The
European Union’s (EU) Drinking Water Directive has more recently been
instrumental in bringing pesticide examination of surface waters and ground
waters to the attention of scientists, policy makers and pressure groups. The
Directive established a very low maximum admissible concentration (MAC) of 0.1 µg/l (one part per ten billion) for any
individual pesticide and 0.5 µg/1 for total pesticide content. The
introduction of MAC, provided the yardstick against which the extent of
pesticide pollution in water could be measured and an EU mandate to monitor
drinking water sources in order to demonstrate compliance. This compliance monitoring
has demonstrated the geographical spread of water pollution from pesticides and
shows that pesticides are being detected more frequently and at higher levels in
surface water than in groundwater.
Contamination
pathways
The effectiveness of any control measure will be
influenced by the processes which pollutants reach water. These can be divided
into subsurface and/or surface dominated processes (runoff).
Subsurface
drainage
Subsurface flow is likely to be the major mechanism for
the transport of soluble pollutants in many catchment areas, especially in the
winter when the water table approaches the soil surface. In the UK intensive
agriculture on clay-based soils is generally accompanied by sub surface drainage
which provide routes for the rapid transport of water pollutants from the soil.
In addition, water movement is often aided by the widely-practised supplementary
technique of mole draining. Subsurface drains have been specifically implicated
in the transport of pesticides with a relatively high mobility. Rose et al(1)
reported high concentrations of isoproturon in subsurface drains in Oxfordshire
and Williams et al(2) maintained that drainage systems played a major
role in the movement of simazine in storm runoff from a catchment area in
Herefordshire.
Runoff
Surface runoff is commonplace where flow pathways converge
as a result of local topography and is greatest in coarser texture
Sources
of Contamination
In 1993 the NFU surveyed current practices undertaken
by farmers for the disposal of pesticides containers(3). As many as 92% of all
respondents disposed of paper and cardboard outers by burning, 6% sent cardboard
outers to local authority landfills and only 2% recycled. Plastic containers
were generally washed and again the moat popular method (75%) of end disposal
was burning, 7% buried containers on the farm, whilst 14% used local authority
landfills. Recycling schemes are in their infancy for agricultural plastics and
respondents noted a reticence from plastic recycling companies to accept
pesticide cans. 3% of the respondents were currently involved in trials of
returnable packs. A total of 90% of respondents buried metal containers but only
17% burnt and crushed cans before burying them. At present 4% of respondents
recycled metal containers via the recycling “scrap man”, whilst 5% of
respondents specifically stated that metal containers were not acceptable and
would not purchase products in metal containers.
Washings
and left-over spray
With well-trained operators the question of left-over pesticide should not occur
as only sufficient product should be mixed for the field in question.
Nevertheless, ‘tank washings’ caused by washing out the sprayer prior to
moving to a new product in a different field are an unavoidable feature of
sprayer operation. The Food and Environment Protection Act Code of Practice
issued by the Ministry of Agriculture indicates the appropriate disposal
techniques is to spray these washings over the previously sprayed crop. In well
organised operations an appropriate area of the field is left for this purpose
in order to avoid crop residue problems. The Ministry of Agriculture Code
suggests that sacrificial areas of wasteland could be used for this purpose.
Although freely mentioned in the Code, there are no soakaways approved by the
NRA in the UK.
In the last few years environmental protection equipment
(Sentinel) has been developed by ICI and Allmans which filters these washings
and captures the pesticide residues in a capsule for subsequent disposal and the
treated water may be discharged normally. Regrettably this installation is
expensive and uptake has been limited.
In the NFU’s survey the average volume of rinsate in the
washing operation was 200 litres, ranging from 10 to 1,000 litres. The average number
of rinses was 2.5, ranging from 1 to 5, and the average amount of water used in
the washing process as a percentage of tank size was 38. In-tank nozzles for
rinsing were fitted to 1 in 5 of respondents sprayers and the average reduction
in washings from such equipment was 64%, ranging from 25% to 95%.
A total of 71% of the respondents disposed of their washings
onto the last crop sprayed; 56% sprayed out washings on waste ground. In many
cases respondents said that the first rinses were sprayed out on the crop, and
the later rinses were emptied on waste ground or in the yard. 16% of respondents
said their washings were solely disposed in the yard and 11% of respondents
maintained they had approved soakaways or sumps under their yards. Whilst 8% of
respondents disposed of tank washings on low grade wildlife areas, only 1% of
respondents had Sentinel cleansing apparatus and a further 3% of respondents
disposed of washings on set-aside land prior to ploughing or used unmetalled
farm roads as a soak-away.
The survey also reported on the
frequency of washing the exteriors of sprayers. Only 14% of respondents wash
their sprayers down routinely after use or at the very least daily and 21%
stated that they washed their sprayers regularly and a further 21% washed their
sprayers infrequently on an “as required” basis. Some respondents were
candid enough to state that they wash their sprayers only at the end of the
season and 26% stated they had no routine cleaning programme.
Diffuse
contamination - normal use and pollution
A study by the British Geological Survey in 1986
recognised that accidental spillage or the disposal of containers were not the
only threat to water quality and concluded that the greatest threat came from
the normal use of agricultural pesticides, especially those relatively soluble
herbicides that are widely applied in the autumn for weed controlling winter
cereals. This is a prime example of diffuse pollution which arise from normal
agricultural practice as pesticides make their way from their target to surface
and groundwater over longer periods of time. It is difficult if not impossible
to assign this type of contamination to a specific farmer.
It is now agreed that groundwater is
primarily at risk from normal spray applications. Only 10%-40% of field-applied
pesticides reach their target organism directly and a significant proportion
‘falls’ to the soil. Soil loading is greater for those materials designed
for root rather than foliar uptake. Autumn-applied herbicides, therefore, pose
the greatest risk not only because they are widely used products but also
because they are often applied directly onto bare or sparsely vegetated soils to
kill weeds before they emerge.
Diffuse contamination can be
exacerbated by poorly calibrated equipment. It is a requirement under FEPA Code
of Practice that the qualified spray operator will ensure that the sprayer is
correctly calibrated and maintained. Nevertheless, localised over-dosing can
occur in ‘short work’ and around obstacles but the almost universal use of
tramlines has led to great accuracy in application.
Spraying in unsuitable wind conditions
leads to drift — an equally undesirable phenomenon. Farmers’ intuition,
underpinned by Meteorological Office research which showed that the number of
‘spray windows’ is far higher in the autumn than in the spring, has led
farmers to favour certain autumn applications to balance the farm workload in
the which in spring is usually much heavier for a variety of crop and farm
management reasons.
Avoidance
Minimising use
Between 1974 and 1982, the quantity of pesticides used
in Britain in terms of tonnes of active ingredient, more than doubled but since
the mid l980s the level of use has fallen. There has been a decline of more than
17% between 1980 and 1989. During this period, the area under treatment has
increased by 5%(4). This decline has been caused by a reduction in use and also
in a gradual shift towards more active products.
There is an overall desire by farmers to minimise pesticides
use if only to reduce their variable costs! The reduction of dose rates by the
adoption of new products which are biologically more active and use materials in
the rate of g/ha rather than kg/ha has now become common place. Sulphonylurea
herbicides and pyrethroid insecticides are some of the groups of chemicals that
have made this possible.
Growers are now reducing the rate of
The adoption of these techniques has been facilitated by a
strong advisory system. The UK is somewhat unusual in Europe in that the
majority of pesticide decisions are made by qualified and experienced advisors.
Engineering
controls
Closed transfer systems
The possibility of accidental spillage of concentrated
formulated product can never be completely alleviated. The key issue is that the
operator should be fully trained as to what course of action to take in the
event of accidental spillage. This is a component of the statutory National Test
Proficiency Council (NTPC) training modules in the certificate of competence for
spray operators.
Systems to transfer agrochemical products from pack to
sprayer within a closed system are currently being developed. These follow two
lines: first, those based on the concept of returnable packaging and second
other systems which are generic and connect directly with the universal neck
sizes of agricultural packs. In some cases, the incremental cost of installing
such installations as original equipment on sprayers is relatively small. A
further benefit to the fanning operation is the increase in speed of refilling
with the appropriate amount of agrochemical product which is substantially
faster than can be achieved by hand.
Novel
application methods
Research progresses in devising and refining
application methods. The principal objectives have been the management of
droplet size to ensure a better targeting and the avoidance of drift. Overseas,
the development of spinning disc controlled droplet applicators and the
electrostatic systems have been to the fore. Meanwhile in northern Europe, the
development of twin fluid and air-assisted sleeve sprayers which make use of
conventional agricultural formulations have predominated. The latter systems
also improve work rates as they operate with lower water volumes and in some
instances reduction of dose rate is possible.
Recent research and development seeks to link closed system
transfer with increased application accuracy. Direct injection
Buffer
zones
One of a number of measures being considered to
control pollution of surface waters, is to reduce pollution from diffused
sources by the establishment of buffer zones between the pollutant source and
the receiving waters. Musscutt et al(5) reviewed a number of research
studies which reported positive effects for buffer zones particularly on
reducing sediment loading and phosphorous content in surface runoff and reducing
nitrogen content in diffused subsurface flow. Where substrate drains provide the
major flow pathway through riparian areas, Musscutt et al suggested that
the effect of buffer zones on pollution will be minimal unless some additional
measures are undertaken.
Groundwater
protection
As non-point source pollution from agriculture occurs
for a range of reasons, buffer zones are likely to vary in their effectiveness
according to the source and transport mechanism. The effects of buffer zones on
water quality are:-
the direct effects achieved by preventing spray in vulnerable areas;
the
indirect effects when the pollutants are transported into the zone —
infiltration into the underlying soil, the reduction of surface flow
velocity due to vegetation and the physical filtering effect of dense
vegetation.
Generally,
buffer zones have been reported to reduce the transport of pollutants in surface
runoff. However, certain aspects of their performance have not been fully
quantified. Dillaha et al(6) identified that buffers are more effective
in removing sediment than pollutant. This may impact on transport in the soluble
phase but probably indicates the importance of fine particles to which pollutant
is adsorbed.
Asmussen et al(7) reported that in US, the amount of
2,4-D was reduced by a 25m grass zone by 77% and 76% in wet and dry conditions
respectively but the losses were of a similar magnitude in the soluble phase
too. Water infiltration, sediment deposition and adsorption to vegetable and
organic matter were thought to be the reasons for this major reduction in
pesticide concentration.
The available evidence from plot
studies indicates that certain pollutants transported in surface runoff may be
retained by a relatively narrow buffer. Much of the sediment load can be
effectively removed by a zone less than 5m wide but the retention of fine
particles is more likely to be achieved by a wide zone. The width required to
intercept soluble pollutants is likely to vary according to the local moisture
conditions. Also, a simple linear strip may not be the most effective shape
either. It has been argued that surface runoff is a spatially intermittent
phenomena and that buffer zones should be oriented to take account of the path
of the runoff water. If the zones are placed across the bottom of fields, or in
valleys, or alongside lakes and rivers they can be effective in retaining soil
particles but it remains to be confirmed that they are capable of restricting
the movement of agrochemical products.
Buffer zone vegetation type has also
has an important bearing on the pollutant removal processes. Few studies have
considered the effect of different vegetation types. Sediment and associated
pollutants are more likely to be removed if the increased flow resistance
imposed by the buffer vegetation is sufficient to cause sediment deposition.
This is likely to depend on the density of the vegetation but a variety of
ecological systems can contribute to this role, for example, forests, hedges,
retention ponds and particularly grass floras.
Patty et at(8) grass buffer
strips in the Loire valley on slopes of 10% in fields of continuous wheat. Two
herbicides were chosen for the study because of their different physiochemical
properties. Results confirm that grass can limit the effects of runoff even with
soluble products moving in the soluble phase. Further trials are under way
focusing on the water circulation patterns caused by local topography.
The long term performance of buffer
zones is also unclear. Where natural riparian areas have survived,
pollutant removal has been observed. Nevertheless, it has been suggested that
such areas may achieve an equilibrium in that pollutant inputs are initially
retained and then subsequently released. This is particularly the case with
fertiliser pollutants, notably phosphorous, whether this is the case for
pesticides remains to be ascertained.
Specific
aqueous hazards
Recently the Pesticides Safety Division (PSD) at the
Ministry of Agriculture, has granted approval to a number of pesticides subject
to a 6m “buffer zone” when sprayed adjacent to ditches or watercourses.
Apparently this stipulation results
from concerns arising from an assessment of acute risk to aquatic life,
resulting from direct over spraying of surface waters. Notwithstanding, the
technical justification for such a restriction on ecotoxicological grounds, this
restriction implies that farmers routinely overspray dykes and watercourses,
which is far for the case!
Economics
current practice
The possibility of reducing agrochemical input per
se has an obvious and immediate benefit. Much of the current trend towards
reduction of pesticide usage is fuelled by sound economic reasons. The
profitability of modern farming is severely challenged and ‘luxury’ use of
pesticide to ensure showpiece fields is no longer economically justifiable. The
use of economic thresholds and tolerance models for weed and pest infestation
levels which do not affect marketable yield is now common.
Agrochemicals represent 8% of European farmers’ turnover
and in the UK 19% of the variable costs of winter cereals are carried in
agrochemical expenditure. The use of generic products which arc cheaper than
proprietary compounds is increasingly appealing to some farmers. In winter
cereals this tends to lead farmers towards autumn herbicide applications as the
price differentials are significant.
Restricted
zones
The use of buffer zones to mitigate the effects on
specific hazards, e.g. waterways, requires careful economic evaluation. Not only
does the impracticality of such restrictions impinge upon management costs but
also in agronomic terms the potential threat for reinfestation of pests and
diseases from the unsprayed zones is real and could lead to increased
agrochemical usage in the rest of the field, for example, reinfection from
potato blight which in turn has a major economic implication to the fanner.
Imposing 6m buffer strips means in practice on some Fen farms that 8% would
remain either permanently unsprayed or unsprayed with the product of choice.
Such economic arguments require careful assessment alongside the technical
rationale on the width required for such specific hazard buffer zones.
PSD by seeking to enforce such a label
restriction seem to presume that sprayer operators are unable or unwilling to
control their equipment in such a way as to avoid contamination in water
courses. It has been suggested that this requirement is unenforceable.
The use of buffer zones to control
diffuse contamination is another matter. Further information is required for
their appropriate design and management. However the removal of riparian areas
from intensive agriculture use may produce a range of other benefits including
enhancement of the conservation, ecological and amenity value of the landscape
through improvements in both aquatic and terrestrial environments.
As yet, there has been little
assessment of the economic benefits of buffer zones. Any research should take an
integrated approach so that buffer zones may be designed to produce the maximum
range of benefits.
The possibility of implementing
restricted zones for pesticides similar to the nitrate vulnerable zones is being
considered in certain quarters. It has been suggested that sets-side under the
CAP reform should be viewed flexibly as an opportunity to offset adverse effects
to buffer zone imposition on farm businesses.
Future
challenges
Farmers throughout the UK accept they have a duty to
steward the land which they farm. Farmers are running small business,
Several economic instruments are under consideration in
Europe to encourage pesticide use reduction. Undoubtedly farmers would like to
see the careful evaluation of risk versus benefit related to the cost of food to
consumers if production costs were increased by scientifically unjustified
measures.