Quantification of erosion sources in a tropical volcanic insular catchment (Galion river, Martinique, France): application of sediment tracing tools to coastal marine environment

Between 1972 and 1993, in the French West Indies (Martinique and Guadeloupe), farmers applied a toxic organochlorine insecticide, chlordecone, to control the banana weevil. In the late 1990s, the intensification of agricultural practices in the West Indies led to accelerated soil erosion and sediment transfers to river systems and the sea (Sabatier et al., 2021). This increase in soil erosion leading in turn to a release of chlordecone stored in polluted agricultural soils. These accelerated lateral transfers of sediments are strongly controlled by land use and agricultural practices. The identification of soil erosion sources is therefore essential to effectively fight against the consequences of erosion on the resurgence of chlordecone. Using sediment tracing tools applied to coastal marine sediment archives, the objective of the current research was to model the potential changes in sediment sources throughout time in the West Indian catchments. Banana and sugarcane crops, forests, channel banks and landslides were targeted here as potential sources of sediment.

To this end, soil samples were collected across the Galion catchment at locations presenting contrasted soil types and land use contexts. In addition, a marine sediment core was collected in the Galion Bay in April 2017. In order to quantify source contributions, a suite of physico-chemical properties was measured in both soil and sediment samples.

Subsoils provided instead the main source of sediment in the Galion catchment (between 40 and 50% of sediment). In contrast, the contribution of cultivated soils increased during the 1960s (15 to 30% of sediment) and showed a second increase phase in the late 1990s (30 to 40% of sediment). These phases of increases were interrupted by decreases and major sediment contributions from subsoils. These increases of cultivated soils contributions can be explained by changes in agricultural practices (mechanization, irrigation) since the 1960s and the glyphosate introduction in the late 1990s, which increased soil erosion under cropland. Subsoils contribution increases correlate well with period of extreme events like Matthew cyclone in 2016.

Overall, the comparison between the calculated sediment contributions and the reconstructed chlordecone fluxes shows that the decreases in subsoil contributions correlate well with those of chlordecone concentrations in marine sediments. In contrast, the increases of cultivated soil contributions to sediment correspond well to increases of chlordecone concentrations in sediment.

Accordingly, these results showed the chlordecone contamination dilution due to increase of subsoil erosion. In the future, river sediment samples, collected with sediment traps, will also be analyzed using the same procedure to provide more detailed spatially-distributed information regarding erosion source contributions across the catchment.