Critical Review

Estimating Dissolved Organic Carbon PartitionCoefficients for Nonionic Organic ChemicalsL A W R E N C E P . B U R K H A R D *

U.S. Environmental Protection Agency, Office of Research and Development,National Health and Environmental Effects Research Laboratory, Mid-ContinentEcology Division, 6201 Congdon Boulevard, Duluth, Minnesota 55804

A literature search was performed for dissolved organiccarbon/water partition coefficients for nonionic organicchemicals (KDOC), and KDOC data were taken from more than70 references. The KDOC data were evaluated as afunction of the 1-octanol/water partition coefficients(KOW). A predictive relationship of KDOC ) 0.08KOW with95% confidence limits of a factor of 20 in either directionwas developed using KDOC data based upon naturallyoccurring dissolved organic carbon. Inclusion of KDOC datafor Aldrich humic acid, a reagent-grade organic carbon,resulted in a slightly different predictive relationship of KDOC) 0.11KOW with 95% confidence limits of a factor of 14in either direction. The large uncertainties in theserelationships are, in part, caused by the variability instructure and composition of dissolved organic carbon(DOC) in sediments, soils, and surface waters. This variabilityis not accounted for by the hydrophobicity parameter.For individual chemicals, ranges in KDOC values approaching2 orders of magnitude were observed among investigationsusing Aldrich humic acid as the DOC. These largeranges of KDOC values suggest that measurement techniquesare also, in part, responsible for the large uncertaintiesin these relationships.

IntroductionThe bioavailability, fate, and behavior of hydrophobic organicchemicals (HOCs) in aquatic ecosystems are directly influ-enced by the dissolved and particulate organic carbonpresent. To understand and quantify the importance ofdissolved organic carbon (DOC), numerous investigators havestudied the binding/sorption of HOCs to DOC using a widevariety of DOCs. Sources of DOC studied include surfacewaters, sediment and soil porewaters, groundwaters, Aldrichhumic acid and other commercially available humic andfulvic acids, and humic and fulvic acids isolated fromsediments, soils and surface waters. Typically, sorption ofHOCs to DOC is expressed as a partition coefficient of thechemical between the DOC and freely dissolved phases (KDOC)on an organic carbon basis, i.e., L/kg of organic carbon.

Determination of the KDOC requires the measurement orcalculation of the amount of chemical sorbed to the DOCand the freely dissolved concentration of the chemical in theaqueous phase. To determine freely dissolved concentrationsof chemicals in solutions containing DOC, a variety of

analytical techniques are available, fluorescence quenching(1), purging or sparging techniques (2, 3), solid-phasemicroextraction (SPME) (4, 5), equilibrium dialysis (6),solubility enhancement (7), ultrafiltration (8), reverse-phaseHPLC separation (9), size exclusion chromatography (2), andliquid-liquid extraction (2, 10). Some of these techniquesmeasure directly the concentration of the freely dissolvedchemicals, while others physically separate the DOC-boundchemical from the freely dissolved chemical. All of themethods for measuring freely dissolved chemical in waterhave some type of limitation, and these limitations can leadto large uncertainties in the determinations of KDOC. Thetechniques that appear to have smaller biases are SPME,sparging, fluorescence quenching, and possibly equilibriumdialysis because these techniques cause the least disruptionin the existing partitioning between the freely dissolved andsorbed phases while making the their measurements. Exceptfor the SPME technique, Suffet et al. (11) has presented anexcellent discussion on the individual techniques and theirlimitations. The reader is urged to consult Suffet et al. (11)as well as the individual references listed above for furtherinformation on the analytical techniques.

The freely dissolved chemical concentrations in sedimentporewaters and surface waters are generally accepted as beingthe best measure, currently available, for the bioavailablefraction of nonionic organic chemicals to aquatic organisms(11, 12). Freely dissolved concentrations can be estimatedusing a three-phase partitioning model (13):

where ffd is the fraction of chemical freely dissolved, [POC]is the concentration of particulate organic carbon, KPOC isthe partition coefficient between particulate organic carbonand freely dissolved chemical, [DOC] is the concentration ofthe dissolved organic carbon, KDOC is the partition coefficientbetween dissolved organic carbon and freely dissolvedchemical, Cw

fd is the freely dissolved concentration of thechemical in the water, and Cw

t is the total concentration inthe water sample, i.e., the sum of the chemical sorbed toPOC and DOC, and the freely dissolved chemical. With properknowledge of the POC and DOC partition coefficients, freelydissolved concentrations of chemical can be estimated.

The above definition of the bioavailable chemical in water,i.e., the freely dissolved chemical, has been used to determineKDOC values with aquatic organisms (14). In performing thesedeterminations, organisms were exposed to chemicals in thepresence of varying concentrations of DOC, and biocon-centration factors (BCFs) were measured. With the measured

* Corresponding author phone: (218)529-5164; fax: (218)529-5003;e-mail:burkhard.lawrence@epa.gov.

ffd ) 1/(1 + [POC]KPOC + [DOC]KDOC)

Cwfd ) Cw

t ffd

10.1021/es001269l Not subject to U.S. Copyright. Publ. 2000 Am. Chem. Soc. VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4663Published on Web 10/21/2000

BCFs, KDOC is determined using nonlinear regression analysiswith the following equation (14):

where BCFDOC is the bioconcentration factor for a chemicalin water containing DOC, and BCF0 is the bioconcentrationfactor for a chemical in water containing none of the DOCof interest, e.g., dilution water used in toxicity testing. Insolving this equation for KDOC, BCFDOC values determinedusing four to eight DOC concentrations are required. Thismethod of determining KDOC differs from the analyticaltechniques listed above in that the freely dissolved chemicalconcentrations do not have to be measured. This methodassumes that no particulate organic carbon is present andthat only the freely dissolved chemical can be taken up bythe aquatic organisms. Biotransformation of the HOC by theorganisms should not be an issue with this technique becauseregardless of the DOC content of the water, all BCFs wouldbe similarly affected (15).

Research performed to date suggests that KDOC is notstrongly dependent upon the hydrophobicity of the chemical,i.e., 1-octanol/water partition coefficient of the chemical(KOW). For example, Kukkonen and Oikari (16) and Evans(17), using a variety of surface water samples with differingDOC concentrations, reported ranges for KDOC values of morethan an order of magnitude for benzo[a]pyrene and PCBcongeners. Similarly, Perminova et al. (18) using 26 differenthumic materials reported ranges for KDOC values of ap-proximately an order of magnitude for pyrene, fluoranthene,and anthracene. Using Lake Michigan data, Eadie et al. (19)developed a log KDOC - log KOW relationship with a slope of0.24, which also suggests a weak dependence of KDOC uponKOW. However, McCarthy and Jimenez (20) and Freidig et al.(21) reported log KDOC - log KOW relationships using Aldrichhumic acid with slopes (correlation coefficient) of 1.03 (0.98)and 0.67 (0.90), respectively, which suggest that there is somedependence of KDOC upon KOW. In contrast, partition coef-ficients between sediments/soils organic carbon and water(KOC) have been found to be strongly dependent upon theKOW of the chemical. In a recent evaluation of the KOC databy Seth et al. (22), KOC was found to be equal to 0.35KOW with95% confidence limits of a factor of 2.5 in either direction.Previously, Karickhoff (23) and DiToro et al. (11) reportedstrong dependence of KOC upon KOW, KOC ) 0.41KOW and KOC

) KOW, respectively.

Humic substances are the major component of the naturalorganic carbon in water, sediment, and soil systems, e.g.,50-80% (18, 24). Therefore, much research has been focusedon determining the sorption/binding affinity of HOCs tohumic substances as related to composition and structureof the humic substances, e.g., the aromaticity, H/C and O/Catomic ratios, UV absorptivity, pH, molecular weight, andNMR descriptors of structure (16, 18, 25-27). In general, theaffinities of HOCs for fulvic acids are smaller than those forhumic acids, thus resulting in smaller KDOC values for fulvicacids in comparison to humic acids. Aldrich humic acid, acommonly used reagent-grade humic acid, has binding/sorption affinities for HOCs that are larger than those forDOC and naturally occurring humic and fulvic acids (11).Unfortunately, a definitive model for predicting KDOC valuesis not available for DOCs from surface waters or sediments.Nevertheless, predictive relationships have been developedbecause of the dire need for such a model. These include arelationship of KDOC ≈ 0.135KOW derived by Kopinke et al.(28) using Hildebrand solubility parameters and a relationshipof KDOC ) 0.1 KOW proposed by the U.S. EnvironmentalProtection Agency for use in deriving freely dissolved chemical

concentrations with the three-phase partitioning model (29).These relationships as well as those based upon Aldrich humicacid above were developed using small KDOC data sets, bothin numbers of chemicals and types of DOC.

In this study, the results of a literature review for KDOC

measurements are reported and evaluated. The objective ofthis report is to develop predictive relationships for KDOC

using KOW with a comprehensive KDOC data set. Sources ofDOC used in the measurements include (a) Aldrich humicacid; (b) humic and fulvic acids isolated from surface waters,sediments, and soils; (c) sediment and soil porewaters; (d)groundwaters; and (e) surface waters, e.g., lakes, rivers, andestuaries.

MethodsA literature search was performed using SciFinder by CAS(30), and the KDOC data, obtained from 73 references, arereported in Table 1 (in Supporting Information). For somereports, KDOC values were estimated from the figures becausenumerical values for the KDOC values were not reported. Inthis analysis, KDOC data for effluents from industrial facilitieswere not used because the DOC in these effluents wasbelieved to be substantially different from that in theenvironment. KDOC data for ionizable (polar) organics werenot used in this analysis as well because the partitioningbehaviors of ionic and nonionic organic chemicals aredifferent (31). These data, even though they were not used,are reported in Table 1 (in Supporting Information). Alsoincluded in Table 1 (in Supporting Information) are KDOC

values determined by model calculations from field measuredKD values, the partition coefficient of the chemical betweenthe POC and the water passing a filter. These values were notused in the data analysis but were plotted in the figures forcomparison purposes. In Table 1 (in Supporting Information),three data sets are reported that were not used in the dataanalysis. These were the fulvic acid data of Burgess and Ryba(32), which differed by an order of magnitude between thetwo measurement techniques used in the study; the Aldrichhumic acid data of Johnsen (10), where equilibrium condi-tions were not obtained; and the ambient water data of Naeset al. (33) determined using polyurethane foam plugs, atechnique not used by any other investigator.

All regression analyses were performed using the geo-metric mean regression technique (34) because both the Xand Y variables were measured with error. These regressionswere performed using the equations provided by Ricker (34)and Webb et al. (35).

Results and DiscussionIn Figure 1, KDOC values are plotted for five DOC sources asa function of the chemicals’ KOW. For DOCs consisting ofAldrich humic acid and sediment porewaters, linear rela-tionships between log KDOC and log KOW were observed withslopes of approximately 1 with correlation coefficients of0.77 and 0.64, respectively (Table 2). In contrast, log KDOC

data for humic and fulvic acids without Aldrich humic acids,soil porewaters and groundwaters, and surface waters havea slight dependence upon log KOW although their slopes aresimilar to 1. For the latter three DOC sources, their correlationcoefficients are rather small, ≈0.3; therefore, their regressionequations should be viewed with some caution. For most ofthe DOC sources, the KDOC data are very unevenly weightedamong the individual chemicals. For example, biphenyl andbenzo[a]pyrene had 4 and 49 KDOC measurements, respec-tively, for DOC from surface waters. This unevenness causedsome difficulties in the regression analyses because theregression fits are skewed toward the chemicals with moremeasurements.

From an analytical and methodological perspective, theAldrich humic acid KDOC data should provide the best measureof overall analytical precision among the investigations

BCFDOC ) BCF0/(1 + KDOC[DOC])

4664 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 22, 2000

because the DOC was the same in all measurements. Forty-one chemicals were found to have more than one measure-

ment, and the 10th, 50th, (median), and 90th percentile andpooled standard deviations for these measurements were0.08, 0.24, 0.47, and 0.40 on a log KDOC basis, respectively.The differences between the smallest and the largest mea-surements ranged from 0.02 to 1.92 log units with a mediandifference of 0.48 among the 41 chemicals. An estimate ofthe 95% confidence limits on a relative scale can be obtainedby doubling the pooled standard deviation and then trans-forming to the antilog scale. This results in a 95% confidencelimits of a factor of 6.3 in either direction. Comparing theselimits to those of Seth et al. (22) for KOC, a factor of 2.5 dem-onstrates that measurement of KDOC is less precise than forother partitioning measurements. Because the Aldrich humicacid studies were all performed using a common source ofDOC, the range of a factor of 6.3 in either direction does notinclude variability due to differences in DOC compositionand structure. Therefore, one should expect even largerconfidence limits when other DOC sources are included inthe analysis. A quick perusal of Figure 1 reveals this trend.

Visual comparison of the KDOC values (Figure 1) suggeststhat the measurement methods, in general, provide similarKDOC data. If any biases exist, the biologically determinedKDOC data might be slightly higher, on average, than thoseobserved with the other techniques; the equilibrium dialysisand solubility enhancement techniques appear to provideslightly lower KDOC values than the other techniques for thehumic and fulvic acids (Figure 1). Given the differences inDOC sources, composition, and structure among the inves-tigations, a robust analysis for analytical and methodologicalbiases is not possible with the data.

The KDOC data arising from surface water DOC appears tohave slightly more scatter or variability than that observedfor the other DOC sources. On a diagenesis basis, surfacewater DOC might be expected to be more variable than DOCfrom sediments, soils, and humic and fulvic acids becausesurface water DOC contains detritus from recently deceasedplankton and algae, macrophytes, etc. Some of the variabilityin all sources of DOC including surface waters is caused bydifferences among the measurement techniques. Compari-sons of the reverse-phase and dialysis techniques by Landrumet al. (9) and Kukkonen and Pellinen (36) suggest thatdifferences of an order of magnitude or more can occurbetween these two methods in some cases. More typically,these investigators found the dialysis technique providinghigher values, i.e., factors of 2-5. Landrum et al. (9) performedside-by-side KDOC measurements with biphenyl, log KOW )4.09, using water samples from Lake Erie and Huron River.The KDOC data differed by factors of 3 and 34 between thesetwo techniques, respectively. Some of the variability in theKDOC data from surface waters might also be related to whenthese measurements were made because most of themeasurements occurred in the 1980s when methodologiesfor measuring KDOC values were evolving or new.

To provide a comparison of the KDOC values for all DOCsources, KDOC values for each chemical were averaged acrossanalytical methods within each DOC source and replotted(Figure 2). Average values were determined in part becauseof the rather unevenness of the data set in numbers ofmeasurements per chemical. The plot of log KDOC versus logKOW shows considerable consistency, and a strong depen-dence of KDOC upon KOW is apparent (Figure 2). The averageKDOCvalues for the Aldrich humic acids are on average higherthat those derived from natural sources (Figure 2), and theseresults are consistent with the differences in affinities forAldrich humic acid and naturally occurring organic carbonreported in the literature for HOCs (11). The data used inthese regressions contained average KDOC values for 14, 110,84, 7, and 22 chemicals for the DOC sources of naturally

FIGURE 1. KDOC values determined using the reverse-phase (circle),equilibrium dialysis (open square), sparging (plus diamond), cal-culated/model-derived (downward triangle), fluorescence quench-ing (upward triangle), solubility enhancement (open diamond),biological (plus square), and solid-phase microextraction (openplus diamond) techniques for DOCs from different sources. Thegeometric mean regression and their 95% prediction confidencelimits are plotted.

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occurring humic and fulvic acids (no Aldrich humic acid),all humic and fulvic acids including Aldrich humic acid,sediment porewaters, soil porewaters and groundwaters, andsurface waters, respectively (Table 2).

On a theoretical basis, the equation log KDOC, KPOC, or KOC

) A log KOW + B has a slope of 1 when the ratio of the activitycoefficients of the chemical in octanol to that in the organiccarbon phase is constant for chemicals with different KOW

values (22). The relationships derived by Seth et al. (22),DiToro et al. (12), and Karickhoff (23) of log KOC ) A log KOW

+ B had slopes of 1 and are clearly consistent with thehypothesis that the ratio of activity coefficients is constant.Given the above theoretical basis and experimental data, aslope of 1 was assumed for the relationship in this investiga-tion, i.e., log KDOC ) log KOW + B. This equation, byrearrangement, results in B ) log KDOC - log KOW ) log-(KDOC/KOW), and B can be found by averaging the differencesof the log KDOC and log KOW for the individual chemicals orby averaging the logarithms of the ratio of the KDOC to KOW

for the individual chemicals. For the data set consisting ofnaturally occurring DOC (no Aldrich humic acid), an averagedifference (standard deviation, number of data points) of-1.11 (0.659, 127) was obtained. Transforming the averagedifference to an antilog scale results in a predictive relation-

ship of KDOC ) 0.08KOW with the 95% confidence limits of afactor of 20 [antilog of (st(R ) 5%,df ) 126) ) 0.659 × 1.979)] ineither direction from the predicted mean KDOC. When Aldrichhumic acids are included, an average difference (standarddeviation, number of data points) of -0.966 (0.578, 223) wasobtained, and after transformation to an antilog scale, apredictive relationship of KDOC ) 0.11KOW with 95% confidencelimits of a factor of 14 in either direction was obtained. Theresiduals for the predictive relationship, experimental logKDOC values minus predicted log KDOC values, with naturallyoccurring DOC have a slight dependence upon the KOW

(Figure 3). The distribution of the residuals is normallydistributed (R ) 10%) with 64 negative and 63 positiveresiduals.

Analyses of the average KDOC values were also performedon a chemical class basis using the subsets of polychlorinatedbiphenyls (PCBs) and polycyclic aromatic hydrocarbons(PAHs). The regressions with and without the Aldrich humicacid data were about the same; therefore, only the regressionswithout the Aldrich data were reported in Table 2. The slopeof the PAH regression was not significant different from 1.0(R ) 0.05) whereas the slope of the PCB regression was slighlylower and significantly different from 1.0. However, PCB dataextend to much higher KOWvalues, and the depression of theslope is caused, I believe, by the difficulties in performingthe freely dissolved measurements for log KOW valuesexceeding 6.5. These difficulties would cause the reportedKDOC values to be too small. With some PCB data, log KDOC

values increase linearly with increasing log KOW (37) whereaswith other data, log KDOC values plateau with increasing logKOW (38). These inconsistencies among PCB data for higherKOW PCBs are suggestive and consistent with a methodologicalbias and/or analytical difficulties. The average differences(standard deviation, number of data points) of the log KDOC

and log KOW for the individual PCBs and PAHs were -1.24

TABLE 2. Geometric Mean Regression Equations for KDOC upon KOW

DOC source geometric mean regression equation na r sxy

Aldrich humic acid log KDOC ) 0.85 ((0.03)b‚log KOW + 0.27 ((0.20) 269 0.77 0.52humic and fulvic acids without Aldrich humic acid log KDOC ) 0.88 ((0.06)‚log KOW - 0.11 ((0.31) 230 0.29 0.65sediment porewaters log KDOC ) 0.99 ((0.04)‚log KOW - 0.88 ((0.23) 396 0.64 0.66soil porewaters and groundwaters log KDOC ) 0.91 ((0.13)‚log KOW - 0.22 ((0.68) 47 0.31 0.61surface waters log KDOC ) 0.97 ((0.06)‚log KOW - 1.27 ((0.40) 210 0.32 0.99all DOC including Aldrich humic acid log KDOC ) 0.85 ((0.04)‚log KOW - 0.11 ((0.21) 223 0.78 0.52naturally occurring DOC (no Aldrich humic acid) log KDOC ) 0.85 ((0.06)‚log KOW - 0.25 ((0.34) 127 0.67 0.60PCBs, naturally occurring DOC (no Aldrich humic acid) log KDOC ) 0.71 ((0.06)‚log KOW - 0.50 ((0.36) 77 0.69 0.41PAHs, naturally occurring DOC (no Aldrich humic acid) log KDOC ) 1.18 ((0.13)‚log KOW - 1.56 ((0.72) 33 0.76 0.73

a n ) number of data points, r ) correlation coefficient, sxy ) standard error of estimate. b ((standard deviation).

FIGURE 2. Average KDOC values for individual chemicals for differentDOC sources: humic and fulvic acids (open diamond), sedimentporewaters (downward triangle), soil porewaters and groundwaters(plus square), and surface waters (upward triangle). The geometricmean regression and their 95% prediction confidence limits areplotted.

FIGURE 3. Residuals between measured log KDOC values and logKDOC values predicted using the relationship of KDOC ) 0.08 KOW. DOCsources: humic and fulvic acids without Aldrich humic acid (opendiamond), sediment porewaters (downward triangle), soil pore-waters and groundwaters (plus square), and surface waters (upwardtriangle). The 95% confidence limits are plotted.

4666 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 22, 2000

(0.529, 77) and -0.58 (0.705, 33), respectively. Transformingthese average differences to the antilog scale results inpredictive relationships of KDOC ) 0.06KOW and 0.26KOW forthe PCBs and PAHs, respectively. Although there is somedifference in the predictive relationships on a class basis,this difference is not statistically significant (R ) 0.1).

The variability of the overall predictive relationship, i.e.,KDOC ) 0.08 KOW, is much larger than the factor of 2.5 reportedby Seth et al. (22) for KOC and is larger than the variabilityobserved with Aldrich humic acid alone (Figure 1), approx-imately a factor 10 in either direction, i.e., antilog of (sxy-t(R)5%,df)268) ) 0.52 × 1.969). This larger variability is clearlyconsistent with the findings of Kukkonen and Oikari (16)and others where hydrophobicity of the chemical is not theonly factor affecting the association of HOCs with DOC. Theabove relationship, KDOC ) 0.08 KOW, provides a useful ruleof thumb for modelers and environmental scientists whenselecting values of KDOC for HOCs in the absence of measuredvalues.

The large 95% confidence limits, a factor of 20 in eitherdirection, associated with the predictive relationship mightor might not result in large uncertainties in outputs frommodels employing this relationship. These uncertainties willbe dependent upon the model, its input parameters, and thehydrophobicity of the chemical. For example, using the three-phase partitioning model (eq 1), the fraction of chemicalfreely dissolved ( ffd) in an ambient water for a chemical witha log KOW of 4 with 2 mg/L DOC, 0.1 mg/L POC, and KPOC )KOW would be 99.7% with a 95% confidence limit of 96.8-99.9%; essentially no difference. In contrast, for a chemicalwith a log KOW of 7, ffd would be 27.8% with a 95% confidencelimit of 2.9-48%. These 95% confidence limits have an overallrange of a factor of 16, and this range is substantially smallerthan the factor of 20 in either direction for the predictedKDOC. Users of the KDOC - KOW relationship must fully evaluateand understand the effects of the large uncertainties in theirindividual applications.

The variability in the structure and composition of DOCin sediments, soils, and ambient waters and the analyticaldifficulties in making KDOC measurements results in sub-stantial variability in measured KDOC values. Predictiverelationships based solely upon the hydrophobicity of thechemical will have large uncertainties as illustrated by theabove predictive relationship. However, the underlyingimportance of hydrophobicity of the chemical is wellsupported by the data. The data suggest that any model forpredicting the association of a wide range of HOCs to DOCwill require the inclusion of a parameter for the chemical’shydrophobicity. To lower uncertainties, substantial improve-ments in characterizing the composition of DOC as well assubstantial improvements in understanding how the differentcomponents of the DOC interact with HOCs will be required.Characterizing DOC by just measuring total dissolved organiccarbon does not adequately describe this phase! Recentresearch results by Perminova et al. (18) suggests for humicacids that the ratio of the aromatic to aliphatic carbon contentof humic acids might be a very useful characterization tool.The relatively large variability observed among investigationsusing Aldrich humic acid as the DOC source suggests thatrefinements and improvements in measurement techniqueswill also be required for lower uncertainties.

AcknowledgmentsI thank David Mount, Patricia Kosian, and Larry Heinis fortheir constructive reviews; Keith Sappington, Philip Cook,and Erik Winchester for helpful discussions; and RobertBurgess and Kristoffer Naes for providing numerical KDOC

data from their reports.

Supporting Information AvailableKDOC data are reported in Table 1 with the chemical name,

DOC source and type classification, DOC (mg/L) (whenavailable), log KOW, log KDOC or KDOC, method used to performthe measurement, and source of data for each entry (29pages). This material is available free of charge via the Internetat http://pubs.acs.org.

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Received for review May 16, 2000. Revised manuscript re-ceived September 6, 2000. Accepted September 11, 2000.

ES001269L

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