September 25th, 2017
Notes on the Hydrogeology of Sable Island, Nova Scotia
Fresh Water on Sable Island
Contributed by Terry W. Hennigar, P.Eng., FEC, who spent many months on Sable Island in the 1970’s researching the hydrogeology of Sable Island. He is the author of the only report on the fresh water supply of Sable Island, entitled “Water Resources and Environmental Geology of Sable Island, Nova Scotia”. Mr. Hennigar last visited Sable Island in 1977.
Although Sable Island has been the subject of many reports on its history, ship wrecks, geological setting, and other sectors of science, its water resources were always taken for granted. It wasn’t until the early 1970’s that Mobil Oil asked the question about the nature and extent of the fresh water resources on the island. As it so happened, the province of Nova Scotia through the then Department of Mines, had recently begun a major new program to inventory, map, and evaluate the Groundwater Resources of the province. In May 1971 a reconnaissance trip was made to the island with Mobil Oil staff to assess the scope, details of a study, and equipment required for an assessment of the fresh water resources of Sable.
The geology of the upper 1,000 feet, or more, is a series of inter-bedded sand and peat deposits. The sand is uniform in size (about 0.5 mm diameter) and mostly clear and white quartz, with a scattering of various feldspars and heavy metals. The sands are porous and permeable which are ideal attributes of an aquifer. The island location, climate, and general landscape are unique to Nova Scotia which makes the hydrogeology a challenge to define and evaluate. Figure 1 shows the setting of Sable on the edge of the Continental Shelf. Three approaches were considered and applied to the water resources study of the island:
1. a review of the dynamics of the vulnerable and ever changing dune systems;
2. drilling and installation of water monitoring equipment; and,
3. interpretation of geological and geophysical data, and “Plain Old Basic Horse Sense”.
Figure 1 The precarious setting of Sable Island on the edge of the Continental Shelf.
The source of fresh water on Sable Island is all from precipitation (rainfall and snow fall). The average annual inflow (precipitation) of fresh water is approximately 1,270 mm. Of this total approximately 38% is lost to evaporation and transpiration annually. The remainder of the freshwater is temporarily stored in the sands while discharging to the ocean under the North and South beaches. Because of the high rates of groundwater flow, and the low hydraulic head in the fresh water reservoir, high tides and storm surges introduce significant volumes of salt water into the edges of the freshwater reservoir. This natural intrusion of salt water, together with the intrusion from over-pumping of water supply wells, poses real threats to the fresh water resources of the island. In addition, the very high permeability of the sand deposits makes the subsurface freshwater resources extremely vulnerable to contamination from land uses such as waste disposal and petroleum spills.
The review and study of the dune systems revealed well drained sands and deep water tables under high topographical areas (ie. tops of dunes). Infiltration rates of the sands were measured and found to be much higher than rates of precipitation experienced on the island. Therefore no surface runoff, i.e., stream flow, occurs on the island. What precipitation is not lost from evaporation and/or transpiration infiltrates (percolates) vertically to the freshwater lenses and the groundwater reservoir. Figure 2 shows the drifting nature of the sable sands and how an obstacle such as a house causes an accumulation of sand on the leeward side of the prevailing wind.
Figure 2 An example of the shifting sands of Sable Island.
The drilling of test holes for geologic samples, and the installation of monitoring wells, allowed a preliminary picture of the nature and characteristics of the aquifer system under the island. Samples of sand were collected at various depths during drilling, and analysed for their geological characteristics. Monitoring wells were constructed that allowed sampling at various depths within the fresh water reservoir. Water samples were collected and analysed for general chemistry. Water level recorders were installed to monitor water level responses in the freshwater reservoir to tidal activity and precipitation events. Figure 3 shows the type of drilling equipment used to drill the test holes and install monitoring wells on Sable during the project.
Figure 3 Drilling equipment used to install observation wells.
To compile a picture of the freshwater system under the island a number of observations were made and conclusions drawn, including but not limited to the following: the application of geology and geophysics; interpretation of drilling results; the results of water quality analyses; monitoring water levels; and “Plain Old Basic Horse Sense”. Note in Figure 4 where unsaturated sands underlie the dunes to considerable depths. The first (shallow) saturated zone consists of fresh water, and salinity increases with depth. In Figure 4 the green coloured zone is fresh water (chloride <250 mg/L), the yellow coloured zone is brackish water (chloride = 250 – 1,000 mg/L), and the brown coloured zone is salt water (chlorides > 1,000 mg/L).
The freshwater reservoir extends under the length of the island where dunes occur, i.e., it does not extend under the East or West spits. The water table is slightly convex with the highest levels along the centre of the island. This geometric curved surface gives rise to groundwater flow from the centre of the island towards the North and South coastlines. The highest recorded elevation of the water table near the centre of the island is approximately 1.4 metres above mean sea level. Surface water flows downhill in a stream; groundwater flows down gradient in an aquifer system.
Figure 4. A diagram showing the fresh water reservoir (green colour) under Sable Island. The white area is unsaturated sand.
Horses know where to dig for a fresh water supply. This is a low topographic area between the North and South dune system, in the centre of the island, where the water table is the highest.
Figure 5 An example of “Plain Old Basic Horse Sense”.
For more details on Sable Island’s unique hydrogeology, Mr. Hennigar’s report on “Water Resources and Environmental Geology of Sable Island, Nova Scotia” is available in many libraries, some used book stores, or on-line at:
Appendix: More photos of the Groundwater Project
A-1 A deposit of heavy metals, inter-bedded with the white silica sand, near the high tide mark.
A-2 Using geophysics (resistivity) to determine the depth of fresh water.
A-3 The Nova Scotia Base Camp, as newly established in 1973. The units are of fibre glass construction and were manufactured in Dartmouth, Nova Scotia.