Toxicology Module 5

Environmental Fate Model

A framework connecting a toxicant's source, environmental transport and transformation, effective exposure or dose, and resulting biological effects.

1. Stationary,

2. Transported,

3. Transformed

Three possible environmental outcomes of a toxicant: remain at its source, move elsewhere, or change into another chemical species.

Dilution

Reduces the concentration of a released chemical by spreading it through a larger volume of environmental medium.

Degradation

Chemical breakdown that can diminish an original contaminant and may produce other substances.

1. Dilution and Degradation,

2. Focusing and Accumulation

Two major processes that decrease or increase the environmental impact of contaminant releases.

1. Chemical Use Pattern,

2. Physicochemical Properties,

3. Environmental Characteristics

Three principal factors controlling a chemical's environmental fate.

Entry Routes

Pathways by which toxicants enter a system; examples include

• skin,

• ingestion and

• inhalation in organisms, and

• runoff or discharges in the environment.

Fluid Dynamics

Movement of fluids, such as blood, water or air, that redistributes dissolved or carried chemicals.

Intermedia Transport Processes

Processes that move or distribute chemicals between environmental compartments, including partitioning and complexation.

Partitioning

Distribution of a chemical between different phases or compartments, such as blood and lipids or water and soil.

Complexation

Association or binding of a chemical, particularly a metal ion, with another substance to form a complex.

1. Hydrolysis,

2. Oxidation,

3. Reduction

Examples of chemical reactions that can transform toxicants in living organisms and environmental systems.

• Point Sources,

• Nonpoint Sources

The two major classes of environmental toxicant sources, distinguished by whether discharges have an identifiable individual origin.

Point Source

A discrete, recognizable and generally measurable pollutant discharge, such as a factory outlet, smokestack or oil spill.

Nonpoint Source

Diffuse contamination from widespread or numerous scattered sources, such as agricultural runoff, atmospheric deposition or vehicles.

Spatial Distribution

Where a toxicant is located

Temporal Distribution

How a toxicant's location or distribution changes over time.

Air and Water

The usual fluids that carry toxicants through environmental transport.

1. Advection,

2. Diffusion

Two major physical transport processes: bulk fluid movement and mixing driven by random motion or disequilibrium.

Advection

Passive transport of a chemical by bulk movement of water or air; it can occur independently of diffusion.

Non-diffusive Transport

Another term for advection, emphasizing that chemicals move with the bulk fluid rather than through random diffusive mixing.

Intraphase or Homogeneous Advection

Passive chemical movement within the same medium, such as in flowing water or moving air.

Interphase or Heterogeneous Advection

Chemical transport involving another phase carried within or between media, such as contaminants attached to suspended particles.

N=GC

Formula of Transport Rate

Homogeneous Advective Transport Rate (N)

Mass of chemical transported per time; calculate it by multiplying fluid flow rate (G) by chemical concentration (C).

Flow Rate (G)

Volume of transporting medium passing per unit time, commonly expressed in cubic metres per hour.

Chemical Concentration (C)

Mass of toxicant per unit volume of transporting medium, such as grams per cubic metre.

Atrazine

An agricultural herbicide whose movement from fields into nearby water increases with rainfall and the presence of drainage ditches.

Air current

The movement of air, often referred to as wind, and is caused by differences in atmospheric pressure or temperature.

Pressure Gradient

Change in air pressure over horizontal distance that drives air from high-pressure toward low-pressure areas.

1. Particle Deposition,

2. Atmospheric Deposition,

3. Ingestion of Contaminated Particles or Food

Three outcomes or pathways of heterogeneous chemical transport that can transfer toxicants to sediments, surfaces or organisms.

Particle Deposition

Settling of suspended particles, along with contaminants sorbed to them, into bottom sediments.

Atmospheric Deposition

Transfer of airborne chemicals to land or water surfaces through settling particles or precipitation.

Heterogeneous Advective Transport Rate

Mass moved per unit time in a secondary phase, calculated from that phase's flow rate and toxicant concentration.

Hydrophobic Toxicants

Water-avoiding compounds that may bind strongly to particles even when their dissolved concentration is low.

DDT

A hydrophobic insecticide that can travel bound to particles, partition into air despite low volatility, and form DDE during hydrolysis.

Benzo[a]pyrene

A hydrophobic toxicant cited as an example of a chemical that may be strongly associated with transported suspended particles.

Colloidal Particles

Very small, relatively mobile particles able to sorb toxicants, often due to high organic carbon content and a large surface-area-to-mass ratio.

Macromolecular Detritus

Dead organic material from plants or organisms containing large molecules such as proteins, polysaccharides, lipids and lignocellulose.

Organic Co-solvents

Water-miscible organic solvents that increase the apparent solubility and transport of poorly water-soluble contaminants, especially in polluted groundwater.

Ethanol

A water-miscible organic solvent listed as a possible co-solvent carrier of toxicants at contaminated sites.

• Wet Deposition,

• Dry Deposition

The two principal forms of atmospheric heterogeneous transport: chemicals absorbed by falling water and chemicals attached to settling solid particles.

Wet Deposition

Removal of atmospheric chemicals through their absorption into falling liquid water droplets.

Dry Deposition

Transfer of airborne contaminants sorbed onto particles that settle onto the Earth's surface without precipitation.

Diffusion

Chemical transport by random motion caused by disequilibrium; it redistributes chemicals within a phase or between different phases.

Diffusion within a Phase

Diffusional transport within a phase can result from random (thermal) motion of the chemical (molecular diffusion), the random turbulent mixing of the transport medium (turbulent diffusion), or a combination of both.

Disequilibrium

A condition in which chemical concentrations or distributions between locations or phases have not yet reached equilibrium.

1. Molecular Diffusion,

2. Turbulent Diffusion

Two fundamental mechanisms of intraphase diffusion, through random thermal molecular motion and turbulent mixing.

Molecular Diffusion

Spreading of chemicals through random motion of individual molecules arising from thermal energy.

Turbulent Diffusion

Chemical mixing caused by irregular fluid motions and eddies; it usually dominates diffusive mixing in air and water.

Eddy

A localized swirling motion or whirlpool in air or water that promotes mixing of chemicals.

Laminar Flow

Relatively orderly fluid flow in which chemical mixing occurs mainly through molecular motion.

Turbulent Flow

Fluid flow containing eddies that produce more vigorous chemical mixing than laminar flow.

Mechanical Diffusion or Dispersion

Mixing as moving water takes varied, winding paths around particles in porous soil or sediment, even without turbulence.

Macrodispersion

Large-scale dispersion caused by differences in flow paths, such as groundwater moving around extensive low-permeability formations.

Tortuous Pathways

Twisting, indirect routes taken by water around soil or sediment particles during transport.

Diffusion between Phases (Interphase)

The movement of a substance from one distinct physical phase (such as gas or liquid) into or across an adjacent phase

Interphase or Intermedia Diffusion

Transfer of a chemical between different media toward equilibrium without changing its molecular identity.

• Air–Water Interface,

• Particle–Water Interface,

• Biological Membrane–Water Interface

Examples of boundaries between phases through which chemicals can diffuse and partition.

Equilibrium Partitioning

Distribution of a chemical between phases until thermodynamic equilibrium is reached and the net transfer rate becomes zero.

Thermodynamic Equilibrium

State in which transfers between phases balance, leaving their relative concentrations stable although individual molecules continue moving.

• soil,

• water,

• air, or

• Biota

Environmental compartments used to predict the concentration of a substance based on its tendency to dissolve or adhere to different phases.

Biota

Living organisms that may absorb and accumulate environmental pollutants.

Porewater

Water occupying the pores of soil, sediment or rock; contaminants dissolved in it can be available to organisms.

Sediment

Solid material deposited in water bodies that may hold contaminants and exchange them with surrounding porewater.

• Solubility,

• Volatility,

• Affinity for Organic Matter

Major physicochemical properties influencing how chemicals partition between environmental phases.

Partition Coefficients

Numerical quantities describing the equilibrium distribution of a chemical between two phases.

1. K_OW (octanol-water partition coefficient)– indicates lipophilicity

2. K_LW (lipid-water partition coefficient)

3. K_OC (organic carbon normalized partition coefficient)

4. K_P (Particle– Water Partitioning)

5. Henry’s law constant– for air-water partitioning

Major partitioning systems described by KOW, KLW, KP, KOC and Henry's Law constant, respectively.

Octanol–Water Partition Coefficient (KOW)

Measure of how strongly a chemical favors octanol rather than water; high values indicate hydrophobicity and lipophilicity.

Octanol

Organic reference liquid used in KOW measurements because its composition makes it a useful approximation of a lipid-like phase.

Hydrophobicity

A chemical's tendency to avoid or associate poorly with water; often corresponds to stronger preference for organic phases.

Lipophilicity

A chemical's tendency to dissolve in or associate with fats and lipids rather than water.

Hydrophilicity

A chemical's tendency to associate with water; relatively low lipid–water partitioning indicates greater water affinity.

Lipid–Water Partition Coefficient (KLW)

Measure of a compound's distribution between fat or lipid and aqueous phases; high values favor lipids.

High KLW, Low KLW

High values indicate lipid-loving substances; low values indicate more water-loving substances.

Phytoplankton

Microscopic aquatic producers that may show unexpectedly high apparent lipid–water partitioning because their large surface area permits considerable surface sorption.

Top Predators

Organisms high in aquatic food chains, such as marine mammals, that may show high apparent partitioning due to slow chemical elimination.

Marine Mammals

Aquatic top predators cited as examples of animals with high apparent lipid–water partition coefficients and slow toxicant elimination.

Surface Sorption

Attachment of chemical molecules to an organism's or particle's exterior, especially influential when surface area is large relative to volume.

Particle–Water Partition Coefficient (KP)

Describes equilibrium distribution of a contaminant between surrounding water and solid soil, sediment or suspended particles.

High KP, Low KP

High values mean strong particle binding and less dissolved mobility; low values indicate more dissolved chemical and greater mobility in water.

Organic Carbon–Water Partition Coefficient (KOC)

Measure of a contaminant's partitioning between organic carbon in soil or sediment and the surrounding water.

High KOC, Low KOC

High values indicate strong organic-matter sorption and low leaching; low values indicate weak sorption and higher leaching potential.

Leaching

Movement of dissolved chemicals through soil or other porous materials, usually carried by infiltrating or percolating water.

Henry's Law Constant

Air–water partitioning parameter linking a gas's partial pressure above liquid to its dissolved concentration at a specified temperature.

Partial Pressure

Pressure attributable to an individual gas in a mixture; used with dissolved concentration in Henry's Law partitioning.

High Henry's Law Constant, Low Henry's Law Constant

____ values indicate greater transfer from water to air; ___ values generally indicate greater retention in water.

Vapor Pressure

A measure of a substance's tendency to enter the vapor phase, relevant to air–water partitioning.

Solubility

The capacity of a chemical to dissolve in water; very low solubility can favor partitioning into air or organic phases.

Chlorinated Solvents

Solvents containing chlorine; many are identified as high-Henry's-Law chemicals that can transfer readily from water to air.

Alcohols

Organic compounds identified as generally having low Henry's Law constants because of comparatively high water solubility.

Polycyclic Aromatic Hydrocarbons (PAHs)

Aromatic compounds with multiple fused rings; larger ___ are listed as low-Henry's-Law chemicals, and aquatic organisms may metabolize them.

Environmental Fate

Where a pollutant persists or travels among environmental compartments, influenced by transport, degradation and phase partitioning.

Bioavailability

The portion of a contaminant accessible to an organism for uptake or interaction; partitioning affects this accessible fraction.

Bioaccumulation

Buildup of a chemical inside organisms over time, often favored for hydrophobic contaminants that partition into lipids.

Risk Assessment

Evaluation of likely exposure and harmful effects on people or ecosystems using chemical toxicity and environmental behavior.

Persistence

Length of time a chemical remains in the environment before transformation or degradation, depending on reaction rates.

• Reversible Reactions,

• Irreversible Reactions

Two major classes of chemical transformation: interconvertible chemical states and lasting formation of daughter chemicals or reaction products.

Reversible Reaction

Chemical change that allows continuing exchange between forms, such as ionization or complexation, without permanently altering the underlying chemical identity.

Irreversible Reaction

Transformation of a parent chemical into a new product, such as through photolysis, hydrolysis or many oxidation–reduction processes.