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explain: forcing function
a forcing function is a function or variable that influence the system (ex. Temperature or pH)
state variable
describe the state of the system (the model result, ex. Element concentrations)
calibration
is done to optimize parameter values to obtain the best fit to measured data
normative mineral
a mineral whos presence in a rock is theoretically possible, based on a chemical analysis of water or rocks. Used in inverse mass balance modelling
colloid
is a small particle suspended in water with dimensions between 1nm 1 1µm
they are dynamic: continiusly generated by physical fragmentin, erosion and precipitation
can be removed by coagulation, attachment, settling and dissolution
conceptual model
describes state variables and forcing functions and shows (in a simplified way) how these are connected by processes in the modelled system
can be in form of a word model, picture model or a box model
what is accuracy?
corresponds to systematic errors. Explains the absolute deviation from the true value
what is precision?
corresponds to the random errors. Explains the spread (standard deviation) around the measured value
how is carge imbalance calculated? what quality is acceptable?
Charge imbalance = ([conc. Cations] – [conc. Anions]) / ([conc. Cations] + [conc. Anions]) *100
For the error to be conciderd acceptable you want it to be less then 5% which means that this error is quite large and adjustments should be done to improve it.
For dilute solutions such as rain water a higher error is acceptable.
how to improve charge imalance?
You could try to add ions to improve the charge balance (or look at the accuracy of the analysis, if ther is a problem there)
You could also try to adjust the concentration of Cl or HCO3-
explain the difference between the solubility product and the saturation index
The solubility product is a value for when a solubility reaction is at equilibrium.
Saturation index is calculated from the solubility product and the ion activity. The SI value indicates if a mineral is at equilibrium or if it will dissolve or precipitate.
How is SI defined adn calculated?
SI = log (IAP / Ksp)
SI > 1 =precipitate
SI < 1 = disolving
Write a balanced chemical reaction for incongruent weathering of potassium feldspar (KAlSi3O8) to kaolinite (Al2Si2O5(OH)4) in the presence of carbonic acid.
2KAlSi3O8 + 2H2CO3 + 9H2O Al2SiO5(OH)4 + 2K^+ + 2HCO3 ^- + 4H4SiO4
what are the proportions of K+ , HCO3 – and Si that will be released into solution from this reaction?
For 1 mole of feldspar that weathers results in: 1 mole of K+ and 1 mole of HCO3 ^- and 2 mole of SiO2
Why is kaolinite usually chosen as the newly formed secondary mineral in silicate weathering reactions?
It is the most common secondary mineral?? Stable at the conditions at the earth surface, it also has a low solubility (and therefore it usually stays as a solid instead of dissolving and being transported away).
Explain how a geochemical model can be used as a scientific tool.
They can be a useful instrument used to better understand complex systems and can reveal system properties. They can also be used to test scientific hypotheses (since the model can simulate reactions that can be compared with observations). The models can also reveal if there are any weaknesses in our knowledge.
parameter
A parameter is coefficients in the mathematical representation of processes, ex equilibrium constants, diffusion coefficients, water flow rates
local equilibrium
A local equilibrium depends on scale and time. A system can have small parts that are close to equilibrium where thermodynamics can be applied, theses are referred to as areas of local equilibrium. In a solutions a local equilibrium can occur some distant away from the dissolving minerals.
describe two important limitations of the database that is included in most modelling softwares
The database contains information on a limited number of selected species, not all species are in the database.
The quality in the database may also vary and some numbers/information may even be incorrect.
describe briefly at least four important steps in the modelling procedure that should be performed before you start using a modelling software to do the actual modelling calculations
- Define the problem
- Define space and time limits
- Define subsystems
- Select model complexity
- Consider quality and quantity of data
It is often said that “all models are wrong”. Give three reasons why geochemical modelling in many cases is still useful.
They can be a useful instrument used to better understand complex systems and can reveal system properties. They can also be used to test scientific hypotheses (since the model can simulate reactions that can be compared with observations). The models can also reveal if there are any weaknesses in our knowledge
Which of the two following systems is more likely to be at equilibrium: 1) a slowly flowing groundwater, or 2) a fast flowing stream water? Explain your answer.
A slow flowing stream has longer contact time where mere reactions can occur. This causes the system to reach a more stable state where minerals will dissolve and precipitate and eventually reach equilibrium.
In a fast flowing water there is not enought time for all reactions to occur and all the products will be washed away
what is the difference between a model and a computer program
A model is produced by a human being (modeller) aided by computer programmes
The term “computer code” is often used for the commands written in the computer
programme /software
what is verification?
Does the model react as expected?
what is validation
Test the model against an independent set of data
what is a sensitivity analysis
How sensitive is the model to changes in input data?
errors or limits with the database?
- The database contains information on a limited number of selected species
- The data quality in the database may vary (it may even be incorrect)
what could a large charge imbalance indicate?
- that a major ion has not been included in the analysis
- problems with sampling or analytical errors
- the presence of OH complexes of Al and other elements (for example, the charge of Al(OH)2
+ is different from that of
Al3+)
what is molarity?
mole/liter of solution
what is molality
mole/kg of solution
what are the redoxreactions in natural waters
CH2O + O2 –> CO2 + H2O
5CH2O + 4NO3- –> 2N2 + CO2 + 4HCO3- + 3H2O
CH2O + 2MnO2 + 3CO2 + H2O –> 2Mn2+ + 4HCO3-
CH2O + 4Fe(OH)3 + 7CO2 –> 4Fe2+ + 8HCO3- + 3H2O
2CH2O + SO42- –> H2S + 2HCO3-
2CH2O –> CH4 + CO2
what is some problems with Eh values
- Natural waters contain many redox couples NOT at equilibrium; it is not always clear to which couple (if any) the Eh electrode is responding.
- Eh values calculated from redox couples often do not correlate with each other or directly measured Eh values.
- Eh can change during sampling and measurement if caution is not exercised.
what is the water quality in pit lakes?
- Many pit lakes high levels of sulfate, acid and dissolved metals
- Depends on surrounding geology
- Oxidation of some sulfides produce acid
- Alkalinity of groundwater can neutralize acid
- Wall rock composition
- Chemistry vadose zone
- Quality and quantity of run off
- Exposed rock can be major source of acid even if is located below GW table before and after
the mine operation
why can scientific models be useful?
- They can lead to new insights and increase our understanding
They help conceptualize and integrate large amounts of data and information
They can be tested by comparing their consequences or their predictions with independent observations
why can the models be wrong to use?
- They can be misleading
- It is possible to demonstrate any preconceived idea with a particular choice of data, codes, and assumptions
- If the results from model concepts and/or calculations cannot be confirmed or tested with observational data, there is no way to determine
the reliability of these results
which minerals are likely to reach solubility equilibrium?
- Minerals dominated by ionic bonding
- Minerals of simple stoichiometry
- Minerals forming at low temperatures
at what pH does different mineral buffer?
Calcite 6–7
Siderite 5–6
Al hydroxide 4–5
Fe hydroxide 3–4
