by addition of nutrients, addition of air/oxygen (a process called ‘bioventing’),
addition of other terminal electron acceptors (e.g. hydrogen peroxide) or addi-
tion of co-metabolic substrates.
On 24 March 1989 the Exxon Valdezoil tanker ran aground on Bligh Reef,
Prince William Sound, Alaska spilling 37 000 tonnes of oil. Despite efforts to
contain the spill, tidal currents and winds caused a significant proportion of the
oil to be washed ashore. Approximately 15% (~2000 km) of shoreline in Prince
William Sound and the Gulf of Alaska became oiled to some degree. Bio-
remediation was one of a number of techniques applied in the clean-up operation.
Bioremediation was favoured because the majority of molecules in crude oil are
biodegradable and because shorelines often support large populations of oil-
degrading microorganisms.
Biostimulation of the shoreline microbes was effected through addition of
fertilizers. Two products were applied: Inipol EAP22, a urea-based product
designed to stick to oil, and Customblen a slow-release granular fertilizer con-
taining ammonium nitrate, calcium phosphate and ammonium phosphate. These
products were selected to minimize nutrient losses with the tides and thereby
optimize nutrient input to the oiled areas. By late summer 1989 approximately
120 km of shoreline had been treated in this way.
Hydrocarbon degradation by this method starts with attack of the methyl
group (–CH 3 ) at the extremity of the hydrocarbon chain (terminal –CH 3 ), a
process called methyl-oxidation, resulting in formation of a carboxylic acid group
(–COOH; Fig. 4.31). The reaction then proceeds by b-oxidation (explained in
Fig. 4.31), a process that cleaves C 2 units from the hydrocarbon chain as ethanoic
(acetic) acid (CH 3 COOH). Ethanoic acid is then utilized in the tricarboxylic acid
(TCA) cycle through which the microbes derive energy. This mechanism is
limited to straight-chain molecules; branch chains have to be removed by other
degradation pathways before b-oxidation can proceed.
Comparison of oil degradation between treated plots and adjacent control
plots indicated that after 109 days the treated plots had experienced about
90% consumption of hydrocarbons, whereas no significant changes had occurred132 Chapter Four
Table 4.11Consideration of typical factors relating to in situor ex situtreatment of
contaminated land.
In situ Ex situ
For Against
Less expensive More expensive
Creates less dust Creates dust during excavation
Causes less release of contaminants May disperse contaminants
Treats larger volumes of soil Limited in scale—batches treated individually
Against For
Slower Faster
Difficult to manage Easier to manage—ensure results
Not suited to high clay soils or Suited to a variety of sites including high clay
compacted sites and compacted sites