File: C:\Cl05\roadsalt
May23-05
Chloride in Fall Creek as influenced by road salt
D. R. Bouldin
Crop and Soil Science, CALS
Cornell Univ
Ithaca NY
SUMMARY
Chloride concentration was measured at irregular intervals between 1972 and 2003 in
Fall Creek, a 129 square mile watershed in central NY. The major sampling point was
about 1/8 mile above a USGS gauging station which provided detailed data on flow
regimes. The concentration of Cl was measured either coulimtrically or by ion
chromatography. The timing of sampling was generally designed to sample flow levels
seasonally and not by a set frequency.
During late spring-summer-early fall when road salt was not applied, the Cl
concentration decreased from on the order of 20 to 25 ppm at low flow to 8 to 10 ppm as
flow increased during a given year. Based on yearly regressions fitted to these samples,
flow weighted concentrations increased systematically from about 11 ppm in 1972 to 19
ppm in 2003.
Several sets of samples were taken at closely spaced intervals during snow melt
or winter rain following applications of road salt. In these situations the concentrations of
Cl increased as flow increased and then decreased rapidly as the runoff continued.
Concentrations were as high as 60 to 70 ppm during some of these intervals. Loading
from this direct runoff is likely to be on the order of 20 to 50% of the flow weighted
concentrations listed above.
Based on limited literature, I judge the summer concentrations of Cl are probably
not an important detriment to macroinvertibrates. Even the winter concentrations are of
questionable importance in my opinion because they are of short duration and even then
are not high enough to be a major factor.
We did not have enough data to judge whether there are portions of
subwatersheds in which the concentrations are high enough to adversely influence
populations of macroinvertibrates nor on the impact of roadsalt on adjacent terrestrial
ecosystems.
SEASONAL EFFECTS
Illustrated in Fig 1 are Cl concentrations and flow plotted against time for winter
and summer seasons. The important aspect of the data is that during the winter when road
salt is being spread, concentrations of Cl tend to increase with increasing flow while
during the rest of the year when road salt is not being spread, the concentrations tend to
decrease as flow increases. Similar results were obtained in other years but not illustrated
here. This observation was hypothesized to illustrate two sets of behavior: a)when salt is
not being actively spread, the decrease in concentration with increasing flow is basically
dilution of the inputs of Cl from aquifers and small inputs of Cl from surface runoff/
shallow aquifers while b)when salt is being actively spread, the dissolution of salt from
roadways effectively swamps the outflow from the major aquifers and hence
concentration increases with increasing flow.
Based on these observations, the data was analyzed in two groups. The first
group was the data judged not to be influenced by direct runoff of road salt and were
analyzed with the objective of estimating, for the period 1972 to 2003, the yearly loading
and flow weighted concentration of Cl from the aquifers which provide base flow. The
second group was judged to be influenced by direct runoff of road salt and analyzed to
provide data on the influence of direct runoff during periods of active spreading of road
salt. For purposes of discussion the first group will be referred to as “nosalt” and the
second group as “runoff”.
The separation of the data into the groups was accomplished as follows. For each
calendar year with sufficient data, a spread sheet was prepared by combining a) daily
flow for Fall Creek at Forest Home (USGS station 04234000) b)daily weather records for
Ithaca NY and c) Cl concentrations when available. The nosalt data was selected as
follows. Based on weather records and flow data, the end of effect of active spreading of
salt was judged to occur (usually) sometime in April when the flow had subsided from
snow melt and recent rain. The nosalt period was begun then and ended when the first
trace of snow was recorded in the fall. The runoff group included all of the data from
January 1 to the beginning of the nosalt period plus the data from the first trace of snow
in the fall to the end of December for that calendar year. As will be illustrated later, the
concentration effects of the direct runoff increase rapidly as flow increases and decrease
rapidly as the flow event continues. This makes the separation of the data into the two
groups relatively easy and clear.
ANALYSIS OF THE DATA NOT INFLUENCED BY DIRECT RUNOFF OF
ROAD SALT
Shown in Fig 2 are regressions of Cl concentration on flow for the nosalt groups
by years together with the parameters in the regressions. The lines are the regression
equations and the points are the calculated concentrations when samples were taken; the
lines are not extrapolations beyond sampled flow.
In order to estimate loadings and flow weighted concentrations across years, the
Fall Creek data was segmented into the number of days per year that specified flow
intervals occurred in the period 1926-1996. The regressions were then used to estimate
the average concentration during each interval and the product of concentration and total
flow during that interval was summed to provide yearly loading of Cl and by summing
flow per year the flow weighted concentration was calculated as total loading divided by
total flow. Note that the average flow and flow distribution for the period 1926-1996 was
used, not flow for the individual years.
Shown in Fig3 is a plot of flow weighted concentration for 1972 through 2003.
During this period, the flow weighted concentration and hence loading essentially
doubled. Note that this will be called the influence of base flow independent of direct
runoff and will be referred to as “nosalt”.
ANALYSIS OF DATA INFLUENCED BY DIRECT RUNOFF OF ROAD SALT
The analysis was carried out as follows. The two periods chosen were February 1
to April 30, 1994 and February 1 to April 15, 2003. The periods before and after the
above periods in calendar years of 1994 and 2003 appeared to relatively free of active
snow melt/ rain and thus probably major impacts of runoff on Cl loads were captured
during these periods.
During these intervals when sample data was available, the loading per 2 hours
was calculated for the 2003 data and regressions of load on flow was calculated. The
regressions were then used to estimate loading for the total period. Using the regression
for nosalt periods the corresponding load was calculated as if there were no runoff
component. The difference was then calculated as the runoff load. Fig 4 illustrates the
relationship between flow and load for February 2003. A similar procedure was used for
1994 except daily loads were used instead of bihourly loads.
The results are given in Table 1. The differences among years is quite large. This
is reasonable for 2 reasons. The first is that the frequency of sampling is critical. As
illustrated in fig 1, the concentrations increase rapidly and die off within 2 to 5 days. This
means that a daily sample is inadequate and only in 2003 was the sampling interval
reasonably suitable. In 1994 only daily samples were taken and the data is less reliable.
A second reason is that the very nature of the load from runoff means it will be
extremely variable. First, the amount of salt on the roads is dependent on frequency of
snow and the effect of any salt on the stream flow depends on the nature of snow melt/
rainfall since it is the transport agent. Some winters there will be lots of snow and hence
lots of salt. During other winters there will be less snow and presumably less road salt.
In my judgment, the runoff as % of total is likely to be in the range of 15 to 50 %
for the reasons listed in the previous paragraph. Presumably the variability in nosalt
loading will be less variable because the concentrations are buffered by the aquifers.
In relation to delivery to Cayuga Lake, over the last 10 years, the nosalt
component averaged about 17 ppm for Fall Creek. We suppose that the runoff component
was 40% of this or the resultant input would be 17*1.4 = 24 ppm +/- ??.
COMPARISON WITH OTHER STUDIES
The highest concentrations in Fall Creek are comparable to those found in a New
England watershed near Hadley MA in a Conneticut River valley (Rhodes et al. 2001).
They summarized their data in a graph of Cl concentration (0-70 ppm) vs road density
(m of roads per m2 of watershed, 0.001 to 0.004) Mason et al (1999) found less
concentration in a small watershed (103 ha) traversed by 0.3 km of a 4 lane major
highway heavily salted during the winter. They also measured increased Na in soils
adjacent to the stream but below the road.
Several studies have been made in the Toronto region. Some winter samples of
streams were as high as 1400 ppm but summer samples were comparable to those in Fall
Creek.. In sixteen springs the average concentration ranged from essentially zero to 1200
ppm and did not vary widely over seasons. Mayer et al (1999) summarized
concentrations across Canada in several ways including measured concentrations in
streams, springs and lakes, and roadsalt applications in watersheds divided by average
annual stream runoff. The measured concentrations ranged from a few ppm to 8900 ppm.
For lakes they report concentrations of 25, 28, and 1.3 ppm for Erie, Ontario and
Superior, respectively. Small isolated lakes were comparable to Superior.
CHLORIDE IN CAYUGA LAKE AND OTHER FINGER LAKES
In 1994, the Lake Source Cooling project reported concentrations of 40 ppm in
Cayuga (Sterns and Wheler. 1994.) Effler et al (1989) carried out a comprehensive study
of Cl in Cayuga Lake. Prior to 1970 a rock salt mining operation disposed of waste salt in
the lake and by 1970 the concentration was about 100 ppm. In 1970 the disposal into the
lake was stopped and the concentration of Cl began to drop. By 1986 the Cl concentration
was about 50 ppm. A model of flushing of Cl was developed and calibrated with the
available data for Cl loading from Likens (1974) and available outflow data; this model
estimates Cl concentration of about 32 ppm by 2000.
The equilibrium level in Cayuga Lake depends upon the ratio of total salt input
divided by average annual outflow; both of these numbers are uncertain. Probably the
loading of Cl as measured by Likens (1974) in 1970-71 did not include any input from
direct runoff and thus is too small. The inflow/ outflow of water for Cayuga Lake is
subject to uncertainty. Perhaps both of these numbers are too small and hence the ratio is
about correct and the predicted level of 32 ppm may be reasonable.
In a comparison of 11 lakes in central NY, Seneca, Cayuga and Conesus had over
10 ppm Cl while the rest were less than 10 ppm. (Shaffner and Oglesby, 1978). Perhaps
the concentrations in some of these lakes have increased from increasing amounts of road
salt since many of the reported analyses were taken.
SOURCES OF Cl OTHER THAN ROADSALT
Over the last 30 years, Cl has been measured in other watersheds in or local to
Fall Creek. One study measured Cl in streams draining uninhabited, mostly wooded
watersheds with no obvious human activities such as farming. Cl concentrations averaged
about 1.0 ppm for 3 watersheds (Bouldin. Unpublished).
A set of shallow wells was established on the Teaching and Research Center at
Harford adjacent to a farmed field that received large amounts of manure. The field was
isolated from runoff from road salt and dilution by inflow from other fields. In this
situation the concentration of Cl averaged about 20 ppm (Bouldin. Unpublished).
Also sampled was 40 foot well in a gravel aquifer which received drainage from
about 2400 acres. About 1200 of these acres received manure from about 600 animal
units while the other 1200 acres were pasture or wooded. There were about 4 miles of
roads. The concentration of Cl averaged about 13 ppm in the 1993-95 period (Bouldin.
Unpublished).
BIOLOGICAL IMPACTS OF CL
Williams et al (2000) assessed tolerance of several invertibrate species/taxa in 16
springs in the Toronto region which varied in concentration from 20 to 1100 ppm Cl and
with low seasonal variation. They derived a biotic index based on frequency of
occurrence of 34 species/taxa and related it to concentration of Cl. There is a clear
decrease in the index over the range of 0 to 1100 ppm but in the 0 to 100 ppm range there
is much variability in the index but no clear trend. This data is not necessarily relevant to
the much lower weighted mean concentrations found in Fall Creek but they do indicate a
major effect is unlikely.
REFERENCES
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Identifying potential land use derived solute sources to stream baseflow using ground
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Christen, Kris. 2005. Road salt may degrade soil, lake quality. Water, Environment and
Technology. 17:16-18.
Effler, Steven W., Martin Auer and Ned Johnson. 1989. Modeling Cl concentration in
Cayuga Lake, USA. Water, Air and Soil Pollution. 44:347-362.
Likens, Gene E. 1974. Water and nutrient budgets for Cayuga Lake, New York.
Technical Report no. 82. Cornell University Water Resources and Marine Sciences
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Mason, Charles F., Stephen A. Norton, Ivan J. Fernandez, and Lynn E. Katz. 1999.
Deconstruction of the chemical effects of road salt on stream water chemistry. Journal of
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Schaffner, W. A., and R. T. Oglesby. 1978. Limnology of eight finger lakes: Hemlock,
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Sterns and Wheler. 1994. Lake Source cooling project. Figure C-1-1.
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