Pasadena engineering firm Tetra Tech Inc. said Tuesday that it acquired an engineering firm based in Diamond Bar, Bryan A. Stirrat & Associates, for an undisclosed amount.

BAS, which has annual revenue of $40 million and 150 employees in California and Arizona, provides landfill design, hazardous waste management, groundwater protection and other services for municipal and commercial customers, Tetra Tech said. The two companies have worked together on landfill remediation programs, including a brownfield redevelopment near Los Angeles.

The BAS acquisition strengthens Tetra Tech’s technical capabilities in watershed protection and adds new customers throughout the southwestern United States, Tetra Tech Chief Executive Dan Batrack said in a statement.

It’s the company’s fourth recent acquisition. Tetra Tech Inc. earlier this month acquired three companies that expanded its geographic coverage and technical services. The largest was Tesoro Corp, a Virginia construction management firm that works primarily for the U.S. Department of Defense and has annual revenue of $80 million.

The other two were Mussetter Engineering of Fort Collins, Colo., and ACI Engineering of Vancouver, B.C. Mussetter provides expertise in water resources and environmental hydraulics. ACI provides high voltage power engineering to support energy transmission.

Tetra Tech shares were down 10 cents, or less than 1 percent, to $29.04 in midday trading on the Nasdaq.