The Responses of Different Types of Soil When Exposed to Salt
Steve Blue, Breah Minor, Hillary Pink, Bridget Thorpe-Kavanaugh - May 2007
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Abstract
Introduction
Methods
Results
Discussion
References
The excess use of salt used on city streets and parking lots during the winter months at Edgewood College and within the Wingra Watershed is an environmental concern. Excess amounts of salt can be destructive to plants and animals. Salt that infiltrates in the soil and into the groundwater can be harmful to humans as well. The purpose of our experiment is to examine salt retention by different types of soils. Our results can help researchers to look at many different aspects of soil interaction with salt including water and salt infiltration, as well as conductivity and NaCl levels. We performed a variety of tests in order to examine soil interaction with salt. We used a vacuum system to pump our salt water solution and deionized water solution through different soil types. After the vacuum system was used, we tested conductivity, chloride, and salt levels. During the experiment we documented the amount of water that flowed through each vacuum test. Our results support our hypothesis that clay will retain the most salt initially but will allow the least amount of total salt to infiltrate into the groundwater because of its lower permeability. Clay does not allow water to infiltrate into the groundwater, however, an excess of stagnant water remains on top of the soil. There is a tradeoff with different types of soil; either the lake and streams are affected by runoff, or the groundwater is affected by salt infiltration. Subsoil does not leave the amount of stagnant water that clay does, but it does allow some salt to infiltrate.
Wisconsin is known for harsh winters, snow
fall and icy conditions. To keep citizens safe sodium chloride, NaCl, is used as
road salt to minimize the risk of accidents. Edgewood College uses excessive
amounts of road salt in parking lots and sidewalks to prevent injury. The use of
road salt has been a cause of contamination in groundwater. Road salt can enter
soil and adversely impact the watershed of an area, human health, and other
inhabitants of the environment. Salt use reduces water quality, kills roadside
vegetation and wildlife, damages soil, road surfaces, bridges, parking ramps,
and automobiles (Foster 2000). The characteristics of a soil determine the
movement of the NaCl through the soil and into the groundwater.
There are two paths that salt can take within the watershed. Either the water
infiltrates into the soil or runoff can occur as result of water sitting on top
of the soil. Salt infiltration into the groundwater can negatively affect humans
as well as plants and animals. The effect of salt intake can also be linked to
high blood pressure and hypertension in humans (American Heart Association
1957).
A high level of salt contamination can be dangerous. The City of Madison Road
Salt Report conducted in 2003 through 2004 done by the Madison Department of
Public Health states, surges of melted ice and snow containing salt has the
potential to harm fish and other aquatic organisms as they enter local lakes and
rivers. The water from snow and ice melt carries dissolved road salt into lakes
and streams through storm water runoff and into groundwater through water
seepage. Neighboring communities obtain their drinking water from groundwater in
this area. At high concentrations, these chemicals can have toxic effects on
plants and fish as well as our drinking water quality.
Our experiment will focus on the question of what are the effects of soil
texture and permeability on retention of NaCl. The soil types used in our
experiment are clay, sand, top soil, and subsoil. Each of these soils has
different physical properties which will determine the infiltration rates of
water into the soil. Soil texture affects properties like soil porosity and
permeability. Soil porosity refers to the amount of pore or open space between
soil particles (Ritter 2006). Permeability is the degree of connectivity between
soil pores. Water will run through a highly permeable soil quite readily. Fine
textured clay soils hold more water than coarse textured sandy soils.
Our objective was to test the ability of different soil types to retain water
and salt, and to test the conductivity and the chloride levels of each soil
after a salt water flush and a deionized water flush have been performed. We
predicted that the sand, top soil and subsoil would retain less salt than clay.
Our hypothesis was that the clay mixture used in the experiment would retain the
most salt initially but the clay mixture would allow the least amount of salt to
infiltrate into the groundwater.
First we gathered four different soil types: clay, sand,
top soil (high organic mix), and subsoil (low organic mix). The clay was
extracted from the ground located in the rain garden behind the Edgedome on the
Edgewood College campus, and the other soils were previously collected by the
science department.
In testing each soil, we began by measuring 130 grams of soil and 1 gram of salt
per 1 liter of water. We first placed our 130 grams of soil into a funnel
containing a 102 sized filter that rested on top of the beaker that was
connected to an electric vacuum. We then measured the conductivity of the salt
water solution we used to filter through the soil. We placed 200 ml of salt
water into our funnel. Next the electric vacuum was turned on to create suction,
allowing the water to flow through the soil and into the beaker. Time is not a
useful reference in our experiment because water flowed straight through sand,
top soil, and subsoil due to their consistency. After ample time was given for
the water to filter through to the beaker, the water that came through was
measured and tested for its conductivity.
Next a series of chloride tests were performed on the water that flowed through
the beaker along with the water that remained on top of the soil. We calculated
the average of the two chloride tests that were taken on both water samples to
ensure less room for error. For the last step, 200 ml of deionized water was
poured into the filter to simulate the flow of rain water. The electric vacuum
was turned on and again the time varied for this step depending on the soil
being tested. Times were not recorded because they were not relevant in our
experiment. The water that flowed through the beaker was measured and tested for
conductivity and chloride levels. Conductivity was tested with a hand held
conductivity meter submerged in the water solution. The Hach chloride test kit
model 8-P was used to determine the amount of NaCl in the water solutions. The
chloride test kit uses silver nitrate titrant. The average of the two chloride
tests were used in our analysis.
Figure 1 – Results of the salt water conductivity test
Figure 1 shows the conductivity of the different soil
types before the salt flush, after the salt flush, and after the deionized water
flush. The starting conductivity of the salt water mixture was 1996 MS/u. Our
results show that clay has the lowest conductivity compared to the other three
soil types after the salt water flush and the deionized water flush. Sand has
the highest conductivity after the salt water flush; however, top soil has the
highest conductivity after the deionized water flush.
Figure 2- Amount of water that flowed through each water flush
Figure 2 shows the amount of water that flowed through
every soil type during each water flush. 200 ml of water was poured on top
during each soil test. Clay again allowed the least amount of water to flow
through, which resulted in the majority of the water remaining on top of the
soil. The greatest amount of water flowed through top soil during both flushes.
The infiltration rate of each soil determined the amount of water that was
allowed to flow through.
Figure 3- Amount of salt that flowed through each flush
Figure 3 shows the amount of salt that was retained by each soil during the salt water flush and deionized water flush. There was 200 mg of salt present in the starting salt water mixture. Clay allowed the least amount of salt to infiltrate, leaving the majority of the salt in the soil and on top where the stagnant water remained. The largest amount of salt flowed through the sand during the water flushes.
Figure 4- Concentration of NaCl (sodium chloride) in each flush
Figure 4 shows the concentration of NaCl in each soil type after each flush. The starting amount of NaCl in the salt water mixture was 1000 mg/l of NaCl. Clay contained the least amount of NaCl after the salt water flush. However, subsoil contained the highest amount of NaCl after the salt water flush. After the deionized water flush top soil allowed the least amount of NaCl to infiltrate through, as opposed to clay, which allowed the most NaCl to flow through.
Our results generally supported our thesis which is that
the clay mixture will retain the most salt initially and will allow the least
amount of salt to infiltrate into the groundwater. Our results show that the
clay mixture only allowed a total of 48.9 mg of salt to infiltrate with both the
salt flush and the deionized water flush. This was compared to top soil which
allowed 151.8 mg, subsoil with 132.5 mg, and finally sand which allowed 172.6 mg
of salt to infiltrate.
The clay mixture also allowed the least amount of water to infiltrate in both
the salt water flush simulation and the deionized water flush. Only 52 ml of
water flowed through the clay during the salt flush, and 30 ml of water flowed
through during the deionized water flush simulation. All of the other soils
allowed most of the water in both the salt flush and the deionized water flush
to infiltrate through. Clay leachate had the lowest conductivity and contained
the least amount of NaCl as well. This is due to the fact that clay’s fine
particles are able to withhold salt from flowing through into the groundwater
(University of Arizona 1998).
After considering and comparing all of the results we gathered we came to the
conclusion that, even though clay allowed the least amount of water and salt to
infiltrate through the soil, clay might not be the best soil for bioinfiltration
systems. The clay may have allowed the least amount of water to infiltrate, but
all the water that did not infiltrate remained on top of the clay. This water
that remains on top of the clay has the potential to do more damage than the
water that infiltrated through. Runoff can carry all the stagnant water full of
salt into lakes, rivers, and streams, harming plant life, and many other
organisms.
We discovered that the best soil for bioinfiltration systems will be a soil that
allows more water to flow through than clay to prevent a large amount of
stagnant water to remain on top of the soil. Subsoil allowed 146 ml of water to
flow through during the salt flush and 112 ml of water in the deionized water
flush. Topsoil allowed 156 ml and sand allowed 164 ml of water to infiltrate.
Subsoil allowed just the right amount of water to infiltrate in order to prevent
a large amount of water from remaining on top of the soil. It also prevents a
large rush of water from infiltrating, which protects the groundwater from this
highly concentrated salt water. Subsoil also allowed the least amount of salt to
flow through when comparing it to topsoil and sand. Clay allowed the least
amount of salt to infiltrate into the soil; however most of that remaining salt
stayed on top of the soil and would be carried by runoff. Top soil contains
higher amounts of macro-pores which allows for sufficient draining. Clay soils
contain more micro-pores which have a higher water holding capacity and can
result in poor drainage (Smiley 1999). Subsoil only allowed a total of 132.5 mg
of salt to infiltrate with both the salt water flush and the deionized water
flush, compared to 151.8 mg with top soil, and 172.6 mg with sand.
We recommend that subsoil be planted around the Edgewood campus to minimize the
damage to the neighboring plant organisms, animals, and humans. Salt is damaging
to the environment when it is used, however subsoil will prevent a large flow of
runoff and a reasonable amount of salt to infiltrate into the groundwater. It is
important to consider the implications of every aspect of salt use and soil
infiltration, and the benefits and disadvantages must be weighed heavily in
order to decide which soil is better for the environment when using salt in the
winter. Our results can help researchers to look at many different aspects of
soil interaction with salt including water and salt infiltration, as well as
conductivity and NaCl levels.
Our results make it clear that there is a trade off associated with each soil
type. Clay will prevent high amounts of NaCl from infiltrating into the
groundwater, but high amounts of salt will then be carried in runoff. Subsoil
will prevent runoff, but it allows some salt to infiltrate into the groundwater.
Our results can potentially guide others to make the best soil choice possible
for rain gardens and other bioinfiltration systems to ensure the health of the
environment.
There are a few potential sources of error in our experiment. Human error could
potentially be a source of problems in our results, as each experiment required
a lot of measuring. The time that each soil was exposed to the salt water and
deionized water flushes is another possible source of error. Time is not a
factor in our experiment because soils like top soil and sand allowed the water
flushes to infiltrate in a matter of seconds, while clay took much longer. The
amount of time that each soil is exposed to the salt was an uncontrollable
factor in our experiment. During the chloride tests misjudgments could be
another source of error. The chloride test requires the tester to determine when
the water has turned a different color, and human error could have resulted in a
miscounting of the number of drops that are required to turn the water a
different color.
Over the course of our experiment there are a few things that we would have done
differently now then when we started. During the course of winter, soil can be
exposed to many salt and rain flushes. A series of salt flushes and deionized
water flushes instead of just one salt flush and one deionized water flush could
have been a more accurate way of testing the soil. There are a variety of
different soils that occur in the environment as well. In our experiment we
could have used more than just four different types of soil, so we recommend
that future researchers test more soil types.
American Heart Association (1957) as cited in
Transportation Research Board Special Report 235 Highway Deicing
http://onlinepubs.trb.org/onlinepubs/sr/sr235/099-112.pdf
E. Thomas Smiley, Ph.D., Thomas R. Martin. Soil
Drainage. Barlet Tree Research Laboratories 1999.
http://www.mygardenguide.com/care/Soil%20drainage.pdf
Effects of Road Salt
http://ewr.vt.edu/environment/teach/gwprimer/roadsalt.html
Is Road Salt a Major Carcinogen?
Harold D. Foster February 2000
http://elements.nb.ca/theme/transportation/salt/salt.htm
University of Arizona. Manual Reference 1998
http://cals.arizona.edu/pubs/garden/mg/soils/soils.html
Ritter, Michael E. The Physical Environment: an Introduction to Physical Geography. 2006. 15 Feb 2007. http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/title_page.html
Road Salt Impacts on Drinking Water
http://onlinepubs.trb.org/onlinepubs/sr/sr235/099-112.pdf
The City of Madison Road Salt Report 2003-2004
Prepared by John Hausbeck, Kristi Sorsa, and Tommye Schneider, Madison
Department of Public Health
http://www.ci.madison.wi.us/engineering/stormwater/SaltReport2004.pdf