because of their high 
 affinity for solid particles , hydrophobic 
 organic contaminants ( hocs ) preferentially deposit in soil or sediment 
 in the environment , where they may cause direct ecological effects 
 or become secondary contamination sources . as shown in recent studies , 
 hocs in the soil or sediment are distributed in heterogeneous regions 
 of organic matter . sequestration of hocs in the  glassy  
 region of organic matter may lead to irreversible sorption or reduced 
 bioaccessibility . therefore , 
 use of the bulk chemical concentration may not convey the actual risk 
 because the total concentration could substantially overexpress bioavailability . thus , predicting bioavailability that quantifies 
 the actual amount of hocs available for bioaccumulation or eliciting 
 toxicity is essential for improving risk assessment . changes in bioavailability may be especially pronounced 
 for recalcitrant 
 hocs , e.g. , dichlorodiphenyltrichloroethane ( ddt ) , polychlorinated 
 biphenyls ( pcbs ) , or polyaromatic hydrocarbon ( pahs ) , from historical 
 contamination episodes , because aging and other sequestration processes 
 are known to decrease contaminant bioavailability . methods such as mild solvent extraction 
 and sequential desorption have been previously tested for measuring 
 hoc bioaccessibility . in mild extraction , a variety of  soft  
 solvents ( e.g. , n - butanol , methanol , ethyl acetate ) 
 are used to estimate the readily extractable fraction as an approximation 
 of the bioavailable concentration . however , in such applications , 
 the measured value invariably depends on the strength of the selected 
 solvent . sequential desorption employing 
 sorbents such as tenax and cyclodextrin has also been used in many 
 studies , where the polymeric sorbent traps the desorbed hoc to facilitate 
 desorption measurement . however , the sequential desorption 
 approach is extremely time - consuming , and laborious . as an alternative , a single - point tenax desorption has 
 been tested , but studies comparing the single- 
 and serial desorption schemes often showed under- or overestimations . therefore , there is a great impetus to identify a simple and readily 
 adoptable method for predicting bioaccessibility of hocs in soil or 
 sediment . we recently explored the use of isotope dilution for 
 predicting 
 bioavailability of hocs and evaluated the method using bifenthrin 
 and pyrene as model contaminants in laboratory spiked freshwater sediments . the principle of isotope exchange was previously 
 used for measuring metal bioavailability . however , sorption of metals to soil or sediment is known to differ 
 mechanistically from that of organic molecules , as metal ions are 
 adsorbed to specific charged sites . in this study , 
 we extended the isotope dilution method ( idm ) to sediments historically 
 contaminated with recalcitrant hocs . specifically , we evaluated the 
 validity and performance of idm for marine sediments from a superfund 
 site contaminated with ddt and pcbs from many decades ago , and demonstrated 
 the predictability of idm for contaminant bioavailability in extensively 
 aged matrices . seven legacy hocs , including 
 four ddt derivatives ( o , p-dde , p , p-dde , o , p-ddd , and p , p-ddd , denoted as ddxs ) and three pcb congeners ( pcb 52 , pcb 
 70 and pcb 153 ) , were considered in this study . standards of pcb 67 
 and pcb 191 were used as external ( recovery ) surrogates and pcb 30 
 and pcb 82 as internal standards . stable isotope labeled hocs ( denoted 
 as * hocs herein ) including c - o , p-ddd , c - o , p-dde , c - pcb 52 , 
 and c - pcb 153 and deuterated compounds p , p-ddd - d8 , p , p-dde - d4 , and pcb 70-d3 were used in the idm method development and evaluation . sources and purities of these chemicals may be found in the supporting information ( si ) . marine sediments 
 ( 015 cm ) historically contaminated with ddt and pcbs were 
 collected at two sites ( denoted as pv6c and pv8c ) of the palos verdes 
 shelf ( pvs ) superfund site off the coast of los angeles , ca . both 
 sediments were sieved through a 2 mm mesh and stored at 4 c 
 before use . contamination of the pvs ocean floor was due to many decades 
 of discharge of wastewater by the los angeles county sanitation district 
 and disposal of wastewater containing ddt by the montrose chemical 
 company , once the largest ddt manufacturer in north america , from 
 1950s to 1970s . as a result , the pvs superfund site encompasses about 
 44 km of sediment that is heavily contaminated with ddxs 
 and pcbs . in some areas , monitoring showed that ddxs and pcbs at pvs have the potential to 
 accumulate into marine benthic invertebrates and fish , posing human 
 health risks via dietary exposure . the pv8c sediment was collected 
 from the hot spot near the outfalls , whereas the pv6c sediment was 
 relatively less contaminated . sediment 
 properties ( texture and organic carbon content ) were analyzed at the 
 university of california davis analytical laboratory using standard 
 methods ( davis , ca ) . the measured organic 
 carbon ( oc ) contents of pv6c and pv8c were 2.34% and 3.54% , respectively . the sand , silt , and clay contents were 36% , 51% , and 13% for pv6c 
 and 48% , 39% , and 13% for the pv8c sediment , respectively . a batch 
 equilibration experiment was carried out to determine the time for 
 each native hoc to reach sorption equilibrium and the apparent sediment briefly , 2 g 
 of the homogenized pv8c sediment ( dry weight basis ) was placed in 
 a 50 ml glass centrifuge tube , and 20 ml of 32  artificial 
 seawater containing 200 mg / l sodium azide was added . the tubes were 
 sealed with aluminum foil lined caps and mixed at 120 rpm on a horizontal 
 shaker . after 0.5 , 1 , 2 , 4 , 6 , 12 , 24 , 48 , or 96 h of mixing , triplicate 
 samples were removed and centrifuged at 670 g for 
 30 min for phase separation . a similar protocol was used for the pv6c 
 sediment , except that the samples were retrieved after 12 , 24 , and 
 48 h of mixing . after centrifugation , the supernatant was collected 
 and extracted using a liquid  liquid extraction ( lle ) method . after addition of internal standards , an aliquot of the final extract 
 was analyzed on a varian 3800 gc equipped with a varian 1200 triple 
 quadrupole mass spectrometer ( gc  ms / ms ) ( varian , sunnyvale , 
 ca ) . sediment samples were freeze - dried and then 
 extracted following a modified epa method 3620c . the details of sediment 
 extraction and method performance may be found in si . the final extracts were reconstituted in 1 ml of hexane 
 and analyzed by gc  ms / ms . in the 
 use of idm for bioaccessibility prediction , the fraction of native 
 hoc reversibly sorbed may be inferred from the sorption of the externally 
 added , isotope - labeled hoc counterpart ( * hoc ) . the principle of idm 
 is based on assumptions that * hocs added to the sediment distribute 
 themselves just between the aqueous phase and the labile fraction 
 after short - term mixing , and that the addition of * hocs does not modify 
 equilibrium of the native hocs . under these assumptions , the distribution 
 of native hocs in sediment may be expressed as1where ct ( g / kg ) 
 is the total hoc concentration in sediment , cw ( g / l ) is the hoc concentration in the solution phase , cse ( g / kg ) is the accessible concentration of sediment - sorbed 
 hoc , csne ( g / kg ) is the nonaccessible concentration of sediment - sorbed 
 hocs , and d ( l / kg ) is the dilution factor equaling 
 the ratio of the solution volume to the solid mass . the ct may be characterized through an exhaustive extraction 
 following the same method as described above . the overall accessible 
 concentration ce ( g / kg ) in sediment 
 is2when * hoc is 
 introduced into the sediment water 
 binary system , it will readily redistribute itself between the solution 
 and labile phases , which are in dynamic equilibrium3where * cw ( g / l ) 
 is the concentration of * hoc in the solution 
 and * cse ( g / kg ) is the sorbed concentration of * hoc . the total 
 concentration of * hoc in sediment ( * c0 , g / kg ) is4combining eqs 3 and 4 , cse may be given as5combining 
 eq 2 and 5 , accessible 
 concentration ce may be given as6furthermore , the ratio of ce over the 
 total chemical concentration ct gives 
 the accessible fraction of the hoc , e:7 a similar protocol to the above batch 
 equilibration experiment was followed to evaluate partition kinetics 
 of externally added * hocs and to estimate ce and e from simultaneous measurements of cw and * cw . briefly , 
 to a 2 g sediment and 20.0 ml of artificial seawater , 10 l 
 of a working solution containing each stable isotope - labeled hoc at 
 5 mg / l was added . the sediment  water slurry was mixed at 120 
 rpm on a horizontal shaker . triplicate samples were removed after 
 0 , 0.5 , 2 , 4 , 6 , 12 , 24 , 48 , 96 , or 192 h for both pv8c and pv6c sediments . samples were centrifuged at 670 g for 30 min for phase 
 separation , and the supernatant was collected . both the sediment and 
 supernatant were separately analyzed for levels of hocs following 
 the same methods as described above . determination of * hocs in both 
 the solution and solid phases allowed construction of isotope dilution 
 kinetics and estimation of the time for isotope tracers to reach dilution 
 equilibrium . when * hocs in the aqueous phase reached a steady state , 
 the measured cw and * cw , along with * c0 , were used 
 to calculate ce and e of the native hocs in the target sediment samples using eqs 6 and 7 . to validate the performance 
 of idm , the same sediment samples were subjected to sequential desorption 
 in which tenax beads were used to recover the desorbed hocs to facilitate 
 measurement . tenax - aided desorption has been previously used in a 
 number of studies , and fit of the desorption curve to a three - phase 
 model gives estimates of the relative distribution of sorbed - hocs 
 in the rapid , slow and very slow desorption pools . briefly , a 
 2-g aliquot of sediment was placed in a 50 ml polyethylene centrifuge 
 tube . tenax beads ( 0.15 g , scientific instrument services , ringoes , 
 nj ) and 20 ml artificial seawater with 200 mg / l sodium azide were 
 added . for pv6c , after 1 , 2 , 6 , 12 , 24 , 48 , 96 , 144 , 192 , or 312 
 h of shaking , the sample with tenax was centrifuged at 670 g for 30 min . the trapped beads were rinsed thoroughly with the deionized water 
 and air - dried before they were transferred to 20 ml glass vials and 
 then mixed in 3 ml of acetone - hexane ( 1:1 , v : v ) , followed by sonication 
 for 5 min by a fisher sonic 550 ultrasonic processor . the extraction 
 was repeated three times . the extracts were combined , concentrated 
 under n2 to near dryness , and reconstituted in 1 ml of 
 hexane . a similar protocol was used for pv8c sediment . however , 
 owing to its high levels of ddxs and pcbs , the sequential desorption 
 was continued up to 837 h to obtain the entire desorption kinetics . the desorption kinetics up to 312 h for the pv6c sediment and up 
 to 837 h for the pv8c sediment were used to construct the desorption 
 curve and for estimating the rapid ( fr ) , slow ( fs ) , and very slow ( fvs ) desorption fractions by fitting data to 
 a triphasic model using sigmaplot 11.0 
 ( san jose , ca)89where st and s0 ( g / kg ) 
 are concentrations of hocs in 
 sediment after desorption time interval t ( h ) and 
 before the initiation of desorption , respectively , and kr , ks , and kvs are the rate constants ( h ) for 
 the rapid , slow , and very slow desorption fractions , respectively . the usefulness of idm was 
 further demonstrated by evaluating the correlation between ce and uptake of hocs into a marine deposit feeder neanthes arenaceodentata . the selected marine polychaete 
 is a predominant benthic invertebrate in the surface and subsurface 
 macrofauna community in the pvs area and is an important food source 
 for the bottom feeding flatfish california halibut ( paralichthys 
 californicus ) . the overall method 
 for the bioaccumulation test was modified from the american society 
 for testing and materials ( astm ) sediment toxicity test with polychaetous 
 annelids . briefly , 23 week old n. arenaceodentata worms ( aquatic toxic support , bremerton , 
 wa ) were acclimated in aerated artificial seawater at room temperature 
 ( 20  1 c ) for 1 week and fed with dry algae . after the 
 acclimation , 15 worms were transferred to a 1 l beaker containing 
 20 g ( dry weight basis ) of pv8c or pv6c sediment and 350 ml artificial 
 seawater . the worms were exposed at 20 c for 4 d with air bubbled 
 into the overlying water . the water level , salinity , and ammonia contents 
 were checked daily and kept constant throughout the exposure . after 4-d of 
 exposure , the worms were retrieved by sieving and depurated in clean 
 artificial seawater ( 32 ) for 48 h. the analysis of the tissue 
 samples for ddxs and pcbs and lipid content are given in si . several practices 
 were used to ensure the accuracy and reproducibility of sample analysis . first , the recoveries of target hocs and surrogate standards spiked 
 in the sediment and worm tissue samples were verified in method development 
 ( si ) . second , both external surrogates 
 and internal standards were spiked in all samples to monitor the extraction 
 efficiency and check for instrument drift in sensitivity ( si ) . in addition , a laboratory blank was included 
 with every batch of nine samples to monitor for potential contamination 
 from laboratory materials and solvents . furthermore , a standard reference 
 material , srm 1947 lake michigan fish tissue , was purchased from the 
 national institute of standards and technology ( nist ) and used for 
 validating the analysis of tissue samples of n. arenaceodentata in this study . the calibration curve standards were prepared daily 
 and were used only when the regression coefficient was greater than 
 0.999 . the difference between different treatments was determined 
 by one - way analysis of variance ( anova ) and tukey test , using spss 
 15.0 ( spss , chicago , il ) . the measurement of ddxs and 
 pcbs in the sediment phase ( cs ) and in 
 the aqueous phase ( cw ) over time allowed 
 the characterization of phase distribution kinetics of the extensively 
 aged hocs in the marine sediments . figures s1 and s2 ( si ) display the relative phase distribution of 
 hocs in the pv8c and pv6c sediments at different time intervals . in 
 the pv8c sediment , an apparent phase equilibrium was reached before 
 12 h of mixing , and the apparent kd remained 
 constant for up to 96 h. one - way anova analysis showed that there 
 was no difference among the distribution values beyond 12 h for pcbs 
 ( p = 0.1630.365 ) , ddes ( p = 0.2270.528 ) , or ddds ( p = 0.4330.911 ) . 
 in the pv6c sediment ( figure s2 , si ) , it 
 appears that o , p-ddd and p , p-ddd reached an apparent equilibrium within 12 h , while all 
 pcbs and ddes reached a steady state after 24 h. this observation 
 was in agreement with other studies , where phase equilibrium was reached 
 between sediment and liquid phases for ddt and its metabolites within 
 4 h. therefore , 24 h was considered as 
 the time duration sufficient for achieving phase distribution equilibrium 
 for the native hocs in the historically contaminated sediments in 
 this study . the time necessary for the externally added * hocs 
 to reach phase distribution equilibrium was subsequently determined . 
 upon introduction to the sediment slurry , * hocs rapidly partitioned 
 to the sediment phase and an apparent phase distribution equilibrium 
 was reached after a short period of mixing . figure 1 shows the changes of aqueous ( * cw/*c0 ) or sediment phase fraction ( * cs/*c0 ) of c - pcb 52 and p , p-dde - d4 as a 
 function of mixing time . the kinetics of other * hocs may be found 
 in the si ( figure s3 ) . the strong sorption 
 of * pcbs and * ddxs to the sediment resulted in rapid dilution of the 
 added isotope tracers in the aqueous phase , as reflected from the 
 rapid decreases in * cw/*c0 ( figure 1 ) . for example , after 
 2 h of mixing , 90.2% and 89.1% of the initially added c - pcb 52 and p , p-dde - d4 were 
 found in the sediment phase for the pv8c sediment or the original 
 concentration in the solution phase was diluted by 10.2 and 9.1 times , 
 respectively . high percentages of dilution were also found for the 
 other isotope - labeled hocs ( 7.928.9 fold ) in the pv8c sediment 
 at 2 h ( figure s3 , si ) . fast partitioning 
 of * hocs from 24 to 192 h , the fractions of * hocs in both sediment 
 and aqueous phases were essentially constant , suggesting that phase 
 distribution of the isotope - labeled tracers reached an apparent equilibrium . one - way anova analysis showed no significant change in the fraction 
 of * hocs in the sediment or solution phases ( p = 
 0.2760.890 for * pcbs , p = 0.5360.624 
 for * ddes , and p = 0.3880.769 for * ddds ) 
 for the pv8c sediment between 24 and 192 h ( figures 1 and s3 , si ) . similar results ( p = 0.1090.878 ) were also observed for the pv6c 
 sediment ( figure s4 , si ) . at the steady 
 state , less than 2% of the introduced * hocs remained in the aqueous 
 phase for either pv8c or pv6c sediment . fractions of isotope 
 tracers ( * hoc ) in the sediment ( solid triangle ) 
 and liquid phase ( open triangle ) in the pv8c sediment slurry as a 
 function of mixing time : ( a ) c - pcb 52 ; ( b ) p , p-dde - d4 . the apparent kd values calculated 
 for 
 native hocs using the derived cs and cw ( figure s5 , si ) 
 showed no detectable influence from the addition of isotope analogues . the kd values of each hoc obtained from 
 the previous batch sorption experiment and the ones after the addition 
 of isotope tracers were found to be essentially identical ( tables 1 and s2 , si ) ( paired t test , p = 0.209 ) . this finding suggested 
 that addition of isotope tracers did not alter the phase distribution 
 of the native hocs , validating one of the prerequisites for idm . this 
 was expected as the tracers were isotope - labeled counterparts of the 
 native hocs and should have very similar physicochemical properties . when the isotope dilution reached an apparent equilibrium ( e.g. , 
  24 h ) , the accessible concentration ce was estimated using eq 6 with the derived cw , * cw , and * c0 ( predetermined ) . the ce values of pcbs or ddxs were much higher in pv8c than those 
 in pv6c ( table 2 ) , which coincided with the 
 overall higher contamination levels of hocs at the pv8c location . the overall ce of 
 ddxs and pcbs were estimated at 14052 and 223 g / kg for the 
 pv8c sediments , respectively , which was consistent with the overall 
 higher levels of ddxs in these sediments than pcbs . the relative accessibility e was calculated as 
 the ratio of ce over ct for each hoc in the sediments ( table 2).the use of e eliminated the effect of different 
 contamination levels , allowing more straightforward comparison of 
 contaminant accessibility . the derived e values were 
 all smaller than 1 , with most e values < 0.50 for 
 the hocs in the pv8c sediment ( table 2 ) , clearly 
 suggesting that a significant fraction of the native hocs was in regions 
 of sediment that were not accessible by the externally added * hocs . the derived e values of pcbs and ddxs in the pv8c 
 sediment were consistently smaller than those in the pv6c sediment , 
 which may be attributed to the higher oc content in the pv8c sediment 
 ( 3.54% ) as compared to that in the pv6c sediment ( 2.34% ) . for example , 
 the averaged e values of ddes and ddds were 0.37 
 and 0.28 , respectively , in the pv8c sediment , which were smaller than 
 those in the pv6c sediment ( 0.60 for ddes and 0.65 for ddds ) . in the 
 same sediment , for example , in the pv6c sediment , e values 
 for pcb 52 ( 0.89 ) , pcb 70 ( 0.62 ) , and pcb 153 ( 0.58 ) displayed a decreasing 
 trend as the chlorine substitution or kow increased . in the pv8c sediment , a 
 similar trend was not observed , and the e values 
 for the different pcb congeners were statistically the same . this 
 suggests that there were other factors ( e.g. , aging time ) affecting 
 the accessibility of hocs . contact time or aging has been often found 
 to diminish the availability of pcbs in sediments . the sequential tenax - aided desorption was applied 
 to the same pv8c and pv6c sediments to evaluate hoc accessibility 
 by fitting the desorption curves ( figure s6 , si ) to the triphasic model to derive fr , fs , and fvs . the regression fit to the model was excellent for all sediment - hoc 
 treatments ( table s3 , si ) . the estimated 
 rapid fraction ( fr ) was consistently smaller 
 in the pv8c sediment than that in the pv6c sediment for the same compound . for example , the averaged fr values were 
 0.15 , 0.08 , and 0.10 for pcbs , ddes , and ddds in the pv8c sediment , 
 which were smaller than those in the pv6c sediment ( 0.30 , 0.30 , and 
 0.33 , respectively ) . however , fr of hocs 
 was only about a half of e given by idm for the same 
 hoc . for example , in the pv8c sediment , the e and fr of pcb 52 were 0.49 and 0.22 , respectively . similar differences were observed in a previous study where spiked 
 sediments ( pyrene and bifenthrin ) were considered . studies showed that in addition to fr , the slow desorption fraction fs also contributed to bioavailability . for instance , when the rapid 
 desorption fraction was removed , pahs in the slow desorption compartment 
 migrated to the rapid desorption pool over time . when benthic deposit feeders were exposed to sediments 
 with the rapid desorption fraction removed , there was still accumulation 
 of pcbs and pahs . these observations 
 together suggest that bioaccessibility of sediment - borne hocs may 
 encompass both fr and fs as estimated from desorption kinetics . a good 
 linear relationship ( r = 0.86 , p < 0.01 ) was observed between the e values and the sum of fr and fs ( figure 2 ) . the linear 
 slope ( 1.12  0.13 ) was close to 1 , implying that the accessibility 
 predicted by idm was close to the sum of fr and fs estimated from desorption kinetics 
 and likely a better indicator for bioavailability than fr . the concentrations of pcb 153 in the pv8c and pv6c 
 sediments were relatively low ( 26.3 and 11.4 g / kg ) , resulting 
 in a large variance in the estimated fr , fs , and fvs values for this pcb congener . for example , fr , fs and fvs of pcb 153 in the pv6c sediment were 0.10  0.33 , 0.38 
  3.42 , and 0.52  3.74 , respectively . if the data points 
 for pcb 153 were excluded from the correlation , the linear relationship 
 further improved substantially ( r = 0.93 , p < 0.01 ) . linear correlation between the accessibility 
 ( e ) given by the isotope dilution method and the 
 sum of rapid and slow 
 desorption fractions ( fr + fs ) derived by tenax desorption ( slope = 1.12  0.13 , r = 0.86 , p < 0.0001 ) . the performance of idm was further 
 evaluated by comparing the measured ce of hocs with 4 d accumulation of hocs into 
 the marine polychaete n. arenaceodentata . a previous 
 study showed that the uptake of hocs by the polychaete reached a steady 
 state after 4 d of exposure . after 4 
 d of exposure , tissue residue ( cb ) of 
 hocs was analyzed and expressed on both dry weight ( dw ) and lipid 
 weight ( lw ) bases . owing to the relatively low levels of pcbs in the 
 sediment , only ddes and ddds were reliably detected in the tissue 
 samples . for example , cb of ddes was 1595.0 
  326.8 g / kg ( dw ) for the pv6c sediment exposure and 3282.0 
  192.9 the values for 
 ddds were 42.1  11.7 and 161.2  20.6 g / kg ( dw ) , 
 respectively . strong linear relationships were found between ce ( g / kg ) normalized by oc content and cb ( g / kg ) normalized by lipid content 
 for ddes and ddds in the pv8c ( r = 0.84 ) and pv6c ( r = 0.94 ) sediments ( figure 3 ) . the slopes for the linear relationships were 1.88  
 0.57 and 1.20  0.22 for pv8c and pv6c sediments , respectively . although more studies may be required to test other hocs or organisms , 
 the accessible concentration ce derived 
 by idm was clearly predictive of the bioaccessibility of ddt derivatives 
 in the historically contaminated marine sediments to the benthic polychaete n. arenaceodentata . linear correlation between the lipid content - normalized 
 tissue 
 residues of ddxs ( log cb , g / kg - lipid ) 
 in n. arenaceodentata and the organic carbon - normalized 
 accessible hoc concentrations ( log ce , 
 g / kg - oc ) in both the pv8c ( solid circle ) and pv6c sediments 
 ( open circle ) . as demonstrated in this 
 study , the use of idm to derive ce or e involves only a few 
 simple steps , i.e. , addition of isotope - labeled analogues to a sediment 
 sample , mixing the sample for a relatively short duration ( e.g. , 24 
 h ) , and analyzing the aqueous concentrations of both labeled and nonlabeled 
 ( native ) hocs using a ms - equipped chromatographic system . standards 
 labeled with c or deuterium are widely available , especially 
 for legacy contaminants . at present , in fact , many 
 laboratories already use stable - isotope labeled standards as recovery 
 surrogates or internal standards in routine analysis of environmental 
 samples . therefore , the measurement of ce or e may require only minor modifications to existing 
 procedures . in this study , the pv6c and pv8c sediments contained 
 very different levels of ddxs and pcbs . the isotope - labeled analogues 
 were added at a fixed rate ( i.e. , 25 g / kg ) to the sediment 
 samples , regardless of the bulk sediment concentrations of the individual 
 hocs . when expressed as the ratio to the native hoc level , * c0/ct varied from 
 0.09% to 82.4% for the pv8c sediment and from 1.3% to 301% for the 
 pv6c sediment . therefore , results of this study suggested that it 
 is not necessary to control the ratio of * c0/ct in the use of idm . this offers the 
 advantage that the concentrations of native hocs in a contaminated 
 sample need not to be known beforehand and that a single rate of isotope - labeled 
 tracers may be used irrespective of the levels of native hocs . this 
 lends a great versatility to the method because field - contaminated 
 samples often contain a mix of hocs over a wide range of concentrations . another important consideration in the use of idm is the time for 
 the externally introduced isotope tracers to reach dilution equilibrium . 
 in the present study , there was no significant difference in 
 the time to equilibrium among the different hocs or between different 
 sediments . as shown in this study , isotope dilution did not change 
 appreciably during 24192 h for both sediments and all hocs 
 considered . therefore , after dilution equilibrium is reached , the 
 specific time interval used for mixing is not critical , which adds 
 flexibility to the method . moreover , a mixing interval of 24 h is 
 relatively short and convenient , contributing to a high sample throughput 
 and hence feasibility . the pv shelf superfund site represents 
 a vast contaminated area 
 where a multitude of factors could have affected the concentration 
 profiles of ddxs and pcbs at a given location , including effluent 
 discharge history , distance to the outfalls , topography of the ocean 
 floor , and sediment deposition , burial , and resuspension caused by 
 ocean currents . the two locations considered in this study , pv8c and 
 pv6c , were about 2 km apart , with pv8c close to the outfalls . as shown 
 in this study , contaminant accessibility as estimated by idm ranged 
 from 0.28 to 0.89 and was substantially smaller than 1 for most hocs , 
 suggesting that aging and other factors may have significantly decreased 
 the bioavailability of hocs at this superfund site . thus , the use 
 of bioavailability - relevant measurements over bulk sediment levels 
 may provide improved risk assessment that is valuable in decision 
 making , in , e.g. , designation of specific areas for remediation . in 
 addition , findings from the current study also offered a preliminary 
 comparison between locations at the pv shelf site . the relative accessibility 
 of ddxs and pcbs in the pv8c sediment was consistently smaller than 
 in the pv6c sediment . although this study only considered 
 marine sediments and a select group of hocs , the principle of idm 
 should be similarly applicable to other solid matrices ( e.g. , soil ) 
 and contaminants and should be further explored .