Sewage and fertiliser in a river (eutrophication) sim

What the sim shows

Sewage and fertiliser in a river (eutrophication). A model river runs across the bench from upstream (left) to downstream (right), with sampling stations at 0, 1, 2, 4, 8 km. A farm field drains into it at 0.5 km, and an untreated sewage outfall enters at the same place. First take before readings: drag the oxygen probe to each station and press Record reading. Then drag the fertiliser sack onto the field (it is spread just before heavy rain, so the runoff washes into the river), or lift the outfall valve to let out one pulse of untreated sewage. Press Next week and follow the chain, in order: nitrates and phosphates enter; algae grow fast (an algal bloom) and cover the surface; light can't reach the plants below, so they die; the algae die too; decomposer bacteria feed on the dead matter and multiply; the bacteria respire aerobically and use up the dissolved oxygen; fish and other animals that need oxygen die or move away. The oxygen falls because of the decomposers' respiration, not because of the algae: algae make oxygen in daylight. Sewage feeds the decomposers straight away, because it is organic matter, so the oxygen falls sooner. Further downstream the input is diluted and used up, and oxygen dissolves back in from the air, so the river recovers: plot your oxygen readings against distance to see the oxygen sag, and compare them with your before readings. Plants and fish take longer to come back than the oxygen: fish return only once the oxygen is back to 6.0 mg/L, and after fertiliser the plants at 1 km are still at 45 % cover (clean is 60 %) in week 12, when the oxygen there is back to 8.5 mg/L. Simplified on purpose: the numbers are illustrative (≈): real values differ from river to river and week to week, and real rivers vary far more; one pulse of input: the fertiliser is washed in once and the outfall lets out one pulse, then the river is left to recover; constant temperature (20 °C) and constant flow, so the saturation level of oxygen stays at 9.1 mg/L; time moves in whole weeks, one click at a time; each week is worked out in hourly steps and stored to the precision shown; the oxygen is a weekly daytime average, never above saturation; in a real bloom it can rise above saturation by day and fall low at night; only the decomposers' respiration uses oxygen here; in a real river, animals, and plants and algae at night, use some too; the dose reaching each station falls by the same share every 2.2 km below the input (dilution, uptake on the way and settling); each station is a stretch of river with its own algae, plants, mud and bacteria; water flowing from one station to the next is not followed; the bacteria count is one number for all the decomposers, given as times the clean level; one nutrient number (nitrate) stands for nitrates and phosphates; fish are present at 5.0 mg/L or more, stressed below 5.0 and gone below 3.0, and come back from upstream only at 6.0 mg/L or more; real fish differ by kind; the river is shortened: the stations are spaced in proportion to their distance, the bench is not to scale, and the close-up is not to scale.

• Lit: the outfall valve's handle
• Close-up: the river from above, and a slice of the river at the probe
• Green on the water: algae cover; fish drawn: present (3), stressed (2, at the surface) or none

Use it in your lessons

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