A little-noticed seven-week-old Panamanian government document has thrown yet another - possibly final - monkey wrench into the approval process of the Panama Canal treaties.

Hard-line treaty opponents in Washington are attempting to use the document, a Foreign ministry communique published in Panama which they say appears to reject Senate reservations and understandings attached to the treaties, to delay President Carter's trip to Panama next Friday.

Carter and Panamanian head of state Gen. Omar Torrijos are scheduled to exchange instruments of ratification then, indicating final joint acceptance by the two countries of the treaties and their attachments.

The U.S. Senate ratified the treaties by extremely close votes in March and April.

Administration sources said they view the new opposition effort, led by Sen. Jesse Helms (R-N.C.), as an ineffective last-ditch tactic to stop the treaties. Although they admitted the Panamanian document "muddies the waters" on the issue of whether Panama accepts the substantive changes added to the treaties by the Senate, they said that only the Panamanian ratification instrument itself constitutes that government's official word on the accords.

In remarks on the Senate floor last Monday, Helms said the document signified Panama's "repudiation" of the additions, including the reservation authored by Sen. Dennis DeConcini (D-Ariz.) authorizing U.S. military intervention to keep the canal open, and said that Panamanian "interpretations" of some of the reservations were "diametrically opposed to the Senate's intention."

Specifically, the Foreign Ministry communique states that the "right of intervention" does not exist in international law.

Helms, saying that the United States will be "mousetrapped" into acceptance of this interpretation if the ratification instruments are exchanged as scheduled, called on Carter to ask that the communique be withdrawn, with written assurances that Panama understands what the additions mean.

Helms also suggested that Panama hold a new plebiscite on the treaties - which were ratified by the voters there last October before the Senate modified them - or that they be sent back to the Senate for reconsideration with Panama's intepretations. Helms presumably thinks that one group or the other would now reject the treaties.

Administration sources characterized the Panamanian communique as "for local consumption" and described it as an equivocally worded attempt to make what were considered offensive treaty additions by the Senate palatable to the Panamanian people.

One administration source described the document, which Panamanian newspapers published in April, as "gobbledygook."

While State Department spokesman Hodding Carter III said last week that Carter would respond directly to Helms' concerns, the administration sources said that the complaint was not viewed as a serious problem. They noted that "the text of a communique by a foreign office is not legally binding in any way" and that Panama's government could say whatever it wanted to its own people.

An aide to Sen. DeConcini, whose reservation Panama considered the most objectionable, said that office was not concerned about the communique. "The bottom line was not the vote on the treaty," the aide said, "or a vote to pass or not to pass a reservation or resolution of ratification by the Senate," but the instruments of ratification to be exchanged Friday.

Instruments of ratification are the official documents that two countries exchange in acceptance of a binding bilateral accord. Each country writes its own instrument, but it cannot change the wording of the actual treaties and it must be accepted in full or not at all.

The administration sources said the first draft of the Panamanian instrument was received for review several weeks ago. While one source who participated in the subsequent negotiations said that Panama "had some stuff in the first draft" that the United States requested be changed, he described the disputed passages as "nothing really contentious."

"We went through a whole negotiating process with" the Panamanians, the source said. "We had to kind of educate them to what we had in mind," but he said the Panama government "is over" its attempts to object to the reservations.

"They want to put on the best show they possibly can for Carter," he said. "They're not interested in a fuss."

The source said a final draft of the Panamanian instrument has now been shown to and approved by the Senate leadership and Foreign Relations Committee. He said it contains none of the ostensibly objectionable wording of the Foreign Ministry communique, and does not mention the DeConcini, or any of the other five understandings and reservations alluded to in the ministry communique.

News services reported the following from Panama:

Former President Arnulfo Arias returnedto a hero's welcome in Panama City yesterday after 10 years' exile in the United States.

He was greeted by native bands playing salsa music and thousands of persons waving flags of the Panamenista Party, one of the largest political groupings in Panama.

Thousands of spectators who could not get into th terminal pressed against the airport's chain fence to get a glimpse of Arias, who served three times as president and was ousted three times by the military.