Sen. Charles Mathias Jr. (R-Md.) has asked for $7.5 million to begin dredging the Baltimore harbor, in attempt to stay ahead of Reagan administration plans to limit federal spending for state water projects.

The money, if approved by Congress, would pay for the first phase of dredging the harbor channel from a depth of 42 feet to 50 feet.

The dredging project, held up for years by lawsuits filed by environmental groups opposed to the state's choice of a site for dumping the dredging spoils, is expected to cost $270 to $300 million, said Monica Healy, a special assistant to Mathias.

Mathias requested the funds in a letter to Sen. Mark Hatfield, (R-Ore.), chairman of the Senate Appropriations Committee. The money would be included in the fiscal year 1982 Energy and Water Development Appropriations bill.

The Mathias letter follows introduction by the Reagan administration of legislation that would require local jurisdictions to reimburse fully the federal government for the cost of such projects in the future.

Mathias said testimony at hearings on the U.S. role as a world coal trader indicates the country must have deep water ports to remain competitive.

The Baltimore channel's depth of 42 feet is too shallow for most coal freighters to pass through once they are loaded.

Dredging of the Baltimore port has been planned for at least a decade, but was held up by lawsuits opposing state plans to dump the spoils on Hart and Miller islands, just off the coast of Baltimore County and east of the city. m

A federal judge gave the state the go-ahead on the project last December.

he state hopes to begin dredging the 52 million cubic yards of spoil that will be placed behind a man-made dike and later converted into a beach and recreation area.