dc.description.abstract |
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. |
en_UK |