Remediation of Soil and Groundwater Contaminated by Oil

"Oil" covers many different kinds. Among petroleum products, for example, there are kerosene for heaters and gasoline for cars. Petroleum is separated at refineries into gasoline, kerosene, diesel, heavy fuel oil and so on, but it exists as petroleum as a collection of similar oils.

Oil, then, is not a collection of molecules of a single structure but a mixture of many components. Its molecular structure is a complex mixture of hydrocarbons; an oil such as gasoline has a name, but gasoline is not a single molecular structure: a collection of oils with similar molecular structures is given the name gasoline.

Animal and vegetable oils are oils too. Fish oil, for instance, contains large amounts of two similar components, EPA and DHA. Animal and vegetable oils differ in structure from petroleum hydrocarbons and biodegrade easily.

The oils that become a soil or groundwater contamination problem when they leak are the "mineral oils": gasoline, kerosene, diesel, heavy fuel oil, machine oil, lubricating oil and so on.

Some oils, typified by PCBs, benzene and polycyclic aromatic hydrocarbons, are designated as carcinogens or hazardous substances.

Oils recognised as hazardous in this way are subject to detailed regulations on storage, transport and disposal in Japan and other countries.

Even when oil is stored with great care, however, it can leak, and soil and groundwater can become contaminated with it.

If such soil is left as it is, it may later harm people and the environment. Land contaminated with oil might become a park, for example, and harm children's health.

In the case of groundwater contamination, the oil may also spread over a wide area. Soil and groundwater contaminated with oil should be remediated as soon as possible.

Earlier, using gasoline as an example, we noted that gasoline is made up of oils with similar molecular structures. Around the world there is a tendency to examine the polycyclic aromatic components in oil and set an environmental standard for each.

In many countries, including the United States, Europe, China and some countries in Southeast Asia, a whole list of standard values is set not only for benzene but also for components such as toluene, xylene and ethylbenzene.

For some reason, however, Japan has no such detailed standards as other countries. As a result, apart from specific oils recognised as hazardous such as PCBs and benzene, oils are not designated as regulated substances.

Compared with other countries, Japan seems to give little consideration to the effects of the various components of oil on the environment, on plants and animals, and on people. The tendency is to remove oil because something out of the ordinary is there, a strange smell or a visible oil film, and because it is unpleasant or feels wrong.

For that reason, soil and groundwater remediation in Japan is often carried out for reasons such as the following.

Oil remediation is carried out mainly in cases like these.

• Leased land or a neighbouring property was contaminated with oil
• An oil leak at a public facility
• The company's own oil standard was exceeded, so it remediates
• Remediation out of social responsibility after withdrawing from a business
• Oil contamination was discovered when the land was to be sold

Typical cases of oil remediation in Japan include the following.

Soil contamination is contamination in which oil has soaked into the soil. Groundwater contamination is contamination in which oil has soaked down as far as the groundwater. In Japan, almost any piece of land has groundwater. Whether the contamination stays in the soil or reaches the groundwater depends on the depth of the groundwater and how far the oil has soaked in.

For soil contamination there are two main approaches: carrying the contaminated soil elsewhere for treatment, or treating it where it is. Treating it where it is is called in-situ remediation.

With groundwater contamination, because oil is basically lighter than water, once it reaches the water table it spreads sideways and may contaminate a wide area. The water table also rises and falls with the seasons, so the contamination forms a vertical band.

Groundwater remediation is mainly done in situ.

Little oil dissolves into groundwater, so if a large amount of oil has pooled on the water table, physically recovering that oil is effective.

There are many ways to remediate oil-contaminated soil and groundwater, such as digging up the contaminated soil or injecting a remediation agent into the ground. Each has advantages and disadvantages.

Methods that spread a remediation agent on the land or inject it into the ground include the use of oxidants and bioremediation. Here we compare their advantages and disadvantages.

Oil remediation with oxidants

One chemical method of oil remediation is to spread an oxidant, but it is hard to use where there is aquatic life, such as ponds, rivers and the sea.

It also turns the land acidic, so its use is limited for remediating fields. And because oxidants corrode metal, they may affect metal buildings and structures.

An oxidant also reacts only for an instant: remediation happens when the oxidant meets the oil, but only in that instant. If there is not enough oxidant, more has to be added.

Oxidants react with organic matter other than oil, so in soil rich in organic matter they do not react only with the oil to be removed, and more oxidant is needed.

Oil remediation with bioremediation

Bioremediation uses the power of microorganisms to remediate oil. Because the rate of remediation depends on the condition of the microorganisms, the period is hard to predict, which is a disadvantage, but it has no effect on the environment and it can be used on land with buildings, which are advantages. Oil is remediated by spreading or injecting an agent containing microorganisms onto the contaminated land and mixing it with the soil.

The advantage of bioremediation is not only its low environmental load but also the fact that the microorganisms are alive.

Unlike an oxidant, in other words, as long as the microorganisms are alive they keep remediating the oil in that land.

For soil or groundwater remediation by bioremediation, please contact us.