Publications—Water-Resources Investigations Report 99–4077
By Gregory E. Granato and Kirk P. Smith
U.S. Geological Survey Water–Resources Investigations Report 99–4077
Prepared in cooperation with the Massachusetts Highway Department
This report is available in Portable Document Format (PDF):
WRIR 99–4077 (556 KB) – 27
pages
Discrete or composite samples of highway
runoff may not adequately represent in–storm
water–quality fluctuations because continuous
records of water stage, specific conductance, pH,
and temperature of the runoff indicate that these
properties fluctuate substantially during a storm.
Continuous records of water–quality properties can
be used to maximize the information obtained
about the stormwater runoff system being studied
and can provide the context needed to interpret
analyses of water samples.
Concentrations of the road–salt constituents
calcium, sodium, and chloride in highway runoff
were estimated from theoretical and empirical
relations between specific conductance and the
concentrations of these ions. These relations were
examined using the analysis of 233 highwayrunoff
samples collected from August 1988
through March 1995 at four highway–drainage
monitoring stations along State Route 25 in southeastern
Massachusetts.
Theoretically, the specific conductance of a
water sample is the sum of the individual conductances
attributed to each ionic species in solution—the product of the concentrations of each ion
in milliequivalents per liter (meq/L) multiplied by
the equivalent ionic conductance at infinite dilution—thereby establishing the principle of superposition.
Superposition provides an estimate of
actual specific conductance that is within measurement
error throughout the conductance range of
many natural waters, with errors of less than ±5
percent below 1,000 microsiemens per centimeter
(µS/cm) and ±10 percent between 1,000 and 4,000
µS/cm if all major ionic constituents are accounted
for.
A semi–empirical method (adjusted superposition)
was used to adjust for concentration
effects—superposition–method prediction errors at
high and low concentrations—and to relate measured
specific conductance to that calculated using
superposition. The adjusted superposition method,
which was developed to interpret the State Route
25 highway–runoff records, accounts for contributions
of constituents other than calcium, sodium,
and chloride in dilute waters. The adjusted superposition
method also accounts for the attenuation
of each constituent's contribution to conductance
as ionic strength increases. Use of the adjusted
superposition method generally reduced predictive
error to within measurement error throughout the
range of specific conductance (from 37 to 51,500
µS/cm) in the highway runoff samples. The effects
of pH, temperature, and organic constituents on
the relation between concentrations of dissolved
constituents and measured specific conductance
were examined but these properties did not substantially
affect interpretation of the Route 25 data
set.
Predictive abilities of the adjusted
superposition method were similar to results
obtained by standard regression techniques, but
the adjusted superposition method has several
advantages. Adjusted superposition can be applied
using available published data about the
constituents in precipitation, highway runoff, and
the deicing chemicals applied to a highway. This semi–empirical method can be used as a predictive
and diagnostic tool before a substantial number of
samples are collected, but the power of the
regression method is based upon a large number of
water–quality analyses that may be affected by a
bias in the data.
Abstract
Introduction
Field and Laboratory Methods
Highway-Runoff Monitoring Records
Theoretical and Empirical Analyses of Specific Conductance Data
Regression Analysis
Superposition
Concentration Effects
Other Effects
Adjusted Superposition
Estimation of Road-Salt Constituent Concentrations
Conclusions
References
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Suggested Citation:
Granato, G.E., and Smith, K.P., 1999, Estimating Concentrations of Road–Salt
Constituents in Highway–Runoff
from Measurements of Specific Conductance: U.S. Geological Survey Water-Resources
Investigations Report 99–4077.
For additional information write to:
Director,
USGS Massachusetts–Rhode Island Water Science Center
10 Bearfoot Road
Northborough, MA 01532or visit our Web site at:
http://ma.water.usgs.gov
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