Predicted impact of climate change: pesticides in groundwater

The British Geological Survey has undertaken an assessment for the Department of the Environment Food and Rural Affairs of the impacts of climate change on the fate and behaviour of pesticides in the environment. The approach taken was to look individually at the sources of pesticides, the movement of pesticides through the subsurface (pathways) and the receptors, specifically the groundwater resources used for water supply and the rivers fed by groundwater. This source-pathway-receptor analysis is presented below to illustrate the complexities resulting from the interrelationship between different environmental factors, and between these and man’s activities. The study concluded that, in the long term, land-use change driven by changes in climate may have a more significant effect on pesticides in the environment than the direct impacts on pesticide fate and transport.

Sources

• Changes to cropping patterns mean that changes in pesticide use will be limited to a shift in the amount of certain classes of already approved pesticide. Thus there is likely to be little change to the range of active ingredients used in the UK as a result of climate-induced change in cropping patterns
• Increased prevalence of existing pests, diseases and weed species is predicted under climate change, implying wider and more frequent applications of existing pesticides or the introduction of products that are new to the UK
• The tolerance of crops to pesticides may reduce under higher levels of ozone. If this is the case, increased pesticide applications may start to reduce crop yield and this may lead to the manufacture of new active ingredients
• Higher temperatures would mean a greater proportion of pesticides in the gas phase, and consequently longer potential atmospheric transport distances from sources, however, degradation processes will also be accelerated, so the overall effect of higher temperatures is not clear
• The rate of atmospheric degradation of vapour-phase pesticides may increase due to higher concentrations of tropospheric ozone
• Changes in seasonal rainfall may lead to changes in the spatial and temporal distribution of wet deposition of airborne pesticides and their degradation products.

Pathways

• Higher temperatures will give rise to increased volatilization and faster degradation of pesticide residues in the soil and surface waters
• Increased winter rainfall will lead to wetter soils and so pesticide-rich water will move more rapidly from the surface through the soil matrix
• More by-pass flow due to more intense rainfall events will increase the likelihood of rapid pesticide movement to drains, surface waters and vertically to below the soil layer
• Surface runoff may become more significant leading to increased erosion of pesticide-rich soil particles from fields to drains and surface waters
• Shorter recharge periods could allow less opportunity for pesticide transport and more time for degradation
• Higher soil water contents will increase degradation rates in soils re-enforcing temperature effects while warmer dryer summers may lead to increased cracking of shrink-swell clay soils and may increase the potential for by-pass flow in autumn and winter. The net effect may be to increase the speed at which pesticides are moved by by-pass transport routes in the winter
• Dry soils have a lower biodegradation potential than wet soils and pesticide residues from spring application could persist through the summer to the autumn
• More frequent and intense storm events will affect groundwater recharge. Periodic flushing into aquifers of pesticides sorbed onto colloids and sediments may be enhanced.
• High groundwater levels may occur more frequently due to increased winter rainfall. Periodic high groundwater levels can intercept pesticides and other diffuse agricultural pollutants in the unsaturated zone and soil zone, reducing the time for pesticide degradation and leading to seasonal increases in the concentration of pesticides in groundwater
• More frequent localised groundwater flooding of agricultural land may mobilise pesticides sorbed onto colloids and sediments.

Receptors

• In summer, mean river flows may decrease significantly contributing to a reduction in the dilution potential of surface water bodies potentially increasing pesticide concentrations if runoff or spray drift events occur
• Reduction in annual minimum groundwater levels by the 2080s are expected (e.g. up to about 2 m in the Chalk) but the implications for source yields and pesticide exposure at receptors is uncertain
• The impact of climate change on baseflow to groundwater-dominated rivers has not been systematically studied, and the implications for changes in pesticide exposure is very uncertain.

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