Transfer of phytosanitary products from the soil to the water.

The quality of water migrating towards aquifers depends on the ability of the soil and subsoil it passes through to eliminate the pollutants it contains. The mechanisms that enable surface or underground water to be transformed into drinking water, with its varying levels of dissolved or suspended mineral or organic matter, are the soil's purifying properties.

The quality of France's surface and, above all, groundwater is such that it is a prime resource for supplying water to the population. Where they exist, groundwater is often accessible and abundant, providing water that requires little treatment before it can be distributed to consumers. This natural quality is largely due to the soil, which acts as a filter.

However, on the ground, animal life and, above all, human activity generate quantities of pollutants, whether natural (excrement) or chemical, occasional (accidents) or diffuse (agricultural origin). Is this filter sufficient to depollute water from surface activities? Knowledge of the potential transfer of pollutants from soil to groundwater is obviously fundamental to the sustainable protection of water resources. However, the deterioration of groundwater and improved knowledge of transfer mechanisms are leading us to take a more realistic view. The soil, between the surface and the water table, is an imperfect "living filter". Thus, the migration of pesticides into groundwater is now a matter of course, and the concept of soil filtering all molecules between the surface and the water table is being called into question.

Pollutant transfer modes and times vary widely from one type of pollutant to another and from one type of soil to another. They depend on soil characteristics and humidity, the chemical reactions of molecules with water and the environment, and microbial activity. As a result, an aquifer may be protected against one type of pollution but not another. For example, the Fontainebleau sand table is well protected against microbiological pollution thanks to the filtering power of the sand, but it is ineffective against water-soluble chemical pollution.

This variability in transfer mechanisms makes it difficult to understand filtration processes properly, and calls for a multi-disciplinary approach (pedology, chemistry, microbiology, etc.).

A good understanding of transfer mechanisms is fundamental in helping decision-makers to preserve groundwater resources in a sustainable way, and to make the necessary trade-offs: the extent of protection perimeters, the nature of any changes in agricultural practices to be implemented, the possible selection of sites to be frozen for groundwater protection... The choices are open-ended, but start from a simple and bitter observation:...