Causal Analysis/Diagnosis Decision Information System (CADDIS)

• Common Candidate Causes
• Interactive Conceptual Models

CC.1. Metals

Metals and metalloids are electropositive elements that occur in all ecosystems, although natural concentrations vary according to local geology. Land disturbance in metals-enriched areas can increase erosion and mobilize metals into streams. Human activities redistribute and concentrate metals in areas that are not naturally metals-enriched. These metals can reach water bodies when they are released into the air, water, and soil. Unlike sediment and nutrient impairments, there is often no visible evidence of metals contamination (Image CC.1-1).

While some metals are essential as nutrients, all metals can be toxic at some level and some metals are toxic in minute amounts. Impairments result when metals are biologically available at toxic concentrations affecting the survival, reproduction, and behavior of aquatic organisms.

This module addresses water column contamination by metals and metalloids that commonly cause toxic effects. These include arsenic, cadmium, chromium, copper, lead, inorganic mercury, nickel, selenium, and zinc. Organic mercury and tributyl tins are special forms of metals that are beyond the scope of this module. Salts of abundant and relatively nontoxic metals are discussed in the module on ionic strength while metals in complex toxic mixtures are discussed in the module on toxic substances in the water column.

You may go directly to a specific section of interest by clicking on the topic outline in the right navigation bar.

CC.1.1. What to Consider When Determining if Metals Should be Included as a Candidate Cause

Metals are addressed in this module as proximate stressors that should be listed as a candidate cause in Step 2 when potential human sources and activities, site observations, or observed effects support portions of the source-to-impairment pathways in the conceptual model for metals (Figure CC.1-1). The conceptual model and some of the other information are also useful for Step 3, Evaluate Data from the Case.

CC.1.1.1. Checklist of Sources, Site Evidence and Biological Effects

This checklist will help you determine whether metals should be listed as a candidate cause based on the presence of sources and activities, site evidence, or biological effects. Click on the title of each column to be directed to more detailed information. Like the conceptual model above, this list can also guide you in collecting evidence that reduces or enhances support for metals as a candidate cause.

If you know of other reasons to list metals as a candidate cause or even screen metals from the analysis, please share your insights using the comments section.

Consider contributing, modifying, and related factors as candidate causes when listing metals as a candidate cause:

• Unspecified Toxic Chemicals
• Ionic Strength

Consider other causes with similar evidence:

Colored water:	Nutrients (sulfate reducing bacteria can color water)
Spinal abnormalities:	Temperature
Aquatic life kills:	Other toxics, low dissolved oxygen, pH
Blackened tails:	Whirling disease
Gill damage/mucous:	pH, pathogens

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CC.1.1.2. Sources and Activities that Suggest Listing Metals as a Candidate Cause

One reason to list metals as a candidate cause is the presence of metal sources or other evidence of metals in a stream or watershed. When identifying sources of metals, consider both non-point and point sources.

Non-point source contributions of metals are dispersed and variable over time (Marsalek et al., 2006). Metals are transported to surface waters in storm water runoff from roadways and parking lots (Image CC.1-2). Other non-point sources include runoff from waste sites, mines, and land where metal-containing sludge, fertilizers, and pesticides have been applied. Atmospheric contaminants from non-point or stack releases also enter waterways through direct wet and dry deposition or indirectly through overland storm water runoff. In colder climates, snow disposal sites may act as seasonal sources for metals associated with atmospheric deposition, road use and snow management.

Disturbance and redistribution of metals-contaminated sediments by dredging can result in repartitioning of metals into the water column. It is important to consider that sediment may contain legacy metals-contamination from past land uses.

The relative distribution of urbanized areas contributing non-point metals and other toxicants within a watershed can be identified using EPA's EnviroMapper for Water (zoom in to state and region of interest).

Industrial sources of atmospheric releases of metals can be identified by querying the Toxics Release Inventory. Information from this database should only be used to identify which metals may be present, because stack releases in an area do not necessarily result in actual exposures due to differing release-specific fate and transport pathways. In addition, location metadata might identify the company headquarters rather than the point of release, so confirming the location of actual release is recommended.

Point source contributions of metals to surface waters include releases by different industries and by wastewater treatment facilities. Point source contributions from water treatment facilities may be identified through permits granted under the National Pollutant Discharge Elimination System.

Other evidence of metals in the watershed or site area may be identified by examining state or tribal databases, the STORET, US Geological Survey-NAWQA and EMAP data repositories.

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CC.1.1.3. Site Evidence that Suggests Listing Metals as a Candidate Cause

One might also list metals as a candidate cause when they have been measured at the site. The availability of data on metals concentrations in biota, sediments, or water suggests that metals co-occur with the impairment.

Acid mine drainage is an extreme case of metals contamination that results in visible evidence (e.g. Image CC.1-3). When acid mine drainage mixes with the higher pH water of a receiving stream, the metal salts precipitate from the water column as a floc that coats the stream bed. This is a physical cause of impairment because floc can smother organisms and their benthic habitat. While metal precipitation removes much metal from the water column, the water still may carry toxic concentrations of metals downstream. If flocculates occur in the watershed, metals may be present in elevated concentrations in associated streams. Note that blooms of sulfate-reducing bacteria may also alter the color of water.

When listing metals, also consider how site water chemistry conditions influence metal bioavailability and toxicity. This will be important in later stages of the assessment. The Issue Paper on the Environmental Chemistry of Metals (PDF) (Langmuir et al., 2004) (113 pp, 1.7 MB, About PDF) reviews important environmental chemistry factors influencing metal bioavailability.

The fraction of metals present as biologically available free metal ions is particularly important. High concentrations of free metal ions in the water column are toxic because they compete with nutrient cations (e.g. calcium, potassium, magnesium, etc.) for binding sites located on the chloride cells of gill epithelia (biotic ligands). This impairs gill respiratory function and the ability to regulate blood pH and ion concentrations. Metals are not available for gill binding when they exist as organic compounds or are bound to sulfates, organic acids, other anions, or negatively charged particles (abiotic ligands).

Data for individual ions may be used in calculating ion balance while alkalinity, hardness, and dissolved organic carbon are common aggregate measures of abiotic ligands. Low pH of the water favors metal solubility, increasing free metal ion content, particularly when abiotic ligands are not abundant. Taken together, the concentration of metal ions relative to nutrient ions, the abundance of abiotic ligands, and pH determine the biologically available fraction and toxicity of metals in water (DiToro et al., 2001).

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CC.1.1.4. Biological Effects that Suggest Listing Metals as a Candidate Cause

Metals could also be listed as a candidate cause when the impairment involves gross pathologies or community changes that are indicative of adverse metals effects. For example:

• Mucous streaming from fish gills, due to gill injury or impaired ionoregulation (Hunn and Schnick, 1990),
• Degeneration of caudal chromophores resulting in black tails in fish (Bengtsson and Larsson, 1986; Sippel et al., 1983). However, blackened tails also are symptomatic of whirling disease, a parasitic infection of trout and salmon (Bartholomew et al., 2003),
• Skeletal deformities and impaired growth and development, for example due to metals acting as analogs for calcium and sulfur (Sorensen, 1991),
• Abnormal organ appearances or parasite loads (Sures, 2001) (although organ pathologies are seldom available),
• Reduced abundance of metal-sensitive invertebrate taxa, such as mayflies (Pollard and Yuan, 2005), and
• Increased abundance of relatively metal-tolerant taxa such as caddisflies and many stoneflies (Clements et al., 1992).

Metals alter communities because species differ in sensitivity. Different taxa have different chloride cell densities on their gills. This influences their vulnerability to effects on respiration and the regulation of blood pH and ion concentrations (DiToro et al., 2001). Different taxa also have different metabolic mechanisms for detoxifying, sequestering and excreting metals. Some taxa can acclimate to chronic metals exposure by increasing the capacity of these metabolic mechanisms. Metals accumulated by organisms enter the food chain and can contribute to toxic effects through dietary exposures. For a general review on ecological metal effects, examine the Issue Paper on the Ecological Effects of Metals (PDF) (Kaputska et al., 2004) (113 pp, 1.7MB, About PDF).

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CC.1.1.5. Site Evidence that Supports Excluding Metals as a Candidate Cause

There are no site observations that specifically provide evidence of the absence of metals. 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 metals from your initial list of candidate causes, because different species have different metal requirements and different sites have different naturally occurring levels of metals.

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CC.1.2. Ways to Measure Metals

Unlike the water quality parameters such as pH, temperature, conductivity, and dissolved oxygen, metals are not commonly measured on site. Water, sediment, and tissue samples must be returned to the laboratory for processing and analysis. Download EPA Method 1669: Sampling Ambient Water for Trace Metals at EPA Water Quality Criteria Levels (PDF) (37 pp, 144K, About PDF) for details on how to obtain uncompromised water samples. Information on appropriate sample handling and methods for analysis can be found in EPA's Methods for the Determination of Metals in Environmental Samples (PDF) (308 pp, 11.9MB, About PDF), and the 1994 supplement (PDF) (260 pp, 11.9 MB, About PDF).

Water samples are often filtered to measure dissolved metal concentrations and acidified to keep metals in solution. Operationally, a metal is considered to be dissolved if it remains in solution after filtration through a 0.45 μM filter. Suspended metals are those that remain on the 0.45 μM filter. “Acid extractable metals” refers to the concentration of metals after acidification of an unfiltered sample. It is used primarily to estimate drinking water exposures rather than aquatic life exposures.

Biota may be prepared as whole organisms, or individual organs may be dissected out and analyzed. Biological samples are often dried to a constant mass before analysis and data are reported as “dry mass”. Percent moisture [i.e., (1-mass after drying / mass before drying) *100] for these samples may be used to back calculate the original tissue concentration [i.e., 1-(%moisture/100)*[metal]. Samples may be analyzed fresh and reported as “fresh” or “wet” mass. Dry mass may be calculated using the percent moisture reported for a sub-sample. Preparation of biota and sediment for analysis involves dissolution of organic materials through digestion in nitric acid followed by hydrogen peroxide. Sometimes perflouric acid is used when digesting sediment to ensure that mineral compounds dissolve and to release metals bound in crystalline matrices.

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CC.1.3. Literature Reviews of Stressor-Response Information for Metals

We provide an annotated bibliography of literature reviews that relate biological effects to metals concentrations to help you identify useful information for Step 4 of the SI process: Evaluate Data from Elsewhere. Documents are included if they contain information on stressor intensities and biological responses in the field or if they compile field-relevant stressor-response information not yet covered in the ECOTOX database. The U.S. EPA has published Ambient Water Quality Criteria (AWQC) documents on many metals that also provide helpful reviews. The AWQC documents are discussed along with available links in the Unspecified Toxic Chemicals page of CADDIS.

If you know of other useful literature reviews relating biological effects to metals concentrations, please provide us with the citation and one or two descriptive sentences using the comments section.

Beak International Incorporated (2002) Literature Review of Environmental Toxicity of Mercury, Cadmium, Selenium and Antimony in Metal Mining Effluents (PDF) (142 pp, 410K, About PDF). Prepared for: The TIME Network and sponsored by Natural Resources Canada, The Mining Association of Canada and Environment Canada.

This report characterizes the metal content and other properties of mine effluents and summarizes speciation (including Eh-pH diagrams), fate, transport and toxicity (acute laboratory tests). Includes information on mercury, cadmium, selenium, and antimony concentrations in untreated and treated mine effluent, and a review table of acute toxicity in fish and invertebrates. Speciation figures and text are provided for mercury, cadmium, selenium and antimony.

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Borgmann, U; Couillard, Y; Doyle; et al. (2005) Toxicity of sixty-three metals and metalloids to Hyalella azteca at two levels of water hardness. Environmental Toxicology and Chemistry 24(3):641-652.

This paper is not a literature review. It reports data from toxicity tests of chemicals in water and will eventually be included in the ECOTOX database. It is included here because it is an extensive work that examines the influence of water hardness on metal toxicity. We provide a data extraction (Borgmann et al. 2005 data.xls)(42K, .xls file) to facilitate data comparisons that are not possible in ECOTOX. In particular, we calculated relative breadth (mean divided by span of 95% confidence interval) to provide a weighted relative uncertainty in LC50 estimates to facilitate such comparisons.

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Naimo, TJ. (1995) A review of the effects of heavy metals on freshwater mussels. Ecotoxicology 4:341-362.

This review characterizes metal concentrations in different water bodies and describes the distribution and metabolism of metals in freshwater mussels. Text on page 352 ff. reviews the data on chronic effects and table 2 reports data for acute laboratory tests for Cd, Cu, Ni, and Zn: These data also are in ECOTOX, but we suggest examining the original document to compare toxicity data and review how metals interact with biota at a metabolic level. The review also lists citations reviewing alternative causes (other than metals) of mussel decline. This citation is not available in electronic format.

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Rowe, CL; Hopkins, WA; Congdon, JS. (2002) Ecotoxicological implications of aquatic disposal of coal combustion residues in the United States: a review. Environmental Monitoring and Assessment 80:207-276.

This paper characterizes coal combustion residues, concentrations observed in associated water, sediment and biota, and toxicity to biota. Data for selenium are relatively abundant. Table VII lists population- and community- level effects. Note that few information sources include a complete suite of data (that is, toxicity data accompanied by data for concentrations of toxicants in the environment and in biota). We provide a data extraction (Rowe et al. 2002 data.xls) (2.2MB, .xls file) to facilitate data comparison.

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Bartholomew, JL; Lorz, HV; Sollid, SA; et al. (2003) Susceptibility of juvenile and yearling Bull Trout to Myxobolus cerebralis and effects of sustained parasite challenges. J Aquat Anim Health 15:248-255.

Beak International Incorporated. (2002) Literature review of environmental toxicity of mercury, cadmium, selenium and antimony in metal mining effluents. The TIME Network and sponsored by Natural Resources Canada, The Mining Association of Canada and Environment Canada.

Bengtsson, BE; Larsson, A. (1986) Vertebral deformities and physiological effects in Fourhorn Sculpin (Myxocephalus quadricornis) after long-term exposure to a simulated heavy metal-containing effluent. Aquat Toxicol 9:215-229.

Borgmann, U; Couillard, Y; Doyle; et al. (2005) Toxicity of sixty-three metals and metalloids to Hyalella azteca at two levels of water hardness. Environ Tox Chem 24(3):641-652.

Clements, WH; Cherry, DS; Van Hassel, JH. (1992) Assessment of the impact of heavy metals on benthic communities at the Clinch River (Virginia): evaluation of an index of community sensitivity. Can J Fish Aquat Sci 49:1686-1694.

DiToro, DM; Allen, HE; Bergman, HL; et al. (2001) Biotic ligand models of the acute toxicity of metals I: technical basis. Environ Toxicol Chem 20(10):2383-2396.

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.

Kapustka, LA; Clements, WH; Ziccardi, L; et al. (2004) Issue paper on the ecological effects of metals. US Environmental Protection Agency, Risk Assessment Forum: Papers Addressing Scientific Issues in the Risk Assessment of Metals. Available at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=86119.

Langmuir, DL; Chrostowski, P; Chaney, RL; et al. (2004) Issue paper on the environmental chemistry of metals. US Environmental Protection Agency, Risk Assessment Forum: Papers Addressing Scientific Issues in the Risk Assessment of Metals.

Marsalek, J; Jiménez-Cisneros, BE; Malmquist, PA; et al. (2006) Urban water cycle processes and interactions International Hydrological Programme (IHP)of the United Nations Educational, Scientific and Cultural Organization (UNESCO)IHP-VI Technical Document in Hydrology N°78 UNESCO Working Series SC-2006/WS/7.

Naimo, TJ. (1995) A review of the effects of heavy metals on freshwater mussels. Ecotoxicology 4:341-362.

Pollard, A; Yuan, L. (2005) Community response patterns: evaluating benthic invertebrate composition in metal-polluted streams. Ecol Appl 16(2):645-655.

Rowe, CL; Hopkins, WA; Congdon, JS. (2002) Ecotoxicological implications of aquatic disposal of coal combustion residues in the United States: a review. Environ Monit Assess 80:207-276.

Sippel, AJA; Geraci, JR; Hodson, PV. (1983) Histopathological and physiological responses of rainbow trout (Salmo gairdneri richardson) to sublethal levels of lead. Water Res 17(9):1115-1118.

Sorensen, EMB. (1991) Metal poisoning in fish. Boca Raton, FL: CRC Press 374 pp.

Sures, B. (2001) The use of fish parasites as bioindicators of heavy metals in aquatic ecosystems: a review. Aquat Ecol 35(2):245-255.

US EPA (Environmental Protection Agency). (2005) Methods/indicators for determining when metals are the cause of biological impairments of rivers and streams species sensitivity distributions and chronic exposure-response relationships from laboratory data. U.S. Environmental Protection Agency, Office of Research and Development, National Center for Environmental Assessment, Cincinnati, Ohio.

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