U.S. Geological Survey (USGS)
Dr. Bradley is a Research Ecologist with the U.S. Geological Survey (since 1991). His research focuses on exposure and effects of contaminant mixtures in drinking water and on contaminant mixtures as stream quality stressors.
Mixed Contaminant Exposure and Predicted Effects in Wadeable Streams of the Southeastern United States
Complex contaminant mixtures and associated aquatic-health effects are growing concerns. Replicate (n = 3-10) water samples were collected from 54 small Piedmont (USA) streams over 10 weeks in 2014 and analyzed for 489 organic analytes (470 unique chemicals), including 110 pharmaceuticals and degradates, 226 pesticides and degradates, 84 volatile organic chemicals, and 69 organic waste indicator compounds. Of these, 264 (56%) were detected at least once across all sites. Approximately 70% of the detected water contaminants were designed-bioactive organic chemicals (e.g., pharmaceuticals, pesticides). Cumulative detections and concentrations ranged up to more than 150 compounds (median greater than 80) and 160 Âµg L-1 (median greater than 10 Âµg L-1) respectively, and correlated significantly with wastewater discharge and watershed development metrics. Bed-sediment samples (n = 1) from each stream also were analyzed for 117 current-use pesticides, 55 halogenated-organic chemicals, and 38 polycyclic aromatic hydrocarbon compounds. Due to multiple modes of action, high bioactivity, biorecalcitrance, and direct environment application (pesticides), frequent detection of designed-bioactive organics (range up to more than 100; median greater than 40 per site) at cumulative concentrations up to Î¼g L-1 raise concerns for sub-lethal effects to sensitive aquatic species and lifecycle stages. The possible effects of the complex chemical exposures observed in this study were computationally predicted using knowledgebase-leveraging tools.