Risk Characterization

As part of risk characterization, one line of evidence includes consideration of risk quotients to identify potential for ecological effects. Other factors that affect current or potential risks, such as persistence, bioaccumulation and trends in ambient concentrations, are also considered.

Risk Quotient Analysis

Critical exposure and effects results and risk quotients are summarized in Table 10 and described in more detail below.

¹ AF = application factor.

Pelagic Organisms

For pelagic organisms, a risk quotient was developed using the average 96-hour LC₅₀ values of rainbow trout reported by Mayer and Ellersieck (1986) (0.066 mg/L) and Sewell et al. (1995c) (0.45 mg/L). The average of the two studies, which is the CTV, is 0.26 mg/L.

For the industrial release scenario, if STP treatment is considered (27% removal efficiency), the EEV will be 0.0014 mg/L. Using an application factor of 100 on the CTV to account for acute to chronic extrapolation and intra- and interspecies variations, differently sensitive biological endpoints and laboratory to field extrapolations, the ENEV is calculated to be 0.0026 mg/L.

The risk quotient is therefore calculated as:

EEV / ENEV = 0.0014 mg/L / 0.0026 mg/L = 0.54

Even with STP removal considered, this represents a conservative scenario due largely to the very high quantity of DNOC assumed to be used by a single facility.

The maximum EEV under the defined rainfall scenario was determined to be 0.0025 mg/L with no STP treatment due to the assumption of a heavy rainfall. As rainfall represents an acute exposure scenario, the application factor does not need to account for acute to chronic extrapolation. Therefore, using an application factor of 10 and the same CTV of 0.26 mg/L for rainbow trout, an ENEV of 0.026 mg/L is calculated. The risk quotient is therefore:

EEV / ENEV = 0.0025 mg/L / 0.026 mg/L = 0.096

Soil Organisms

There are no quantified amounts of DNOC concentrations in Canadian soils. OMEE (1994) did not detect DNOC in 161 soil samples collected from soils in Ontario. The method detection limit of 0.1 mg/kg (100 ng/g) will be used as a surrogate for the level of DNOC in Canadian soil and is selected as the EEV.

One study was located in the literature on the effects of DNOC on terrestrial organisms. The LC₅₀ from a 14-day acute toxicity study on the earthworm is 15 mg/kg of soil. This value is selected as the CTV for exposures of soil organisms to DNOC. Dividing the value by a factor of 100 to account for extrapolation from laboratory to field conditions, acute to chronic ratio and interspecies and intraspecies variations in sensitivity gives an ENEV of 0.15 mg/kg.

The risk quotient for soil organisms is therefore:

EEV / ENEV = 0.1 mg/kg / 0.15 mg/kg = 0.67

Aquatic Wildlife

The EEVs for the mink and river otter were estimated to be 0.0004 mg/kg-bw per day and 0.000 007 mg/kg-bw per day, respectively. The ENEV for the mink was estimated to be 0.035 mg/kg-bw per day, and the ENEV for the river otter was calculated to be 0.0047 mg/kg-bw per day.

The risk quotients for aquatic wildlife are thus calculated to be:

EEV_mink / ENEV_mink = 0.0004 mg/kg-bw per day / 0.035 mg/kg-bw per day = 0.011

EEV_otter / ENEV_otter = 0.000 007 mg/kg-bw per day / 0.0047 mg/kg-bw per day = 0.0015

Benthic Organisms

No monitoring data for DNOC in sediments in Canada were identified. IPCS (2000) has stated that if released to water, DNOC is only moderately adsorbed onto aquatic sediments. Level III multimedia fate simulation estimated that only about 1% of DNOC is expected to partition to sediments. It is therefore believed that there will be minimal exposure of benthic organisms to DNOC.

Weight of Evidence Analysis

The risk quotient analyses for pelagic and soil organisms and wildlife have shown that it is unlikely that organisms are currently exposed to concentrations of DNOC above known effect thresholds. This conclusion is based on current import levels, the locations where DNOC is being used industrially and the current state of knowledge of its atmospheric chemistry.

A conservative scenario based on concentrations of DNOC in precipitation that could be expected to enter Canadian receiving water indicated that the potential for risk to aquatic organisms from this source is low.

In addition, modelling estimates of industrial releases to the St. Clair River indicate that DNOC is not likely to cause adverse effects on pelagic or benthic organisms. This is based on a conservative release scenario developed for the one company importing the substance that currently reports releases of DNOC to the NPRI, although the company has not reported releases to water.

Although sorption is low at environmentally relevant pHs, little leaching to groundwater has been found, likely due to biodegradation.

Potential sources of release of DNOC to the environment are to air and water. Based on its properties, DNOC is persistent in air but not bioaccumulative. Long-range transport modelling estimates that it will be transported over moderate distances, and a decreasing concentration with increasing latitude is expected.