![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Candidate Causes
CC.8. Unspecified Toxic Chemicals
The toxicity of a substance refers to its potential to harm living organisms. Toxicity is a function of concentration and duration of exposure, and varies by species, age and condition of exposed organisms. Toxic chemicals, as considered here, are individual chemicals, mixtures of chemicals and their by-products that originate from human activities (Image CC.8-1). These are toxic chemicals that are not yet identified and are capable of adversely affecting living organisms either directly or indirectly. They may be unknown because they have not been measured or measurement is difficult (e.g., due to episodic occurrence, unique chemistry, or low concentrations). Their effects may be suspected but, because of absent or incomplete chemical monitoring data, exposure cannot be confirmed. Under these circumstances, "toxic chemicals" should be listed as a candidate cause. Note that toxic metals are treated separately, see CC.1. Metals.
CC.8.1. What to Consider When Determining if Toxic Chemicals Should be Included as a Candidate Cause
Toxic chemicals as addressed in this module should be listed as a candidate cause when potential human sources and activities, site observations, or observed effects support portions of the pathways in the conceptual model (Figure CC.8-1).
CC.8.1.1. Checklist of Sources, Site Evidence and Biological Effects
A checklist is provided below to help you identify key data and information useful for determining whether to include toxic chemicals among your candidate causes. The title for each column links to more detailed descriptions. The list is intended to guide you in collecting evidence to eliminate or enhance support for toxic chemicals as a candidate cause. You also may be aware of other indicators of toxic chemicals; please share your insights using the comment section.
Consider listing toxic chemicals as a candidate cause based on the presence of sources and activities, site evidence, and biological effects.
Sources and Activities
- Non-point sources from industrial, agricultural, mining, logging, urban and residential activities, and related development
- Historical sources and landfills
- Spills and illegal dumping
- Point source discharges from industries, municipal treatment facilities, animal husbandry operations
Site Evidence
- Odors, sheens, discoloration of water
- Deposits on stream banks or bottoms
- Abnormal levels of water quality characteristics such as pH, conductivity, hardness, or dissolved oxygen
- Reports of past chemical spills or episodes of toxic releases, such as treatment plant failures
- State, county records of pesticide applications, business operations
Biological Effects
- Abrupt increases in fish or invertebrate mortality
- Other significant community changes, such as large reductions in species richness or abundance
- Abnormal behaviors, such as fish leaping from the water, gasping at the surface, or crowding into tributaries
- Gross pathologies not typical of pathogens, such as tumors, deformities, or sloughing of gill tissues
- Appearance of new parasites, disease
- Toxic effects in tests of effluents, ambient waters or sediments
Consider contributing, modifying, and related factors as candidate causes when toxic chemicals are selected as a candidate cause. These factors can influence concentration and toxicity, and are important for understanding the dynamics of distribution and effects of toxic chemicals:
- Dissolved oxygen
- Flow alteration
- Ionic strength
- Sediments
- Temperature
- UV exposure
- Change in pH
CC.8.1.2. Sources of Toxic Chemicals
Humans have processed, concentrated, and released thousands of chemicals to the environment since the industrial revolution. The enactment of environmental legislation in the 1970s (e.g., Clean Water Act, Clean Air Act) reduced chemical releases, yet toxic chemicals continue to cause impairments in aquatic environments because there are:
- A limited number of water quality criteria supporting regulation. Only a fraction of existing chemicals have established criteria (see Table CC.8-1),
- Legacies of contaminated sediments and soils exist from past management practices. These contaminants can become biologically available or mobilized under certain conditions (e.g., oxidation, leaching, flooding events),
- Changes in toxicity, with some chemicals only becoming toxic when transformed or mixed with others in the environment,
- Undocumented pulse events, and accidental and illegal releases to surface waters, and
- Long range transport of chemicals via water or air.
You should consider direct inputs, transformations (e.g., photolysis, hydrolysis, metabolism), and transport mechanisms when evaluating whether to include toxic chemicals as a candidate cause. Additional investigations will normally be required to identify what toxic chemicals may be important and where they originate. Useful sources of information on toxic chemicals released to the environment may be found through the Toxics Release Inventory. Additional information is available from Web sites on hazardous waste, pesticides, Superfund sites and health related issues, among many others. Due to the difficulty of conducting effective on-site investigations, the services of specialists in aquatic toxicology and environmental chemistry are often essential for successful evaluations.
Most human activities can become a source of toxic chemicals (Images CC.8-2 and CC.8-3), so the following examples are not all inclusive. In addition, a natural setting without obvious human disturbance or activity does not preclude the possibility that toxic chemicals are causing observed effects. Contamination in undisturbed sites may be present because chemicals can be distributed over long distances in the atmosphere (Image CC.8-4), chemicals such as pesticides may be aerially applied to forests or range lands, or historic sources of toxic chemicals may be obscured over time. Generally there are two classes of source delivery: nonpoint and point.
Industrial, agricultural, mining, logging, urban and residential activities, and related development - These land uses are all potentially non-point sources of toxic chemicals. Chemicals in smoke stack emissions can spread widely across the landscape or region (Image CC.8-4). Building materials, solvents, fuels, cleaning materials, drugs, and pesticides enter the environment in the course of regular use or operation (e.g., Images CC.8-5). Pesticides used on crops or lawns, around building foundations, or applied to reduce nuisance insects or vegetation can contaminate surface waters, particularly when used improperly (e.g., application immediately before rain events). Chemical spills on soils, improper disposal of chemicals in drains, cleaning of residences, animal husbandry operations that use various antibiotics or hormones, leaks in pipes or holding tanks, and many other potential exposure pathways can result in episodic or sustained releases of chemicals to surface waters. Hardened surfaces and storm drains facilitate chemical transport to aquatic environments (see CC.7. Flow Alteration).
Historical sources and landfills - Past industrial and commercial operations (e.g., tanning operations, slaughterhouses, lumber mills, service stations, and drycleaners) may have contaminated soils and sediments, allowing toxic chemicals to leach into surface waters through surface or subsurface pathways. Active landfills that are failing, old landfills created before current technologies, buried wastes from commercial or military operations, or lands contaminated by former smokestack emissions or transportation corridors can contribute toxic chemicals to the aquatic environment. Releases of contaminants may occur slowly over several years or, under some conditions, at high concentrations over relatively short time frames (e.g., during intense precipitation events, flooding, dredging).
Spills and illegal dumping - Spill events (Image CC.8-6) and dumping can introduce chemicals in concentrated pulses at levels that are acutely toxic to aquatic biota, or result in enduring non-point releases for chronic exposures. Even relatively non-toxic substances, such as sodium sulfate or sodium chloride, can be toxic when present at very high concentrations. When highly toxic substances are involved, the results can be far-reaching and long-lasting. These sources tend to occur in specific locations, like point sources, but are often subsequently distributed in the environment by landscape processes similar to non-point sources, potentially creating lethal episodic exposures.
Enhanced delivery from land uses - Land use changes across the landscape linked to commercial and residential development and other human activities can directly impact delivery of toxic chemicals to surface waters. These factors influence concentration and toxicity, and are important for understanding the dynamics of distribution and effects of toxic chemicals from regular commercial, residential, and agricultural activity.
Industries, municipal treatment facilities, commercial establishments, and animal husbandry operations - These point sources often discharge mixtures of chemicals directly into surface waters at specific locations (Image CC.8-7). While direct surface water discharges are normally operated under permit, accidental releases, intermittent changes in chemical composition of effluents, and/or combined sewer overflow events can contribute to long-term or temporary increases in chemical concentrations.
CC.8.1.3. Site Evidence that Suggests Listing Toxic Chemicals as a Candidate Cause
If you observe the following evidence during site reconnaissance, or there are records of past contamination, toxic chemicals should be considered [adapted from Hunn and Schnick (1990)]:
- Odors, sheens, or discoloration of the water (Image CC 8-8),
- Sludge or discolored deposits on stream banks or bottoms,
- Abnormal levels of water quality characteristics such as pH, conductivity, hardness or dissolved oxygen, or
- Reports of past chemical spills or episodes of toxic releases, such as treatment plant failures.
Keep in mind that most chemicals do not leave visible signs in the environment, and the introduction of many toxic chemicals may not result in readily-detected changes in commonly-measured water quality parameters. Thus, the absence of these site observations does not preclude concerns over toxic chemicals. However, in most cases, toxic chemicals co-transport with other relatively benign materials and should be considered a potential cause when changes occur in the relationships among commonly measured parameters of water quality such as conductivity, alkalinity, and hardness (Stewart, 2001).
CC.8.1.4. Biological Effects that Suggest Listing Toxic Chemicals as a Candidate Cause
Effects of toxic chemicals on aquatic life can be classified into one of several categories [see Hunn and Schnick (1990)]. Acutely toxic chemicals act quickly and may cause extensive mortality. Some chemicals act generally and kill both plants and animals, whereas other chemicals act specifically and may affect only plants, or only animals, or only certain life stages of some kinds of organisms. Mortality or other effects attributable to exposure to these chemicals may be rapid, progressive or lingering, especially if the chemical operates via a chain of adverse environmental changes. Sub-lethal concentrations of toxic chemicals result in more subtle changes. Although these effects are seldom detected in the field, over time they can result in reduced abundances and even local extirpation of species and changes in community structure.
Specific biological effects cannot be identified for toxic chemicals, but may be implied as a cause by certain impairments or other observed effects, especially if the observed impairment(s) are more severe than would be suggested by known, quantified stressors. Examples include:
- Abrupt increases in fish or invertebrate mortality (e.g., fish kills, see Image CC.8-9),
- Other significant community changes, such as large reductions in species richness or abundance,
- Abnormal behaviors, such as fish leaping from the water, gasping at the surface, or crowding into tributaries,
- Gross pathologies not typical of pathogens, such as tumors, deformities, or sloughing of gill tissues,
- Appearance of parasites or diseases, potentially due to toxic stress or immunotoxicity, and
- Toxic effects in tests of effluents, ambient waters, or sediments.
Species that are known to be susceptible or resistant to particular classes of chemicals, by their presence or absence, can signal impact by toxic chemicals. For those chemicals with aquatic life criteria, more is known about species sensitivities (see criteria list in Table CC.8-1). Clinical signs may also be associated with fish toxicity. Some examples are listed below, followed by their potential chemical causes in parentheses (U.S. Department of Interior, 1970). Note that in some cases, similar biotic effects may be caused by other types of stressors.
- White film on gills, skin, and/or mouth (trinitrophenols)
- Sloughing of gill epithelia (detergents, quinoline)
- Clogged gills (ferric hydroxide)
- Bright red gills (cyanide)
- Dark gills (phenol naphthalene, hydrogen sulfide)
- Hemorrhagic gills (detergents)
- Distended opercles (phenol, cresols, cyanide)
- Blue stomach (molybdenum)
- Pectoral fins in extreme forward position (organophosphates, carbamates)
Some types of behaviors suggest exposure to specific chemical classes such as pesticides (Image CC.8-10) [adapted from Hunn and Schnick (1990)]:
- Organochlorine pesticides cause disorders of the central nervous system that, in fish, can lead to increased ventilation rates, rapid jerky movements of body and fins, uncoordinated movements, spasms, convulsions, racing, increased sensitivity to external stimuli, and high excitability. Eventually, fish exposed to organochlorine pesticides may lose equilibrium, and respiration typically stops shortly thereafter.
- Organophosphorus pesticides cause lethargy and loss of equilibrium in fish. Exposed fish are dark, often showing reddish discoloration with hemorrhaging in muscles and beneath dorsal fins. They become hypersensitive; startled fish involuntarily swim in rapid circles and may have tremors or convulsions and begin coughing. They also may display involuntary forward extension of the pectoral fins and opercula, and may exhibit spinal abnormalities. Typically, smaller individuals tend to die first, but eventually all sizes of fish may die (Hunn and Schnick, 1990).
CC.8.1.5. Site Evidence that Supports Excluding Toxic Chemicals as a Candidate Cause
There are no site observations that specifically provide evidence of the absence of toxic chemicals. General reasons for excluding a candidate from the list are described in Step 2.2 of the Step-by-Step guide and in Tips for Listing Candidate Causes.
We strongly caution against using benchmarks of effects (e.g., water quality criteria) as evidence for excluding toxic chemicals from your initial list of candidate causes, because different species have different toxic chemical requirements and different sites have different naturally occurring levels of toxic chemicals.
CC.8.2. Ways to Identify the Presence of Toxic Chemicals
Unknown toxic chemicals, by definition, are either unmeasured or inadequately measured. Aside from direct measurement for specific chemicals in the environment, related measures may be useful in determining if toxic chemicals should be considered as a candidate cause:
- Presence of chemicals in sediments may suggest that they occurred in the water column in the past,
- Presence of chemicals at low concentrations may suggest that they occur at higher concentrations intermittently, or
- Presence of by-products or decomposition products are possible indicators of toxic chemicals.
If tests for toxic chemicals have not been analyzed previously, other types of water analyses may suggest the presence of sources of toxic chemicals. For example, elevated ionic strength or biological oxygen demand may suggest the presence (or historical presence) of an effluent or other source.
CC.8.3. Literature Reviews of Toxic Chemicals as Candidate Causes of Aquatic Life Impairment
A causal analysis of toxic chemicals may benefit from information concerning the characteristic effects of commonly occurring toxic chemicals and the concentrations at which they occur, even if there is uncertainty about the identity of the specific chemicals involved. The section below discusses EPA's ambient water quality criteria documents, which provide access to high quality data and narrative summaries for many chemicals that have caused adverse effects on aquatic life. However, it is important to remember that the criteria values themselves are not indicative of the occurrence of effects or the absence of effects.
Ambient Water Quality Criteria Documents as Literature Reviews
Since the early 1980's, EPA has been developing ambient water quality criteria designed to protect aquatic organisms from harmful exposure to chemicals. The EPA's water quality criteria documents are valuable sources of exposure-response information for aquatic organisms, and each contains a comprehensive review of the aquatic toxicity literature for a particular chemical at the time of publication. The aquatic toxicity data used to derive the criteria are screened to ensure they meet certain toxicity test practices and data quality objectives. More information on how data are screened can be found in U.S. EPA (1985d). Finally, the toxicity data contained in the criteria documents are arrayed from least sensitive to most sensitive, which is useful for comparing the effects on species in the field to relative sensitivities in the laboratory. Additional attributes of EPA's water quality criteria documents include:
- Acute and chronic toxicity data conforming to standard test durations and endpoints in separate tables for freshwater and saltwater animals. [Acute toxicity data usually reflect severe endpoints from short-term exposures (e.g., 2- to 4-day LC50s or EC50s). Chronic toxicity data usually reflect exposure over all or part of an organism's life cycle and include sublethal (e.g., growth, reproduction, development) and lethal (survival) endpoints.],
- Data on chemical toxicity to aquatic plants (algae, macrophytes) and on bioaccumulation in aquatic organisms, and
- Discussions of the environmental chemistry of the chemical, speciation, best measures of the chemical in water, and relative sensitivities of species.
Since most of the toxicity data contained in the criteria documents reflect exposures to a single chemical under standardized conditions in the laboratory, caution should be exercised when comparing such data to effects on organisms in the field, because other factors may increase or decrease a chemical's toxicity in the natural environment. Older criteria documents should be supplemented by more recent publications.
Table CC.8-1, below, contains the name and source of EPA water quality criteria to protect aquatic life. Documents without links are not posted but may be ordered from Office of Water Shopping Cart Home Page.
You will need the free Adobe Reader to view some of the files on this page. See EPA's PDF page to learn more.
| Chemical (pp, file size) or (report number for ordering) | Year |
|---|---|
| 2-Chlorophenol (PDF) (61 pp, 2.9MB) | 1980 |
| 2,3,7,8-Tetrachlorodibenzo-p-dioxin (296 pp, 9.2MB) | 1984 |
| 2,4-Dichlorophenol (PDF) (69 pp, 3.3MB) | 1980 |
| 2,4-Dimethylphenol (PDF) (60 pp, 2.8MB) | 1980 |
| Acenaphthene (PDF) (50 pp, 1.4MB) | 1980 |
| Acrolein (PDF) (102 pp, 4.8MB) | 1980 |
| Acrylonitrile (PDF) (154 pp, 7.4MB) | 1980 |
| Aldrin/Dieldrin (PDF) (154 pp, 4.9MB) | 1980 |
| Aluminum (PDF) (54 pp, 1.2MB) | 1986 |
| Ammonia - 1999 Update (PDF) (153 pp, 350K) | 1999 |
| Ammonia (Saltwater) - 1989 (PDF) (67 pp, 2.1MB) | 1988 |
| Antimony (PDF) (114 pp, 5.6MB) | 1980 |
| Arsenic - 1984 (PDF) (71 pp, 3.2MB) | 1984 |
| Atrazine - 2003 Revised Draft (PDF) (178 pp, 350K) | 2003 |
| Benzene (PDF) (125 pp, 6.1MB) | 1980 |
| Benzidine (PDF) (71 pp, 3.3MB) | 1980 |
| Beryllium (PDF) (85 pp, 3.9MB) | 1980 |
| Cadmium - 2001 Update (PDF) (166 pp, 320K) | 2001 |
| Carbon Tetrachloride (PDF) (130 pp, 6.4MB) | 1980 |
| Chlordane (PDF) (67 pp, 3.1MB) | 1980 |
| Chloride - 1988 (PDF) (46 pp, 1.9MB) | 1988 |
| Chlorinated Benzenes (PDF) (216 pp, 10.2MB) | 1980 |
| Chlorinated Ethanes (PDF) (146 pp, 6.9MB) | 1980 |
| Chlorinated Naphthalene (PDF) (68 pp, 2MB) | 1980 |
| Chlorinated Phenols (PDF) (182 pp, 7.7MB) | 1980 |
| Chlorine - 1984 (PDF) (64 pp, 2.6MB) | 1984 |
| Chloroalkyl Ethers (PDF) (108 pp, 3.2MB) | 1980 |
| Chloroform (PDF) (68 pp, 3.3MB) | 1980 |
| Chlorpyrifos - 1986 (PDF) (71 pp, 3.2MB) | 1986 |
| Chromium (PDF) (113 pp, 5.5MB) | 1980 |
| Copper - 2007 Revision (PDF) (204 pp, 2.2MB) | 2007 |
| Cyanide - 1984 (PDF) (64 pp, 2.7MB) | 1984 |
| DDT (PDF) (174 pp, 8.3MB) | 1980 |
| Dichlorobenzenes (PDF) (105 pp, 3.1MB) | 1980 |
| Diazinon - 2005 (PDF) (89 pp, 350K) | 2005 |
| Dichlorobenzidine (PDF) (48 pp, 2.0MB) | 1980 |
| Dichloroethylenes (PDF) (60 pp, 2.7MB) | 1980 |
| Dichloropropane and Dichloropropene (PDF) (59 pp, 1.8MB) | 1980 |
| Dinitrotoluene (PDF) (91 pp, 4.1MB) | 1980 |
| Diphenylhydrazine (PDF) (35 pp, 1.5MB) | 1980 |
| Dissolved Oxygen, freshwater (PDF) (62 pp, 3.1MB) | 1986 |
| Dissolved Oxygen, Salt Water (PDF) (55 pp, 310K) | 2000 |
| Endosulfan (PDF) (154 pp, 7.3MB) | 1980 |
| Endrin (PDF) (102 pp, 4.6MB) | 1980 |
| Ethylbenzene (PDF) (50 pp, 2.0MB) | 1980 |
| Fluoranthene (PDF) (84 pp, 2.3MB) | 1980 |
| Haloethers (PDF) (30 pp, 1.1MB) | 1980 |
| Halomethanes (PDF) (135 pp, 6.6MB) | 1980 |
| Heptachlor (PDF) (104 pp, 5.4MB) | 1980 |
| Hexachlorobutadiene (PDF) (55 pp, 2.5MB) | 1980 |
| Hexachlorocyclohexane (PDF) (107 pp, 4.8MB) | 1980 |
| Hexachlorocyclopentadiene (PDF) (99 pp, 5.2MB) | 1980 |
| Isophorone (PDF) (49 pp, 2.1MB) | 1980 |
| Lead (PDF) (158 pp, 7.7MB) | 1980 |
| Lead - 1984 (EPA#: 440/5-84-027) | 1984 |
| Mercury - 1984 (PDF) (143 pp, 6.4MB) | 1984 |
| Naphthalene (PDF) (74 pp, 1.8MB) | 1980 |
| Nickel (PDF) (101 pp, 2.5MB) | 1986 |
| Nitrobenzene (PDF) (73 pp, 2MB) | 1980 |
| Nitrophenols (PDF) (160 pp, 7.6MB) | 1980 |
| Nitrosamines (PDF) (88 pp, 4.4MB) | 1980 |
| Nonylphenol (PDF) (96 pp, 350K) | 2005 |
| Parathion (PDF) (72 pp, 1.8MB) | 1986 |
| Pentachlorophenol (PDF) (135 pp, 3.5MB) | 1986 |
| Phenol (PDF) (94 pp, 2.6MB) | 1980 |
| Phthalate Esters (PDF) (110 pp, 3.2MB) | 1980 |
| Polychlorinated Biphenyls (PDF) (204 pp, 7MB) | 1980 |
| Polynuclear Aromatic Hydrocarbons (PDF) (200 pp, 6.2MB) | 1980 |
| Selenium - 2004 Draft (PDF) (334 pp, 1.2MB) | 2004 |
| Silver (PDF) (219 pp, 7MB) | 1980 |
| Tetrachloroethylene (PDF) (59 pp, 1.6MB) | 1980 |
| Thallium (PDF) (12 pp, 0.3MB) | 1980 |
| Toluene (PDF) (94 pp, 3.1MB) | 1980 |
| Toxaphene (PDF) (92 pp, 2.5MB) | 1986 |
| Tributyltin (PDF) (138 pp, 367K) | 2004 |
| Trichloroethylene (PDF) (66 pp, 1.9MB) | 1980 |
| Vinyl Chloride (PDF) (97 pp, 2.7MB) | 1980 |
| Zinc (PDF) (165 pp, 4.8MB) | 1980 |
Hunn, JB; Schnick, RA. (1990) Toxic substances. In: Meyer, FP; Barclay LA, eds. Field manual for the investigation of fish kills. Washington, DC: US Fish and Wildlife Service. Resource Publication 177.
Stewart, AJ. (2001) A simple stream monitoring technique based on measurements of semiconservative properties of water. Environ Manag 27(1):37-46.
US Department of the Interior. (1970) Investigating fish mortalities. Washington, DC: Division of Technical Support, Federal Water Pollution Control Administration, CWT-5, 1970. 21 pp.
US EPA (Environmental Protection Agency). (1985d) Technical support document for water quality-based toxics control. U.S. Environmental Protection Agency, Office of Water, Washington, D.C.; EN-336.