The following provides discussions of a number of case studies in Alternative Cap or ACAP designs.
Under current Title 27, California Code of Regulations, (27 CCR), section 21090, all closed landfills are required to have installed a landfill cap or cover. The landfill cover is intended to maintain a protective seal to keep moisture and rain from penetrating the landfill waste and prevent exposure of the public and the environment to the disposed waste.
The cover must be of a thickness to prevent moisture intrusion into the waste, failure of the cover by erosion and structural or integrity failure, and to prevent the cover from being breached by digging or other activities by wildlife or humans. The cover is intended to protect public health and safety and the environment.
Current Title 27 regulations require a "prescriptive" cover design, one that is established by regulation and intended for use in closure of all landfills. The prescriptive cover, as outlined in Title 27, section 21090 a (1-3), shall contain:
- A foundation layer of not less than 2 feet of appropriate materials.
- Permeability layer not less than 1 foot thick and of hydraulic conductivity not more than 1.0 x 10-6 cm/sec.
- Erosion resistant layer not less than 1 foot thick capable of sustaining vegetation and resistant to wind, raindrop impact, or runoff or mechanically resistant.
In recent years, some design exceptions have been allowed that fall outside the prescriptive standards imposed by Title 27, section 21090. These new cover designs can employ plastic high-density polyethylene (HDPE) layers, clay geotextiles and monolithic or mono-covers. The advent of bioreactors is creating another frontier in cover design. The Yolo County Landfill is one instance where a bioreactor is being employed.
- Evapotranspiration Landfill Cover Systems Fact Sheet: This resource from the U.S. Environmental Protection Agency (EPA) provides current information on final cover design evapotranspiration data.
- Site-Specific Design Report Alternative Cover Demonstration Project-Altamont Sanitary Landfill and Resource Recovery Facility: Site-specific data on mono-cover at the Altamont Landfill, Alameda County.
- Alternative Earthen Final Covers (AEFC) Theory and Practice: A presentation by Bill Albright of the Desert Research Institute, Nevada discusses cover designs.
- Alternative Cover Assessment Program, U.S. EPA: The goal of the Alternative Cover Assessment Program (ACAP) is the development of field-scale performance data for landfill final cover systems.
Workshops in Southern California covered the topic of alternative caps and discussed the evapotranspirative concept (ET) of plants and final covers on landfills. The following links provide information covering this concept:
- Landfill Gas and ET Covers: A Florida State University presentation.
- Introduction to Water Balance Covers: A presentation on this type of landfill cover by Craig Benson, PhD, University of Wisconsin-Madison.
- Life Cycle Assessment of a Bioreactor and an Engineered Landfill for MSW Treatment: A presentation on a bioreactor lifecycle assessment.