak events (maximum recorded of 2500 m3/s). In addition, the river receives the effluent discharges of more than 55 WWTPs, and at some places the effluents may represent almost 100% of the total flow, especially during drought periods. This fact can explain the high levels of emerging organic contaminants detected on the river including PhACs, increasing together with the volume of effluent discharged by WWTPs when moving downstream along the river (Ginebreda et al., 2010). Furthermore, according to the predictions of the Intergovernmental Panel on Climate Change (IPCC) (Christensen et al., 2007), such tendency is expected to increase in the medium/long term in the Mediterranean area (Acuña and Tockner, 2010).
As far as contamination is concerned, and as a result of the hydrological situation above described, different physical phenomena may occur at the same time: first, the lack of dilution during water scarcity periods may increase the concentration of pollutants; second, and working in the opposite direction, low flows increase the hydraulic residence time, thus facilitating natural degradation processes (Lam et al., 2004); finally, floods may contribute to remobilization of pollutants from sediments (Petrovic et al., 2011).
In this context, the present study aimed (a) to trace the presence of PhACs in sewage impacted surface waters in the lower course of the Llobregat River as a representative example of a stressed Mediterranean River, and (b) to determine some quantitative relationships between levels of PhACs and flow under different hydrological conditions. To this end, we applied a rough modeling approach based on the plug-flow model as proposed by Pistocchi et al. (2010), in order to have a preliminary quantitative assessment on (a) the load of each pollutant generated by the sewage systems upstream from the point under control, and (b) the overall observed decay of the different compounds in the river channel.
Section snippets
Basin and site description
The Llobregat River is the second longest river in Catalonia (NE Spain), with a total length of 156 km and a catchment area of 4957 km2. Its hydrology is characterized by a high variable flow, which is strongly influenced by seasonal rainfall. The mean annual bulk precipitation is 3330 hm3 and it has an annual average bulk discharge of 693·106 m3. The year-round hydraulic conditions are characterized by several peak flow events that are highly variable, from 50 m3/s on May 2004, to 1 m3/s on March
Occurrence
More than 50 of the 73 target compounds were present in all analyzed samples (Table S-3). Concerning therapeutic groups (Fig. 2), anti-inflammatory drugs were generally the most ubiquitous compounds and the therapeutic group with the highest total concentration along the river section studied and throughout the monitoring campaigns. Levels of anti-inflammatories ranged from 200 to 1100 ng/L in ABR, whereas in SJD the range of concentrations was from 200 to 1800 ng/L. Individual concentrations of
Conclusions
Occurrence of pharmaceutical compounds in Mediterranean rivers like the Llobregat is subjected to a pronounced seasonal variation. This fact can be partially explained in terms of the extreme flow changes characteristic of the Mediterranean hydrology. For instance, in the case studied flow peaks during the rainy season exceeded by up to two orders of magnitude with the base river flow directly affecting concentration. However several other concurrent phenomena (eventually operating in opposite
Acknowledgments
This work has been funded by the Spanish Ministry of Economy and Competitiveness through the projects VIECO (2008 009/RN08/011), SCARCE (Consolider-Ingenio 2010 CSD2009-00065) and CEMAGUA (CGL2007-64551/HID). It reflects only the authors' views and the European Community is not liable for any use that may be made of the information contained therein. Merck is acknowledged for the gift of LC columns. SP acknowledges the contract from the Ramón y Cajal Program of the Spanish Ministry of Economy
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