R. van Egmond

Fate of triclosan in field soils receiving sewage sludge.

The anti-microbial substance triclosan can partition to sewage sludge during wastewater treatment and subsequently transfer to soil when applied to land. Here, we describe the fate of triclosan in a one-year plot experiment on three different soils receiving sludge. Triclosan and methyl-triclosan concentrations were measured in soil samples collected monthly from three depths. A large fraction of triclosan loss appeared to be explained by transformation to methyl-triclosan. After 12 months less than 20% of the initial triclosan was recovered from each soil. However, the majority was recovered as methyl-triclosan. Most of the chemical recovered at the end of the experiment (both triclosan and methyl-triclosan) was still in the top 10 cm layer, although there was translocation to lower soil horizons in all three soils. Between 16.5 and 50.6% of the applied triclosan was unaccounted for after 12 months either as a consequence of degradation or the formation of non-extractable residues.

Integration of bioaccumulation in an environmental risk assessment

Abstract The potential of a substance to bioaccumulate in the aquatic environment into an organism depends particularly on its lipophilicity and its metabolism within the organism. Bioaccumulation is an exposure-related parameter and will determine the body burden but it is not an ‘effect’ or hazard in itself. Therefore, when appropriate, bioaccumulation should be included as an exposure related parameter in the environmental risk assessment of substances. The “direct” toxicity assessment should consider “time to steady state” in evaluating PNEC values. Depending on the steady-state criteria used, the duration of a 96h acute fish LC50 test may be insufficient for substances with a log Kow above the range of 3.8–4.5. If exposure and uptake are possible, the BCF is estimated in an iterative approach in order to address the potential for bioaccumulation and resultant “indirect” (dietary) toxicity. When the BCF value is above 1000, a PEC/PNEC assessment for predators is made. A step-wise approach is recommended. In practice, substances which are widely dispersed in the environment, which potentially can be taken up by biota, which are persistent, lipophilic and exhibit negligible metabolism will be selected by this scheme for a more detailed evaluation.

Dynamic multi-phase partitioning of decamethylcyclopentasiloxane (D5) in river water.

The behaviour of decamethylcyclopentasiloxane (D5) in river water was evaluated by measuring concentration changes in open beakers. Effective values for the partition coefficient between organic carbon and water (K(OC)) were derived by least-squares optimisation of a dynamic model which accounted for partitioning between the sorbed and dissolved phases of D5, and for losses via volatilisation and hydrolysis. Partial mass transfer coefficients for volatilisation were derived from model fits to controls containing deionised water. Effective values of log (K(OC)) were between 5.8 and 6.33 (mean 6.16). These figures are higher than some other experimentally-derived values but much lower than those estimated from the octanol: water partition coefficient using single-parameter linear free energy relationships (LFERs). A poly-parameter LFER gave a predicted log (K(OC)) of 5.5. Differences in partitioning are believed to be due to the nature of the organic matter present. The new value for effective K(OC) was employed in a simple model of D5 behaviour in rivers to ascertain the extent to which a higher affinity for organic carbon would depress volatility. The results suggest that despite the revised K(OC) value, volatilisation of D5 remains a significant removal mechanism in surface waters.

Effect of Aldrich humic acid on water-atmosphere transfer of decamethylcyclopentasiloxane.

The behaviour of the cyclic volatile methyl siloxane (cVMS) decamethylcyclopentasiloxane (D5) in surface waters is explored using a combination of laboratory experimentation and mathematical modelling. In the laboratory experiment, changes were observed in the concentration of radiolabelled D5 in open stirred beakers containing mineral medium with different concentrations of added Aldrich humic acid over 120 h. Although D5 is very volatile, its strong affinity for dissolved organic carbon (DOC) reduced the rate of water to air transfer significantly. The data were well described using a simple partitioning model which accounted for hydrolysis and for depth and DOC changes resulting from sampling and evaporation, although there was some evidence for the formation of a hypothetical bound residue with increasing time. The model was used to derive effective values for the partition coefficient between DOC and water [log(K(DOC))]. These values were relatively consistent across five treatments and varied between 5.04 and 5.40 log(L kg(-1)), with no systematic treatment bias. These values are significantly higher than previously published experimental estimates of K(OC) for D5 but more than two orders of magnitude lower than some K(OC) estimates based on the octanol:water partition coefficient (K(OW)). The data confirm that volatilisation will be an important loss mechanism from surface waters for D5, although the rate of loss will decrease with increasing DOC concentration.

The effect of triclosan on microbial community structure in three soils

The application of sewage sludge to land can expose soils to a range of associated chemical toxicants. In this paper we explore the effects of the broad spectrum anti-microbial compound triclosan on the phenotypic composition of the microbial communities of three soils of contrasting texture (loamy sand, sandy loam and clay) using phospholipid fatty-acid (PLFA) analysis. Each soil type was dosed and subsequently re-dosed 6 weeks later with triclosan at five nominal concentrations in microcosms (10, 100, 500, 1000 mg kg(-1) and a zero-dose control). PLFA profiles were analysed using multivariate statistics focussing on changes in the soil phenotypic community structure. Additionally, ratios of fungal:bacterial PLFA indicators and cyclo:mono-unsaturated PLFAs (a common stress indicator) were calculated. It was hypothesised that triclosan addition would alter the community structure in each soil with a particular effect on the fungal:bacterial ratio, since bacteria are likely to be more susceptible to triclosan than fungi. It was also hypothesised that the PLFA response to re-dosing would be suppressed due to acclimation. Although the microbial community structure changed over the course of the experiment, the response was complex. Soil type and time emerged as the most important explanatory factors. Principal component analysis was used to detect phenotypic responses to different doses of triclosan in each soil. As expected, there was a significant increase in the fungal:bacterial ratio with triclosan dose especially in treatments with the highest nominal concentrations. Furthermore, the PLFA response to re-dosing was negligible in all soils confirming the acclimation hypothesis.

Vertical variation of mixing within porous sediment beds below turbulent flows

Abstract River ecosystems are influenced by contaminants in the water column, in the pore water and adsorbed to sediment particles. When exchange across the sediment‐water interface (hyporheic exchange) is included in modeling, the mixing coefficient is often assumed to be constant with depth below the interface. Novel fiber‐optic fluorometers have been developed and combined with a modified EROSIMESS system to quantify the vertical variation in mixing coefficient with depth below the sediment‐water interface. The study considered a range of particle diameters and bed shear velocities, with the permeability Péclet number, PeK between 1000 and 77,000 and the shear Reynolds number, Re*, between 5 and 600. Different parameterization of both an interface exchange coefficient and a spatially variable in‐sediment mixing coefficient are explored. The variation of in‐sediment mixing is described by an exponential function applicable over the full range of parameter combinations tested. The empirical relationship enables estimates of the depth to which concentrations of pollutants will penetrate into the bed sediment, allowing the region where exchange will occur faster than molecular diffusion to be determined.

Assessment of bioconcentration and secondary poisoning of surfactants

The relevance of the bioconcentration behaviour of surfactants for the secondary poisoning assessment and for the risk characterisation in the bird and mammalian food chain has been investigated. The approach used is described in the recently revised EU Technical Guidance Document for the Risk Assessment of Substances. The results demonstrate that, based on experimentally derived bioconcentration factors, environmental concentrations and effects in animals, there is a clear level of safety for both linear alkylbenzene sulphonate (LAS) and alcohol ethoxylates (AE), the most important surfactants by volume. To assess other surfactants used in detergents, a bioconcentration factor that would need to be attained for secondary poisoning to be of concern has been estimated from predicted environmental concentrations and known long-term effects data in animals. Based on the known structural similarity of these surfactants to LAS and AE and the ubiquitous nature of the enzymatic systems that are present in biotransformation processes in organisms, it is concluded that bioconcentration of these surfactants to these levels is highly unlikely. Therefore the potential for secondary poisoning effects of these surfactants is extremely low.