AREA A/B ENGINEERING REPORT - Waste Management

Geosyntec Consultants
Leachate is liquid that drains through the disposed waste as the waste compresses in the landfill.
When designing leachate management systems, the potential for leachate formation is assessed
through the preparation of a water balance for the landfill. This involves summing inputs (the
amount of water entering the landfill) and subtracting other outputs (the amount of water
consumed in chemical reactions and the quantity leaving as water vapor). A water balance study
at a landfill helps identify the significance of these various water components. This is important to
leachate management in terms of determining needed storage, treatment, and disposal capacities
and configuration. 7 Peak leachate flow rates generally decrease over time as active landfill
areas are closed and improved infiltration control is achieved by the final cover system.
An active gas management system (GMS) is designed and sized to collect generated LFG and to
maintain appropriate negative pressures within the landfill to optimize gas collection efficiency
and to reduce the potential of landfill gas migration or surface emissions. Design factors include
current and future MSW intake, corresponding LFG generation and yield potential, field
observations and testing, the size and characteristics of the permitted landfill area, surrounding
terrain, and subsurface conditions. These factors will determine system performance and the
means to control LFG migration and emissions.
As with any civil engineering structure, consideration of the long-term geotechnical stability of the
landfill is a key focus during the design process. Demonstrating both short- and long-term landfill
stability are components of the design calculations and modeling performed during preparation
of a permit application.
2.2.3 Construction
Following state agency approval of the landfill’s planning and design
specifications, a permit is issued to begin construction, starting with the base liner,
leachate collection and recovery system (LCRS), site access roads, and accessory
operations facilities. Leachate is contained and recovered for proper disposal
from the earliest phases of landfill use, thus supporting the effectiveness of the
liner system. Liner systems are designed and constructed to contain leachate and gas within the
landfill as well as to direct leachate and gas to their controls systems.
Liner systems are typically constructed as a composite system of low permeability materials,
including natural soils (compacted clay), geomembranes, and/or geosynthetic clay liners
(materials used in construction of containment systems are discussed in more detail in Section 3.1).
Both the liner system and LCRS are installed in accordance with a construction quality assurance
(CQA) program. An independent program of third party CQA is required to inspect, test,
document, and certify that the liner system is installed in accordance with design specifications
and regulatory requirements.
7 Estimates of leachate generation rates can be performed using several computer programs of which the USEPA’s
Hydrogeologic Evaluation of Landfill Performance (HELP) model is the best known and most widely used. Seminal
references on leachate generation include Peyton & Schroeder (1993) and Schroeder, et al (1994).
MD10186.doc 29 29 March 2009