3.2 Unintended spreading on or through the soil

As a result of regular spraying, pesticides inevitably will come into contact with the soil. This is also the case when pesticides are emptied or spilled on the field, in watercourses or in other places. Such direct discharges can cause particularly high and acute pesticide concentrations. What follows below is an outline of how pesticides can be spread to surrounding areas by surface water, or by being transported by the water through the soil, or by the root systems of plants, before ultimately being released into the soil.

a) Surface leaching: Plant protection agents generally have the same leaching pattern as nutrients and soil particles. The leaching pattern can vary from year to year, from place to place, and depends on weather conditions, topography and the nature of the soil.

Sorption to soil particles. The degree of the pesticide's sorption to soil particles depends both on the pesticide and the nature of the soil. Pesticides with strong sorption to soil particles are especially likely to be transported by surface water. This usually occurs immediately after heavy rainfall, and can result in pesticidic ponds in low-lying areas, or in polluted brooks and rivers.

Soil preparation: The extent of soil preparation usually has an effect on the surface transport of pesticides. Less soil preparation reduces the loss of pesticides with strong sorption capacity, which mainly are transported by soil particles. However, the extent of soil preparation has no influence on the loss of pesticides with weak sorption to the soil, since they can be transported easily by surface water without being attached to soil particles.

Soil erosion has an effect on the spreading of pesticides. If there is heavy erosion many soil particles disappear, and along with them the pesticides. Certain methods and the time of soil preparation can increase the risk of erosion. Erosion is often caused by heavy precipitation (see booklet No. 1 "Agriculture" and No. 7, Water supply").

b) Spreading through the soil: The following conditions can affect the pesticide transport through the soil: the chemical nature, drainage and structure of the soil and the root systems of plants.

The chemical nature of the soil: The mobility of the pesticide is affected by its solubility in water and the binding to soil particles - the greater solubility and weaker binding, the greater mobility. A phenoxide such as MPCA is highly soluble, whereas DDT is practically non-soluble. The solubility depends among other things on whether the pesticide is acidic or alkaline. In the tropical climates the soil is generally acidic and contains iron and aluminium oxides (see booklet No.1 "Agriculture"). Kaolinite is the most common clay soil. Kaolinite, oxide and hydroxide are contributing factors to low ion exchange capacity. When this occurs in acidic soils, pesticides with negative or neutral ion charge are easily leached out. The soil in tropical climates generally contains little organic material and clay. This is a highly contributing factor to increasing the mobility of the pesticide or causing it to be leached out of the soil.

The drainage and structure of the soil: The transport of pesticides through the soil also depends on the amount of water in circulation and the saturation capability of the soil. If the soil contains a lot of water, which can occur after heavy rainfall, pesticides will be leached out easily. Pesticides can thus pollute the ground water (see chapter 3.3). The saturation of the soil depends on the amount of cracks, worm holes or old root systems. When there is a surplus of water in the soil, tile drains can also transport water containing pesticides. Both water soluble pesticides as well as pesticides with strong binding to soil particles will be transported through soil profiles with many cracks. However, in compact soil such as clay soil, which is rich in organic materials, it is mainly water soluble pesticides and pesticides with weak binding to soil particles which will be washed out. Soil preparation and a more permanent vegetation surface will prevent the water from flowing downwards and thus reduce the vertical transport of highly soluble pesticides.

Spreading by the root systems: Pesticides can be absorbed by some plants and transported through the plant to the root system where it is subsequently released into the soil. This can happen relatively quickly, but is usually of little importance because only small quantities are being transported.

c) Mitigative measures: The following measures can be taken to hinder unintended transport of pesticides by water:

· To hinder acute discharges by:

- Establishing safe conditions for diluting the pesticide and filling it on sprayers.
- Establishing safe pesticide handling routines.
- Establishing safe routines for cleaning the spraying equipment.

· Pesticide resits in sprayers may be avoided by diluting a smaller quantity of pesticide than is normally recommended for the last portion, and then using it more sparingly.

· Very poisonous pesticides should not be used on fields sloping down to watercourses during seasons with heavy precipitation.

· The risk of soil erosion and surface flow should be reduced.

· Pesticides with high risk of leaching, such as triazines, should not be used on porous soil above important ground water sources. The ground water then risks being polluted for several decades.