R.L. Stollar & Associates, a Santa Ana ground-water consulting firm, said it has received a $15-million contract from the U.S. Army to monitor contamination at the Rocky Mountain Arsenal in Commerce City, Colo. The contract, part of $1 billion being spent for cleanup of the contaminated area, calls for Stollar to monitor air, water, and plant and animal life over the next three years. The land was used by the government in the 1940s to manufacture chemical warfare agents and munitions, and later the property was leased to industrial firms for pesticide and herbicide manufacturing. The contract is expected to boost Stollar's work force by six, to 38 employees.