EC & Lake Stratification

Electrical Conductivity (EC) of water is a readily obtained measure of its salt content. EC values in the area of 30-60 uS/cm are typical of pristine lakes in the Halifax region. EC values for Sandy Lake were in that range in 1955 and 1971, and 1980 (not in 1977 however) but samples taken from 1985 onward were well above 100 with an overall upward trend. View Lakes for details.

EC is one of the variables, along with temperature, dissolved oxygen and pH measured by probes used characterize the vertical or limnological profiles of lakes, such as those obtained for Sandy Lake in Oct of 2017.

One feature of the Sandy Lake profiles that raises concern but was not identified in the AECOM 2014 Report* is an elevated EC or salt content in the deeper layers of the lake (the “hypolimnion”). Sandy Lake is a “dimictic” lake meaning that it normally turns over (mixes from the top to the bottom), twice a year.  In late spring into summer,  the deeper parts of the lake become “thermally stratified” – as water warms it becomes lighter and stays on top of the lake, leaving the heavier cold water at the bottom, with a “thermocline” – a narrow band in which the temperature changes sharply from warm to cold – in between. The deep, cool layer is called the hypolimnion.
* AECOM 2014, p 46: Road salt application: road salts pose a risk to plants and animals in the aquatic environment. Road salt application can also impact groundwater quality, leading to elevated concentrations of chloride in drinking water. HRM recognizes the potential impacts to surface and groundwater quality and utilizes several best management practices to reduce the impacts when possible (HRM 2012). However, the application of road salts along Hammonds Plains Road and to a lesser extent on secondary residential roads contributes to chloride loading in Sandy Lake. ref is to: HRM Staff Report. 2012. Road Salt Impacts on Lakes. Environmental and Sustainability Standing Committee. April 16, 2012. 47 pp. There is no mention of possible impacts of salt on lake stratification in this document.

This is a desirable feature because the deeper, cooler waters provide a summer refuge for cool water fish, especially salmon and trout. Oxygenation of the hypolimnion is reduced over time, but then as the lake cools in the fall it  becomes “isothermal” (the same temperature and same density from top to bottom), and a good wind will cause it to mix from top to bottom, re-oxygenating the deep layers.

In winter, the lake again stratifies, but this time it is because water has a maximum density at 4 degrees C, and when surface water goes below 4 degrees, the cooler water sits on top and then freezes and forms ice when it gets down to zero. In the spring as the ice melts and the water warms, the lake reaches a point at which it is again isothermal and and can be turned over by the wind.

Dimictic lakes mix from the surface to bottom twice each year.  From Wikipedia

We say such lakes are “thermally stratified”.  If salt accumulates in the deeper waters, those waters become heavier because of the salt and as the salt content increases, there is more resistance to the normal, temperature induced turnover, and the lake may not turn over, usually the spring is affected first. Then the deeper waters are more likely to go anaerobic (devoid of oxygen), making them unsuitable for cold water life. As well, that can trigger release of phosphorous  a plant nutrient-  from the sediments, causing the lake to produce an excess of algae which in turn further degrades the lake.

EC values in the area of 30-60 uS/cm are typical of pristine lakes in the Halifax region. 6 EC values for Sandy Lake were in that range in 1955 and 1971, and 1980 (not in 1977 however) but samples taken from 1985 onward were well above 100 with an overall upward trend. The low values in 1955 and 1971 suggest the lake was likely well below the mesotrophic range (re: figure above), i.e. it was oligotrophic in those earlier years – see this post

Some literature and links on lakes and road salt

To see an example of a local lake affected by multiple factors including heavy roadsalt loading, View

Oathill Lake Lake Restoration
Slide Presentation from the Oathill Lake Conservation Society, 2018.

The slide presentation is no longer posted. EC values were very high, circa 500 uS/cm for surface waters, and circa 1200 uS/cm for deep water when the lake was well stratified.

It’s a remarkable story of how citizens got together to restore the lake to the extent possible given that the area is heavily settled.

More examples, not local

A reduction in spring mixing due to road salt runoff entering Mirror Lake (Lake Placid, NY)
by B. Wiltse et al., 2019, Lake and Reservoir Management 36:109–121. Normal spring mixing in Mirror Lake (50 ha, 18 m max depth) failed to occur in 2017 when hypolimnion (deep water) chloride was approx, 70 to 110 mg/L (~316-490 uS/cm) compared to 30 to 50 mg/L (~131-220 uS/cm) at the surface. Surface water values for Sandy Lake are similar to those for Mirror Lake which is of similar size and depth; deep water Sandy values are not there yet but EC of incoming streams are well above those values, especially in winter. The authors of the Mirror lake study conclude: “The incomplete spring mixing resulted in greater spatial and temporal extent of anoxic conditions in the hypolimnion, reducing habitat availability for lake trout. Restoration of lake mixing would occur rapidly upon significant reduction of road salt application to the watershed and improvements in stormwater management.”

Road Salt Effects on the Water Quality of Lakes in the Twin Cities Metropolitan Area
Novotny et al. 2007. ST. ANTHONY FALLS LABORATORY Engineering, Environmental and Geophysical Fluid Dynamics Project Report No. 505 “Density current intrusions have the ability to cause chemical stratification of a lake. NaCl concentrations of 1 g/L increase the specific gravity of water by approximately 0.0008 (Wetzel 2001). This change is significant in relation to temperature-induced density changes. For example, a temperature change from 4 to 5o C produces the same density change (0.000008) as a salt concentration of 10 mg/L (Wetzel 2001). Density stratification caused by salt can be much stronger than that caused by temperature causing lakes to become “meromictic”. This condition is typically found in lakes with a small surface area, yet fairly deep. Large lakes are less vulnerable to becoming meromictic than small lakes because large lakes have a greater fetch which can create more powerful wind mixing and dilution of intruding density currents (Environment Canada Health Canada 1999). Shallow lakes are less susceptible to meromixis than deeper lakes due to the smaller amount of energy required to fully mix the lake.”

Increase of urban lake salinity by road deicing salt
Eric V.Novotny et al., 2008. Science of The Total Environment 406:131-144 “Over 317,000 tonnes of road salt (NaCl) are applied annually for road deicing in the Twin Cities Metropolitan Area (TCMA) of Minnesota. Although road salt is applied to increase driving safety, this practice influences environmental water quality. Thirteen lakes in the TCMA were studied over 46 months to determine if and how they respond to the seasonal applications of road salt. Sodium and chloride concentrations in these lakes were 10 and 25 times higher, respectively, than in other non-urban lakes in the region. Seasonal salinity/chloride cycles in the lakes were correlated with road salt applications: High concentrations in the winter and spring, especially near the bottom of the lakes, were followed by lower concentrations in the summer and fall due to flushing of the lakes by rainfall runoff. The seasonal salt storage/flushing rates for individual lakes were derived from volume-weighted average chloride concentration time series. The rate ranged from 9 to 55% of a lake’s minimum salt content. In some of the lakes studied salt concentrations were high enough to stop spring turnover preventing oxygen from reaching the benthic sediments. Concentrations above the sediments were also high enough to induce convective mixing of the saline water into the sediment pore water. A regional analysis of historical water quality records of 38 lakes in the TCMA showed increases in lake salinity from 1984 to 2005 that were highly correlated with the amount of rock salt purchased by the State of Minnesota. Chloride concentrations in individual lakes were positively correlated with the percent of impervious surfaces in the watershed and inversely with lake volume. Taken together, the results show a continuing degradation of the water quality of urban lakes due to application of NaCl in their watersheds.”

Lake stratification caused by runoff from street deicing
John H.Judd. Water Research Volume 4, Issue 8, August 1970, Pages 521-532

By Robert C. Bubeck and Richard S. Burton U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 87-4223

Road Salt Impact on LakeStratification and Water Quality
Novotny, E.V and Stefan, H.G. 2012.
Journal of Hydraulic Engineering 138: 1069-1080.

Redox stratification and salinization of threeKettle Lakes in Southwest Michigan, USA
Koretsky, C.M et al. 2012. Water Air & Soil Pollution

Cultural meromixis: Effects of road salt on thechemical stratification of an urban kettle lake.
Sibert, R.J. et al., 2015. Chemical Geology 395:

More Literature

How common road salts and organic additives alter freshwater food webs: in search of safer alternatives
Matthew S. Schuler et al, 2017. Journal of Applied Ecology 2017, 54, 1353–1361

Review of operation of urban drainage systems in cold weather: water quality considerations
J Marsalek et al., 2003. Water Sci Technol. 48(9):11-20 “Cold climate imposes special requirements on urban drainage systems, arising from extended storage of precipitation and pollutants in the catchment snowpack, processes occurring in the snowpack, and changes in catchment surface and transport network by snow and ice. Consequently, the resulting catchment response and runoff quantity differ from those experienced in snow- and ice-free seasons. Sources of pollutants entering urban snowpacks include airborne fallout, pavement and roadside deposits, and applications of de-icing and anti-skid agents. In the snowpack, snow, water and chemicals are subject to various processes, which affect their movement through the pack and eventual release during the melting process. Soluble constituents are flushed from the snowpack early during the melt; hydrophobic substances generally stay in the pack until the very end of melt and coarse solids with adsorbed pollutants stay on the ground after the melt is finished. The impacts of snowmelt on receiving waters have been measured mostly by the snowmelt chemical composition and inferences about its environmental significance. Recently, snowmelt has been tested by standard bioassays and often found toxic. Toxicity was attributed mostly to chloride and trace metals, and contributed to reduced diversity of benthic and plant communities. Thus, snowmelt and winter runoff discharged from urban drainage threaten aquatic ecosystems in many locations and require further studies with respect to advancing their understanding and development of best management practices.”

Urban Environment and Sewage Salinization.
A. Vengosh, 2003. In Treatise on Geochemistry, 2003.Two major sources of salinity are identified in the urban environment: sewage and road salt. The salinity of domestic wastewater is derived from both the salinity of the source water supply to the municipality and the salts added directly by humans

Relating Road Salt to Exceedances of the Water Quality Standard for Chloride in New Hampshire Streams.
Philip R. Trowbridge et al., 2010. Environ. Sci. Technol. 2010, 44, 13, 4903–4909 “Six watersheds in New Hampshire were studied to determine the effects of road salt on stream water quality. Specific conductance in streams was monitored every 15 min for one year using dataloggers. Chloride concentrations were calculated from specific conductance using empirical relationships. Stream chloride concentrations were directly correlated with development in the watersheds and were inversely related to streamflow. Exceedances of the EPA water quality standard for chloride were detected in the four watersheds with the most development. The number of exceedances during a year was linearly related to the annual average concentration of chloride. Exceedances of the water quality standard were not predicted for streams with annual average concentrations less than 102 mg L−1. Chloride was imported into three of the watersheds at rates ranging from 45 to 98 Mg Cl km−2 yr−1. Ninety-one percent of the chloride imported was road salt for deicing roadways and parking lots. A simple, mass balance equation was shown to predict annual average chloride concentrations from streamflow and chloride import rates to the watershed. This equation, combined with the apparent threshold for exceedances of the water quality standard, can be used for screening-level TMDLs for road salt in impaired watersheds.” Another summary*: According to a 2010 study in the journal of Environmental Science & Technology, continued sampling in New Hampshire showed that streams with average annual chloride concentrations greater than 100 mg/L are a good predictor of what will become 230 mg/L water quality standard violations. Researchers found that impairments occur when at least 15% of the watershed is dedicated to developed land and transportation uses, and salt loading rates are about 70 metric tons per square kilometer.

A Fresh Look at Road Salt: Aquatic Toxicity and Water-Quality Impacts on Local, Regional, and National Scales
Steven R. Corsi et al., 2010. Environ. Sci. Technol. 2010, 44, 19, 7376–7382 “Research on the influence of urban land use on aquatic life in streams has identified a level of 7−12% impervious surface where decreases in biological integrity were observed (19-21).” Good discussion of challenges of dealing with increasing salt loading.

Comparison of the effects of temperature and solutes on water density
An example

Density values were generated from temperature and TDS by Water Density Calculator


Do Road Salts Cause Environmental Impacts?
Stranko,S. et al. 2013. Maryland Dept of Natural Resources. 35 pages

*Solving Slick Roads and Salty Streams
Water Environment Federation Stormwater Report, March 4, 2015

Water quality measurements on Williams Lake and Colpitt Lake (Halifax, N.S.) Dec 7-13, 2015 with reference to possible impacts of road salt
Patriquin, D. 2016. Report to Williams Lake Conservation Company.

Salting our freshwater lakes
Hilary A. Dugan et al., 2017 Proceedings of the National Academy of Sciences (USA) 114: 4453-4458.
Press coverage
Road salt threatening health of freshwater lakes, study finds
Margo McDiarmid · CBC News · Posted: Apr 10, 2017 “Study of 371 North American lakes finds melting snow and runoff put ecosystems at risk”
Highway Salt Is Polluting Our Lakes
Joseph Blumberg in Dartmouth News “… The report also emphasized the critical role pavement is playing. In places with a snowy winter and heavy use of road salt, there is a direct connection between the amount of development around the lake and the lake’s increasing salt concentration over time. Rather than being absorbed into the ground, the salty runoff drains across the paved surfaces and into neighboring water bodies. Lakes with surrounding land cover that was more than 1 percent paved were found to show increasing levels of chloride content.”

A review of the species, community, and ecosystem impacts of road salt salinisation in fresh waters.
William D. Hintz & Rick A. Relyea 2019  Freshwater Biology Volume64 (6):1081-1097

 Simple Model of Changes in Stream Chloride Levels Attributable to Road Salt Applications
Stephen B. Shaw et al., 2012. Journal of Environmental Engineering Volume 138 Issue 1 – January 2012/ “Increasing stream chloride (Cl−) concentrations have been observed over the last several decades in regions that receive regular road salt. In many cases, these increases occur even when road salt application has remained nearly constant, indicating the presence of multiyear attenuation within watersheds. This paper presents a simple mixing model to interpret the relationship between Cl− inputs and Cl− in stream discharge. The model was applied to data collected between 1972 and 2003 from Fall Creek in central New York, and the results indicate that stream salt concentrations may continue to increase for several decades. The estimated average residence time of road salt in the watershed was approximately 50 years, although the uncertainty in road salt application history suggests residence times of 40–70 years are reasonable. Hydrologists may be able interpret historical road salt applications and stream salt responses as essentially a regional tracer experiment to gain insights into macroscale watershed characteristics that could dominate average water residence time.

Regulations are needed to protect freshwater ecosystems from salinization
Matthew S Schuler et al., 2018 Philos Trans R Soc Lond B Biol Sci 2018 Dec 3;374(1764):20180019.

Salt in freshwaters: causes, effects and prospects – introduction to the theme issue
Miguel Cañedo-Argüelles et al., 2019. Philos Trans R Soc Lond B Biol Sci. 2019 Jan 21; 374(1764): 20180002.

Seasonal and Long-Term Dynamics in Stream Water Sodium Chloride Concentrations and the Effectiveness of Road Salt Best Management Practices
Victoria R. Kelly et al., 2019. Water, Air, & Soil Pollution volume 230, Article number: 13 “We use a 32-year dataset from a rural, southeastern New York stream to describe the effect of long-term road salt use on concentrations of sodium (Na+) and chloride (Cl-“). Mean annual stream Na+ and Cl-” concentrations initially increased, reached a plateau, and then increased again. Trends in summer and winter stream concentrations were similar but summer concentrations were higher than winter, indicating that salt entered the stream via groundwater discharge. Seasonal and inter-annual variability in stream Na+ and Cl” concentrations and export were high in the latter years of the study and can be explained by increased variability in stream discharge. Stream water Na+ and Cl-” concentrations were positively correlated with conductivity, and conductivity was negatively correlated with discharge during all seasons (p < 0.001). We used road salt application data from a local agency to examine effects of best management practices. Despite reductions in salt application, there was no commensurate decrease in stream water Na+ and Cl-” concentrations. We estimate that the legacy of long-term salt accumulation in groundwater and soils may delay a decline in stream water Na+ and Cl-” concentrations by 20- 30 years. Continued research to develop road salt reduction practices is important to mitigate impacts on freshwater ecosystems and drinking water supplies.

A Review of Road Salt Ecological Impacts
Athena Tiwari & Joseph Rachlin. 2018. Northeastern Naturalist 25(1):123-142 “Road-salt runoff is an increasing problem in areas of North America that receive snow. Its effects include groundwater salinization, loss or reduction in lake turnover, and changes in soil structure. Road-salt runoff can affect biotic communities by causing changes in the composition of fish or aquatic invertebrate assemblages. It also poses threats to birds, mammals, and roadside vegetation.”

Minnesota Stormwater Manual: Cold climate impact on runoff management
Minnesota State Document. Comprehensive. “This section introduces national and international research and experience on stormwater practices maintained in cold climate regions, and presents principles for adapting BMPs to provide effective pollutant removal and runoff control during cold-weather months. It also introduces some recent findings from within Minnesota on the impact of climate change on stormwater and meltwater runoff. For more information on this topic, see Chapter 5 of MPCA’s Protecting Water Quality in Urban Areas. Minnesota stormwater managers must recognize that runoff from snowmelt has special characteristics, and that BMP design criteria addressing only rainfall runoff might not work well during cold periods. This becomes a major problem because a substantial percentage of annual runoff volume and pollutant loading can come from snowmelt.”

Road Salt – Review of Best Management Practices
Report Prepared for: Halifax Regional Municipality, Sustainable Environment Management by Stantec, 2011. (There is no mention of possible impacts of roadsalt on lake stratification in this document).

Environment Canada on Roadsalts