Water quality from contrasting drained clay soils : the relative importance of sorbed and aqueous phase transport mechanisms

A full understanding of the processes controlling the movement of pesticides within the soil 1 water regime is critical to the successful reduction of losses of these substances to surface waters. Until recent years the importance of absorbed phase transport of pesticides has largely been ignored with respect to drainflow and the appearance of strongly absorbed hydrophobic compounds in riverine sediments has hitherto been largely attributed to surface runoff 1 erosion. Field studies were setup for the 1994/5 winter season to investigate the hydrology and chemistry associated with drainflow from the lighter clay soils of Boarded Barns Farm, with two calcareous (Hanslope and Stretham) and two non-calcareous (Melford and Ludford) soils being chosen for study. Instrumented study plot areas were established in each field, with drainflow measurement and water sampling being automated. Laboratory based investigations (including rainfall simulation and desorption) were carried out to support, extend and clarify the field data, in particular the processes involved in the transport of isoproturon and sediment. A striking feature seen in the field studies was the appearance of large amounts of suspended sediment in drainflow from the Melford soil, in contrast to the clear drainflow from the calcareous soils. The sediments in drain flow were identified as being of surface origin, generated by rainfall impact and transported via preferential by-pass flow. Laboratory investigations have indicated that the chemical nature of the topsoil is a dominant factor controlling the transport of sediments and clay flocculation due to the presence of calcium carbonate is proposed as a mechanism for preventing sediment transport in the calcareous soils. Where sediment was found in drainflow, it was shown to be responsible for the transport and losses in the absorbed phase of not only diflufenican, a strongly absorbed pesticide, but also isoproturon, a weakly absorbed pesticide. The laboratory studies indicate that isoproturon is transported predominantly absorbed to sediment and later desorbs 1 diffuses slowly into the aqueous phase of the receiving waters. The results of the field experiments were confirmed by the rainfall simulation studies on 20cm deep (20cm diameter) topsoil cores taken from the field, which showed sediment transport only in the non-calcareous soil. The extent of this phenomena of sediment transport in the UK is not well defined, due to the historical lack of research, however soils known to show the effect have a land coverage of 6% of England and Wales. The present study highlights an additional mechanism by which pesticide losses to surface waters may occur, but the relative importance will depend heavily on the hydrological, structural and chemical nature of the soil system.