Is the Adit the Only Source? Iron and Acidity in Clearwater Creek
A disused copper mine discharges water from an adit into Clearwater Creek, which flows at 0.35 m³/s. Sampling sites: A is 200 m upstream of the adit, B is 500 m downstream. The adit flows at 0.020 m³/s and its water is pH 3.2 with 3200 mg/L Fe2+. A laboratory report gives Site A pH 6.4, Fe2+ 0.4 mg/L, dissolved oxygen 8.6 mg/L; Site B pH 3.9, Fe2+ 940 mg/L, dissolved oxygen 4.1 mg/L. The council asks whether the mine is the sole source of the iron and acidity. A 25.00 mL sample from Site B was titrated with 0.0100 mol/L KMnO4 in acid solution; mean titre 8.40 mL. Field officers note an orange precipitate coating the creek bed for about 300 m below Site B, and no fish.
1. Calculate the Fe2+ concentration at Site B from the titration, then use the flow data to test whether the adit discharge can account for it. What does your answer imply for the monitoring program?
Read the worked answer
Titration
MnO4- + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ + 4H2O
- n(MnO4-) = 0.0100 × 8.40 × 10^-3 = 8.40 × 10^-5 mol
- n(Fe2+) = 5 × 8.40 × 10^-5 = 4.20 × 10^-4 mol
- c(Fe2+) = 4.20 × 10^-4 / 0.02500 = 1.68 × 10^-2 mol/L
- mass = 1.68 × 10^-2 × 55.85 = 0.938 g/L ≈ 940 mg/L
The titration confirms the laboratory value, so the analysis is not the problem.
Dilution test
Expected concentration if the adit were the only input:
(0.020 × 3200 + 0.35 × 0.4) / 0.37 = 64.1 / 0.37 ≈ 170 mg/L
Measured is 940 mg/L, about 5.5 times the prediction. The adit cannot be the only source: unmonitored seepage must be entering between A and B, most plausibly through the spoil heap.
Implication
The design is at fault, not the chemistry: a control point above the adit says nothing about inputs below it. Add a site within the 500 m reach and sample any seep before remediation money is committed, otherwise the wrong source gets treated.
2. Compare the acidity at the two sites and explain why pH continues to fall below Site B even if no further discharge enters the creek.
Read the worked answer
Acidity comparison
- pH 6.4 → [H+] = 3.98 × 10^-7 mol/L
- pH 3.9 → [H+] = 1.26 × 10^-4 mol/L
- ratio ≈ 320, i.e. 2.5 pH units
Predicted mixed acidity from the adit alone (pH 3.2, [H+] = 6.3 × 10^-4):
(0.020 × 6.3 × 10^-4 + 0.35 × 4.0 × 10^-7) / 0.37 = 1.27 × 10^-5 / 0.37 = 3.4 × 10^-5 mol/L → pH 4.5
The measured pH 3.9 is about 3.6 times more acidic than dilution allows, matching the iron result and pointing again to a second, unmapped input.
Why it gets worse downstream
4Fe2+ + O2 + 10H2O → 4Fe(OH)3(s) + 8H+
Dissolved oxygen falls from 8.6 to 4.1 mg/L, showing this oxidation is already running, and it releases two H+ for every Fe2+ oxidised. The orange coating is the Fe(OH)3 product. So Site B understates the eventual impact: the creek becomes more acidic with travel time, not less. Monitoring must extend further downstream and record travel time as well as distance.
3. The mine proposes dosing the adit with calcium hydroxide. Estimate the daily lime and sludge quantities and judge whether this is the right response.
Read the worked answer
Reaction
Ca(OH)2 + H2SO4 → CaSO4 + 2H2O, and raising pH precipitates iron as Fe(OH)3, so dosing removes both acid and metal.
Order-of-magnitude dose
- Fe2+ flux = 1.68 × 10^-2 mol/L × 0.37 m³/s ≈ 6.2 × 10^-3 mol/s
- Each Fe2+ releases 2H+ on oxidation, so OH- needed ≈ 1.24 × 10^-2 mol/s
- Ca(OH)2 needed = 6.2 × 10^-3 mol/s × 74 g/mol = 0.46 g/s ≈ 40 kg/day
- Sludge = 6.2 × 10^-3 mol/s × 107 g/mol ≈ 0.66 g/s ≈ 60 kg/day dry Fe(OH)3
Judgement
40 kg/day of lime and 60 kg/day of sludge are small, so dosing at the adit is a defensible short-term measure for the 300 m reach. But it treats the symptom: precipitated sludge armours the streambed and smothers habitat, and nothing stops the sulphide oxidation generating the acid. Poorly controlled dosing also risks pH overshoot, which is as lethal to fish as acidity.
Better value comes from source control (covering the spoil to exclude O2 and rainwater, diverting surface flow), with lime used only until that is in place. Any dosing must be pH-controlled with monitoring above and below the dose point.