Bioremediation: enhanced bioremediation In Situ

The first three methods are preferred because they stimulate the naturally occurring indigenous micro-organisms, already adapted to the environment. 

Lightly oiled sediment (sand, gravel, soil, mud).
Oiled seaweed and vegetation (even fauna) may be treated

May be easily implemented on any polluted site (usually considered for coastal sheltered sites with slow natural clean-up by waves or inland sites).

- it is relatively inexpensive with low energy requirements
- it can be carried out without elaborate equipment

Oil with a high asphaltene and resin content degrades slowly due to the molecular recalcitrance of the hydrocarbons while oil with a high aliphatic and aromatic content is a much more nutrient-dependent process and will degrade more rapidly within the adequate environment. It is recommended to carry out a GC/ MS analysis to define the composition of the oil and evaluate its biodegradability.
To achieve maximum biodegradation, sediment pore water should exhibit concentrations of 1.5 mg nitrate/litre, Phosphorous concentrations of approximately one-tenth of the nitrate levels, with oxygen levels above 2 mg/litre (Source: AMSA).
High permeability soils are required to allow the nutrients to reach the indigenous microorganisms (avoid fines clays).

Easy access to the treatment site.
Bio degradation is less efficient at low temperature.
Soil must be humid.
Pollutants must not be adsorbed to clay and/ or mud. In this case, they are unavailable for the micro organisms.

Under anaerobic conditions, contaminants may be degraded to a product that is more hazardous than the original contaminant.
Nitrate injection to groundwater is of concern because nitrate is a regulated compound. Bio-augmentation using non-native micro-organisms is also controversial.
The circulation of water-based solutions through the soil may increase contaminant mobility and necessitate treatment of underlying groundwater.

Refer to those applying to the management of polluted soils in situ. Special authorisation should be delivered for such work.

Bioremediation is a long term process (months to year(s)).
Bioremediation degrades aromatics, N-alkanes and iso-alkanes. Resins and Asphaltenes are usually resistant to bioremediation. Cyclic hydrocarbons (Saturated and Aromatics) are partially biodegraded.
The efficiency of biodegradation can reach satisfactory levels when correctly implemented on biodegradable material.

Limited, less than 30 euros / m3 (Source: KOLLER), 15 to 75 euros/ ton (Source: Bocard)
Related to the manpower, equipment for the spreading and purchase of stimulating agent.