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Lead/Coolant Machins Shop Issue

I agree with the thought that the brass is the lead source.

Are brass fines from the cutting operation carried into the rinse?  Although
most of the fines residue may remain in the cutting fluid (does the waste
cutting fluid show elevated lead levels?) any fines that would wash off in
the first bath would maximize possible lead exposure in the rinse.  The
residue would also continue to disolve in the first rinse, long after the
part moved to the second rinse, further increasing the lead level in the
first bath.  (This is not a sure thing but look at the bottom of the first
and second rinse baths.  Does the first bath develop more of a bottom sludge
over time?)  

pH differences between the cutting fluid and the rinse water may explain
different lead concentrations in each liquid over time.  I would expect the
rinse water to be more aggressive at dissolving lead from brass.