Look at the sediment at the base of the metal catch basin in the prior drill rig photo. Whatever material they are using, the color of the material is consistent with a grey to whitish residue. Below is a photo where there is whitish grey residue on the ground, on the rig, and on the guide chute for the drill there looks to be a grey clumpy residue. This was at a core sample drill location in front of the emergency spillway.
They may be using a low viscosity mix as I've seen metal catch basins blend in color to the drill area location - especially when the top layer is the reddish soil. It turns the basin fluid to a light orange color. In any case, the voiding and the "lift" of the perforated drains under the slabs would allow a volume of this drill mix material to accumulate without entering into the underslab drains. Only under pressurized "wash" conditions, when the spillway is in operation, will this residual volume of material be flushed out. The revelation of the whitish grey residue in the emergency spillway photo infers a link to the mystery of the "milky white" coloring that entered into the Feather River.
I see the grey white residue. I suspect if they are drilling into the blue/grey rock that the fines would look like that, think roto-hammer into concrete leaves grey white fines. The tote on the right is the same type of container 250 gallons of drilling fluid come(s) in. I do not recognize the labeling - the picture is too bad. The black and white 55 gallon drums are typically used to collect waste or hazmat materials in.
The report will be interesting. In the ocean similar foam plumes form due to organic material being agitated.