Hi Jim 0216, During this flow period most of the water was "jumping" over the shotcrete. :-)
The best fit answer is the "drilling mud" from the numerous core holes drilled into the Upper Main Spillway - these array of coring sample "holes" that DWR prevented anyone from photographing (Drone, Gallery, Press..). To effectively "core sample", the large holes had to be drilled to a depth to where the under slab rock quality could be determined. This likely would be 10's of feet or greater. During the core drilling, a "drilling mud" is pumped into the coring bit. The viscosity of the "mud" is not as fluidic as simple water. Thus, this "mud" could flow into the known issues of void areas below the slab, into the drain rock along all of the linear drains, and into areas that the BOC described in their reports as "washing areas" or "piping channels" (voids in sheet form). If you consider the square footage of area that was core sampled, (178 ft wide by close to 1,650 feet long = 293,700 sq ft), there is a potentially very large volume area that could be intruded with this "drilling mud". Normally, the excess "drilling mud" flows back to the surface and is captured in metal catch basins. In the case of the spillway, the underslab area is its own catch basin of any escaping "drilling mud".
Because of the viscosity of the drilling mud, it would not flow very easily into the drains. In fact, the drain pipe is lifted by the bell couplings to where a "void layer" of near 2 inches in height needs to be reached before any liquid would surface through the perforation holes in the VCP pipe.
So when the extensive drilling & mudding was completed, the escape "drill mud" would be sitting under the slabs in a large potential volume. When the spillway was activated, the pressurized water under the slabs would "wash" and dilute the drilling mud while mixing with the water. This "dilution" would flow through the sidewall outlet drains into the main chute & then down to the river.
I suspect if the drilling mud was not fully Eco-friendly, this could become another controversy for DWR. It may develop into a controversy just from the fact that DWR hasn't said anything (keeping quiet).
Core Sample rig: Requires tanks & drums - "drilling mud" is injected while coring to facilitate the "cutting process" - excess material retained in a metal catch basin - "escape" drilling mud likely penetrated in void and drain areas under the Upper Spillway from the large number of core holes drilled.
