Contaminated Land and Bioremediation - Environmental Biotechnology: Theory and Application - page 101

The effectiveness of natural attenuation has been demonstrated by 20 years or
more of research in the USA, which gave rise to the 'Part 503 Rule'. Issued in
February 1993, the Clean Water Act , specifically the part of it called Title 40 of the
Code of Federal Regulations, Part 503 - The Standards for the Use or Disposal
of Sewage Sludge , which is commonly referred to as the 'Part 503 Rule' or even
simply 'Part 503', sets out benchmark limits for the USA.
Typical European regulations follow a precautionary limits model, at times
referred to as the 'no net gain or degradation' approach, meaning that there should
be no overall accumulation of contaminants in the soil, nor any degradation of
the soil quality, compared with original levels.
Part 503 is based on risk assessment of selected key pollutants which pose
a threat to humans, other animals or plants, making evaluations of a number
of different possible pathways, from a direct, 'single incident' scenario, to a
lifetime of possible exposure via bioaccumulation. The standard which is set as
a result is based on the lowest concentration which was deemed to present an
acceptable risk.
In this way, higher heavy metal concentrations and cumulative loading rates
are permitted than would be allowed under the Europe model, since the ability of
soil to lock them up effectively indefinitely has been demonstrated by extensive
research. Accordingly US legislation is based on the principle that even if the
background level of a given heavy metal species increases over time, its migra-
tion or availability for uptake by plants or animals would be precluded by the
combined action of the resident microbes and other general soil characteristics.
In many ways this has strong echoes of the soil modification of contaminant
effect previously discussed.
The engineered solution
If natural attenuation is not appropriate, then some form of engineered response
is required, the selection of which will depend on a number of interlinked factors.
Thus, the type and concentration of the contamination, its scale and extent, the
level of risk it poses to human health or the environment, the intended eventual
site use, the time available for remediation, available space and resources and
any site-specific issues, all influence this decision. Many of the key issues have
already been discussed and the earlier Figure 5.1 sets out the factors governing
technology transition between the in situ and ex situ techniques.
Essential Features of Biological Treatment Systems
All biotechnology treatments have certain central similarities, irrespective of
the specific details of the technique. The majority of applications make use of
indigenous, resident microbes, though in some cases the addition of specialised
organisms may be warranted. Thus, the functional biology may be described as a
process of bioenhancement or bioaugmentation, or occasionally a mixture of both.