Effect of agricultural pesticides on the degradation of medium spill concentrations of Bonny light crude oil in a tropical rain forest soil

Abstract

The effect of three agricultural pesticides; K-othrin (deltamethrin), dichlorvos (2,2 dichlorovinydimethy phosphate) and carbofuran (2,3–dihydro–2,2-dimethylbenzofuran-7-y1methylcarbamate, on the degradation of medium spill concentrations of Bonny light crude oil in a tropical rain forest soil in the Niger Delta was investigated. Hydrocarbon (crude oil) loss within a 56-day period was monitored using photometric method. Mineralization (ultimate biodegradation) for the same period was monitored using the ratio of inorganic carbon to that of total organic carbon. Two positive control systems were employed one consisting of soil impacted with 5000 ml/m2 of olive oil while the other consisting of soil impacted with 5000 ml/m2 of crude oil. Negative control consisted of soil impacted with 5000 ml/m2 of olive oil and 200 g of sodium azide. Other treatment options involved soils impacted with 5000 ml/m2 of Bonny light crude oil with pesticide at manufacturers recommended doses and soil impacted with pesticides alone. Soil with no treatment also served as a test system. Results suggested that there was a significant difference at 0.05 probability levels in the percentage hydrocarbon left when the mean value of treatment with pesticide alone was compared with treatment with pesticide/crude oil mixtures. There was also a significant difference in the percentage hydrocarbon left when pesticide/crude oil mixtures treatments were compared with the two positive controls. Percentage hydrocarbon left decreased in the following order; Negative control > pesticide alone > pesticide/crude oil mixtures > crude oil alone > olive oil alone. Results suggested that percentage mineralization of organic carbon was greater in options involving pesticide alone when compared with options containing pesticide/crude oil mixtures. Percentage mineralization levels in pesticide/crude oil mixtures treatments were similar to values obtained in negative control and positive control involving crude oil. Percentage mineralization in treatments decreased in the following order; olive oil alone > pesticide alone > pesticide/crude oil mixtures = crude oil alone = negative control. Results showed that the application of pesticide to tropical soil impacted with medium spill levels of Bonny light crude reduced hydrocarbon degradation and mineralization in a 56 day period. Hydrocarbon degradation (loss) was significantly lower in pesticide alone options than in pesticide/crude oil mixtures suggesting possible enhancement of crude oil degradation in the latter treatments. Hydrocarbon loss was greater in crude oil alone options than in the other two treatments suggesting toxic influence of pesticide. Mineralization was, however, greater in pesticide alone options than in pesticide/crude oil mixtures suggesting greater degradability of these pesticides over pesticide/crude oil mixtures. Key words: Pesticides, Bonny light crude oil, mineralization, hydrocarbon, treatment.