SCIENCE · INVESTIGATE AND EXPLAIN

Trace pollution from source to receptor

Distinguish source strength, transport, contact and biological response.

Learning outcome: Draw a source-pathway-receptor hypothesis and specify observations that could challenge it.

Starting recap

The air, water and soil cards are hypothetical; the model has no organism or product chemistry.

All lessons are open directly. Your saved experiments remain when you leave and return.

Try

12 g uniformly distributed in 600 L is 20 mg/L, not a measured internal dose.

Open the experiment

Predict

Does no visible plume establish that no exposure occurs?

Your prediction and reason

Experiment

Draw one pathway, choose measurements for its arrows and inspect what leaves the model boundary under export.

Trace a pollutant pulse and its fate accounts

Original batch pulse · separate observed water-quality record · Original hypothetical parent substance in a well-mixed 1000 L store. Four discrete operations add supplied parent mass, export an aliquot and replace it with equal clean water, then transform a fraction of remaining parent. Step duration is unspecified. Transformation tracks original-parent equivalent, not product mass or harmlessness. No dose, ecological effect or safety inference is calculated.

Supplied parent source: Single 40 g pulse Ordered export and transformation: Export only: 25% exchange

Reset keeps your prediction, captures and practice history.

Where did the parent substance go?

Single 40 g pulse; Export only: 25% exchange. Predict the final parent mass and its destinations before comparing captures. This fictional, well-mixed water store starts with zero parent substance in 1000 L. The four steps have no specified physical duration. Inputs and fractions are supplied teaching assumptions, not measured chemical rates.

Add the supplied parent mass with negligible water volume.
Export 25% of the mixed water and its parent substance, then replace it with the same volume of parent-free water. The store returns to 1000 L.
Transform 0% of the parent remaining after exchange. Track the amount converted as original-parent equivalent.
Record remaining parent and both cumulative fate accounts before the next addition.

Exported parent leaves this boundary; its downstream fate is not calculated. Transformed-parent equivalent means grams of original parent converted. It is not product mass, destruction of matter, mineralization or proof of harmlessness. Co-reactants and products are not explicitly modeled. This hypothetical transformable parent must not be relabeled an element such as lead that ordinary chemistry cannot destroy.

Final parent-equivalent balance: 40 g supplied = 12.65625 g parent remaining + 27.34375 g parent exported + 0 g original-parent equivalent transformed. Residual: 0 g.

Final parent concentration is 12.65625 mg/L. Convert grams to milligrams, then divide by 1000 L. Mass in grams and concentration in mg/L happen to have the same numerical value only because this store holds 1000 L. They remain different quantities.

Exact arithmetic, not implied measurement precision. Mass columns use g; concentration columns use mg/L.
Step	Source g	Before fate g	Before fate mg/L	Exported parent g	After exchange g	End parent g	End concentration mg/L
1	40	40	40	10	30	30	30
2	0	30	30	7.5	22.5	22.5	22.5
3	0	22.5	22.5	5.625	16.875	16.875	16.875
4	0	16.875	16.875	4.21875	12.65625	12.65625	12.65625

At each step, cumulative source = end parent + cumulative export + cumulative transformed equivalent. Water export is followed by equal replacement.
Step	Cumulative source g	Cumulative export g	Water exported L	Clean replacement L	End water L
1	40	10	250	250	1000
2	40	17.5	250	250	1000
3	40	23.125	250	250	1000
4	40	27.34375	250	250	1000

Across the four operations, 1000 L leaves and 1000 L replaces it; final volume remains 1000 L. This is ordered batch exchange, not continuous flow. The transformation account does not track products' subsequent export or reactions.

Equal parent histories can hide different fates

Compare the single pulse with export only and transformation only. Both end at 30, 22.5, 16.875 and 12.65625 g parent. In the export case, 27.34375 g parent moved outside the store. In the transformation case, 27.34375 g original-parent equivalent was converted. Identical parent concentration histories do not establish identical products, exposures or effects.

Compare the single pulse and repeated additions under the same fate profile. Each supplies 40 g total, but its timing differs. A higher final parent store is an accounting result, not a prediction of organism uptake, food-chain magnification or biological harm.

Unavailable outputs: biological effect, internal dose and product mass are not calculated. No safe/unsafe label follows from these accounts, including the zero-source control. There is no duration from which to calculate exposure-time integrals or half-life.

Check the sequential operations and a dilution transfer

For a 40 g pulse with both operations, export removes 10 g, leaving 30 g. Transformation converts 7.5 g of that remainder, leaving 22.5 g. The two operations account for 43.75% of the original parent, not 50%; the second percentage uses a different starting amount.

Separate paper case: 12 g in 600 L is 20 mg/L. Adding 600 L clean water without exporting any parent gives 10 mg/L in 1200 L, while parent mass remains 12 g. That is dilution, not transformation or destruction. This paper case changes volume and is not a selectable recipe in the fixed-volume lab.

Final parent 12.65625 g; concentration 12.65625 mg/L in 1000 L.

40 g supplied parent = 12.65625 g remaining + 27.34375 g exported + 0 g transformed-parent equivalent. Products and biological effects are not calculated.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Follow a substance beyond its release point

A pipe, an air emission and a contaminated soil patch can all start environmental pathways. Draw source → transport or storage → receptor before deciding what a measurement means. A receptor is an organism or other target that could contact the substance. The diagram is a hypothesis about connections, not proof that exposure or harm occurred.

Use the three hypothetical cards below. Choose one measurement for each arrow and say what finding could challenge the proposed connection. No card supplies a measured concentration or an observed biological response.

Hypothetical mechanism cards: no observed release, exposure or effect is asserted. Arrows propose pathways to investigate; they do not measure transport speed or amount.

Water: a hypothetical pipe discharge

Text pathway: Pipe release → Mixing / transport → Aquatic organism.

A point source enters water. Compare chemical-specific samples with flow and timing; downstream presence alone does not uniquely identify this pipe.

Your evidence question: Which upstream comparison or other-source record could challenge the proposed source link?

Soil: a hypothetical stored residue

Text pathway: Soil / sediment store → Runoff / remobilization → Downstream habitat.

Distributed stored material can move in dissolved or particle-associated form. New emissions may stop while an older store remains.

Your evidence question: What paired soil, suspended-material and water measurements distinguish storage from current export?

Air: a hypothetical deposition pathway

Text pathway: Air emission → Transport / deposition → Soil or water receptor.

Airborne material can deposit into another medium. Emitted mass, ambient concentration and deposited amount are different quantities.

Your evidence question: Which wind, deposition and chemical-identity observations would test this pathway?

Two mechanisms requiring different evidence

Nutrient hypothesis: added nutrients may support greater production; respiration and decomposition of organic material can raise oxygen demand and contribute to oxygen depletion. Measure relevant nutrients, production, oxygen and competing influences. No oxygen threshold or effect magnitude is supplied.

Toxicant hypothesis: chemical form and availability, receptor contact and biological response matter. Request chemical-specific evidence and appropriate comparisons; environmental concentration alone is not internal dose. Transformation of a parent does not establish harmless products.

Distinguish a source from its distribution

A discharge pipe is a localized point source. Runoff can combine contributions from many places across a landscape, making its source pattern diffuse. An identifiable pipe does not imply that every downstream concentration comes from that pipe; background inputs and other pathways may also matter.

For the pipe card, compare appropriately timed upstream and downstream samples while recording flow and other inputs. For the soil card, inspect stored material as well as exported water or sediment. For the air card, investigate emissions, atmospheric transport and deposition separately. A substance released to air can later enter water or soil; an environmental medium is not a sealed compartment.

The EPA metals causal-assessment resource illustrates the importance of pathways, stored material and chemical availability. It does not attach measured metal concentrations to these original cards. A sediment store can remain after new emissions stop, and later disturbance can remobilize material. “The source stopped” and “all receiving environments are clear” are different claims.

Identify the quantity at each step

Source strength may be mass released per interval. Transport describes movement and redistribution. Environmental concentration states an amount of substance per specified amount of medium: for example, mg/L in water or mg/kg of soil. For soil or tissue, specify the wet- or dry-mass basis where relevant. Exposure involves contact through a route over an interval; internal dose concerns what enters an organism. A biological effect needs evidence about a response. These quantities are related but are not interchangeable.

As a paper example, a fictional release of 12 g distributed through 600 L gives 12,000 mg / 600 L = 20 mg/L. This calculation describes a stipulated well-mixed concentration. It does not determine what an organism absorbs, how long contact lasts, or whether an effect occurs. Return to solution concentration for the mass/volume refresher.

The pulse lab uses a hypothetical transformable parent substance. It must not be renamed lead or another element that ordinary chemical transformation cannot destroy. Changing an element’s chemical form can change availability while preserving its elemental mass. The model does not track that speciation problem.

Compare pathways without inventing transport physics

Inspect the lab’s export-only profile. Export moves parent substance beyond the water-store boundary; it does not calculate the receiving pathway. On paper, add a second box downstream and mark its fate unknown. A lower local concentration can coincide with material being transferred elsewhere.

The groundwater travel lesson supplies an ideal advective comparison, not dispersion, sorption or an arrival-concentration prediction. Identify which missing processes would matter for your chosen card instead of borrowing its travel time as a complete contamination forecast.

Transfer: challenge a visual claim

A fictional survey sees no visible plume after a discharge stops. Write two reasons this observation alone does not establish no exposure, and choose an additional measurement. Distinguish an unseen substance from an unmeasured one.

Check the pollution-pathways transfer

A substance can be dissolved or otherwise not visually apparent; previously stored material may also persist or remobilize. Chemical-specific sampling in relevant media, timed to the proposed pathway, could test the hypothesis. Exposure still needs route and duration information. No visible plume is not proof of absence, and these hypothetical possibilities are not a diagnosis of an actual site.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Explain stored-material and cross-media pathways without inventing concentrations or effects.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Test demand and explain resource limits Next: Close a pollutant pulse account

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