MINE DEWATERING IN THE HUMBOLDT RIVER BASIN
OF NORTHERN NEVADA

By Tom Myers, Ph.D.

Hydrologic Consultant

 

The Humboldt River basin in northern Nevada provides more than half of the current United States’ gold production. Most of this mining occurs in deep, open pit mines excavated up to 1000 feet below the groundwater table. Eighteen open pit mines and at least three underground shafts extend below the groundwater table in the Humboldt River basin.

When mines extend below the water table, they must pump the local aquifers to keep their pit or shafts dry. This lowers the water tables near the mine. Dewatering and open pit mine construction cause three types of water deficit that may affect surface water flows in the future after mining ceases. First is the disposal or consumptive use of dewatering water. This creates a drawdown whose volume equals the sum of consumptive use and discharge volumes. Second, the volume of the pit below the water table also represents a volume which will fill with water. Because this volume was mostly rock prior to mining, the pit lake represents another substantial deficit. Third, water will evaporate from the surface of the pit lakes. This is a perpetual loss to the water balance of the basin.

Potential impacts on surface water resources result from three activities, or stresses. First is dewatering. This is the process of removing water around the pits and shafts to keep the mines dry. This water is either consumptively used, recharged (in some cases) or discharged to surface water effectively removing it from the local basin. Second is the refill of pit lakes and the associated drawdown cones. The volume of drawdown cones around the mines should approximate the water withdrawn for dewatering in excess of recharge. This deficit will be made up from recharge, surface water and movement of recharged water. Third is the long-term deficit caused by evaporation from pit lakes. This is a permanent draw from the basin’s water supply.

Between 1997 and 1999, approximately 200,000 acre-feet of water was pumped from the aquifers in the Humboldt River basin. When water is removed in excess of the safe yield, groundwater mining results. The Nevada State Engineer requires mines to recharge dewatering water when it is feasible. However, when it is not feasible, the State Engineer allows disposal. Therefore, dewatering water is either used consumptively in mining and milling operations, recharged to the local basin to offset deficits created by the dewatering, irrigated with in lieu of existing groundwater permits or discharged to surface water.

McCoy Cove recharges up to 90% of its pumped water (see the picture). This is the way it should be done. The Pipeline Deposit mine tries to infiltrate a similar amount but underestimated the required area and has had overflows and seeps form. There is no indication that water recharged through infiltration basins will reach the bedrock aquifers that are being dewatered. The Betze/Post mine infiltrated and irrigated with most of their dewatering water up to 1996, but it was done downhill from the mine so it is not likely that much of this water will be available to fill the pit when dewatering ceases. During much of 1997 and 1998, Betze/Post discharged straight to the Humboldt River. Lone Tree discharges straight to the Humboldt River the 90% of dewatered water not consumptively used or sent to the Valmy Power plant and the Trenton Canyon Project. The Gold Quarry Mine discharges straight to Maggie Creek about 40,000 acre-feet/year which flows into the Humboldt River.

Water not recharged is lost to the basin. While the extra water may benefit downstream ranchers and other users, it will not be available to fill the deficit when mining ceases. The deficit consists of the volume of water required to fill the pit and the drawdown cone created by dewatering. (When a well pumps, water flows to the well from all directions. In three dimensions, the water table resembles an inverted cone. The volume of unsaturated soil within the cone but above the water table is the deficit. Around open pit mines, the drawdown cones extend for many miles and are as deep as the mine, up to 1200 feet in some instances.)

By 1994, the total deficit expected on the Humboldt River basin due to dewatering was approximately 1,000,000 acre-feet. An acre-foot is one foot of water covering an acre and is approximately the amount of water used by a family of four. By 1997, the expected deficit in the Humboldt River basin reached 4,000,000 af. Two additional expansions are the Betze-Post expansion and the Leeville Project. The Betze/Post pit lake will be 580,000 af (HCI, 1997) which represents a 380,000 af expansion. The Leeville expansion is an underground mine which will pump 55,000 af/y with a 90% discharge rate for 18 years. The current deficit estimate for the Humboldt River basin is close to 5,400,000 af. The top 100 feet of surface aquifer in the basin holds approximately 8,000,000 af. The potential for impacts caused by replenishing the deficits is high and depends on the source of the replenishment.

There are three major unanswered questions about this deficit. First, what is the source of recharge to pits and drawdown cones? All of the pits intersect alluvial, unconfined aquifers with connections to the river. If 5,000,000 af is drawn from the 8,000,000 af stored near the river, the effects would be pronounced. However, the pits also intersect many bedrock layers. Some dewatering discharge reports suggest that, due to high temperatures and geochemistry, much of the dewatering occurs in bedrock aquifers. The aquifers are poorly mapped; their recharge zones and fractures are uncertain. Some may be dewatered and unable to provide the recharge.

Second, what is the rate of refill? Longer refill times suggest less impact. In a simple computer model, Myers varied the hydraulic conductivity by two orders of magnitude which caused the time to refill to vary from 11 to over 300 years. Estimates for refill at the Pipeline Deposit Mine vary from 10 to 50 years. For the Gold Quarry Mine, initial estimates in environmental documents were that by 2001, pumpage would equal 42,000 gallons/minute (gpm). As of 1993, two years after mining commenced, pumpage exceeded this amount. Industry consultants also predicted the 150,000 af Gold Quarry pit will be 95 percent full within 18 years. This average annual refill amount exceeds 40 percent of the annual flow from the Maggie Creek basin. Because most of the dewatering is discharged to surface water and lost to the local basin, it seems unlikely that there will be enough water available to fill the pit lake.

Third, what is the connection of the pits to the Humboldt River and its surface tributaries? Drawdowns in the bedrock under Maggie Creek exceed hundreds of feet. There is a clay layer between the surface alluvium and the bedrock, therefore the question is what is the connection? Industry experts predict that Maggie Creek and tributaries will loose up to all of its baseflow when mining ceases and the pit lake begins to form. At the Lone Tree Mine, which causes a 1000 foot drawdown less than 2 miles from the Humboldt River, in the vicinity where water levels are less than 10 feet from the surface, the total predicted decrease in river flow is only 0.45 cfs during pit refill. However, modeling by Great Basin Mine Watch suggests the river flow will be decreased by substantial amounts.

Once pits refill with water, they will evaporate in perpetuity. Evaporation rates will be about 2.25 ft/y. For the Humboldt River basin, based on 1998 figures, total evaporation will exceed 7000 af/y when all pits are full. Combined surface and groundwater flow at Winnemucca is 185,000 af/y, therefore evaporation could be 3.7 % of the total flow in basin. It will be a much larger fraction of flow from some sub-basins.

The ultimate question to be answered regarding dewatering in the Humboldt basin is how much of the deficit, water pumped from the aquifer and wasted will be made up from surface water flows. If all of it is, it will require almost 30 years of the total flow in the Humboldt River to replenish the deficit. The environmental and economic future of the entire northern portion of Nevada depends on the answer to the question.

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