Re: BTEX addendum II

[hwilshire@isdmnl.wr.usgs.gov]

Posting from hwilshire@isdmnl.wr.usgs.gov (Howard Wilshire)

I will toss in a few comments to this discussion from a geological
perspective. As I am sure you are aware, there are soils and soils, and
lots of alluvium (accumulations of sand, gravel, silt, clay) that's called
soil but isn't. Important questions in residence time of any contaminant
are the nature and thickness of the material above the water table
(unsaturated zone), what kinds of clay minerals are present and in what
abundance (they vary radically in their sorptive properties), and the
mineralogy of non-clay components. Stratification and fractures are of
critical importance inasmuch as they may control the rate of movement of
contaminants through the unsaturated zone, and transport through fractures
may bypass the sorptive properties of the body of the "soil".

>From: Don Zweifel <zweifel@nexus.chapman.edu>
>
>Jeffrey,
>
>The data we were referencing came from the USEPA's RREL or Risk Reduction 
>Engineering Laboratory in Cincinnati, Ohio, Treatability Database. And 
>from USEPA document EPA-440/4-79-029 (Water-related Environmental Fate of 
>129 Priority Pollutants).
>
>There are numerous mitigating factors that infuence the biochemical decay
>coefficient: What type of aquifer is the pollutant residing in? Confined
>or unconfined? What is the soil stratification look like. What is its 
>permeability factor? 
>
>Our principle aquifer at MCAS, El Toro, Ca. is described as
>having an occasional discontinuous lenses of clayey and silty sands and
>gravels contained within an assemblage of sany clays and sandy silts. The
>Dept. of the Navy has dismissed as unacceptable the `No Action'
>alternative because natural attentuation will not be a time-effective
>methodology for the TCE and benzene plume that is contaminating our 
>principle aquifer.
> 
>Pls reply.
>
> Don Zweifel