under environmentally relevant conditions where an indeterminable number of
compounds co-exist together and which are able to enhance or reduce the effects
of individual compounds, their concentration can contribute to the overall effect of
environmentally available contaminants.
5.3 Case Study: Acute and Sublethal Toxicity of Sediment
Sorbed DF and IB in the Midge Chironomus riparius
In order to perform environmentally relevant risk assessment, the final fate of
contaminants has to be taken into account. Apart from being dissolved within the
water column, chemical compounds may tend to be sorbed onto sediments or
particulate matter affecting the organisms dwelling within. The factors governing
this tendency are generally compound-specific and are defined by its molecular
structure, but also by the nature of the sediment onto which the compound is sorbed.
Thus, the extent of sorption of the compound is related to its physicochemical
properties and other factors such as the sediment organic matter content, surface
sorption to mineral constituents, ion exchange capacity, pH value, and complex
formation with metal ions such as Ca, Mg, Fe or Al, and H bonding [88].
An idea about the lipophilicity is given by the octanol-water distribution coefficient K ow , with very soluble compounds (hydrophiles) generally having a log K ow < 3
indicating they preferentially remain in water and low-soluble compounds
(hydrophobes) having a log K ow > 3 and a tendency to associate with the sediment.
The range of K ows within the group of pharmaceuticals varies significantly, with log
K ows from negative values up to values of 6. The log K ows of DF and IB are
comprised between 1.90 and 2.48, respectively, indicating a relatively low tendency
to potentially adsorb onto sediments. A small proportion of the environmentally
available DF and IB, however, are able to be sorbed onto sediments.
Within the group of sediment-dwelling organisms, the amount of sorbed chemical
affects species differently depending on their living and feeding habits. Sediment
exposure can occur through the fraction reversibly bound to the sediment which is
available through the interstitial water, whereas those organisms that ingest sediments have to affront exposure through two different ways and are generally more
sensitive [89]. In the past years, the number of studies that have reported the negative
effects of sediment sorbed pharmaceutical compounds on sediment-dwelling organisms has increased, but only a limited number of standardized tests are available.
Standardized protocols for sediment toxicity covering a wide range of sublethal,
environmentally relevant responses have been developed for the midge Chironomus
riparius (OECD 218/219: Sediment-Water Chironomid Toxicity Test Using Spiked
Sediment/Spiked Water; OECD 233: Sediment-Water Chironomid Life-Cycle Toxicity Test Using Spiked Water or Spiked Sediment; OECD 235: Chironomus sp.,
Acute Immobilisation Test). Chironomus species occur in aquatic habitats in high
abundance and diversity and are easily bred in laboratory cultures, with their larval
178
C. Trombini et al.
compounds co-exist together and which are able to enhance or reduce the effects
of individual compounds, their concentration can contribute to the overall effect of
environmentally available contaminants.
5.3 Case Study: Acute and Sublethal Toxicity of Sediment
Sorbed DF and IB in the Midge Chironomus riparius
In order to perform environmentally relevant risk assessment, the final fate of
contaminants has to be taken into account. Apart from being dissolved within the
water column, chemical compounds may tend to be sorbed onto sediments or
particulate matter affecting the organisms dwelling within. The factors governing
this tendency are generally compound-specific and are defined by its molecular
structure, but also by the nature of the sediment onto which the compound is sorbed.
Thus, the extent of sorption of the compound is related to its physicochemical
properties and other factors such as the sediment organic matter content, surface
sorption to mineral constituents, ion exchange capacity, pH value, and complex
formation with metal ions such as Ca, Mg, Fe or Al, and H bonding [88].
An idea about the lipophilicity is given by the octanol-water distribution coefficient K ow , with very soluble compounds (hydrophiles) generally having a log K ow < 3
indicating they preferentially remain in water and low-soluble compounds
(hydrophobes) having a log K ow > 3 and a tendency to associate with the sediment.
The range of K ows within the group of pharmaceuticals varies significantly, with log
K ows from negative values up to values of 6. The log K ows of DF and IB are
comprised between 1.90 and 2.48, respectively, indicating a relatively low tendency
to potentially adsorb onto sediments. A small proportion of the environmentally
available DF and IB, however, are able to be sorbed onto sediments.
Within the group of sediment-dwelling organisms, the amount of sorbed chemical
affects species differently depending on their living and feeding habits. Sediment
exposure can occur through the fraction reversibly bound to the sediment which is
available through the interstitial water, whereas those organisms that ingest sediments have to affront exposure through two different ways and are generally more
sensitive [89]. In the past years, the number of studies that have reported the negative
effects of sediment sorbed pharmaceutical compounds on sediment-dwelling organisms has increased, but only a limited number of standardized tests are available.
Standardized protocols for sediment toxicity covering a wide range of sublethal,
environmentally relevant responses have been developed for the midge Chironomus
riparius (OECD 218/219: Sediment-Water Chironomid Toxicity Test Using Spiked
Sediment/Spiked Water; OECD 233: Sediment-Water Chironomid Life-Cycle Toxicity Test Using Spiked Water or Spiked Sediment; OECD 235: Chironomus sp.,
Acute Immobilisation Test). Chironomus species occur in aquatic habitats in high
abundance and diversity and are easily bred in laboratory cultures, with their larval
178
C. Trombini et al.