Mine water Flashcards
What is mine water impacted by?
- 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
What are some possible contaminants/problems with mine water?
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
How is mine water classified?
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
Which processes affect the metal concentration in mine water?
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
Which metals point to pyrite, sphalerite, galena and chalcopyrite being relevant minerals?
Fe, Zn, Cu, Pb
What is congruent vs incongruent reactions?
- Congruent reaction
Reversible
E.g. gypsum dissolution - Incongruent
Irreversible – a completely new mineral formed
Which processes are catalyzed by microbial activity?
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
Describe the consequences of buffering of mine water
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)
What are some field indicators of AMD contaminated water?
o Colored precipitates along water
o Affected or absent fauna or flora
o Biofilms of algae
Describe the evolution of mine water
- Reactive primary minerals present
Dissolution of reactive primary minerals -> precipitation of secondary minerals (works as a sink) -> early-term drainage chemistry - Reactive primary minerals exhausted
Dissolution of secondary minerals (works as a source) and slow dissolution of less reactive primary minerals -> middle term drainage chemistry - Reactive primary and secondary minerals exhausted
Slow dissolution of less reactive primary and secondary minerals -> long term drainage chemistry
Which factors affect the water quality?
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
What is the equilibrium constant and the ion activity product?
- Equilibrium constant K
Temperature dependent
Solubility product Ksp - Ion activity product IAP
The actual concentration of ions in the solution multiplied with eachother
What is the saturation index?
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
When does precipitation occur?
o When the solubility product of the species is exceeded.
o Most secondary minerals in mines are iron minerals
o Causes hard-pan formation
What are some effects of iron hydroxide formation?
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
What is the precipitation of elements affected by? In other words, what changes the saturation index?
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
What is the first flush in mining?
o After mining -> secondary minerals covered by water again -> dissolved -> enrichment of metals -> decreasing concentrations shortly after
Where does radionuclides come from and what are they?
o From uranium mines
o Thorium, radon, radium, uranium
o Usually low concentrations
o Issue of radon in mine air
Which are the two different types of pit lakes?
o Terminal
Where water only leaves through evaporation
o Flow through
Groundwater enters on one sites and exits on another side
Which differencies are there between a pit lake and a natural lake?
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
Which processes affect pit lakes?
Groundwater inflow
Surface water runoff
Rainwater inflow
Evaporation
Precipitation
Adsorption
Algal growth
What are some important hydrological questions regarding pit lakes?
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?
What can isotopes be used for when it comes to pit lakes?
Investigating the telative contribution of surface water and groundwater since they have different isotopic compositions
Investigating the amount of evapoconcentration
What are some remediation or prevention stategies for pit lakes?
Flooding
Backfilling
Active methods (liming)
Passive methods (constructed wetlands)
Increasing primary production
Induced stratification
* Dense bottom water to accelerate processes
