Besides meeting some of the requirements of the CE4594
(Soil and Groundwater Pollution) course offered by the Charles Via Department of Civil
Engineering at Virginia
Polytechnic Institute and State University and taught by Dr.
Daniel Gallagher, this page serves to inform the public about
the fact that the convenience of being able to move freely on the
roads even during snowy conditions, comes with a price to pay.
Picture this, it is mid December in upstate New York about 10
degrees below zero with a wind chill factor of 20. On top of all
this the roads are covered in about 4 feet of snow. How is all
this snow removed? The answer is by way of road salting or
deicing. The second questions are what happens to all this salt
once it is applied to get rid of the snow? Most importantly what
effects does salt have on the environment and groundwater
systems?
Road deicing chemicals, have detrimental effects on the
environment. The United States Senate in 1988 requested a study
by the Transportation Research Board into the economic impacts of
using rock salt and calcium magnesium acetate (CMA) for highway
deicing. The research board as part of their study stated some of
the environmental impacts of road salting. In this paper, we will
focus on how road salting impacts the groundwater system.
The process of road salting, which involves the application of
large quantities of salt to the roads to deice them, has negative
effects on the environment, human health, and ground water
systems. According to the transportation board study mentioned
earlier, salt from the highway is introduced into the groundwater
through a number of ways:
1)When runoff occurs from highways, flows are sometimes carried
to ditches and unlined channels through which the water
infiltrates in to the soil and eventually into the groundwater (Road Transport, 1991).
2) Also, when snow is plowed together with the salt, the pile
that is accumulated on the roadside melts during warmer weathers.
The water that results contains dissolved salt which can also
infiltrate(Road Transport, 1991). Plowing and
splashing of salt causes the salt to deposit along the pavement,
especially near the shoulders. Salt particles that are carried by
run off tend to be dependent on such factors as direction of
slope. Figure 1 below is an animation that
shows the infiltration process and how it ultimately affects the
groundwater system. In the animation, the pink material
represents the deposited mixture of salt and melted snow which
starts from the ground surface and works its way into a drinking
water system.
Figure 1. Animation of How Salt
Water from Deicers Contaminate Groundwater
Besides these modes of dispersion of road salt, action of
vehicular movement in contact with the salt and snow on pavement
produces a brine that is highly mobile. Aerial dispersion of
small droplets by spray and splash from traffic also help carry
the salt particles farther from their initial source. Ultimately
these particles are washed into the soil system either by rain or
melted snow. Plowing pushes salt-laden snow to the side of the
road where it melts causing runoff to enter drainage ways and
then the groundwater system. The roadway is the point source from
which salt is deposited in the landscape then to groundwater
systems (D'Itri, 1992). Examples of
groundwater pollution from deicing of salt can also be seen
through research on salt in urban sewage and road systems. The
salt infiltrates the groundwater by being discharged through
sewers and land drainage channels. When the snow melts, salt
concentrations in urban areas may rise to levels of the order of
100 mg/liter. Water drained from roads that is discharged to
lakes have been found to be as high as 5000 mg/ liter.
Groundwater has shown that the increase in the chloride
concentration naturally present due to deicing salt may be 20-60
mg/liter. Other measurements indicate 2-3 mg/ liter (Road Transport, 1989).
The characteristics of the soil mainly determine the movement of
the salt through the soil down to the groundwater. According the Jones et al.(1986), course textured soil allows
infiltration fast while finer textured soils are characterized by
slow infiltration. Also, the charges on the sodium and chloride
are also important determinants. Clay particles which are
negatively charged tend to repel the negatively charged chloride
while the positively charged sodium undergoes ion exchange with
other positively charged soil particles. This process, according
the work done by Jones et al.(1986), and the
Maryland Dept. of Transportation, accounts for the retention of
higher percentage of sodium in the soil.
Sodium chloride accumulation tends to diminish permeability of
the soil and tends to increase alkalinity. It can also ten to
reduce the aeration of the soil. It increases alkalinity by
reducing the ion exchange capability of the soil. In general,
chloride is less detrimental than sodium. High levels of sodium
also cause the loss of vital plant nutrients such as K, Ca, Mg. (Road Transport, 1991)
A case study, conducted in Wisconsin illustrates the damaging
effects of the use of salt as a deicer. Like most northern
states, Wisconsin has been using common halite (road-salt) as a
deicing agent, and in much larger quantities since 1957-87 (D'ltri, 1992). Studies in Wisconsin show an
increase in chloride concentrations of the surface waters.
Several lakes in Northern Wisconsin were the focus of
investigations showing groundwater flow components entering the
lakes from nearby highways (see Figure 2).
Effects on groundwater resources have shown concentration levels
of chloride as high as 200 m/l. (D'ltri, 1992).
Most of the high values came from shallow water tables but one
well in particular had higher levels at a greater depth
representing the downward sinking plume of chloride rich water
from an easterly source, Highway 51 and Vilas County highway.
Drainage integration is poor and many lakes are topographically
isolated from one another. Groundwater serves as the major link
among most of the lakes with about 90-95% of the total water in
the region in groundwater and about 5% in lakes (Figure
2) (D'ltri, 1992).
Evidence for chloride contamination can be seen by looking at
Trout Lake and Sparkling Lake where the focus on contamination
entering the groundwater system by way of road salting is from
Highway 51 and Vilas County Highway. Sparkling lake demonstrates
the largest chloride anomaly and because it was the subject of a
detailed isotopic groundwater budget. It was chosen to study the
mechanisms of chloride contamination from groundwater systems to
the lakes. Sparkling lake was typical of many of the lakes in the
area and had no surface inflow or outflow. Spring snow melt
essentially goes directly into groundwater by seepage
(groundwater recharge). Background lake and groundwater chloride
concentrations range from .3 to .5 mg/l in the Vilas County area
and concentrations above this range generally indicate chloride
contamination from road salt (D'ltri, 1992).
Because these contaminated lakes receive their chloride loading
from the roads up gradient of the lake, it is indicative that the
direction of flow of groundwater is from the road toward the
lakes. Thus, these elevated chloride levels can generally be used
to detect vectors of flow through the local groundwater flow
system(D'ltri, 1992).
In 1991 Sparkling Lake had a concentration of 3.7 mg/l; up from
2.6 mg/l when it was first determined in 1982. The average lake
increase is 0.15 mg/l chloride per year, which represents an
average net chloride loading rate of 1,200 kg/yr., or a nine year
average of 2.3 tons of road salt per year accumulation into the
groundwater system and lakes. Ion balance calculations for the
groundwater system indicates that ion exchange is an important
process. Loss of sodium while moving through the GW is matched by
the exchange release of other major cations such as Mg, Ca, K,
and ammonium (D'ltri, 1992). The correlated
increases in chloride and sodium suggest a common linkage between
the 2 dissolved constituents, and indicate road salting as the
source. Dramatic increase in chloride entering the lake result
from greater inputs of salt per unit drainage in urbanized areas
surrounding the lakes. Significant pumping in the Madison area
has resulted in increase well chloride levels due to two things.
One, reversal of hydraulic heads by pumping, resulting in
recharge water to enter into city aquifers. Two, drawdown by
wells creates a cone of depression that allows recharge into the
well from seepage with in the cone of depression surrounding the
well. (See Figure 3) (D'ltri,
1992).
The latter mechanism provides a local groundwater recharge system
that captures surface water and dissolved salts and leads them
directly to the well at the center of the cone of depression.
Chloride concentrations are observed in some wells that are
higher than the lake water. Such waters could not possibly have
come from nearby lakes unless additional salt was added after
entering the groundwater, a highly unlikely situation (see Figure 3) (D'ltri, 1992).
Impacts of deicers in the environment from the application of CMA
has been found to have a major impact on vegetation. For example,
in western Europe 700,000 trees die annually due to a result of
deicing salt applications to roads. This figure does not
represent diseased trees or trees affected directly due to pests.
Damage to vegetation occurs through deicing chemicals deposition
on the soil profile making chemicals available for absorption by
plant roots. Chloride affects leaf margins and shoot tips causing
marginal scorching. Sodium affects the sodium pump located in the
plasmalemma, which is responsible for reducing cell turgor
pressure (D'ltri, 1992). Deicing starts by
affecting the vegetation along the roadway where the salt is
being applied. Figure 4 below illustrates the
process of how water from deicers or salt affect vegetation.
Figure 4 . Animated Slide Show
of Salt Water Affecting Vegetation.
Road
Salt's Effects on Ground Water Quality
Charles Seawell is an
undergraduate in the Department of
Forestry and Wildlife with a second major in Environmental Science and a
minor in Geology. His chosen
focus is environmental resource management with concentrations in
forest stewardship and hydrology. Employment opportunities can
include jobs with state agencies, private consultant firms, the
Environmental Protection Agency, the Soil Conservation Service,
or the Forest Service.
Newland Komla
Agbenowosi is a Graduate student in the Department of Civil Engineering.
His area of specialty is in Hydrosystems
Engineering. Newland has done research on the application of
GIS in the Design of Sewer Systems. He has also been involved in
the monitoring of the effectiveness of the use of cattail plants
in remediating acid mine drainage. Currently He is looking into
the use of Global Positioning Systems (GPS) to estimate
precipitatable water in the atmosphere and ultimately predict
rainfall.
Send comments or suggestions to:
Student Author: Charles Seawell cseawell@vt.eduand
Newland Agbenowosi newland@vt.edu
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
