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CADDIS Volume 2: Sources, Stressors & Responses

Unspecified Toxic Chemicals

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Authors: S. M. Marcy, G.W. Suter II

Fifty-five gallon storage barrels. Photo by Eric Vance, U.S. EPA.
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.
Courtesy of E. Vance, U.S. EPA.

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 or mixtures of chemicals and their by-products that originate from human activities (Figure 1). These are toxic chemicals have not yet been identified in the assessment and are capable of adversely affecting living organisms. 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, insecticides, and herbicides are treated separately.

A Simple Conceptual Diagram for unspecified toxic chemicals.
Figure 2. A simple conceptual diagram illustrating causal pathways, from sources to impairments, related to unspecified toxic chemicals. Click on the diagram to go to the Conceptual Diagrams tab and view a larger version.

Checklist of sources, site evidence and biological effects

Toxic chemicals as addressed in this module should be listed as a candidate cause when potential human sources and activities, site observations, or observed biological effects support portions of the source-to-impairment pathways in the conceptual diagram for unspecified toxics (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 sources and activities, site evidence, and biological effects listed in the checklist, click on checklist headings or go to the When to List tab of this module.

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:

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