CSCE 2021 Annual Conference Inspired by Nature – Inspiré par la Nature
26-29 May 2021
BEHCHOKO, NWT – WATER SUPPLY PLANNING STUDY Johnson, K 1,3, Lanteigne, M2, and Keung, C1 1
EXP, Canada AECOM, Canada 3 ken.johnson@exp.com 2
Abstract: Water Supply for the Edzo settlement of Behchoko in the Northwest Territories currently consists of a surface water intake and pumphouse located in a river channel between Marian Lake and Great Slave Lake. Water and sewer are conveyed through a buried pipe system. In late winter there is a reoccurring issue of complete freezing of the water channel, and in response, water must be trucked to the water plant at a considerable cost. Edzo initiated a water supply planning study of the relative economic costs of alternate water sources for the community. Six alternate options were considered to replace the existing water supply system. The options ranged from various surface water sources adjacent to Edzo as well as ground water. The water quality of the various surface water bodies is similar with elevated concentrations of colour, iron, aluminum, and turbidity. The water quality of the well water is anticipated to have elevated iron, manganese, and hardness. The existing water treatment facility will be able to treat all the surface water sources. The six water supply options were analysed, and the well water option was eliminated from further consideration because of quantity limitations and water treatment requirements. Based upon analyses, it is recommended that the existing water supply from the West Channel be maintained with additional infrastructure, including a raw water reservoir. This option has the lowest anticipated capital cost, and operation and maintenance costs. 1
BACKGROUND
Edzo is a community located at 62o 40’ north latitude, and 116o 04’ west longitude, and is one of two settlements associated with the Tłı̨ chǫ Community Government of Behchokǫ̀, which also includes the larger community of Rae. Edzo is 106 km northwest from Yellowknife along Highway 3 and is located on a peninsula in Marian Lake approximately 24 kilometres away from Rae. The Edzo settlement is bounded on the north by Marian Lake, on the south by the North Arm of Great Slave Lake, and on the east by the West Channel between Marian Lake and the North Arm of Great Slave Lake. Edzo was constructed by the Government of the Northwest Territories in the late 1960’s and early 1970’s as an alternative townsite to Rae. The Rae townsite had planning issues with limited space for community expansion, as well as public health issues because of concerns about contaminated surface runoff impacting the drinking water supply. The first roads in Edzo were built in 1969 and the construction of the water and sewer systems began in 1970. At that time two wells were drilled within the community for the supply of potable water. Soon after the wells were commissioned, it was found that they did not have sufficient capacity to supply the community. This led to the development of a nearby surface water source which pumps raw water from the West Channel on the east edge of the community to a water treatment facility through a buried pipeline, and then subsequently distributes treated water through a buried water supply system. Low water levels in West Channel occurring in late winter, compounded by freezing of the channel have caused problems with the water supply for several decades. In the past, when this situation occurred, an alternate water supply was sourced by either trucking raw water from other locations to the water GEN 410-1
treatment plant, or pumping raw water to the intake location from the other side of the highway, a distance exceeding 200 m. This chronic water supply interruption was identified as unsustainable for the long-term water supply to the community for Edzo. 2
EXISTING SYSTEMS, ISSUES, AND SUPPLY OPTIONS
Edzo’s West Channel supply, water treatment and distribution system include a 150 mm diameter, intake pipe that brings raw water to a wet well located beneath a pumphouse on the edge of West Channel. The intake is approximately 25 m long and extends approximately 15 m into West Channel. The water from the wet well is pumped to the water treatment plant located approximately 900 m away, through a 100 mm diameter buried HDPE water line. Freeze protection to the water line is accomplished by recirculating the flow from the water treatment plant back to the pumphouse through a 50 mm buried recirculation HDPE line. The water treatment plant is located within the community and utilizes a 230 litre/min package treatment unit. Treated water is stored in a 900 m3 buried concrete storage reservoir. The Edzo raw water supply infrastructure is approaching 50 years of age and is near the end of its service life. The raw water supply/return lines however, are HDPE pipe and buried, and are expected to have experienced little to no deterioration over their current operating life and therefore, are expected to be in adequate condition to serve the community for the next 20 years. Over the winter months, the West Channel has been reported to freeze-up as early as September, restricting potential flow to the Edzo raw water intake from both Marion Lake to the north, and the North Arm of Great Slave Lake to the south. Ice break-up in the channel is reported to be in mid-April or May. The period of no flow in the channel based upon the historical freeze-up and break-up observations of September through to the end of April is 210 days. Based on the results of the bathymetric survey carried out in 2018, the depth of water under the highway located 160 m upstream from the intake is only 0.5 m, therefore it can be expected that at this location the channel would freeze to the bottom in early winter, cutting off the flow from Marian Lake. As well, 700 m downstream of the intake the depth of water also reduces to 0.5 m, isolating the intake from replenishment from the North Arm of Great Slave Lake at about the same time (see Figure 1).
Figure 1. Existing Edzo water supply from West Channel
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ALTERNATE WATER SUPPLY OPTIONS
Alternate water supply options were identified in consultation with the community (see Figure 2), and are as follows: 1. Surface Raw water supply from West Channel filling a seasonal earthen water storage reservoir to provide raw water for the period from October through May and connecting to the existing water treatment facility. 2. Surface water supply from Marian Lake using an intake pipeline extending out into the lake and a shoreline pumphouse conveying the raw water to the existing water treatment plant. 3. Surface water supply from the Frank Channel north using an intake pipeline extending out into the channel and a shoreline pumphouse conveying the raw water to the existing water treatment plant. 4. Surface water supply from the Frank Channel south using an intake pipeline extending out into the channel and a shoreline pumphouse conveying the raw water to the existing water treatment plant. 5. Surface water supply from the North Arm of Great Slave Lake using an intake pipeline extending out into the lake and a shoreline pumphouse conveying the raw water to the existing water treatment plant. 6. Well water supply from an existing well system that was established in the early 1970’s and discontinued because of insufficient capacity.
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Figure 2. Edzo water supply options Option 1 The West Channel water supply option (see Figures 1, 2, and 3) would utilize a new pumphouse installed beside the existing pumphouse, with a new wet well and new water intake pipe into West Channel, and will maintain the existing HDPE raw water supply/return pipes to the water treatment plant. To provide sufficient water supply over the anticipated 210 day winter period when flow from the West Channel is limited, additional raw water storage capacity would be developed in the form of a new earth storage reservoir located near the existing raw water supply/return lines, across Highway 3 from the existing water treatment plant. It is calculated that an additional live storage capacity of approximately 45,000 m 3 is GEN 410-3
required to satisfy the water demand to the year 2035. The earth reservoir would be filled with water prior to freeze-up every year.
Figure 3. West Channel water supply plan view (Option 1)
Figure 4. West Channel water supply profile view (Option 1) Assuming there is sufficient depth of overburden to construct a reservoir southwest of the intake, the required live storage capacity of 45,000 m3 (excluding ice and dead volume) may be constructed with an HDPE lined rectangular pond 130 m long, 100 m wide, 6 m deep, with side slopes of 3H:1V. An ice depth of 1.0 m was assumed.
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Figure 5. West Channel water supply system details (Option 1) Option 2 The Marian Lake water supply option (see Figures 2 and 6) would utilize a new water intake pipeline into Marian Lake, supplying raw water to a wet well/pumphouse located on the Edzo shoreline. To provide sufficient water supply without the flow limitations of the West Channel, it is proposed that a submarine water intake line be extended into Marian Lake. A 3.5 kilometre intake line terminated in the proximity of the Rae intake, in an area where the lake’s depth can provide an unrestricted flow of raw water to the water treatment system. The new water intake pipeline would be anchored with weights and extend approximately 500 metres onto the shore to a new wet well and intake pumphouse. The final position of the intake pumphouse would be selected based upon ground conditions. Raw water from Marian Lake would flow by gravity through the intake pipe and into a wet well beneath a new intake Pumphouse. The intake pumphouse would pump the raw water to the water treatment plant approximately 1.5 kilometres from the Pumphouse. This option also includes the construction of an access road to the pumphouse and installation of a power line.
Figure 6. Marian Lake water supply (Option 2) GEN 410-5
Option 3 The Frank Channel North water supply option (see Figures 2 and 7) would utilize a new pumphouse installed beside the Frank Channel with a wet well and water intake pipe into the Frank Channel. The new pumphouse would discharge into a 3800 metre raw water supply line that would traverse undeveloped terrain for approximately 1800 metres and along the existing highway for approximately 2000 metres. The pipeline would be a shallow buried and insulated HDPE pipe with an independent recirculation line from the water treatment facility to the new pumphouse to provide freeze protection. The features of the pipeline would include several air release valves dictated by high points in the terrain and a crossing of the West Channel. This option also includes the construction of an access road to the pumphouse and installation of a power line
Figure 7. Frank Channel north water supply (Option 3) Option 4 The Frank Channel South water supply option (see Figure 2) would utilize a new pumphouse installed beside the channel with a wet well and water intake pipe into the Frank Channel. The new pumphouse would discharge into a 5000-metre raw water supply line that would traverse undeveloped terrain for approximately 1000 metres and along the existing highway for approximately 4000 metres. The pipeline would be a shallow buried and insulated HDPE pipe with an independent recirculation line from the water treatment facility to the new pumphouse to provide freeze protection. The features of the pipeline would include a crossing of the West Channel. This option also includes the construction of an access road to the pumphouse and installation of a power line.
Option 5 The North Arm of Great Slave Lake water supply option (see Figure 2) would utilize a new water intake pipeline into the North Arm, supplying raw water to a wet well/pumphouse located on the Edzo shoreline. To provide enough water supply without the flow limitations of the West Channel, it is proposed that a submarine water intake line be extended 5 km into the Great Slave Lake to an area where the lake‘s depth can provide an unrestricted flow of raw water to the water treatment system. The new water intake pipeline would be anchored with weights and extend approximately 500 metres onto shore to a new wet well and intake pumphouse. The final position of the intake Pumphouse would be selected based upon ground conditions. Raw water from the Great Slave Lake would flow by gravity through the intake pipe and into a wet well beneath a new intake Pumphouse. The intake pumphouse would pump the raw water to the water GEN 410-6
treatment plant approximately 500 metres from the Pumphouse. This option also entails construction of an access road to the pumphouse and installation of a power line. Option 6 The well water supply option (see Figure 2) would require the redevelopment and expansion of the existing two water wells that served the community in the 1970’s. 4
PHYSICAL ENVIRONMENT
The geology and terrain in Edzo consist of sandy sloping ground bounded by muskeg to the north and west. This geology and terrain contrast with the bedrock terrain within and around the Community of Rae. The top 1.2 to 1.5 m of soil in Edzo generally consists of granular material (sand and gravel), which is underlain by a layer of shale bedrock. Some areas of the muskeg are underlain by impervious clay. Edzo is in the discontinuous permafrost zone. The vegetation within the Edzo area is heavily forested with spruce, poplar, jack pine, willow and other hardy species. Mosses, low bushes, and grasses cover open areas. A seven-month long winter is the dominant season between October to April, and the mean daily temperature in January is -30°C. The summers, from June to August, are cool, with a mean daily temperature of 16°C in July. The mean frostfree period is 113 days. Ice forms on the lakes in October and leaves in early June. The average annual precipitation is about 250 mm of which 60 % falls as rain. Prevailing winds are from the north and east. 5
AVAILABLE WATER QUALITY AND WATER QUANTITY
The raw water of Marian Lake and the West Channel are similar for all practical purposes because the West Channel is an outlet from Marian Lake to the North Arm of Great Slave Lake. The water quantity available from the West Channel, Marian Lake and North Arm is, in principle, unlimited. Since the West Channel is directly connected to Marian Lake and the North Arm, this water source should also be, in principle unlimited; however, the channel is subject to freezing which isolates the raw water intake from a replenishing water supply. Water demand in 2015, referred to as winter water demand over this period was estimated to be 27,000 m3. The projected winter water demand in 2035 is estimated to be 44,000 m3. The current and projected water demand exceeds the estimated available water in the freeze isolated section of the West Channel, which ranges from 18,000 m3 to 25,000 m3 depending upon the ice thickness (0.9 m to 1.2 m of ice). The raw water quality of the Frank Channel and the North Arm is expected to be similar, for all practical purposes, to the water quality of Marian Lake. Water quality has been compiled most recently as part of the Rae water treatment facility construction in 2009. The water quality parameters of colour, iron, aluminum, and turbidity were above the Guidelines for Canadian Drinking Water Quality. The well water supply has typical water quality characteristics of groundwater with high levels of iron, hardness, and total dissolved solids. It is anticipated that the recommissioning of the well system and expansion of the well system would not provide enough water for the community because the Edzo well water is limited to 38,000 m3 per year. 6
WATER TREATMENT NEEDS
The water treatment plant currently serving Edzo was designed to treat the water from Marian Lake / West Channel. The anticipated similarity of the water quality in the Frank Channel and the North Arm of Great Slave Lake would allow this facility to treat the water from these sources as well. The water treatment plant would require a complete reconfiguration and optimization to treat the well water supply if this source was selected. The water treatment plant applies the processes of mixing, coagulation, flocculation, tube settler assisted clarification and gravity filtration. The treatment process consists of two mechanical flocculation units, one GEN 410-7
tube settler sedimentation process train and two dual media gravity filters rated to produce 218 m3 of treated water per day. The treatment system is comprised of the following unit processes: raw water flow control; chemical coagulation and rapid mixing; caustic injection for pH adjustment; clarification within a tube settler; filtration through dual media filters; post treatment by sodium hypochlorite injection for maintaining chlorine residual. 7
CONCLUSIONS AND RECOMMENDATION 1. The estimated annual water quantity required by the community of Edzo is currently 47,000 m3, and the future water quantity required is 70,000 m3 in 20 years (2035), and 107,000 m3 in 40 years (2055) years. 2. The water quality of Marian Lake, Frank Channel, the West Channel and the North Arm of Great Slave Lake is anticipated to be very similar with elevated concentrations of colour, iron, aluminum and turbidity. The water quality of the Edzo well water is anticipated to have elevated iron, manganese and hardness. 3. Water treatment needs for Marian Lake, the West Channel, Frank Channel and the North Arm of Great Slave Lake water may be achieved with the current water treatment facility in Edzo which applies the processes of mixing, coagulation, flocculation, tube settler assisted clarification and gravity filtration. The water treatment needs for the Edzo well water may not be achieved by the current treatment plant, and an upgraded treatment process would be needed to remove iron and/or manganese and hardness. 4. The water quantity available Marian Lake, Frank Channel and the North Arm of Great Slave Lake is unrestricted; the West Channel is limited to 18,000 to 25,000 m3 by the channel freeze up between the period of October through May; the Edzo well water is limited to 38,000 m 3 per year. 5. Six water supply options were analysed, and the well water option was eliminated from further consideration because of quantity limitations and water treatment requirements. The West Channel, Marian Lake, the Frank Channel North and South and the North Arm of Great Slave Lake supply options were further analysed to develop conceptual design configurations. 6. All options will both require environmental measures associated with the construction of each option. The Marian Lake and Great Slave Lake options will require considerable environmental measures because of the extensive in-water construction required. 7. The estimated capital cost for the options are: West Channel water supply system $4.0 million, Marian Lake water supply system $8.9 million, Frank Channel north water supply system $6.9 million, Frank Channel south water supply system $7.7 million, and North Arm of Great Slave Lake water supply system $11.4 million. 8. Based upon the conclusions, the West Channel water supply option is most economical, and, on that basis, it is recommended that this option be advanced to provide the community of Edzo with a reliable raw water supply with the lowest capital, and operation and maintenance costs.
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