Solution Exercise 3
The water balance for a tailings storage facility is calculated by comparing all the volume inflow for a given time period with all the volume outflows. The difference between these volumes corresponds to the change in the volume of water held in the tailings storage facility over that time period. The water balance can be calculated either for the free water in the tailings facility, or the total volume of water, which includes the volume of water entrained within the tailings solids.
The water balance equation for the free water in the facility shown in the Figure can be written:
Inflows
|
Qp |
volume added from direct precipitation |
|
Qrunoff |
volume added by undiverted surface runoff in catchment |
|
Qts |
volume of water transferred in with the tailings discharge |
|
Qrc |
volume of water returned from seepage collection pond |
Outflows
|
Qpp |
water volume sent to process plant |
|
Qwt |
water volume released to environment, after treatment (in any) |
|
Qent |
water volume entrained with tailings solids |
|
Qep |
water lost to evaporation from pond |
|
Qeb |
water lost to evaporation from beach |
|
Qs |
seepage to groundwater, recovered |
|
Qsnr |
seepage to groundwater not recovered |
Change in Storage
|
ΔS |
Change in volume of water held as free water in the pond |
The water balance equation for free water in the pond would be:
ΔS = (Qp + Qrunoff + Qts + Qrc) – (Qpp +Qwt + Qent + Qep + Qeb + Qs + Qsnr)
Many of these quantities can be measured directly (e.g., volumes of water pumped between facilities) while other quantities can be estimated using climate data (e.g., evaporation). Seepage losses to groundwater are sometimes back-calculated after conducting bathymetric surveys of the tailings pond to calculate the change in pond volume over a given time period, and using the water balance equation to estimate seepage loss.