A bill being introduced in the House of Representatives would authorize up to $50 million in federal funds toward construction of a $120-million project to preserve the water quality of the Suisun Marsh east of San Francisco. Rep. Vic Fazio (D-West Sacramento) announced that he would introduce the bill today to implement a preservation plan agreed upon Sept. 6 by state and federal agencies. Salt water from San Francisco Bay has been increasing the salinity of the brackish marsh because fresh water is being diverted from the wetlands, Fazio said. He said less fresh water means fewer plants to feed waterfowl. The plan calls for the construction of levees, gates and other devices to prevent salt water encroachment. The Sept. 6 agreement requires California to pay 20% of the cost, state water contracts to pay 40% and the federal government to pay 40%. Fazio said his bill also would require the federal government to pay 40% of the continuing operation costs.