W A House, B V Zhmud, D R Orr, G K Lloyd and G P Irons.

MAY 1998

Colloids are naturally occurring particles or large polymer molecules in the size range of 1-1000 nm, i.e. <1µm in diameter, which do not settle from fresh waters on standing. Larger particles are generally considered to be suspended in water and are kept in this state by the movement of the water. Colloids are generally more mobile in the environment compared with sediments and behave in a similar way to chemicals which are dissolved in water. Some pesticides, such as the synthetic pyrethroids which are extremely toxic to aquatic organisms, are only very slightly soluble in water but are very strongly bound to sediment particles and colloids. The association of such compounds with colloids has important implications for their translocation in the environment and movement from rivers to lakes and estuaries, and through the soil to groundwater. The current research was aimed at obtaining more information about the role of colloids in the fate and behaviour of pesticides and the development of practical methods to assess their relative importance. The methods developed will be of value in the assessment of the importance of colloid transport in the environmental fate of pesticides considered for registration purposes.

Routine analysis of natural waters for pesticides normally involves filtration of the samples prior to analysis and therefore neglects the contributions of suspended solids and a large fraction of the colloid component to the pesticide content of the sample. When samples are not filtered but treated as "whole water" samples, depending on the method used to extract the pesticides from the sample, some part of the colloid and sediment associated pesticides will be measured. The alternative method of separating the colloids and sediments from the water, and extracting these independently of the water sample, is a complex procedure, very time consuming and is impractical on a routine basis. Future development of analytical techniques to ensure that pesticides associated with suspended clays and other colloids are include in the reported concentrations to the appropriate regulators is needed. This study shows that the analysis of "whole water" samples is essential if the pesticides which are bound to colloids and suspended material are to be determined. The analysis of filtered water samples will produce a very incomplete picture of the occurrence of pesticides in rivers and fate of pesticides applied in agriculture.

This project examines in detail the interaction of selected pesticides with three major groups of colloids (a) clay particles, (b) macromolecules of humic/fulvic acid, i.e. part of the organic colloids and (c) inorganic colloids consisting of predominantly mineral particles. The study shows the importance of organic colloids in transporting a range of pesticides in rivers. Even relatively weakly adsorbed herbicides such as simazine, have a relatively strong affinity with organic colloids when compared with their affinity to organic carbon in soils. In view of the complex nature of colloids and sediments, a pragmatic approach is suggested in which routine water quality measurements are used to estimate the transport of pesticides with colloids and other particulates in rivers. Suspended solids measurements by filtration of the sample through a 0.45 µm membrane include clay and mineral colloids with the larger sediment particles. Standard field measurements of the suspended solids concentration and dissolved organic carbon content may then be used with the appropriate distribution coefficients, to estimate the relative amounts of bound pesticide.

• An automated ultra-filtration system has been developed and used to measure the distribution of a range of triazine and organophosphorus pesticides between the dissolved phase and organic colloids in river water.
• Organic colloids were found to interact strongly with a range of pesticides. Measurement of the distribution coefficients using the ultra-filtration cell containing fresh waters from the Humber rivers (Aire, Calder, Trent, Don and Swale) with atrazine. simazine, propazine, prometryn, desmetryn, terbutryn, cyanazine, parathion, fenitrothion and malathion, gave high distribution coefficients in the range of 7900 to 36,000 ml g¹ normalised with respect to dissolved organic carbon. Differences in the sorption affinities between the pesticides were generally in accordance with their octanol-water partition coefficients .
• Field data from the rivers Aire (2 sites), Calder, Ouse, Trent and Don, from samples collected during storms and weekly monitoring, showed the presence of a wide range of pesticides including simazine, atrazine, prometryn, desmetryn, terbutryn. malathion, parathion and fenitrothion, cis and trans perrnethrin, dieldrin and lindane, e.g. over 90 % of the samples from the R. Calder contained lindane, permethrin and simazine at concentrations greater than 0.01 µg dm^-3. The data were consistent with the mobilisation of permethrin and dieldrin with suspended sediments during storms and the strong affinity of permethrin with suspended sediments.
• The importance of organic colloids in the transport of triazine and organophosphorus pesticides in the R. Aire was estimated using the distribution coefficients measured with the ultra-filtration cell together with field data of dissolved organic carbon and suspended sediments from the NERC LOIS (Land Ocean Interaction Study). The majoritv of the bound material was predicted to be with organic colloids measured as dissolved organic carbon, leading to maximum concentrations in periods of low-flow. Up to 20 % of the triazine load was transported with colloids and suspended matter compared with up to 30 % for the selected organophosphorus compounds.
• The good relationship between the distribution coefficients and the octanol-water partition coefficients for the triazines, organophosphorus compounds and literature data for polyaromatic hydrocarbons and DDT, permitted an estimate of the appropriate distribution coefficients. This led to a prediction of betveen 70 and 90 % of permethrin and dieldrin in the R Aire water associated with organic colloids and suspended sediments - with the majoritv sorbed to organic matter. The predictions for lindane were similar to the values obtained for the organophosphorus compounds.
• Suspended sediments in the rivers Aire, Ouse and Swale were mainly composed of material of less than 900 µm diameter with the clay fraction,<2µm, generally < 10 % by volume but up to 30 % by mass. The R. Aire sediment was mainly clays such as montmorillonite. kaolinite, chlorite and illite. The majoritv of this material was separated during centrifugation leaving onlv the organic colloids in the centrifugate. The interaction of selected pesticides with clay colloids was exothermic and the isotherms linear at low concentrations. The specific interactions of flutriafol with clay colloids was investigated using molecular modelling. The interaction was found to be dominated by hydration and hydrogen bonding with a possibilitv for acid-base interactions with surface hydroxyl groups on the clay. The herbicide is unlikely to enter the interlayer spacing of the expandable clays such as montmorillonite. Sorption was not found to be strongly dependent on salinity with the greatest increase between 0.001 and 0.01 M NaCl, e.g. rainwater to hard water. Subsequent increases in dissolved solids to sea water concentrations, had a very small effect on sorption but such increases are expected to lead to particle aggregation in the water column. It is concluded that molecular modelling is a powerful tool to gain insight to the binding mechanisms between surfaces and pesticides.

Copies of this report are available from the Foundation, price £35.00, less 20% for FWR Members.