Water Management at Alaska's Operating Mines: Process water, groundwater, and runoff all managed with care.
The Division of Water, part of the Alaska Department of Environmental Conservation, is tasked with improving and protecting the quality of Alaska's water. Specifically, the Alaska Pollutant Discharge Elimination System (APDES) program issues permits to authorize the appropriate discharge of water to the environment.
Fort Knox, an open-pit gold mine north of Fairbanks that's been in operation since 1994, has a closed system for process water (water which cannot be classified as drinking water and is used in connection with technical plants and industry processes) and maintains an APDES permit to discharge extracted, non-contact, non-process groundwater from pit dewatering wells.
The mine processes ore onsite at a carbon-in-pulp mill; other onsite facilities include the tailings storage facility (TSF), constructed wetlands complex, freshwater reservoir, and the Walker Creek Valley heap leach facility.
Fort Knox operates as a zero-discharge facility, which is possible because the TSF and mill form a closed system for process water. Water that has been used at the carbon-in-pulp mill to process mined material is sent to the tailings facility as part of the tailings slurry, and water is pumped to the mill from the TSF decant pond (a structure that uses sedimentation to remove settleable matter and turbidity--aka cloudiness/haziness--from wastewater). In plain terms; none of this water is discharged to the environment.
According to the "Fort Knox Annual Activity Report for Reporting Year 2016" published in February 2017, the mine maintains dry conditions via a system of thirty-four dewatering wells (of which three are inactive) and four Fish Creek wells. For 2016 the average pumping rate from the dewatering system was 2,120 gallons per minute. The majority of that water, 70 percent, was pumped directly to tailing impoundment; 16 percent was pumped directly from Fish Creek wells to the mill; and 14 percent was discharged to the Old Fish Creek Channel, from where it flows into the mine's freshwater reservoir.
Fort Knox applied for and was granted an APDES permit to discharge non-contact, non-process groundwater to the Old Fish Creek Channel in 2012, but discharge of groundwater (not requiring treatment) only began in March 2015. In June of 2016 a reverse osmosis water treatment system became operational for dewatering groundwater that does require treatment. In total in 2016, 607 acre-feet (an acre-foot is a unit of volume equal to a sheet of water one acre in area and one foot deep) of treated and non-treated dewatering well groundwater was discharged to the Old Fish Creek Channel.
A bathymetrie (the study of water depth) survey was conducted at the decant pond at the Fort Knox TSF and showed the decant pond contains approximately 10,095 acrefeet of water, an increase in the pond's volume attributed to abnormally high rainfall in 2014-2016. Fort Knox engages in ongoing water management activities including storm water control and the dewatering well groundwater discharge to reduce the additional volume created by an abnormal increase in rainfall.
Red Dog is an open-pit zinc and lead ore mine that's been in operation since 1989. It's located approximately ninety miles north of Kotzebue in the Delong Mountains, and its onsite water treatment system includes industrial water treatment plants and a sewage treatment plant.
The Red Dog water treatment plants are comprised of the TSF, water treatment tanks, high density sludge clarifier, and sand filters. Water from many sources feed the TSF, including mill tailings, mill process water, water treatment sludge, mine waters, and ambient precipitation runoff. TSF water is often referred to as reclaim water, and the first treatment step is to remove cadmium, which is accomplished by adding sodium sulfide to a water pipeline while the water is transported to water treatment tanks. The sodium sulfide reacts with the dissolved cadmium and forms insoluble cadmium sulfide, which remains stable throughout the water treatment process.
The reclaim water then flows into a 6,500 cubic-foot rapid-mix tank, where reacted lime and recycled solids are added to the water to adjust the pH to approximately 10.3. The water then flows into a 50,000 cubic-foot lime reactor where the water is sparged with compressed air (meaning air bubbles stir up and mix the water). This ensures the full oxidation of all ions in the solution--also known as precipitation, the ions are altered in form from a dissolved state to a solid state, essentially from soluble metals to insoluble metal-hydroxides. From this tank the water flows to a clarifier feed well; at some point flocculant is added, either in the clarifier feed well or in a floc mix tank immediately upstream of the clarifier. Flocculant coalesces the smaller particles into larger solids. In the clarifier the solids are allowed to settle and are then removed through the underflow, while the treated water leaves the clarifier through the overflow. Finally, the treated reclaim water runs through sand filters, which remove any residual solids. Automated pH and turbidity meters take final measurements to determine whether the water meets mandated standards and can be discharged into Red Dog Creek or if the water needs to be returned to the tailings impoundment.
At Red Dog water is processed at two plants: Water Treatment Plant 1 operates year-round at a rate of 6,000 gallons per minute. This water is not discharged to the environment but is directed to the mill for ore processing. Water Treatment Plant 2 operates seasonally, and it is this plant that discharges water at Outfall 001 at the Middle Fork of Red Dog Creek at a maximum capacity of 14,500 gallons per minute (this plant has the ability to provide water to the mill when necessary).
In 2016 Red Dog completed construction of a new lime slaker building in time for the discharge season. The lime slaker project allowed for complete treatment of all captured main waste stockpile rock drainage water, which increased in 2015 compared to previous years. In the Q1/Q2 2016 Red Dog-Suvisi (Red Dogs' newsletter), General Manager Henri Letient said, "I am pleased with the way we started the annual discharge season. Discharge of treated water to Middle Fork Red Dog Creek was initiated on May 1 with well over 600 million gallons of treated water already discharged from the pond, 100 percent in compliance with regulatory limits."
Usibelli Coal Mine is Alaska's only operating coal mine. It was founded in 1943 by Emil Usibelli and remains a family-run business today. The mine site is located near Healy, 115 miles south of Fairbanks and 250 miles north of Anchorage.
Usibelli operates year-round and averages production between 1.2 million and 2 million tons of coal a year. One unique aspect of Usibelli is that the company has any water management requirements at all considering it doesn't use any water in its mining process.
To access the coal seams, overburden (material that lies on top of a coal seam) and interburden (material between the coal seams) need to be removed. A 1300W Bucyrus-Erie Walking Dragline, nicknamed "Ace-in-the-Hole" by Healy school children, removes the bulk of the overburden, while dozers move overburden short distances or from locations difficult for the dragline to access. Shovels then strip the overburden and load the uncovered coal into haul trucks. These, of course, aren't hand-held shovels; they're large machines with buckets that can haul approximately 26 cubic yards of material and can load a 150-ton haul truck in about four passes. The coal is then crushed and it's ready for transport.
The water that Usibelli is responsible for drains from active mining areas or is runoff water from recently reclaimed lands. Usibelli has an onsite laboratory, and half of that lab's work is dedicated to water testing. According to the company, water is collected during the summer season from May to October. Once collected, the water is tested for iron, manganese, settleable and suspended solids, turbidity, and pH.
At Usibelli operations, water is directed into a series of settling ponds, wherein the water is brought into compliance with Usibelli's APDES permit. Water in the settling ponds is treated with flocculant to enhance the removal of sediment. After settling, the water passes through a treatment train consisting of passive aeration, which removes iron; dolomite beds to remove manganese; and a polishing pond before discharge.
Mining can be done right, and in Alaska there is appropriate and consistent oversight to ensure that mine operations do not pose unnecessary risk to the environment. When mining is approached in a responsible and safe way, it's a huge economic boon for the state, especially for the rural communities in which much of the mining industry's exploration and production takes place.
According to "The Economic Benefits of Alaska's Mining Industry," prepared in consultation with the McDowell Group and published in January 2017, in 2016 the mining industry was responsible for 4,350 direct mining jobs and 8,600 direct and indirect jobs. The industry paid $675 million in total direct and indirect payroll and made $111 million in payments to Alaska Native Corporations in addition to $81 million in state government related revenue through rent, royalties, fees, and taxes. The report states that direct mining jobs are "mostly year-round jobs for residents of more than fifty communities throughout Alaska, half of which are found in rural Alaska where few other jobs are available." Those jobs are also some of Alaska's highest paying at an estimated average annual wage of $108,000.
Additionally, it's estimated the mining industry spent more than $65 million on exploration in 2016 and has spent more than $3.4 billion on exploration since 1981. Last year the report estimates that $120 million was spent on mine construction and capital investment. In 2015, the commodities claimed through mining activity had an estimated export value of $1.5 billion, or 32 percent of Alaska's total exports. According the Alaska Miners Association, "Alaska is a state steeped in a tradition of exploration, mining, and production... Roads, docks, bridges, buildings, renewable energy sources like the hydro plants in Southeast--much of Alaska's vital infrastructure--was built on mining. Mining today plays an active role in Alaska's growth and economic well-being."
By Tasha Anderson
Tasha Anderson is the Associate Editor for Alaska Business.
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|Title Annotation:||SPECIAL SECTION Mining|
|Comment:||Water Management at Alaska's Operating Mines: Process water, groundwater, and runoff all managed with care.(SPECIAL SECTION Mining)|
|Publication:||Alaska Business Monthly|
|Date:||Nov 1, 2017|
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