2611 Environmental Toxicology and Chemistry, Vol. 20, No. 11, pp. 2611–2616, 2001  2001 SETAC Printed in the USA 0730-7268/01 $9.00 + .00 LABORATORY ANALYSES OF THE POTENTIAL TOXICITY OF SEDIMENT-ASSOCIATED POLYDIMETHYLSILOXANE TO BENTHIC MACROINVERTEBRATES KEVIN S. HENRY,*² W ILLEMIEN H. WIELAND,‡ DAVID E. POWELL,§ and JOHN P. GIESY² ²Department of Zoology, National Food Safety and Toxicology Center, Institute for Environmental Toxicology, Michigan State University, East Lansing, Michigan 48824, USA ‡Wageningen University, Toxicology Section, 6700 EA Wageningen, The Netherlands §Environmental Sciences (C03101), Dow Corning Corporation, Midland, Michigan 48686, USA ( Received 24 October 2000; Accepted 17 April 2001) Abstract—Polydimethylsiloxane (PDMS) is widely used in a number of industrial processes and consumer products that result in down-the-drain disposal. The log p value for the PDMS used in the present study was 10, and the vapor pressure and water solubility values were below detection limits. These physicochemical characteristics and a measured degradation rate of 3% after six months in moist soils suggest that PDMS may accumulate in aquatic sediments. Sediment toxicity tests with the amphipod Hyalella azteca and with larvae of the midge Chironomus tentans were used to assess the potential for toxicity of PDMS-amended sediments to benthic invertebrates in short-term (10-d) and whole-life-cycle (28 d for H. azteca, 50–65 d for C. tentans) exposures. Endpoints for short-term tests included survival and growth, while life-cycle assays considered survival, growth, reproduction, and, for C. tentans only, emergence. Short-term and life-cycle exposures to concentrations of 1,000 mg PDMS/kg sediment (dry wt) indicated that PDMS will not reduce survival, growth, or reproduction in H. azteca or C. tentans. Keywords—Chironomus tentans Hyallela azteca Chronic toxicity testing Polydimethylsiloxane INTRODUCTION Polydimethylsiloxane (PDMS) fluids are synthetic poly- mers consisting of an alternating silicon–oxygen backbone with methyl side groups (Fig. 1). Polydimethylsiloxane is used in a variety of industrial and consumer applications, including heat transfer fluids, antifoams, cosmetics, waxes, and polishes [1]. Many of these uses result in down-the-drain disposal. The most common linear siloxanes are insoluble in water and ex- tremely hydrophobic; therefore, siloxanes become associated with sludge solids on reaching sewage conduits and wastewater treatment plants [2,3]. Approximately 3% of the PDMS disposed to wastewater treatment plants in the United States is released to surface waters adsorbed to sludge particles in the treated effluent, rep- resenting a total annual loading of approximately 294,000 kg [4]. On entering surface waters, the majority of the suspended sludge and associated PDMS will eventually deposit into bot- tom sediments. The small fraction of PDMS not sorbed to suspended particles in the effluent would partition to sus- pended solids in the river or lake water and also eventually become deposited into the bottom sediments [4]. Degradation of PDMS has been demonstrated in soils and is believed to occur by hydrolysis to yield monomeric di- methylsilanediol [2], though the rate of this reaction is in- versely related to soil moisture content [5]. The potential for PDMS degradation in freshwater sediments was demonstrated in the laboratory [6]. After a year of incubation in aerated sediments, 5 to 10% of the PDMS present was hydrolyzed to dimethylsilanediol (DMSD)—results similar to those for deg- * To whom correspondence may be addressed (kshenry@dow.com). The current address of K.S. Henry is Dow Chemical Company, 1803 Building, Midland, MI 48674, USA. radation studies in moist soils, where 3% of the PDMS was hydrolyzed to DMSD in six months [5]. The rate of PDMS degradation was less in sterile sediments, suggesting that this process may be related to sediment microflora [6]. The mo- bility of PDMS in aquatic systems will be limited primarily to resuspension and deposition of sediment particles with which it is associated. Concentrations of PDMS decrease with depth in sediments and are not detectable at depths that cor- respond to periods before the production and use of PDMS fluids [7,8]. While PDMS tends to accumulate in sediments, concentrations can decrease in response to mass dilution and degradation [6] or if inputs are reduced or discontinued, for example, by enhanced wastewater treatment technology [9]. Mean measured PDMS concentrations in sediments from de- positional areas downstream from wastewater treatment plant (WWTPs) range from less than the detection limit to 78 mg/ kg (dry wt) at sites receiving point source inputs [9]. However, 90% of the sediments analyzed contained less than 26 mg PDMS/kg (dry wt) [9]. The biota exposed to PDMS in these areas would primarily be benthic invertebrates intimately as- sociated with these sediments. Polydimethylsiloxane appears to be relatively nontoxic to benthic invertebrates (Table 1). In part, this nominal level of toxicity is due to the fact that little bioaccumulation occurs (Table 1). This low potential to bioaccumulate is due to the high molecular weight of PDMS molecules, which are too large to cross biological membranes [10,11]. While PDMS has been found to be nontoxic in short-term studies, little infor- mation was available on the chronic toxicity of PDMS to sed- iment-dwelling benthic invertebrates. Since PDMS is surface active, it was postulated that PDMS in sediments could change the physical characteristics of the matrix. For instance, it has been reported that PDMS can reduce the bioavailability of