HI Mary, I am not 100% familiar with methods of collecting oil from soil. I think @eustatic might know better. I saw he had some diluted soil samples with him at the Barnraising. Anyways I would approach it in two separate ways: 1. I think collecting samples from frozen soil would be pretty much same process as collecting samples from not frozen soil. I would say just defrost the sample in a room temperature. Do not add more water to it. I would even try to dry the sample in the room temp, to get rid of as much water as possible. After that I will use the mineral oil which should attract the oil from the targeted sample. I would try to purify the sample to get rid of the soil remains and than try to measure the spectra. 2. The other way around might be actually (as you are saying) defrost the soil and mixing it with more water. I would wait until the mixture settles down and than I would try to freeze it and collect oil from the surface with a cotton swab. ... another way might be adding a centrifugation process to help the filtering / settling of the soil particles.

@pdhixenbaugh: There are different oil concentrations which can be detected by naked eye. We had a very interesting debate about this at this last Barnraising. Here is a link to a note Jeff wrote during the session: https://publiclab.org/notes/warren/11-21-2015/oil-sample-concentration-session-at-barnraising Also the concentration might depend on may different factors. Oil is hydrophobic and floats on the top, but that is not always the case. in case of an oil spill (eg. BP) it took really long time for the oil to reach the surface, sometimes many miles away from the source. That means that oil could be virtually in any layer of the water level. Also I probably depends on what kind of oil, etc. We chose the ratio 1:20 as a testing model in more less "ideal" environment. That said it was 1.5ml of oil in 30ml of water. I tried to recover as much oil as possible. Recently even in repeated cycles, but I was not able to recover all. In the best case I was able to recover 1ml max. or less. So that means that the rest of the oil stayed in the water.

Yes, my 1:20 mixes after some time form two layers.

Is it enough in the field to skim liquid from the surface, and let gravity separate the oil samples? - yes that is what I would try to do but it is good to freeze the mixture because you will get more actual oil. And you can very easily collect it with the cotton swab from the ice, avoiding collecting the water by accident. Also right now I am testing the PIG Oil Only absorbent mats. That might be interesting for you. Will post that in couple of days.

check out my new research note on re-heating and re-freezing the samples here: https://publiclab.org/notes/Matej/12-15-2015/separating-oil-from-water-by-freezing-the-water-part-2 - you can find here spectra of the reheated samples. Also look at my Spectral Workbench profile. I have there much more measurements than published here in the research note: https://spectralworkbench.org/profile/matej

what gets spilled in different types of situations (bakken crude spill, gulf oil spill, industrial contaminants...) and where would you expect to find those contaminants -- on the surface, dissolved or suspended, or on the bottom? And if you are doing a general investigation, where should you sample from? - See Jeff's research note from the barnraising posted on the top of this comment. also look at the HAZCAT guide from Randy Austin, Chief, Spill Response Program, Region 2 (NY City), Division of Environmental Remediation, New York State Department of Environmental Conservation. Ideally I would take larger samples from different layers of water, but also in different places. There might be quite a difference between sampling moving body of water (river) and a static body of water where most of the oil will most likely float on the surface. Generally the oil is always trying to float to the top. Good approach would be trying to locate the suspected source and sample there. Also sometimes happens that a "pocket" of oil is trapped under ground and that can leak from the bottom of the river bed. There is really a lot of variables that come into play here. I remember that @eustatic https://publiclab.org/profile/eustatic mentioned lot of interesting things related to your question. Perhaps try to contact him. This is certainly worth writing a separate research note. I will look into that.

Oh, just noticed -- on your dielectric fluid scan, you need to select a new cross section -- the top row of pixels is blank. Great post!

And do you have a link to the HAZCAT guide from Randy? Thanks!