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CADDIS Volume 2

Unspecified Toxic Chemicals

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The toxicity of a substance refers to its potential to harm living organisms. Toxicity is a function of exposure concentration and duration, and varies by species, age and condition of exposed organisms. Toxic chemicals, as considered here, are individual chemicals or mixtures of chemicals and their by-products that originate from human activities (Figure 1). These are toxic chemicals that have not yet been identified in the assessment but may be adversely affecting organisms.

These chemicals 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. Toxic metals, insecticides, and herbicides are considered separately in their own modules.Photo illustrating 55 gallon drums that may be used to store toxic waste, ultimately becoming a potential source.Figure 1. Fifty-five gallon drums of chemicals, when buried or left in the open to rust, may become a ready source of toxic chemicals. 

Checklist of Sources, Site Evidence and Biological Effects

Toxic chemicals should be considered a candidate cause when potential human sources and activities, site observations or observed biological effects support portions of the source-to-impairment pathways (Figure 2). This diagram and some of the other information also may be useful in Step 3: Evaluate Data from the Case.

The checklist below will help you identify key data and information useful for determining whether to include unspecified toxics among your candidate causes. The list is intended to guide you in collecting evidence to support, weaken or eliminate toxic compounds as a candidate cause. For more information on specific entries, go to the When to List tab.

A simple conceptual diagram illustrating causal pathways, from sources to impairments, related to unspecified toxic chemicalsFigure 2. A simple conceptual diagram illustrating causal pathways, from sources to impairments, related to unspecified toxic chemicals. Click on the diagram to view a larger version.Consider listing toxic chemicals as a candidate cause when the following sources and activities, site evidence and biological effects are present:

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
  • pH

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Sources and Activities that Suggest Listing Toxic Chemicals as a Candidate Cause

Since the industrial revolution, humans have processed, concentrated and released thousands of chemicals to the environment. The enactment of environmental legislation (e.g., Clean Water Act, Clean Air Act) in the 1970s reduced chemical releases. However, toxic chemicals continue to cause impairments in aquatic environments for numerous reasons:

  • There are a limited number of water quality criteria supporting regulation, and only a fraction of existing chemicals have established criteria.
  • 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).
  • Often substances undergo changes in toxicity, becoming toxic only when transformed or mixed with others in the environment.
  • Undocumented pulse events and accidental and illegal releases to surface waters occur.
  • There is long range transport of chemicals via water or air.

Photo of a worker striping paint off a building may be a source of toxic waste in rivers and streams.Figure 3. Chemicals and water used to strip grime and paint off a building may enter ditches for easy transport to surface waters. 
Photo courtesy of HUD
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 which toxic chemicals may be important and where they originate.

Useful information on toxic chemicals released to the environment may be found on U.S. EPA websites on the following topics:

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.

Photo of a someone dumping oil in the wrong containers may be a source of toxic waste in rivers and streams.Figure 4. Improper recycling of anti-freeze and oil may release of chemicals into the air and soil. Most human activities can become a source of toxic chemicals (Figures 3 and 4), 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 atmospherically distributed over long distances (Figure 5), chemicals such as pesticides may be aerially applied to forests or rangelands, and historic sources of toxic chemicals may be obscured over time. Generally there are two classes of contaminant delivery: nonpoint and point.

Industrial, agricultural, mining, logging, urban and residential activities and related development: These land uses are all potential non-point sources of toxic chemicals. Chemicals in smoke stack emissions can spread widely across the landscape or region (Figure 5). Building materials, solvents, fuels, cleaning materials, drugs and pesticides enter the environment during regular use or operation (e.g., Figure 6).

Photo showing how smokestack air emissions may become nonpoint source inputs to surface waters. Figure 5. Smokestack air emissions may become nonpoint source inputs to surface waters. Pesticides used on crops or lawns, around building foundation, 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.

Historical sources and landfills: Past industrial and commercial operations (e.g., tanning operations, slaughterhouses, lumber mills, service stations, dry cleaners) may have contaminated soils and sediments, allowing toxic chemicals to leach into surface waters through surface or subsurface pathways. Photo illustrating how chemicals in runoff from road and home maintenance are carried by snow melt and storms.Figure 6. Chemicals in runoff from road and home maintenance are carried by snow melt and storms. 
Photo courtesy of C.K. Racine
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 (Figure 7) 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.Photo show the results of a spill event on rivers and streams.Figure 7. Slurry is heavy in a river after a 2001 coal spill in Martin County, Kentucky. Contamination from such spills is often long enduring. 
Photo courtesy of S. Webb, University of Kentucky

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 (Figure 8). 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.Photo illustrating how an end-of-pipe effluent release may contribute to chemical loads in surface waters. Figure 8. End-of-pipe effluent releases may contribute to chemical loads in surface waters. 

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 (Figure 9)
  • Sludge or discolored 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

Photo illustrating how trash can impaired a ditch with a sheen and sludge which suggests a chemical contamination.Figure 9. An impaired ditch with a sheen and sludge suggests chemical contamination. Keep in mind that most chemicals do not leave visible signs in the environment, and 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).

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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. Other chemicals act specifically and may affect only plants, 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 species extirpation and community structure changes.Photo of several dead fish.Figure 10. A fish kill. 

Specific biological effects cannot be identified for toxic chemicals, but may be implied as a cause by certain impairments or other observed effects. This is especially true 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 Figure 10)
  • 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
  • Toxic effects in tests of effluents, ambient waters or sediments

Species 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 water quality criteria, more is known about species sensitivities. 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 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)
Photo of a crop duster airplane spraying agricultural fields with pesticides.Figure 11. A crop duster airplane is spraying agricultural fields with pesticides. 

Some types of behaviors suggest exposure to specific chemical classes such as pesticides (Figure 11):

  • 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 (Hunn and Schnick 1990).
  • 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, with startled fish involuntarily swimming in rapid circles. Fish also may have tremors or convulsions and begin coughing. They 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).

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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 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. Different species have different toxic chemical requirements and different sites have different naturally occurring levels of toxic chemicals.

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Unknown toxic chemicals, by definition, are either unmeasured or inadequately measured. Thus, this section might be more accurately titled Ways to Identify Unspecified Toxic Chemicals. 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.
  • 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.

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About Conceptual Diagrams

Conceptual diagrams are used to describe hypothesized relationships among sources, stressors and biotic responses within aquatic systems.

Simple Conceptual Model Diagram

Example of an unspecified toxic chemicals simple conceptual diagram.Figure 2. A simple conceptual diagram illustrating causal pathways, from sources to impairments, related to unspecified toxic chemicals. Click on the diagram to view a larger version.Human activities introduce large numbers of chemical compounds to the environment, making it impractical (scientifically and economically) to test for all possible contaminants in an impaired waterbody. Toxic chemicals and chemical mixtures can be significant stressors and ultimately contribute to biological impairment in aquatic ecosystems.

Human activities can introduce chemicals to aquatic environments via point and non-point sources. Point sources result in direct discharges of toxics into aquatic systems; examples include industrial or wastewater treatment plants.

Non-point sources (e.g., runoff from agricultural fields) may introduce toxic chemicals to aquatic systems via soil transport, surface runoff, subsurface waters, and atmospheric deposition. Land cover and channel alterations (e.g., land clearing associated construction and development, increased channel incision) also can contribute to toxic contamination, by increasing surface runoff, erosion, and subsequent delivery of contaminants to surface waters.

Stress resulting from one or more toxic chemicals may be due to episodic or sustained increases in chemical concentrations. Episodic exposures can be especially difficult to assess, because measurements must be taken at very specific times to catch these short-term events. However, the distinction between episodic versus sustained increases in toxic chemicals is important, as it may influence the types of biological effects observed.

Additionally, the bioavailability and toxicity of a toxic chemical is influenced to varying degrees by other factors (and/or stressors), including water temperature, pH, hardness, alkalinity, ionic strength, DO, suspended sediment, and dissolved organic matter.

Toxic chemicals, individually or in mixtures, may directly or indirectly impair aquatic plants, invertebrates, and/or fish assemblages. Impairments may include increased mortality, decreased reproduction, behavioral changes, or poor physiological condition, especially in sensitive taxa or life stages; these effects can serve as modes of action for community-based biotic effects.

Decreases in condition can contribute to lower reproductive success, greater susceptibility to other stressors, or increased vulnerability to natural predation and climatic events, further altering population and community structure. Population dynamics may be influenced directly through emigration or through changes in community structure, which subsequently affect food webs and other biotic interactions.

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Detailed Conceptual Model Diagram

Example of an unspecified toxic chemicals detailed conceptual diagram.Figure 13: Click on the diagram to view a larger version of the Detailed Conceptual Model Diagram for Unspecified Toxic Chemicals.This conceptual diagram examines surface water and sediment contamination by non-metal toxic chemicals that originate from human activities. The diagram depicts sources and land use alterations that directly and indirectly contribute toxic chemicals to surface waters near the top of the figure, leading down the diagram to steps in the causal pathway, proximate stressors (i.e., increased toxic chemicals), other stressors influencing contaminant concentrations, modes of action, and eventually biological responses at the bottom.

Although natural biotic and abiotic processes may contribute toxic substances to the environment, these substances are not considered here; toxic effects of metals, ammonia, and pH are discussed in separate modules. This narrative generally follows the diagram from top to bottom, left to right.

Linking Sources to Proximate Stressors

Most human activities result in some release of chemicals into the environment; these releases may introduce toxic chemicals to aquatic systems adsorbed to soil particles, in surface runoff, in subsurface waters, by deposition on water surfaces, or by direct discharge into surface waters. Several potential activities leading to chemical contamination are shown across the top of the diagram, linked to an array of specific sources. Both point and non-point sources may contribute toxic chemicals.

Non-point sources include waste incineration, fossil fuel combustion, fertilizer and pesticide applications, road salting and animal wastes. These activities spread chemicals across the landscape or emit them into the air with later deposition on land and water surfaces. Surface and subsurface transport processes move these chemicals to surface waters. Septic systems, landfills, and industrial and mining wastes pollute surface waters through more localized runoff and leaching processes.

Finally, point source discharges (e.g., end of a pipe discharges) include industrial effluents, wastewater treatment plant effluents, stormwater overflows, and other sources that can deliver chemicals directly into surface waters. Point sources are normally, but not always, discharged under permit. This array of potential sources suggests that any single body of water can be contaminated by many sources, and thus exposed to a mixture of chemicals – some of which may be causing observed effects.

Historical sources are sources that are no longer active. Past deposition of contaminants onto soil, leachates from poorly designed landfills, buried toxic wastes from commercial or military operations, former spills, illegally disposed wastes, or past contamination released from any of the other potential sources may continue to leach toxic chemicals into surface waters through surface or subsurface transport. Many of these sites have been obscured by time, natural vegetative growth, and lost or destroyed records, so their discovery may only occur because of observed biotic effects.

A number of important processes can modify the delivery and toxicity of chemicals released to the environment, although they are not actually sources of chemicals themselves. Channel alteration can lead to channel incision and erosion that may deliver contaminated sediments to streams, creating episodic events. Loss of natural land cover along riparian zones and across the watershed, as well as hardening of land surfaces, enhance delivery of chemicals to water bodies through surface and subsurface transport processes.

Delivery of chemicals to surface waters also is influenced by precipitation patterns, a contributing environmental condition (hexagon). Precipitation events may wash toxic chemicals into surface waters; decreased precipitation, as well as water removal for human use, can decrease dilution and increase concentrations of existing chemicals. Dissolved organic carbon also may influence bioavailability and uptake of contaminants.

The toxicity of chemicals can be enhanced or diminished by other stressors, including increased ionic strength and suspended sediments, decreased dissolved oxygen, or changes in temperature. Changes in these additional stressors can alter the impact of chemical toxicity, and may also interact with transport and exposure modifying factors noted above.

Linking Proximate Stressors to Biological Impairment

The proximate stressor in this model is “toxic chemicals” in surface waters (center blue box). Both episodic and sustained exposures to toxic chemicals can lead to the array of effects shown. High concentrations of a toxic chemical delivered in episodic runoff events, effluents released during treatment failures, spills, or other such events may lead to acute toxicity and death of some or all biota.

Continuous point sources, atmospheric deposition, contaminated groundwater or other sustained sources may cause impairment through chronic toxic effects of low level concentrations (see hexagons, center). It is likely that different chemicals will be delivered to a water body through multiple transport mechanisms.

Clues as to the nature of the chemical and how exposure is occurring may be found in processes (hexagons) that lead to alteration of biotic communities. Emigration from, or avoidance of immigration to, areas within a water body may suggest localized concentrations. Abrupt mortality may suggest acute toxicity; acute exposures often also cause injuries and increase susceptibility to other stressors.

Episodic exposures may cause gill injuries that increase susceptibility to pathogens. Sustained exposures may lead to injuries and deformities, increased parasitism and disease, and a general increase in sensitivity to other stressors. Lowered reproductive success may suggest sustained exposure. Further evidence may be found in evaluating the presence or absence of sensitive taxa or life stages, and changes in population age structure. In some cases loss of one species will influence community structure (e.g., because a critical food source is lost).

Toxic effects depend on numerous factors, including characteristics of the chemical; species, age and condition of the biota; characteristics of the surface water; exposure pathway; amount and mode of delivery; and environmental concentrations of the chemicals. Chemical toxicity can lead to biologically impaired plant, invertebrate and/or fish assemblages. When chemical toxicity is suspected, further investigation is needed to determine which chemical or mixture of chemicals may be the cause of impairment.

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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. U.S. EPA's ambient water quality criteria documents provide access to high quality data and narrative summaries for many chemicals that have caused adverse effects on aquatic life.

Since the early 1980s, U.S. EPA has been developing ambient water quality criteria designed to protect aquatic organisms from harmful exposure to chemicals. These 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 that 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 (1991). 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 these 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.
  • Discussions of the environmental chemistry of the chemical, speciation, best measures of the chemical in water and relative species sensitivities.

Since most of the toxicity data contained in the criteria documents reflects 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.

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You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.
  • Hunn JB, Schnick RA (1990) Toxic substances. in: Meyer FP, Barclay LA (Eds). Field Manual for the Investigation of Fish Kills. U.S. Fish and Wildlife Service, Washington DC. Resource Publication 177.
  • Stewart AJ (2001) A simple stream monitoring technique based on measurements of semiconservative properties of water. Environmental Management 27(1):37-46.
  • U.S. Department of the Interior (1970) Investigating Fish Mortalities. Division of Technical Support, Federal Water Pollution Control Administration, Washington DC. CWT-5. 21 pp.
  • U.S. EPA (1991) Technical Support Document for Water Quality-Based Toxics Control. U.S. Environmental Protection Agency, Office of Water, Washington DC. EPA/505/2-90-001.

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