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.
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* 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 at all. Then the deeper waters can 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

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).

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 EC values are very high, circa 500 uS/cm for surface waters, and circa 1200 uS/cm for deep water when the lake is 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. A lot of what they have begun to get back, with a lot of effort, are features Sandy Lake still retains. We need to ‘work on it’ to keep it that way, also to restore the lake to a more pristine condition.

Another example:
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.”

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

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

Sibert, R.J. et al., 2015. Cultural meromixis: Effects of road salt on thechemical stratification of an urban kettle lake. Chemical Geology 395:
126-137.

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

Koretsky, C.M et al. 2012. Redox stratification and salinization of threeKettle Lakes in Southwest Michigan, USA. Water Air & Soil Pollution
223:1415–1427.

Philip R. Trowbridge et al., 2010. Relating Road Salt to Exceedances of the Water Quality Standard for Chloride in New Hampshire Streams. 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.
*Solving Slick Roads and Salty Streams Water Envioronment Federation Stormwater Report, March 4, 2015

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

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

A. Vengosh, 2003. Urban Environment and Sewage Salinization. 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

Stephen B. Shaw et al., 2012. Simple Model of Changes in Stream Chloride Levels Attributable to Road Salt Applications
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.

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.

Conductivity, Salinity and Total Dissolved Solids.”
Fondriest Environmental, Inc. 3 Mar 2014. Web. Units of measurement etc.

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.

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.

–Electrical Conductivity (EC) as a measure of the TDS content (view conversion factors).