Estimates of oil dispersal
Jan Erik Kaarø
A study carried out by NTH's Professor Erik I. Hjeldnes with his project and M.Sc. students may simplify the task of deciding whether oil contamination in sandy soil threatens groundwater
The Norwegian State Pollution Control Authority estimates that about 4,000 buried oil tanks leak some of their contents every year. This could mean as much as 20,000 cubic metres of oil infiltrating the soil. Just a few litres are sufficient to affect a source of drinking water. There are probably about 300,000 buried oil tanks in Norway half of them elderly steel tanks.
The Trandum leakage
The study was started by NTH's Dept. of Geotechnical Engineering in the wake of a leakage that was discovered at Trandum military camp in autumn 1990. Some 19,000 litres of fuel oil leaked out, and investigations carried out by the Norwegian Geotechnical Institute (NGI) showed that the oil had contaminated about 1,000 cubic metres of soil. The contamination penetrated to depths of 20 - 30 metres, but did not reach the groundwater magazine.NGI has started remediation of the ground by means of soil microbes that were already present in the ground. This method of biodegradation does work, but it is slow. In Trandum, the oil has contaminated a volume of soil that is about 50 times as great as the leakage itself. Such a ratio is normal for relatively coarse masses of this type.
Experiments
Hjeldnes's model tests, which used dry and moist sand, have provided valuable information about the rate of progress of the oil front. The tests have also demonstrated that the ability of the sand to contain oil depends on its porosity and water content. The rate of advance of the contamination drops off rapidly. However, the most important observation is that the shape of the bubble of contamination is highly dependent on whether the sand is dry or moist. Oil penetrates most deeply in moist sand, as shown in this figure.This is of decisive importance when we want to assess the danger of groundwater contamination," stresses Hjeldnes.