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To
ensure that no significant amounts of contaminants leave
the site, a landfill operator needs to continuously
monitor his facility. This article provides a broad
overview of the monitoring requirements for each type
of landfill waste product.
By
Daniel P. Duffy
Landfill
Gas
Groundwater
Surface
Water
Air
and Dust
Leachate
Conclusions
It would
be a mistake to view a landfill as a static structure.
Landfills should be considered systems that accept disposed
waste and cover soils and in return generate liquids
and particulates that can adversely affect the environment.
These generated
products include landfill gas (LFG), leachate, surface-water
runoff, and airborne dust. They have the potential to
harm the health of workers and neighbors as well as
pollute the air, water, soil, and groundwater adjacent
to the landfill. To ensure that no significant amounts
of contaminants are leaving the site, a landfill operator
needs to continuously monitor his facility.
Landfill
Gas
LFG results
from the anaerobic decomposition of deposited waste.
The actual cycle of aerobic/anaerobic digestion and
conversion of organic waste constituents into methane
and other LFGs is beyond the scope of this article.
Many factors affect LFG formation: moisture content
of the waste, percent of total waste that is organic
and decomposable, percentage of the disposal volume
that is cover soil, in-place density of the waste, and
so on. The best models can only give a rough estimate
of potential LFG formation, and significant gas can
still be generated years or decades after the landfill
is closed.
LFG migrates
underground via natural and manmade pathways. Natural
pathways include fracture zones normally associated
with karst topography, significant cavernous structures,
dry pockets or strata of sand and gravel, and soil strata
interfaces. Note that migrating LFG cannot travel at
a depth lower than the current groundwater table unless
a manmade structure is provided. These structures include
the trenches associated with all types of buried utilities
(sanitary sewers, storm sewers, electrical service lines,
cable TV lines, telephone lines, and water mains). Usually
the granular or aggregate bedding for these buried utilities
is sufficiently porous to allow easy migration of LFG
along the trench line. Landfill design permits almost
always require that the locations of manmade or natural
potential pathways for LFG migration be located and
properly surveyed. Included with manmade structures
would be basements or other deep foundations where LFG
can accumulate to dangerous levels.
Asphyxiation
and explosion are the two main health risks associated
with LFG. When LFG accumulates in a trench, excavation,
or other enclosed space, an extremely dangerous situation
is present. Gas infrared analyzers ("sniffers")
are used to make sure that the air in the enclosed space
is safe to breathe, and they are used to measure gas
accumulations in monitoring probes. Gas can also accumulate
in the foundations, basements, and closed rooms of nearby
buildings. Such places can accumulate LFG until it exceeds
the mandated Lower Explosive Limit (LEL). LFG migrating
through soil at shallow depths tends to kill root systems,
resulting in visible vegetative stress along the path
of migration. Such dead or dying vegetation is typically
a clear indication of migrating gas, and monitoring
probes are usually installed in these areas to directly
measure the amount of escaping LFG.
Regulatory
standards vary from state to state. Most states, however,
have not established design standards for LFG extraction
systems, though a few have established standards for
LFG monitoring. Ohio EPA has established such standards.
OEPA requires each landfill to submit an LFG monitoring
plan consisting of a description of the monitoring system
and the procedures for monitoring and sampling. The
monitoring system design document addresses site environs,
property description, geological information, landfill
characteristics, gas generation potential, temporary
monitors, and construction standards. Operational procedures
must conform to regulatory mandated schedules and manufacturer’s
instructions. OEPA requires monthly monitoring during
the operational life of the landfill, followed by quarterly
monitoring during postclosure, semiannual monitoring
for five years of postclosure, and a grant of authorization
to cease monitoring.
Monitoring
devices include manual devices used during the exploration
of areas that might have accumulated dangerous levels
of LFG and permanent probes used for the measurement
of LFG in soils adjacent to the landfill. Probes come
in two varieties: barhole and set. Barholes are manually
set. They typically consist of 5/8-in.-diameter screened
drive points driven into the ground using a slide hammer.
Probes are usually driven until probe refusal and can
go as deep as 8 ft.
Probes provide
more permanent monitoring stations. They are typically
constructed of PVC pipe with a _-in. internal diameter,
and the slotted section of the pipe is screened at a
predetermined depth. PVC probes are typically set with
a hollow stem auger mounted on an all-terrain vehicle
drill rig. The annulus (the space between the walls
of the boring and the pipe) is filled with washed sand
to the top of the screened interval. Either bentonite
or concrete is used to cap the borehole. A removable
cap is installed on the end of the pipe, and a Teflon
tube is epoxied to a hole in the opening of the cap.
A steel protector pipe and a concrete pad provide protection
against damage from vehicles and equipment. Additional
protection in the form of bollards and visual flagging
is often provided.
Sampling
and analysis of LFG are performed on a regular schedule.
Sample readings are taken monthly or quarterly during
the operational lifetime of the landfill. During the
postclosure care period, sample readings are taken quarterly
or semiannually. Infrared analyzers measure concentrations
of LEL, oxygen, and carbon dioxide, both as a percent
and as a volume. A manually operated positive pressure
pump is used to draw gas samples from the screened interval
via a Teflon tube connected to a barb on the screened
drive point or to the Teflon tube attached to the PVC
probe cap. Measurements last for 60 seconds or until
stabilized, recording both peak and sustained levels.
After each reading, the infrared analyzer is purged
with ambient air for 60 seconds. Should LFG monitoring
indicate a significant amount of LFG is migrating from
the landfill, either the landfill operator can modify
the mechanical components of the landfill extraction
system or the structural elements of the landfill containment
structures. An unbalanced LFG extraction system can
result in the accumulation of LFG in one portion of
the landfill. The resultant pressure buildup can either
drive LFG off-site or cause damage to landfill structures
(e.g., when a bubble of LFG accumulates under a geomembrane
component of the final cover). Since MSW is very heterogeneous,
gas production rates can only be predicted engross.
Certain areas of the landfill might be generating more
gas than others. The disposal of sludges, introduction
of significant moisture content (e.g., during leachate
recirculation), and excessive use of operational cover
soils can affect LFG generation rates. Balancing the
LFG extraction system–adjusting the valve openings
at each extraction well to ensure consistent pressure
heads–is the first step. Should balancing the extraction
system fail to solve the problem, the system should
be augmented with additional extraction wells or physical
barriers.
There are
many suppliers of LFG detectors on the market. The following
are a few examples (based on each manufacturer’s
information).
Bascom-Turner
Instruments Inc. provides a range of detectors for measuring
methane and optionally oxygen in LFG. These detectors
determine the extent of subsurface LFG migration and
monitor the safety of ambient air in confined and enclosed
spaces by providing accurate %LEL and %GAS. Catalytic-combustion
and thermal-conductivity sensors detect the full range
of methane in the air. The catalytic-combustion, thermal-conductivity,
and oxygen sensors automatically calibrate.
CEA Instruments’
Portable Landfill Gas Analyzer model LMS-40 provides
simultaneous monitoring of gases (CH4, CO2,
and O2), atmospheric pressure, and temperature.
It has an internal sampling pump and a datalogger run
by internal data management software. All site parameters
can be simultaneously measured and then stored along
with the current time, date, borehole, and site location.
The LMS-40 combines a nondispersive infrared gas analyzer
for measuring concentrations of methane and carbon dioxide
and a long-life electrochemical cell for measuring oxygen
concentrations. Pressure is measured using a vacuum-referenced
pressure transducer, and an external thermistor temperature
probe may be used to obtain site temperatures.
The GEM2000
is CES-Landtec’s newest instrument for analyzing
LFG composition and calculating flow. Combining the
capabilities of the now-discontinued GA-90 for monitoring
gas migration probes and the GEM500 for monitoring gas
extraction systems, it offers improved speed and accuracy.
Groundwater
Contaminated
groundwater is usually the result of a leak in the liner
(or no liner at all in older landfills), which allows
leachate to migrate downward into a regional aquifer.
Depending on the hydrological characteristics, the resultant
contaminant "plumes" can be very extensive,
projecting thousands of feet beyond the limits of the
landfill.
Health risks
manifest themselves primarily in drinking water. Contaminant
aquifer plumes can adversely impact nearby public well
fields and individual water-supply wells. As a secondary
impact, contaminated groundwater can also pollute soils
with which it comes into contact. Accidental ingestion
or respiration of contaminated soil particles is another
health-risk pathway.
Most state
regulations require each landfill to have an established
groundwater monitoring system. The plan is based on
information derived from a detailed site hydrogeological
investigation. The hydrogeological study should determine
the depth, thickness, and flow directions of regional
aquifers, as well as information concerning intervening
strata of low-permeability soils. The plan should designate
a sufficient number of wells at necessary depths. A
minimum of one "background" well in each aquifer
located upgradient from the landfill and three downgradient
monitoring wells set in the same aquifer(s) are typically
required. Each well is designed, installed, and developed
to allow representative groundwater samples. Groundwater
monitoring wells made of PVC or HDPE slotted pipe are
set in boreholes. The annular space between the wall
of the borehole is sealed near the surface. The slotted
section of the pipe is screened and surrounded by sand
or gravel.
A groundwater
monitoring plan should also include a description of
equipment and such procedures as calibration of sampling
equipment, decontamination, and chain of custody. The
frequency of sampling is usually quarterly, though some
parameters can be tested annually. Most states also
require an annual update of the physical measurement
of the groundwater-table elevations. The resultant data
can be used to create a program for assessing groundwater
quality. This information is submitted to the state
agency each year.
Sampling
and analysis are performed to provide a statistically
significant data set. The values of each parameter tested
for in the sample will be subjected to a statistically
applicable analysis, such as a parametric analysis of
variance. This is done to identify statistically significant
parameters with elevated values indicating possible
contamination. The parameters tested for may vary from
state to state, but most are taken from the list provided
in federal regulation 40 CFR. Alternate lists of parameters
may be allowed for inorganic pollutants or for low-yield
wells.
Often there
is nothing that can be directly done to repair the failed
landfill component that is allowing leachate to migrate
out and contaminate local groundwater. Excavating in-place
waste to find and repair a breach in the liner system
is prohibitively expensive. Usually the best leachate/groundwater
contamination control system is a good landfill cap.
Older landfills that have no liner system at all should
consider a composite (geomembrane and compacted clay)
cap to prevent percolation of rainfall and leachate
formation in the first place. Exterior barriers such
as low-permeability slurry walls can also be added to
block the path of contaminated groundwater migration.
Should a severe groundwater contamination occur, usually
the only acceptable remedy is a complete remedial action
plan including groundwater extraction and treatment.
Any proposed corrective measure has to be approved by
a state agency. The approval process includes a plan
submittal, schedule for the implementation, public meetings,
and an allowance for immediate interim corrective measures.
Groundwater
monitoring equipment (most of which can also be used
for leachate monitoring) is available from a variety
of suppliers and meets a variety of needs (based on
each manufacturer’s information).
Hydrolab
Corporation’s Quanta·G water-quality instrument
is designed specifically for groundwater monitoring.
The Quanta·G monitors up to eight different water-quality
parameters simultaneously. It is equipped with a 1.75-in.
316 stainless steel housing. Users can monitor temperature,
specific conductance, salinity, total dissolved solids,
dissolved oxygen, pH, oxygen-reduction potential, depth,
and vented level. The display logs 100 frames of data.
Voss Technologies
offers SingleSample disposable bailers, accessories,
and the EasyPump system for taking representative samples
without purge water.
Solinst Canada
Ltd. manufactures groundwater monitoring equipment for
site characterization, spill investigations, and long-term
groundwater monitoring, including its Water Level Meter,
Interface Meter, Integra Bladder Pump, Waterloo Multilevel
System, Peristaltic Pump, Drive-Point Piezometer, groundwater
sampler, and Flow-Through Cell.
GeoLog Inc.
manufactures groundwater sampling equipment, including
bladder pumps, purge pumps, bailers, and such related
accessories as air compressors, pump-cycle controllers,
tubing, well caps, and electronic static water-level
indicators. Equipment has been field-tested by EPA under
its Environmental Technology Verification Program.
QED Environmental
Systems Inc. is the world’s largest manufacturer
of pneumatic pumping systems for groundwater sampling,
control of leachate and condensate liquids at landfills,
and recovery of contaminated groundwater including fuels
and solvents. Specializing in low-flow groundwater sampling
systems, QED’s equipment offers accurate methods
to conduct long-term dedicated monitoring.
Surface
Water
Precipitation
that does not percolate through final or intermediate
cover will run off the landfill as surface water. Such
runoff is usually collected in diversion channels that
direct the flow to a sedimentation basin. There the
runoff is detained for enough time to allow sediment
particles within the runoff to settle out prior to discharge.
The basin’s discharge point is the primary source
of contaminants, suspended sediment, or other debris.
Should other contaminants be detected (volatile organic
compounds, metals, high levels of biochemical oxygen
demand [BOD] or pH, and so on), it is usually an indication
that the runoff has encountered exposed waste. Should
this occur, the runoff is usually considered to be leachate
and measures are taken to either cover the exposed waste
or ensure that the contaminated runoff does not leave
the landfill.
Assuming
that the runoff control structures were designed and
built correctly and are in good repair, the only pathway
for surface-water contaminants to exit the landfill
is via the sedimentation-basin discharge structure.
The basin itself should have been properly designed
with enough retention time to ensure that sediments
carried by the surface water are settled out. Basins
are designed to handle only a certain peak storm, however;
the 25-year/24-hour or 50-year/24-hour rainfall events
are typical. A larger storm might result in excessive
flows and the passing of sediment to natural waters
beyond the basin.
Compared
to other potential landfill discharges, excessive sediment
has a relatively minimal health impact. Besides increasing
the turbidity of downstream waters, excessive sediments
could have an adverse effect on stream-derived drinking-water
supplies and local biota. Usually surface-water discharges
are retained in settling basins long enough to ensure
that clay-size particles have settled out. A prohibitively
large retention basin is usually necessary to settle
out silts.
Bucket sampling
of discharging surface water is the most common form
of sampling. Sampling and analysis is usually done quarterly
or after excessive rainfalls (25-year/24-hour rainfall
events or greater). In addition to visual observations
of turbidity (comparing the color of the discharge to
a standard color chart) and measurement of suspended
sediment, organic and inorganic parameters, such as
those listed in 40 CFR, may also be tested for.
Should testing
indicate the presence of organic or inorganic pollutants,
the probable cause is a breach in the cover system that
has allowed surface water to come into contact with
exposed waste. Technically this liquid is now considered
leachate and should be prevented from leaving the landfill
if at all possible. The breach in the cover system should
be repaired as quickly as possible. Should discharges
contain excessive amounts of sediment, there are several
repair options, such as vegetation of the cover to reduce
erosion and modifying the basin to increase retention
time.
Surface-water
monitoring has different goals than groundwater or leachate
monitoring. As such, the equipment used differs in design
and operation (based on each manufacturer’s information).
Advanced
Measurements and Controls Inc. manufactures a Dissolved
Oxygen Sensor. The sensor design requires no moving
parts and features linearity and repeatability, supplementary
water-temperature reading, and toleration of oil-contaminated
environments such as the long-term monitoring of landfill
sites. The unit’s chemical sensor provides inherently
greater stability than does conventional polarographic
technology. Signal conditioning electronics are designed
with temperature compensation to provide true output
in ppm of dissolved oxygen or percent saturation.
The Greenspan
Turbidity Sensor offers a sensitive system based on
an array of high-gain infrared optics that provide excellent
accuracy at low turbidities. The design is based on
an infrared emitter and receiver module and an electronic
detection and measurement unit. The optical system transmits
an infrared beam of 860 nm and detects the backscatter
intensity to determine turbidity.
Air
and Dust
Dust and
blown litter are typically a problem only during active
waste disposal operations. While blowing litter is not
considered a pollutant that requires monitoring, most
states do require monitoring of dust. Dust is primarily
produced by equipment operations during the construction
of compacted soil liners and caps. Though constantly
wetted by water trucks to achieve the required moisture
content, the clays used to construct the liner inevitably
generate significant dust clouds. The roads used by
the haul trucks, and later by the trash trucks, are
also a major source of dust. Again, water trucks can
continuously wet a road’s surface, but dust is
never completely eliminated.
The primary
pathway for air and dust contaminants is in the form
of windborne particulates. Even on nonwindy days, dust
can be a problem. It tends to hang in the air obscuring
operators’ vision, cause choking and coughing among
unprotected operators, and generally degrade the appearance
of the site (an important concern relating to public
acceptance of the landfill).
Long-term,
repeated exposure to even nonhazardous airborne particulates
can cause serious health problems. Even simple silicates
can impact the respiratory system of unmasked workers.
Wearing a filter mask or filtering the air supply to
operators in their cabs is always recommended.
Very few
states have hard and fast regulatory standards for dust
control at sanitary landfills. Most simply require a
written dust-control plan from the operator as part
of the permit design application. Far more stringent
are the standards at hazardous or nuclear waste sites.
At these facilities a timed observation rule is typically
used.
Monitoring
often takes the form of simple visual observations.
When equipment operations or vehicle traffic generate
significant quantities of dust, the site safety and
health officer will visually observe the resultant dust
cloud. Using a stopwatch, he can measure the amount
of time until the cloud settles and normal visibility
is restored. The amount of time allowed for dust clouds
to obscure visibility before it is considered a problem
can vary from 30 seconds to two minutes, depending on
site conditions. Should dust be considered excessive
by site standards, additional and more frequent water-spray
truck operations will be required.
Where physical
sampling and air monitoring are required, filter canisters
meeting PM10 requirements are typically used.
A background canister is set upwind from the project
to establish local conditions. At least three other
canisters are set downwind from the operations area
along the site’s property line. Usually, however,
such strict monitoring procedures are not necessary
at a landfill.
Leachate
Leachate
results when percolation (either rainfall or snowmelt)
infiltrates through the landfill’s cover, comes
into contact with deposited waste, picks up contaminants,
and then accumulates within the landfill. The primary
means for estimating leachate production is the Hydraulic
Evaluation of Landfill Performance (HELP) model. While
the estimates made by this model are not to be taken
as literal predictions, the values generated are useful
in providing comparisons for different landfill designs.
Given local climate conditions, the configuration of
the landfill’s cap, waste’s thickness, and
liner system can be altered using the HELP model to
determine which factor has the greatest impact on leachate
production. Once these factors have been determined,
modifications can be made to the landfill design and
construction to minimize the potential for accumulated
leachate or leachate outbreaks.
Leachate
outbreaks occur on the sideslopes of a landfill and
are usually the result of leachate accumulating above
a semipermeable or impermeable soil stratum within the
waste. This stratum is typically leftover intermediate
cover that was never stripped away prior to further
waste disposal operations. Once the leachate can no
longer percolate downward, it follows the sideways path
of least resistance until it reaches an exterior slope.
If the leachate collection and extraction system also
fails, leachate can accumulate on the floor of the landfill
in excess of regulatory rules. Subtitle D and most state
regulations allow no more than 12 in. of leachate-head
accumulation on the bottom of the landfill. Heads greater
than this amount are thought to represent a potentially
excessive driving head that would force leachate through
a liner system into underlying soil and groundwater.
Depending
on its constituents, leachate can pose a serious threat
to groundwater supplies. Leachate outbreaks on sideslopes
can also contribute to potential slope instability and
sliding failure.
Aside from
visual observations with "dipsticks" placed
down the leachate extraction riser into the sump area,
leachate head accumulation is usually measured by pressure-sensitive
depth gauges typically built into the extraction sumps.
These extraction pumps are designed to cycle on and
off depending on the depth of leachate that accumulates
within the collection sump.
Sampling
and analysis are primarily performed for the benefit
of the recipient of the leachate and are usually done
on a quarterly basis. Local publicly owned treatment
works that manage the community’s wastewater, as
well as the regional sanitary sewer system, have standards
regarding what type of leachate they can accept. Constituents
of concern include fats, oils, and grease; BOD; phosphorus;
metals; polychlorinated biphenyls; pesticides; and other
contaminants listed in federal regulation 40 CFR Part
136. Most leachate contains these substances, though
not typically in amounts in excess of regulatory levels.
The exception typically is BOD, which can be two to
four times higher than regulatory acceptance standards.
Most onsite pretreatment performed prior to final discharge
is designed to reduce the leachate’s BOD to acceptable
levels for final treatment.
Should excessive
leachate heads accumulate in the landfill sumps, the
leachate extraction and collection system should be
either repaired or upgraded. The existing pumps may
be undersized for actual needs. This can commonly occur
during the early stages of a disposal cell’s life
when waste does not completely cover the sand and piping
of the cell’s leachate collection system. Once
a thick layer of waste has been deposited over the entire
cell, the absorptive capacity of the waste can greatly
reduce the amount of free liquid that eventually becomes
leachate.
(See section
on groundwater monitoring for a listing of typical leachate
monitoring equipment.)
Conclusions
Liquid and
airborne contaminants from a landfill are capable of
contaminating local air, soil, surface water, and groundwater
to the point of posing a danger to human health and
the environment. Landfills should be treated like dynamic
systems instead of static piles of waste. Waste, cover
soil, and percolating rainfall cross the boundary into
the landfill. There they interact and create leachate,
LFG, and surface-water runoff. By diligently monitoring
the landfill for these potential contaminants, the operator
can ensure that the landfill’s systems are functioning
properly. The monitoring provides a system feedback
information loop that can be used as the basis for redesign
and field modifications.
Daniel
P. Duffy, P.E., is a professional environmental engineer
in Cincinnati, OH.
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