D6 · Publication Volume 22

Waste Rock and Acid or Metalliferous Drainage

sulfides, oxidation, testwork and source–pathway–receptor reasoning

Learning objectives

By the end of this lesson, the learner should be able to frame a defensible decision about sulfides, oxidation, testwork and source–pathway–receptor reasoning; distinguish observation, interpretation, assumption, obligation and decision; construct a causal pathway with explicit spatial and temporal boundaries; use quantitative evidence without false precision; identify distributional and long-duration consequences; specify controls with triggers and accountable responses; and design a versioned evidence package that can be independently reviewed.

The objective is transferable reasoning, not memorisation of a jurisdiction, organisation or operating procedure. A learner must state where current law, rights, permits, engineering authority or specialist review governs a real decision. The tutorial supplies no universal threshold and authorises no field activity, disclosure, facility or closure outcome.

Decision context

The decision is how to classify and manage excavated material when sulfide oxidation, acidity, salinity or metal release may create long-duration drainage. A defensible plan distinguishes source potential, reaction rate, water and oxygen pathways, attenuation, receiving environments, uncertainty and management time. It does not infer behaviour from total sulfur or paste pH alone.

Write a decision contract before analysis. It should name the decision owner as a role rather than a person, affected systems, lifecycle phase, spatial and temporal support, applicable authority to be verified, evidence cut-off, alternatives, uncertainty, dependencies, irreversible choices and review trigger. Separate what the analysis can inform from what it cannot approve. If the boundary excludes a pathway or affected group, record the reason and the evidence needed to reconsider it.

Core concept: system and boundary

Waste Rock and Acid or Metalliferous Drainage: simplified institution-neutral teaching model
Waste Rock and Acid or Metalliferous Drainage: simplified institution-neutral teaching model

Acid or metalliferous drainage develops through coupled mineral reactions, gas and water transport, microbiological influence, heat, particle size and secondary mineral formation. Acid generation and neutralisation can occur at different rates and locations. Near-neutral drainage may still carry environmentally relevant metals or sulfate, while acidic material may remain dormant until exposure conditions change.

Map the system as linked objects rather than a flat issue list. For every object, ask what state can change, which process causes the change, how quickly it propagates, what feedback exists and which observations could distinguish competing explanations. Keep physical, ecological, social, legal and governance relationships connected without pretending that one discipline can decide for another.

Core concept: pathways and obligations

The source–pathway–receptor model separates what can react, how products move and what can be affected. Static tests screen composition and neutralisation potential; kinetic tests explore reaction through time under specified conditions; field cells and operational observations provide scale evidence. None alone guarantees field performance, and sample selection must represent lithology, alteration, weathering, grain size and uncertainty.

Obligations and controls also have a lifecycle. Record their origin, exact wording, intended outcome, affected interest, responsible role, dependencies, start condition, evidence, review point and release condition. Do not convert a conditional commitment into an unconditional claim, or a professional recommendation into an approved requirement. Where rights or consent apply, preserve the conditions and authority attached to them.

Quantitative reasoning

A common screening quantity is net neutralisation potential, NNP=ANC-AP, where acid-neutralising capacity and acid potential use the same basis. The result is conditional on analytical method, mineral reactivity and selected conversion factors. It must not be used as a universal boundary. Load at a receptor remains L=QC, with uncertainty from both flow and chemistry.

Before calculating, declare system boundary, support, units, time zone, reporting period, denominator, treatment of missing and censored data, uncertainty model and rounding. Compare raw, adjusted and modelled values rather than overwriting one with another. Sensitivity analysis should vary plausible drivers jointly where they are dependent. A neat number is not evidence that the underlying model is complete.

Evidence and uncertainty

Evidence includes mineralogy, sulfur and carbon forms, elemental composition, acid-base accounting, leach extraction, kinetic trends, particle-size effects, oxygen ingress, seepage quantity, pore-water chemistry and field analogues with justified comparability. Preserve raw test cycles, maintenance events and censored results. A composite can conceal a small high-risk population that controls seepage quality.

Classify evidence as direct observation, laboratory result, derived value, model output, stakeholder or rights-holder input, requirement, expert judgement or assumption. Attach method, date, location or population support, quality state, access restriction and lineage. Confidence should explain both variability in the system and knowledge uncertainty. Conflicting evidence remains visible until a documented decision resolves or bounds it.

Lifecycle controls

Controls act on source, pathway or receptor: selective handling, minimising exposure, blending only with verified kinetics, covers, encapsulation, drainage routing, water exclusion, oxygen limitation, collection, treatment and monitoring. Every concept must be tested against settlement, erosion, climate variability, construction quality, material availability, changing reaction rates and post-closure maintenance.

Use a control record with unwanted event, causal pathway, prevention or mitigation function, performance requirement, leading and lagging indicators, verification frequency, trigger, immediate response, escalation authority, recovery action and evidence of effectiveness. Controls must survive foreseeable change in climate, schedule, staff, contractors, data availability and lifecycle phase. A monitoring point without a response rule observes risk but does not control it.

Interfaces and data

Link each material unit to origin, volume, mass, mineralogical class, test samples, uncertainty, destination, placement geometry and construction verification. Link water observations to the placed materials and flow pathways that can explain them. If classification criteria change, keep the old state and recalculate affected inventories rather than rewriting history.

The minimum exchange contract specifies identifier, geometry or population support, coordinate and vertical reference where relevant, time basis, unit, vocabulary, null semantics, method, uncertainty, quality status, sensitivity, licence or use condition, version and checksum. Preserve raw evidence and make transformations reproducible. A dashboard, map or report is a view of controlled evidence; it must not become the only surviving record.

Integration checkpoint

The lesson checkpoint passes only when another reviewer can follow the chain from decision and affected interest through sulfides, oxidation, testwork and source–pathway–receptor reasoning, evidence, uncertainty, alternatives, control and residual obligation. Every claimed control must have an observation that can test it, and every material observation must have a pre-agreed response path.

Ask four integration questions: What can change the conclusion? Who experiences the outcome and who has authority? Which lifecycle handover could lose the evidence or obligation? What remains after the proposed control succeeds? If any answer is hidden in narrative, convert it to a controlled record before advancing.

Synthetic worked example

A synthetic waste-rock population contains three provisional classes. Class A has low sulfide and abundant reactive carbonate; Class B has moderate sulfide with variable neutralisation; Class C is only eight percent of mass but shows delayed acidity in kinetic testing. A bulk-weighted average appears benign. The learner demonstrates why separate placement and confirmatory testing of Class C may control the outcome.

The numbers and labels are synthetic and intentionally incomplete. Recalculate them from the stated basis, show any residual, and create at least two plausible explanations before selecting an action. Mark the evidence that would discriminate among explanations. Do not transplant the illustrative quantities, triggers or acceptance language to real work.

Practice task

Create a source–pathway–receptor register and sampling design for a fictional waste-rock inventory. Include material hypotheses, static and kinetic tests, spatial support, classification uncertainty, placement rules, construction evidence, seepage monitoring, triggers and contingency actions. Identify one way a benign average could hide a critical subpopulation.

Submit the artefact with a one-page decision statement, data dictionary, assumptions register, alternative explanation, control table and change log. A peer should be able to locate every input and challenge every conditional step. The task is incomplete if it relies on an unnamed rule, a private conversation, an unexplained score or a figure that cannot be traced to versioned evidence.

Common failure modes

Common failures are treating total sulfur as reactive sulfide, treating all neutralising minerals as equally available, using one composite for a heterogeneous inventory, ending kinetic tests before delayed behaviour appears, transferring laboratory rates directly to field scale, assuming a cover remains intact, and reporting treatment as the first option without evaluating source and pathway controls.

A cross-cutting failure is institutional storytelling: wording that implies a named organisation, person or website owns, endorses or supplies the tutorial or its conclusions. Another is site mimicry, where an invented example looks like a real property and borrowed parameters appear authoritative. Keep examples explicitly synthetic, roles anonymous, external names in source notes only and every real application dependent on current local evidence and authority.

Lesson summary

Drainage risk is a time-dependent source–pathway–receptor problem. Reliable decisions combine representative mineralogical and geochemical tests, scale-aware interpretation, verified placement, water evidence, explicit uncertainty and controls that remain credible after closure.

The durable output is not a final-looking score. It is a reviewable chain connecting purpose, affected systems, evidence, uncertainty, alternatives, decisions, controls, responsibilities and remaining obligations. That chain must be capable of change without erasing the earlier state.

Review questions

  1. What boundary and decision contract are required for sulfides, oxidation, testwork and source–pathway–receptor reasoning?
  2. Which observations would distinguish the two most plausible causal explanations?
  3. Which quantities can be conserved or reconciled, and which judgements cannot be reduced to a score?
  4. How can a missing observation differ from a confirmed absence or zero?
  5. Which control has the earliest useful trigger and what action follows it?
  6. What evidence and obligation must survive the next lifecycle handover?
  7. How would you communicate the residual consequence without implying approval or certainty?

Sources