Effects of clay microstructure and compost quality on chlordecone retention in volcanic tropical soils: consequences on pesticide lability and plant contamination

The scientific and economic context of our study is related to the pollution of the soils, fresh and marine water by a persistent organochlorine pesticide (chlordecone) in a tropical context (French West Indies). The former application of chlordecone results today in a diffuse pollution in agricultural soils, which are sources of contamination for cultivated roots, tubers, vegetables and terrestrial and marine ecosystems. Chlordecone is a very though and stable molecule (considered as a POP), it is mainly present in solid phase and has a strong affinity with organic matters. To prevent consumers and ecosystems exposure, it is thus necessary for us to evaluate the factors that influence chlordecone migration in the environment. In our research, we studied the impacts of clay microstructure on the chlordecone retention, comparing allophanes (amorphous clays present in andosols) and halloysite clays (type 1/1). We showed that allophane aggregates had a greater ability to trap chlordecone mainly due to their fractal structure. We also measured the effects of added composts on soil microstructure and on chlordecone lability and transfer rate from soil to plant 3 and 6 months after incorporation. The intensity and persistence of these effects were related to the initial quality and richness of the added composts. (Résumé d'auteur

Soil microstructure and added organic matter: keys for chlordecone sequestration

The scientific and economic context of our study is related to the pollution of the soils, fresh and marine water by a persistent organochlorine pesticide (chlordecone) in a tropical context. The former application of chlordecone results today in a diffuse pollution in agricultural soils, which are sources of contamination for cultivated roots, tubers, vegetables and terrestrial and marine ecosystems. Chlordecone is a very though and stable molecule (considered as a POP), it is mainly present in solid phase and has a strong affinity with organic matters. To prevent consumers and ecosystems exposure, it is thus necessary for us to evaluate the factors that influence chlordecone migration in the environment. In our research, we studied the impacts of clay microstructure on the chlordecone retention. We showed that allophane aggregates had a great ability to trap chlordecone mainly due to their fractal structure. We also measured the effects of added composts on chlordecone lability and transfer rate from soil to plant . We show that compost addition modifies the fractal structure of allophane clays favouring the chlordecone retention. These results allow us to propose a new strategy in opposition to the complete soil decontamination: the chlordecone sequestration. ( Résumé d'auteur

Approche physique de la décontamination des sols

Les sols contaminés par la chlordecone sont à l'origine des contaminations secondaires de produits entrants dans l'alimentation de la population antillaise (eau de consommation, fruits, légumes, viandes). Les sols sont donc le paramètre clé de ce problème sanitaire et différents travaux de recherche essayent de proposer des solutions pour décontaminer les sols. Cependant, les techniques de remédiation connues, comme l'excavation, la phytoremédiation, la biodégradation, la réduction chimique sont encore à l'étape d'études pour la chlordecone. D'autres part, tous les types de sols ne semblent pas réagir de la même manière avec ce pesticide et notamment leur capacité à retenir cette molécule est dépendante du type de sols. Des travaux précédents ont montré que les andosols (sols volcaniques jeunes) sont fortement pollués mais paradoxalement seraient moins contaminant pour l'eau de ruissellement et les légumes qui y sont cultivées. Les andosols se différencient des autres types de sols parce qu'ils sont constitués d'une argile spécifique formée de nano matériaux : l'allophane. Nous nous intéressons à la structure physique particulière de cette argile (fractale de type labyrinthe) à des échelles comprises entre 5 et 100 nm. Nous calculons les propriétés de transport dans les argiles et montrons que les propriétés physiques (perméabilité, conductivité hydraulique, coefficient de diffusion) sont très faibles (diminution de 1 à 3 ordres de grandeur) à l'intérieur des agrégats d'allophane. Cette structure nano poreuse confère à l'argile des propriétés spécifiques : une forte tortuosité, des faibles capacités de transfert et le confinement des pesticides. L'approche physique démontre que l'argile allophane est capable de piéger à long terme la chlordécone et ces caractéristiques de confinement et de piégeage nous ont conduit à : 1) poser la question de l'accessibilité de la chlordecone présente dans les sols à d'éventuelles techniques de décontamination, 2) conclure que pour certains sols il sera physiquement pratiquement impossible de décontaminer, 3) proposer une alternative à la décontamination : la séquestration accrue de la chlordecone dans les sols, par l'ajout de matière organique. Nous montrons que l'ajout de matières organiques dans les sols diminue notablement le transfert sol /legume et sol/eau (facteur 3 à 10)

Compost addition to polluted soils to ensure fruit and vegetable safety

Pollution affects food safety, environmental quality and human health. ln the case of persistent soil pollution such as chlordecone in the French West Indies, horticultural crops can be so polluted as to exceed maximum residue limits. Generally, there is no remediation solution for these polluted soils. Interactions between organic matter and pesticides are known to help reduce the bioavailability of pesticides in soils. Sequestering pesticides in soils by adding compost could be an alternative way of reducing their diffusion into food chains. We added 5% organic matter to the soil upper layer in horticultural fields and pots. The soil to crop transfer for radish, lettuce and cucumber decreased from 33 to 75% according to the crop and the soil type. We demonstrated that sequestration depended on the soil type and the clay physical properties. In nitisols, chlordecone sequestration was mainly due to chemical interactions with the compost particles whereas in andosols, it was partly due to the physical impact of the added compost (allophane collapse). Increasing soil organic matter content reduces pesticide mobility in the soil and hence its bioavailability for plants. Adding organic amendments to soils is a promising way to reduce the risk of pesticide contamination of food and thus decrease human exposure to potentially harmful chemicals

Caractérisation de l'hétérogénéité d'une pollution persistante : cas de la chlordécone aux Antilles

L'utilisation de la Chlordécone, polluant organique persistant, pour lutter contre le charançon du bananier il y a plus de vingt ans a abouti à une pollution durable et diffuse des compartiments environnementaux et à l'exposition chronique des populations et des écosystèmes. Aujourd'hui, le polluant se disperse à partir du sol des parcelles polluées où il est stocké. Aussi, la caractérisation du niveau de pollution de chaque parcelle est une étape incontournable pour évaluer et gérer le risque de transfert vers les cultures et les eaux. Dans ce travail, nous avons mis en évidence l'hétérogénéité spatiale, horizontale et verticale, de la pollution à l'échelle de la parcelle et ses déterminants. Le type de sol et sa teneur en matière organique, la stratégie de l'exploitation, les pratiques postérieures à l'application du pesticide (travail du sol... ) modifient les niveaux de pollution ou la disponibilité de la molécule des horizons du profil de sol considérés. Le travail du sol profond et régulier dilue la pollution sur le profil. Ce travail a contribué à élaborer un outil d'aide à la décision pour l'échantillonnage. Il servira de base à la caractérisation d'autres pollutions diffuses issues d'applications non homogènes dans l'espace. (Résumé d'auteur

Sequestering a persistent organochlorine with organic fertilizer and organic amendment to increase food safety in Martinique

Chlordecone is an environmentally persistent insecticide that was intensively used in banana cropping systems. Although its use was restricted 20 years ago, chlordecone permanently pollutes soils and consequently continues to contaminate crops, water resources, and food chains. This pesticide was recently suspected of being implicated in the increasing incidence of prostate cancer and in the impaired development of young children. Given the proven risks to human health, the release of chlordecone from polluted soil needs to be controlled to reduce contamination of the different food chains. Natural decontamination of the soil through lixiviation will take decades to centuries. The persistence of chlordecone in soils is explained by i) its physicochemical properties: low solubility in water, hydrophobicity, which gives it a high affinity for organic matter and ii) its poor biodegradability related to its peculiar chemical structure with high steric hindrance. Chlordecone pollution concerns a large area, meaning chlordecone-polluted sites require efficient cost-effective in situ treatments. To date, phytoextraction and microbial degradation have not really been efficient in the case of diffuse pollution of chlordecone in the soil. Thus, sequestering chlordecone in the soil could be an alternative way to reduce its availability for crops and water resources. We then chose to study the ability of two different organic matters to sequestrate this molecule in the two main contaminated soil types presenting different characteristics: andosols, with amorphous clays (allophane), and nitisols, with crystalline 1/1 clays. We tested two types of organic matter: an organic fertilizer and an organic amendment. For each experiment (in microcosm, in pots and at field scale), the same quantities (5% w/w) of organic fertilizer or organic amendment were incorporated into both contaminated soils. During these experiments, we measured during three months the effects of organic fertilizer and organic amendment incorporation into soil on (i) chlordecone leaching by water, (ii) chlordecone bioavailability for susceptible crops (radish, cucumber, lettuce) and contamination of their different organs (roots, tuber, fruits, leaves) (iii) chlordecone distribution in different soil fractions (0-50?m, 50-200?m, 200-2000?m), (iv) soil microstructure (pore size distribution, specific surface area, mesopore volume and nanoscale structure). A quick decrease in water extractable chlordecone was observed in amended soils and then its transfer from soils to plants. Depending on the soil and the crop, the soil-plant transfer was also reduced by a factor of 1.9 to 15 when organic matter was incorporated. Chlordecone distribution was modified by the incorporation of both organic materials leading to a transfer of the molecule from the finest to the coarser fractions according to the organic matter size distribution, thus raising a "sponge" effect, mainly observed on the nitisol, of the added organic matter on the chlordecone probably due to the high affinity to organic matter and hydrophobicity of this molecule. For the andosol, the results showed that the changes in chlordecone distribution in the different soil fractions were not as dramatic as those in the nitisol. This raises the question of the specific physical and structural properties of the allophanic clays. Considering the impact of organic matter on soil microstructure seen though the pore size distribution, after the addition of organic matter, the mesopore structure was not really affected in the nitisol, even after 3 months. These results clearly differed from those observed on andosol where the addition of compost led to a marked reduction in mesoporosity. After 90 days, the pore size distribution was still strongly affected by both composts (loss of 80% of the mesopores). Results also show a clear decrease in the specific surface area and specific mesopore volume of the andosol after addition of orga

Long term organochlorine soil pollution in agriculture: The lessons learnt from the Chlordecone pollution in French West Indies

The French West Indies face nowadays a diffuse and long term environmental pollution related to historical use of organochlorine insecticide in banana fields, Chlordecone (CLD). Due to its stability, it now pollutes soil, waters, agricultural products and leads to a global exposure of people and ecosystems. We wonder how to manage such a complex pollution system involving all environmental compartments, linked each other. For that, we have been conducting research studies for more than 15 years in the field of agronomy and environment to explore the fate of the molecule in the environment, the impact on food safety, and also the remediation options. Three main points are addressed: i) how to characterize the pollution and to make easier the diagnosis? ii) how to assess the impacts on environmental compartments and agricultural products? iii) How to manage the pollution? The tools we developed make a diagnosis of such pollution easier at different scales (field, farm, watershed, and territory). We built monitoring support systems for the water quality of rivers with models helping to understand variability of water contamination. We built also decision support systems to farmers to manage their soil pollution and choose the crops that will ensure food security. Now CLD content of local crop products on the market complies with the Maximum Residue Limit. Management is more complex for animal products, and further investigations are needed. We investigated alternative solution to enhance CLD soil sequestration, using physical properties of French West Indies volcanic soil and organic matter amendment. Increased organic matter content in soil reduced drastically the CLD transfer from soil to water, but this effect was not perennial and amendment had to be regularly applied. Our results show that an integrative approach is needed to build efficient policies to manage such pollution than to prevent new ones