Environmental risk assessment is a structured quantitative and qualitative approach to the estimation of environmental hazard potential, environmental exposure potential, adverse environmental impact potential, and overall environmental risk. The cornerstone for performing an environmental risk assessment is to obtain environmental fate and effects data.1–5Fate and effects parameters are presented in Table 19.1 with typical expected ranges for these compounds.6These data can be broadly divided into three groups:
1. Mobilitydescribes the probable movement of a chemical among and between the various environmental compartments, such as water, air, soil, sediment, biomass, organisms.
2. Persistencedescribes various environmental transformation processes that chemicals may undergo and which may result in the removal of those chemicals from the en- vironment, including such processes as hydrolysis, photolysis, and biodegradation.
3. Ecotoxicity describes various effects on environmental organisms that chemicals may have. Organisms used typically for ecotoxicity testing include algae, daphnids, other microorganisms, fish, and other invertebrates, such as earthworms.
19.1.1 Assessing Environmental Mobility and Persistence
If a chemical is introduced into the environment, there is a certain probability that it may move from the point where it is released. Eventually, the compound may be distributed over a broad geographical area as the original parent molecule or as different degradants and/or metabolites. The different environmental compartments and their relationships are illus- trated in Figure 19.1, where:
. Koc is the organic carbon/water distribution coefficient, which can be measured directly or estimated from the octanol/water distribution coefficient.
. Kwis the water/air distribution coefficient, the reciprocal of Henry’s constant (H), which is the air/water distribution coefficient.Hcan be measured directly or estimated from the water solubility and the vapor pressure.
. BCF is the bioconcentration factor, which is the organism/water distribution coeffi- cient and can be measured directly or estimated from the octanol/water partition coefficient, logKow(logP).
TABLE 19.1 Fate and Effects Parameters and Typical Expected Ranges
Parameter High Low
Sẳwater solubility >1000 mg/L <10 mg/L
HẳHenry’s constant >107atmm3/mol <107atmm3/mol LogKow, logDow, logKd, logKoc >3 <3
UV/visibleẳabsorption >290 nm <290 nm
Hydrolysis half-life,t1/2 Short: minutes Long: weeks
Photolysis half-life,t1/2 Short: minutes Long: weeks
Biodegradation half-life,t1/2 Short: minutes Long: weeks
Toxicity LC50<1 mg/L LC50>100 mg/L
Assigning volumes or sizes to the various compartments allows calculation of the predicted equilibrium concentrations in each compartment as a function of the input load.
Thus, the primary compartments that chemicals will tend to migrate toward, or accumulate in, can be identified. The next step in assessing the environmental fate of a chemical is to consider the transformation or degradation processes that it may undergo. Such processes include hydrolysis, photolysis, and biodegradation. All of these will serve to decrease the concentration of a chemical in a particular compartment and allow estimation of the persistence of the chemical in the environment. Examples of inferences that may be made based on fate data are shown in Table 19.2.
KOC
KOC
WATER SOIL BCF
FISH
SUSPENDED SEDIMENT SEDIMENT
FIGURE 19.1 Model ecosystem.
TABLE 19.2 Examples of Inferences Based on Fate Dataa
LogKow H
Depletion
Mechanism(s) Exist Inference
<3 <107 Yes Chemical will distribute primarily to the water compartment and will degrade over time.
>3 <107 Yes Chemical will distribute to biomass, soils, and sediments and will degrade over time.
>3 >107 Yes Chemical will distribute to air, biomass, soils, and sediments and will degrade over time.
<3 >107 No Chemical will distribute to the air and water compartments and may be persistent.
<3 <107 No Chemical will distribute to the water
compartment and may be persistent.
>3 <107 No Chemical will distribute to biomass,
soils, and sediments, may bioconcentrate in organisms, and may be persistent.
aIn all cases, acute and subchronic toxicity may be an issue.
Example 19.1 A wastewater stream contains certain amounts of an active pharmaceutical ingredient7(API). This stream is treated in a wastewater treatment plant (WWTP). What is the likely fate of this API as the waste stream undergoes treatment in this WWTP?
Solution We may use models to assess WWTP scenarios. High logKow,Dow,Kd, orKoc would suggest that a chemical may tend to undergo sorption to the activated sludge biomass and that one depletion mechanism may be removal of the chemical from the system with the waste sludge. Knowing the parameters around the WWTP would allow calculation of the likely concentrations in the sludge and water. Although these models are equilibrium models and assume equilibrium conditions, which would generally not be the case in practice, they are useful first approximations to the fate of a chemical in the environment or in a WWTP. For this example, we used the activated sludge model WW-Treat, developed by Cowan et al.,8with the results shown in Table 19.3.
TABLE 19.3 Wastewater Treatment Model for an API
Code Units Value
Parameter
Influent concentration Ci g/m3 1.7
Sludge/water partition coefficient Kp — 871
Fraction sludge removed in primary clarifier Rp — 0.6
Total suspended solids in influent S g/m3 220
Gas flow rate G m3/h 0.45
Henry’s low constant H m3atm/mol 7.00109
Hydraulic retention time HRT h 8
Biodegradation rate of dissolved substance K1 h1 0
Biodegradation rate of adsorbed substance K2 h1 0
Mixed liquor suspended solids MLSS g/m3 2500
Water flow rate Q m3/h 250
Gas constant R m3atm/molK 0.0821
Fraction of solids removed in reactor Ra — 0.95
Sludge retention time SRT h 216
Temperature T K 293
Primary treatment module
Concentration on primary sludge Cps g/m3 2.73101
Concentration in effluent from primary settling Ce1 g/m3 1.54 Activated sludge module
Volatilization loss term Kv — 3.031010
Biodegradation loss term Kb — 0.00
Dissolvedþabsorbed compound concentration Cr g/m3 1.54
Compound concentration absorbed to sludge Cs g/m3 1.05
Final effluent concentration Ce2 g/m3 5.36101
Final distribution
Total removed 68%
Volatilization 0%
Biodegradation 0%
Adsorption 62%
Effluent 32%
mean that the biodegradation percent is zero?
19.1.2 Assessing Ecotoxicity
Aquatic toxicity tests are used to detect and evaluate the potential toxicological effects of chemicals on aquatic organisms. Since these effects are not necessarily harmful, a principal function of the tests is to identify chemicals that can have adverse effects on aquatic organisms at relatively low exposure concentrations or body residues. These tests provide a database which can then be used to assess the risk associated with a situation in which the chemical agent, the organisms, and the exposure conditions are defined. Organisms commonly used in aquatic toxicity tests include the following:
. Microorganisms, used in tests such as microbial respiration inhibition and Microtox, are useful as toxicity screening tools and are particularly applicable to chemicals in waste streams intended for treatment by activated sludge or other biological treatment systems.
. Algae, such as green algae and blue-green algae, are simple photosynthetic organisms found in many terrestrial and aquatic habitats. Algae are extremely important in the functioning of aquatic ecosystems because they serve as a foundation of most aquatic food chains.
. Daphnids, such as Daphnia magna, are freshwater microcrustaceans commonly referred to as water fleas. They are ubiquitous in temperate fresh waters and are an ecologically important species because they convert phytoplankton and bacteria into animal protein and form a significant portion of the diet of numerous fish species.
. Fish, such as warm-water fathead minnow and bluegill and cold-water rainbow trout and brook trout, are ecologically and economically important organisms that are widely distributed throughout most aquatic environments. Fish fill diverse ecological roles and represent an essential link in the food chain by converting aquatic matter into protein that is able to be harvested as human or animal food.
. Other organisms, such asHyallela azteca, a benthic amphipod, and earthworms, are used in tests for chemicals likely to concentrate in sediments and soils.
The most common aquatic toxicity tests are short-term (acute) tests, using lethality or immobility (in the case of daphnids) as the endpoint. These tests provide a means of comparing substances whose mechanisms of action may be quite different and indicate whether further toxicity studies should be conducted. An acute toxicity test is conducted to estimate the median lethal concentration (LC50) of the chemical in water to which the test organisms are exposed. The LC50is the concentration estimated to produce 50% mortality of a test population over a specific time period. The length of exposure is usually 24 to 96 h, depending on the species. When effects other than mortality are measured, the general expression EC50is used. The EC50(median effective concentration) is the concentration of a chemical estimated to produce a specific effect (e.g., behavioral or physiological) in 50% of a population of test species after a specified length of exposure (e.g., 24 or 48 h). Typical effect criteria include immobility, a developmental abnormality or deformity, loss of equilibrium, failure to respond to an external stimulus, and abnormal behavior. The general guidelines shown in Table 19.4 may be useful in evaluating acute toxicity data.