On April 8, 2000, Spadra Landfill, located in the City of Walnut, closed to the general public after serving the eastern San Gabriel Valley since 1957. The Spadra Landfill began operations on a 128-acre site formerly known as Smith Ranch. It was operated by the Sanitation Districts under a Joint Powers Agreement with the California State Polytechnic University (Cal Poly Pomona) and the County of Los Angeles. The Sanitation Districts purchased additional acreage over the years to add to the landfill. In addition to disposal, the Sanitation Districts implemented various materials recovery programs at the site. These programs recovered 400,000 tons of greenwaste, 475,000 tons of asphalt, 576 tons of metallic discards, 380 tons of tires and 55,000 gallons of oil. The greenwaste and asphalt were beneficially reused at the site.

In 1985, the Sanitation Districts entered into the Spadra Landfill and Resource Conservation Agreement with Cal Poly Pomona and the County to create LandLab, a 320-acre center for education and research in the sustainable use of resources while providing for recycling, the diversion of waste materials, and the efficient use of refuse capacity. As part of their responsibilities outlined in the 1985 agreement, Cal Poly Pomona prepared a Master Plan which identified the post closure land uses for the site as a combination of education, research, recreation and open spaces.

The Sanitation Districts continue to monitor and maintain the environmental controls, which include groundwater and surface water monitoring, and landfill gas control at the Spadra Landfill.