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Abstract: A field experiment was conducted on a Calcaric Cambisol soil to study the consequences of the penetration depth and properties of pesticides on the risk of subsequent leaching. Three pesticides with different mobility characteristics and bromide were injected at 30cm (where soil organic matter (OM) was 2%) and 80cm (soil OM 0.5%) on irrigated plots without a crop. The migration of injected solutes was assessed for two years by sampling the soil solution using six porous cups installed at 50 and 150cm depth and by relating solute contents to drainage water flux estimated by the STICS model (Simulateur mulTIdisciplinaire pour les Cultures Standard). Pesticides injected at 30cm were strongly retained so that no metolachlor or diuron was detected at 50 and 150cm. The ratio of atrazine peak concentration in the soil solution to concentration in the injected solution (C/C 0) was 1×10−3 and 0.2×10−3, respectively, at 50 and 150cm. When injected at 80cm, (C/C 0) of atrazine, metolachlor and diuron were 10×10−3, 1×10−3 and 0.3×10−3 at 150cm, respectively; 1/(C/C 0) was correlated with K oc values reported from databases. The ratio of drainage volume to the amount of water at field capacity in the soil layer between the injection point at 30cm and the water sampling level (V/V 0) at 50 and 150cm was 0.6 and 0.9, respectively, for bromide and 1.6 and 1.0 for atrazine. V/V 0 of the injected solutes at 80cm was for bromide, atrazine, metolachlor and diuron 0.6, 0.9, 1.2 and 1.7, respectively; pesticide V/V 0 was correlated with K oc. The retardation factor was a good indicator of migration risk, but tended to overestimate retardation of molecules with high K oc. Atrazine desorption represented an additional leaching risk as a source of prolonged low contamination. The large variability in soil solution of bromide and pesticide concentrations in the horizontal plane was attributed to flow paths and clods in the tilled soil layer. This heterogeneity was assumed to channel water fluxes into restricted areas and thereby increase the risk of groundwater contamination. The methodology used in the field proves to provide consistent results. [Copyright &y& Elsevier]

The important effect of soil wetting and drying cycle (WDC) on soil structure, and the consequent effect on pollutant fate is underexplored. We thoroughly investigated the changes in soil structure and in leaching of Alion (indaziflam) and Express (tribenuron methyl), pre and post WDC, from two clayey soils and two loamy soils under different land uses (uncultivated, field crops, and orchards). Soil stability was quantified by an aggregate durability index we recently developed. WDC did not affect the stability of the sandy-loam soils, as expected. However, for the sandy-clay-loam with high CaCO 3 content aggregation was observed. For the clayey soils with similar CaCO 3 , aggregation and disaggregation were obtained, for a soil with relatively low and high SOM, respectively. The stability trends are reflected by the ratio between the contents of inorganic carbon and soil organic matter (SOM), CaCO 3 /SOM, normalized to the clay content. Aggregation was explained by CaCO 3 cementation, while disaggregation was attributed to high clay content and to alterations in SOM conformation post WDC. These opposite trends, obtained for the two clayey soils, were confirmed by analyzing changes in soil packing employing X-ray tomography (micro-CT). Our results clearly demonstrated that soil aggregation and disaggregation, induced by a WDC, suppresses and enhances herbicide mobility, respectively. However, the effect of WDC on herbicide leaching was not noticeable for Alion upon its high adsorption to a clayey soil, indicating that herbicide physical-chemical properties may dominate. Finally, WDC induces micron-scale changes in aggregate structure, which have a notable effect on pollutant mobility and fate in the environment. Image 1 • Soil wetting and drying induces micro-changes in aggregate stability. • CaCO 3 and organic matter are key factors in aggregation of clayey soils. • Soil stabilization, aggregation, may reduce pesticide mobility. • Soil disaggregation may enhance pesticide mobility. • Pesticide physical-chemical properties dominate their mobility. [ABSTRACT FROM AUTHOR]

Soil columns were collected from a blueberry field, and insecticide solutions were allowed to leach through these columns. Insecticides from four different chemical classes were applied at two different rates: the concentration at which the insecticides wash off blueberries under rainfall conditions and the labeled field rate at which they are sprayed. The soil columns were divided into thirds; top, middle and bottom. Soil bioassays using Eisenia foetida Savigny, as an indicator species, were set up to determine the toxicity of the insecticides at a top, middle and bottom layer of the soil column. The mass of E. foetida was also measured after the bioassay experiment was completed. The concentrations at which insecticides wash-off of blueberries from rainfall were not lethal to E. foetida. In order to support mortality data, insecticide residues were quantified in the soil layers for each insecticide. Under field rate leaching conditions, carbaryl showed the high levels of toxicity in the top and middle layers of soil suggesting that it has the highest risk to organisms from leaching. This study will help blueberry growers make informed decisions about insecticide use, which can help minimize contamination of the environment. • Insecticide leaching from rainfall wash-off were not lethal to E. foetida. • Carbaryl showed the highest level of toxicity to E. foetida. • Insecticide recovery generally declined as soil depth increased. [ABSTRACT FROM AUTHOR]