Spurred by growing international conflicts over the environment, the United States is proposing a treaty that would require signatory governments to prepare environmental assessments and consult with other countries on potentially harmful projects.

Assistant Secretary of State Lucy Benson left last week for Nairobi where she will present the idea to representatives of more than 50 nations gathered for a U.N. Environment Programme meeting.

The proposal, which could potentially affect millions of dollars in industrial and development activity around the world, could be controversial, a State Department official said last week. "We are setting it forward rather gently so we don't scare people off," he said."Governments are very possessive about their prerogatives."

The treaty idea was first outlined in a resolution sponsored by Sen. Claiborne Pell (D-R.I.) and passed unanimously by the Senate last July. It would export the basic principles of the United States' landmark environmental law, the 1969 National Environmental Policy Act.

A treaty would address such problems as:

The acid rain from power plants in Europe that has poisoned the lakes of several Nordic countries.

Dams built in one country that affect nations downstream. The United States, for example, is wrangling with Canada over North Dakota's Garrison Diversion project which could pollute Canadian rivers.

The clearing of tropical forests in one country that might cause erosion and flooding in a neighboring country.

Nuclear reactors that could release radiation across international borders.

A Soviet proposal to divert rivers that freed into the Arctic Ocean, thus affecting the polar icecap.

As outlined in the Pell resolution, a treaty would require the preparation of an environmental assessment-a concise document-for any major project that would affect "the global commons" or the environment of another country. Global commons include the oceans and the atmostphere.

Before undertaking a project, countries would consult with potentially affected states or, in the case of the global commons, with the U.N. Environment Programme. Countries would delay the project for 90 days to allow for consultation. They would agree to "employ a good faith effort to settle disputes."

The treaty would apply only to projects within a country's boundaries, not projects it might understake in another country. An executive order last January, however, requires U.S. agencies to write environmental assessments for projects undertaken abroad.

However, the treaty proposal would not require environmental impact statements, the elaborate analyses called for by law for U.S. projects. "The assessment procedures contemplated would be modest and flexible," according to a State Department cable to 20 embassies.

If the treaty proposal is well-received at the Nairobi meeting, the United States would seek this fall to have the United Nations instruct the Environment Programme to develop principles for a treaty. Negotiations could then begin in 1981.

Negotiations could be especially sensitive with Third World countries, which often suspect environmental programs of hampering their development.

"Extensive and coordinated international efforts wll have to be made by the countries of the world if the planet is to escape becoming a sink-hole of pollution incapable of sustaining human life," said Leonard Meeker, of the Center for Law and Social Policy.