**1.2. Objective**

This chapter explores the multifactorial nature of soil pollution and its evolution at field and regional scale. We focus on two key factors that determine CLD contamination and disper‐ sion in ecosystems. One is physical and the other anthropogenic. The first is **the influence of clay microstructure** on (1) the concentration of CLD in the soil, (2) the bioavailability of the

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pollutant for crops and water resources, and (3) the efficiency of the remediation process. The second factor is past and present **farming practices** through their effects on (1) the stock of pollutant and its components in the soil and its distribution at the plot and regional scales, and (2) the diffusion of the pollutant in the different compartments of the environ‐ ment. These two factors determine the contributing areas and their pollutant stock, the po‐ tential availability and fluxes of the pollutant, and hence its potential transfer to different environmental compartments.

**Figure 1.** The chlordecone molecule (C10Cl10O)

#### **2. Influence of clay microstructure**

The studied soils come from Guadeloupe (16°15 N, 61°35 W) and Martinique (14°40 N, 61°00 W) in the French West Indies. These volcanic islands rise to 1467 m and 1388 m elevation respectively. Rainfall is high and ranges from 1000 to 10 000 mm/year depending on the ele‐ vation and geographical area. All primary minerals of andesitic rocks are weathered, so that soils have a high content of secondary minerals (clays): halloysite for nitisol, halloysite and Fe-oxihydroxides for ferralsol, and allophane for andosol, the three main soil types contami‐ nated in French West Indies [1, 10, 16].

These volcanic soils have a high infiltration capacity (saturated hydraulic conductivity greater than 60 mm h−1) [1, 10]. However in the ''clay'' matrix of andosols, there are pores smaller than 1 micron, where water and solute transfers are slow. All these soil types are acidic (pH = 4.5 -6), which prevents clay dispersion and sheet erosion [1]. Among these soils, the carbon content of andosol is particularly high, which may influence the retention of the compound (stock) and its availability.

Indeed all soils are not equivalent in terms of pesticide contamination and in their ability to transfer the pollutant to water and to plants [17, 18]. For example, although andosols are highly polluted [1, 10, 16], data show they release less pesticide to percolating water and crops than other soils [1, 19]. In the case of CLD, one explanation for the retention effect re‐ ported in the literature is the high organic content of these soils and the high affinity of the pesticide for soil organic carbon [1, 8].

However, these volcanic soils contain amorphous clay, allophane, whose structure and physical properties differ from those of the crystalline clays found in nitisols and ferralsols. Allophane clay is amorphous and has physical features that closely resemble those of nano porous materials: large pore volume and water content, a broad pore size distribution, a high specific surface area and a fractal structure [20-22]. One objective of this chapter is to show the influence of the clay microstructure on the accumulation and retention of chlorde‐ cone in soils.
