An evaluation of measured and predicted concentrations of 17α ethinyl estradiol (EE2) in surface waters of the United States (U.S.) and Europe was conducted to develop expected long term exposure concentrations for this compound. Measured environmental concentrations (MECs) in surface waters were identified from the literature. Predicted environmental concentrations (PECs) were generated for European and U.S. watersheds using the GREAT-ER and PhATE™ models, respectively. The majority of MECs are non-detect and generally consistent with model PECs and conservative mass balance calculations. However, the highest MECs are not consistent with concentrations derived from conservative (worst case) mass balance estimates or model PECs. A review of analytical methods suggests that tandem or high resolution mass spectrometry (MS) methods with extract cleanup result in lower detection limits and lower reported concentrations consistent with model predictions and bounding estimates.\ud Based on model results using PhATE and GREAT ER, the 90th percentile low flow PECs in surface water are approximately 0.2 and 0.3 ng/L for the U.S. and Europe, respectively. These levels represent conservative estimates of long-term exposure that can be used for risk assessment purposes. Our analysis also indicates that average concentrations are one to two orders of magnitude lower than these 90th percentile estimates. Higher reported concentrations (e.g., greater than the 99th percentile PEC of ~1 ng/l) could result from methodological problems or unusual environmental circumstances; however, such concentrations are not representative of levels generally found in the environment, warrant special scrutiny, and are not appropriate for use in risk assessments of long-term exposures.\u
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