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Emerging organic chemicals (EOCs) have received increasing attention in the last decade due to their possible adverse affects on ecosystems and human health. Several studies have suggested wastewater as a primary contributing source of EOCs to the water environment, but few have quantified their occurrence, especially in onsite wastewater treatment systems and associated receiving environments. In Colorado there are over 600,000 onsite wastewater systems (OWS) in operation, resulting in over 100 billion liters of wastewater that is being processed by OWS and then discharged to the environment each year. The focus of the research described in this paper is to determine the occurrence of a select group of EOCs in OWS and their receiving environments. Thirty OWS, 11 groundwater wells, and 9 surface waters for a total of 50 sites in the Colorado Rocky Mountain region are being analyzed for a suite of EOCs and wastewater indicator compounds using two methods: (1) continuous liquid-liquid extraction with methylene chloride, and (2) evaporation followed by derivatization to form propyl esters. Preliminary results from analyses of samples collected from twelve of the selected sites have identified 10 compounds, including ethylenediaminetetraacetic acid, nitrilotriacetic acid, and nonylphenolpolyethoxylates. Five EOCs were identified in OWS effluents: 4-nonylphenol (510- 28,000 ng/L), caffeine (5,300 – 11,000 ng/L), triclosan (14-130 ng/L), 3-â-coprostanol (1,100- 99,000 ng/L), and cholesterol (2,800-38,000 ng/L). The samples from each of the 50 sites have been characterized with respect to general water and wastewater parameters and are presently being analyzed for the suite of EOCs and wastewater indicator compounds. The results will aid in defining the potential risks to ecosystems or human health due to EOCs discharged in OWS effluents.