Does sea-dyke construction affect the spatial distribution of pesticides in agricultural soils? - A case study from the Red River Delta, Vietnam.

The Red River Delta is a major agricultural production area of Vietnam with year-round use of pesticides for paddy rice cultivation and other production systems. The delta is protected from flooding, storm surges and saline water intrusion by a sophisticated river and sea-dyke system. Little is known about the effects of such a dyke system on pesticide pollution in the enclosed landscape. Our aim was to address this gap by i) determining pesticide prevalence in soils and sediments within a dyked agricultural area, and by ii) assessing whether and to which degree this dyke system might affect the spatial distribution of pesticides. After sampling paddy rice fields (topsoil) and irrigation ditches (sediment) perpendicular to the dyke in Giao Thuy district, we analysed 12 of the most commonly used pesticides in this area. In soils, we detected most frequently isoprothiolane (100% detection frequency), chlorpyrifos (85%) and propiconazole (41%) while in sediments isoprothiolane (71%) and propiconazole (71%) were most frequently found. Maximum concentrations reached 42.6 μg isoprotiolane kg-1 in soil, and 35.1 μg azoxystrobin kg-1 in sediment. Our results supported the assumption that the dyke system influenced residue distribution of selected pesticides. More polar substances increasingly accumulated in fields closer to the sea-dyke (R2  = 0.92 for chlorpyrifos and 0.51 for isoprothiolane). We can thus support initiatives from local authorities to use the distance to dykes as a mean for deliniating zones of different environmental pollution; yet, the degree at which dykes influence pesticide accumulation appear to be compound specific.