Rep. Pat Schroeder (D-Colo.) called yesterday for a formal investigation into conflict-of-interest charges against the nominee for the No. 3 job at the Environmental Protection Agency, James W. Sanderson.

Sanderson, a Denver attorney, was nominated last month to be assistant administrator for policy and resource management at EPA. He was a consultant to EPA Administrator Anne M. Gorsuch on personnel matters for most of last year.

In a telephone interview, Sanderson denied any conflict, saying that if the key incident occurred at all it was during a period when he was not working for EPA and before he was asked to take the policy chief's job.

In a letter to EPA Inspector General Matthew Novick, Schroeder asked for an investigation of charges raised in a three-page anonymous letter dated Dec. 18 and sent to Senate Environment and Public Works Committee Chairman Robert T. Stafford (R-Vt.), with copies to Schroeder and Sen. Gary Hart (D-Colo.)

The author, claiming to be an EPA employe, charged that Sanderson had "ordered" EPA's Denver regional administrator, Steven J. Durham, on Sept. 19 to postpone a decision on stream classifications that would have been inimical to the interests of two clients of Sanderson's law firm, the Denver Water Board and Snowmass Inc.

Durham obeyed, the letter alleged, and asked for more information instead of making the decision. Durham could not be reached for comment.

Sanderson said he did not recall contacting Durham on Sept. 19, but said some contact may have been made by his law firm, Saunders, Snyder, Ross and Dickson. In any case, Sanderson added, he had worked for EPA as a consultant during the spring until June and again beginning in October, but not during September.

"In any case I never ordered Durham to do anything," he said.

Schroeder asked Novick to check all of Sanderson's actions in 1981 and 1982 before EPA on behalf of any of his clients.

Byron Nelson, speaking for EPA, called Schroeder's letter "a cheap political shot" related to the fact that Sanderson chaired the campaign for Schroeder's opponent in the 1980 elections, Naomi Bradford.

An aide to Stafford said the letter was being evaluated for possible use during Sanderson's Senate confirmation hearing, expected early next month.