Spatially explicit river catchment modelling of decamethylcyclopentasiloxane

Decamethylcyclopentasiloxane (D5) belongs to a group of chemicals known as cyclic volatile methylsiloxanes (cVMS). D5 is used in a variety of applications including personal care and household care products. EUSES has been used to estimate concentrations of D5 in effluent and river water immediately downstream of a Sewage Treatment Plant (STP), assuming a per capita chemical consumption (PCC) of 11.6 mg/cap/day and STP removal efficiency of 95.3% (based on SimpleTreat). Two surface water monitoring campaigns have been conducted on the rivers Nene and Great Ouse in the Anglian region of the UK on two different occasions. The surface water catchment model LF2000-WQX was used to estimate D5 concentrations in the river Nene and Great Ouse. LF2000 allows natural flow estimates to be derived for any river reach in the UK and for the impact of artificial influences on the flow regime to be modelled. LF2000-WQX (water quality extension version) incorporates the ‘Mode 0’ functionality of the GREAT-ER 1 simulator. The results of a sensitivity analysis indicate that the assumed environmental load and/or the removal rate of D5 in STP are incorrect and lead to modelled concentrations that significantly over-estimate the observed concentrations of D5 in surface water. Monitored concentrations were better estimated through the use of measured effluent concentrations, but additional samples (effluent and surface water) are needed to refine the assessment. Large variation in D5 exposure concentrations were observed in the river Nene, with higher concentrations associated with summer monitoring results. The effect of seasonal variations in effluent dilution will be explored in an attempt to explain the effect of temporal variation on observed concentrations in the two UK rivers.