Breaking Down Leachate

Ed Repa

In 1993, The U.S. Environmental Protection Agency (EPA) promulgated criteria for municipal solid waste (MSW) landfills that included closure and post-closure care (PCC) requirements (40 CFR Part 258). These requirements established a 30-year PCC period that a state could increase or decrease. The PCC requirements were intended to protect human health and the environment from releases from closed landfills using four primary safeguards: leachate management; monitoring landfill gas (LFG) migration; groundwater monitoring; and maintaining the integrity and effectiveness of the final cover.

Because leachate management is critical in PCC and leachate quality in part determines the length of the PCC period, the Environmental Research and Education Foundation, Alexandria, Va., funded research to find out how operational practices, waste type, waste age and other physical factors affect leachate quality with respect to typically regulated inorganic compounds, volatile organic compounds (VOCs) and biochemical constituents at MSW landfills. The research was conducted by Dr. Robert D. Gibbons of the University of Illinois at Chicago; David D. Slain of Terra-Dynamics Consulting in Lewiston, N.Y.; and Dr. Jeremy W.F. Morris of Geosyntec Consultants in Columbia, Md.

The study characterized leachate from MSW landfills located in a variety of climates and from sites that accept different waste streams to determine if leachate components were affected by final capping, age of the waste at final capping and the presence of an active LFG collection system. The study used analytical data collected over an extended period of time to show leachate quality trends.

As part of the study, leachate from MSW-only cells was compared with leachate from cells containing MSW and construction and demolition (C&D) waste, and cells containing MSW and special waste (e.g., industrial wastes) to determine if these additional materials significantly impact leachate quality. The findings revealed that leachate from sites containing different kinds of waste could not be differentiated from one another.

Based on probability, the statistical analysis suggested that the time following final capping had the largest effect on changing the concentrations of leachate components. Time since final capping was found to have statistically significant effects on two inorganic indicators (chloride and sulfate); all organic indicators [biological oxygen demand (BOD), chemical oxygen demand (COD) and total organic carbon (TOC)]; most VOCs (with the exception of ethyl benzene); and two metals (chromium and zinc). Significant time effects were not found for the remaining metals or semi-volatile organic compounds (SVOCs) that were represented by phenols.

The effect of an active LFG collection system on constituent concentration was significant for three metals (barium, chromium and nickel) and two inorganic indicators (ammonia and sulfate). But, these systems had little or no effect on chloride, organic indicators, phenols, and most VOCs. The effect of age of waste at the time of closure capping was significant for nickel and barium and the classes of organic and inorganic indicator constituents. Age of waste had little or no effect on the VOCs and SVOCs.

To determine the significance of rates of change in concentrations, both the probability values and the magnitude of the rate of change were considered jointly. The average annual estimated rates of change for the leachate components allowed evaluation of their environmental impact. For the purposes of evaluation, an environmentally significant annual rate of change was determined to be plus or minus 5 percent or more per year.

The statistical analyses of data indicated that both time after final capping with low permeable materials and age of the waste at the time of capping were both statistically and environmentally significant for:

Ammonia and sulfate were the only indicators that were both statistically and environmentally significant for the effects of LFG collection alone, not surprising since both contain compounds that are typical trace constituents in LFG. None of the constituents evaluated in the study exhibited significant effects because of age of waste at the time of final capping.

In addition, the effect of time indicated a decreasing average annual rate of change for all constituents and compound classes over the PCC period, with many found to be statistically significant.

The findings provided additional validation to previous studies conducted in laboratory settings and reinforced the concept that MSW leachate quality trends are predictable. In general, the concentrations of many constituents of concern decrease over time following landfill closure.

The study results support proactive landfill management to return a site in a condition that is both beneficial to the community and protective of human health and the environment. However, the results relate to trend only and not specific long-term threats. Actual evaluation of threats is more appropriately based upon site-specific data and evaluation methodologies that define the landfill's condition — either in terms of functional or organic stability. Nevertheless, the study's conclusions certainly support using performance-based methods to determine a landfill's potential threat to human health and the environment.

The information should allow decision makers — landfill designers, regulators and operators — to more accurately anticipate the effect that different landfill design features, management options and components will have on leachate quality over time. This, in turn, will support future operational changes — such as leachate recirculation and liquids addition and green energy opportunities — as well as long-term management strategies for closed landfills (e.g., application of all soil covers or phytocaps, use of passive or low-energy control elements).

The predictability of leachate quality over time provides a sound technical basis for decision making as the landfill industry focuses on threat-based or performance-based objectives for establishing and maintaining PCC programs at MSW landfills.

Ed Repa is vice president, environmental programs, at the Enviromental Research and Education Foundation. He can be reached at [email protected]. Copies of the reports “Municipal Solid Waste Landfill Leachate Characterization Study” and “Performance-Based System for Post-Closure Care at MSW Landfills: A Procedure for Providing Long-Term Stewardship under RCRA Subtitle D” are available by contacting Sarah Stancliff at (703) 299-5139, ext. 10, or [email protected].