health officials are tasked with identifying 
 and mitigating uncontrolled 
 releases of chemicals that pose unacceptable levels of risk to public 
 health , such as exposure to polycyclic aromatic hydrocarbons ( pahs ) 
 at superfund sites or following an oil spill . in cases where the public 
 can be exposed to contaminated fish and/or shellfish , it is necessary 
 to collect and evaluate tissue levels of hazardous chemicals in order 
 to protect consumers . however , obtaining necessary amounts 
 of tissue data is logistically challenging , time - consuming , expensive , 
 and can further stress compromised ecological communities . furthermore , careful consideration must be given to organism home 
 range , sample availability , and spatial and temporal contamination 
 trends in order to obtain samples that capture site - representative 
 variability in tissue concentrations . having a predictive 
 approach to estimate concentrations of hazardous chemicals in aquatic 
 organism tissues would be useful to risk assessors in instances when 
 tissue data are limited . one approach to estimating chemical 
 concentrations in aquatic biota 
 is through the application of empirical steady - state bioaccumulation 
 factors ( bafs ) , typically calculated as the quotient of tissue concentrations , 
 expressed on a wet weight , dry weight , or lipid normalized basis , 
 and total aqueous phase concentrations . however , applying 
 published bafs to predict site - specific contaminant concentrations 
 in aquatic organisms can produce estimates that vary by orders of 
 magnitude for contaminants of a given log kow . this is due in part to variable chemical bioavailability , physiological differences between organisms used to derive bafs 
 and those of interest in risk assessments , and the error incurred 
 from applying average regression - based bafs to site - specific data . additionally , using bafs to estimate chemical 
 concentrations in tissues is particularly problematic when concentrations 
 in the aqueous phase are below detection limits . passive samplers , 
 such as semipermeable membrane devices ( spmds ) , 
 uptake and concentrate the bioavailable fraction of lipophilic contaminants 
 in a time - integrated and site - specific fashion , where the rate of 
 chemical uptake is related , in part , to the log kow of the contaminant and physical properties of the sampler . it is recognized that physical processes that control chemical uptake 
 into spmds , namely chemical partitioning and diffusion , are similar 
 to those that control chemical uptake by aquatic organisms . as a result , 
 a large effort has been made to compare spmds to aquatic biota in 
 side - by - side controlled experiments using chemical uptake mechanisms , exchange kinetics , and accumulation signatures to determine if spmds could be used as surrogates 
 for biomonitoring organisms in water quality assessments . these types 
 of studies have been conducted for fish and several species of bivalve . however , few studies have tried 
 to mathematically relate the chemical concentrations and signatures 
 captured by spmds directly to tissue levels ; only 
 one has tried to predict concentrations in resident , or wild , aquatic 
 organisms , and none have combined concentrations 
 measured in the freely dissolved phase , converted from spmds ( cw , spmd ) , with multivariate regression techniques . in the present study , we sampled pahs in the freely dissolved phase 
 using spmds and collected colocated resident crayfish ( pacifastacus leniusculus ) at several locations within 
 and outside the portland harbor superfund mega - site . the sampling 
 design was chosen to capture cw , spmd of 
 pahs that ranged 100 or more and pah sources that are common 
 to superfund sites . crayfish were targeted 
 because they have less efficient cyp450 systems , lower rates of chemical 
 excretion compared to aquatic vertebrates , and relatively small home 
 ranges ; they also represent a direct route of pah exposure 
 to the public . differences between freely 
 dissolved and crayfish tissue pah profiles are described based on 
 results from principal components analysis ( pca ) . the predictive performance 
 of several calibration models that relate pah concentrations in the 
 freely dissolved phase to crayfish concentrations are presented , along 
 with recommendations for handling measurements that are below method 
 detection limits . the 
 study was conducted on the lower 18.5 miles of the willamette river , 
 just prior to its confluence with the columbia river . in 2000 , a portion 
 of the river ( river mile 3.5 to 9.2 ) was designated the portland harbor 
 superfund mega - site and was later extended to include river mile 2 
 through 11.8 . this stretch of the willamette 
 and its shorelines has seen nearly 100 years of heavy industrial , 
 commercial , and urban modification . areal and satellite images showing 
 approximate sampling locations are provided in the supporting information , figure 1 . sampling was performed 
 during the fall of 2003 at nine sites that were known to vary in their 
 degree of pah contamination . samples within the mega - site were collected 
 from river miles 3.5 , 7 west ( 7w ) , 7 east ( 7e ) , and 8 , whereas those 
 upstream of the mega - site were from river miles 13 , 17 , and 18.5 . a more focused sampling event was carried out near the former mccormick 
 and baxter creosoting company site at river mile 7e prior to sediment 
 capping in 2004 . spmds and crayfish cages were placed upriver ( 7e - s ) , 
 in close proximity to ( 7e - c ) , and downriver ( 7e - n ) of known sediment 
 contamination hot spots and nonaqueous phase liquid plumes that resulted 
 from creosoting operations . in addition , sites within the mega - site 
 receive inputs from coal tar contaminated industrial sites , petroleum 
 spills , urban runoff , combined sewer overflows , and atmospheric deposition . standard triolein containing ( 1 ml 
 and 95% pure ) spmds 
 were purchased from environmental sampling technologies ( st . joseph , 
 mo ) and consisted of 91106 cm strips of 2.5 cm wide low - density 
 polyethylene lay - flat tubing with a wall thickness of 7095 
 m , surface area of 450 cm , and total weight of 
 4.5 g. spmd samples were collected in october and early november , 
 2003 in 14- to 19-day deployments . stainless steel cages containing 
 five spmds were colocated with crayfish traps at each site and deployed 
 3 m from the river bed . crayfish ( p. leniusculus ) samples were collected over three sampling campaigns from september 
 to october , 2003 using standard operating procedures adapted from 
 epa s guidance for assessing chemical contaminant data 
 for use in fish advisories . crayfish 
 were captured using crayfish traps fashioned with chicken breast as 
 bait ; traps were retrieved within 24 h of deployment . upon retrieval , external crayfish surfaces were cleaned of foreign material using 
 ambient and 18 m cm water and inspected 
 for physical damage . samples were subsequently euthanized using liquid 
 nitrogen , individually wrapped in aluminum foil , and stored on frozen 
 ice packs in labeled zip - lock bags for transport . crayfish pah concentrations 
 were assumed to be at steady state because crayfish were likely exposed 
 to site - specific pah mixtures for the entirety of their lives . in the laboratory , frozen crayfish were brought to room temperature , 
 sexed , sized for body and carapace lengths , weighed , and dissected 
 to isolate visceral tissue . whole visceral samples were frozen individually 
 with liquid nitrogen and homogenized to a fine powder using a prechilled 
 stainless steel mortar and pestle . crayfish extraction proceeded using 
 a previously described method with modifications ( described in the supporting information ) . spmd extracts and 
 crayfish tissue samples were analyzed for the following 16 pahs : naphthalene 
 ( nap ) , acenaphthylene ( acy ) , acenaphthene ( ace ) , fluorene ( flo ) , phenanthrene 
 ( phe ) , anthracene ( ant ) , fluoranthene ( fla ) , pyrene ( pyr ) , benz[a]anthracene ( baa ) , chrysene ( chr ) , benzo[b]fluoranthene ( bbf ) , benzo[k]fluoranthene ( bkf ) , 
 benzo[a]pyrene ( bap ) , indeno[1,2,3-cd]pyrene ( ipy ) , dibenz[ah]anthracene ( dba ) , and benzo[ghi]perylene ( bpy ) . spmds were extracted and chemically 
 analyzed using high performance liquid chromatography ( hplc ) with 
 diode - array and florescence detectors as described previously , whereas crayfish samples were analyzed using 
 a previously described gc  ms method . method detection limits ( mdls ) for crayfish were calculated as 10*sd 
 of replicate low - level standard measurements or as the lowest standard 
 that reproducibly produced signal - to - noise ratios > 3 and ranged 
 from 
 0.05 to 1.2 ng / g wet weight . quality control samples met data quality 
 objectives and are further described in the supporting 
 information . pah concentrations 
 in spmds ( cspmd , ng / g ) were converted 
 to freely dissolved water concentrations ( cw , spmd , ng / l ) using the following established equation : eq 1where rs are laboratory 
 determined pah sampling rates ( l / d ) , vspmd is the sampler volume ( ml ) , and t is the deployment 
 time in days ( d ) . laboratory - based sampling rates 
 were temperature adjusted using methods described by sower and anderson . all statistical analyses and modeling experiments 
 were performed using cw , spmd in order 
 to account for deployment time , chemical specific sampling rates , 
 and the effect of site - to - site differences in particulate and dissolved 
 organic matter conditions on pah bioavailability . crayfish pah concentrations , cw , spmd , and crayfish morphological data were 
 checked for normality using shapiro  wilk tests . differences 
 between upriver and superfund site pah concentrations and crayfish 
 morphology were evaluated using mann  whitney rank sum tests 
 for data that failed shapiro all pairwise spatial 
 differences in pah concentrations and crayfish morphology , and at 
 individual superfund sites compared to upriver control sites , were 
 evaluated using kruskal wallis one way anovas on ranks incorporating 
 dunn s procedure for multiple comparisons when there were unequal 
 treatment group sizes , or standard one way anovas when site data were 
 normally distributed . summary statistics were calculated 
 for individual pahs , the summed concentration of pahs not including 
 flo ( pahs ) , and summed levels of carcinogenic pahs ( c - pahs ) , 
 which included fla , chr , baa , bbf , bkf , bap , ipy , and dba . descriptive 
 statistics , graphing , and simple linear regression were performed 
 using sigmaplot 11.0 ( systat software inc . ) . principal components 
 analysis ( pca ) was used to investigate similarities and differences 
 between freely dissolved and crayfish pah profiles . the goal of pca 
 is to identify a few uncorrelated linear combinations of original 
 data ( i.e. , principal components ) that capture as much multivariate 
 response variation as possible . freely dissolved and crayfish tissue 
 pah concentrations were standardized to total sample pah concentration 
 ( pah ) and 4th root transformed prior to pca because preliminary 
 tests indicated that this transformation improved overall data visualization 
 and reduced the impact of large scaling differences between pah proportions . values < mdls were inputted as zeros , whereas flo and bap were excluded 
 from pca due to chromatographic interferences in spmd extracts . in total , pca was performed on a mathematical 
 matrix composed of 88 rows ( 60 crayfish samples , 26 cw , spmd samples ) and 14 columns ( pahs ) . score and loading 
 plots were used to help visually interpret patterns in the data . several empirical univariate and multivariate regression models 
 were investigated for calibrating concentrations of pahs in the dissolved 
 phase to concentrations in crayfish tissues . prior to modeling , predictor 
 variables ( x , cw , spmd ) were 4th root transformed and averaged by site , while response 
 variables ( y , crayfish tissue concentrations ) were 
 first averaged by site and then 4th root transformed . we determined 
 empirically that this combination of transformations maximizes model 
 performance , while providing a simple procedure for converting predicted 
 pah tissue concentrations from modeled units into original wet weight 
 concentration units using a 4th power transformation . pah concentrations 
 that were less < mdls were input as either  zero  
 or the method detection limit before calculating site averages . thus , 
 regression analyses were performed using x and y matrices that were each composed of 9 rows ( sampling sites ) 
 and 13 or 15 columns ( average site concentration of individual pahs ) , 
 for univariate regression and pls , respectively . partial least - squares 
 calibration ( pls ) is a multivariate regression 
 method , that seeks to predict a set of response variables y , from a set of predictor variables , x. pls overcomes overfitting problems often encountered during multiple 
 linear regression analyses when predictor variables are highly multicollinear . 
 to avoid overfitting , the pls algorithm constructs predictive models 
 indirectly , extracting new variables called factors from the x data , and expresses the predictive models 
 as linear combinations of these factors . like principal components 
 extracted by pca , pls factors are labeled sequentially , explaining 
 decreasing variation in the x variables . however , 
 unlike pca , the pls algorithm seeks factors which also contribute 
 to the prediction of the y variables . the optimal number of pls factors to include in the models was 
 determined using leave - one - out - at - a - time - cross - validation because 
 our data set was relatively small . here , each sample was withheld 
 in turn as a sample and predicted using a model 
 parametrized with the remainder of the predictor data ( x  1 row ) . this process was repeated with models set to systematically 
 select one additional factor up until the pls model composed of all 
 possible factors was evaluated . predictive performance was assessed 
 using the predicted residual sum of squares ( press ) statistic and 
 hotelling s t statistic , where 
 the predicted residual is calculated as the difference between the 
 predicted and measured value and the t statistic is the multivariate equivalent of the student s t statistic . ideally , as factors are added , both the press 
 and t statistics will decrease and then 
 begin to increase indicating the onset of overfitting . the number 
 of pls factors associated with the smallest press and t statistic represents the best number of factors to include 
 in the final model . predictive performance was evaluated by inputting 
 the transformed cw , spmd data into the 
 final cross - validated models and comparing residuals between pah concentrations 
 observed in crayfish tissues and those predicted by pls . multivariate 
 analyses and graphing were performed using a combination of primer 
 ( version 6 , primer - e , united kingdom ) , sas ( version 9.2 , usa ) , and 
 tibco spotfire s+ ( version 8.1 , usa ) software . a total of 60 wild - caught resident 
 crayfish , composed of 28 males and 32 females , were evaluated during 
 the course of this study ( supporting information , table 1 ) . it has been reported that organism age , body size , and 
 physiological state can influence contaminant uptake and resulting 
 tissue concentrations . the morphological 
 data are consistent with the hypothesis of no differences in crayfish 
 body length , carapace length , and body weight between individual sites 
 or between crayfish captured within and outside ( controls ) the designated 
 superfund mega - site ( kruskal  wallis one way anova on ranks , p > 0.1 across all tests ) . crayfish carapace lengths 
 observed 
 in this study ranged from 3 to 6.4 cm with a mean of 4.5  0.8 
 cm , suggesting that organisms were generally adults greater than 2 
 years of age . studies conducted in british 
 rivers reported that p. leniusculus generally migrates less than 225 m over a 2 year period and may 
 have a more limited home range in high water flow environments . as a result , pah concentrations in both crayfish and spmds are taken 
 to be site - specific and represent time - weighted average contaminant 
 concentrations . taken together , these results 
 suggest that crayfish evaluated in the present study were similar 
 in age and physical attributes regardless of sampling location , and 
 display site - specific contamination patterns . it is not readily apparent 
 how well crayfish analyzed in the present study represent other wild 
 crayfish populations or if chronic exposure to environmental contamination 
 has affected their development , physiology , behavior , or ecology . however , it is likely that fishermen using this section of river capture 
 and consume crayfish with morphological attributes similar to those 
 reported here . the range of pah concentrations measured 
 in crayfish and the freely 
 dissolved phase across all sites is presented in figure 1a , b , respectively , and further described in thesupporting information , tables 2 and 3 . the median 
 summed concentration of pahs in crayfish tissue collected within the 
 mega - site ( 214 ng / g w.w . ) was similar to previous reports and significantly larger compared to upriver 
 sites ( 12 ng / g w.w . ) . this was true for roughly 90% of the individual 
 pahs ( supporting information , table 4 ) . comparison of individual and pah crayfish concentrations 
 between individual sites within the mega - site and upriver  control sites revealed significant differences between river miles 7en , 7ec 
 and in a few instances 3.5 and 7es . median pah concentrations 
 were generally 13 to 30 times larger ( p generally 
 < 0.001 ) within the mega - site than upriver . tissue concentrations 
 of carcinogenic pahs were high in crayfish collected within the superfund , 
 while they were frequently below detection at upriver sites ( 52 to73 
 ng / g w.w . crayfish pah profiles 
 at river mile 7e were comparable to source profiles for creosote contaminated 
 sediments reported for the wyckoff / eagle harbor superfund site and support earlier findings by sower and anderson . pah concentrations measured in tissues 
 were not strongly correlated ( r < 
 0.25 ) with crayfish body length , carapace length , or whole - body weight . however , concentrations were similar in magnitude to those measured 
 in crayfish exposed to effluent from a u.s . the cw , spmd data paired to crayfish tissue concentrations in 
 the present study were reported in detail by sower and anderson and are summarized in the supporting information , table 3 . pah concentrations measured 
 in colocated ( a ) resident crayfish 
 and ( b ) freely dissolved phase ( cw , spmd ) samples . samples were collected along a gradient of pah contamination 
 within and outside of the portland harbor superfund mega - site . measurements 
 below detection limits are represented by  x  , whereas 
 pahs that were not quantified due to chromatographic interferences 
 are indicated by cw , spmd samples were collected in tandem with a larger study by sower and 
 anderson . curved red lines highlight 
 differences in the range of high molecular weight pahs measured in 
 crayfish tissues and the freely dissolve phase . pca was used to explore 
 similarities and differences between crayfish and cw , spmd pah profiles across the study area . several key 
 trends were revealed when data were standardized to total pah content 
 and plotted against pc1 and pc2 ( figure 2 ) . the scores of cw , spmd pahs tended to cluster 
 together , indicating that pah profiles in the dissolved phase were 
 more similar across sites compared to crayfish ( figure 2a ) . in contrast , the large spread of crayfish pah scores indicates 
 that crayfish pah profiles were more variable between sites . crayfish 
 collected from within the mega - site were clearly unique compared to 
 those collected upriver . differences are attributed to the influence 
 of high concentration point - source contamination within the harbor 
 and low , variable , nonpoint source inputs at upriver sites . the pca also revealed that pah profiles in both 
 crayfish and the aqueous phase varied according to collection site , 
 where moving from upriver through the superfund tended to be negatively 
 correlated with pc1 and pc2 . pca on 4th root transformed pah profiles using 
 the first two principal 
 components , pc1 and pc2 , which together account for 55.4% of data 
 set variation . the pca scores plot for sample profiles ( a ) is provided 
 with numbers indicating sampling location and colored triangles differentiating 
 freely dissolved (  ) and crayfish (  ) samples . the pca 
 loadings plot ( b ) shows directions of increasing weight for each indicated 
 pah variable in terms of pc1 and pc2 . the length of the vectors corresponds 
 to the percentage of that pah s variation explained jointly 
 by pc1 and pc2 , with the perimeter of the reference circle indicating 
 100% . the loadings diagram ( figure 2b ) shows that 
 pah profiles in the freely dissolved phase ( scores in figure 2a ) were enriched in acy , phe , pyr , and fla ( log kows > 4 and < 5.2 ) compared 
 to crayfish profiles . the relatively diminished proportion of high 
 molecular weight pahs ( log kows > 5.5 ) in the dissolved phase is likely related to 
 solubility 
 limitations and competitive sorption to dissolved- and particulate - bound 
 organic carbon . similar findings were reported by axelman et al . for 
 water samples and caged blue mussels ( mytilus edulis l. ) impacted by aluminum smelting operations . but crayfish pah profiles in the present study had larger 
 relative proportions of chr than mussels . this may be because crayfish 
 were site residents with complex pah exposure scenarios that occurred 
 at the sediment water interface where high molecular weight 
 pahs predominate , whereas the data reported by axelman et al . were 
 for caged mussels that were exposed to pahs in the water column for 
 a relatively short period of time . in 
 addition to high relative proportions of chr , crayfish pah profiles 
 were also enriched in nap compared to the freely dissolved phase . this could be because sediments that crayfish interacted with in the 
 mega - site have been impacted by point - source inputs from creosoting 
 and manufactured gas production , both of which can have relatively 
 large proportions of nap depending on the type of process residue 
 and degree of material weathering . not surprisingly , 
 the loadings diagram shows that in both crayfish and water samples 
 traveling from upriver through the superfund is associated with an 
 increased proportion of pahs , such as ace , baa , bbf , ipy , bpy , and 
 dba , which are constituents of known contamination sources to the 
 harbor . although crayfish within the superfund were 
 not readily distinguishable from one another by pca , pah profiles 
 in resident crayfish and the dissolved phase both indicate a dominant 
 contamination region that is unique to the mega - site . several approaches were investigated 
 for mathematically linking cw , spmd to 
 crayfish pah concentrations , including ordinary least - squares regression 
 ( ols ) , principal components regression ( pcr ) , and partial least - squares 
 regression ( pls ) . though the predictive capability of pcr was analyzed , 
 the focus of further discussion will be on pls because it consistently 
 outperformed pcr in terms of cross - validation and r values ( data not shown ) . results for ols and pls are 
 presented in figure 3 for average crayfish 
 pah concentrations by site , calculated by inputting zeros for measurements 
 that were below detection ( pls-zero  ) . plots of predicted 
 versus measured values were used to evaluate modeling performance 
 graphically , where values that fell closer to the solid diagonal line 
 indicated strong predictive capacity . ideally , if the model explained 
 the data perfectly , all values would fall on the diagonal line . predicted and 
 measured pah levels in crayfish tissues determined 
 using pls on averages calculated by inputting  zero  
 for values below mdls (  ) compared to ( a ) ordinary least - squares 
 regression (  ) or ( b ) pls on averages calculated by inputting 
 mdls for nondetects (  ) . asterisks indicate that flo , bap , 
 ipy , bpy , and dba were unable to be modeled using ordinary least - squares 
 regression . the solid 1:1 diagonal line indicates perfect prediction , 
 whereas the dotted ( 0.5 from 1:1 ) and dashed ( 1 from 1:1 ) 
 lines are included simply to assist the reader in comparing general 
 trends between the predictive models . data points are in modeled units . as shown figure 3a , flo , bap , and other 
 high - molecular weight carcinogenic pahs this is because levels in spmd extracts were either unable to 
 be quantified due to chromatographic matrix interferences or because 
 levels were < mdls . as a result , predictions could not be made because cw , spmd were unable to be calculated for these 
 pahs . of the remaining pahs , 50% appeared not to be linearly related 
 to concentrations in the freely dissolved phase and would be better 
 modeled as the average concentration across all sampling sites . this 
 was the case for nap , ant , chr , bbf , and bkf . although it does appear 
 that ols provided some predictive capacity for ace , phe , pyr , fla , 
 and baa , r values were consistently low 
 with values ranging from < 0.001 to 0.69 , where most were below 
 0.4 and half were below 0.2 . for pls calibration , leave - one - out - at - a - time 
 cross - validation was 
 used to identify the appropriate number of pls factors to include 
 in the final model . root mean press and t values plotted against increasing number of model factors indicated 
 that a two - factor pls model provided good predictions ( overall r value of 72.5% ) , while simultaneously guarding 
 against overfitting ( supporting information , figure 2 ) . correlation - loading plots of the two - factor pls model 
 revealed that pahs frequently above detection limits in both crayfish 
 and the freely dissolved phase provided the strongest predictions , 
 where 70 to 95% of the variation was explained for these pahs ( supporting information , figure 3 ) . pls grouped 
 sites similarly to pca , but also provided improved resolution between 
 sites within the superfund that vary in terms of pah profiles , such 
 as river mile 7 west and 7 east . compared 
 to low molecular weight pahs that were frequently detected , the model 
 accounted for a smaller amount of variation for several high molecular 
 weight pahs that were detected infrequently and at low levels in aqueous 
 phase samples , such as chr , bpy , ipy , and bap . this is likely because 
 data for these residues were more variable as a result of their lower 
 relative solubility and increased interaction with organic - rich solid 
 phases . pah concentrations in crayfish tissues predicted using 
 pls-zero  
 showed remarkable improvements compared to ols regression . pls overcame 
 challenges associated with data that were below detection in the aqueous 
 phase ( i.e. , predictor block ) , at low to sub part - per - trillion levels . this is because pls generates a multidimensional calibration curve 
 composed of linear combinations ( factors ) of predictor variables using 
 all of the predictor pahs . these so - called factors are mathematically 
 orthogonal to one another , which allow them to simultaneously overcome 
 challenges associated with multicollinearity . for example , though 
 bap was not quantified in the aqueous phase due to matrix interferences 
 during instrumental analysis , crayfish tissue concentrations were 
 estimated using pls models composed of two factors , each of which 
 was composed of a unique linear combination of predictor pahs for 
 which data in the aqueous phase did exist . tissue concentrations of 
 flo , ipy , bpy , and dba were estimated in the same way and agreed well 
 with measured tissue concentrations . r values for pahs included in the present study ranged from 0.52 to 
 0.77 and were generally greater than 0.65 ( supporting 
 information , figure 3 ) . the influence of measurements 
 that were at , or below , detection 
 on calibration performance was evaluated using a second pls model , 
 developed by substituting pah method detection limits for zeros in 
 the data set ( pls-mdl  ) . measurements were pretreated 
 as described previously , subjected to pls with cross - validation , and 
 modeled using two - factors , which accounted for 71.8% of the variation 
 in response variables . consistency between predicted and measured 
 values are presented in comparison to the pls-zero  
 model in figure 3b . it is important to keep 
 in mind that average measured concentrations were increased in the 
 pls-mdl  model compared to the pls-zero  
 model . in terms of performance , both models described > 70% of response 
 variation , provided a mechanism for estimating pahs in crayfish tissues 
 for pahs not measured in the freely dissolved phase , and provided 
 balanced model fits for tissue concentrations that ranged 200 
 on average across all pahs and sampling locations ( supporting information , figure 4 ) . however , substituting method 
 detection limits for zero values led to systematically larger averages 
 for sites that had high frequencies of pah measurements below detection . this was the case for pahs with molecular weights greater than 228 
 g / mol ( i.e. , > baa ) and log kows > 5.5 , where measurements in the aqueous phase and 
 crayfish 
 for these contaminants were more variable between replicates . to compare and contrast the utility of the pls-zero and pls-mdl  calibration models , pls - predicted and 
 measured crayfish pah concentrations were 4th power transformed to 
 wet weight tissue concentrations ( ng / g w.w . ) , and plotted together 
 ( figure 4a ) . predictions from both pls models 
 successfully recreated the range of pah concentrations measured in 
 resident crayfish across the sampling area . for sites with mid - to - high 
 levels of contamination , downriver of rm 7e - c , both models predicted 
 tissue concentrations on average to within 2 of measured values 
 across all 15 pahs evaluated . predicted crayfish pah concentrations 
 for samples collected at upriver sites , where pah concentrations were 
 lower and more variable , differed more substantially from measured 
 values , but were generally within 19 and 3 for the pls-zero  
 and  mdl  models , respectively improved estimates of 
 site - specific tissue concentrations were observed using the pls-mdl  
 model compared to a previous approach developed by thomann and komlos that relied on equilibrium partitioning - based 
 crayfish biota - sediment accumulation factors ( bsafs ) , especially for 
 pahs with log kows > 
 6 . this is significant because pahs with log kows in this range are typically drivers in human health risk 
 assessments that include consumption of contaminated seafood as an 
 exposure pathway . ( a ) predicted crayfish tissue pah concentrations 
 compared to measured 
 concentrations expressed in wet tissue weight and ( b ) close - up view 
 of measurements near method detection limits for four representative 
 pahs . data points represent (  ) average site concentrations 
 calculated by inputting  zero  for measurements  
 mdls and (  ) average site concentrations calculated after inputting 
 mdls for measurements that were below detection . although both pls models provided good estimates for pahs 
 that 
 were frequently above mdls , differences were apparent for pahs below 
 or near mdls . for instance , nap was detected in every crayfish sample 
 ( > 6.5 ng / g w.w . 60 above the mdl ) and a majority 
 of 
 spmd extracts . as a result , differences between pls-zero  
 and -mdl  predictions were relatively small , with the 
 largest differences found at sites that had large frequencies of measurements 
 below detection in the aqueous phase ( figure 4b ) . this was found to be the case for other low molecular weight 
 pahs as well ( ace , flo , phe , pyr ) . however , because these pahs are 
 classified as noncarcinogenic and their human health screening levels 
 in tissues are relatively large , the differences between the two models 
 do not have significant impacts on their interpretation in a risk 
 assessment framework . in contrast , high 
 molecular weight pahs , such as bap , ipy , and dba , were frequently 
 below or near mdls in crayfish , the freely dissolved phase , or both . as expected , substituting mdls into the dataset produced average crayfish 
 concentration estimates that were larger in magnitude than observed 
 using zeros . from a screening level risk assessment stand point , the 
 pls-mdl  modeling approach will provide tissue concentrations 
 that are more health protective and guard against false negatives . 
 this is especially important for high molecular weight pahs that often 
 exhibit carcinogenic activity because these compounds have low tissue 
 concentration screening levels and are often drivers of risk assessments 
 at superfund sites . however , it is important 
 to recognize that actual pah concentrations fall somewhere between 
 zero and mdls , which affords risk assessors some discretion in the 
 treatment of measurements that fall below detection limits . in practice , human health and ecological risk assessments 
 are often challenged 
 by a limited amount of site - specific data for contaminant concentrations 
 in resident aquatic biota . combining passive sampling with pls calibration 
 appears to provide a means to fill this knowledge gap . additionally , 
 using passive samplers would provide assessors dissolved phase concentration 
 estimates , extracts suitable for detecting site - specific toxicity 
 to aquatic systems , and a means for identifying contamination 
 sources based on chemical ratios and profiles , thus increasing the cost effectiveness of 
 each field sampling event . the approach may be especially useful during 
 early project planning and screening - level assessments , when resources 
 for performing field sampling may be in short supply . though our models 
 were developed using pah concentrations in visceral tissue , the approach 
 may perform comparably for tissues more commonly consumed by the public . future research should focus on validating the current model using 
 a larger data set , comparing lipid filled spmds to nonlipid filled 
 samplers that employ performance reference compounds , evaluating the 
 predictive capacity of the approach in other aquatic biota and tissue 
 types , and extending the approach to additional contaminants that 
 drive risk assessments .
the current work sought to develop 
 predictive models between time - weighted 
 average polycyclic aromatic hydrocarbon ( pah ) concentrations in the 
 freely dissolved phase and those present in resident aquatic organisms . 
 we deployed semipermeable membrane passive sampling devices ( spmds ) 
 and collected resident crayfish ( pacifastacus leniusculus ) at nine locations within and outside of the portland harbor superfund 
 mega - site in portland , or . 
 study results show that crayfish and aqueous 
 phase samples collected within the mega - site had pah profiles enriched 
 in high molecular weight pahs and that freely dissolved pah profiles 
 tended to be more populated by low molecular weight pahs compared 
 to crayfish tissues . 
 results also show that of several modeling approaches , 
 a two - factor partial least - squares ( pls ) calibration model using detection 
 limit substitution provided the best predictive power for estimating 
 pah concentrations in crayfish , where the model explained 72% 
 of the variation in the data set and provided predictions within 3 
 of measured values . 
 importantly , pls calibration provided a means 
 to estimate pah concentrations in tissues when concentrations were 
 below detection in the freely dissolved phase . the impact of measurements 
 below detection limits is discussed .