Impact of herbicides used in olive groves on waters of the Guadalquivir river basin (southern Spain)

A long-term programme was carried out to monitor the water concentrations of at least 4 of 6 herbicides (atrazine, simazine, terbuthylazine, diuron, oxyfluorfen, and diflufenican) and 2 of their metabolites (desethylatrazine and desethylsimazine) and relate them to the impact of olive crops in the Guadalquivir river basin. The mean surface water concentrations found were mostly above the UE recommended limit for drinking water (0.1 μg L−1), but showed a decrease with time: diuron from 2.36 μg L−1 in 2003 to 0.03 μg L−1 in 2010, and terbuthylazine from 0.89 μg L−1 in 2008 to 0.20 μg L−1 in 2010. The mean herbicide concentrations for groundwaters were lower than those for surface waters, but some were still above the limit for drinking water: diuron ranged from 0.39 μg L−1 in 2003 to 0.01 μg L−1 in 2010, and terbuthylazine from 0.70 μg L−1 in 2008 to 0.22 μg L−1 in 2010. The maximum herbicide water concentrations, in both surface and ground waters, were measured in winter and spring, coinciding with rainfall periods. Herbicide concentrations were found to be related: (a) to soil runoff processes in surface waters and (b) to leaching or preferential flow through soil into groundwaters. Herbicide solubility, half-life, and field dose were also related with their respective water levels. The acceptable evolution of herbicide concentration in surface waters from 2002 to 2010 is associated to the current regional and national regulations and actions, which should be maintained.