Section A: Life cycle assessment
Question 1
What is risk?
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Answer 1
What is risk?
Risk is the possibility of a negative outcome.
A risk therefore is a possible negative outcome that can cause damage. The damage can be to human life,
ecosystems or economic in nature.
Question 2
Name the three stages which make up risk analysis.
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Answer 2
Risk analysis has the following stages:
1. Hazard identification – Identifying risk agents and the conditions under which they are harmful
2. Risk assessment – Describing and estimating the risk
3. Risk evaluation – Comparing and judging the significance of the risk
Question 3
Briefly discuss the three links in the risk chain. Remember that the a risk chain is a simplified model for a risk assessment that leads to the risk estimate.
Answer 3
Three conditions are required for a risk to exist. First, a risk source is required. A risk source is anything that can
release or otherwise introduce a risk into the environment. An example of a risk source is a nuclear power plant or
a hazardous chemical installation.
Secondly, an exposure route or pathway is necessary. In this example, the pathway may be wind dispersion of
radionuclides and toxic chemicals or the biomagnification through a food web.
Thirdly, a negative effect will have to exist. The causal process in this example is the development of cancer or
radiation poisoning from radiation or a neurological impairment caused by the toxic chemical.
Risk source à Exposure pathway à Effect à Risk estimate
Fig 1: The risk assessment chain.
Each of these three conditions – releases from a risk source, exposure and effect may be thought of as links in a
risk chain. (Merkhofer, 1987) The level of risk depends on the specific nature of the risk source, exposure and
effect. It is therefore necessary to look at aspects of all the above links including the potential of the source to
release the risk agent; the intensity, frequency and duration of the exposure and the nature of the target
ecosystem of human population exposed; and the effect of the agent on the target (Covello, VT and Merkhofer,
MW, 1993).
Risk estimation is the combination of the different elements of the risk chain and the calculation of the estimated
risk.
Question 4
What are the four steps that need to be taken to conduct a complete risk assessment? Also briefly discuss what each of the steps entails.
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Answer 4
Based on this model, a complete risk assessment consists of four distinct steps:
1. Release assessment
Release assessment will look at the possibility or probability of a risk source to release or otherwise introduce risk into the environment. It will also include the type, amount and timing of the release of whatever risk agent is involved. Risk agents can be toxic substances, kinetic energy or other risk agents.
2. Exposure assessment
Exposure assessment looks at the conditions at the time of release of the risk agent. It links the release of the risk agent with the 'uptake' or action of the risk agent by the target.
3. Consequence assessment
This step takes into account the damage effect of the risk agent on the target. In this case the target can either be humans or ecosystems. It affects human health and ecosystem integrity.
4. Risk estimation
Risk estimation requires integration of the results from release assessment, exposure assessment and consequence assessment to produce a risk estimate. (Covello, VT and Merkhofer, MW, 1993)
Question 5
How are man-made risks often released into the environment?
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Answer 5
Man-made risks are often caused by the breach of a physical barrier or less than effective containment and subsequent release of risk agents into the outside environment. (Covello, VT and Merkhofer, MW, 1993)
Question 6
Name any four natural risk factors.
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Answer 6
Natural forces are risk factors too. Earthquakes, mudslides, rock falls, avalanches, floods, hailstorms, blizzards, tropical cyclones, tsunamis, meteor impacts and a large number of others threaten human life. Ecosystems are somewhat less affected by these natural disasters and yet five mass extinctions have occurred in earth history caused by factors like climate change or impact events.
Question 7
The risk agents released can be chemical, physical, biological or forms of energy. Give two examples of each of the types of risk agent.
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Answer 7
The risk agents released can be chemical, physical, biological or forms of energy. Chemical risks include corrosive substances, explosives, flammable liquids or gases and toxins. Physical risk agents like rock falls, flood waters or volcanic ash can be just as dangerous. Biological risks agents include genetically modified organisms or natural pathogenic bacteria and viruses. Forms of energy include mechanical energy, sound energy, electromagnetic energy and radiation, extreme heat or cold and atmospheric pressure. (Covello, VT and Merkhofer, MW, 1993)
Question 8
Briefly discuss monitoring for release assessment. Give any two examples of monitoring for release assessment. You don't necessarily have to give the ones in the notes.
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Answer 8
Monitoring gathers data in order to detect changes in some of the variables of interest. Most risk-source monitoring is focused on information related to releases or release potential.
Examples of monitoring activities used in release assessment include verifying the integrity of hazardous or radioactive waste at a disposal site or compiling records on the production or import of toxic chemicals. Critical issues in monitoring include determining where and how to take representative samples, deciding how to handle the samples in route to laboratories, selecting analytical methods, choosing a format for the results and for conveying the degree of confidence in the data and deciding how to interpret data on the quantity, transformation, and migration of the risk agents (Covello, VT and Merkhofer, MW, 1993).
Monitoring provides the baseline information required by decision makers. Actions can be deemed necessary if the indication is that releases are above safe limits. Action can be in the form of changes to laws and technologies forcing a predictable change in the risk source. For example, if regulations require installing technology for purifying the emissions produced by some production process, and if the technology is known to have some level of efficiency, then comparison with existing releases forms the basis for future assessing risk with and without the new regulation. (Covello, VT and Merkhofer, MW, 1993)
Question 9
What is performance testing and accident investigation for release assessment? Can you think of ways in which performance testing can be conducted for windscreen wipers and motorcycle crash helmets?
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Answer 9
3.3 Performance testing and accident investigation
Monitoring systems for release assessment are usually designed to collect measurements of the condition of the risk
source while it is functioning normally. Sometimes it is useful to gather information while deliberately subjecting the risk source or its component systems to stress, that is, to investigate the behavior of a system under conditions that are extreme or otherwise of special concern. Performance testing therefore entails collecting data about a system under controlled, usually stressful, conditions (Covello, VT and Merkhofer, MW, 1993).
It is often used for mechanical or electrical components. If performance testing has generated sufficient data about specific types of components, statistical methods can be used to derive the operating and failure characteristics of those components used in the technological system under study. Sometimes the failure of individual components is of less concern and therefore the entire system is subjected to performance tests. Even if only limited experiments can be performed, such testing can still be useful.
If the reliability of a component or subsystem of a risk source is very high, accelerated-life tests are often used to avoid lengthy tests or the need to test many units to obtain useful failure data. With accelerated-life tests, the unit is subjected to operating conditions designed to produce a large number of failures in a short period of time.
Key to the design of accelerated tests is ensuring that the same failure mechanisms occur under the more stressful conditions as under normal conditions. Then, the only change is that time is effectively accelerated (Covello, VTand Merkhofer, MW, 1993).
For example, if corrosion happened at a specific rate at typical temperature and humidity, then the same type of corrosion may happen much more quickly in a humid laboratory oven at elevated temperature, or if you want to test the locking mechanism for a new lock that is expected to be locked and unlocked two times daily, you can accelerate it by opening and closing the lock a number of times a minute over a shortened time (Covello, VT and Merkhofer, MW, 1993). Accelerated testing requires interpreting failure data obtained under extreme conditions.
For this methods had to be developed. Acceleration models are used to translate the results of accelerated performance tests to more realistic, lower-stress conditions of use.
Accident investigation is another release assessment method. The investigation of accidents that have happened can provide much useful information for risk assessment. An important element of accident investigation isdetermining exactly what caused the system to fail. The approach means that the accident needs to be reconstructed under the guidance of physics and the laws of nature. To help guide the investigation, the investigation often employs a method known as mental movies. A mental movie is a reenactment of the accident that is played out in the investigator's mind. Gaps in the movie indicate unknowns that the investigator needs to resolve to understand or explain the accident. Other methods used by accident investigators include structuring methods; chemical, thermal or metallurgical analysis; reconstruction of surviving parts and simulations likeexplosive tests and reenactment of hypothesized accident scenarios (Covello, VT and Merkhofer, MW, 1993).
Question 10
The following questions are based on modeling methods for release assessment.
10.1 What is a model
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10.2 What is a Failure-Mode and Effects Analysis?
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10.3 What are fault trees and event trees and what is the biggest difference between the two?
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10.4 What are discharge models used for?
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Answer 10
10.1 A model is a synthetic representation of reality. They can be done on paper or in complex computer simulations.
10.2 This method can be used for predicting risks that have not yet occurred before. They are based on the specific conditions and sequence of faults that need to take place for an accident to take place. Many of the methods used are rather qualitative in nature. Examples of the different methods are Failure mode and effects analysis (FMEA), preliminary hazards analysis (PHA), fault hazard analysis (FHA), hazards and operability analysis (HAZOP), and criticality analysis (CA). (For detailed descriptions see, Green and Bourne, 1972; McCormick, 1981; ALChE/CCPS, 1985; and Henley and Kumamoto, 1991)
EFEA studies a system or subsystem to determine what failures of which components will result in what effect. An EFHA studies a system or a subsystem to determine the impact of its failure on other parts of the system and to try to quantitatively rate hazards. A CA studies a system, or subsystem to determine the relative magnitude of each potential failure, based on the failure rate and effect on the system or subsystem (Covello, VT and Merkhofer, MW, 1993).
10.3 Fault tree analysis, event-tree analysis, and related methods
Fault-tree analysis and event-tree analysis are the methods most frequently used for creating models of sources of discrete risks. Fault-tree analysis is based on a specialized model that may be represented as a diagram of binary (yes/no) logic that traces backward in time the different ways that a particular event could occur. Logic gates connect the branches of the event tree together. The OR gate is often used in which either one or the other of the branches can be followed but not both. With a fault-tree, the first step is to define the final failure event. From this point possible faults leading to the defined failure are defined.
Event trees are also graphic tree structures. An event tree starts with a particular undesired initiating event and predicts how the system will respond to the initiating event. Each branch represents a state or condition of the system. Each node in an event tree represents a possible course of action or subsequent failure of the safety systems that will be called upon as the accident progresses. Probabilities of occurrence are given to each branching. A series of probability calculations then provides the probabilities of occurrence of the final accident.
The use of fault trees and event trees in modeling discrete risk sources tends to be complementary. In addition to fault trees and event trees, related modeling methods useful for characterizing risk sources include reliability block diagrams, GO methodology and Markov models. Markov models are very useful effective for system that has time-dependent failure rates.
10.4 Discharge models
To model the materials released in the event of containment failure, discharge models are used. This type of models is used very extensively for applications in chemical engineering. It can be used to model both controlled and accidental releases like the rupturing of a pressurized tank. The models tend to be based on well-developed theories of engineering and thermodynamics but sometimes formal theory is lacking and simplifications are performed. Different discharge models are based on well defined mathematics. The mathematical formulae
contain variables such as pressure gradient and liquid/gas density. Hole size is a key point in the discharge model as well as the phase of the released material. (Covello, VT and Merkhofer, MW, 1993)
Gas discharge models differ depending on whether the source is a containment vessel or a long pipe. Other conditions distinguishing gas discharge models include thermodynamics of the release. In the case of a long pipe, the mathematics is more complicated because of the introduction of frictional resistance which causes the discharge rate to vary with time even if the containment pressure does not. Fires have an entirely new set of complications. Heat flux through vessel walls need to be taken into account. (Covello, VT and Merkhofer, MW, 1993)
Discharge models are also used to estimate the vapor releases from pools of volatile liquids, such as from hazardous waste lagoons or spills. Mass and energy balance form the basis of these models. The models also take into account the thermodynamic properties of the liquids and the surface on which the liquid rests (Covello, VT and Merkhofer, MW, 1993). If the release involves a pressurized liquid exposed to the atmosphere at a temperature higher than its boiling point it will start to flash, or rapidly vaporize. If the volume of liquid is small simplified models assume that all of the liquid would simply evaporate. If the volume on the other hand is quite large, a twophase release will be triggered in which a large percentage of the liquid will enter the atmosphere as tiny droplets, which remain suspended as an aerosol or rain out onto the ground. This affects the subsequent dispersion of the cloud, since the droplets cause the cloud to have a higher density and possibly a lower temperature (Covello, VT and Merkhofer, MW, 1993). Flash models typically provide the vapor/liquid split for a discharge of superheated liquid. Models of aerosol and rainout must then be used to provide estimates of the characteristics of the cloud, which would typically be input to atmospheric dispersion models (Covello, VT and Merkhofer, MW, 1993).
Discharge models are also often used for estimating the characteristics of atmospheric releases in cases where the released gases may have a significant momentum or positive buoyancy. Plume models are useful for continuous releases, while puff models are used when the release occurs over a short period of time. Like other discharge models, plume and puff models are designed to provide the initial conditions of the release cloud for atmospheric dispersion models, including temperature, aerosol content, density, size, velocity and mass (Covello, VT and Merkhofer, MW, 1993). Atmospheric dispersion models have additional inputs like local weather conditions, atmospheric composition, heat flow, moisture content, dew points and topography.
Discharge models are also available for modeling fires and explosions. Explosion models are concerned with estimating the characteristics of the shock wave overpressure effects and projectile effects. Fire models are concerned with thermal radiation effects. Several different types of models for explosion and fire have been developed. Flash fire models describe the ignition of a cloud formed from the release of a volatile flammable gas.
The models provide thermal radiation zones, which provide input for the thermal effects model, or peak overpressure as a function of distance which would be an input into an explosion consequence model. A special kind of explosion is for the rupture of a vessel containing superheated liquid or liquefied gas. The boiling liquid expanding vapor explosion (BLEVE) model may be used for modeling it (Covello, VT and Merkhofer, MW, 1993).
Question 11
What is the difference between risks agents acting additively, synergistically and antagonistically?
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Answer 11
Another important aspect of exposure assessment is determining which groups in the population may be exposed to a risk agent; some subgroups may be especially susceptible to adverse health effects. Risk agents normally act additively i.e. the total effect is the combined effect of the individual risks. On the other hand, risk agents can act synergistically i.e. they enhance the effects and the total effect is bigger than the combined effect of the individual risks. Antagonistically acting pollutants partially cancel out the others effect.
Question 12
What information do models for exposure assessment provide?
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Answer 12
Concentration, uptake, frequancy, duration etc.
Question 13
What makes exposure assessment so difficult?
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Answer 13
An important factor making exposure assessment so difficult is the strong influence that individual personal habits can have in human exposure. (Covello, VT and Merkhofer, MW, 1993)
If the risk agent is absorbed when the consumer product is used, the patters of use will affect exposure levels. For risk assessments associated with consumer products, exposure assessment focuses on understanding how the products may be used and how different use patterns affect exposure levels. (Covello, VT and Merkhofer, MW, 1993)
If exposure occurs through the air or water, then exposure assessment must take into account how the risk agent moves from its source through the environment and how it is changed over time. Chemical risk agents normally become diluted and may degrade after release. The aim of exposure assessment is this case is to determine the concentration of toxic materials in space and time where they interface with target populations. With some risk agents, understanding the changes that occur following released is all-important. For example, genetically engineered organism released into the environment will seek to reproduce and if successful may introduce new genes into other species and primary air pollutants may undergo photochemical reactions to form secondary pollutants (Covello, VT and Merkhofer, MW, 1993). Physical risk agents such as mechanical energy or heat can also be the risk in an exposure assessment.
Exposure assessments for environmental risks have additional difficulties. One complicating factor in this regard is the fact that the environment is not a single point source, but a complex interacting system with components of varying sensitivities. An exposure assessment must describe the levels of exposure and all conditions that might be needed to assess the effect of those exposures, including 1) the magnitude, duration, timing and route of exposure; and 2) the size, nature and sensitive subpopulations of the population exposed (Covello, VT and Merkhofer, MW, 1993). For environmental risks, the exposure assessment will have to include the spatial distribution and differential sensitivities of the exposed elements of the environment. In addition, because of the different pathways that a risk agent can follow, the exposure assessment will have to add up the exposures form different environmental media.
Question 14
The following questions are based on monitoring methods for exposure assessment.
14.1 How can monitoring for exposure assessment be conducted for a leaking liquid hazardous waste lagoon?
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14.2 What is the difference between direct and indirect methods of monitoring for exposure assessment. Give an example of direct and indirect monitoring methods for chemistry students' exposure to sulphuric acid vapours in the lab.
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Answer 14
14.1 By drilling boreholes at different distances and in different directions from the lagoon, drawing samples and analyzing them in the lab. Surface waters can also be monitored for any contaminants.
14.2 Direct monitoring methods attach monitors to the students themselves and indirect methods monitor the labs and have to take into account the movement of students into and out of the lab. You can have a base or alkali with a indicator as a reactive media to detect exposure levels. Passive air pollution samplers or dragar tubes can be used to establish ambient concentrations in the lab.
Question 15
Briefly discuss the following exposure assessment models.
You can include some of the following information in your answer: What are they, what do they model, what input variables do they require, what types of transformations do they take into consideration, what information can be output from them and how do they calculate this?
15.1 Atmospheric distribution models
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15.2 Surface water models
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15.3 Ground water models
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15.4 Food chain models
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Answer 15
15.1 Atmospheric models
Atmospheric models address the processes of air-pollutant transport, diffusion and deposition. Transport is the movement of the suspended pollutant through air currents. Air currents typically cause emissions from a point source to follow a zigzag course initially, and then to veer off to one side in an arc. (Covello, VT and Merkhofer, MW, 1993)
Diffusion refers to the motion of individual particles and molecules that tend to spread and dilute the pollutant.
Deposition is the dropout of particulate matter to the ground, water bodies and vegetation. Deposition is caused by gravity and makes individual particles fall to the ground or as pollutants contained in falling raindrops or snowflakes.
Models distinguish between wet and dry deposition because the rate of deposition is higher during rain or snow than normal. (Covello, VT and Merkhofer, MW, 1993)
One important factor influencing atmospheric dispersion is atmospheric stability i.e. conditions that tend to resist or enhance the vertical movement of air. At most times and in most places, the atmosphere gets steadily colder with increase in altitude. Since warm air rises, this produces a general upwelling of warm air; however, if for some reason the temperature at the surface is colder than the temperature higher above, then the dense cooler air near the surface cannot get enough lift to rise (Covello, VT and Merkhofer, MW, 1993). This is called an atmospheric inversion and upwelling stops. Without upwelling, any pollutant injected into the atmosphere at the surface from chimneys and tailpipes builds up in the air near the surface. Atmospheric stability is particularly important for estimating
atmospheric concentrations near the ground, because it determines where the plume will first reach the ground. (Covello, VT and Merkhofer, MW, 1993)
The outputs of atmospheric models are: 1) atmospheric concentrations of the pollutant and 2) deposition rates, both wet and dry. Atmospheric concentrations determine exposure. Deposition rates determine how much of the pollutant emitted to the atmosphere reaches the soil, surface and groundwater. (Covello, VT and Merkhofer, MW, 1993)
Most models simplify the process of computing atmospheric concentrations and depositions by dividing the dispersion process into two phases: initial plume rise and subsequent dispersion and deposition. Plume rise is assumed to occur without significant dispersion. For industrial stacks, the amount of rise is added to the actual stack height to determine the effective stack height. Briggs (1975) has developed equations to describe plume rise under different atmospheric stability conditions (Covello, VT and Merkhofer, MW, 1993).
Since so many variables affect the diffusion process, no rigorous mathematical solution has been found. Instead, the most common modeling approach, called the Gaussian plume model is used. The model assumes that the plume from the emission will become more diffuse as it spreads out in a cone shape. Gaussian models often assume that the wind is distributed uniformly whenever it is within a specific angular sector. Consequently, the estimates pollutant concentration varies according to the downwind distance from the source but is constant in the crosswind direction within each angular sector.
Gaussian models have limited applicability when the terrain is complex or uneven and when release rates are highly variable. They are fairly accurate however, for predicting ground-level concentrations where reasonably flat terrain exists and where and average, fairly stable release rate can be assumed (Covello, VT and Merkhofer, MW, 1993).
Validation studies have concluded that Gaussian plume models can predict within a factor of two the annual average atmospheric concentrations at distances of 10 kilometers or less over relatively flat terrain. (Hoffman et al., 1979) Predictions over shorter time scales tend to be less accurate.
Another, more complex class of atmospheric models attempts to compute the trajectory that a pollutant might follow.
Trajectories are typically composed of a connected sequence of trajectory segments estimates for fixed intervals of time and based on historical wind data. Running the models requires solving complex systems of equations that represent continuity of motion and conservation of mass. Trajectory models are most often used for long-range predictions of atmospheric concentrations. Short-term concentrations are assumed to follow some specified frequency distribution (Covello, VT and Merkhofer, MW, 1993).
To handle cases where the terrain is not flat and clearly has an effect on transport and dispersion, complex terrain models have been developed. In addition for accounting for plume rise, atmospheric stability, dispersion and deposition, these models account for plume interaction with terrain, including plume deflection around objects, plume
lifting over terrain, plume impingement on terrain and terrain contours (Covello, VT and Merkhofer, MW, 1993).
To represent rapid, short-duration emissions, such as those that might result from explosions, 'puff' transport models have been developed. Puff transport models frequently adopt the simplifying assumption that transport and dispersion of a pollutant in the crosswind and vertical directions are uncoupled from the downwind direction. Typically, dispersion in the crosswind and vertical directions is assumed to have a Gaussian distribution. Pollutant distribution in the downwind direction is most simply represented as 'plug flow', wherein it is assumed that the pollutant is carried by the wind with no dispersion in the downwind direction and no change in the size of the puff. More complicated models account for downwind dispersion and growth of the puff. (Covello, VT and Merkhofer, MW, 1993)
Models for short-term or transient releases that are neither puffs not continuous plumes have also been developed.
One way of doing this is by treating a release like a series of puffs. The results are then numerically integrated to obtain the concentration. Another approach is to simplify it mathematically.
Atmospheric models have also been developed for the indoor air environment. Indoor emission may occur in various ways. Movement of outdoor pollutants indoors and the generation of pollutants by indoor activity are the most important. Accounting for differences between indoor and outdoor concentrations can be crucial. The levels of some pollutants can be a lot lower indoors than outdoors and for others it can be a lot higher than the outdoor concentration. Few models are available for estimating indoor pollutant concentrations originating from outside sources.
For determining vapor concentrations from indoor emissions, compartmental models have been developed. These models account for the ventilation of the different volumes of air in buildings. Indoor air quality models can become very complex if it incorporates the effects of air filter efficiencies, particulate agglomeration, nonspecific heterogenous decay of reactive pollutants, multiple sources and sinks, and varying source strengths (Covello, VT and Merkhofer, MW, 1993).
In addition to transport and dispersion, many atmospheric models account for transformations of the airborne risk agents i.e. the production of secondary pollutants. The introduction is necessary because sometimes the secondary pollutants are less or more hazardous than the primary one. The formation of acid rain is one example. Sulfuric acid and nitric acid, the two acids that make up acid rain, are formed in the atmosphere from the gaseous pollutants sulfur dioxide and nitrogen oxide, which are emitted from fossil fuel burning. The formation of these acids in the atmosphere occurs during the chemical reaction of oxidation. Chemical transformation processes and changes in the populations of microorganisms are often approximated by first-order rate equations (Covello, VT and Merkhofer, MW, 1993).
15.2 Surface water models
Surface-water transport and fate models model the movement of pollutants in surface waters and their endpoint in the environment. Pollutants in surface water can originate from point sources or from non-point sources. As with atmospheric models, the concentrations produces by surface water depend on the flow and mixing of the water body and any physical, chemical, radiological, or biological transformations that may occur (Covello, VT and Merkhofer, MW, 1993. There are however key differences between surface water and atmospheric models. Pollutants released to water may dissolve or either float on the surface or descend to the bottom depending on the solubility and density. Volatile pollutants will tend to escape from water into the atmosphere by turning into vapor. Pollutants in dissolved form may be adsorbed from solution onto sediments, or removed by biochemical cycles. Pollutants in the form of particulate matter may settle, or precipitate to the bottom. Adsorption and precipitation tend to reduce the concentrations in surface water, but create deposits that become longer-term pollutant sources through desorption or resuspension processes (Covello, VT and Merkhofer, MW, 1993). The mixing processes in water also differ from that of the atmosphere. Pollutants entering surface water are initially dispersed by currents and turbulence. Subsequent concentrations depend on flushing characteristics. In a lake, for example, the rates at which new water enters from incoming streams and leaves by an outlet stream are important. A shallow lake will tend to be well stirred because
wind keeps the waters mixed. A deep lake on the other hand will have a well mixed surface layer due to the action of wind and an unmixed deeper layer (Covello, VT and Merkhofer, MW, 1993).
Chemical processes can be very important in water. Ionization occurs when water molecules, in effect, pull apart the crystal of a contaminant. The ions produces are reactive, and combine with other ions to create different chemicals that may be more or less toxic, soluble, volatile or likely to be biomagnified. The rates at which such chemical processes occur often depend on water temperature, oxygen levels and the pH of the water. The more acidic the water is, the more soluble the metal. Dissolved metal are more likely to be taken up by plants and ingested by animals. Warmer water typically causes reaction rates to increase (Covello, VT and Merkhofer, MW, 1993). Surface water models must account for such effects.
The objective of most surface-water models is to predict the movement, dilution, partitioning and degradation of contaminants in water. Typical applications include a) estimating the position and size of plumes and or mixing zones for point-source discharges; b) estimating the downstream movement rate, dispersion and fate of contaminants following spills or other pulse discharge events, and c) estimating average concentrations as a function of distance for continuous point source or non-point source discharges. The models are generally designed for specific types of water bodies such as streams, rivers, estuaries, coastal water, impoundments, reservoirs, or lakes (Covello, VT and Merkhofer, MW, 1993).
15.3 Groundwater models
Groundwater models are used to estimate the fate of contaminants that enter groundwater flows. Most groundwater models have been developed to analyze the movement of pollutant plumes. Once a contaminant enters a groundwater flow system, its motion is determined largely by underground water-flow patterns. These patterns depend on local geohydrological features like the location of aquifers, natural water recharge, infiltration and withdrawal rates, and factors than influence the density of the fluid. Whether or not groundwater flow is present, contaminants can be dispersed through molecular diffusion (Covello, VT and Merkhofer, MW, 1993). Often, contaminants are adsorbed onto the soil, thereby retarding their movement relative to that of the groundwater. In addition, contaminants in groundwater can be transformed through chemical reactions or biological breakdown.
The objective of most groundwater models is to calculate the distribution of contaminants in the unsaturated soil above the water table near point-sources of pollution and in the saturated soil in the aquifers under the groundwater table. One output of primary concern in risk assessment is the concentration of pollutants in groundwater used for irrigation and drinking. Groundwater models are often complex due to the mechanisms that affect transport including contaminants leaching from the surface, advection (including infiltration, flow through the unsaturated zone, and flow
with groundwater), dispersion, sorption (including adsorption, absorption and ion exchange), and transformations (including biodegradation, hydrolysis, oxidation, reduction, complexation, dissolution and precipitation) (Covello, VT and Merkhofer, MW, 1993). The fact that not all chemicals may be dissolved in water means that groundwater models may have to represent chemicals that float on top of groundwater or sink to the bottom of the aquifer (Covello, VT and Merkhofer, MW, 1993).
Groundwater transport and fate models typically include two components: a model for groundwater flow and a model for contaminant transport. Groundwater flow models define the convective flow field and provide estimates of water velocities for flow paths and travel times. The equations forming the foundation of the models are based on geohydrology theory. Based on the outputs from the groundwater flow model, the contaminant transport model estimates the migration of contaminants, taking into account dispersion and retention. The contaminant transport
15.4 Food chain models
Low ambient concentrations of contaminants can reach very high concentrations if they are taken up and stay in the food chains. This is called biomagnification, bioaccumulation or bioconcentration. Food chain models explore the links between organisms at different trophic levels and how the contaminant moves to higher trophic levels. It therefore stepwise looks at the number and weight of smaller lower organism ingested by those at a higher trophic level, the concentration in those lower organisms and the ratio of the contaminant that will reside in the organism. A simplified method is used for aquatic organisms in which the concentration of the contaminant in the water is used to infer the concentration of the contaminant in living organism that live in the water. This ratio is called the partition coefficient for that chemical species. It can be estimated based on the octanol/water partition coefficient. The coefficient measures the tendency of a chemical to remain in water or to partition to octanol (Covello, VT and Merkhofer, MW, 1993).
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