Methods for evaluating in-stream attenuation of trace organic compounds Jeffrey H. Writer a,b,⇑ , Steffanie K. Keefe a , Joseph N. Ryan b , Imma Ferrer b , Michael E. Thurman a,b , Larry B. Barber a a US Geological Survey, 3215 Marine Street, Boulder, CO 80303, United States b University of Colorado, Department of Civil, Environmental, and Architectural Engineering, 428 UCB, Boulder, CO 80309-0428, United States article info Article history: Available online 23 March 2011 abstract Wastewater treatment plants are often the most substantial contributor of trace organic compounds including pharmaceuticals, steroidal hormones, and surfactants to surface waters. Studying stream reaches below wastewater treatment plants provide valuable information on the environmental persis- tence of these compounds. Three methods for conducting field investigations to evaluate in-stream atten- uation of trace organic compounds are presented: (1) using intrinsic tracers in wastewater, (2) environmental sampling coupled with dye studies to assess travel times between sample locations, and (3) Lagrangian sampling. Advantages and limitations of each method are discussed, along with key findings from several investigations. Published by Elsevier Ltd. 1. Introduction The identification of a multiplicity of trace organic compounds in surface waters worldwide, including pharmaceuticals, steroidal hormones, and surfactants illustrates society’s profound influence on water resources. Environmentally observed concentrations of these compounds have been observed to adversely impact aquatic organisms (Jobling et al., 1998; Kidd et al., 2007; Vajda et al., 2008). The majority of previous research on environmental fate of trace organic compounds has focused on individual mechanisms (e.g. biodegradation, photolysis, sorption), generally in controlled labo- ratory experiments (Liu and Liu, 2004; Matsuoka et al., 2005; Kwon and Armbrust, 2006; Lu et al., 2008; Bradley et al., 2008, 2009). Loading rates based on estimated use of trace organic compounds and removal efficiency during wastewater treatment have been incorporated into mathematical modeling of hydrologic systems to provide estimates on expected environmental concentrations and persistence (Anderson et al., 2004; Oakes et al., 2010; Johnson, 2010; Escher et al., 2011). Limited studies incorporate hydrologic analysis with an evaluation of in situ biogeochemical transforma- tion (Williams et al., 2003; Labadie and Budzinski, 2005; Fona et al., 2006). Although laboratory investigations and mass balance and modeling techniques are important, in situ determination of trace organic attenuation aid in evaluating assumptions on domi- nant attenuation mechanisms and calibrating models. The purpose of the presented research is to describe several of the methods that the authors have used to understand fate and transport of trace or- ganic compounds, and highlight some of the important findings. Municipal wastewater treatment plants (WWTP) effectively re- move the vast majority of the collected inorganic and organic com- pounds. Nevertheless, WWTP are often the most substantial source of trace organic compounds to surface waters; therefore, studying stream reaches below WWTP provide valuable information on the environmental persistence of trace organic compounds. Coordinat- ing water quality sampling with hydrologic analysis is essential to understand the fate and transport of trace organic compounds in surface waters. Challenges in assessing environmental fate of trace organics include low (but environmentally relevant) concentra- tions, transformation from parent compounds, variable loading, and multiple sources. Methods used for quantitative evaluation of trace organic compound attenuation in rivers downstream from WWTP are presented, along with advantages and limitations of each method. Three methods have been utilized in the research on the fate and transport of trace organic compounds: (1) intrinsic tracers in wastewater, (2) dye studies to assess travel times be- tween sample locations, and (3) Lagrangian sampling. 1.1. Intrinsic tracers Boron and Gd are trace elements that are generally elevated in wastewater effluent, and are often used as tracers of wastewater influence due to their conservative chemical behavior (Neal et al., 1998; Barber et al., 2006; Verplanck et al., 2005). Anti-depressant pharmaceuticals (including bupropion, carbamazepine, venlafax- ine) are consistently identified in wastewater effluent and are not typically found from other sources. Comparison of the environ- mental persistence of these two classes of compounds provides information on in-stream attenuation downstream from WWTPs, with limited hydrologic analyses. Key assumptions when using this 0883-2927/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2011.03.071 ⇑ Corresponding author at: US Geological Survey, 3215 Marine Street, Boulder, CO 80303, United States. Tel.: +1 303 541 3094. E-mail address: jwriter@usgs.gov (J.H. Writer). Applied Geochemistry 26 (2011) S344–S345 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem