Effect of Open Pit Mine Dewatering and Cessation on a Semi-arid River Flows
Tom Myers, PhD, Hydrologic Consultant, Reno NVĀ  June, 2016lone tree pit annotated


Executive Summary
Open pit mining that extends below the groundwater table captures groundwater and can cause a very large change to groundwater relations. At mines in northern Nevada, groundwater levels near open pit mines have been lowered as much as 1500 feet and drawdown has extended 50 miles from the pit. As pit lakes fill after mining ends, groundwater flows into the pit forming a lake.

The total amount of dewatering that occurred in the Humboldt River basin since the early 1990s is 3,900,000 acre-feet (af), or 1.3 billion gallons. The total volume of the seven largest pit lakes in the Humboldt River basin once full will be about 1,000,000 af, or 326 million gallons. Evaporation from pit lakes will be about 10,000 af/y. The water to replenish these groundwater storage and evaporation deficits must come from existing groundwater storage, recharge, or surface water flow in the Humboldt River. For comparison, the average flow rate on the Humboldt River is 280,000 af/y and the average groundwater recharge is 223,000 af/y
There is little published literature regarding how mine dewatering affects nearby river flows. It can be difficult to find a location with adequate uncomplicated data because of drought and the fact that dewatering discharge into the river can mask how the dewatering draws water from the river. At the Lone Tree Mine along the Humboldt River in northern Nevada, surface water flow gages above and below the mine allow an assessment of the amount of water lost from the river beginning at the end of mine dewatering.
Double mass curve analysis indicates that 176,000 af of water was lost to the river since dewatering ceased to help recover the aquifers from dewatering. This would could have replenished either the bedrock or alluvial aquifer drawdown or gone directly to the pit lake, which currently totals about 50,000 af. Patterns of groundwater level recovery suggest that water is drawn toward the recovering pit lake from a distance in the bedrock, although the flow into the filling pit lake is certainly supplemented by leakage from the alluvium through vertical fractures into the bedrock which thereby draws water from the river. At Lone Tree, water level measurements verify a connection between bedrock and alluvial aquifers.
The average additional loss to the Humboldt River has been about 22,000 af/y, or about 30 cfs, for eight years. This additional loss rate about 8 percent of the total average flow in the Humboldt River. The flow loss is to groundwater discharge to the river, which is baseflow, so most losses are apparent during low flow periods during the late summer or fall.
It is not possible to estimate how much water will ultimately be lost to the river, but as of 2015 there is no sign the loss rate is decreasing. Losses depend on the hydraulic gradient near the pit and the groundwater storage capacity of the dewatered bedrock near the pit.

For the entire report, click here: http://gbrw.org/new/wp-content/uploads/2016/06/Myers-Lone-Tree-Research-2016-.pdf