Investigations into the causes of residue variability on carrots in the UK

A pilot study was established to determine whether the preferential flow of water and thus insecticide in carrot beds might be responsible for the high residues of organophosphorus insecticides detected in individual carrot roots grown in the UK. A field site typical of UK carrot growing conditions was selected on a sandy soil with a low organic carbon content. Brilliant blue dye was applied in water to a small number of field plots located in the carrot beds to trace water movement through the bed and not to simulate insecticide application or irrigation. The plots were excavated following sufficient time for infiltration and drainage. Horizontal and vertical cross-sections through the soil profile were cut and descriptions of the dye presence in relation to soil features and the position of the carrot roots were made. Dye tracing and soil analyses showed there was a clear mechanical cultivation effect which generated a preferential movement of dye and water within the bed. The subsequent growth of carrots also created additional pathways of preferential movement due to stem-flow or canopy drip. A second study which increased replication of samples and allowed analysis of triazophos and chlorfenvinphos residues in the carrots could not identify any single factor which was conclusively responsible for initiating high residues in individual roots.