Water–rock interaction and environmental geochemistry

What reactions control dissolved composition, mobility and exposure risk in a catchment?

Synthetic field investigation. A fictional groundwater transect crosses carbonate, shale and sulfide-bearing strata; students separate aqueous concentration from flux and hazard claims. The setting, sampling values and numerical observations are invented for instruction; they do not describe a monitored site or an operational geochemical assessment. Compare at least two hypotheses before drawing conclusions, and make every material balance dimensionally explicit.

By the end of this unit, students should be able to name the relevant reservoirs, express governing relations with units, calculate at least one interpretable quantity, check a competing explanation, and specify what extra observation would increase confidence. The worked calculations are tightly simplified and do not constitute a validated geochemical speciation model, a professional environmental investigation or a public-health threshold determination.

Mechanism and why it matters

Water composition evolves as minerals dissolve, secondary phases precipitate and gases exchange across boundaries. Identify the named chemical phase and its exchange boundaries. Distinguish measured species totals from inferred reactions; one pattern can have several mechanisms.

Sampling and measurement protocol

Build a stoichiometric reaction list and record pH, alkalinity, temperature and rock mineralogy. Record phase, sampling support, date, concentration or ratio units, calibration and detection limit. Define which quantities are observed and which depend on an assumed model.

Competing explanations and identifiability

A water chemistry trend can reflect mixing, evaporation and biological activity as well as mineral reaction. Specify an independently testable observation capable of distinguishing this explanation from an alternative source, reaction or laboratory bias.

Worked numerical example — Calcium yield

Problem. If dissolution of 2 mmol calcite releases one mol Ca per mol calcite, find Ca mmol.

Mathematical setup: 2*1. After checking the listed mass, time and amount units, the calculated answer is 2 mmol. Write down the inputs separately from the chosen constant or reference so an independent reviewer can recompute the result. If the expression uses a logarithm or a ratio, confirm that its argument is dimensionless; if a molar amount is involved, convert grams to moles before applying reaction coefficients. Because the values are invented for this lesson, the arithmetic demonstrates the physical scale and not a measurement-derived regional conclusion.

Sensitivity check. Compare the sign and magnitude with a limiting case and perturb the main input by ten percent. Report whether the relationship is linear or nonlinear, and avoid overstating numerical precision.

Targeted investigation and practical interpretation

Calculate theoretical dissolved load from mineral dissolution. Create a six-row synthetic table with inputs, predicted outputs and one replicate. Compare a rival process and document a falsifiable difference.

Advanced reasoning: Aqueous geochemical setting

Concentration, speciation and contaminant mobility depend on pH, redox context, ligand supply, mineral surfaces and residence time. The simplified arithmetic here omits many coupled effects. An observed ion total is not itself evidence that a particular water is safe or unsafe; such decisions require validated sampling methods, applicable standards and site-specific assessments.

Knowledge check and research questions

1. Recalculate the numerical example using consistent units.
2. Which competing physical process could reproduce the observation?
3. Which new measurement would discriminate the mechanisms?
4. Which assumed parameter creates the greatest model sensitivity?

Instructor benchmark. 2 mmol, conditional on the stated model and units.

Mechanism and why it matters

Alkalinity is acid-neutralizing capacity; it is not identical to bicarbonate concentration under all pH conditions. Identify the named chemical phase and its exchange boundaries. Distinguish measured species totals from inferred reactions; one pattern can have several mechanisms.

Sampling and measurement protocol

Specify the titration endpoint and whether non-carbonate species contribute to alkalinity. Record phase, sampling support, date, concentration or ratio units, calibration and detection limit. Define which quantities are observed and which depend on an assumed model.

Competing explanations and identifiability

Organic acids, phosphate, borate and high-pH carbonate can bias the simplistic bicarbonate-only conversion. Specify an independently testable observation capable of distinguishing this explanation from an alternative source, reaction or laboratory bias.

Worked numerical example — Alkalinity as CaCO3

Problem. 2.4 meq/L alkalinity times 50.04 mg/meq gives mg/L as CaCO3.

Mathematical setup: 2.4*50.04. After checking the listed mass, time and amount units, the calculated answer is 120.096 mg/L as CaCO3. Write down the inputs separately from the chosen constant or reference so an independent reviewer can recompute the result. If the expression uses a logarithm or a ratio, confirm that its argument is dimensionless; if a molar amount is involved, convert grams to moles before applying reaction coefficients. Because the values are invented for this lesson, the arithmetic demonstrates the physical scale and not a measurement-derived regional conclusion.

Sensitivity check. Compare the sign and magnitude with a limiting case and perturb the main input by ten percent. Report whether the relationship is linear or nonlinear, and avoid overstating numerical precision.

Targeted investigation and practical interpretation

Convert meq/L alkalinity to CaCO3 equivalents. Create a six-row synthetic table with inputs, predicted outputs and one replicate. Compare a rival process and document a falsifiable difference.

Advanced reasoning: Aqueous geochemical setting

Concentration, speciation and contaminant mobility depend on pH, redox context, ligand supply, mineral surfaces and residence time. The simplified arithmetic here omits many coupled effects. An observed ion total is not itself evidence that a particular water is safe or unsafe; such decisions require validated sampling methods, applicable standards and site-specific assessments.

Knowledge check and research questions

1. Recalculate the numerical example using consistent units.
2. Which competing physical process could reproduce the observation?
3. Which new measurement would discriminate the mechanisms?
4. Which assumed parameter creates the greatest model sensitivity?

Instructor benchmark. 120.096 mg/L as CaCO3, conditional on the stated model and units.

Mechanism and why it matters

Electrical neutrality demands balanced equivalent charges even when ion reports have different mass units. Identify the named chemical phase and its exchange boundaries. Distinguish measured species totals from inferred reactions; one pattern can have several mechanisms.

Sampling and measurement protocol

Convert each measured species into milliequivalents per liter before checking anion–cation closure. Record phase, sampling support, date, concentration or ratio units, calibration and detection limit. Define which quantities are observed and which depend on an assumed model.

Competing explanations and identifiability

A large charge balance error might indicate missing ions or incorrect unit conversion, not just a broken instrument. Specify an independently testable observation capable of distinguishing this explanation from an alternative source, reaction or laboratory bias.

Worked numerical example — Charge balance error

Problem. Cations=4.4 meq/L, anions=4.0 meq/L; compute 100(C-A)/(C+A).

Mathematical setup: 100*(4.4-4.0)/(4.4+4.0). After checking the listed mass, time and amount units, the calculated answer is 4.7619 percent. Write down the inputs separately from the chosen constant or reference so an independent reviewer can recompute the result. If the expression uses a logarithm or a ratio, confirm that its argument is dimensionless; if a molar amount is involved, convert grams to moles before applying reaction coefficients. Because the values are invented for this lesson, the arithmetic demonstrates the physical scale and not a measurement-derived regional conclusion.

Sensitivity check. Compare the sign and magnitude with a limiting case and perturb the main input by ten percent. Report whether the relationship is linear or nonlinear, and avoid overstating numerical precision.

Targeted investigation and practical interpretation

Calculate percentage charge balance error. Create a six-row synthetic table with inputs, predicted outputs and one replicate. Compare a rival process and document a falsifiable difference.

Advanced reasoning: Aqueous geochemical setting

Concentration, speciation and contaminant mobility depend on pH, redox context, ligand supply, mineral surfaces and residence time. The simplified arithmetic here omits many coupled effects. An observed ion total is not itself evidence that a particular water is safe or unsafe; such decisions require validated sampling methods, applicable standards and site-specific assessments.

Knowledge check and research questions

1. Recalculate the numerical example using consistent units.
2. Which competing physical process could reproduce the observation?
3. Which new measurement would discriminate the mechanisms?
4. Which assumed parameter creates the greatest model sensitivity?

Instructor benchmark. 4.7619 percent, conditional on the stated model and units.

Mechanism and why it matters

Dissolved ions can partition to mineral surfaces; simple distribution coefficients are empirical and condition dependent. Identify the named chemical phase and its exchange boundaries. Distinguish measured species totals from inferred reactions; one pattern can have several mechanisms.

Sampling and measurement protocol

Measure dissolved and solid-associated inventories separately with filtration and extraction controls. Record phase, sampling support, date, concentration or ratio units, calibration and detection limit. Define which quantities are observed and which depend on an assumed model.

Competing explanations and identifiability

pH, competing ions, oxide surface area and organic coatings can change sorption dramatically. Specify an independently testable observation capable of distinguishing this explanation from an alternative source, reaction or laboratory bias.

Worked numerical example — Solid-water partition

Problem. With Kd=12 L/kg and aqueous level=0.08 mg/L, compute solid concentration.

Mathematical setup: 12*0.08. After checking the listed mass, time and amount units, the calculated answer is 0.96 mg/kg. Write down the inputs separately from the chosen constant or reference so an independent reviewer can recompute the result. If the expression uses a logarithm or a ratio, confirm that its argument is dimensionless; if a molar amount is involved, convert grams to moles before applying reaction coefficients. Because the values are invented for this lesson, the arithmetic demonstrates the physical scale and not a measurement-derived regional conclusion.

Sensitivity check. Compare the sign and magnitude with a limiting case and perturb the main input by ten percent. Report whether the relationship is linear or nonlinear, and avoid overstating numerical precision.

Targeted investigation and practical interpretation

Compute adsorbed concentration using a given Kd. Create a six-row synthetic table with inputs, predicted outputs and one replicate. Compare a rival process and document a falsifiable difference.

Advanced reasoning: Aqueous geochemical setting

Concentration, speciation and contaminant mobility depend on pH, redox context, ligand supply, mineral surfaces and residence time. The simplified arithmetic here omits many coupled effects. An observed ion total is not itself evidence that a particular water is safe or unsafe; such decisions require validated sampling methods, applicable standards and site-specific assessments.

Knowledge check and research questions

1. Recalculate the numerical example using consistent units.
2. Which competing physical process could reproduce the observation?
3. Which new measurement would discriminate the mechanisms?
4. Which assumed parameter creates the greatest model sensitivity?

Instructor benchmark. 0.96 mg/kg, conditional on the stated model and units.

Mechanism and why it matters

Changes in oxygen, electron donors and mineral phases redistribute iron, manganese and some trace species. Identify the named chemical phase and its exchange boundaries. Distinguish measured species totals from inferred reactions; one pattern can have several mechanisms.

Sampling and measurement protocol

Measure dissolved oxygen, Fe(II)/total Fe, sulfide and site-specific species along a flow path. Record phase, sampling support, date, concentration or ratio units, calibration and detection limit. Define which quantities are observed and which depend on an assumed model.

Competing explanations and identifiability

An element concentration alone cannot establish oxidation state, toxicity or regulatory exceedance. Specify an independently testable observation capable of distinguishing this explanation from an alternative source, reaction or laboratory bias.

Worked numerical example — Dissolved export

Problem. Concentration 0.20 mg/L carried by 1000 L/s for an hour; mass in grams.

Mathematical setup: 0.2010003600/1000. After checking the listed mass, time and amount units, the calculated answer is 720 g. Write down the inputs separately from the chosen constant or reference so an independent reviewer can recompute the result. If the expression uses a logarithm or a ratio, confirm that its argument is dimensionless; if a molar amount is involved, convert grams to moles before applying reaction coefficients. Because the values are invented for this lesson, the arithmetic demonstrates the physical scale and not a measurement-derived regional conclusion.

Sensitivity check. Compare the sign and magnitude with a limiting case and perturb the main input by ten percent. Report whether the relationship is linear or nonlinear, and avoid overstating numerical precision.

Targeted investigation and practical interpretation

Estimate a dissolved mass flux without making a safety diagnosis. Create a six-row synthetic table with inputs, predicted outputs and one replicate. Compare a rival process and document a falsifiable difference.

Advanced reasoning: Aqueous geochemical setting

Concentration, speciation and contaminant mobility depend on pH, redox context, ligand supply, mineral surfaces and residence time. The simplified arithmetic here omits many coupled effects. An observed ion total is not itself evidence that a particular water is safe or unsafe; such decisions require validated sampling methods, applicable standards and site-specific assessments.

Knowledge check and research questions

1. Recalculate the numerical example using consistent units.
2. Which competing physical process could reproduce the observation?
3. Which new measurement would discriminate the mechanisms?
4. Which assumed parameter creates the greatest model sensitivity?

Instructor benchmark. 720 g, conditional on the stated model and units.

('Groundwater chemistry transect', 'Sample an imagined aquifer across carbonate and reducing shale zones. Tabulate major cations, anions, sulfate, dissolved oxygen and filtered total iron. Convert each major ion to equivalents and compute charge balance residuals. Propose one reaction path and one two-water mixing path with predictions for alkalinity and iron species. Do not use invented results to state potability or remediation requirements.')

Design and controls

Before computing any numbers, list the intended mechanistic question, the sampling unit, plausible confounding influences and the criteria for rejecting a proposed explanation. Separate control treatments from replicates. Specify one independent check that was not used to choose model coefficients, since a model cannot be validated merely by predicting its own calibration inputs. Synthetic scenarios are safe for classroom inference but do not replace field permissions, contamination control or specialist laboratory work.

A complete synthetic data record

Use a header with the following fields: sample_id, phase, dry_mass_g or volume_L, chemical_species, measured_value, analytical_unit, uncertainty, model_assumption, replicate_id and observation_status. Every derived column should include a formula and an audit trail. Avoid blanks that could ambiguously mean zero; label missing or below-quantification values explicitly. The class should archive its original input table and a derived working table so updates to calculations cannot overwrite evidence.

Expected result and alternatives

Analyze the invented measurements with the simplest mass balance or equilibrium relationship appropriate to the question. Choose plausible parameter ranges before looking at the pattern. Next deliberately generate an alternative dataset under a different process that yields a similar headline gradient. Explain which additional variable, sampling depth, time series or reference material would distinguish them. This step addresses the practical reality that geological and chemical processes are often nonunique when seen through sparse observations.

Sensitivity and uncertainty

Build a one-at-a-time sensitivity table varying at least two inputs by 5%, 10% and 20%. Use a separate scenario for systematic calibration shift, because correlated bias does not vanish by averaging independent samples. Include sampling uncertainty when a large reservoir is represented by a few rock chips or bottles. Report the resulting range as a model-conditioned bound rather than an objectively measured confidence interval unless a statistical sampling model was justified.

Interpretation and reporting

The final deliverable is a short field-style memo with a figure, a unit-checked table, a reproducible calculation and two rival explanations. Distinguish measured variables from estimated parameters by typography or column label. Provide the domain where the simple model is defensible, including temperature, composition and phase assumptions. Avoid making health, engineering or management decisions from synthetic data; those require validated data and appropriate regulatory and professional guidance.

Further experimental design

Propose two follow-up observations and rank them by discrimination power, cost and likelihood of analytical bias. A new sample that merely repeats the same confounded measurement may reduce noise without reducing the ambiguity of the hypothesis. A strategically chosen cross-phase, isotope or seasonal measurement can be more informative. Set a stopping rule: what numerical discrepancy or missing diagnostic signature would be sufficient to reject the favored model?

Research notebook prompts

1. At checkpoint 1, identify which assumptions in investigation 5 are directly tested, which still require external constraints, and how one alternative reaction or transport pathway could change the conclusion without necessarily altering the measured primary tracer. Record the expected sign of the change and the observation that would distinguish it.
2. At checkpoint 2, identify which assumptions in investigation 5 are directly tested, which still require external constraints, and how one alternative reaction or transport pathway could change the conclusion without necessarily altering the measured primary tracer. Record the expected sign of the change and the observation that would distinguish it.
3. At checkpoint 3, identify which assumptions in investigation 5 are directly tested, which still require external constraints, and how one alternative reaction or transport pathway could change the conclusion without necessarily altering the measured primary tracer. Record the expected sign of the change and the observation that would distinguish it.
4. At checkpoint 4, identify which assumptions in investigation 5 are directly tested, which still require external constraints, and how one alternative reaction or transport pathway could change the conclusion without necessarily altering the measured primary tracer. Record the expected sign of the change and the observation that would distinguish it.
5. At checkpoint 5, identify which assumptions in investigation 5 are directly tested, which still require external constraints, and how one alternative reaction or transport pathway could change the conclusion without necessarily altering the measured primary tracer. Record the expected sign of the change and the observation that would distinguish it.
6. At checkpoint 6, identify which assumptions in investigation 5 are directly tested, which still require external constraints, and how one alternative reaction or transport pathway could change the conclusion without necessarily altering the measured primary tracer. Record the expected sign of the change and the observation that would distinguish it.
7. At checkpoint 7, identify which assumptions in investigation 5 are directly tested, which still require external constraints, and how one alternative reaction or transport pathway could change the conclusion without necessarily altering the measured primary tracer. Record the expected sign of the change and the observation that would distinguish it.
8. At checkpoint 8, identify which assumptions in investigation 5 are directly tested, which still require external constraints, and how one alternative reaction or transport pathway could change the conclusion without necessarily altering the measured primary tracer. Record the expected sign of the change and the observation that would distinguish it.

The correct output of a geochemical investigation is not just a fitted number. A defensible conclusion links an observation to a specific physical or chemical mechanism while making conditional assumptions explicit. In this unit, write a three-column comparison: observed or invented data, quantitative model result, and possible geological explanation. Any statement that relies on a presumed endmember, reference standard or equilibrium assumption belongs in the last two columns. For a stronger claim, propose observations that would separate mechanism from coincidence.

Independent short assessment

• Choose one of the five numerical cases and reconstruct it using a different sequence of unit conversions. Confirm that the dimensions still cancel.
• Choose the most important environmental, geological or laboratory confounder from the five lessons. Explain whether it creates random scatter, systematic bias or a genuine alternative Earth process.
• Choose two variables of different spatial scales and explain why plotting them on the same grid would not establish causation.
• Write a transparent uncertainty statement: data are fictional, reference assumptions are specified, sensitivity has been tested, and a professional application requires additional measurements.

Suggested group discussion

Compare a precise but unrepresentative measurement with a noisy but representative sampling design. The former may look more convincing in a graph yet produce a more biased inventory or flux. Discuss a case where adding more significant digits does not improve geological accuracy. Discuss whether a model with a near-perfect fit has proved its reaction pathway or merely fitted several free parameters. End by identifying one prediction not used in the original fit, and describe how a prospective measurement could potentially falsify the process model.