Moradtours

G.L. Harris

_{and A. Forster}

1_{ADAS Land Research Centre, Meden Vale, Mansfield, Nottinghamshire NG20 9PF, UK.} 2_{Huntingdon Life Sciences, PO Box 2, Huntingdon, Cambridgeshire PE18 6ES, UK.}

Abstract

The movement of pesticides to surface waters has become an area of concern across Europe and other countries where pesticide usage is a key part of crop management. Pesticide losses to surface waters can be rapid; as a consequence, remedial measures may have a more or less immediate effect in reducing contamination, and resulting environmental impact. One such measure attracting increasingly widespread interest is the use of buffers generally considered to be best located close to, or adjacent to, surface water courses. However, the mechanisms by which buffer zones can control pesticide loss are not well understood, neither is the optimum design and function of buffers always clear. This review paper considers the mechanisms and importance of pesticide transport to surface waters and assesses the evidence that indicates whether buffers can be effective in protecting both water quality and the environment. In particular, the paper examines research which addresses the appropriate design of buffers and assesses the potential long-term role for these landscape features.

INTRODUCTION

As agriculture has intensified, an increasingly wide range of pesticides has been found in surface waters draining agricultural catchments at concentrations above that considered appropriate for potable water and wildlife (White and Pinkstone, 1995). In the EU, Williamset al. (1991), Gillet (1991), Harris et al. (1993) and Traub-Eberhard et al. (1995) are among many who have reported studies showing appreciable pesticide losses. In the US, Asmussenet al. (1977), Neely and Baker (1989) and Bengston et al. (1990) all reported pesticides in surface runoff at concentrations considerably above 0.1µg/l. Most workers have reported that peak pesticide concentrations were found soon after application and the initiation of surface runoff or drainflow, often with concentrations higher in the US than in the EU. Leonard (1988) in particular, suggested that these higher peak pesticide concentrations in the US were due to more intense storms than are found in Europe.

Pesticide movement has been reported both in solution and in the particulate phase. Flow paths for each can be different and the likelihood of a pesticide being absorbed to particulates will depend on its adsorption properties (Marshall et al., 1996). As a consequence, measures introduced into the landscape to address pesticide movement must reflect these different flow paths.

The conversion of streamside margins and riparian areas to provide a buffer to pesticides reaching watercourses has been reported widely (Muscutt et al., 1993; Norris, 1993). In particular, in the US, vegetated filter strips are an approved ‘Best Management Practice’ which is part-funded by the US Department of Agriculture. Also, in the US, widespread installation has occurred along all perennial streams (Dillaha, 1989). Other countries have also adopted similar approaches. For example, in some Scandinavian countries vegetated buffer zones are already widely used alongside lakes to control contamination (Keskitalo, 1990) whereas in New Zealand a policy of ‘retirement’ of riparian zones has been recommended to protect aquatic habitats (Smith, 1989).

This paper reviews the information available on the mechanisms of diffuse pesticide transport from agricultural soils and assesses the impact that buffers and a buffer policy could have on pesticide concentrations and the biodiversity of surface waters.

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Buffer zones to reduce pesticide contamination

MECHANISMS OF PESTICIDE MOVEMENT

Surface runoff

Surface runoff can occur when the surface soil becomes saturated or when rainfall intensity exceeds the infiltration capacity of the surface soil aggregates. Agricultural management can influence water movement. For example compaction of the soil surface by machinery or the use of permanent tramlines (Harris, 1995) can increase surface runoff.

Many pesticides are readily carried in surface runoff at concentrations that could affect both catchment water quality and stream biodiversity; this process applies particularly to pesticides only weakly adsorbed (Asmussen et al., 1977; Wauchope and Decoursey, 1986; Leonard, 1990; Harris et al., 1994; Brownet al., 1995).

Sub-surface runoff

Sub-surface flow is widespread in many surface water catchments, particularly in clay-based soils without pipe drainage. Where permeability is particularly low, permanent pipe drains will be linked to a secondary drainage treatment to improve water movement (Harris, 1995). In such soils the presence of macropores is particularly important to the flow of water (Beven and Germann, 1982). In contrast in the US, and in more permeable EU soils, surface drainage, or drainage without secondary treatments, is more common and considerable research has focused in these regions on surface runoff (Leonard, 1990).

Pesticides are often applied in the autumn, just before the onset of drainage, or in the spring when soils are still relatively wet and hence rapid losses can occur. Once a pesticide is applied to the soil, the likelihood of movement to sub-surface drainage will depend on the properties of the chemical itself (especially mobility and degradation), soil structure and organic matter content, the period of time between application and drainflow, and the background soil moisture and temperature conditions (Jones et al., 1995). However, some groups of pesticides are more likely to be lost than others, for example herbicides, such as isoproturon, which are readily mobile and remain in the soil for many months (Monke et al., 1989; Williams et al., 1991; Harris et al., 1994). In contrast strongly adsorbed pesticides have only been found at very low concentrations and with low overall losses (Joneset al., 1995).

Particulate transport

Wauchope (1978) has suggested that only pesticides with a solubility in excess of 10 ppm are likely to be lost primarily in the water phase. However, it is evident that many pesticides are found in particulate matter and that this may be an important transport route in some conditions. Buttle (1990) found 20-46% of the losses of the pesticide metachlor (a moderately soluble herbicide) were carried in sediments over the reporting period. Houseet al. (1992) and Worralet al. (1993) have also highlighted the importance of this mode of transport. Houseet al. (1992) found simazine and atrazine in water in three UK catchments, but lindane, DDT and other strongly adsorbed pesticides were also detected in bed and suspended sediments. Recent studies, for example those reported by Marshallet al. (1996), are investigating the source of the particulate matter involved and will be important in determining the most appropriate control measures for this route of pesticide transport.

Spray drift

A further source of contamination not always considered is that from spray drift, directly into the watercourse, which can pose a further risk to aquatic life. Numerous studies have shown that spray can drift over considerable distances (Harris et al., 1992; Ganzelmeier, 1993; Lloyd and Bell, 1993), although for a typical arable crop the volume of the original spray application that travels in excess of 6 m may be no more than 1%.

POTENTIAL FOR BUFFER ZONES TO REDUCE PESTICIDE LOSSES