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CADDIS Volume 2: Sources, Stressors & Responses

Unspecified Toxic Chemicals

Sources and activities that suggest listing toxic chemicals as a candidate cause

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 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.
Chemicals and water used to power-wash the exterior of a building. Source: http://www.hud.gov/images/photos/download/hud-picp-2004-hud54.jpg.
Figure 3. Chemicals and water used to strip grime and paint off a building may enter ditches for easy transport to surface waters.
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 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 websites on hazardous waste, pesticides, Superfund sites and health related issuesExit EPA Disclaimer, 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 (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 distributed over long distances in the atmosphere (Figure 5), 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.

A mechanic recycles used oil. Photo by Eric Vance, U.S. EPA.
Figure 4. Improper recycling of anti-freeze and oil may release of chemicals into the air and soil.
Courtesy of E. Vance, U.S. EPA.
Smoke-stack air emissions. Photo courtesy U.S. EPA.
Figure 5. Smokestack air emissions may become nonpoint source inputs to surface waters.
Courtesy of U.S. EPA.
Dense residential development. Photo by C.K. Racine.
Figure 6. Chemicals in runoff from road and home maintenance are carried by snow melt and storms.
Courtesy of C.K. Racine.

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 (Figure 5). Building materials, solvents, fuels, cleaning materials, drugs, and pesticides enter the environment in the course of regular use or operation (e.g., Figure 6). 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 Flow Alteration).

Slurry is heavy in a river after a coal spill.  Photo by Suzanne Webb, University of Kentucky, http://www.geocities.com/kyriverpal/martincoky.html.
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.
Courtesy of S. Webb, University of Kentucky.
Polluted water coming from a pipe into a waterbody. Photo courtesy U.S. EPA.
Figure 8. End-of-pipe releases of effluents may contribute to chemical loads in surface waters.
Courtesy of U.S. EPA.

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 (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.

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.

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Site evidence that suggests listing toxic chemicals as a candidate cause

Polluted ditch. Photo by Eric Vance, U.S. EPA.
Figure 9. An impaired ditch with a sheen and sludge suggests chemical contamination.
Courtesy of E. Vance, U.S. EPA.

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, 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).

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Biological effects that suggest listing toxic chemicals as a candidate cause

Dead fish in a stream channel. Photo courtesy U.S. EPA.
Figure 10. A fish kill.
Courtesy of U.S. EPA.

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 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, 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 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, 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)
A crop duster airplane spraying agricultural fields. Photo courtesy U.S. EPA.
Figure 11. A crop duster airplane is spraying agricultural fields with pesticides.
Courtesy of U.S. EPA.

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

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

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