good overview of env. fate of pcdd/f

[ttweed@wildrockies.org (Tony Tweedale)]

  Michael McLachlan (U. Bayreuth, Germany) 'A simple model to predict
  accumulation of pcdd/f's in an agricultural food chain' _chemosophere_
  34:5-7:1263-76 ('97)
  
  is actually, in its 1st half, a brief but packed summary of the literature
  on deposition, uptake & metabolism.  among the interesting facts:
  
  * root/uptake/translocation does not seem to be the major pathway to
  plants, as once thought [except for the cucurbita family--squash, cukes &
  melons)!].
  
  * cow uptake direct from soil depends a lot on the harvesting method for
  their feed [if self-employed (free-range), cows are efficient at ingesting
  more plant than soil!].
  
  * Cl4dd/f apparantly partition to gas phase in atmosphere, Cl8dd/f to
  particles.  this is followed by: "The importance of dry gaseous deposition
  was supported by field studies which showed that the relative contribution
  of the lower chlorinated congeners to the pcdd/f levels was much higher in
  plants than in atmospheric particles [5, 24].  While it is possible that
  particle bound deposition plays a more important role for the lower
  chlorinated congeners at locations close to sources where there are very
  high levels of atmospheric particles, dry gaseous deposition would appear
  to be the dominnat mechanism for the lower chlorinated pcdd/f in rural
  areas where  the bulk of agricultural productioin occurs."  it is unclear
  to me if he saying the Cl4dd/f congeners *preferentially deposit*, based on
  observation of their higher levels in plants than in atmospheric particles.
  
  *gaseous uptake is described by a simple linear model, plants w/ large
  surface areas doing the most.
  
  * "Only 2,3,7,8 substituted pcdd/f are usually found in cattle...other[s]
  are usually metabolized [28, 31]."   maybe he meant excreted, as such has
  been said for humans, that the flatter 2,3,7,8 congeners get through the
  gut easier.
  
  * dairy cattle are close to a steady state.  beef cattle store, "However
  the growth rate..is so large that there is extensive growth dilution of
  pcdd/f {concentrations, presumably]...are generally not much higher than
  the [C] measured in milk fat."
  
  * in the modeling section (his model agrees well with his observed
  concentrations),  fig. 3 & 4 show that 2,3,7,8-tcdd is ~ 11% of ocdd and
  ~4% of total pcdd, in milk.  i assume this is elevated over typical env.
  levels?  roughly the same proportion is shown for furans, and for dioxins
  and furans in beef. if there were typical environmental levels to compare
  to, and the concentrations were expressed in TEQ, it would become apparent.
  
  
  tony tweedale