As a result of an accident a tanker lost its load of petrol and heating fuel.
A large quantity of fuel approximately 2000 litres soaked into the soil. Analyses revealed that the average level of hydrocarbons present was in the order of 0.55 %.
The pollution affected an area of 100 m2 and to a depth of 0.6 m. Our goal was to reduce hydrocarbon levels to the lowest possible value in order to meet the legal limit. In order to test the results of our treatment samples were taken from two selected points.
The first stage of the clean up operation took place during January. Exceptionally high rainfall prior to and during the incident resulted in infiltration of hydrocarbons into the sandy soil 7.5kg of bacteria (EU70) was dehydrated in 120 litres of luke warm water and injected into the ground. Larger quantities were applied where the fumes were most powerful and where the soil was very black due to the fuel. The soil was then turned in order to oxygenate it and to mix the soil and bacteria. Turning of the soil also allowed the elimination of volatile matter. In order to gain as much warmth as possible and help the degradation process, straw and a liner were spread over the treated surface.
A sample was taken from three different points and labelled. See figure 1.
The second stage operation consisted of:
- Turning the soil with a mechanical shovel.
- Second inoculation of EU70 in order to maximize the degradation of hydrocarbons.
- Tilling of the soil/bacteria mixture with a motorised cultivator to achieve good homogenisation.
The following changes in soil condition were noted:
- Appearance of brown patches in the black fractions of the soil.
- The presence of heavy fractions (when the aqueous solution of bacteria were most numerous).
- An aromatic odour indicating volatilization of light compounds and/or a transformation of a quantity of different compounds into by products of degradation.
A second set of samples was taken. The gap between the second and third treatment was fifteen days.
Stage 3 treatment took place as in stage 2 but on this occasion a hydrocarbon solving detergent was added in order to dissolve the adsorbed hydrocarbons on the soil particles and make them more available to the bacteria. (This fraction is often difficult to degrade because, at this stage, the carbon source is less available for bacteria).
Macroscopic observations revealed that:
- Odour was reduced compared to the first and second treatments.
- The soil was becoming browner.
No further samples were taken at this time because it was decided to wait for two months to allow bioremediation to take place. Three months after the first injection, a control analysis was performed.
Results of the analyses
The results of the analyses are presented in chart below
Fig 1 (blue line) shows the change in hydrocarbons at the first sampling point where the spill was the strongest. The bioremediation process lowered the hydrocarbon concentration from 0.55% to 0.072% (a reduction of 87% in three months).
Fig 1 (red line) shows the change in hydrocarbon levels at the second sampling point. In this case the percentage in total hydrocarbons reduced from 0.09 % to 0.046 % (a reduction of 49%). This is to be expected because, in this case, the hydrocarbons were less available and the contamination was lower than in the first sampling point.