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The sediment quality guideline, ERL, is not a chemical concentration at the threshold of sediment toxicity

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Abstract

While it is being used as such, the sediment quality guideline ERL (effects range low) is not a threshold of any chemical concentration in sediment at which the probability of toxicity shows an abrupt increase. Similarly, while it has been done, there is no basis for assuming that multiple concentrations above an ERL increase the probability of toxicity.

Introduction

There are several instances (Van Dolah et al., 1999; EPA, 2001a; SFEI, 2003; Kiddon et al., 2003) where authors have attributed undue biological significance to the sediment quality guideline called the effects range low (ERL) (Long and Morgan, 1990). The point of this short note is to clarify that an ERL is not a threshold. Rather, an ERL is simply a low point on of a continuum of bulk chemical concentrations in sediment that roughly relate to sediment toxicity. For illustrative purposes I use concentrations of Cu, though any chemical with an ERL could serve as well.

Section snippets

Connecting sediment chemistry with toxicity

The ERL and ERM (effects range median) are the 10th and 50th percentiles, respectively, on an ordered list of concentrations in sediment found in the literature that co-occur with any biological effect. For Cu, the original ERL (Long and Morgan, 1990) was 70 ppm because 51 concentrations were found in association with an effect and 7 were less than 70 ppm (to be precise that ERL was the 14th percentile but 3 of the lower 7 concentrations were almost identical at 67, 68.2 and 68.4 ppm). Long et

Influence of particle size

It is well established that chemical concentrations in sediment vary inversely with particle size. One expects concentrations to be greater in fine- relative to coarse-grained sediment. Given that no account is taken of grain-size when determining ERLs, one would further expect ERL exceedances to increase in proportion to the fine-grained portion of sediment. That is, in fact, the case as shown on Table 1 based on the 2165 samples with grain-size determinations from the dataset used by O’Connor

Critiques of ERL uses

It is a mistake to treat ERLs as thresholds. This was done, for example, by the San Francisco Bay Estuarine Institute in their Pulse of the Estuary Report (SFEI, 2003), and by Kiddon et al. (2003) in their assessment of the ecological condition of the US Mid-Atlantic estuaries. In both of these any ERL exceedance was judged as an indicator of degradation. SFEI (2003) uses ERLs as “contaminant guidelines [that] are generally intended to indicate if … sediment is safe.” Kiddon et al. (2003) refer

Conclusion

ERLs are not thresholds below which sediment toxicity is impossible and above which it is likely. They are just concentrations at the low end of a continuum roughly relating bulk chemistry with toxicity. Categorizing sediments on the basis of whether their chemical concentrations include one or more ERL exceedance leads to misperceptions of the actual probability that such sediments are toxic.

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