Water percolation in different sediment media: a study of sediment capabilities in
landfill systems
By: Dayton Brown and Tyler Greenwood
ED 4260
For: Matt Roy and Wayne Melville
February 28, 2014
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Brown and Greenwood
Water percolation in different sediment media: a study of sediment capabilities in
landfill systems
Introduction
The process of water percolation effect’s everyday life on earth as water
slowly makes its way from the earth’s surface from rainwater toward the
groundwater system. However, the contents in the water are not always beneficial
to the surrounding areas health and well being, with landfill sites acting as potential
threats of pollution to an ecosystems overall health. In an attempt to keep the
ground water clean, landfill sites use techniques to contain contamination above the
groundwater system. Clay is used in a variety of forms to decrease the amount of
contaminant movement through subsurface soils (Sarabian and Rayhani 2013;
Hamdi and Srasra 2013; Zhan et al. 2014). These forms include a compact clay layer
that absorbs water, while also creating a barrier between the topsoil contaminants
and the subsoil beneath the layer (Hamdi and Srasra 2013). Along with the compact
layers, a technology known as a geosynthetic clay liner, a layer made from a mesh
material and clay, is laid down prior to the development of a landfill with a similar
goal of keeping contaminants out of the groundwater system in mind (Sarabian and
Rayhani 2013).
When developing landfills there are selected sediment characteristics that
are adhered to when constructing a layer that will restrict the movement of
contaminated water to the groundwater system. The ideal material consists of small
sized sediment particles that create small pores or “pore spaces” that make it
difficult for water to move through the layer due to hydrogen bonding of the water
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molecules and the surrounding sediment (Hamdi and Srasra 2013; Sarabian and
Rayhani 2013). The material also must not expand or contract significantly when
water is added or removed (Hamdi and Srasra 2013). If this occurs the layer is
susceptible to leaking, allowing the contaminated solution to move toward the
groundwater system.
Geosynthetic clay liners, made of mesh materials and clay, have been
implemented into landfill sites in an attempt to create a less permeable layer to
restrict the movement of contaminated water to the groundwater system (Sarabian
and Rayhani 2013). In attempt to create a model to show how geosynthetic clay
liners are affected in the environment, Sarabian and Rayhani (2013) looked at the
effect annual weather patterns have on the clay liners. The study revolved around
subjecting one geosynthetic layer to constant temperatures, and another to varying
cycles of heat to study the differences in water retention between each liner
(Sarabian and Rayhani 2013). It was found that heat caused the geosynthetic layer
to have adverse effects when studying the water retention and permeability of the
layer (Sarabian and Rayhani 2013). It was concluded that prolonged exposure to
environmental conditions creates problems in the layers permeability (Sarabian and
Rayhani 2013). As a result, it was determined that the clay liners should not be
exposed above ground for a long period of time in order to combat against the
adverse affects that the environment places on the materials (Sarabian and Rayhani
2013). While the study we conducted did not delve into the specifics of different
environmental conditions on the sediment, the study has the capabilities to be
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expanded on in the future to incorporate how different climatic zones act on the
sediment types.
Studies found that uncontrolled landfill areas account for a substantial
amount of solution movement through the sediment (Zhan et al. 2014). This
wastewater can be hazardous to the surrounding area if the water is left
unrestrained. An uncontrolled landfill site in southeastern China relied on the
natural clay sediment of the area to contain the contaminated surface water of the
landfill (Zhan et al. 2014). In this particular case the pollution had travelled 9m into
the sediment over a 17-year period (Zhan et al. 2014). While this is a single case,
there appears to be a correlation between the effect of contamination and the
absence of an impermeable clay layer that restricts the movement of water from the
upper horizons (Zhan et al. 2014). The implementation of clay barriers appears to
be an integral part of a landfill site to stop contamination of ground water.
While the impermeable barrier that the clay produces is an essential part to
containing the pollution on site, there is also an emphasis on the material that can
be used to cover the clay boundary (Sarabian and Rayhani 2013). It has been
determined that a “poorly graded sand subsoil” is the best material to place above
the barrier to ensure the correct amount of moisture reaches the clay layer,
ensuring that the barrier does not fail due to environmental forces (Sarabian and
Rayhani 2013). The goal here is to incorporate sediment that will allow water to
keep the layer moist, while allowing the clay compound to contain the contaminated
water in the upper horizons (Sarabian and Rayhani 2013).
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Furthermore, sand can act as a water filter when it is implemented with the
correct mixture of materials (Janzen et al. 2009). Janzen et al. (2009) researched the
affect of micro-pollutants from wastewater on soils, specifically on how to filter the
contamination that was present in the water. By layering gravel, sand, and peat, it
was found that micro-pollutants could be retained and broken down in the
boundary layer due to bacteria (Janzen et al. 2009). The layers of media allow for
optimal water retention that replicates the characteristics of a natural wetland
(Janzen et al. 2009). The research took the simulation a step further and created a
reed filled top layer to allow for further retention (Janzen et al. 2009). While this
may not be a feasible implementation strategy for landfills as bulldozers are
continually moving the topsoil, it may be something that landfills can find a solution
to ensure pollutants are separated as much as possible.
By subjecting different types of sediment to higher than normal quantities of
water in the environment, we will be modeling the water percolation and retention
capabilities of different sediment media. By using an exaggerated amount than what
would be seen in nature, we are able to see how the sediments react to extreme
conditions to find which sediment types are able to create a boundary layer in
landfill sites. The sediments being studied are: sand, potting soil, grade “A” gravel
and clay. By using the different sediment types our aim is to develop a spectrum that
will identify which sediments retain water or restrict water movement.
Furthermore, by subjecting the sediment samples to a contaminated mixture
of 60 parts antifreeze and 40 parts water, we can test the pH of the residual liquid to
see the filtration capabilities of the different sediment samples. To replicate rainfall
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the sediment will be subject to 3 trials of water treatment to see if the pH remains
the same over time, or if the contamination is flushed from the sediment. By
conducting this we hope to determine which sediments can be used as an acceptable
impermeable layer in landfill sites and which sediments can be used as a topsoil to
help remove harmful contaminants.
Hypothesis:
We believe that the test will show differentiation in sediment permeability.
Based on an understanding of water movement through sediment, we believe the
A1 gravel will be the most permeable material and let the most water through. This
will be followed by sand, soil and finally clay as the material that will be the least
permeable, letting minimal amounts of water through, if any at all.
When studying the effect the sediments have on filtration we believe that the
clay will filter the liquid the best, bringing the pH of the antifreeze-water solution
back to an acceptable pH of around 7. This is due to the small size of the clay
particles that allow the sediment to pack together and retain the contaminants. The
second best filter sediment will be soil, followed by sand, and A1 gravel last due to
its inability to retain water, allowing the liquid to flow through readily.
Methods
Preparation for the following experiment required the collection of the four
sediments (sand, potting soil, grade “A” gravel and clay) being investigated from an
area in the Fort Frances/Rainey River District in Northwestern Ontario. Each media
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deposit was found in an open environment thus the consistency of foresaid media is
indefinite due to possible natural influences. Approximately 1,182.5 ml of each
sediment was collected to account for the potential for error and multiple trials.
Gravel
Sand
Soil
Clay
Figure 1: Sediment samples from Rainy River District,
Northwestern Ontario
The experiment was commenced in a closed environment so no external
environmental forces could affect the results. Each experimental trial was subject to
error due to the approximate measurement of the media as a volume quantity.
The simulated landfill area was modeled by cutting a 2 litre Pop bottle into
two sections. A full cut was made at the point where the bottle begins to taper into
the spout; the top section served as our simulated landfill where each sediment
form was individually tested, while the bottom served as a collecting basin for the
liquids that leached through. Medical gauze was then cut into approximately 10cm x
10cm squares creating a screen to hold sediment within the upper section of the
bottle, yet allow liquid to percolate through the sediment. This screen was then
securely wrapped onto the nozzle of the pop bottle with the use of an elastic band.
After the screen was secured, the top section of the bottle was then placed within
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the remaining section of the bottle with the nozzle facing down to ensure sediment
was able to be retained, creating our simulated landfill.
Figure 2: Pop bottle simulated landfill apparatus filled with sand media.
`
Each sediment type was investigated independently with 236.5 ml of
sediment poured into the pop bottle top. Before each trial we ensured a wastebasket
appropriate for holding the mix of liquid and sediment was available.
Water Trials
236.5 ml of water was measured and poured over the sediment at a
consistent rate for all trials. At this point we began the stopwatch, timing for 2
minutes for each trial. After the 2 minutes had expired we removed the simulated
landfill, being careful not to spill the sediment or liquid that may still be percolating.
After the top “landfill” section of the pop bottle was removed we poured the
liquid that had collected into the bottom basin for measurement. After recording the
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results we rinsed all equipment of residue so no contamination with future tests
occurred.
Antifreeze Trials
The initial set up was identical for both trials in regards to simulated landfill
and screen procedures. Basin procedures differed due to nature of the liquid and pH
assessments. An alternative wastebasket was available to dispose of all
contaminated media. After the liquid percolated through the sediment and collected
in the basin a sample of the liquid was inserted into the pH test kit tube. 2 drops of
phenol red was then introduced and the resulting color of the liquid is interpreted
on the pH scale.
Procedures were duplicated for all sediment types and appropriate pH levels
were recorded. The sediment contaminated with antifreeze was then disposed of in
an environmentally friendly manner.
Figure 3: pH testing kit analyzing antifreeze sample after
percolation through sand.
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Results
From our trials we looked to assess the differences between 4 media types in
regards to the amount water allowed to percolate through the sediment. As an
extension we studied if the different medias served as a filter for contaminated
liquids by measuring the pH of the residual percolated liquid. From our results we
observed how the different media types affected the rate of water percolation with
sand allowing the most water, 150ml, to travel through the media within the 2minute trial time. Clay was the most impermeable media type allowing only 5 ml of
water to percolate through within the 2 minutes. This is depicted in Figure 4,
showing the different amount of liquid that percolated through the media contents
in the 2 minutes.
In regards to the antifreeze, pH trials were unfortunately inconclusive. The
sand trials showed small improvements with the residual water pH becoming
slightly less basic in trial 2, but due to procedural faults the margin of error
discounts any results.
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H2O
Antifreeze 1
Antifreeze 2
Antifreeze 3
Antifreeze 4
(mL)
(ph)
(ph)
(ph)
(ph)
Gravel
20
N/A
N/A
N/A
N/A
Soil
100
7.7
N/A
N/A
N/A
Clay
5
N/A
N/A
N/A
N/A
Sand
150
7.7
7.6
7.7
7.7
Table 1: Data Results of h20 percolation & ph trials between media types.
H2O (ml)
H20 Percolation Through Media
Types
160
140
120
100
80
60
40
20
0
Gravel
Soil
Clay
Media Types
Sand
Figure 4: Graph depicting the amount of water (ml) that percolated through each of the
media types.
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Antifreeze pH
Antifreeze Percolation Through
Sand Media
7.72
7.7
7.68
7.66
7.64
7.62
7.6
7.58
7.56
7.54
Trial 1
Trial 2
Trial 3
Test Trial Number
Trial 4
Figure 5: Graph depicting the change in antifreeze pH after four trials of percolation
through sand sediment.
Discussion
In an attempt to recreate the effect that sediments can have on restricting
groundwater contaminants, we subjected different types of sediment to higher than
normal quantities of water. By doing so our intention was to create a model showing
the percolation of water through various types of sediment to assess retention and
filtration abilities of the different sediment types. Controlling the amount of
groundwater contamination is essential to preserve the surrounding ecosystem and
potential threats of pollution to societies, thus the development of landfills requires
the consideration of the surrounding sediment composition.
By comparing the retention of different sediment types (sand, potting soil,
grade “A” gravel and clay) we determined that clay serves as the most impermeable
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media, while sand served as the least. This supports the basis of landfills largely
being placed on clay beds to decrease the amount of contaminant movement
through subsurface soils, as outlined by Hamdi and Srasra (2013) and Sarabian and
Rayhani (2013). The “A” gravel was determined to be the 2nd least permeable, which
was a surprise for us since was assumed it would be the most permeable sediment
type in our hypothesis. We credit this to the composition of the sediment mixed in
with the gravel, which when saturated became a type of impermeable material
made up of small sediment particles. The sand showed that it allowed a significant
quantity of liquid to percolate through its media, while the soil permitted slightly
less. This shows that soil and sand are poor choices to control ground water
contamination within a landfill.
Throughout our Antifreeze pH trials we encountered various complications
when attaining results. The sand had the most recordable results because the pH
color comparison was somewhat readable, where the antifreeze became slightly
more transparent after each filtration cycle. We can suspect that this was due to the
sand acting as a filter and retaining some antifreeze, allowing the water to travel
though, thus diluting the sample. For all the other sediment types the
contamination of the sediment discolored the liquid making the color comparison
unreadable. This was a complication that we were not expecting, but may have been
able to be remedied by using pH testing strips rather than a phenol drop method.
We are able to hypothesize that this would give a better result, however sediment
contamination may result in similar inabilities to confidently read the pH. The most
practical way of reading the results would be to subject the residual liquid to further
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filtration to strain sediment particles even further, which in turn would skew the
data. Future studies may benefit by testing for something other than pH, as the
indicating process may be difficult to determine.
Our rational for each trial being two minutes long was based on initial sand
and soil trials where the liquid had percolated through the media in less than two
minutes. Although unrealistic compared to a real life application, we felt that on the
scale of our design two minutes was adequate. In future trials we could implement
more trials over an extend period of time to account for the persistent amounts of
groundwater that an actual landfill receives, which also could examine how different
sediment types become packed down. Other adjustments can be applied in future
studies to avoid potential errors. Most importantly we feel increasing the surface
area of the model landfill will allow more liquid to pass through the media, rather
than all the sediment and liquid being bottlenecked into a 1 inch opening. The use of
various other screens could also be investigated to determine if the size of the holes
affected the flow of the liquid into the basin.
Testing the permeability of the sediments was a successful experiment with
results that challenged our hypothesis. By implementing the adjustments to the
logistical development of the experiment we believe that further investigations
regarding sediment percolation can be more successful.
Conclusion
By subjecting the sediment samples to the water tests we were able to
determine that the best sediment for creating an impermeable layer in a landfill is
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either clay or grade “A” gravel due to the water retention qualities of both. While
both the clay and gravel reacted similarly to the water simulations, if we determine
the sediment type based on the characteristics outlined by Hamdi and Srasra (2013)
and Sarabian and Rayhani (2013) the homogenous small particle size of the clay
would be favoured over the mixture of large and small sediment sizes that make up
the gravel. However, it appears that if clay were unavailable, the “A” gravel would
act as a sufficient layer to restrict contaminant movement. While further studies are
needed to view the differences in percolation over a longer time period of time than
2 minutes, our initial findings appear to determine that it would make an acceptable
layer.
The soil and sand showed that they are unable to prevent large quantities of
water to percolate through the sediment samples. While this is the case, the sand
was able to retain 76.5 ml of water, and the soil was able to retain 86.5 ml of water.
By comparing how much water was initially poured over the sediment, and the total
water yield following percolation we discovered these quantities. This proves that
these soil types do have the capacity to retain a finite amount of water. Under
normal conditions layers of sand and soil may be able to retain a certain amount of
water before experiencing a surplus that would lead to deeper water percolation.
Having mentioned this, we understand the importance of the impermeable clay
layer below the topsoil to ensure percolation of contaminated water does not
persist to the groundwater system.
It is important to note that the filtration portion of the study, while
unsuccessful in all cases but the sand, was able to provide us with future inquiries
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surrounding the affect of sediment on filtrating contaminated water. The sand
showed small improvements on the pH of the contaminated liquid, although
because of procedural error we must discount the data. Future studies will need
better pH and contamination identification procedures to measure the cleanliness of
the residual percolated liquids. Furthermore, more trials will need to be done for
longer periods of time to see how environmental factors would act on contaminated
soils.
In order to determine the effect of water percolation in different sediment
media it was apparent that soils and sand retain a small amount of water, while the
gravel and clay prevent the water from percolating through. While this was a very
simplistic representation of how sediments are able to manipulate the percolation
of water, more advanced studies can be produced off of the concept. The 2-minute
time frame selected only worked for some tests, it will be necessary to determine
the length of time necessary for water to percolate through the clay and the gravel.
This will give us insight on the effect of the water on the clay and gravel over a
prolonged period of time. By doing so, further conclusions can be made to prove
that clay and gravel are acceptable boundary layer sediments.
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References
Hamdi, Noureddine, and Ezzeddine Srasra, 2009. Hydraulic conductivity study of
compacted clay soils used as landfill liners for an acidic waste. Waste
Management. 33, 60-66.
Janzen, Niklas, Stefan Banzhaf, Traugott Scheytt, Kai Bester, 2009. Vertical flow soil
filter for the elimination of micro pollutants from storm and waste water.
Chemosphere 77, 1358-1365
Sarabian, Tahmineh, and Mohammad T. Rayhani, 2013. Hydration of geosynthetic
clay liners from clay subsoil under simulated field conditions. Waste
Management 33, 67-73
Zhan, T.L.T., C. Guan, H.J. Xie, Y.M. Chen, 2014. Vertical migration of leachate
pollutants in clayey soils beneath an uncontrolled landfill and Huainan, China:
A field and theoretical investigation. Science and Total Environment 470-471,
290-298.