I though I might share with you a
cautionary note concerning the use of CLEA UK. I have noticed this effect before, and
I know others have too, but I am not aware of any discussion of the
problem.
It concerns the variation in
results if you run the same thing several times. This is to be expected as it is a
probabilistic program. In some
cases, the variability is not excessive, but it is in others and it is
necessary to carry out many repeated runs before reporting a value. I have not done enough research to
determine what is to blame, or why.
It depends on the chemical and the land use and I have not yet detected
a pattern.
Consider anthracene as an
example. I have run all 4 land
uses at all 3 SOMs to calculate GACs. The land uses
with exposure to plant uptake are highly variable, but the others are
stable. There is no real
variation with SOM for residential without plant and commercial. The variation between runs for the
other two is so great that it masks any potential variation with SOM, and so I
am going to lump them all together.
I have made 5 runs at each SOM or 10 runs at each SOM, but not being a
statistician I am not sure if these are enough to be viable. The findings are
as follows.
Residential without plant uptake
varies from 19100 to 20700 mg/kg (mean 19693) – the range is 8.1% of the
mean. No. of runs =
15.
Commercial varies from 434000 to
452000 (mean 440667) – the range is 4.1% of the mean. No. of runs =
15.
Residential with plant uptake
varies from 17600 to 57800 (mean 33330) – the range is 120.6% of the
mean. No. of runs =
30.
Allotments varies from 15400 to
83800 (mean 42700) – the range is 160.2% of the mean. No. of runs =
30.
As you can see, there is a huge
range in the latter two cases.
I assume this variability will be
removed once CLEA UK is updated to remove the
probabilistic functions, as suggested might be the case in The Way
Forward.
Regards,
Kevin
Privett.
Dr
Kevin Privett
Geo-Environmental
Associate
Hydrock
Consultants Ltd
Over
Court Barns
Over
Lane
Almondsbury
Bristol
BS32
4DF
Tel:
(01454) 619533
Fax:
(01454) 614125
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(07799) 430870
Offices in
Bristol, Plymouth, Northampton, Stoke-on-Trent. www.hydrock.com
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