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Annex A

Envionmental Impact Indicators for Road Salts

Introduction

The purpose of Annex A is to provide guidance by identifying concentrations of chloride in the environment at which certain negative environmental impacts are likely to occur. A series of thresholds have been identified for different environmental compartments: surface water, groundwater and soil. Concentrations above these levels have the potential to result in negative impacts. In all cases, natural background concentrations should be considered in evaluating regional and local impacts. Data in this annex are based on findings presented in the Road Salts Priority Substances List Assessment Report.

Surface Water

The following paragraphs present certain thresholds associated with concentrations of chloride in surface water.

Figure 1 presents background concentrations of chloride in Canadian surface waters and concentrations of chloride that cause adverse biological effects. The column on the left provides a range of average background concentrations for five regions in Canada. The variation in background concentrations of chloride is greatest in western Canada and markedly decreases moving eastward to the Great Lakes area and Atlantic Canada. The lowest variation in chloride concentration is reported on the Canadian Shield.

The right column of Figure 1 is useful for identifying the levels of chloride in surface waters above and below concentrations reported to cause certain negative impacts. Concentrations of chloride of approximately 140 mg/L should be protective of freshwater organisms for short-term exposure; concentrations less than 35 mg/L are likely protective during long-term exposures. Overall, approximately 5 percent of species are predicted to experience effects from chronic exposure to concentrations of chloride of about 210 mg/L, while 10 percent of species would be affected at concentrations of about 240 mg/L.

Other jurisdictions have derived guidelines for the exposure of aquatic organisms to chlorides (Figure 1). The United States Environmental Protection Agency (EPA) developed a similar guideline. Overall, the EPA guideline indicates that biota, on average, should not be affected unacceptably if the four-day average concentration of chloride does not exceed 230 mg/L more than once every three years. Similarly, the biotic impacts would be minimal if the one-hour average chloride concentration does not exceed 860 mg/L more than once every three years.

Lakes located in Canada typically undergo vertical mixing every spring and fall as a result of a change in water temperature. Dissolved salts can hinder the vertical mixing of water bodies as dense, salt-laden water sinks to deeper layers (meromixis). The absence of vertical mixing can ultimately lead to a depletion of oxygen in the lower layers of lakes and a reduction in the cycling of nutrients. Meromictic conditions have developed in lakes with salt concentrations of approximately 60 mg/L Na and 105 mg/L Cl. Small, deep lakes are the most vulnerable, although concentrations associated with meromixis will vary greatly, depending on local conditions.

Figure 1 - Comparison of natural background concentrations of chloride in Canadian surface water and thresholds for adverse biological effects. The column on the left provides an overview of average background concentrations for five regions in Canada. The column on the right identifies levels at which certain impacts may occur. Data in this figure helps characterize average background concentrations for different areas and impacts that can occur at different concentrations.

Figure 1 - Comparison of natural background concentrations of chloride in Canadian surface water and thresholds for adverse biological effects.

  1. British Columbia (Mayer et al., 1999)
  2. Prairies (Mayer et al., 1999)
  3. Canadian Shield (Mayer et al., 1999)
  4. St. Lawrence Lowlands and Great Lakes (Mayer et al., 1999)
  5. Atlantic Canada (Mayer et al., 1999)
  6. Species should not be negatively affected if one-hour average concentration of CI does not exceed value more than once every 3 years (U.S. EPA 1988)
  7. Species should not be negatively affected if 4-day average concentration of CI does not exceed value more than once every 3 years (U.S. EPA 1988)
  8. Based on predicted data, 5% of species would be negatively affected (median lethal concentration) [Evans and Frick, 2001]
  9. Based on predicted data, 10% of species would be negatively affected (median lethal concentration) [Evans and Frick, 2001]
  10. Estimated no affects value derived from Ceriodaphnia dubia 4-day LC50 (Cowgill and Milazzo, 1990)
  11. Estimated no affects value derived from 33-day LOEC survival for fathead minnow (Birge et al., 1985)
  12. Chloride concentration in lower layers of water body associated with meromixis (Smol et al., 1985)
  13. To protect freshwater aquatic life from acute and lethal effects, the maximum concentration of total chloride at any time should not exceed this value. (BC Ambient Water Quality Guidelines for Chloride, 2002)
  14. To protect freshwater aquatic life from chronic effects, the 30-day average concentration of total chloride should not exceed this value. (BC Ambient Water Quality Guidelines for Chloride, 2002)

Ground Water

Chloride concentrations identified for freshwater biota will likely be protective of groundwater biota and groundwater that emerges into surface water.

A significant proportion of road salts may be contained within the groundwater system. The time taken to reach an equilibrium where salt inputs are balanced by salt outputs depends on local hydrogeological conditions and may take from a few years to hundreds of years.

Soils

Soil integrity, soil organisms and vegetation will generally be protected at concentrations of about 60 mg Na/L and 90 mg Cl/L. Damage to plants has also been observed at soil concentrations of 16 mg Na/kg and 30 mg Cl/kg (dry weight). Changes in natural plant communities have been recorded in areas affected by road salts runoff and liquid salt spray from moving vehicles.

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