Groundwater chemistry (VTGN10)

GW chemistry lectures 1-4

How can you estimate the TDS by measureing electricial currence?

TDS (ppm) = 0.725* EC(uS/cm)

Which are the major cations and anions in groundwater?

Cations: sodium, calcium, magnesium. Anions: bicarbonate, sulphate, chloride

Describe silicate hydrolysis

Is most important process when it comes to weathering of silicate minerals. For example, weathering of olivine (Mg2SiO4) leads to magnesium ions in solution and silic acid

Describe clay weathering

Clay weatering is a silicate weathering process. For example K-feltspars weathers into clay and forming bicarbonate and K+ ions. Also plagioclase weathers into clay and forms bicarbonate and sodium ions.

How do transfer the temperature and pressure of crystallisation of a mineral to their weathering potential?

Minerals formed at lower P and T such as quartz are less suspectible for chemical weathering in contrast to minerals formed at higher P and T

Describe carbonate weathering

Calcite, CaCO3+ + carbonic acid —> Ca2+ and bicarbonate

How will water in aquifer composited in magmatic/metamorphic rock vs sedimentary rock differ in TDS?

Magmatic/metemorphic rocks consisting of mainly silicate minerals will result in lower TDS, with ions such as Na, Ca, Mg, HCO3. Sedimentary rock will aside from silicate mineral usually consist of calcite, halite, more easiliy dissolveble, leading to higher TDS with addition of SO4- and Cl- as well.

How do you conduct a collin diagram?

Is a bar diagram of concentrations of ions in meq for cations and anions separately

How do you see the TDS in a circular diagram?

Size of the diagram - radius should show it

What is a stiff diagram

visualization of ion compostion in water by plotting concentrations on different sides of a 0-axis

What is a piper plot?

Consists of two triangles, one for anions and one for cations, and one diamond. First one plot the concentrations on the triangles which is used to find a position on the diamond leading to a classification of the water

What is a hold back process? How can we see that it is happening in the water?

Hold back process is a process that alters the ion composition without increasing TDS. I.e it is not a weathering process. There are two main hold back processes, ion exchange and sulphate reduction.

What is sulphate reduction?

is a hold back process or a redox reaction where sulphate SO4 is used by bacteria in anaerobic conditions and forms sulphuric hydrogen gas and biocarbonate. Thus sulphate rich waters can become rich in bicarbonates. This happens in bogs, fens, old and deep aquifers. In mEq there will be an equal removal of sulphate ions as increase in bicarbonate.

Give some possible sources of the acidity needed to dissolve minerals

Main acid is carbonic acid formed by carbon dioxide dissolved in water from air or from cellular respiration. Other sources are acid rain (Combustion of N and S leading to nitric acid HNO3- and sulphuric acid. Other less common sources could be oxidation of Ammonium and iron sulphides (FeS2 from pyrite).

Why is soil horizon considered an acid pump?

Roots in upper soil layer consumes oxygen and produces carbon dioxide in respiration leading to acidification (carbonic acid).

How do we calculate the temporary and permanent hardness

Permanent hardness = sum of mg+ca -hco3. Temporary hardness = HCO3- conc.

What is the relation between Eh and oxidation level? and unconfined/confined aquifer.

Positive redox potential indicates more oxidating environment. Negative redox potential indicates more reducing environment. If we have if we have reduced conditions, it is a good indicator of a confined aquifer. Unconfined aquifers have direct contact with air and is usually oxidized conditions.

Describe the forms Fe, Mn, N and S are in reduced vs oxidated states

Reduced vs oxidated state
Fe2+ (aq) <—> Fe3+ (solid)
Mn2+ (aq) <—> Mn3+ (solid)
NH3/NH4+ <—> NO3-

H2S (g) <—> SO4(2-)

What is a redox buffer?

Redox buffers keeps the redox potential Eh stable, even though oxidation level changes. However, when the redox buffer is overrun there is large changes occurs in redox potential

How can fens and bogs nearby a groundwater well affect the water?

There is a risk of iron precipitation (reduced iron precipitates when oxidized), low pH, mobile metals, plant remains, sulphuric acid in water H2S.

Which three processes generate carbon dioxide and results in hydrogen ion production?

athmosperic co2 dissolving in water, respiration by animals and plants produce co2 gas, dead plants are oxidized generating co2 gas.

give example of three redox processes adding to hydrogen production (aside from carbon dioxide)

• Mineral pyrite (FeS2) oxidized by oxigen resulting in sulphate and hydrogen ions

• Nitrifrication, ammonium is oxidated by oxygen gas to nitrate and hydrogen ions

• Iron oxidation by nitrate (denitrification) give production of nitrogen gas and hydrogen ions

which are the ions produced in weathering of feldspars?

thca++. mg++, na+,k+. hco3-

Mention four important aspects on redox processes in G.w

• Oxygen gas comes with infiltrating water into soil zone and will give shallow G.w an amount of oxygen. This is then consumed in both redox reactions and in decay of organic matter

• What kind of organic matter that is available impacts the redox processes, as some matter have large resistance to oxidation

• Redox buffers can stabilize Eh even though oxidation level changes

• Circulation time in aquifer is important. It takes very long time to have reduced conditions in aquifer. Thus old, very deep G.w can have reduced conditions.

How can postglacial land upheaval be of groundwater chemical importance (in sweden)

Land upheaval can cause previous reduced environments to be in contact with air and thus become oxidized environments. This is a problem in northern sweden as sulphuric soils containing FeS2 is oxidized to SO4- and causes acificidation.

Give example of where we can find groundwaters with chemically reduced characters

Old and very deep aquifers with long recirculation time can have reduced conditions. Also, shallow groundwaters below fens and bogs can have reduced conditions.

Difference oxidation level recharge/discharge areas?

Higher oxidation level in recharge areas compared to discharge areas

What is ion exchange?

Is a hold back process. It happens because soil particles are negatively charged and thus positively charged ions will be adsporbed to the particles. We can see that it happens if the composition of cations changes but not the TDS. The reaction increases with surface area, thus clay minerals and organic matter are effective media for this process. The process also depend on the ions, the exchangability order Na > K > Mg > Ca. Calcium rich waters can become Na rich waters due to this.

Too high amounts of nitrate can be a health problem. GIve examples of reasons for nitrate amounts in groundwater

• nitrogen load from atmosphere due to burning of fossil fuels

• fertilizer use leakage

• point sources as sewage pipes

• mineralization of organic N

at which pH is bicarbonate the dominating carbonate species?

ph 7-9