Evaluation of the Environmental Impact of Spent Engine Oil on Soil Ecosystem

Abstract

Ecosystems being widely subjected to growing manmade stresses, the question of acclimation capacities of soil organisms to chemical pollution is of major concern. This study aims at evaluating the impact of spent engine oil (SEO) on soil ecosystem using Lumbriscus spp and leguminous plants as indicator organisms. Uncontaminated soil sample (1kg) was spiked with SEO at different concentrations (0, 1, 5, 10, 15, 20, 25, 30 and 50g) and its physicochemical properties, soil metabolic functions, total petroleum hydrocarbon (TPH) and total microbial population were analyzed. The plants were grown in the contaminated soil samples and monitored for 28days for their growth parameters, chlorophyll content and bioaccumulation of heavy metals. Earthworms were exposed to the contaminated soil for 28 days, during which mortality rate, body mass index, activities of antioxidant enzymes and lipid peroxidation were assessed. Histological studies on the earthworms were also carried out at the end of exposure period. SEO at different concentrations resulted in significant increase (p< 0.05) in the total petroleum hydrocarbon (TPH) in all the concentrations which significantly decreased (p< 0.05) with time. Total bacteria population in the soil decreased significantly (p<0.05) following contamination on day zero. On day 7 and 14, there was proliferation of bacteria. The bacteria countable colony forming unit reached 70.0 x 107cfu/ml on day 14, which began to decline thereafter. Soil fungal count on day 0 decreased significantly (p<0.05) with increase in SEO concentration from 18.0 x 107cfu/ml to 13.0 x 10 cfu/ml and followed the same trend with bacteria. Spent engine oil had stimulatory effect on soil dehydrogenase activities (DHA) throughout the study period in all concentrations. Soil lipase activities increased with respect to the concentrations on days 0 and 7, peaked on day14, (790.19µg pNp/(gx10min)) and declined thereafter. Soil catalase, phosphatase and urease activities were observed to fluctuate. SEO in the soil adversely affected the performance of the test plants (mung bean, soy bean and cucumber). The tolerance level for each plant differed in the following order: mung bean > soy bean > cucumber. Chlorophyll content of the plants studied decreased and the decrease was well correlated with the increase in levels of pollution. The plants bioaccumulated the following elements (Al, Ni, Zn, Fe, Cu, Cr, and Mg) and their concentrations were affected by the concentration of SEO. Histological studies carried out on the seminal vesicle and hindguts of earthworms revealed some damage to the organs at higher a degree of contamination (from 20 g/kg soil and above). The result of this study showed that SEO may have a negative impact on soil organisms and hence soil productivity.