The impact of existing large-scale mine dewatering that has occurred through 2015 on Humboldt River streamflows will be evaluated using the middle and lower HRB models. Because future mine-dewatering is not known, this evaluation will only be for the impact of historical and current mine-dewatering activity on Humboldt River streamflow.
- All mine dewatering is within the domain of the middle HRB model.
- Lower HRB model will be used to translate the impacts simulated in the middle HRB model to the Imlay gage.
- Criteria for evaluation is dewatering operations that have pumped more than average of 1,000 acre-feet per year for 5 or more years.
- 9 mine dewatering operations to be evaluated.
- Starting in 2016, mine dewatering will be specified as 0 over a prediction period of 100 years to evaluate the response and recovery of the hydrologic system.
Long-term impact and recovery from the mine-dewatering that has occurred to date (2015) will be evaluated by:
- Creating mine-dewatering scenarios for each mine-dewatering operation.
- Scenarios will only simulate pumping associated with dewatering operation. Simulated mine dewatering activities include
- groundwater withdrawal rates and amounts from dewatering mine locations in the Humboldt River basin;
- the discharge of this water to areas of use for irrigation, injection as aquifer storage, and/or discharge to streams; and
- formation of pit lakes after mine dewatering has discontinued.
- Evaluating stream depletion (and accretion) by differencing streamflows at the Imlay gage between mine dewatering scenarios and reference scenario.
- Producing plots showing streamflow accretion and depletion over time for each dewatering operation evaluated and all operations together.
It is anticipated that plots will show substantial increase in flows during early periods of mine dewatering when much of the water was being discharged to the Humboldt River or tributaries of the Humboldt River followed by a period of depleted flows as mine dewatering is discontinued and groundwater removed from storage is replenished.
The effect of evaporation from pit lakes will be briefly evaluated. This will be done by creating a scenario in which evaporation from pit lakes is not simulated and calculating the difference in streamflow from the model that simulates evaporation from pit lakes.