Paper Title
Perfluorooctane Sulfonate (PFOS): A Contaminant of Emerging Concern in Guam’s Groundwater

Abstract
Perfluorooctane sulfonate (PFOS) is a fully fluorinated anion that was once widely used in industry. It is very persistent and has substantial bioaccumulation and biomagnification properties, particularly in humans. PFOS is moderately water soluble (~600 mg/L) and has recently emerged as a drinking water contaminant of potential concern. In 2009, USEPA issued a provisional drinking water health advisory (HA) for PFOS of 200 ng/L. A final HA of 70 ng/L was promulgated in May 25, 2016. The Guam Waterworks Authority (GWA) began monitoring PFOS in Guam’s groundwater in March 2015, in response to USEPA's third Unregulated Contaminant Monitoring Rule, (UCMR3). Overall, five of their production wells were identified as PFOS contaminated and levels in two of them were consistently above the USEPA's 70 ng/L benchmark. Values recorded in the most contaminated well ranged from 160-410 ng/L. Both wells were taken offline in June 2016 and all further monitoring was suspended. WERI saw merit in continuing to monitor both wells in order to decide upon best management strategies prior to bringing them back on line. As a first step in this direction WERI implemented monthly monitoring of the most contaminated well in June 2017. A preliminary assessment of data obtained thus far is presented here. A highly significant, positive relationship between well PFOS levels and rainfall was observed. A delayed well response time of at least one month following major rain events was also evident. Well PFOS concentrations substantially declined over a similar time frame during dry weather conditions but remain above 200 ng/L as of this writing. Potential sources of PFOS and their locations within the watershed are briefly discussed together with management options. (275 words) Keywords - Perfluorooctane sulfonate, PFOS, groundwater contamination, seasonal influences, Guam.