Suspended Particulate MatterA Source or Sink for Chemical Mixtures of Organic Micropollutants in a Small River under Baseflow Conditions?

Suspended particulate
matter (SPM) plays an important role in the
fate of organic micropollutants in rivers during rain events, when
sediments are remobilized and turbid runoff components enter the rivers.
Under baseflow conditions, the SPM concentration is low and the contribution
of SPM-bound contaminants to the overall risk of organic contaminants
in rivers is assumed to be negligible. To challenge this assumption,
we explored if SPM may act as a source or sink for all or specific
groups of organic chemicals in a small river. The concentrations of
over 600 contaminants and the mixture effects stemming from all chemicals
in in vitro bioassays were measured for river water,
SPM, and the surface sediment after solid-phase extraction or exhaustive
solvent extraction. The bioavailable fractions of chemicals and mixture
effects were estimated after passive equilibrium sampling of enriched
SPM slurries and sediments in the lab. Dissolved compounds dominated
the total chemical burden in the water column (water plus SPM) of
the river, whereas SPM-bound chemicals contributed up to 46% of the
effect burden even if the SPM concentration in rivers was merely 1
mg/L. The equilibrium between water and SPM was still not reached
under low-flow conditions with SPM as a source of water contamination.
The ratios of SPM-associated to sediment-associated neutral and hydrophobic
chemicals as well as the ratios of the mixture effects expressed as
bioanalytical equivalent concentrations were close to 1, suggesting
that the surface sediment can be used as a proxy for SPM under baseflow
conditions when the sampling of a large amount of water to obtain
sufficient SPM cannot be realized.