Understanding the impact of farming on aquatic ecosystems
Download 432.61 Kb.
|
PesticidesAgriculture and horticulture use over 80% of all pesticides in England and Wales.112 The quantity of pesticides used by agriculture and horticulture has remained relatively stable over recent years though the area that they are applied to has increased and the unit area rate of active ingredient application decreased.113 There are other significant sources of pesticides such as herbicide use in urban areas and on railways. For example, diuron is used only in the amenity sector (not in agriculture) and in 2005 there was a significant increase in the incidence of detection for diuron in water bodies. Pesticides can enter water bodies through a combination of sources and pathways in a catchment including114: run-off through field and urban drainage systems and leaching through soil; spills, spray drift and ‘over-spray’ of water courses; pollution incidents; direct discharges/inputs, disposal routes or via sewage treatment works. There are critical or determining factors (such as pesticide chemical characteristics, climatic, agronomic practices, topographic, soil, and subsoil variability) that control the environmental fate, behaviour and pathways of a pesticide, and the subsequent risk posed to aquatic ecosystems. There appears to be no information on the total loads or apportionment of pesticides in the surface waters of England and Wales. The EA’s WFD Article 5 risk assessment estimated that diffuse pesticide use (agricultural uses and sheep dip) were putting 20.8% of river water bodies (20.4% by length) at risk of failing environmental objectives, and 2.9% (7.4% by length) were at risk because of diffuse pesticide EQS failures. In terms of point sources of pesticides, 0.3% of river water bodies were assessed to be at risk. No separate risk assessments were undertaken for pesticides in lakes or transitional and coastal waters. It should be noted that the risk assessment relied solely upon water quality data to assess the level of exposure pressure and does not take into account ecological impacts that might occur through pesticide contamination of sediments or via the food chain in biota. The EA monitors over 250 different pesticides at about 2,500 sites across England and Wales.115 The great majority of the sites monitored are rivers rather than lakes, for which there is relatively little information. The monitoring covers the pesticides that are used most frequently and in the greatest quantities for both agricultural and amenity use. Data from 2006 shows that for nine indicator pesticides (atrazine, chlorotoluron, 2-4-D, dichloroprop, diuron, isoproturon, MCPA, mecoprop and simazine), 6.5% of freshwater samples had pesticide concentrations exceeding the 0.1 µg l-1 threshold value (the Drinking Water Directive maximum allowable concentration standard for any individual pesticide and hence not necessarily related to possible effects on aquatic organisms). Between 6% and 8% of samples exceeded the threshold value each year from 1998 to 2006. These exceedances were mainly due to the herbicides diuron, isoproturon and mecoprop. The changes between years may be related to weather patterns at the time pesticides are applied since watercourses are more vulnerable to pesticide contamination when application coincides with rainfall. The highest levels of diuron were found in Thames and North East regions. In the case of Thames this was probably linked to the more urban nature of this region and correspondingly greater and more frequent use of this substance. Diuron will be taken off the UK market during 2008. Sheep dip chemicals containing either cypermethrin or diazinon caused over a third of all EQS failures in freshwaters in 2006.116 There were two main sources: sheep-dipping, especially in Wales and the north of England, and effluents from the wool processing industries centred in Yorkshire. In 2006 there were 64 failures of EQS standards for cypermethrin and 16 failures for diazinon in England and Wales, compared with a total of 22 failures in 2005. This large increase is due to the increased monitoring effort of the EA at targeted locations as part of its Pollution Reduction Programme for sheep dip. Although cypermethrin was withdrawn from sale in February 2006, farmers were still able to use up existing stocks. Lower numbers of EQS failures for sheep dip chemicals are expected in 2007. In terms of serious pollution incidents, out of 49 category 1 (most serious) and 2 (significant) incidents in the years 2000 to 2005, 43 were attributable to agricultural uses of pesticides.117 The UKTAG has assessed current compliance with proposed WFD standards for pesticides using monitoring data and face value concentrations.118 The results indicate that the following percentages of freshwater monitoring sites (total sites in brackets after percentages) in England and Wales, respectively, would fail the standards: 21% (316) and 19% (257 sites) for cypermethrin, 4% (485) and 8% (247) for diazinon, and 2.4% (619) and 0% (78) for mecoprop. The equivalent figures for salt water sites in England were 6% (16) cypermethrin, 5% (88) diazinon and 3% mecoprop (73); there were no data for Wales. As monitoring is often targeted where problems are expected, the proportion of failures was considered to be not necessarily representative of all water bodies. It should also be noted that pesticide use that could result in transient, but still damaging, contamination could be associated with seasonal or episodic activities such as spraying with insecticides. Such activity might not have been captured in the current monitoring programme in which sampling is relatively infrequent. EA groundwater monitoring data from 2005119 showed that the most commonly detected pesticide was the herbicide atrazine and its degradation products desethyl atrazine and desisopropyl atrazine. Although the ecological effects of substances in groundwater are not directly considered in the WFD, the achievement of good chemical status of groundwater includes the requirement that associated surface water bodies are able to meet the surface water EQSs or do not suffer any significant diminution of ecological or chemical quality. There is some evidence of improving groundwater quality and a general decline in pesticide concentrations in the Thames region over the last ten years. Current monitoring of transitional and coastal waters for pesticides is carried out as part of the UK marine monitoring programme. The most recent summary of environmental monitoring data for UK estuarine and coastal waters120 concluded that in UK estuaries the concentrations of metals and organic substances rarely exceeded the standards set under the EU Dangerous Substances Directive. No median values exceeded any EQS for the period 1999 to 2001. In addition, estuarine biological water quality measured by toxicity to oyster embryo-larval development was generally very good. The oyster embryo larval test is a robust measure of the biological water quality of discrete water samples. This information indicates that risks from agricultural pesticides to the ecological quality of transitional and coastal waters are likely to be low. However, in 2006 there were 25 EQS failures for tributyl tin (TBT) in marine waters compared with 54 in 2005 and 50 in 2004.121 Problems in the marine environment often occur close to docks and boat yards, such as in the Thames and Mersey estuaries. However, TBT use is not usually associated with farming. The impact of agricultural pesticides on different elements of an aquatic ecosystem will be dependent on the mode of action of the substance(s) involved and, therefore, the taxonomic group(s) which are most susceptible to the substance at typical environmental concentrations. In the case of herbicides, phytoplankton, phytobenthos, macrophytes and macroalgae would be expected to be the taxonomic groups most affected by exposure in the receiving water. For insecticides and molluscicides, invertebrates, crustaceans and molluscs would be expected to be those most likely to be affected by exposure to these substances at typical environmental concentrations. Otters are top predators and are seen as an important biological indicator of the health of rivers. The main factor in the decline of the otter population in England during the 1950’s is believed to have been the impact of organochlorine pesticides though degradation of river and riparian habitats in England, mostly as a result of agricultural intensification, has also been a contributory factor since then. More recently, otter numbers have increased in England and pollution is no longer generally believed to be limiting fish populations sufficiently to constitute a barrier to the spread of otters in England.122 EQSs are established for the protection against both long term, chronic effects and for short term, direct and acute ecotoxic effects. If the EQS is exceeded in the receiving water, the severity of the impact will be dependent on the magnitude and duration (hours, days or months) of the exceedance. Where low concentrations of a chemical are released over a long period sub-lethal effects on growth, development and reproduction in the most sensitive taxonomic groups may be observed whilst releases of high concentrations over a short time scale may result in mortalities of indigenous organisms. It should be recognised, however, that even within the most sensitive taxonomic group not all the species will respond in the same manner to a given concentration and there will be an order of sensitivity. In reality this means that loss of the most sensitive species from a taxonomic group will result in a change in the species composition of the community but will not necessarily result in a change in biomass. This is because a slightly more tolerant species may increase its population to take advantage of the available resources following the loss of the more sensitive species. The nature of the change in species composition will be site-specific and will depend on a range of factors related to the site (including habitat structure and water chemistry) as well as the pesticide involved and the exposure concentration and duration. The EU has currently identified 33 substances (or groups of substances) as Priority Substances under the WFD of which 12 substances or groups are pesticides. EQSs will be established at the European level for these substances in a Daughter Directive to the WFD. The EQSs will be expressed as Annual Averages (AA) or Maximum Allowable Concentrations (MACs). The EQSs as AA concentrations are designed to protect against sub-lethal effects of the substance resulting from long-term exposures whereas the EQSs as MACs are designed to protect against transient short-term exposure at higher concentrations which may result in mortality. These standards are also designed to protect against the secondary poisoning of predators (e.g. birds and mammals) resulting from the consumption of aquatic biota that has accumulated the chemical following exposure. An aim of the WFD is the progressive reduction (or for certain of these substances the cessation or phasing out) of discharges, emissions and losses of Priority Substances to surface waters. In the EU, all pesticides are presently being reviewed to ensure that they satisfy modern standards for safety and effectiveness. As a result some pesticides are being taken off the EU market, either for certain uses or altogether. Atrazine and simazine have been withdrawn in 2007, diuron will be withdrawn around the end of 2008 and isoproturon will be withdrawn in the UK in 2009. EU Member States are required to establish national EQSs for other relevant pollutants not included in the Priority Substance list. To that end, the UKTAG has identified 33 substances for early consideration, partially chosen on the basis of those most often regulated for discharges to water across the UK. So far, the UKTAG has evaluated 18 substances and the UKTAG Consultation Report proposed new standards for nine of these substances including six pesticides. These substances are all currently used in the UK for different agricultural applications with 2,4-D, linuron and mecoprop being used as herbicides and cypermethrin, diazinon and dimethoate being used as insecticides. Download 432.61 Kb. Share with your friends:
|