Mine water

Question 1

What is mine water impacted by?

Answer 1

• Can be impacted by mine waste outside the process, waste rock, tailings dam, open pit mines and underground mines
• Can be impacted by surface water or groundwater or come into contact with primary and secondary minerals

Question 2

What are some possible contaminants/problems with mine water?

Answer 2

o Metals
o High salinity (from e.g. salt and coal mines)
o Nitrogen (from explosives) -> eutrophication
o Process chemicals
o Hydrocarbons (from machinery)
o PCBs

Question 3

How is mine water classified?

Answer 3

o ARD: pH < 6 and moderate or high metal concentrations
Associated with VMS (volcanogenic massive sulphides), porphyry copper, skarn, coal etc
o NMD: elevated metals in solution at near neutral pH
o SD: high levels of e.g. sulphate (>1000 mg/l) at neutral pH without significant metal concentrations
Associated with Mississippi-Valley type, low-sulphide gold quartz vein, skarn etc

Question 4

Which processes affect the metal concentration in mine water?

Answer 4

o Mineralogy, hydrology, climate, redox, pH, alkalinity
o Increasing evaporation -> decreased pH, increased metal conc
o Increasing base metal sulphide content -> mainly increased metal conc
o Increasing pyrite concentration -> decreased pH, increased metal conc
o Increasing carbonate concentration -> increased pH, decreased metal conc
o Increasing dilution by water -> increased pH, decreased metal conc

Question 5

Which metals point to pyrite, sphalerite, galena and chalcopyrite being relevant minerals?

Answer 5

Fe, Zn, Cu, Pb

Question 6

What is congruent vs incongruent reactions?

Answer 6

• Congruent reaction
  Reversible
  E.g. gypsum dissolution
• Incongruent
  Irreversible – a completely new mineral formed

Question 7

Which processes are catalyzed by microbial activity?

Answer 7

o Sulphur and iron oxidation
Causes AMD
Abiotic processes starts the reaction chain. Then comes the microbial activity.
o Sulphate and iron reduction
Decreases concentrations in waters

Question 8

Describe the consequences of buffering of mine water

Answer 8

o In the same environment -> Iron(III) precipitates before it can oxidize pyrite -> decreased rate of pyrite oxidation
o Down-stream from mine -> neutral mine water enriched in sulphate
o Buffering ranges specific for minerals (sequential pH buffering)

Question 9

What are some field indicators of AMD contaminated water?

Answer 9

o Colored precipitates along water
o Affected or absent fauna or flora
o Biofilms of algae

Question 10

Describe the evolution of mine water

Answer 10

1. Reactive primary minerals present
   Dissolution of reactive primary minerals -> precipitation of secondary minerals (works as a sink) -> early-term drainage chemistry
2. Reactive primary minerals exhausted
   Dissolution of secondary minerals (works as a source) and slow dissolution of less reactive primary minerals -> middle term drainage chemistry
3. Reactive primary and secondary minerals exhausted
   Slow dissolution of less reactive primary and secondary minerals -> long term drainage chemistry

Question 11

Which factors affect the water quality?

Answer 11

o Minerals present
o Trace element concentrations and distribution in minerals
o Mineral resistance (depends on physical/chemical conditions and the properties of the mineral)
o Chemical conditions (redox potential, pH). pH doesn’t change equilibrium constant, but it changes what we have in the water
o Element mobility (element properties, chemical composition, adsorbent present, flow rate and volume, complexing agents)
o Changes in chemical conditions

Question 12

What is the equilibrium constant and the ion activity product?

Answer 12

• Equilibrium constant K
  Temperature dependent
  Solubility product Ksp
• Ion activity product IAP
  The actual concentration of ions in the solution multiplied with eachother

Question 13

What is the saturation index?

Answer 13

o SI = log(IAP/Ksp)
o Negative value -> mineral dissolves
o Positive value -> mineral precipitates
o 0 -> equilibrium
o Use together with geological knowledge about e.g. mobility to learn about the phases in water

Question 14

When does precipitation occur?

Answer 14

o When the solubility product of the species is exceeded.
o Most secondary minerals in mines are iron minerals
o Causes hard-pan formation

Question 15

What are some effects of iron hydroxide formation?

Answer 15

o Staining of streams/wetlands
o Covers microorganisms -> macroorganisms leave -> biologically dead waters
o Two protons released per mole of iron for precipitation -> decrease in pH if acidic anaerobic wetlands are treated

Question 16

What is the precipitation of elements affected by? In other words, what changes the saturation index?

Answer 16

o Evaporation
o Oxidation of reduces species
o pH changes
o Redox potential changes
o Flocculation or coagulation
o Mixing of two waters
o Adsorption

Question 17

What is the first flush in mining?

Answer 17

o After mining -> secondary minerals covered by water again -> dissolved -> enrichment of metals -> decreasing concentrations shortly after

Question 18

Where does radionuclides come from and what are they?

Answer 18

o From uranium mines
o Thorium, radon, radium, uranium
o Usually low concentrations
o Issue of radon in mine air

Question 19

Which are the two different types of pit lakes?

Answer 19

o Terminal
Where water only leaves through evaporation
o Flow through
Groundwater enters on one sites and exits on another side

Question 20

Which differencies are there between a pit lake and a natural lake?

Answer 20

 High relative depth
Depth relative to surface area
 Often low pH
 Often high elemental concentrations
 Tendency to develop meromixmis – no complete mixing of water
Wind not enough to mix the deep waters
Salinity increases with depth -> increased meomixmis
 Low nutrient input
Often clearcut surrounding the lake and no inflowing stream

Question 21

Which processes affect pit lakes?

Answer 21

 Groundwater inflow
 Surface water runoff
 Rainwater inflow
 Evaporation
 Precipitation
 Adsorption
 Algal growth

Question 22

What are some important hydrological questions regarding pit lakes?

Answer 22

 How fast will it fill?
* From 1 yr
* Longer fill rates -> more exposed areas -> higher elemental concentrations
 How large are the different inflows/outflows?
* Residence time of elements
 What is the volume of the lake?

Question 23

What can isotopes be used for when it comes to pit lakes?

Answer 23

 Investigating the telative contribution of surface water and groundwater since they have different isotopic compositions
 Investigating the amount of evapoconcentration

Question 24

What are some remediation or prevention stategies for pit lakes?

Answer 24

 Flooding
 Backfilling
 Active methods (liming)
 Passive methods (constructed wetlands)
 Increasing primary production
 Induced stratification
* Dense bottom water to accelerate processes