Contaminated Land and Bio-Remediation - Environmental Biotechnology: Theory and Application - page 102

Biotechnology Selection
Although the primary focus of remediation methods commonly falls on technolo-
gies dependent on a relatively high engineering component, there is one purely
biological treatment option which can be a very effective means of clean-up.
Known variously as 'natural attenuation', 'passive remediation', 'bioattenuation'
or 'intrinsic remediation', it is appropriate for sites where the contamination
does not currently represent a clear danger to human health or the environment.
Though it is not an engineered solution, neither is it a 'do nothing' approach
as is sometimes stated, since it is not an exercise in ignoring the problem, but
a reasoned decision on the basis of the necessary site investigations, to allow
nature to take its course. The approach works with natural cycles and the pre-
existing indigenous microbial community to bring about the required treatment.
The need for a good initial survey and risk assessment is clear, and typically a
comprehensive monitoring programme is established to keep a check on progress.
The effectiveness of natural attenuation has been demonstrated by 20 years
or more of prior research in the United States, 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' set out benchmark limits for the US.
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 inter-linked factors.