FR/D 0030

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. <1m 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 m 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.


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