I think you are really asking for trouble if you don't get a registered civil (geotechnical) engineer with local experience with foundation design to do an excavation plan for you. The fact that you asked the question here sort of indicates to me that you don't really understand what you're...
Roctest (www.roctest.com) makes all sorts of water level measurement devices for slope projects. If they aren't familiar with it, I'd be surprised. I used vibrating wire piezometers at a landfill site once (worked like a charm). Halcrow buckets sounds like a bubbler-type depth gage (I've...
Donovoid is right of course that in-situ bio will work on the dilute plume down gradient of the source. Sorenson's work at INEEL was for a waste injection well where they had thousands of gallons (if not more) of DNAPL. But that was at some large depth (greater than 80 meters as I recall) in...
At your concentrations, you probably have DNAPL. I don't think in-situ bio will do anything for free product. Normally, I would then recommend some form of chemical oxidation. But if you have a confining layer at 9 feet, why don't you just dig it up?
I'm not confident either. Chameau and Hassiotis have a methodology for pinning slide planes:
Hassiotis, Sophia, Chameau, J.L. [1988] Stabilization of Slopes Using Piles, Joint Highway
Research Project, Purdue University, Report No. FHWA/IN/JHRP-84/8
Hassiotis, Sophia, Chameau, J.L...