United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81 -122 July 1981
Project Summary
Liners of Natural Porous
Materials to Minimize
Pollutant Migration
Wallace H. Fuller
This study concerned the relative
effectiveness of natural low-cost
liners—(a) crushed limestone, (b)
clayey soil, (c) hydrous oxides of iron,
and (d) crushed pecan hulls—to
minimize the migration of beryllium,
cadmium, chromium, iron, nickel,
zinc, and total organic carbon consti-
tuents of municipal solid waste landfill
leachates. Several leachate variables
such as aqueous dilution, aeration,
pH, and flux were also studied for their
effect on movement of metals through
11 representative U.S. soils. The
research was conducted on a
laboratory scale with the use of soil
columns as a first step in screening for
potential liners and leachate manipu-
lation practices.
This Project Summary was develop-
ed by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the re-
search project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
When landfills are not completely
impermeable then underlying ground-
water may be contaminated by leachate
constituents if these are not adequately
retained by soils between the landfill
and groundwater. Research on indus-
trial waste disposal has centered on use
of manufactured products such as
plastics and cementing roadbed mate-
rials (such as asphalt and concrete) to
alleviate the leaching problem by
completely preventing liquid movement
out of the landfill. Little attention has
been given to use of natural low-cost
materials as barriers for minimizing
pollution migration by retaining con-
taminants from liquids as they pass out
of the landfills.
The objectives of the project were to
1 Identify further those parameters
in soil and municipal solid waste
landfill leachate that influence
movement of contaminants
through soils.
2. Evaluate crushed limestone as a
liner to limit metal movement from
landfills.
3. Evaluate hydrous oxides of iron,
natural clays, and pecan hulls as
liners to limit metal and organic
constituent movement from land-
fills.
4. Study the use of control of flux
(flow rate) to limit migration of
metals and organic carbon consti-
tuents through soils.
The elements that were considered
include: arsenic (As), beryllium (Be),
cadmium (Cd), chromium (Cr), iron (Fe),
nickel (Ni), selenium (Se), vanadium (V),
and zinc (Zn) as well as total organic
carbon (TOC).
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Materials and Methods
Table 1 presents the properties of
some of the soils used in the study.
Leachate was generated in two 4000-L
tanks that had been packed with typical
municipal solid waste (MSW). The soils
were uniformly packed into PVC cyl-
inders. MSW leachate was perfused
through soil using a peristaltic pump
regulated to deliver fixed fluxes ranging
from 1.2 to 17.1 cm/day. The effluents
were collected and analyzed each hour
by use of a fraction collector. Where
necessary, leachates were enriched to a
concentration of 100 mg/L for the
element in question to ensure that
metal migration would occur through
the soil columns. The following barrier
materials were considered:
Limestone
Unsieved, commercial-grade lime-
stone (98% CaC03) from Cedar Bluff,
Kentucky, was used. The limestone,
developed for commercial agricultural
soil application, has a particle density of
2.72 g/cm3 and a fairly broad range of
size distribution. When used as a leach-
ate barrier, the Cedar Bluff limestone
was compacted to an average density of
1.67 g/cm3.
Pecan Hulls
Pecan hull waste from a local com-
mercial pecan production enterprise
was further crushed by hand in a
ceramic mortar to pass a 2-mm sieve. In
this form, it was layered over soil in a
column.
Hydrous Oxides of Iron
Iron rust was scraped from a gas
boiler, ground in a steel mortar to pass a
0.5-mm sieve, and used without further
alteration. No carbon was identified as a
contaminant. Also, waste ferrous'
sulfate was sprinkled on some columns
and allowed to oxidize.
Results
Layers of crushed limestone retarded
movement of contaminants through
soil. Concentrations of contaminants
increased more rapidly in effluents from
untreated soil columns than they did in
effluents from limestone-treated
columns. See Figure 1 for an example of
the effect of movement on Fe. The retar-
dation of Cr movement by limestone and
soil combined was greater than the sum
of the effects of each acting alone (Table
2).
Iron hydrous oxide applied as iron rust
was an effective barrier, retarding
movement of contaminants so that they
appeared much later in effluents from
treated columns than in effluents from
untreated columns. Figure 2 shows an
example of this effect on two soils for Cr
and Ni. Waste ferrous sulfate treated
soils likewise retained metals from
leachate, but increased levels of iron
were noted in column effluents. Be-
cause of this leaking effect, ferrous
sulfate waste is not recommended as a
liner. No iron leakage was noted from
the iron rust treated columns.
Conclusions
• The composition of MSW leachate
(particularly with reference to TOC
content, total ion concentration,
and pH) exercises a profound in-
fluence on retention of the metals
As, Be, Cd, Cr, Fe, Ni, Se, V, Zn,
and the more common elements
Ca, K, Na, and Mg.
• An abundance of electron donors
(TOC and Fe+2) in leachates
combined with acidity reduction,
either naturally by'aging or by in-
serting a limestone barrier,
encourages chromium retention
by soil through the mechanism of
valency change of CrVI to Crlll.
Crlll, a cation, is less mobile than
CrVI, an anionic form.
• When placed in layers over soil,
crushed agricultural limestone;
—significantly slows the rate of
movement of As, Be, Cd, Crlll,
Table 1. General Characteristics of Some of the Soils
Soil
Series
Soil
Paste,
pH
Cation
Exchange
Capacity
meg/WOg
Elec.
Cond. of
Extract,
fjmhos/cm
Column
Bulk
Density, Silt,
g/cc %
Soil
Surface
Clay, Area,
% mz/g
Predominant
Clay Minerals*
Davidson 6.4 9 169 1.40 20 52 51.3 Kaolinite
Mohave 7.8 12 510 1.54 28 40 127.5 Mica,
(Ca) montmorillonite
Ava 4.5 19 157 1.45 60 31 61.5 Vermiculite,
kaolinite
Anthony 7.8 6 328 2.07 14 15 49.8 Montmorillonite.
mica
Mohave 7.3 10 615 1.78 37 11 38.3 Mica, kaolinite
Kalkaska 4.7 10 237 1.53 4 5 8.9 Chlorite, kaolinite
Wagram 4.2 2 225 1.89 8 4 8.0 Kaolinite, chlorite
* Listed in order of dominance.
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1.0 .-
.8
.6
.4
.2
/Mohave Sandy Loam
—• No Limestone
-— Limestone
/Ava Silty Clay
•s
.8
.6
.4
.2
.01
1.0
.8
.6
.4
.2
.0
Mohave Clay Loam
I i L/U>|
- i
I
i
-i
T Wagram Loamy Sand
I i i
t Anthony Sandy Loam
j i
Kalkaska Sand
10 14 18 22 26 2 6 JO
Pore Volume Displacement N
14 18 22
Figure 1. Effect of crushed limestone la yered (2 cm) o ver six soils on the retention
of Fe from MSW landfill Leachate II.
Fe, Ni, Se, V, and Zn through
soil,
—affects metal retention in the
following order of decreasing
values:
Crlll>Se>As>V>Be>Zn>Ni>
Fe>Cd>CrVI>TOC
—exhibits metal retention effects
several times greater than
when limestone or soil alone is
used (a synergic effect),
—is poorly effective for TOC atten-
uation in MSW leachate, and
—does not effectively attenuate
the TOC in MSW leachate.
Thick layers of crushed limestone (5,10,
15 cm) provide a more effective barrier
than thin layers (2 cm) for MSW leach-
ate metal migration through soils.
• Leachate flux through soil
columns has a significant influ-
ence on attenuation of certain
metal lions in MSW leachates
when flow rates are between 2
and 12 cm/day. The effect of other
leachate variables such as pH or
concentration of TOC or inorganic
salts so overshadows the flux
effect that it is difficult to separate
flux effects on metal attenuation
from the effects of other variables.
• Hydrous oxides of iron, either
formed by spraying soils or sands
with ferrous sulfate or iron rust
placed as a 1-mm layer over soil,
delay migration of soluble Fe and
Cd.
• Other studies indicate that natural
clay soils (20%<2// clay), com-
pacted to known homogeneous
densities with sufficient sodium
salts (Na2COa, NaCI) to provide
greater than 20% of cation-
exchange capacity saturation,
may be so manipulated as to pre-
vent downward movement of
MSW leachate, solution, and all
pollutants nearly completely.
• The data reported here (except the
item above) are the result of labor-
atory studies only and are
intended as a first step in screen-
ing potential liners. They'do not
form a sufficient basis for exact
liner design. The complex nature
of the interactions of soil, leach-
ate, and pollutants require field
testing before final applications
can be recommended.
Recommendations
• Because soils are highly stratified
under natural conditions, soils
underlying the disposal site must
be made as homogeneous as prac-
tical by mixing and compacting to
ensure minimum migration of
pollutants from solid waste land-
fills.
• The use of crushed limestone
should be considered for
minimizing movement of poten-
tially hazardous pollutants such as
As, Be, Cd, Cr, Fe, Ni, Se, V, and
Zn.
• Layering limestone over soil as a
barrier between MSW landfill
leachates and geological mate-
rials underlying the landfill is
recommended over mixing lime-
stone and soil together or mixing
limestone with solid waste.
• A layer of crushed limestone
about 15 cm (6 in.) thick is sug-
gested as minimum at most
disposal sites, but specific wastes
and soils should be tested before a
precise thickness is finally selec-
ted for a given site.
• The particle sizes of crushed agri-
cultural limestone that are
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size distribution, (2) optimum
thickness, (3) cementation
characteristics, (4) channeling,
and (5) mechanics of placement.
The full report was submitted in ful-
fillment of Grant No. R 803988 by the
University of Arizona, Tucson, AR
85721, under the sponsorship of the
U.S. Environmental Protection Agency.
generally commercially available
appear to be suitable for use, but
field verification studies are sug-
gested. The most desirable
particle sizes range from coarse
gravel to medium sand (5 to 0.05
mm), with most of the material in
the middle group.
• High-quality limestone is recom-
mended because polluting metals
may otherwise be present and be-
cause, when compared with lime
(CaCOs), dolomite (MgCOa) is
highly insoluble and quite unreac-
tive at field temperatures.
• Field studies should be conducted
to verify the usefulness of crushed
agricultural limestone and should
include evaluation of (1) particle
Table 2. Effect of Limestone Alone and Limestone Layered Over Soils on the
Attenuation of Cr in MSW Landfill Leachate II*
Series
Soil
Clay. %
Ratio of
Cr in
Influent and
" — Effluent.
pH C/Co
#Pvd* Number When Cr Concen-
tration in Effluent is at
C/C0 Given in Col. 4
Limestone Soil Soil and
Alone Alone Limestone
Davidson c
52
6.2
0.13
28
Ava sicl 31
4.5 0.38
21
49
Anthony si 15
7.8 1.00 57
39
125
Mohave si 11
7.3 0.76 46
25
144
Kalkaska s
4.7
1.00
57
17
111
Wagram s
4.2
1.OO
57
15
109
* Leachate II had a pH value of 4.0, TOC of >3000, and Fe of 300 ppm.
*Pvd, Pore volume displacement number, is the number of pore volumes of
leachate that have passed through the column.
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Treatments
Soil Alone
Soil FeO
6 8 10 12 0 2 4 6 8 10 12 14 16 18 20
Pore Volume Displacements
Figure 2. Effect of the hydrous oxides of iron precipitated on Anthony si and
Wagram s on attenuation of CrVI and attenuation ofCrVlandNiofMSW
Leachate I.
Wallace H. Fuller is with the Arizona Agricultural Experiment Station, The
University of Arizona, Tucson, AZ 85721.
Mike H. Roulier is the EPA Project Officer (see below).
The complete report, entitled "Liners of Natural Porous Materials to Minimize
Pollutant Migration," (Order No. PB 81-221 863; Cost: $9.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
0 as.aOVERNMEKTmiKTlNOOFFICE: 1«61 -757-012/7236
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