CADDIS Volume 2: Sources, Stressors & Responses
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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 (Figure 3). 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 U.S. 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.
One also may 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. Figure 4). 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).
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).
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 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.