A Beltsville water and sewer contractor pleaded guilty yesterday to failing to pay state taxes on income that prosecutors said he received through a scheme to divert thousands of dollars in company money to his own use.

Otha D. Upchurch, secretary-treasurer of the Suburban Utility Contractors, Inc., a water and sewer contracting firm, pleaded guilty in Anne Arundel County Circuit Court to one count of providing false information on his 1980 state income tax returns. Prosecutors from the state attorney general's office said that Upchurch failed to report income of more than $40,000 for 1977, 1978 and 1980.

Upchurch was one of several contractors involved in allegations of bid rigging to win contracts to build water and sewer lines in the Maryland suburbs.

Judge Raymond G. Thieme immediately sentenced Upchurch to 30 days in the county's detention center. He was ordered to devote 200 hours to community service and to amend his tax returns for those three years, including paying all unpaid taxes, interest charges and penalties.

Upchurch was the fourth water and sewer contractor convicted of tax evasion charges since the start of an investigation by the attorney general's office in late 1980. That probe came after a series of articles in The Washington Post in August 1980 detailed price fixing and bid rigging among contractors who bid on contracts from the Washington Suburban Sanitary Commission.

Also convicted of tax evasion to date was Upchurch's boss, Marvin F. Bosley, the president of Suburban Utility Contractors. Two officials of Tri-County Utilities, Inc., of Prince George's County, Frank T. Hauser, Jr. and Lee Ganey, Jr., pleaded guilty to similar tax evasion charges and received sentences similar to Upchurch.