According to public comment by the Upper Los Angeles River Area Watermaster in a 1993 document on the USEPA site page 11 here, the Walt Disney Co. has been using Headworks as a Spreading Ground for its treated water since 1961.
The doc concerns a cleanup strategy at the San Fernando Valley Superfund Site (Region 9). Since it only outlines basic plans, some contingencies and comment from early 1993, there is not much to conclude other than LADWP appears to have had oversight while Disney appears to have held the rights.
Exactly who put the Hex in Headworks is still anybody's guess.
