Effect of suspended soil material and pig slurry on the facilitated transport of pesticides, phosphate and bromide in sandy soil

The colloid-facilitated transport of pesticides and other pollutants to groundwaters is a source of concern to authorities in the European Union and other countries. It has been suggested that colloids are good sorbents for pesticides and other pollutants, and it is now suspected that the spreading of animal manures increases the potential for colloid-facilitated transport. In experiments in lysimeters of sandy soil to which phosphate, triallate, chlordane and bromide were applied to the surface, the first concentration peaks of the other chemicals always appeared before that of bromide, suggesting that their transport was facilitated by colloidal or suspended soil material. Chlordane was transported only when sorbed on suspended soil material and was associated most closely with the coarsest of suspended material (>1.2 mum), possibly because it contained the most organic matter. Phosphate was also associated with this class but probably because of the clay it contained. Applying pig slurry to the soil significantly increased the loss of triallate from the lysimeters but not that of chlordane, probably because the latter was held strongly by the soil's organic matter. The loss of triallate was not associated with that of any of the size classes of suspended soil material, which may imply that it was transported in association with dissolved or colloidal organic matter. Neither the pig slurry nor an equivalent amount of inorganic phosphate significantly increased the loss of phosphate. The amounts of triallate and chlordane applied were 100 times the standard agricultural applications, but the largest concentration of chlordane measured was still less than the European Union limit of 0.1 mug l(-1). That of triallate was greater than the limit but would almost certainly have been less than it with the standard application. The greatest cumulative losses of phosphate, triallate and chlordane were, respectively, 1%, 0.003% and 0.00002% of the amounts applied. All the results suggest there is only a very small pollution risk to groundwaters from the pesticides and phosphate.