Ecotoxicological risk estimation of new chemical compounds in sediments: an exploratory model study

An exploratory model study has been carried out in order to evaluate the possibility that, upon discharge of WWTP-effluents into surface water, sediment organisms may be exposed to greater risk than water organisms. The sensitivity of the risk quotient PEC/NEC for a number of suppposedly important model variables has been investigated. It is presumed that sediment organisms are at greater risk than water organisms if the concentration of the emitted chemical in pore water is higher than the concentration in surface water. It is concluded that this may occur when: 1. the sediment-water partition coefficient KP(sed) has a smaller value than the suspended particle-water partition coefficient Kp (susp); this may be so: a difference of a factor of 2 seems realistic 2. particulate matter in the WWTP-effluent settles selectively in the vicinity of the emission point ; great uncertainty exists in this respect: in this study 90% selective settling is considered possible. Should all influences operate together in the same direction, then the PEC/NEC for sediment may become up to three orders of magnitude greater than PEC/NEC for water. Such a coincidence is highly unprobable ; 2 to 10 times higher risk for sediment organisms should be taken into account. The most important influences are: - the ratio Kp9susp)/Kp(sed) - the persistence against transport from and transformation in the sediment. In order to assess the possibility of increased risk for sediment organisms critical evaluation of the possibility that one of these variables takes an extreme value is necessary and, in the light of the great uncertainty associated with the risk assessment of new chemicals, presumably also sufficient.DGM/S