pcdd/f: environmental levels & env. fate

[ttweed@wildrockies.org (Tony Tweedale)]

  >Bill Carroll, Chlorine Chemistry Council sez:
  
  >Nope.  TCDD.  Greater than 30 ppt, some mortality in sac fry.  Over 100,
  >total mortality.  Current Lake Ontario--surprisingly, perhaps--5 ppt (10
  >TEQ).  The trout are coming back.  Compare to levels ca. 100 thirty years
  >ago.
  
  right, and 60 ppt tcdd is the LC50 for lake trout (the more sensitive
  species), according to the recent piece by janet ralloff in _science news_
  (17 may).  when lake ontario concentrations used to be 2.5 times the level
  needed for 100% mortality (!), and when many other p.o.p.'s and p.i.p's add
  to the burden of the fry of such a critical species to lake ecosystems but
  are not considered, and when trout populations are still suffering other
  pcdd/f/etc induced stresses -- where does that leave the importance of a
  reduction in some e.d.'s in the environment?
  
  
  >The decay data is recent, and most has so far been done on PCBs.  Hites also
  >did some a few years ago.  There was a paper at Dioxin '96 whose author's
  >name escapes me right now.  Lots more difficult experiments to do, but it's
  >coming.
  
  i have czuczwa & hites (_es&t_ '86, 20:2:195-200) which corroborates what
  you say about lower Cl congeners possibly being more susceptable to
  cleavage (more C-H bonds, more water soluble).  kwok et al. (_es&t_ '95,
  29:6:1591-99) investigate Rx rates w/ the ubiquitous atmospheric radical,
  -OH, and also confirming your statements.
  
  however, what are the rules after deposition from gas phase to soil or
  water, or on surfaces generally?  only ~10% of the key question--actual
  exposure--occurs from inhalation, so what if the planar more Ah toxic ones
  are more available for exposure, possibly even transformed *from* higher
  halogenated ones (i've read one _es&t_ piece that shows at least some
  transformation into 2,3,7,8 congeners)?  and what about the *pattern* of
  substitution?  i know that planar pcdd/f/pcb are much more readily absorbed
  through the gut than the non-planar less Ah toxic ones.
  
  i don't know about the effect of substitution pattern on environmental
  transport, fate and transfer, however.  in fact, i don't even know the
  trends in their physical characteristics, eg o/w coefficients, according to
  substitution degree & pattern (perhaps the higher the halogenation the less
  water soluble?  but what about as to the pattern of substitution?).
  
  can anyone offer some general guidance?
  
  
  tony tweedale