Do bear in mind the following. If you take soil from the site to use in a 
greenhouse pot trial, homogenise it so all replicates receive the same 
treatment (as is usually done), then the bioavailability of cadmium to 
plants will be affected.

I have done a lot of work for my thesis that has clearly shown that the 
uptake of cadmium by lettuce (a typical home grown crop) is significantly 
higher (i.e. double) when the soil contains a homogeneous distribution of 
cadmium, as opposed to cadmium distributed heterogeneously within the pot. 
This is despite pots containing the same overall total concentration of 
cadmium.

What this means is that the pot trials are likely to provide an 
over-estimation of the true in-situ bioavailability.

If you are interested in reading my paper, it has just been published in 
Science of the Total Environment

Heterogeneity of cadmium concentration in soil as a source of uncertainty 
in plant uptake and its implications for human health risk assessment  ? 
ARTICLE
Science of The Total Environment, Volume 326, Issues 1-3, 29 June 2004, 
Pages 49-53
Peter R. Millis, Michael H. Ramsey and Elizabeth A. John

A copy is available on-line as a pdf file through www.sciencedirect.com , 
or I am happy to email a copy to anyone who would like it.

Peter Millis
Postgrad
Centre for Environmental Research
School of Life Sciences
University of Sussex
Falmer
Brighton BN1 9QJ
tel +44 1273 606755 ext 7688 (direct dial +44 1273 877688)


--On 29 September 2004 12:39 +0100 "Wilkinson, Christiaan" 
<[log in to unmask]> wrote:

> Stefan,
>
> in my opinion greenhouse trials are the preferred option for assessing
> bioaccessibility.
>
> for example,
>
> greenhouse trial control light temperature and moisture and can be
> compared with other similar trials. from a scientific point of view this
> is preferable.  you could also control the pH but this is not recommended
> as the sample would not represent the garden soil.
>
> insitue trial are open to the variability of the prevailing weather
> conditions of which you have no control over. the data collected is non
> comparable as it is site specific.
>
> your objective seems to be to assess the bioaccessibility of Cd to plants
> under the best growing conditions therefore greenhouse trials should suit
> your requirements.
>
> as for other methods like PBET testing i am unsure of their reliability
> and believe PBET simulates the human digestive system with a calculation
> to estimate the relative bioaccessibility
>
> QU do you have elevated concentration of Cu and Zn?
>
> reminder: heavy metals with in plants can be stored in several no edible
> sections e.g roots/tubers, stems and leaf. this may affect the data
> depending on which plant is selected for trials.
>
> Christiaan
>
> -----Original Message-----
> From: Stefan Hueneke [mailto:[log in to unmask]]
> Sent: 29 September 2004 10:33
> To: [log in to unmask]
> Subject: The cadmium question
>
>
> We are currently investigating a site where there are several properties
> that are affected by elevated cadmium (amongst other heavy metals). None
> of the residents are currently growing crops in their gardens but I feel
> that we should use the residential with plant uptake SGV given the advice
> in CLR10 that it is prudent to assume this unless there are constraints on
> people doing so. We have taken several samples from each garden and
> calculated the US 95 using the mean value test. In each case this exceeds
> the SGV (although not all of the individual samples do) which generally
> comes out at 1 or 2 as the soils are quite acidic. However in SGV3 it
> states that at pH of less than 6.5 the SGV is likely to overestimate plant
> uptake and that the assessor should consider bioavailability testing and
> crop sampling if possible. How do people feel we should proceed in this
> case. Would bioavailablity testing really tell us anything? We could grow
> crops in a sample of the soil off-site but I feel the conditions then
> would not be site specific. We could ask one of the residents to grow
> some crops in their garden for sampling but I do not feel this is
> controllable and could be open to interference. Any suggestions?
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