Analysis of composition, distribution and origin of hexachlorocyclohexane residues in agricultural soils from NW Spain

Concentrations of the isomers of the organochlorine pesticide hexachlorocyclohexane (HCH) were determined in 252 surface soil samples collected within a sampling network covering agricultural areas in Galicia (NW Spain). The concentration of total HCH (sum of α + β + γ + δ) ranged between 4 and 2305 ng g−¹ (dry weight), with the α-HCH and γ-HCH isomers predominating (< 1-1404 ng g−¹ and < 1-569 ng g−¹, respectively). The distribution of the pesticide residues was very heterogeneous, with the largest concentrations present in one of the studied areas (the province of A Coruña). The distribution of HCH was not found to be related to any soil property (organic matter, pH, clays, and metals). Multivariate statistical analysis of the data revealed that three populations of samples with a defined composition of HCH, can be related to the source of HCH: technical HCH (α/γ > 3), lindane (99% γ-HCH), or both. The existence of a third population consisting almost exclusively of α-HCH suggests that background contamination of anthropogenic origin dates from several decades ago. The detailed analysis of these populations enabled the possible temporal scale of the application of these pesticides to be deduced.