poly- and perfluoroalkyl 
 substances ( pfass ) make up a large group 
 of persistent anthropogenic chemicals used in industrial processes 
 and commercial products over the past 60 years . widespread use and extreme resistance to degradation have 
 resulted in the ubiquitous presence of these compounds in the environment . 
 the 20112012 u.s . national health and nutrition examination 
 survey reported detectable serum pfas concentrations in virtually 
 all individuals ( 97% ) . human pfas exposure has been linked 
 to cancer , elevated cholesterol , obesity , immune suppression , and 
 endocrine disruption . health concerns in the early 2000s prompted manufacturers 
 in europe and north america to phase out production of some long - chain 
 pfass . declines in production of these compounds have been offset by increases 
 in developing regions such as asia . limited 
 available data suggest widespread exposure to replacement ( short - chain ) 
 pfass may also adversely affect human health . human pfas exposure includes dietary sources , household dust , 
 air , 
 and drinking water . exposure from drinking water 
 is a serious concern because of the high aqueous solubility of many 
 pfass . drinking water have 
 been reported in numerous regions , especially near industrial 
 sites that produce or use them . for example , perfluorooctanoic acid ( pfoa ) concentrations 190-fold 
 higher than the lifetime health advisory ( 70 ng / l ) recommended by 
 the u.s . environmental protection agency ( us epa ) were measured in drinking water near a fluorochemical facility 
 in washington , wv , where pfoa was used in fluoropolymer production . many civilian airports and military fire 
 training areas have been 
 contaminated by pfass contained in aqueous film - forming foams ( afffs ) 
 that are widely used during firefighting training activities . groundwater 
 and surface waters surrounding these sites containing pfas concentrations 
 that are 34 orders of magnitude higher than the us epa health 
 advisory level for drinking water have been reported . wastewater treatment plants ( wwtps ) are another important pfas source 
 because these compounds are not removed by standard treatment methods and labile precursors biodegrade , increasing 
 concentrations in effluent relative to influent . land application of approximately half of the biosolids generated 
 by wwtps may contribute to human exposure through subsequent contamination 
 of water , food , livestock , and wildlife . understanding nationwide pfas exposures from drinking water 
 is 
 important for identifying potentially vulnerable populations . however , 
 previous studies have mainly focused on individual point sources of 
 pfas contamination and site - specific drinking water exposures . here we develop a statistical framework for investigating whether 
 increased pfas concentrations in drinking water are associated with 
 the number of point sources within a watershed ( represented by an 
 eight - digit hydrologic unit code , hereafter abbreviated huc ) . we used 
 publicly available drinking water concentration data for six pfass 
 from the us epa s third unregulated contaminant monitoring 
 rule ( ucmr3 ) , including perfluorobutanesulfonic acid ( pfbs ) , perfluorohexanesulfonic 
 acid ( pfhxs ) , perfluoroheptanoic acid ( pfhpa ) , pfoa , perfluorooctanesulfonic 
 acid ( pfos ) , and perfluorononanoic acid ( pfna ) ( table s1 ) . we discuss the utility 
 of the ucmr3 database for identifying sources of pfass to u.s . drinking 
 water supplies , locations of vulnerable populations , and priorities 
 for future monitoring . our analysis included analytical 
 results for six pfass in 36149 drinking water samples from the us 
 epa s ucmr3 program collected between january 2 , 2013 , and 
 december 9 , 2015 . data are also 
 available 
 for 800 public water supplies serving < 10000 individuals , but this 
 represents only a small fraction ( 0.5% ) of the 144165 in this category . 
 minimum reporting levels ( mrls ) for the six pfass analyzed are listed 
 in table s1 . one limitation of the 
 ucmr3 database is that national data on system intakes for public 
 water supplies are classified , making 
 it difficult to place them within a specific hydrological network . we therefore extracted the zip codes for areas served and aggregated 
 data within eight - digit hucs to capture 
 the most detailed hydrologic information that exceeds the spatial 
 resolution of pfas data ( zip code areas ) . we used the highest reported 
 pfas concentrations when multiple systems were located within a single 
 zip code and/or when multiple zip code areas were located within the 
 same huc . our spatial analysis ( figure s1 ) included point source information 
 for ( a ) 16 industrial sites listed in the us epa s 2010/2015 
 pfoa stewardship program ( table s2 ) , ( b ) 8572 wwtps , ( c ) 290 military fire training areas that contain 664 military fire 
 training sites , and ( d ) 533 civilian 
 airports that are compliant with title 14 code of federal regulations , 
 part 139 for personnel trained in the use of afff ( hereafter termed 
  afff - certified airports  ) . pfass produced and/or used vary across industrial sites , and not 
 all compounds were associated with all sites . for example , a fluorochemical 
 manufacturing facility in decatur , al , produced both pfos and pfoa , while only pfoa was used in the manufacturing 
 process of another fluorochemical production facility in parkersburg , 
 wv . we conducted a sensitivity analysis 
 to examine the potential production misclassification bias by limiting 
 industrial sites to include the ones that only produced or used each 
 specific compound ( table s3 ) . we used the 
 google maps application program interface ( api ) to geocode coordinates 
 based on addresses . potentially important pfas sources such as landfills , 
 biosolids , and small industrial pfas users could not be included in 
 this analysis because comprehensive geospatial data are not available . we used arcmap 10.3.1 
 ( esri ) to explore statistical differences between the number of point 
 sources in eight - digit hucs with pfas levels above and below the level 
 of detection . we developed a multivariate spatial regression model 
 for watersheds with detectable pfass that adjusts for correlations 
 and co - location among point sources . pfna and 
 pfbs were excluded from the spatial regression analysis due to a low 
 detection frequency ( 15 and 14 of 1601 watersheds , respectively ) . we used moran s i statistic to test for spatial dependence 
 in the model residuals from an ordinary least - squares ( ols ) regression 
 and correct for spatial dependence in the final spatial regression 
 model . akaike information criterion was 
 used to compare the ols and spatial regression models , where a lower 
 value implies a better model fit . a series of cross - validation tests 
 were also completed to assess the predictive capacity and stability 
 of the final set of models . the ols and spatial regression models 
 were constructed using geoda 1.6 software , and cross - validation was implemented in r version 3.1.3 . pfass 
 were detected at 
 or above the mrls in 194 of 4864 public water supplies , serving 16.5 
 million residents in 33 different states , three american territories 
 ( american samoa , northern mariana islands , and guam ) , and the salt 
 river pima - maricopa indian community . drinking water from 13 states 
 accounted for 75% of detections , including , by order of frequency 
 of detection , california , new jersey , north carolina , alabama , florida , 
 pennsylvania , ohio , new york , georgia , minnesota , arizona , massachusetts , 
 and illinois ( figure 1 ) . detection frequencies for pfass across the 4864 public water supplies 
 were 2.2% for pfoa , 2.0% for pfos , 1.7% for pfhpa , 1.1% for pfhxs , 
 and < 0.003% for others . hydrologic unit codes ( eight - digit hucs ) used 
 as a proxy for watersheds 
 with detectable pfoa and pfos in drinking water measured in the us 
 epa s ucmr3 program ( 20132015 ) . many detectable pfas concentrations in the ucmr3 database 
 are above 
 chronic drinking water and water quality standards for other regions 
 ( i.e. , surface water european union , pfos , < 1 ng / l ; drinking water 
 sweden , sum of seven pfass , < 90 ng / l ; groundwater state of new 
 jersey , pfna , < 10 ng / l ; drinking water state of vermont , sum of 
 pfos and pfoa , < 20 ng / l ) . a recent analysis developed a benchmark dose for 
 immunotoxicity in children and suggested a drinking water limit of 
 approximately 1 ng / l for pfos and pfoa . data from rodents that measured sensitive end points such as mammary 
 gland development support a similar level . six million people were served by 66 public water supplies 
 that 
 have at least one sample at or above the us epa s 2016 health 
 advisory for pfos and pfoa ( 70 ng / l individually or combined ) . concentrations 
 ranged as high as 349 ng / l for pfoa , 1800 ng / l for pfos , and 56 ng / l 
 for pfna . the detection frequency in drinking water sourced 
 from groundwater 
 was more than twice that from surface water ( table s4 ) . long - chain pfass ( pfhxs , 
 pfos , pfoa , and pfna ) were more frequently detected in groundwater , 
 and short - chain compounds ( pfhpa and pfbs ) were detected more frequently 
 in surface waters . this may be due to both the original mode of environmental 
 release ( as an aerosol , application to soil , and aqueous discharge ) 
 and the inverse relationship between pfas mobility and chain length . ng / l ) in the ucmr3 database 
 are up to 2 orders of magnitude higher than the limit of quantitation 
 in most published studies , and more than 10 times higher than the drinking water limit ( 1 ng / l ) 
 suggested by human and animal studies . because pfass 
 are detectable in virtually all parts of the environment , we infer that the large 
 fraction of samples below reporting limits ( table s4 ) is driven in part by high mrls . our analysis indicates point sources 
 are significantly more abundant 
 in hucs with detectable pfass [ two - sided t test , p < 0.05 ( table 1 and figure s2 ) ] . this includes 
 drinking water samples from 1601 of the 2158 total u.s . hucs . for 
 example , hucs with detectable pfoa levels ( 8% of the total ) have more 
 industrial sites , military fire training areas , afff - certified airports , 
 and wwtps than those with concentrations below detection . similarly , hucs with point sources 
 have higher detection frequencies for pfass ( table s5 ) . for example , the presence of a military fire training 
 area within a huc increases the frequency of detection of at least 
 one pfas from 10.4% to 28.2% . one caveat is that imprecise information 
 about public water supply intakes can cause misclassification bias . systems that draw water upstream from point sources , such as minneapolis 
 and st . paul in minnesota , may not actually be affected as indicated 
 by the aggregated spatial analysis . the mean abundance is calculated 
 as the mean number of point sources within hucs with pfass above or 
 below the level of detection . only the major industrial sites 
 participating in the us epa s 2010/2015 pfoa stewardship program 
 were included . wastewater 
 treatment plant . two - sample t - test p - values . spatial regression 
 modeling explains 3862% of the variance in drinking water 
 concentrations for the four pfass considered ( table 2 ) . each additional industrial site within 
 a huc is associated with an 81% increase in pfoa ( p < 0.001 ) , which is the strongest statistical association across 
 compounds and point sources . increasing pfos concentrations are positively 
 associated with the number of industrial sites , but this relationship 
 is not statistically significant ( p = 0.124 ) . the 
 small number of sites that have manufactured or used pfos likely accounts 
 for the lack of a statistically significant relationship . only the major industrial sites 
 participating in us epa s 2010/2015 pfoa stewardship program 
 were included . results have been transformed to 
 reflect expected changes in drinking water concentrations per increase 
 in the abundance of different sources . positive coefficients in the 
 results indicate increasing concentrations with an increasing abundance 
 of point sources within the same hydrologic unit . the spatial error term is used to 
 incorporate spatial autocorrelation structures into a linear regression 
 model . the number of military 
 fire training areas within each huc is positively 
 associated with increasing levels of all pfos , pfoa , pfhxs , and pfhpa , 
 and is statistically significant for pfhxs ( p = 0.045 ) 
 and pfos ( p = 0.007 ) . each additional military fire 
 training area within the same huc is associated with a 20% increase 
 in pfhxs ( p = 0.002 ) , a 10% increase in pfhpa ( p = 0.155 ) , a 10% increase in pfoa ( p = 
 0.111 ) , and a 35% increase in pfos ( p < 0.001 ) . afffs typically contain relatively high concentrations of pfos and 
 pfhxs and their polyfluorinated precursors compared to the concentrations 
 of other perfluorinated carboxylates , which is consistent with these statistical results . we find 
 a small but significant increase in pfos and pfoa ( 2% ; p < 0.01 ) with each additional wwtp within the same huc . this is consistent with the greater abundance but smaller quantities 
 of pfass released by wwtps . show pfas 
 releases from wwtps are important but less significant than those 
 from fluorochemical manufacturing facilities in italy . the number 
 of wwtps may also be a proxy for other population - driven pfas sources . the number of afff - certified airports is not significantly associated 
 with pfas concentrations in the current data set . this may reflect 
 misclassification bias because the certification used to identify 
 airports indicates eligibility but not actual use of afff . the ucmr3 
 database contains limited data for smaller drinking water systems 
 where localized reports of contamination from airports have been most 
 abundant . the ucmr3 database has several limitations that restrict 
 its predictive 
 power for identifying u.s . drinking water supplies likely to contain 
 elevated levels of pfass . classification of geospatial data on intakes 
 for public water supplies limits the spatial resolution of the current 
 data set and associated statistical models to a radius of 50 km ( median 
 radius of watersheds ) . many of the impacted drinking 
 water systems are groundwater systems , and contaminated groundwater 
 plumes are often much smaller than 50 km . geospatial data are lacking for many 
 potentially important pfas point sources such as a wide range of industries , 
 landfills , biosolids application , and other afff - impacted sites where 
 relatively smaller volumes of afff were released . data on pfas releases from smaller industrial facilities ( e.g. , 
 plastics , textiles , paper , and lubricants ) are usually withheld as 
 confidential business information , and little information about airborne 
 emissions is available for characterizing the importance of atmospheric 
 releases and potential long - range transport . for example , biosolids 
 application resulted in one of the largest pfas drinking water contamination 
 events in europe but could not be included 
 in this analysis because u.s . sources not included in our spatial analysis are represented 
 by 
 the highly significant lambda (  ) coefficients ( table 2 ) . areas with high model residuals 
 ( greater than 1.5 standard deviation ) mean that current information 
 about sources can not fully explain the high observed pfas concentrations . the map of model residuals ( figure s3 ) 
 can thus be used to guide high - priority sampling regions in future 
 work . we found a statistically greater abundance of point sources 
 in 
 watersheds with detectable pfass , including afff - certified airports . however , multivariate spatial regression models did not show a significant 
 association between afff - certified airports and concentrations of 
 pfass in nearby drinking water . other studies have reported elevated 
 pfas concentrations in groundwater wells adjacent to afff - certified 
 airports . small drinking water systems 
 and private wells may be disproportionately affected by pfass originating 
 from afff use at civilian airports , but representative data for these 
 small drinking water systems are not included in the ucmr3 program . approximately 44.5 million u.s . individuals 
 rely on private drinking 
 water wells , and 52 million individuals 
 rely on smaller public water supplies ( < 10000 served ) . the ucmr3 
 program includes 0.5% testing incidence for smaller public water supplies and no testing of private wells , meaning that 
 information about drinking water pfas exposures is therefore lacking 
 for almost one - third of the u.s . population .
drinking water contamination with 
 poly- and perfluoroalkyl substances 
 ( pfass ) poses risks to the developmental , immune , metabolic , and endocrine 
 health of consumers . 
 we present a spatial analysis of 20132015 
 national drinking water pfas concentrations from the u.s . 
 environmental 
 protection agency s ( us epa ) third unregulated contaminant 
 monitoring rule ( ucmr3 ) program . 
 the number of industrial sites that 
 manufacture or use these compounds , the number of military fire training 
 areas , and the number of wastewater treatment plants are all significant 
 predictors of pfas detection frequencies and concentrations in public 
 water supplies . among samples with detectable pfas levels , each additional 
 military site within a watershed s eight - digit hydrologic unit is associated with a 20% increase 
 in pfhxs , a 10% increase in both pfhpa and pfoa , and a 35% increase 
 in pfos . the number of civilian airports with personnel trained in 
 the use of aqueous film - forming foams is significantly associated 
 with the detection of pfass above the minimal reporting level . 
 we 
 find drinking water supplies for 6 million u.s . 
 residents exceed us 
 epa s lifetime health advisory ( 70 ng / l ) for pfos and pfoa . 
 lower analytical reporting limits and additional sampling of smaller 
 utilities serving < 10000 individuals and private wells would greatly 
 assist in further identifying pfas contamination sources .