**4. Discussion**

When dealing with agricultural soil pollution, two major tasks must be included in the risk and management analysis: the characterization and analysis of soil physical properties and farmers' past and present practices (Figure 10). These two domains provide information on the level of soil pollution, on the behavior and fate of the pollutant in the soil, and on its remediation potential.

**Figure 10.** Key determinants of CLD soil pollution.

**Figure 9.** CLD transfer in radish organs (leaves, tubers and fine roots) with added compost (in grey) and without (in

These experiments also showed that adding compost closed the microstructure of allophane clays, thus favouring CLD retention in allophanic soils. Adding compost altered the porosi‐ ty of the allophane clay in the size range 10 to 60 nm, while the intensity of this effect varied with the allophane content. We suggest that these pore changes are the consequence of ca‐

Organic matter (OM) amendments aim to modify chemical conditions within the soil pro‐ file. As mentioned above, the OM soil status influences the availability of the pollutant, a factor of primary importance for both pollutant transfer and degradation [55]. At field scale, OM amendments modify the potential sequestration of persistent organic pollutants in the soil by enhancing the soil's sorption capacity for CLD [56]. This sorption capacity depends on the quantity of OM supplied, the type of OM (stable vs. labile) and the frequency of the application. This practice needs to be studied over time, as OM degrades and could modify

Finally, all these practices depend on farm strategies. When the aim is to modify these prac‐ tices, both the scale and the type of farm need to be taken into account. In the case of CLD pollution, analyses revealed a strong "farm effect" [10]. A typology should be built includ‐ ing the farms' overall strategy and objectives, the types of crops grown, practices (more or less intensive) and the farms' specific field orientation (how often the land is used for each

When dealing with agricultural soil pollution, two major tasks must be included in the risk and management analysis: the characterization and analysis of soil physical properties and farmers' past and present practices (Figure 10). These two domains provide information on

pillary stress and of the low mechanical properties of the fractal structure [54].

white).

crop).

**4. Discussion**

the OM – pollutant relationships.

628 Environmental Risk Assessment of Soil Contamination

For soil physical properties, porosity can range from micro-, to meso- and macro-porous in volcanic soils. Fluids containing inorganic and organic solutes and gaseous species can occu‐ py the pores and several factors (size, shape, distribution and connectivity of the pore geo‐ metries) determine how fluids migrate into and through the porosity and ultimately adsorb and react with the solid surfaces. The low hydraulic conductivity calculated for fractal allo‐ phane aggregates thus explains the high pesticide content of these materials. Because of the resulting low hydraulic conductivity, fluid exchange is slow and pesticide bioavailability is consequently reduced. This leads to the accumulation of pesticides that are not easily chemi‐ cally or biologically transformed. Future studies on pesticide degrading microorganisms [57] and other bioremediation tools to clean up polluted soils should take the high soil or‐ ganic carbon content and CLD accessibility in volcanic soils into account.

At field scale, past pesticide application practices account for the potential stock of pollutant in the soil, and an historical analysis will improve the initial diagnosis in terms of the quan‐ tity (doses) of pollutant and its horizontal distribution. Modeling and mapping are appro‐ priate tools to roughly simulate current levels of pollution in the field and to identify the parts of a field that contribute to environmental pollution (water, food, animals). Current practices explain the diffusion of pollution from the soil reservoir: tillage accounts for the vertical distribution of the pollutant, i.e. dilution of the pollutant in the soil profile, and or‐ ganic matter amendments account partially for the availability of the pollutant. In this way, current practices modify the level and volume of polluted soil and the environmental state of the field (surface conditions, OM content, soil profile, etc.), which in turn, influence the fate of the pollutant in the environment. Variability among and within farms also needs to be taken into account by building a farm typology including the general orientations and strategies of the farm and farm practices.

Finally, at the regional scale, all these factors combine to determine soil pollution. In the case of CLD, in agreement with the results of physical analyses, andosols, which contain allo‐ phanic clay and have high organic matter content, are the most polluted. In addition, the ef‐ fect of agricultural practices was evidenced by a strong farm effect and an effect of how long the land was used for banana cultivation.

These two aspects (agricultural practices and soil physical properties) proved to be effective in the diagnosis of CLD pollution and in management analysis and could make sense for other forms of agricultural pollution, especially in the case of persistent pesticides. To illus‐ trate this point, the WISORCH model, which accounts for agricultural loads as well as soil and climate characteristics, can simulate changes in soil pollution over time. In the case of CLD contamination, simulation results showed that soils will remain contaminated for six hundred years, with andosols decontaminating at a slower rate than other soils.
