Peats influence on downward migration of free product?

[stevie714]

I have an site where we have encountered petroleum contamination to depths of at least 100ft bls. The typical water table is roughly 12ft. bls. The site is underlain by silts, clays, and sandy to roughly 20ft bls. In many areas, the surficial material is underlain by peat and highly organic silts and clays. The samples recoverd from within the peat layers were dry. The peat, where present is underlain by sand which is in-turn underlain by limestone. The limestone varies greatly in induration and depth. Numerous large production wells (no longer in use) operated within +/-1500 ft. of the site. Out client feels that the peat may have "dried out" during the period of water production which may have created a cone of depression that would have influenced the site. Does anyone have any experience with sites similar to this? Additionally, what role might the peat layer play in this puzzle and where could I get some good info on peat characteristics? Sorry for the long length of this question but it is a relatively complex problem. Any info would be greatly appreciated.

Thanks in advance,

Steve

 

[CarlB]

I recall at least 2 peat textbooks, both Canadian publications. One was "Muskeg Engineering", the other something like "Engineering Characteristics of Peat". A search at

http://www.google.com

gets some hits on the Muskeg book. A search for '+peat +characteristics +petroleum' or other combination of relevant terms also has many hits.
Good luck,
Carl

 

[Dubliner]

Steve,

I am concerned with the nature of the petroleum contamination. If the water table is 12' bgl and the contamination is found up to 100' bgl then you must be dealing with halogenated hydrocarbons of some sort which are more dense than water (DNAPLs; sp.gr. >1).
If these were spilled directly onto the peat-covered area when it was water wet then they would have displaced the interstitial water anyway and continued their downward migration.
If the nearby pumping caused a major lowering of the water table and the peat was essentially dry at the time of the contamination episode then the peat would become 'wetted' by the hydrocarbons. This means that even with subsequent resaturation (i.e. heavy rain, flooding) some of the contaminating hydrocarbon liquid would have remained (cf. irreducible water saturation). You said that the recovered peat samples were dry. Did you mean literally or did you mean no contamination?
Water table fluctuations do effect waterlogged ares such as bogs and peatlands, some cases may exist within the archives of the UK Environmental Agency (

http://www.environment-agency.gov.org).

If the water table is at a depth of 12' now (with no pumping occurring) and it is relatively stable (i.e. not still rebounding) then was the bogland wet due to a perched aquifer situation? Has the dense pollutant migrated by moving downslope along the lower confining layer of the perched aquifer until it found a way into the lower, confined aquifer?
Interesting problem Steve, I hope my rambling has been of some use. Please contact me for further discussion if you wish.

Regards

Adam