Audit sulfide acid potential without pretending to predict pH

1. Define the evidence and process

Excavating or fracturing sulfide-bearing rock can change contact with oxygen and water. Pyrite oxidation can release acidity and mobilize other constituents. The geological distribution of sulfides and neutralizing minerals matters, but a total mineral inventory alone does not specify reaction rates, drainage paths or receiving-water effects.

2. Connect mechanism, geometry and boundary

A formal fully oxidized endpoint is FeS₂ + 15/4 O₂ + 7/2 H₂O → Fe(OH)₃ + 2 SO₄²⁻ + 4 H⁺. Check each element and charge: the product sulfate charge −4 balances the +4 from hydrogen ions. This endpoint combines intermediate reactions; it is not a claim that one elementary reaction directly makes all products or that every field sample reaches that endpoint.

For the separate supplied neutralization endpoint, CaCO₃ + 2 H⁺ → Ca²⁺ + CO₂ + H₂O. With a moles of fully reacted pyrite and b moles of available reactive calcite, generated acid equivalents are 4a. Calcite use is the smaller of b and 2a; residual acid is 4a minus twice the calcite used. Excess calcite remains a solid inventory, not negative acidity.

The chosen CO₂ endpoint is explicit: a different dissolved inorganic-carbon endpoint changes acid accounting. Potential equivalents cannot be converted directly into pH without water volume, equilibria, buffers and speciation. Neutralizing acid also does not erase sulfate or prove removal of all metals. No materials are handled, reacted or discharged; this is a virtual stoichiometric comparison, not a treatment design.

3. Reconstruct the worked packet

For 2 mol pyrite and 2 mol calcite, generated acidity is 8 mol H⁺ equivalents; calcite consumes 4, leaving 4. Sulfate production is 4 mol and ferric hydroxide endpoint 2 mol. Neither residual acidity nor orange precipitate alone supplies a quantitative pH.

4. Predict and investigate

Predict first: Does doubling available calcite always double the amount that actually reacts?

Check atom and charge balance before varying inventories. Locate the limiting inventory and retain excess solid separately. Identify missing observations needed to move from potential equivalents to a defensible field-water interpretation.

5. Test changed evidence

Apply the evidence to a changed case

For 1 mol pyrite and 8 mol reactive calcite, what reacts and remains?

Retain the evidence frame, calculations and assumptions before opening public feedback.

Compare a worked response

Generated acid is 4 equivalents; only 2 mol calcite react, leaving 6 mol solid calcite and zero residual acid equivalents in the supplied endpoint. Sulfate remains 2 mol. This does not certify clean water or give a pH.

6. Retain a bounded decision

Preserve the first plan, source identity, original observations, units and competing interpretations. All numerical packets and diagrams in this category are invented teaching cases, not measured site conditions. Retain earlier actual USGS archives separately; do not merge unrelated sites into an invented actual history. Human review, unfamiliar transfer and any chosen supervised practical pathway remain separate. No mining, treatment, slope access or environmental clearance is authorized by these exercises.

Mechanism reference, checked 30 October 2026. Original prose, calculations, questions and diagrams.