AREA A/B ENGINEERING REPORT - Waste Management

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sites of different ages under various operating condition have been published (e.g., Farquhar,
1989; Christensen, et al, 1994; Robinson, 1995; Rowe, 1995; Reinhart & Grosh, 1998; Raininger,
et al 1999; Kjeldsen & Christophersen, 1999; Knox, et al, 2000; Christensen, et al, 2001; Ehrig &
Kruempelbeck, 2001; Kjeldsen et al, 2003; Robinson & Knox, 2001 and 2003; Bone et al,
2003). The body of knowledge related to this topic is very extensive.
It is important to recognize that although leachate data from older, pre-Subtitle-D landfills are
often cited as being representative of long-term leachate quality from modern MSW landfills,
these older landfills were constructed prior to the enactment of RCRA and thus often accepted
organic solvents and other hazardous wastes that are no longer permitted in MSW landfills
(except in the very limited quantities found in household waste). In support of this, a number of
recent U.S. studies show an all-around improvement in leachate quality since the enactment of
Subtitle-D, which refutes the opinion that older landfill leachate is representative of leachate at
Subtitle-D landfills. For example, Othman, et al. (2002) found that average VOC concentrations
were generally lower in leachate from post-1990 landfills than leachate from pre-1990 landfills,
and almost always lower than leachate from pre-1985 landfills. A study by Gibbons, et al.
(1999) concluded that MSW and hazardous waste leachate are easily distinguishable, based on
the lower detection frequency and constituent concentrations of 16 key VOCs in MSW leachate.
A2.1.3 Mechanisms Affecting MSW Leachate Quality
The release of compounds from a solid to a solution (i.e., leaching) involves a number of
interrelated transport mechanisms which can be grouped into those predominantly controlled by
diffusion and those predominantly controlled by percolation and kinetics. The factors controlling
leaching also affect the composition of the resulting leachate. These factors include: (i) the
leaching mechanism; (ii) the pH and Eh (redox potential) of the leaching environment; (iii) the
nature and rate of movement of percolating liquids; and (iv) properties of the waste material,
particularly with regards to physical, chemical, and/or biological changes occurring (Heasman,
1997). As described in Section A1.1, the last factor depends significantly on the age of the
landfill and the extent of biodegradation achieved.
A2.1.4 Long-Term Trends in MSW Leachate Quality
Numerous findings in literature on long-term leachate constituent trends (e.g., Kjeldsen, et al,
2003; Morris, et al, 2003a) demonstrate the predictability of these trends over time, broadly
consistent with the stages of waste decomposition shown on Figure A-1. For example, Rowe
(1995) examined the leachate concentration history for three landfills and reported that
concentrations increase to a peak value and then decrease within a monitoring period of 10 to 15
years. Several researchers have investigated the characteristics of dissolved organic matter
(DOM) in leachate (e.g., Ehrig, 1983 and 1988; Pohland, et al, 1986; Kjeldsen & Grundtvig,
1995; Barlaz, et al, 2002) and assign a BOD/COD value of less than 0.1 to a “stable leachate.”
However, ammonia typically accumulates in leachate because there is no mechanism for its
biodegradation under anaerobic conditions, even in “stable leachate” (see Robinson, 1995;
Burton & Watson-Craik, 1998; Kruempelbeck & Ehrig, 1999; Barlaz et al, 2002). At sites where
MD10186.doc 110 29 March 2009