Hi Marika
I had a long to and fro with the Agency over this
https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind0811&L=CONTAMINATED-
LAND-STRATEGIES&T=0&O=A&X=5A045D7A88456D0ABF&Y=cdainton%
40wdenvironmental.com&P=31734
This surface soil pathway is based on the Jury equation/research. Essentially
states that average VF over time t for surface soils will decrease
exponentially.
The Agency has set the default value as the receptor averaging time.
Which, when you think about, is not that clever or logical.
Especially for a commercial receptor.
Technically, the algorithm, should only be valid for top 10cm soils - Agency
has fudged this in CLEA 1.04 by using to depth of 1m before employing the
deeper soil equation to calculate VF (which is not time dependant). They did
this to avoid anomalies when going from 10cm to 10.1 cm depths (this is how
it was stupidly set up in CLEA 1.03)
After hassling the EA about CLEA 1.03, CLEA 1.04 now allows a site specific
value to be entered for this parameter. Its all disussed in 2009 SR4.
Questions for your conceptual model on how to decide on a site specific value
could include - how long since release to surface soils and what would be a
reasonable turn-over time of soils from depth in the mixing zone to the surface.
I tend to go for six years for both residential & commercial (well at least the
VF is the same for both scenarios!).
ASTM RBCA uses longer averaging times (I would need to dig out reference).
Hope this helps
Chris Dainton
WD Environmental
|