Article pubs.acs.org/est
Micropollutant Dynamics in Vidy BayA Coupled HydrodynamicPhotolysis Model to Assess the Spatial Extent of Ecotoxicological Risk Florence Bonvin,† Amir M. Razmi,‡ David A. Barry,‡ and Tamar Kohn*,† †
Environmental Chemistry Laboratory, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland Ecological Engineering Laboratory, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
‡
S Supporting Information *
ABSTRACT: The direct discharge of effluent wastewater into Vidy Bay (Lake Geneva) results in the formation of an effluent plume with locally high concentrations of wastewater-derived micropollutants. The micropollutant hotspots above the wastewater outfall present a potential ecotoxicological risk, yet the spatial extent of the plume and the associated ecotoxicological risk zone remain unclear. This work combines the two main processes affecting the spreading of the plume, namely dilution of micropollutants due to mixing and degradation by photolysis, into a coupled hydrodynamicphotolysis model, with which we estimated the spatial extent of the risk zone in Vidy Bay. The concentration of micropollutants around the wastewater outfall was simulated for typical wind scenarios and seasons relevant to Vidy Bay, and the resulting ecotoxicological risk was evaluated. Specifically, we determined the direct and indirect photolysis rate constants for 24 wastewater-derived micropollutants and implemented these in a hydrodynamic particle tracking model, which tracked the movement of water parcels from the wastewater outfall. Simulations showed that owing to thermal stratification, the zone of ecotoxicological risk is largest in summer and extends horizontally over 300 m from the outfall. Photolysis processes contribute to reducing the plume extent mainly under unstratified conditions when the plume surfaces. Moreover, it was shown that only a few compounds, mainly antibiotics, dominate the total ecotoxicological risk.
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period. As such, the findings cannot be generalized to the whole year, nor do they indicate how specific stratification and wind conditions affect plume spreading. The sampling effort needed to adequately capture the plume spreading under specific meteorological conditions and during different seasons would be a costly and tedious task. Measurements of conservative wastewater tracers such as electrical conductivity can be applied to map the plume, but they do not capture the degradation of micropollutants as the plume spreads from the outfall. As an alternative, models are increasingly used in the field of environmental risk assessment of organic micropollutants to overcome the limitations of sampling or to augment punctual environmental measurements.3,4 To model the dispersion of the wastewater-derived micropollutant plume and the associated ecotoxicological risk zone, a thorough understanding of the various processes affecting the fate of micropollutants is essential. On the one hand, physical
INTRODUCTION Recent work revealed that the direct discharge of wastewater effluent into Lake Geneva’s Vidy Bay results in the formation of an effluent plume, which does not mix readily with the surrounding water column, thus causing locally high concentrations of wastewater-derived micropollutants.1 The authors showed that the plume depth followed the thermocline, which moved deeper over the course of the seasons. In the absence of thermal stratification, between November and January, the plume surfaced or was not detected due to enhanced mixing of the water column. The locally elevated concentrations of micropollutants above the wastewater treatment plant (WWTP) outfall were found to present a potential ecotoxicological risk. However, the spatial extent of the zone bearing an ecotoxicological risk remains unclear. Additional sampling of the Bay would be necessary to better characterize the risk zone. For example, recent sampling within Vidy Bay revealed that over the course of six weeks, the plume could be found over an area of at least 1 km2 surrounding the WWTP outfall.2 These results, however, represent an aggregation of different sampling days and thus reflect the totality of wind and stratification conditions encountered during the sampling © 2013 American Chemical Society
Received: Revised: Accepted: Published: 9207
March 26, 2013 July 15, 2013 July 19, 2013 July 19, 2013 dx.doi.org/10.1021/es401294c | Environ. Sci. Technol. 2013, 47, 9207−9216
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Article
pollutants in a model. The coupled model was used to estimate the concentration of micropollutants in the vicinity of the WWTP outfall under the dominant meteorological conditions encountered in Vidy Bay. Specifically, we determined the direct and indirect photolysis rate constants for 24 wastewater-derived micropollutants and implemented them into a particle tracking model, which followed the movement of water parcels from the WWTP outfall through Vidy Bay. The ultimate goal of this study was to spatially and temporally define the area of potential ecotoxicological risk exerted by micropollutants in Vidy Bay under the dominant meteorological conditions and to determine the contribution of photodegradation to attenuating this risk.
mixing processes contribute to the dilution of micropollutants, and in Vidy Bay, these are largely influenced by local and embayment scale mixing processes (i.e., turbulent mixing and currents).5 On the other hand, biotic and abiotic degradation and removal processes affect micropollutant concentrations6 within the Bay. The hydrodynamics of water masses are the object of a recent study investigating the effect of dominant meteorological conditions on the currents in Vidy Bay.5 In this study, water flow in Lake Geneva was modeled using a 3D finite-difference hydrodynamic model (Delft3D-FLOW), forced by highresolution meteorological data (COSMO-27). The simulations revealed that the main currents in the Bay were largely affected by wind conditions and were generally parallel to the shore (eastward and westward) above the WWTP outfall. Moreover, certain wind conditions were found to result in the formation of a gyre.5,8 Aside from dilution, photolysis, biodegradation, sorption, and volatilization processes may influence micropollutant concentrations. The compounds of interest for this study were selected in part due to their low removal efficiencies during wastewater treatment.9 Accordingly, these compounds are likely not susceptible to sorption to settling particles or biodegradation. Indeed, sorption can be considered significant only for very hydrophobic compounds with log Kow > 5.5,6,10 which is greater than the log Kow for the compounds considered herein (Table S.1, Supporting Information (SI)). Little to no removal was observed for the large majority of the targeted micropollutants during wastewater treatment,11 indicating that biodegradation is not a relevant process either. This notion was confirmed in biodegradation experiments conducted in water collected above the WWTP outfall in Vidy Bay (see SI for details). Finally, given their low Henry’s law constants (Table S.1, SI), the chosen compounds are unlikely to undergo volatilization.6 For these reasons and according to past work,12−14 photodegradation can be expected to be the main elimination mechanism of these compounds over the time scale relevant for the mixing of wastewater effluent within Vidy Bay (generally 1, red) and precautionary risk zone (0.1 < RQm < 1, orange) for the four scenarios and two weather conditions (cloudy, left panel; sunny, right panel) in the layer of release (0 to 2 m depth for unstratified conditions and 14 to 16 m depth for stratified conditions). (a) Vent-unstratified, (b) Vent-stratifed, (c) Bise-unstratified, (d) Bise-stratified. RQm, mixture risk quotient (eq 7). Arrows indicate the wind direction, and axes show the latitudinal and longitudinal coordinates (Swiss Grid system with datum CH1903).
entrapment of the plume at 15 m depth, photolysis processes contributed only slightly to reducing the potential risk zone, with a reduction of 5 and 27% of the affected area under stratified Vent and Bise conditions, respectively. The predicted extent of the ecotoxicological risk for these specific scenarios compared well to more recent field measurements of micropollutant concentrations around the WWTP outfall
during lake stratification. The potential risk zone was found to extend ca. 400−500 m around the WWTP outfall when considering the totality of all measurements obtained during the sampling campaign (June to August 2011).2 Under well-mixed conditions (Figure 4a,c), the area of potential adverse effects (red) was significantly smaller. Under the influence of Vent (Figure 4a), this zone was slightly bigger 9214
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ACKNOWLEDGMENTS Alfred “Johny” Wüest and Benoit Crouzy are gratefully acknowledged for internal review of the manuscript and Dominique Grand-Jean for laboratory assistance. This project was funded by the Swiss National Science Foundation (Project nos PDFMP2-123028/1 and PDFMP2-123034/1) and the éLEMO-project.
compared to the Bise (Figure 4c). Under these conditions, the area of potential risk could be reduced by up to 77% (unstratified Bise) owing to strong photodegradation of the surface plume. The mixture toxicity near the WWTP outfall was dominated by five substances, namely, in order of the importance of their contribution, the antibiotics ciprofloxacin, sulfamethoxazole, and azithromycin; carbendazim, an urban pesticide; and diclofenac, an anti-inflammatory drug. With increasing distance from the source, we observed a shift in the compounds that dominate the overall risk, owing to photodegradation. For instance, under unstratified conditions, 1 km from the WWTP outfall, sulfamethoxazole, azithromycin, and carbendazim still contributed significantly to RQm, but other compounds such as paracetamol, propranolol, and carbamazepine became increasingly relevant. Environmental Implications. This work illustrates the importance of photolysis as well as the key factors affecting the fate of organic contaminants in aquatic environments, namely, solar irradiance (influencing photolysis rates and lake stratification), water absorbance (influencing photolysis kinetics), and mixing processes (influencing distance of the contaminant from the surface). These factors were also found crucial in determining the extent of the wastewater plume in Vidy Bay. The coupled hydrodynamic-photolysis model allowed for the identification of critical scenarios. Simulations show that owing to the thermal stratification, the zone of ecotoxicological risk is largest in summer and extends horizontally over 300 m from the WWTP outfall. Although the affected area is small, the environmental implications of such localized ecotoxicological risk zones are still largely unknown, and the precautionary principle should be applied. Finally, modeling the ecotoxicological risk helped to pinpoint priority substances which contribute significantly to increasing the zone of potential adverse effects. Several antibiotics contributed significantly to the total ecotoxicological risk. Antibiotics thus exhibit a dual problematic role in the environment: not only may they promote the well-known problem of antibiotic resistance but they are also among the most relevant compounds with respect to detrimental ecotoxicological effects. Clearly, these are the substances that should be the focus of future monitoring campaigns and pollution mitigation strategies.
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ASSOCIATED CONTENT
S Supporting Information *
List of target compounds and properties, map of Vidy Bay, biodegradation experiments, wind analysis, direct and indirect photolysis calculations, model parameters and calculations, photodegradation results, and modeling results (time to steady state, depth profiles, plume extent). This information is available free of charge via the Internet at http://pubs.acs. org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: tamar.kohn@epfl.ch. Notes
The authors declare no competing financial interest. 9215
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