Effect of soil water content on heavy mineral oil biodegradation in soil

We investigated the limits and extent of heavy mineral oil biodegradation at different soil water levels and evaluated soil biological activity with regard to its usefulness for monitoring bioremediation processes in a soil contaminated with aged heavy mineral oil. To examine the effects of soil water content on heavy mineral oil biodegradation, a bench-scale investigation was conducted with different water levels (30, 50, and 80% water holding capacity) over a 105 day testing period. During the experiment, hydrocarbon concentration and microbial activity were monitored. At the end of the experiment (105 days), the initial level of contamination of 7858 +/- 343 mg kg(-1) was reduced to 4607 +/- 290 mg kg(-1) in soil with 30% water holding capacity (WHC), whereas only 28% of the hydrocarbons were eliminated in soils with 50 and 80% WHC. The hydrocarbon degradation rates declined with time, with 42% of the initial concentration of hydrocarbons remaining at the end of the experiment. The aged heavy mineral oil biodegradation process could be effectively monitored by soil biological parameters. There were significant correlations between the hydrocarbon concentration and selected biological parameters (soil respiration, dehydrogenase). Oxygen diffusion was inversely related to the soil water content. This affects hydrocarbon degradation and microbial activity. It is imperative to maintain a moderately low soil water content to maintain the effectiveness of bioremediation. Soil biological methods should complement chemical methods when assessing the success of bioremediation process.