Transport and distribution of lindane and simazine in a riverine environment: measurements in bed sediments and modelling

Abstract

Aquatic sediments often remove hydrophobic contaminants from fresh waters. The subsequent distribution and concentration of contaminants in bed sediments determines their effect on benthic organisms and the risk of re-entry into the water and/or leaching to groundwater. This study examines the transport of simazine and lindane in aquatic bed sediments with the aim of understanding the processes that determine their depth distribution. Experiments in flume channels (water flow of 10 cm s(-1)) determined the persistence of the compounds in the absence of sediment with (a) de-ionised water and (b) a solution that had been in contact with river sediment. In further experiments with river bed sediments in light and dark conditions, measurements were made of the concentration of the compounds in the overlying water and the development of bacterial/algal biofilms and bioturbation activity. At the end of the experiments, concentrations in sediments and associated pore waters were determined in sections of the sediment at 1 mm resolution down to 5 mm and then at 10 mm resolution to 50 mm depth and these distributions analysed using a sorption-diffusion-degradation model. The fine resolution in the depth profile permitted the detection of a maximum in the concentration of the compounds in the pore water near the surface, whereas concentrations in the sediment increased to a maximum at the surface itself. Experimental distribution coefficients determined from the pore water and sediment concentrations indicated a gradient with depth that was partly explained by an increase in organic matter content and specific surface area of the solids near the interface. The modelling showed that degradation of lindane within the sediment was necessary to explain the concentration profiles, with the optimum agreement between the measured and theoretical profiles obtained with differential degradation in the oxic and anoxic zones. The compounds penetrated to a depth of 40-50 mm over a period of 42 days.

Interactions with freshwater biofilms cause rapid removal of common herbicides through degradation – evidence from microcosm studies

“…Modern pesticides are relatively small organic molecules that can be readily used by heterotrophic microbes (e.g. 30,31 ). Our 14 C-labeled herbicides had one or multiple 14 C-atoms well integrated in their molecules (Table 1), leaving mineralization as the single option for the observed large loss of label from our experimental units.…”

Diffusion of the Synthetic Pyrethroid Permethrin into Bed-Sediments

“…This is further demonstrated by permethrin diffusing to only approximately 5-10 mm deep in the sediment in 42 days, while in the same experimental approach, simazine and lindane were shown to diffuse to 30-50 mm deep in the sediment (15). Sharom and Solomon (11) investigated the diffusion of permethrin into lake sediment using column experiments.…”

“…In all cases, the trans isomer was observed to disappear more rapidly than the cis isomer, and losses were highest in light conditions. As average temperatures were similar for both experiments (15), it may be unlikely that losses were due to temperature-induced higher microbial metabolism (8). This accelerated degradation of transpermethrin in the light suggests that processes linked to the increase in biological activity in surficial sediments under light conditions affect the fate of trans-permethrin in sediments.…”

“…With a diffusion coefficient of 3.38 × 10 -10 m 2 s -1 obtained using the Wilke-Chang equation (15), estimates were calculated for porosities varying between 0.4 and 0.8 (see Figure 3a), Kd's between 50 and 5500 L kg -1 , and a sediment density of 2.5 kg L -1 . With an upper Kd limit of 5500 L kg -1 , permethrin was estimated to diffuse to a depth of 0.4-0.5 mm, while with a Kd of 50 L kg -1 , the depth of diffusion was calculated between 3.9 and 5.4 mm (Table 2).…”

Interactions with DOM and biofilms affect the fate and bioavailability of insecticides to invertebrate grazers

“…Biofilms also contain a large fraction of extracellular polymeric substances (EPS), i.e., polysaccharides, proteins, nucleic acids, and lipids, which originate from microorganisms and their excretion products within the biofilm (Battin et al 2007;Decho 1990). Carboxylic ligands on EPS-polysaccharides mediate the association of DOM (Decho 1990), metals (Schlekat et al 1998;Selck et al 1999), and pesticides (Allan et al 2004) with biofilms through binding or colloidal trapping. Furthermore, DOM can affect the association of contaminants with biofilms through rapid and efficient sorption (Carter and Suffet 1982;McCarthy and Jimeneze 1985), and thus negatively affect their bioavailability (e.g., Schlekat et al 1998;Selck et al 1999).…”