A coalition of 27 labor and civil rights groups yesterday petitioned Labor Secretary William E. Brock to overturn the department's April 12 decision rejecting a federal standard requiring farms employing more than 10 farm workers to provide toilets and clean drinking water.

The petition said the decision by Robert A. Rowland, head of the Occupational Safety and Health Administration, was "clearly contrary" to the evidence of need and may have been tainted by a conflict of interest. Rowland owns $15,000 to $50,000 in stock in Tenneco Co., whose agribusiness subsidiary could be affected by the decision. Rowland earlier said through a spokesman that he saw no conflict of interest because his decision will have "little or no impact" on the value of Tenneco stock.

"Never in the history of OSHA has a decision been made which was so clearly contrary to the weight of the evidence . . . ," said the petition by the AFL-CIO, the U.S. Catholic Conference, the Farm Worker Justice Fund and others. "Every single medical, public health, or scientific expert who testified [at OSHA hearings] supported the need for a strong sanitation standard."

At his confirmation hearing last month, Brock pledged to review the decision. Spokesman David Demarest said yesterday that Brock intends to review the matter "as soon as possible."

Rowland's decision followed a 13-year court battle by the Migrant Legal Action Program. Uncontested testimony at 1984 OSHA hearings showed that because of poor sanitation, farm workers suffered excessive incidence of parasitic disease, diarrhea and other maladies. OSHA estimates said the facilities would cost less than $1 per worker. OSHA's decision said that most workers are already covered by state standards; that enforcing a new rule would have a low priority because of more pressing health concerns; and that coverage actually could be reduced by an OSHA ruling because it would apply only to employers of 11 or more, while states already cover smaller farms.