FACTORS AFFECTING THE FATE OF CIPROFLOXACIN IN AQUATIC FIELD SYSTEMS L. A. CARDOZA 1 , C. W. KNAPP 2 , C. K. LARIVE 1 , J. B. BELDEN 3 , M. LYDY 3 and D. W. GRAHAM 2,∗ 1 Department of Chemistry, University of Kansas, Lawrence, KS 66045; 2 Department of Civil, Environmental and Architectural Engineering, University of Kansas, Lawrence, KS 66045; 3 Fisheries and Illinois Aquaculture Center and Department of Zoology; Southern Illinois University, Carbondale, IL 62901 ( ∗ author for correspondence, e-mail: dwgraham@ku.edu, Fax: 785-864-5379, Tel: 785-864-2945) (Received 12 October 2003; accepted 26 June 2004) Abstract. Ciprofloxacin (cipro) is a broad-spectrum antibiotic used in human and veterinary medicine that is readily transported into the environment via domestic wastewaters and through direct runoff. Although factors governing cipro fate are becoming understood, an integrated evaluation of disappear- ance mechanisms in aquatic systems has not been performed. Here we examined cipro disappearance rate in surface waters using both laboratory and field systems under different light, and dissolved (DOC) and particulate organic carbon (POC) conditions to determine when photodegradation versus adsorption dominates cipro fate. Initial laboratory experiments showed that cipro rapidly photode- graded (t 1/2 ∼ 1.5 h) with numerous photodegradation products being noted when POC levels were low. However, even moderate water column POC levels resulted in reduced photodegradation (no break- down products detected) and soluble cipro disappearance rates were accelerated. 14 C-ciprofloxacin studies confirmed significant adsorption onto aquatic POC ( K OC values of 13,900 to 20,500 L/kg at neutral pH). In contrast, a follow-up mesocosm-scale field study using low POC water showed that photodegradation could also dominate cipro fate. In conclusion, both adsorption and photodegrada- tion strongly influence cipro fate in aquatic systems, although the dominant mechanism appears to depend upon the ambient POC level. Keywords: aquatic systems, ciprofloxacin, field mesocosm, fluoroquinolone antibiotics, particulate organic carbon, photodegradation 1. Introduction Fluoroquinolones (FQs) are a class of synthetically produced antibiotics that pos- sess broad-spectrum antibacterial properties. The environmental fate of such an- tibacterial agents is important because of their continued use in both human and veterinary medicine, and their potential for migration into the environment and the possible development of resistance in environmental pathogens (Kidwai et al., 1998; Yoshida et al., 1993; Halling-Sørensen et al., 2000; Alonso et al., 2001). The fate of FQ compounds is of particular significance because FQs are often “drugs of last resort” in medicine and they have been shown to potentially be genotoxic in hospital wastewaters (Hartmann et al., 1998). Ciprofloxacin (Figure 1), a common Water, Air, and Soil Pollution 161: 383–398, 2005. C  2005 Springer. Printed in the Netherlands.