Multimedia Environmental Models - Chapter 3 potx

©2001 CRC Press LLCEnvironmental Chemicals and Their Properties 3.1 INTRODUCTION AND DATA SOURCES In this book, we focus on techniques for building mass balance models ofchemical fate in

McKay, Donald "Environmental Chemicals and Their Properties"

Multimedia Environmental Models

Edited by Donald McKay

Boca Raton: CRC Press LLC,2001

©2001 CRC Press LLC

Environmental Chemicals and

Their Properties

3.1 INTRODUCTION AND DATA SOURCES

In this book, we focus on techniques for building mass balance models ofchemical fate in the environment, rather than on the detailed chemistry that controlstransport and transformation, as well as toxic interactions For a fuller account ofthe basic chemistry, the reader is referred to the excellent texts by Crosby (1988),Tinsley (1979), Stumm and Morgan (1981), Pankow (1991), Schwarzenbach et al.(1993), Seinfeld and Pandis (1997), Findlayson-Pitts and Pitts (1986), Thibodeaux(1996), and Valsaraj (1995)

There is a formidable and growing literature on the nature and properties ofchemicals of environmental concern Numerous handbooks list relevant physical-chemical and toxicological properties Especially extensive are compilations onpesticides, chemicals of potential occupational exposure, and carcinogens Govern-ment agencies such as the U.S Environmental Protection Agency (EPA), Environ-ment Canada, scientific organizations such as the Society of Environmental Toxi-cology and Chemistry (SETAC), industry groups, and individual authors havepublished numerous reports and books on specific chemicals or classes of chemicals.Conferences are regularly held and proceedings published on specific chemicalssuch as the “dioxins.” Computer-accessible databases are now widely available forconsultation Table 3.1 lists some of the more widely used texts and scientificjournals Most are available in good reference libraries

Most of the chemicals that we treat in this book are organic, but the massbalancing principles also apply to metals, organometallic chemicals, gases such asoxygen and freons, inorganic compounds, and ions containing elements such asphosphorus and arsenic Metals and other inorganic compounds tend to requireindividual treatment, because they usually possess a unique set of properties Organiccompounds, on the other hand, tend to fall into certain well defined classes We areoften able to estimate the properties and behavior of one organic chemical from that

Table 3.1 Sources of information on chemical properties and estimation methods (See

Chapter 1.5 of Mackay, et al., Illustrated Handbooks of Physical Chemical Properties and Environmental Fate for Organic Chemicals, cited below, for more details)

The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals (Annual), S Budavarie, ed Whitehouse Station, NJ: Merck & Co., 1996.

Handbook of Chemistry and Physics, D R Lide, ed., 81/e Boca Raton, FL: CRC Press.

Verschueren’s Handbook of Environmental Data on Organic Chemicals. New York: John Wiley

& Sons, 1997.

Illustrated Handbook of Physical Chemical Properties and Environmental Fate for Organic Chemicals (in 5 volumes) D Mackay, W Y Shiu, and K C Ma Boca Raton, FL: CRC Press, 1991–1997 Also available as a CD ROM.

Handbook of Environmental Fate and Exposure Data for Organic Chemicals (several volumes),

P H Howard, ed Boca Raton, FL: Lewis Publications.

Handbook of Environmental Degradation Rates, P H Howard et al Boca Raton, FL: Lewis Publications.

Lange’s Handbook of Chemistry, 15/e, J A Dean, ed New York: McGraw-Hill, 1998.

Dreisbach’s Physical Properties of Chemical Compounds, Vol I to III Washington, DC, Amer Chem Soc.

Technical Reports, European Chemical Industry Ecology and Toxicology Centre (ECETOC) Brussels, Belgium.

Sax’s Dangerous Properties of Industrial Materials, 10/e R J Lewis, ed New York: John Wiley & Sons.

Groundwater Chemicals Desk Reference, J J Montgomery Boca Raton, FL: Lewis Publications, 1996.

Genium Materials Safety Data Sheets Collection. Amsterdam, NY: Genium Publishing Corp.

The Properties of Gases and Liquids, R C Reid, J M Prausnitz, and B E Poling New York: McGraw-Hill, 1987.

NIOSH/OSHA Occupational Health Guidelines for Chemical Hazards. Washington, DC: U.S Government Printing Office.

The Pesticide Manual, 12/e C D S Tomlin, ed Loughborough, UK: British Crop Protection Council.

The Agrochemicals Handbook, H Kidd and D R James, eds London: Royal Society of Chemistry.

Agrochemicals Desk Reference, 2/e, J H Montgomery Boca Raton, FL: Lewis Publications ARS Pesticide Properties Database, R Nash, A Herner, and D Wauchope Beltsville, MD: U.S Department of Agriculture, www.arsusda.gov/rsml/ppdb.html.

Substitution Constants for Correlation Analysis in Chemistry and Biology, C H Hansch (currently out of print) New York: Wiley-Interscience.

Handbook of Chemical Property Estimation Methods, W J Lyman, W F Reehl, D H Rosenblatt (currently out of print) New York: McGraw-Hill.

Handbook of Property Estimation Methods for Chemicals, R S Boethling and D Mackay Boca Raton, FL: CRC Press, 2000.

Chemical Property Estimation: Theory and Practice, E J Baum Boca Raton, FL: Lewis Publications, 1997.

Toolkit for Estimating Physiochemical Properties of Organic Compounds, M Reinhard and A Drefahl New York: John Wiley & Sons, 1999.

IUPAC Handbook Research Triangle Park, NC: International Union of Pure and Applied Chemistry.

Website for database and EPIWIN estimation methods, Syracuse, NY: Syracuse Research Corporation ( http://www.syrres.com )

of other, somewhat similar or homologous chemicals An example is the series ofchlorinated benzenes that vary systematically in properties from benzene tohexachlorobenzene.

It is believed that some 50,000 to 80,000 chemicals are used in commerce Thenumber of chemicals of environmental concern runs to a few thousand There arenow numerous lists of “priority” chemicals of concern, but there is considerablevariation between lists It is not possible, or even useful, to specify an exact number

of chemicals Some inorganic chemicals ionize in contact with water and thus losetheir initial identity Some lists name PCBs (polychlorinated biphenyls) as onechemical and others as six groups of chemicals whereas, in reality, the PCBs consist

of 209 possible individual congeners Many chemicals, such as surfactants andsolvents, are complex mixtures that are difficult to identify and analyze One des-ignation, such as naphtha, may represent 1000 chemicals There is a multitude ofpesticides, dyes, pigments, polymeric substances, drugs, and silicones that havevaluable social and commercial applications These are in addition to the numerous

“natural” chemicals, many of which are very toxic

For legislative purposes, most jurisdictions have compiled lists of chemicals thatare, or may be, encountered in the environment, and from these “raw” lists ofchemicals of potential concern they have established smaller lists of “priority”chemicals These chemicals, which are usually observed in the environment, areknown to have the potential to cause adverse ecological and/or biological effectsand are thus believed to be worthy of control and regulation In practice, a chemicalthat fails to reach the “priority” list is usually ignored and receives no priority ratherthan less priority

These lists should be regarded as dynamic New chemicals are being added asenthusiastic analytical chemists detect them in unexpected locations or toxicologistsdiscover subtle new effects Examples are brominated flame retardants, chlorinatedalkanes, and certain very stable fluorinated substances (e.g., trifluoroacetic acid) thathave only recently been detected and identified In recent years, concern has grownabout the presence of endocrine modulating substances such as nonylphenol, whichcan disrupt sex ratios and generally interfere with reproductive processes Thepopular book Our Stolen Future, by Colborn et al (1996) brought this issue to publicattention Some of these have industrial or domestic sources, but there is increasingconcern about the general contamination by drugs used by humans or in agriculture.Table 3.2 lists about 200 chemicals by class and contains many of the chemicals ofcurrent concern

3.2 IDENTIFYING PRIORITY CHEMICALS

It is a challenging task to identify from “raw lists” of chemicals a smaller, moremanageable number of “priority” chemicals Such chemicals receive intense scrutiny,analytical protocols are developed, properties and toxicity are measured, and reviewsare conducted of sources, fate, and effects This selection contains an element ofjudgement and is approached by different groups in different ways A common threadamong many of the selection processes is the consideration of six factors: quantity,

Table 3.2 List of Chemicals Commonly Found on Priority Chemical Lists

Volatile Halogentated Hydrocarbons Monoaromatic Hydrocarbons

trans-1,2-Dichloroethene Polycyclic Aromatic Hydrocarbons

1,2,4,5-Tetrachlorobenzene Halogenated Phenols

Halogenated Biphenyls and Naphthalenes 2,4,5-Trichlorophenol

Polychlorinated Biphenyls (PCBs) 2,4,6-Trichlorophenol

Polybrominated Biphenyls (PBBs) 2,3,4,5-Tetrachlorophenol

Tetrachlorinated dibenzo-p-dioxins 2,4-Dinitrotoluene

Pentachlorinated dibenzo-p-dioxins 2,6-Dinitrotoluene

Hexachlorinated dibenzo-p-dioxins

Heptachlorinated dibenzo-p-dioxins 1-Nitronaphthalene

Octachlorinated dibenzo-p-dioxin 2-Nitronaphthalene

Brominated dibenzo-p-dioxins 5-Nitroacenaphthalene

Chlorinated Dibenzofurans Fluorinated Compounds

Tetrachlorinated dibenzofurans Polyfluorinated alkanes

Pentachlorinated dibenzofurans Trifluoroacetic acid

Hexachlorinated dibenzofurans Fluoro-chloro acids

Heptachlorinated dibenzofurans Polyfluorinated chemicals Octachlorodibenzofuran

Phthalate Esters Nitrosamines and Other Nitrogen Compounds Dimethylphthalate

Pesticides, including biocides, fungicides, rodenticides, insecticides and herbicides

Table 3.2 List of Chemicals Commonly Found on Priority Chemical Lists

persistence, bioaccumulation, potential for transport to distant locations, toxicity,and a miscellaneous group of other adverse effects.

3.2.1 Quantity

The first factor is the quantity produced, used, formed or transported, includingconsideration of the fraction of the chemical that may be discharged to the environ-ment during use Some chemicals, such as benzene, are used in very large quantities

in fuels, but only a small fraction (possibly less than a fraction of a percent) isemitted into the environment through incomplete combustion or leakage duringstorage Other chemicals, such as pesticides, are used in much smaller quantitiesbut are discharged completely and directly into the environment; i.e., 100% isemitted At the other extreme, there are chemical intermediates that may be produced

in large quantities but are emitted in only minuscule amounts (except during anindustrial accident) It is difficult to compare the amounts emitted from these variouscategories, because they are highly variable and episodic It is essential, however,

to consider this factor; many toxic chemicals have no significant adverse impact,because they enter the environment in negligible quantities

Central to the importance of quantity is the adage first stated by Paracelsus,nearly five centuries ago, that the dose makes the poison This can be restated inthe form that all chemicals are toxic if administered to the victim in sufficientquantities A corollary is that, in sufficiently small doses, all chemicals are safe.Indeed, certain metals and vitamins are essential to survival The general objective

of environmental regulation or “management” must therefore be to ensure that thequantity of a specific substance entering the environment is not excessive It neednot be zero; indeed, it is impossible to achieve zero Apart from cleaning up pastmistakes, the most useful regulatory action is to reduce emissions to acceptablelevels and thus ensure that concentrations and exposures are tolerable Not even theEPA can reduce the toxicity of benzene It can only reduce emissions This impliesknowing what the emissions are and where they come from This is the focus ofprograms such as the Toxics Release Inventory (TRI) in the U.S.A or the NationalPollutant Release Inventory (NPRI) system in Canada There are similar programs

in Europe, Australia, and Japan Regrettably, the data are often incomplete A majorpurpose of this book is to give the reader the ability to translate emission rates intoenvironmental concentrations so that the risk resulting from exposure to these con-centrations can be assessed When this can be done, it provides an incentive toimprove release inventories

3.2.2 Persistence

The second factor is the chemical’s environmental persistence, which may also

be expressed as a lifetime, half-life, or residence time Some chemicals, such as DDT

or the PCBs, may persist in the environment for several years by virtue of theirresistance to transformation by degrading processes of biological and physical origin.They may have the opportunity to migrate widely throughout the environment andreach vulnerable organisms Their persistence results in the possibility of establishing

relatively high concentrations This arises because, in principle, the amount in theenvironment (kilograms) can be expressed as the product of the emission rate intothe environment (kilograms per year) and the residence time of the chemical in theenvironment (years) Persistence also retards removal from the environment onceemissions are stopped A legacy of “in place” contamination remains.

This is the same equation that controls a human population For example, thenumber of Canadians (about 30 million) is determined by the product or the rate atwhich Canadians are born (about 0.4 million per year) and the lifetime of Canadians(about 75 years) If Canadians were less persistent and lived for only 30 years, thepopulation would drop to 12 million

Intuitively, the amount (and hence the concentration) of a chemical in theenvironment must control the exposure and effects of that chemical on ecosystems,because toxic and other adverse effects, such as ozone depletion, are generally aresponse to concentration Unfortunately, it is difficult to estimate the environmen-tal persistence of a chemical This is because the rate at which chemicals degradedepends on which environmental media they reside in, on temperature (whichvaries diurnally and seasonally), on incidence of sunlight (which varies similarly),

on the nature and number of degrading microorganisms that may be present, and

on other factors such as acidity and the presence of reactants and catalysts Thisvariable persistence contrasts with radioisotopes, which have a half-life that isfixed and unaffected by the media in which they reside In reality, a substanceexperiences a distribution of half-lives, not a single value, and this distributionvaries spatially and temporally Obviously, long-lived chemicals, such as PCBs,are of much greater concern than those, such as phenol, that may persist in theaquatic environment for only a few days as a result of susceptibility to biodegra-dation Some estimate of persistence or residence time is thus necessary for prioritysetting purposes Organo-halogen chemicals tend to be persistent and are thereforefrequently found on priority lists Later in this book, we develop methods ofcalculating persistence

3.2.3 Bioaccumulation

The third factor is potential for bioaccumulation (i.e., uptake of the chemical byorganisms) This is a phenomenon, not an effect; thus bioaccumulation per se is notnecessarily of concern It is of concern that bioaccumulation may cause toxicity tothe affected organism or to a predator or consumer of that organism Historically, itwas the observation of pesticide bioaccumulation in birds that prompted RachelCarson to write Silent Spring in 1962, thus greatly increasing public awareness ofenvironmental contamination

As we discuss later, some chemicals, notably the hydrophobic or “water-hating”organic chemicals, partition appreciably into organic media and establish high con-centrations in fatty tissue PCBs may achieve concentrations (i.e., they bioconcen-trate) in fish at factors of 100,000 times the concentrations that exist in the water inwhich the fish dwell For some chemicals (notably PCBs, mercury, and DDT), there

is also a food chain effect Small fish are consumed by larger fish, at higher trophiclevels, and by other animals such as gulls, otters, mink, and humans These chemicals

may be transmitted up the food chain, and this may result in a further increase inconcentration such that they are biomagnified.

Bioaccumulation tendency is normally estimated using an organic phase-waterpartition coefficient and, more specifically, the octanol-water partition coefficient.This, in turn, can be related to the solubility of the chemical in the water Clearly,chemicals that bioaccumulate, bioconcentrate, and biomagnify have the potential totravel down unexpected pathways, and they can exert severe toxic effects, especially

on organisms at higher trophic levels

The importance of bioaccumulation may be illustrated by noting that, in watercontaining 1 ng/L of PCB, the fish may contain 105 ng/kg A human may consume

1000 L of water annually (containing 1000 ng of PCB) and 10 kg of fish (containing

106 ng of PCB), thus exposure from fish consumption is 1000 times greater thanthat from water Particularly vulnerable are organisms such as certain birds andmammals that rely heavily on fish as a food source

3.2.4 Toxicity

The fourth factor is the toxicity of the chemical The simplest manifestation oftoxicity is acute toxicity This is most easily measured as a concentration that willkill 50% of a population of an aquatic organism, such as fish or an invertebrate (e.g.,

Daphnia magna), in a period of 24–96 hours, depending on test conditions Whenthe concentration that kills (or is lethal to) 50% (the LC50) is small, this corresponds

to high toxicity The toxic chemical may also be administered to laboratory animalssuch as mice or rats, orally or dermally The results are then expressed as a lethaldose to kill 50% (LD50) in units of mg chemical/kg body weight of the animal.Again, a low LD50 corresponds to high toxicity

More difficult, expensive, and contentious are chronic, or sublethal, tests thatassess the susceptibility of the organism to adverse effects from concentrations ordoses of chemicals that do not cause immediate death but ultimately may lead todeath For example, the animal may cease to feed, grow more slowly, be unable toreproduce, become more susceptible to predation, or display some abnormal behav-ior that ultimately affects its life span or performance The concentrations or doses

at which these effects occur are often about 1/10th to 1/100th of those that causeacute effects Ironically, in many cases, the toxic agent is also an essential nutrient,

so too much or too little may cause adverse effects

Although most toxicology is applied to animals, there is also a body of knowledge

on phytotoxicity, i.e., toxicity to plants Plants are much easier to manage, and killingthem is less controversial Tests also exist for assessing toxicity to microorganisms

It is important to emphasise that toxicity alone is not a sufficient cause for concernabout a chemical Arsenic in a bottle is harmless Disinfectants, biocides, and pesti-cides are inherently useful because they are toxic The extent to which the organism

is injured depends on the inherent properties of the chemical, the condition of theorganism, and the dose or amount that the organism experiences It is thus misleading

to classify or prioritize chemicals solely on the basis of their inherent toxicity, or onthe basis of the concentrations in the environment or exposures Both must beconsidered A major task of this book is to estimate exposure A healthy tension often

exists between toxicologists and chemists about the relative importance of toxicityand exposure, but fundamentally this argument is about as purposeful as squabblingover whether tea leaves or water are the more important constituents of tea.Most difficult is the issue of genotoxicity, including carcinogenicity, and terato-genicity In recent years, a battery of tests has been developed in which organismsranging from microorganisms to mammals are exposed to chemicals in an attempt

to determine if they can influence genetic structure or cause cancer A major difficulty

is that these effects may have long latent periods, perhaps 20 to 30 years in humans.The adverse effect may be a result of a series of biochemical events in which thetoxic chemical plays only one role It is difficult to use the results of short-termlaboratory experiments to deduce reliably the presence and magnitude of hazard tohumans There may be suspicions that a chemical is producing cancer in perhaps0.1% of a large human population over a period of perhaps 30 years, an effect that

is very difficult (or probably impossible) to detect in epidemiological studies Butthis 0.1% translates into the premature death of 30,000 Canadians per year fromsuch a cancer, and is cause for considerable concern Another difficulty is thathumans are voluntarily and involuntarily exposed to many toxic chemicals, includingthose derived from smoking, legal and illicit drugs, domestic and occupationalexposure, as well as environmental exposure Although research indicates that mul-tiple toxicants act additively when they have similar modes of action, there are cases

of synergism and antagonism Despite these difficulties, a considerable number ofchemicals have been assessed as being carcinogenic, mutagenic, or teratogenic, and

it is even possible to assign some degree of potency to each chemical Such chemicalsusually rank high on priority lists As was discussed earlier, endocrine modulatingsubstances are of more recent concern It seems likely that ingenious toxicologistswill find other subtle toxic effects in the future

3.2.5 Long-Range Transport

As lakes go, Lake Superior is fairly pristine, since there is relatively little industry

on its shores In the U.S part of this lake is an island, Isle Royale, which is aprotected park and is thus even more pristine In this island is a lake, Siskiwit Lake,which cannot conceivably be contaminated No responsible funding agency wouldwaste money on the analysis of fish from that lake for substances such as PCBs.Remarkably, perceptive researchers detected substantial concentrations of PCBs.Similarly, surprisingly high concentrations have been detected in wildlife in theArctic and Antarctic Clearly, certain contaminants can travel long distances throughthe atmosphere and oceans and are deposited in remote regions

This potential for long-range transport (LRT) is of concern for several reasons.There is an ethical issue when the use of a chemical in one nation (which presumablyenjoys social or economic benefit from it) results in exposure in other downwindnations that derive no benefit, only adverse effects This transboundary pollutionissue also applies to gases such as SO2, which can cause acidification of poorlybuffered lakes at distant locations A regulatory agency may then be in the position

of having little or no control over exposures experienced by its public The politicalimplications are obvious

There is therefore a compelling incentive to identify those chemicals that canundertake long-range transport and implement international agreements to controlthem A start on this process has been made recently by the United Nations Envi-ronment Program (UNEP), which has identified 12 substances or groups for inter-national regulations or bans These substances, listed in Table 3.3, are also identified

as persistent, bioaccumulative, and toxic Others are scheduled for restriction orreduction They may represent merely the first group of chemicals that will be subject

to international controls Most contentious of the 12 is DDT, which is still widelyand beneficially used for malaria control

3.2.6 Other Effects

Finally, there is a variety of other adverse effects that are of concern, including

• the ability to influence atmospheric chemistry (e.g., freons)

• alteration in pH (e.g., oxides of sulfur and nitrogen causing acid rain)

• unusual chemical properties such as chelating capacity, which alters the availability

of other chemicals in the environment

• interference with visibility

• odor (e.g., from organo-sulfur compounds)

• color (e.g., from dyes)

• the ability to cause foaming in rivers (e.g., detergents or surfactants)

• formation of toxic metabolites or degradation products

3.2.7 Selection Procedures

A common selection procedure involves scoring these factors on some numerichazard scale The factors then may be combined to give an overall factor and

Table 3.3 Substances Scheduled for Elimination, Restriction, or Reduction by UNEP

Scheduled for Elimination Scheduled for Restriction

Scheduled for Reduction

determine priority This is a subjective process, and it becomes difficult for twomajor reasons.

First, chemicals that are subject to quite different patterns of use are difficult tocompare For example, chemical X may be produced in very large quantities, emittedinto the environment, and found in substantial concentrations in the environment,but it may not be believed to be particularly toxic Examples are solvents such astrichloroethylene or plasticizers such as the phthalate esters On the other hand,chemical Y may be produced in minuscule amounts but be very toxic, an examplebeing the “dioxins.” Which deserves the higher priority?

Second, it appears that the adverse effects suffered by aquatic organisms andother animals, including humans, are the result of exposure to a large number ofchemicals, not just to one or two chemicals Thus, assessing chemicals on a case-by-case basis may obscure the cumulative effect of a large number of chemicals.For example, if an organism is exposed to 150 chemicals, each at a concentrationthat is only 1% of the level that will cause death, then death will very likely occur,but it cannot be attributed to any one of these chemicals It is the cumulative effectthat causes death The obvious prudent approach is to reduce exposure to all chem-icals to the maximum extent possible The issue is further complicated by thepossibility that some chemicals will act synergistically, i.e., they produce an effectthat is greater than additive; or they may act antagonistically, i.e., the combinedeffect is less than additive As a result, there will be cases in which we are unable

to prove that a specific chemical causes a toxic effect but, in reality, it does contribute

to an overall toxic effect Indeed, some believe that this situation will be the rulerather than the exception

A compelling case can be made that the prudent course of action is for society

to cast a fairly wide net of suspicion (i.e., assemble a fairly large list of chemicals)then work to elucidate sources, fate, and effects with the aim of reducing overallexposure of humans, and our companion organisms, to a level at which there isassurance that no significant toxic effects can exist from these chemicals The riskfrom these chemicals then becomes small as compared to other risks such as acci-dents, disease, and exposure to natural toxic substances This approach has beenextended and articulated as the “Precautionary Principle,” the “Substitution Princi-ple,” and the “Principle of Prudent Avoidance.”

One preferred approach is to undertake a risk assessment for each chemical.Formal procedures for conducting such assessments have been published, notably

by the U.S Environmental Protection Agency (EPA) The process involves fying the chemical, its sources, the environment in which it is present, and theorganisms that may be affected The toxicity of the substance is evaluated and routes

identi-of exposure quantified Ultimately, the prevailing concentrations or doses are sured or estimated and compared with levels that are known to cause effects, andconclusions are drawn regarding the proximity to levels at which there is a risk ofeffect This necessarily involves consideration of the chemical’s behavior in an actualenvironment Risk is thus assessed only for that environment Risk or toxic effectsare thus not inherent properties of a chemical; they depend on the extent to whichthe chemical reaches the organism

mea-3.3 KEY CHEMICAL PROPERTIES AND CLASSES

3.3.1 Key Properties

In Chapter 5, we discuss physicochemical properties in more detail and, inChapter 6, we examine reactivities It is useful at this stage to introduce some ofthese properties and identify how they apply to different classes of chemicals

It transpires that we can learn a great deal about how a chemical partitions inthe environment from its behavior in an air-water-octanol (strictly 1-octanol) system

as shown later in Figure 3.2 There are three partition coefficients, KAW, KOW, and

KOA, only two of which are independent, since KOA must equal KOW/KAW These can

be measured directly or estimated from vapor pressure, solubility in water, andsolubility in octanol, but not all chemicals have measurable solubilities because ofmiscibility Octanol is an excellent surrogate for natural organic matter in soils andsediments, lipids, or fats, and even plant waxes It has approximately the same C:H:Oratio as lipids Correlations are thus developed between soil-water and octanol-waterpartition coefficients, as discussed in more detail later

An important attribute of organic chemicals is the degree to which they are

hydrophobic This implies that the chemical is sparingly soluble in, or “hates,” waterand prefers to partition into lipid, organic, or fat phases A convenient descriptor ofthis hydrophobic tendency is KOW A high value of perhaps one million, as applies toDDT, implies that the chemical will achieve a concentration in an organic mediumapproximately a million times that of water with which it is in contact In reality,most organic chemicals are approximately equally soluble in lipid or fat phases, butthey vary greatly in their solubility in water Thus, differences in hydrophobicity arelargely due to differences of behavior in, or affinity for, the water phase, not differences

in solubility in lipids The word lipophilic is thus unfortunate and is best avoided The chemical’s tendency to evaporate or partition into the atmosphere is primarilycontrolled by its vapor pressure, which is essentially the maximum pressure that apure chemical can exert in the gas phase or atmosphere It can be viewed as the

solubility of the chemical in the gas phase Indeed, if the vapor pressure in units of

Pa is divided by the gas constant, temperature group RT, where R is the gas constant(8.314 Pa m3/mol K), and T is absolute temperature (K), then vapor pressure can

be converted into a solubility with units of mol/m3 Organic chemicals vary mously in their vapor pressure and correspondingly in their boiling point Some(e.g., the lower alkanes) that are present in gasoline are very volatile, whereas others(e.g., DDT) have exceedingly low vapor pressures

enor-Partitioning from a pure chemical phase to the atmosphere is controlled by vaporpressure Partitioning from aqueous solution to the atmosphere is controlled by KAW,

a joint function of vapor pressure and solubility in water A substance may have ahigh KAW, because its solubility in water is low Partitioning from soils and otherorganic media to the atmosphere is controlled by KAO (air/octanol), which is con-ventionally reported as its reciprocal, KOA Partitioning from water to organic media,including fish, is controlled by KOW Substances that display a significant tendency

to partition into the air phase over other phases are termed volatile organic chemicals

or VOCs They have high vapor pressures