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.
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 some concern 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.
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.
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.
1. Values of electrical conductivity, pH and temperaturearound the perimeter of Sandy Lake on Aug 10, 2017.
Above: Values of electrical conductivity (yellow numerals; units are uS/cm), pH (red) and temperature (red) of surface water at different locations on Sandy Lake on Aug 10, 2017.
Click on image for larger version
The highest EC values occur near the mouths of stream draining settled areas at the south of the lake. (view Stream Map)
2. Values of electrical conductivity, pH and temperature for surface waters on various dates in 2017.
Above: Values of electrical conductivity, pH and temperature for surface waters on various dates in 2017.
Click on image for larger version
Note the higher EC values for streams on the west side of the lake than on the east side of the lake and, in general, that pH values for streams are lower than values for Sandy Lake (see also figure above) except for Bob’s Brook (also known as Johnson’s Brook; it enters the lake at the southwest corner of the lake) and Peverill’s Brook flowing to Marsh Lake.
3. Values of electrical conductivity, pH and temperature for streams associated with Johnson’s (Bob’s) Brook on Nov 8, 2018
The EC value at the 2 culverts where the Johnson’s Brook system enter Sandy Lake was 81 uS/cm, much lower than the 200 uS/cm observed June 21, 2017, and reflects the high rainfall in the preceding month or so; likewise, the EC value at Sandy Lake Beach Park was 134 uS/cm on Nov 8, 2018, compared to 163 uS/c, on Sep 8, 2017.
Three streams converge at “Murphy’s Pit” (see Map page 3 in Thompson 2001). I estimated the volume of flow in Upper Johnson’s Brook (#1 in map above) to be 3 x that in the stream draining the Uplands Park wastewater treatment area, and at least 30 times that draining the construction/trucking yard and community just to the SE of the Dairy Road. Upper Johnson’s Brook had a very low EC value, while those of the other two brooks were elevated (125 uS/cm for #2, 410 uS.cm for #3/4); those streams are major sources of pollutants going into Sandy Lake.
The sites above should be sampled in the winter when salt is being applied to roads.
Also, the quality of water entering at site 4 (on Nov 8 with high electrical conductivity and very cloudy and full of particulate materials) should be further investigated. Is the cloudy water just temporary? What are the nutrient and bacteria levels? How does the flow rate change with seasons and relative to other flows into “Murphy’s Pit” and then into Sandy Lake, i.e. is it a significant source of pollutants for Sandy Lake?
View related photos: Sandy Lake: stream EC etc Nov 8, 2018
View Dec 13, 2018 Observations: Sandy Lake frozen over on Dec 13, 2018 & salt signal increases in streams entering lake via Johnson’s Brook
Info on Uplands Park, Timber Trails and The Dairy Treatment Facilities
Sandy Lake Watershed Study Final Report
AECOM, 2014. 131 pages. History, maps etc. From the document:
Uplands Park – WWTF
The Uplands Park Wastewater Treatment Facility (Figure 1) was built in 1969 and consists of a primary clarifier, a trickling filter with rock media, a secondary clarifier, and hypochlorite disinfection. The plant has a rated capacity of 91 m3 /day with a peak capacity of 178 m3 /day, and serves a population of approximately 170 people. The effluent discharge criteria is 20 mg/L for both biochemical oxygen demand (BOD) and total suspended solids (TSS). In 2009 the plant was upgraded to ultraviolet disinfection (Halifax Water 2008). The plant discharges approximately 40 m3 of treated effluent per day into a wetland/creek approximately 3.5 km upstream of Sandy Lake (T. Blouin, Halifax Water, pers. comm.). In the early 2000s sewage leaks from broken sewer lines southwest of Sandy Lake affected lake water quality. Boil water and no swim advisories for Sandy Lake were issued by the municipality. Following the breaks, the force mains were replaced by Halifax Water (K. Mackenzie, Halifax Water, pers. comm.).
The Timber Trails mobile home park is serviced by communal septic systems. In 2008, North West Community Council entered into a development agreement to enable an expansion of the Timber Trails mobile home park in support of upgrading its old sewage system. The old system had reportedly suffered from overflows and seepage during heavy rain events. As of August 2012, the park expansion has not occurred but detailed engineering of the new waste water treatment facility was underway (HRM Staff Report 2012). Since then, the waste water treatment facility has been upgraded but is not yet operational (A. Bone, HRM, pers. comm.)
[View also: Development Agreement Timber Trails Mobile Home Park Northwest Planning Advisory Committee Sep 11, 2009]
The Farmers Dairy facilities may have had impacts on water quality since it was constructed in the early 1970s. Erosion from areas logged prior to construction may have resulted in lake siltation in the early 1970s. Treated wastewater discharges from the dairy were associated with water quality impacts as reported by lakeside residents in the 1980s (Dalhousie 2002). The Farmers Dairy currently has a primary wastewater treatment facility consisting of two open-air lagoons. Discharge from the lagoons is directed to the municipal sanitary and the Mill Cove Wastewater Treatment Plant (T. Blouin, Halifax Water, pers. comm.).