CASE 005
:
THE CASE OF THE POISONED RIVER
Something is killing the fish in Millbrook River. Three factories operate upstream. Each one has passed its environmental inspection. Each one denies any wrongdoing. But the river does not lie — and neither does chemistry. Can you trace the poison to its source before more damage is done?
CASE BRIEFING
The first report came from a fly fisherman on a Tuesday morning.
He had fished the Millbrook River every weekend for eleven years. He knew its bends, its pools, its currents. He knew what it looked like when it was healthy.
What he found that morning was not healthy.
Dead fish — dozens of them — floating along a three-hundred-metre stretch of the river between Caldwell Bridge and the old mill weir. Brown trout. Perch. Three pike. All dead within what appeared to be a short window of time.
He photographed everything and called the Environment Agency.
By afternoon investigators were at the river. By evening they had a problem.
The Millbrook River runs through an industrial zone before reaching the stretch where the fish were found. Three manufacturing facilities operate along its banks upstream — all legally permitted, all regularly inspected, all currently within their licensed discharge limits according to their own monitoring records.
Facility 1 — Caldwell Metals
A metal processing plant operating on the river's north bank. Licensed to discharge treated cooling water and low-concentration process wastewater.
Facility 2 — Greenfield Agricultural Supplies
A fertiliser blending and packaging facility on the south bank. Licensed to discharge treated surface runoff from its storage yards.
Facility 3 — Hartstone Paper Mill
A paper manufacturing facility at the upstream end of the industrial zone. Licensed to discharge treated process water from its pulp and bleaching operations.
When investigators contacted each facility, the responses were identical.
Caldwell Metals: "Our discharge has been continuously monitored. All readings are within permitted levels."
Greenfield Agricultural Supplies: "We had a minor storage tank overflow two days ago but our containment system captured everything. Nothing reached the river."
Hartstone Paper Mill: "Our treatment systems are fully operational. We discharge within our licence at all times."
Three facilities. Three denials. Dozens of dead fish.
And a river that had been running clear for as long as anyone could remember.
The lead investigator, Dr. Sarah Okonkwo, stood at the riverbank that evening and looked upstream.
"The river knows what happened," she said. "We just need to ask it the right questions."
The questions, it turned out, were chemical ones.
Water samples had been collected at seven points along the river — from above the industrial zone to below the fish kill zone. Sediment samples had been taken from the riverbed. The dead fish had been collected for analysis.
And one detail had already caught Dr. Okonkwo's attention.
The fish kill was not uniform along the entire stretch. It was concentrated. The density of dead fish was highest at a specific point — approximately 140 metres downstream from the outflow pipe of one of the three facilities.
She had not yet told the facilities which one.
Your investigation begins now.
Three factories. Three denials. One poisoned river.
The chemistry of water does not care about permits or monitoring records or what facility managers tell investigators. It records what actually happened — and it keeps that record in the water, the sediment, and the bodies of the fish themselves.
Examine all five Evidence Files. The river has been talking since Tuesday morning. Can you understand what it is saying — and prove which facility is responsible?
Instructions:
Play solo or with a team of up to 5 investigators. Examine the five Evidence Files together, or divide them among your team and have each investigator analyze different clues. Share what you discover, compare theories, and decide which evidence matters most.
When you've examined all the evidence, work individually or as a team to reach your final verdict—then reveal the solution to see if you cracked the case.
EVIDENCE FILES
EVIDENCE FILE 01 +
EVIDENCE FILE 01 — FISH KILL INCIDENT REPORT
Case Reference: MRV-2024-1022
Location: Millbrook River — Caldwell Bridge to Old Mill Weir
Prepared By: Dr. Sarah Okonkwo, Environment Agency Investigator
Date: Tuesday — 6:30 PM
BACKGROUND
At 8:14 AM on Tuesday morning, a member of the public reported an unusual fish kill along the Millbrook River. Environment Agency investigators arrived at the scene at 11:30 AM and conducted a full incident assessment.
INCIDENT FINDINGS
Dead fish were observed along an approximately 300-metre stretch of the river between Caldwell Bridge and the Old Mill Weir.
Species identified:
Brown trout — 34 individuals
Perch — 21 individuals
Pike — 3 individuals
Juvenile fish — numerous — not individually counted
Total estimated mortality: 60 to 80 fish
All observed fish appeared to have died within a relatively short timeframe — bodies showed no significant decomposition and were found floating at the surface rather than settled on the riverbed.
SPATIAL PATTERN
The distribution of dead fish along the 300-metre stretch was not uniform.
Fish mortality was heaviest in a concentrated zone approximately 140 metres downstream from the outflow pipe of one of the three upstream facilities.
Fish mortality decreased progressively with distance from this zone — both upstream and downstream.
This pattern is consistent with a point source contamination event — a discharge originating from a single location rather than a diffuse or gradual pollution source.
WATER OBSERVATIONS
Investigators noted the following visual observations at the scene:
Water clarity: reduced — slight turbidity noted in the kill zone
Water colour: faintly discoloured in the immediate vicinity of the kill zone — described as having a slight grey-green tint
Odour: mild chemical smell reported by investigators at the kill zone — not present upstream
River flow: normal
UPSTREAM CONDITIONS
Investigators walked the river upstream to the industrial zone.
Above the industrial zone the river appeared normal — clear water, no dead fish, no unusual odour.
The abnormal conditions began immediately downstream of the industrial zone and were most severe in the kill zone 140 metres below the outflow pipe.
FACILITY POSITIONS — UPSTREAM TO DOWNSTREAM
Hartstone Paper Mill — furthest upstream
Caldwell Metals — middle position
Greenfield Agricultural Supplies — furthest downstream — closest to kill zone
WATER SAMPLES COLLECTED
Investigators collected water samples at seven points along the river:
Point A — above the industrial zone — control sample
Point B — immediately below Hartstone Paper Mill outflow
Point C — immediately below Caldwell Metals outflow
Point D — immediately below Greenfield Agricultural Supplies outflow
Point E — kill zone — 140 metres below Greenfield outflow
Samples were sealed and submitted for urgent laboratory analysis.
Results will be documented in Evidence File 02.
YOUR TASK
The spatial pattern of the fish kill is your first piece of evidence.
The heaviest mortality is concentrated 140 metres downstream from one specific outflow pipe. Fish mortality decreases with distance from that zone. Above the industrial zone the river is healthy.
What does a concentrated fish kill pattern tell you about the likely source of contamination? Research what a point source pollution event is and how investigators use spatial distribution of damage to identify its origin.
Consider the positions of the three facilities — Hartstone furthest upstream, Caldwell Metals in the middle, Greenfield closest to the kill zone. Does the spatial pattern alone identify the responsible facility — or does it simply narrow the possibilities?
And consider this: Greenfield acknowledged a storage tank overflow two days ago but claimed their containment system captured everything.
If containment had failed — even partially — what would you expect to see in the water sample results?
EVIDENCE FILE 02 +
EVIDENCE FILE 02 — WATER SAMPLE ANALYSIS
Case Reference: MRV-2024-1022
Evidence Item: EV-02 — River water chemical analysis
Prepared By: Dr. James Whitfield, Environmental Chemistry Laboratory
Date: Wednesday — 10:00 AM
BACKGROUND
Five water samples collected by Dr. Okonkwo on Tuesday afternoon were submitted for urgent chemical analysis. Results were returned the following morning.
Each sample was tested for pH, dissolved oxygen, nitrogen compounds, heavy metals, and bleaching agents — the primary chemical indicators associated with the three upstream facilities.
All results are compared against Point A — the control sample taken above the industrial zone — representing the river's natural baseline.
POINT A — ABOVE INDUSTRIAL ZONE — CONTROL SAMPLE
pH: 7.4 — normal for a healthy river
Dissolved oxygen: 9.8 mg/L — healthy
Nitrates: 2.1 mg/L — natural background level
Heavy metals: below detection threshold
Bleaching agents: not detected
This is the river before it reaches any of the three facilities. Every downstream reading will be measured against these figures.
POINT B — BELOW HARTSTONE PAPER MILL OUTFLOW
pH: 7.2 — slight decrease
Dissolved oxygen: 8.9 mg/L — minor reduction
Nitrates: 2.3 mg/L — negligible change
Heavy metals: below detection threshold
Bleaching agents: trace detected — 0.003 mg/L
A small bleaching agent trace is consistent with Hartstone's paper bleaching operations. The level detected is within acceptable environmental limits and is diluting as the river flows downstream.
POINT C — BELOW CALDWELL METALS OUTFLOW
pH: 6.8 — notable decrease
Dissolved oxygen: 7.2 mg/L — meaningful reduction
Nitrates: 2.5 mg/L — minimal change
Heavy metals: zinc detected — 0.18 mg/L · copper detected — 0.09 mg/L
Bleaching agents: trace — continuing to dilute
Zinc and copper appear for the first time below Caldwell Metals — consistent with metal processing discharge. Both readings exceed background levels but remain below the acute toxicity threshold for most river fish species.
POINT D — BELOW GREENFIELD AGRICULTURAL SUPPLIES OUTFLOW
pH: 6.2 — sharp decrease
Dissolved oxygen: 4.1 mg/L — significant reduction
Nitrates: 47.3 mg/L — dramatic spike
Heavy metals: zinc 0.11 mg/L · copper 0.06 mg/L — diluting from Point C
Bleaching agents: not detected
The chemistry changes dramatically below Greenfield's outflow pipe.
Nitrates jump from 2.5 mg/L to 47.3 mg/L — an increase of more than twenty times the natural background level.
Dissolved oxygen drops sharply from 7.2 mg/L to 4.1 mg/L.
pH falls from 6.8 to 6.2.
POINT E — KILL ZONE — 140 METRES BELOW GREENFIELD OUTFLOW
pH: 5.9 — dangerously acidic for river life
Dissolved oxygen: 2.8 mg/L — critically low
Nitrates: 41.6 mg/L — still highly elevated
Heavy metals: zinc 0.09 mg/L · copper 0.05 mg/L — continuing to dilute
Bleaching agents: not detected
The kill zone shows the most severely degraded water chemistry in the entire sample sequence.
Dissolved oxygen at 2.8 mg/L is below the 4.0 mg/L threshold considered the minimum for sustaining most river fish species.
A BRIEF NOTE ON DISSOLVED OXYGEN AND NITRATES
Fish breathe oxygen dissolved in water through their gills. When dissolved oxygen falls below critical levels fish suffocate — even though they are surrounded by water.
Nitrates are nutrients — at low levels they are a natural part of river chemistry. At very high levels they trigger a process called eutrophication — an explosive growth of algae and microorganisms that consume dissolved oxygen as they decompose. This oxygen depletion is one of the most common causes of fish kills in rivers near agricultural facilities.
Fertiliser runoff is the most frequent source of nitrate spikes in river systems.
YOUR TASK
The water sample sequence tells a story — point by point along the river.
Hartstone Paper Mill left a trace of bleaching agents — within acceptable limits. Caldwell Metals introduced measurable heavy metals — below acute toxicity thresholds. Greenfield Agricultural Supplies produced a dramatic nitrate spike and a collapse in dissolved oxygen.
Does the water chemistry sequence identify the facility responsible for the fish kill? Which readings are significant, which are within acceptable limits, and which represent a change severe enough to explain the deaths of 60 to 80 fish?
Research what dissolved oxygen levels are lethal to river fish and what nitrate concentrations can trigger eutrophication. Consider whether the chemical evidence alone is conclusive — or whether the facilities' own explanations need to be examined further before reaching a verdict.
EVIDENCE FILE 03 +
EVIDENCE FILE 03 — FACILITY INSPECTION REPORTS
Case Reference: MRV-2024-1022
Evidence Item: EV-03 — On-site inspection findings
Prepared By: Dr. Sarah Okonkwo, Environment Agency Investigator
Date: Wednesday — 4:00 PM
BACKGROUND
Following the water sample results, Dr. Okonkwo conducted unannounced inspections of all three upstream facilities on Wednesday afternoon. Each facility was required to provide access to discharge records, monitoring logs, and site infrastructure.
HARTSTONE PAPER MILL
On-site contact: Plant Manager — Gerald Foss
Discharge records were presented and reviewed. Hartstone operates a continuous discharge monitoring system that logs outflow chemistry every 15 minutes.
Wednesday's records showed bleaching agent concentrations in the discharge consistently below 0.005 mg/L — within the facility's licensed limit of 0.01 mg/L.
The treatment plant was inspected and found to be functioning correctly.
No evidence of unreported discharge events was identified.
Gerald Foss provided three months of historical discharge records. Analysis showed consistent compliance throughout.
Inspection finding: No evidence of licence violation. Discharge consistent with water sample results at Point B.
CALDWELL METALS
On-site contact: Operations Director — Patricia Yuen
Discharge records were presented. Caldwell operates a batch discharge system — treated wastewater is held in settlement tanks and released at scheduled intervals.
Records showed the most recent scheduled discharge occurred on Monday evening — approximately 36 hours before the fish kill was reported.
Metal concentrations in the discharge were logged at:
Zinc: 0.22 mg/L — within licensed limit of 0.30 mg/L
Copper: 0.11 mg/L — within licensed limit of 0.20 mg/L
Patricia Yuen noted that the metal readings in the river water samples — zinc 0.18 mg/L and copper 0.09 mg/L at Point C — were consistent with dilution of their Monday discharge over 36 hours of river flow.
The settlement tanks were inspected. No overflow or emergency discharge events were recorded.
Inspection finding: No evidence of licence violation. Discharge timing and concentrations consistent with water sample results at Point C.
GREENFIELD AGRICULTURAL SUPPLIES
On-site contact: Site Manager — Barry Nolan
The inspection at Greenfield was more complex.
Barry Nolan confirmed the storage tank overflow he had mentioned during the initial contact. He maintained that the facility's bund — a contained wall surrounding the storage tanks designed to capture any spills — had held the overflow and prevented it from reaching the river.
Dr. Okonkwo requested to inspect the bund.
What she found was not consistent with Barry Nolan's account.
BUND INSPECTION FINDINGS:
The primary bund surrounding Tank 3 — a 40,000-litre liquid fertiliser storage tank — showed evidence of a recent overflow event.
Dried fertiliser residue was visible on the outer bund wall and on the ground beyond the bund perimeter.
A drainage channel running from the bund area toward the river bank showed fresh staining consistent with liquid fertiliser.
The drainage channel outlet — a concrete pipe — was positioned directly above the river at its normal level.
Soil samples taken from alongside the drainage channel showed nitrate concentrations of 312 mg/kg — significantly elevated above background soil levels.
When shown these findings Barry Nolan was initially silent.
He then stated: "The overflow was larger than I initially reported. I believed the bund contained the majority of it. I did not know the drainage channel had been compromised."
He could not confirm how much fertiliser had reached the river.
He could not confirm when the drainage channel had last been inspected.
LICENSED DISCHARGE LIMITS — REFERENCE
Each facility operates under specific licensed limits for its discharge.
Hartstone Paper Mill is licensed to discharge bleaching agents at a maximum of 0.01 mg/L. Wednesday's monitoring showed consistent readings below 0.005 mg/L — well within limit.
Caldwell Metals is licensed to discharge zinc at a maximum of 0.30 mg/L and copper at a maximum of 0.20 mg/L. Monday's discharge recorded zinc at 0.22 mg/L and copper at 0.11 mg/L — both within limit.
Greenfield Agricultural Supplies is licensed to discharge nitrates at a maximum of 10.0 mg/L.
The river water at Point D — immediately below Greenfield's outflow — showed nitrates at 47.3 mg/L.
That is 4.7 times Greenfield's licensed limit.
Greenfield's licensed nitrate discharge limit is 10.0 mg/L.
The river water at Point D — immediately below Greenfield's outflow — showed nitrates at 47.3 mg/L.
That is 4.7 times Greenfield's licensed limit.
YOUR TASK
All three facilities were inspected. Two provided discharge records consistent with the water sample evidence and showed no evidence of licence violations.
One did not.
The bund inspection at Greenfield revealed physical evidence that directly contradicts Barry Nolan's initial account. What does the drainage channel staining, the fertiliser residue beyond the bund perimeter, and the soil nitrate concentration tell you about what happened at Greenfield's storage tank?
The water sample at Point D showed nitrates at 47.3 mg/L — 4.7 times Greenfield's licensed limit. The facility's manager has now partially revised his account.
Is the physical evidence at the site consistent with the water chemistry evidence from Evidence File 02? And does the combination of the two build a sufficient case — or is further evidence needed?
EVIDENCE FILE 04 +
EVIDENCE FILE 04 — SEDIMENT AND FISH ANALYSIS
Case Reference: MRV-2024-1022
Evidence Item: EV-04 — Riverbed sediment and fish tissue analysis
Prepared By: Dr. James Whitfield, Environmental Chemistry Laboratory
Date: Thursday — 9:00 AM
BACKGROUND
Water chemistry captures conditions at a single moment in time. A river's water is constantly moving — chemical readings taken hours or days after a discharge event may show diluted or partially recovered values rather than peak contamination levels.
Sediment and fish tissue tell a different story.
River sediment accumulates and retains chemical signatures over time. Fish tissue absorbs compounds from the water they inhabit. Both provide evidence of what the river was exposed to — not just what it looks like now.
Dr. Okonkwo submitted sediment samples from three locations and tissue samples from six of the recovered fish for laboratory analysis.
SEDIMENT ANALYSIS
Sediment samples were collected from three locations on Tuesday afternoon — the same day as the fish kill.
Sediment Sample S1 — 50 metres above Greenfield outflow
Nitrates: 3.2 mg/kg — consistent with background levels
Heavy metals: zinc 4.1 mg/kg · copper 1.8 mg/kg — low level accumulation consistent with long-term Caldwell Metals discharge within licensed limits
Bleaching agents: not detected
Sediment Sample S2 — Directly below Greenfield outflow
Nitrates: 284 mg/kg — dramatically elevated
Heavy metals: zinc 4.3 mg/kg · copper 1.9 mg/kg — consistent with S1
Bleaching agents: not detected
The nitrate concentration in the sediment directly below Greenfield's outflow is nearly 90 times the level recorded 50 metres upstream.
Sediment Sample S3 — Kill zone
Nitrates: 198 mg/kg — still highly elevated, declining with distance
Heavy metals: zinc 4.0 mg/kg · copper 1.7 mg/kg — consistent with upstream samples
Bleaching agents: not detected
The sediment nitrate gradient — highest directly below Greenfield, decreasing downstream — is consistent with a point source discharge originating at Greenfield's outflow pipe.
FISH TISSUE ANALYSIS
Six brown trout recovered from the kill zone were submitted for tissue analysis.
Tissue analysis examines what compounds have accumulated inside an organism's body — evidence of prolonged or acute chemical exposure that water samples alone cannot provide.
Key findings:
Nitrate accumulation in gill tissue: elevated in all six fish — consistent with acute nitrate exposure
Methemoglobin levels: significantly elevated in all six fish
Heavy metal accumulation: zinc and copper detected at low levels in liver tissue — consistent with background environmental exposure — not at acutely toxic levels
Bleaching agent residue: not detected in any tissue sample
METHEMOGLOBIN — WHY IT MATTERS
Methemoglobin is a form of hemoglobin — the protein that carries oxygen in the blood — that has been chemically altered and can no longer carry oxygen effectively.
High nitrate exposure causes a condition in which nitrates convert normal hemoglobin to methemoglobin. When methemoglobin levels become too high the blood loses its ability to transport oxygen — even when dissolved oxygen is present in the water.
In fish this condition — sometimes called brown blood disease — causes suffocation from the inside. The fish cannot use the oxygen available to them.
Combined with the external oxygen depletion caused by eutrophication — dissolved oxygen falling to 2.8 mg/L in the kill zone — the fish in Millbrook River faced a simultaneous attack on their ability to obtain oxygen and their ability to use it.
The elevated methemoglobin in all six tested fish is direct biological evidence of acute nitrate poisoning.
In the body nitrates are partially converted to nitrites — a related compound — which then react with hemoglobin to form methemoglobin. This conversion is the precise biological mechanism linking the nitrate discharge to the fish deaths.
YOUR TASK
The sediment analysis confirms what the water samples suggested — a dramatic nitrate spike originating at a specific point in the river and decreasing with distance downstream.
The fish tissue analysis goes further — it shows the biological mechanism by which the fish died. Not just low oxygen in the water — but a chemical process inside their own bodies that rendered the oxygen they did have unusable.
How does the sediment evidence strengthen the case established by the water samples in Evidence File 02? And what does the methemoglobin finding add that water chemistry alone could not prove?
Research eutrophication, methemoglobin formation in fish, and how sediment chemistry is used in environmental investigations. Consider what the combination of water samples, sediment analysis, and fish tissue analysis tells you about both the source and the mechanism of this fish kill.
And consider this — Greenfield's manager said the overflow was larger than he initially reported and that he did not know the drainage channel had been compromised.
At what point does a claim of not knowing become a failure of duty of care?
EVIDENCE FILE 05 +
EVIDENCE FILE 05 — DISCHARGE RECORDS AND TIMELINE RECONSTRUCTION
Case Reference: MRV-2024-1022
Evidence Item: EV-05 — Greenfield Agricultural Supplies discharge timeline and historical records
Prepared By: Dr. Sarah Okonkwo, Environment Agency Investigator
Date: Thursday — 3:30 PM
BACKGROUND
Following the facility inspections and laboratory results, investigators focused their attention on Greenfield Agricultural Supplies. Environment Agency officers requested full access to Greenfield's discharge monitoring records, maintenance logs, and tank inspection history going back twelve months.
Barry Nolan provided the records under legal obligation.
What they revealed extended the investigation beyond a single overflow event.
THE TANK OVERFLOW — RECONSTRUCTED TIMELINE
Investigators reconstructed the sequence of events using Greenfield's own internal records, staff statements, and the automated monitoring data from the facility's outflow pipe sensor.
Sunday 6:15 PM
Tank 3 — a 40,000-litre liquid fertiliser storage tank containing high-concentration ammonium nitrate solution — reached capacity during a filling operation. An automated valve failed to close, causing the tank to overflow onto the bund floor.
Sunday 6:15 PM to 7:40 PM
Approximately 3,200 litres of liquid fertiliser overflowed onto the bund floor over a period of 85 minutes before a staff member noticed and manually closed the inlet valve.
Sunday 7:40 PM
The overflow was reported internally to the site manager — Barry Nolan.
Nolan's recorded response in the internal incident log: "Bund contained. No further action required. Will monitor."
No report was made to the Environment Agency.
No inspection of the drainage channel was conducted.
Sunday night to Monday morning
Rainfall of 14mm was recorded at the facility overnight — consistent with regional weather records for that period.
The rainfall entered the bund area — mixing with the fertiliser residue on the bund floor — and flowed toward the drainage channel.
THE DRAINAGE CHANNEL — CRITICAL FINDING
Greenfield's drainage channel — designed to carry clean surface water away from the facility — runs from the bund area to the river bank.
Maintenance records showed the drainage channel had last been formally inspected 14 months earlier.
Its licensed purpose is to carry uncontaminated surface runoff only.
A blocked sluice gate at the drainage channel's internal diversion point — designed to redirect contaminated water to a holding tank rather than the river — was found in a failed closed position.
The sluice gate failure meant that when rainfall washed fertiliser residue from the bund floor into the drainage channel — the contaminated water flowed directly to the river rather than being captured by the holding tank.
Investigators estimated that between 800 and 1,400 litres of high-concentration ammonium nitrate solution reached the river through the drainage channel between Sunday night and Monday morning.
OUTFLOW SENSOR DATA
Greenfield's outflow pipe sensor — required by their licence to monitor discharge chemistry continuously — recorded the following:
Between Sunday 11:30 PM and Monday 4:15 AM the sensor recorded nitrate concentrations in the outflow rising from a baseline of 2.8 mg/L to a peak of 312 mg/L before returning to baseline levels by Monday morning.
Greenfield's licensed nitrate discharge limit: 10.0 mg/L
Peak recorded discharge: 312 mg/L — 31 times the licensed limit.
The sensor data had been automatically logged. It had not been reviewed by Greenfield staff until investigators requested it.
Barry Nolan had not been aware of the sensor peak when he told investigators the bund had contained everything.
Or he had not checked.
WHAT GREENFIELD KNEW AND WHEN
Investigators summarised the timeline of Greenfield's knowledge:
Sunday 6:15 PM — Tank overflow begins. Staff present.
Sunday 7:40 PM — Nolan informed. Records show he classified it as contained. No Environment Agency notification. No channel inspection.
Sunday night — Rainfall washes residue through failed sluice gate into river. Sensor records nitrate spike to 312 mg/L. No staff review of sensor data.
Monday morning — Nitrate levels in river begin driving eutrophication process.
Tuesday 8:14 AM — Fish kill reported by member of public.
Tuesday afternoon — Environment Agency investigators arrive. Greenfield tells investigators: "Nothing reached the river."
Wednesday afternoon — Site inspection reveals drainage channel evidence. Nolan revises account: "The overflow was larger than I initially reported."
Thursday — Sensor data retrieved by investigators shows peak discharge of 312 mg/L — 31 times the licensed limit — on Sunday night.
YOUR TASK
You now have all five Evidence Files.
The outflow sensor recorded a nitrate discharge of 312 mg/L — 31 times Greenfield's licensed limit — on the night before the fish kill. The drainage channel carried contaminated fertiliser runoff directly to the river through a failed sluice gate that had not been inspected for 14 months. The water samples, sediment analysis, and fish tissue results all point to a single source.
And Greenfield's initial response to investigators was: "Nothing reached the river."
Using evidence from all five files — who is responsible for the death of 60 to 80 fish in Millbrook River? Build your case using the chemistry, the timeline, the physical inspection findings, and the sensor data. Which evidence is most conclusive — and what does the gap between what Greenfield reported and what the sensor recorded tell you about the difference between what a facility says and what the science shows?
The river remembered. The sensor recorded it. The sediment held it.
Follow the evidence.
HAVE YOU SOLVED THE CASE?
Review the evidence and decide what you think happened. If you're investigating with others, discuss your findings and agree on your final verdict before revealing the answer.
Once your team has reached a conclusion, reveal the answer.