Evaluating Environmental Impact

Environmental fate processes are about the questions If a particular chemical species is introduced into the environment, what will happen to it? How much can we tell from physical measurements of the chemical's properties, how much can we learn from lab experimentation, and how much do we need to learn directly from measurements on the chemical in the actual environment? Or, more specifically;
  1. Where does it go?
  2. How long will it remain?
  3. What are the products of its reactions?

A. Transport

The movement of chemicals from one environmental compartment to another is referred to as transport. In these processes, the chemical structure of the compound is not changed. The compartments can include not only air, water, and soil, but also biota such as plants, fish, and even humans.

Volatilization. Transport of organic compounds from the solid or aquatic phases to the gas phase (and back again) is a highly important process for the dispersion of chemical compounds around the globe. Dissolution into and volatilization from the aqueous phase is an elaborate process that depends on solubility, vapor pressure, turbulence within the two phases, and other physical and chemical factors. Volatilization of materials from the earth's surface into the troposphere can result in their long-range transport and redeposition, with the outcome being that measurable quantities of such substances can be detected far from their point of release.

Many chemicals escape quite rapidly from the aqueous phase, with half-lives on the order of minutes to hours, whereas others may remain for such long periods that other chemical and physical mechanisms govern their ultimate fates. The factors that affect rate of volatilization of a chemical from aqueous solution (or its uptake from the gas phase by water) are complex, including the concentration of the compound and its profile with depth, Henry's law constant and diffusion coefficient for the compound, mass transport coefficients for the chemical both in air and water, wind speed, turbulence of the water body, the presence of modifying substrates such as adsorbents in the solution, and the temperature of the water. Many of these data can be estimated by laboratory measurements, but extrapolation to a natural situation is often less than fully successful.

Once a chemical becomes airborne, atmospheric mixing processes on regional, elevational, and global scales come into play. East-west mixing of air masses is much more efficient than north-south mixing. Because of the intra-hemispheric constraints on the prevailing winds, air masses seldom mix efficiently across the equator. The atmosphere becomes completely mixed only over very long time scales; for organic compounds with lifetimes of even several years, northern and southern hemisphere variations are measurable if (as is usually the case) one hemispheric source predominates. Compounds of industrial origin are usually localized in the northern hemisphere, whereas substances derived from marine processes are usually more abundant in the southern hemisphere.

We know from studies of gases in solution that the solubility of a gas which does not react with its solvent depends to a considerable degree on its vapor pressure at a given temperature. We can extend these studies to other solutes if we can measure their vapor pressures at higher temperatures and extrapolate them to lower, environmentally realistic temperatures. For the case of air-water partitioning, a simple equation describes the behavior of many substances:

H = P/C

where H is the Henry's Law coefficient for the chemical, P its vapor pressure, and C its water solubility. If we know or can estimate the quantities on the right-hand side of the equation, we can obtain H, and this will allow us to estimate the magnitude of the air-water partition.

Henry's law constants for chemicals of environmental interest have been tabulated by many authors. Click here to see a helpful summary on the Web. If H has a relatively large value for a particular compound, it means that it has a large tendency to escape from the water phase and enter the atmosphere. To get a large value for H, obviously either a high P or a low C (or both) is required. Thus, for example, sec-butyl alcohol and decane have vapor pressures that differ by a factor of 10, with the alcohol being the higher; but because the hydrocarbon's water solubility is negligible, it is much more likely to enter the gas phase than is the alcohol. Similarly, although the pesticide DDT is essentially nonvolatile, its water solubility is far less even than decane's. As a result a small quantity will be volatilized; this accounts for the widespread detection of DDT in environments far from the sites where it was applied. Another heavily applied chemical, the herbicide atrazine, is a little more volatile than DDT, but it is far more soluble, so its tendency to enter the atmosphere is negligible.

Fluid Flow. The movement of a chemical substance within the vapor phase occurs by the combined driving forces of flow and diffusion. An illustration of these effects can be visualized by considering a smokestack plume; in the absence of wind, the plume will rise vertically in a more or less uniform column until it reaches an elevation where density considerations result in its spreading out into a relatively broad and flat mantle. When wind is factored into the equation, the plume may move in a more nearly horizontal direction, more or less parallel to the surface of the ground, and at certain wind speeds the plume structure can break up into loops or bends due to turbulent aerodynamic effects such as eddy formation. In addition, small eddies can result in the breakdown of the coherent plume structure, with the formation of vertical or horizontal regions of increasingly large cross-section and lower concentrations of plume constituents.

The three-dimensional dispersion of a completely soluble organic solute within a volume of pure water will be governed by its rates of diffusion within the water column and by the flow characteristics of the water itself (also called convection or advection). In actual water bodies, complicating factors include the presence of particles of various sizes within the aqueous phase and the effects of boundary layers such as those associated with the air-water and sediment-water interfaces. Further complications occur in soil-water and groundwater systems in which the aqueous phase is a minor component in the presence of an excess of solid material.

Movement of a soluble chemical throughout a water body such as a lake or river is governed by thermal, gravitational, or wind-induced convection currents that set up laminar, or nearly frictionless, flows, and also by turbulent effects caused by inhomogeneities at the boundaries of the aqueous phase. In a temperature- and density-stratified water body such as a lake or the ocean, movement of water parcels and their associated solutes will be restricted by currents confined to the stratified layers, and rates of exchange of materials between the layers will be slow.

The other method of diffusion of a chemical through a liquid phase, molecular diffusion, is driven by concentration gradients. It is normally orders of magnitude slower in natural waters than eddy-driven processes, unless the water body is abnormally still and uniform in temperature. Such situations are found only in isolated settings such as groundwaters and sediment interstitial waters.

Transport of a dissolved substance through a porous medium like a sandy soil, in which interaction between the solute and the solid phase is negligible, is governed by laws of mass transport that are similar to those that apply in solutions. When interactions with a solid phase such as a soil become significant, a situation similar to solid-liquid chromatography develops; solutes with less interaction with the "support," or soil, are moved along with the "solvent front" of water leaching through the medium, whereas others are held back in proportion to their degree of binding.

Partition and Sorption. The transfer of molecules from the air, or from aqueous solution, into an environmental solid phase is referred to as sorption, with the reverse process usually called desorption. A variety of solid phases are available in the environment--small suspended particles both living and nonliving, the anatomical surfaces of larger biota such as fish, and bulk soils and sediments. Each of these surfaces may be thought of as a source or a sink for compounds in solution.

The passage of a compound from solution into a solid environment can be promoted or inhibited by a variety of factors. Sorption and desorption equilibria are, for example, strongly temperature-dependent. In addition, the surface area of the solid, as well as its physicochemical characteristics (charge distribution and density, hydrophobicity, particle size and void volume, water content) are major determining factors that determine the importance and extent of sorption for a particular solute.

Studies of the uptake of organic compounds by many types of natural solid phases (soils and sediments) in the presence of water have clearly shown that only two types of interactions are important; first, a coulombic interaction, in which organic compounds of opposite (positive) charge are sometimes taken up by the (usually) negatively charged solid material; and, generally more important, a hydrophobic interaction in which nonpolar organic compounds are attracted into the solid phase.

There is a very clear correlation between the extent of uptake of a chemical by a natural solid phase and the partition coefficient, Kp, of the chemical, that is, its ratio of concentration or activity between an organic solvent, often octanol, and water:

Kp = [solvent]/[water]

The partition coefficient is not the same as the ratio of the solubilities of a chemical in the two pure solvents, because at equilibrium the solvent phase contains some water and the water phase contains some solvent.

Many data are specifically available on the octanol/water partition coefficients (Kow) of organic molecules. Click here to see a web reference on this. It has been repeatedly demonstrated that chemicals with high Kow's are very readily sorbed by natural sediments. For example, the extremely nonpolar compound DDT has a Kow of about 106 (it is a million times more soluble in octanol than in water), and it is almost completely associated with the solid phase in a two-phase water-sediment system. For atrazine, a chemical of intermediate polarity, the Kow is still high (3 x 103), but far less than that for DDT, so the extent of association with sediments would be expected to be far less pronounced. For quite polar organic compounds, such as acetic acid, the octanol-water partition coefficient is far lower (0.5), and the distribution in the two-phase system would favor water.

B. Reactions

Biotic. The major agents that cause structural change in compounds released to the environment are microorganisms; bacteria, fungi, and (to a lesser extent) algae. The organisms biodegrade contaminents in the environment.

Mineralization by bacteria is the complete breakdown of an organic compound into inorganic ("mineral") substances. An organic compound made up of C, H, N, and O atoms, for example, would be considered mineralized if it was converted entirely to CO2, H2O, and ammonia or nitrate. Readily biodegraded compounds usually are only mineralized to the extent of 80-90% under ideal culture conditions because some of the atoms of the compound are used to biosynthesize structural elements of the cell.

Usually, only a portion of the organic compound is altered by an organism or by the microbial community, so that intermediate organic by-products accumulate and persist.

A useful web site that summarizes biodegradation pathways for a variety of organic compounds is maintained by the University of Minnesota and located at www.labmed.umn.edu/umbbd.

Abiotic. The environment is rich in agents (in addition to living organisms) that bring about chemical reactions that transform organic compounds into by-products; water, oxygen, acids and bases, mineral surfaces, sunlight UV, etc. Water can act as a classic nucleophile, converting, for example, organochlorine compounds into alcohols; or it may hydrolyze esters such as phthalates to their corresponding acids. Oxygen, or more reactive oxidizing species, may convert sulfur-containing pesticides such as phorate to sulfoxide or sulfone derivatives. (Conversely, in regions of the earth's surface that are oxygen-deficient, reductions can occur; nitro compounds such as TNT can be converted to amine derivatives.) Photochemical transformation reactions may convert sunlight-absorbing materials such as polycyclic hydrocarbons into oxygen-containing or rearranged derivatives. These reactions obviously complicate the analysis of risk and environmental exposure of ecosystems or humans to potentially hazardous chemicals.

Professor Patricia Shapley, University of Illinois, 2010