the long-term impact of landfills on the abiotic environment

18 Pages·2010·0.87 MB·English

BRGM
L'ENTREPRISE AU SERVICE DE LA TERRE
THE LONG-TERM IMPACT OF LANDFILLS
ON THE ABIOTIC ENVIRONMENT
BRGM
L'ENTREPRISE AU SERVICE DE LATERRE
THE LONG-TERM IMPACT OF LANDFILLS
ON THE ABIOTIC ENVIRONMENT
Alain CM. BOURGÎ^) and A. Florence LALLEMAND-BARRES^^)
SEPTEMBER 1990
R 31573
GCH SGN 90
..V BRGM
^^' National Geological Survey
,y. Department Geochemistry
^^' Surface and subsurface Service
Department Environment
BP 6009, 45060 ORLEANS Cedex, France - tel. : [phone]
ABSTRACT
Unless wastes are totally isolated from the environment (which
requires unreasonable costs), some migration of leachates from wastes
deposited in landfills will always occur. Active prevention implies
minimizing the entrance of water in the landfill and draining leachate
generated for proper treatment before disposal. Passive prevention
implies the selection of a site where natural attenuation is efficient
(highly adsorbing substratum, thick unsaturated zone for proper degra¬
dation and, finally, drainage into an aquifer of no value) .
This work was performed for the Commission of European Communities
(Directorate General on Environment, Nuclear Safety and Civil Protection)
TABLE OF CONTENTS
1. INTRODUCTION (cid:9) 1
2. IMPACT OF LANDFILLS ON SUBSOILS AND WATER (cid:9) 2
2.1. Origin and nature of pollutants coming out of
landfill sites (cid:9) 2
2.1.1. Lïquid leachate production (cid:9) 2
2.1.2. Gas generation (cid:9) 3
2.2. Hydric balance and the influence of climatic conditions
on the production of effluents (cid:9) 3
2.2.1. Infiltration of leachate (cid:9) 5
2.2.2. Influence of properties of the substratum (cid:9) 8
3. CONCLUSION (cid:9) 13
REFERENCES (cid:9) 14
1 . INTRODUCTION
The disposal of hazardous wastes in landfills poses significant threats
to the environment soil and water pollution and gas release all three sus¬
ceptible to induce serious health hazards. The increasing amount of wastes
to be disposed of, aggravated by their increasing toxicity, has resulted in
a change in waste management policy. Growing concern for better operation
and control of landfills has resulted in the development of fewer but larger
dumping sites. This, in turn, requires state-of-the-art management because
the natural environment can no longer carry out its filtering or attenuation
function in presence of the tremendous quantities of contaminants concentrated
in small areas. Smaller dispersed landfills may be better adapted to the
filtering capacity of the lithosphère. However, this option, as mentioned
before, is not economically feasible if high quality operation and easy
control is sought.
Any landfill is a potential source of contamination. The evaluation
of the risk involved is complex because a landfill is not only the source
of the pollutants it contains but also a reactor where wastes evolve and
are transformed, due to chemical, biological and thermal reactions.
This paper gives technical background infonnation on the impact of
landfills on the abiotic environment. As the primary receptors of land- ,
fill originating pollutants, emphasis will be placed on subsoils and aquifers.
Throughout this paper, the term subsoil is used to identify the unsaturated
zone beneath the landfill site, whereas aquifer designates the saturated zone.
2. IMPACT OF LANDFILLS ON SUBSOILS AND WATER
2.1. Origin and nature of pollutants coming out of landfill sites
The effluents produced by landfill wastes are mostly liquid and gaseous.
Contaminated solids may be dispersed by wind during landfill operation and
colloids may be transported to the underground. But it is usually recog¬
nized that in the long term, with the exception of karstic or other widely
fractured substrata (where colloids can be transported over considerable
distances), the most significant impact of landfills on the environment is
due to the production and leaching out of liquids and gases.
2.1.1. Liquid leachate production (after Cartwright, 1984)
Leachate is produced in landfills when infiltration from rainfall,
surface drainage and/or groundwater inflow combine with the moisture already
present, or formed due to bacterial action, in the waste to exceed the liquid
holding capacity. Compaction reduces the liquid holding capacity of the
waste. This, in addition, may squeeze moisture from the refuse causing
leachate movement prior to infiltration of rainwater.
Typical ranges of leachate composition are given in Table 1. The
composition of the liquid effluent will depend on (a) the nature of the
domestic, industrial or mixed refuse materials and on their interactions,
(b) climatic and hydrogeological settings, and (c) time. During the life
time of a landfill we can observe the following sequence: First an acidifi¬
cation phase and later a fermentation phase, during which organic pollutants
are emitted in high quantities. These products are more or less stable and
their concentration decreases with time.
The transport of pollutants with the leachate is conditioned by their
solubility, which, in turn, can be strongly dependent on the chemistry of
the leachate itself (e.g., precipitation and complexation of heavy metals).
Table 1. Characteristics of Leachate
Liachali-'
Range" Range'' Range''
Coiiütitticnt (mg/L) (mg/L) (mg/L) Fresh Old
Cliloride (Cl) 34-2.800 100-2,400 600-800 742 197
Iron (Fe) 0,2-5,500 200-1,700 210-325 500 1.5
Manganese (Mn) 0.0C-L400 75-125 49
Zinc(Zn) 0-1,000 1-135 10-30 45 0.16
Magnesium (Mg) 16.5-15,600 160-250 277 81
Calcium (Ca) 5-4,080 900-1,700 2,136 254
Potassium(K) 2.8-3,770 295-310
Sodium (Na) 0-7,700 100-3,800 450-5(X)
Phosphate(P) 0-154 5-130 7.35 4.90
Copper(Cu) 0-9.9 0.5 0.5 0.1
Lead (Pb) 0-5.0 1.6
Cadmium (Cd) 0.4
Sulfate (SOj) 1-1,826 25-5(K) 400-6.50
Total N 0-1,416 20-500 989 7.51
Conductivity(jimlios) 6,0()0-9,(K)0 9.200 1.4(H)
Total dissolved solids 0-42,276 10,000-14.(HK) 12.620 l.lll
Total suspended solids 0-2,685 l(K)-7()() 327 2(i(i
pH 0.7-8.5 4.0-8.5 .5.2-6.4 5.2 7.:)
Alkalinity as CaCOj 0-20,8.50 8(M)-1,(K)0
Hardness total 0-22,800 200-5,250 3,5(X)-5.(KK) (cid:9)
lliological oxygen demand 9-54,610 7,.5(K)-1(),(XM)- 14.950
Chemical oxygen demand 0-89,520 1(M)-51,(KM) IG,(KK)-22.(MH) 22.(i.50 81
"U.S. Environmental Protection Agency (1973). 'U.S. Environmental Protection Agency (1975).
''Stein et al. (1971). ''Brunner and Carnes (1974).
2.1.2. Gas generation (after Cartwright, 1984)
The principal gases produced by landfills, caused by the fermentation
of organic wastes, are potentially explosive mixtures of methane (CH.)
and air (explosion risk for mixtures of 5 to 15% of CH.) and the acidifying
carbon dioxide (C0 . Methane is considered to be the greatest concern
since the acidification problem can usually be easily overcome by the
natural buffering capacity of most geological materials.
The typical pattern of gas generation of a sanitary landfill consists
of four phases (Figure 1): Phase I, aerobic; Phase II, anaerobic non methano-
genic; Phase III, anaerobic methanogenic, unsteady; Phase IV, anaerobic
methanogenlc, steady. The nonmethanogenic stage is initiated by hydrolytlc
processes by reducing complex organic matter to soluble components by means
of cellular enzymes. The microorganisms involved in the methanogenic stages
are the common bacterial inhabitants of soil and sewage.
LANDF LL GAS PRODUCTION PATTERN
PHASE
I , n , m EZ
100 1 1
1 1
1 1
1 '
Nz
80
A ~-^ 1
UJ
Z 60 l\ / ' \^
11/ 1
I O 1 y ^!^xciir '
> i/\
40 -- '/\ CHy^ 1
/ i
¡r 20 M1 Vr-x
oz i ^
<
_l
TIME
Figure 1. Sanitary landfill gas production pattern (from
Farquhar and Rovers, 1973)
The generation of gas is controlled by waste composition, moisture,
temperature, alkalinity and pH. The rate of gas production and the duration
of each of the three initial phases can vary considerably, depending on con¬
ditions. Most domestic refuse landfills will reach stable CH, production
(Phase IV) in 200 to 500 days (Farquhar and Rovers, 1973).
The availability of moisture is perhaps the most important controlling
factor. The presence of water favors the aerobic fermentation of organic
refuse, reducing the production of methane.
The methane is slightly less dense than air. But mixed with air it
can migrate not only towards the atmosphere but also towards the bottom and
the sides of the landfill site.
2.2. Hydric balance and the influence of climatic conditions on the
production of effluents
The main carrier of pollutants out of the landfill is liquid leachate.
It is therefore of prime importance to understand the hydric balance in
landfill sites in order to minimize the production of leachate and its
export out of the site.
The current methods of predicting water balance come from those
developed for hydrological and agronomic studies. They are based on the
relationships between rain, evaporation, runoff, underground flow and
water present or produced within the site (Figure 2).
Evaporatio
Landfill
runoff
unsaturated
saturateti
underground flow
infiltration
Figure 2. Hydric balance of a landfill
Leachate in the site = water or moisture contained in the refuse
+ water produced in the refuse
± variation in moisture present in the material
(soil?) layed out to cover the waste during
and after landfill operation
+ rain - evaporation
+ runoff (in and out of the site)
± infiltration in (of clean groundwater) and out
(of contaminated leachate) of the site
If some of the parameters of this equation are rather easily quanti¬
fiable (rainfall. Initial water content of the refuse), other important ones
are not (runoff, evaporation, water content of the aged waste material and
its variations).
Runoff depends on the intensity and the duration of precipitation,
the permeability of the soil, the slope. For a given soil, it can be calcu¬
lated using empirical runoff coefficients. On landfill sites the theoretical
values obtained are rather far from reality (because of the heterogeneity and
the roughness of the surface) .
Evapotranspiration of a soil can also be calculated using empirical
formulas (Thornwaite, Turc, Penmen and others) taking into consideration
the average temperature, the total solar radiation, the relative humidity
of the air, etc. These formulas are not directly applicable to landfill
sites where the évapotranspiration is probably negligible inside of the
landfill, the air in the waste material being immobile (unless there is
ventilation) .
The water content of the waste and its variations depend on the nature
of the waste material, the initial water content and the degree of compact¬
ness. The retention capacity can vary considerably depending on the type
and the physical state of the waste material (capacity of from 0.18 kg/kg
of dry matter for a foundry sand to 2.6 for saw dust and up to 7 for a
surface treatment mud). Domestic refuse has a retention capacity of from
0.5 to 2.5 (depending on the author) at the laboratory scale. In the field
the effective retention capacity seems to be lower. In addition, very often,
leachate appears before the waste materials are entirely saturated (prefer¬
ential circulation by canals and interstices in a very heterogeneous matrix) .
Protection of the environment requires preventing (a) the presence and
the production of leachate and (b) its export out of the site.
Meteorological conditions (rainfall and évapotranspiration) can vary
dramatically. For example, in France, mean efficient rain (i.e., rain minus
evaporation) can quadruple from one place to another. Depending on the rain-
evaporation balance, landfill operations must be carried out so as to minimize
meteoric water input. This may require, for example,
-an efficient drainage system to prevent surface runoff from reaching
the area being filled,
-isolation of the area under exploitation from the rest of the site
and from superficial water outside the site (no surface runoff out
of the site) .
Not only the quantity of leachate produced, but also its composition
and concentration are conditioned by the state of dryness of the refuse.
Indeed, the less leachate volume generated, the lower the fraction of soluble
elements effectively solubilized. In spite of the high concentration of
solubilized chemicals in this small volume of leachate, all the soluble
chemicals have not been mobilized because one can expect some sort of an
equilibrium reaction, controlled by a mass action law, between the substance
considered and the "saturated" solution.
It is also essential to prevent the introduction of groundwater into
the site whether due to the percolation of meteoric water in the unsaturated
zone or to the direct soaking of the bottom of the landfill site in the
saturated zone. Imperméabilisation of the sides of the site should prevent
the first situation. As far as the second is concerned, no site should be
built without a proper thickness of unsaturated zone above any aquifer.
After the production of leachate has been minimized, its movement out
of the landfill should be prevented or minimized. The movement of liquids
and gases outside of the site is mostly governed by gravity. It should,
however, not be forgotten that due to the usual operation of dumping in
layers, there may be a strong component of liquid (or gaseous) flow in a
horizontal direction, towards the sides of the site. If this polluted
liquid flow cannot be prevented, the bottom and the sides of the landfill
should be designed and engineered so that the leachate can be drained and
collected. After proper treatment, the unpolluted liquid can be discharged.
Indeed, the main function of a waterproof layer (artificial or natural)
is to present a sufficient and homogeneous discontinuity in the vertical
permeability profile so that, together with engineered drains, preferential
flow of leachate occurs towards collecting stations.
This drainage of leachate will be necessary as long as polluted
liquids are produced and flowing out of the landfill site.
2.3. Transport of polluted leachate to the subsoil and groundwater
2.3.1. Infiltration of leachate
2.3.1.1. Hydraulic conductivity (or permeability)
The hydraulic conductivity (K) of a geological formation is defined
by its aptitude to be traversed by a fluid under the influence of a gradient
potential. This parameter is thus essential for the evaluation of water
flow through the subsoil underneath or on the sides of a landfill site.
The range of hydraulic conductivity values of various formations is given
in Figure 3.
_Q Homogeneous clay formations, with a hydraulic conductivity lower than
10 m/s, are often classified as impermeable. However, as we will see below,
no rock is fully impermeable.
The hydraulic conductivity in the unsaturated zone varies with the
water content. It reaches a maximum at saturation and therefore measurement
should be carried out under this condition.
Unconsolidated K Permeability
Rocks
deposits (m/s)
r1
very high
10-
ll O 10-^
-10-'
re high
cn
re
t I .10-^
£" 10-^
r «
S 3
a.
-10"* medium
to
"I
= 2 c c -10'' low
-10""
w
-10-*
low
-10-'^
to
(O c
C 10-" very low
10-''
3 ^
- co
111
Figure 3. Range of hydraulic conductivity (after Freeze and Cherry,
1979)
When the subsoil becomes dry, cracks and fissures may form, especially
if the geological formation contains expanding clay minerals such as ben¬
tonite. It is thus a paradox that clay formations should be humid to form
a good hydraulic barrier, but even then the barrier is not totally efficient
(see below) .