Distribution and potential sources of organochlorine pesticides in the karst soils of a tiankeng in southwest China

This research was conducted in an attempt to assess the concentration levels and potential sources of organochlorine pesticides (OCPs) in the karst soils of Dashiwei tiankeng, southwest China. The tiankeng is a karst surface expression that can act as a focal point for introduction of contaminants to groundwater system, which may serve as condenser and receiver for semi-volatile persistent organic pollutants such as OCPs. In this study, surface soil samples from Dashiwei tiankeng were collected and 23 organochlorine pesticide compounds were analyzed. The results showed that the concentration was 0.019-3.605 ng/g for DDTs (sum of p,p'-DDD, p,p'-DDE, o,p'-DDT, p,p'-DDT), 0.001-0.218 ng/g for HCHs (sum of alpha-, beta-, gamma-, delta-HCH), 0.003-0.290 ng/g for CHLs (sum of heptachlor, heptachlor epoxide, TC, CC and trans-nonachlor), 0.001-0.064 ng/g for endosulfan (sum of alpha-endosulfan and beta-endosulfan), 0.008-1.630 ng/g for HCB and 0.023-0.928 ng/g for other OCPs (sum of aldrin, dieldrin, methoxychlor, endrin, endrin aldehyde and endrin ketone). The total OCPs concentrations varied from 0.055 to 5.216 ng/g. The ratio of DDT/(DDE + DDD) in the floor soils of Dashiwei tiankeng ranged from 0.434 to 0.797, suggesting a mostly historical residue of technical DDT contamination. However, the ratio of DDT/(DDE + DDD) in the upper rim soils was higher than one, which that there was fresh DDT application nearby. Both the floor and upper rim soils of Dashiwei tiankeng had high ratios of o,p'-DDT/p,p'-DDT (range of 0.016-10.833 with mean of 5.424 and 4.667-7.714 with mean of 5.723, respectively), which implied that the primary source of DDTs was probably from dicofol-type DDT products. The average ratios of alpha-/gamma-HCH were 24.435 in the floor soils and 1.067 in the upper rim soils, together with the averaged percentages of beta-HCH among the total HCH isomers (accounting for 33.772 %), indicating that the HCHs were a dominant contribution from technical HCH usage in the past.