My question would be, is it seeping through the dam or is it seeping in from the side via surface soil material. Your testing would certainly bear that out. Would there be an easy test to determine an alternative path?
Was the vegetation present during extreme drought when the only source of water at that level might have been through the dam? Could the moisture be coming UP from deeper down? Lots of questions that might help determine the source of the water presence.
Wet Area Analysis (discussion): I've scanned through all of the reservoir water elevation levels & compared the available photographs from (1) Inspection report dates (2) sat image dates (3) other images w/ dates. There is no 1:1 direct answer to cause-effect (of "green vegetation) from just this matrix. However, if you add in a precipitation factor, it does imply a component that facilitates the "greening". One data point on this is the "drought conditions" of 2016 where the precipitation factor was low.
However, since evaporation becomes prominent in very dry conditions (in the embankment), it doesn't answer the question adequately, other than give an indication to the weepage flow volume.
Yes, there is the theoretical possibility that an underground embankment source near the abutting canyon side wall seam may be injecting water into the embankment. The horizontal "seam" nature of the wetspot is the puzzle in this case. You would expect a dispersion pattern downhill (upside down "V") instead of a horizontal embankment origin.
The Palermo tunnel (4:00 position in black circle of 1st photo) has visible waterflow that empties into the Palermo canal. But this tunnel is at an elevation in the side bedrock of the canyon wall that is 99ft lower than the elevation of the "wet area".
This whole question gets back to: With Dam Inspectors stating that this issue should be "investigated" and "action taken" to protect the embankment (depending on findings), why hasn't this been done? (the 2016 Inspection report only states that it had dried up - no answers as to the "remedial repairs" that have fixed the problem).
Investigating this (i.e. "easy test") requires (1) recognition that this is a priority issue (2) procure funding to investigate (within a budget or otherwise) (3) perform the investigation. There is no easy test. Besides, why do Dam Safety Inspections with recommendations/actions if (1)(2)(3) are not followed up upon?