Water treatment "on the rocks" in Yellowknife, NWT

Cryofront - News, Views and Muse from the Far North

Water treatment "on the rocks“ in Yellowknife, NWT Ken Johnson Stantec

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Figure 1

Figure captions Figure 1. The Giant Mine, near Yellowknife, ceased operation in 2005, and has left a legacy of 240 thousand tonnes of arsenic trioxide, which will require perpetual care to keep the toxic material contained (See Summer 2014 WCW magazine article entitled “Managing a ‘Giant’ toxic legacy in the North”) .

Figure 2. The water supply for the City of Yellowknife is currently conveyed by an 8 kilometre submarine pipeline from the Yellowknife River to a pumphouse, reservoir, and soon to be commissioned water treatment plant.

Figure 3. The new water treatment plant for the City of Yellowknife is scheduled for commissioning in 2015, and will include a provision for arsenic removal as a precautionary measure.

The history of Yellowknife’s water supply is intrinsically linked to its start as a hard rock mining town. When gold was discovered on the shores of Great Slave Lake, and the claims were staked, Yellowknife was born as gold mining boomtown. The two most longstanding and productive mines, the Con and Giant Mines, immediately adjacent to the townsite, were both closed by 2005, however both mines have left significant contaminant legacies.

The gold extraction process used in Yellowknife required a ‘roasting’ process to extract the gold from arsenopyrite rock. Until the use of pollution control devices in the 1950’s, this process released uncontrolled quantities of arsenic trioxide and sulphur dioxide into the air around the community, which came to rest on the Canadian Shield around Yellowknife , and the water bodies on the shield.

Despite the fact that arsenic concentrations in the water supply were within the limit for human consumption at the time, Yellowknife decided to change water sources from the adjacent Yellowknife Bay to the mouth of the Yellowknife River in the 1960's. By 1969, a new intake pumphouse was completed, and raw water was pumped through an 8 kilometre submarine pipeline to the townsite, which is still used today.

For many years the Yellowknife River water supply was considered to be a high quality pristine potable water supply requiring only chlorination as water treatment. However, with the new multiple barrier approach to safe drinking water, and increasingly stringent water quality criteria, even the pristine waters of the Yellowknife River demanded an increased level of treatment.

In the 70 years of water supply in the City of Yellowknife, there has been no reported biological contamination in the water supply. The only two events of concern have been the arsenic detection in the 1970’s, and excessive turbidities in the Yellowknife River water during the spring of 2004. The persistence of arsenic in the water, and the probability of another high turbidity event were taken into consideration for the water treatment process selection.

In 2005, the City completed a pilot scale exercise to test different treatment processes on the potential Yellowknife water supplies of Yellowknife River and Yellowknife Bay. The advantage of a Yellowknife Bay water supply would be the decommissioning of the aging submarine pipeline from the Yellowknife River.

Membrane filtration, and direct filtration pilot tests were run on both sources, and based upon pilot testing results, a decision was made to advance a membrane process. The selection and pre-approval of a membrane plant manufacturer prior to the completion of the final design and tendering of the project was completed in 2012, and PALL Canada was chosen as the successful candidate (sourced through DWG Process Supply Ltd).

Given the possibility of changing the raw water source to Yellowknife Bay, the PALL treatment system would include an arsenic treatment system. It was noted that arsenic removal is not be being put in because the arsenic is high in the potential Yellowknife Bay source water, but rather as a precautionary measure.

Arsenic sampling has been ongoing at Yellowknife Bay since 1996, and the arsenic concentrations were all less that 5 parts per billion, with the exception of one sample at 6.5 ppb.

The arsenic in the water of Yellowknife Bay and the Yellowknife River is low, so the City's selection of either source would be acceptable.

Yellowknife Bay has the potential risk of arsenic in the water, which originates from two sources. The first source would be from arsenic "remobilization" from the sediments of the bay. The sediments in the bay contain considerable amounts of arsenic, however the arsenic in the sediments is considered stable, and the low concentrations measured in water from the bay over the past 20 years support this position.

The second arsenic source would be Giant Mine, which has tailings ponds with arsenic concentrations of 20,000 ppb. If a tailings pond breach occurred, water would be discharged into a creek and ultimately into Back Bay, however

dilution would reduce the concentrations to about 100 ppb near the potential site of the raw water intake.

The rationale behind the potential water source change is that the eightkilometre-long submarine pipeline used to bring water to the City from the Yellowknife River intake is reaching the end of its design life, and the underwater pipeline is undersized for future capacity. If the water source remains the same, the pipeline, which has been in place since 1968, needs to be replaced by 2020 for a cost of over $10 million.

Site work on the new plant began in 2011, on the shore of Great Slave Lake, with construction of an access road to the site within the Tin Can Hill parcel. The site already has a 9 million litre water storage reservoir, which was most recently expanded in 2007.

The detailed design process was completed in May of 2013 and the tenders closed on in July, 2013 with award approved by Yellowknife City Council at the end of July. North American Construction was awarded the project for a total cost of $30 million, and the project is expected to be completed in 2015. The risks associated with arsenic in the water supply have been thoroughly considered by the City of Yellowknife, and the precautionary treatment system will provide the City with a flexibility to operate the system with several raw water sources, and provide the residents with the high quality water that the community is known for.