Does sewage still harm the life in Geneva's rivers?

Find your issue, and check it is measured

An environmental issue: something happening, to something, because of something, with proof that somebody measures it.

Sewage still reaches the rivers that drain into Lake Geneva, from overflowing sewers and treatment works. The insect larvae, snails and worms on the riverbed live in that water all year, and the most sensitive of them disappear first.

Proof it is measured

Invertebrates in Geneva's rivers, opened: the canton nets the riverbed at each station up to four times a year and scores what it finds.

And its chemistry

Nutrients and major ions in Geneva's rivers, opened: the same stations and the same station codes, with ammonium among twenty parameters.

The system

Input: ammonium in sewage, from treatment works and from combined sewers that overflow in storms, and some from farmland. Store: the river and the animals living on its bed. Process: bacteria turn ammonium into nitrate and use up oxygen doing it, and some of it is toxic as ammonia, more so in warm water. Output: flow into the lake. Feedback: negative, through people. A river that scores badly is the case for fixing its sewers, which cuts the input.

Write your research question

One sentence naming the indicator and its units, the years and the places. No strategy in it.

The relationship between ammonium (mg N/L, the 90th percentile of each year's samples) and the invertebrate index (IBGN, a score from 1 to 20) across Geneva's river monitoring stations, 1995 to 2022.

Independent variable

Ammonium, in milligrams of nitrogen per litre. Most of it comes from sewage, so it marks where sewage reaches a river. Human.

Dependent variable

The invertebrate index (Indice Biologique Global Normalisé, IBGN): a score from 1 to 20 set by how many kinds of animal are on the riverbed and how sensitive to pollution the most sensitive of them is. Higher is better. The canton's own target for its rivers is at least 12. Natural.

Not obvious

The index answers to the channel as well as the water. A river in a concrete bed can score badly with clean water, and a winding one with some ammonium can score well. So ammonium may explain a lot of the pattern, or only part of it.

Find a real-world strategy and its tensions

One real-world strategy people are genuinely arguing about, two named positions, and what the tension does.

Geneva's river restoration programme, and its first large project: the Aire.

The strategy

In April 1997 the canton added river restoration (renaturation, the word to search for in Geneva's documents) to its water law. Restoration means taking rivers out of concrete channels and giving them room to move. A fund of about 11.8 million francs a year pays for it. The Aire was the programme's first large project: bill PL 8490, debated in Geneva's parliament, the Grand Conseil, on 30 August 2001.

Position 1 · environmental, political

The canton, in the bill's report: stopping treatment works from discharging into the river, and restoring it, meant "la qualité des eaux va donc en principe sensiblement s'améliorer": water quality should improve substantially.

Position 2 · economic, cultural

Marc-André Chevalley and Pierre Dupraz, speaking for the farmers in the same debate: the project was far too big ("sur-dimensionnée, relevant même de la science-fiction"). And the order was wrong: "améliorer d'abord la qualité de l'eau", improve the water first, then free the river.

The tension

Land for the river against land for farming. Giving the Aire room to move takes fields, some of them the canton's protected best arable land.

What it does

Uneven costs: the whole river benefits, but only a few farmers lose land.

For the environment

Both sides agreed the water had to get cleaner. They disagreed about whether room for the river is how you get there.

Link to the question

The farmers said clean the water first; the canton said free the river too. The invertebrate index answers to both, the water and the channel. How closely it follows ammonium says how much of the riverbed's life is about the water alone.

Select and document your data

A dataset you can defend: counted, documented, and cut to a rule fixed before you looked.

Source 1 · invertebrates

Canton de Genève, LCE_HYB_TM_IBGN on the SITG catalogue (Geneva's official map and data service): a CSV inside a zip, semicolon-separated. Read the metadata PDF in the zip's DOC folder. One row is one station in one year: its mean index (IBGN_MOYENNE), how many samplings that mean came from (NB_VALEUR, 1 to 4), and the number of kinds of animal found (TAXON_SOMME).

Source 2 · ammonium

Canton de Genève, LCE_HYB_TM_MAJEURS, the same service. One row is one station in one year; ammonium is N_NH4.

Inclusion rule

Every station-year that appears in both files: 1995 to 2022, because the invertebrates stop in 2022. Take out any ammonium written as −99, the file's code for missing, or it ranks as the cleanest water in Geneva. None of the matched rows had one on 25 September 2026; check yours.

The join

Match the two files on station code (CODEMESURE) and year (ANNEE_MESURE in one, ANNEE in the other), never on the station name: 22 different stations in the invertebrate file are called Embouchure, because every river has a mouth.

Read the column

Ammonium is each year's 90th percentile, not its mean: nine samples in ten were at or below it, so it reports the bad days. The index is a mean of one to four samplings. Say so when you describe the variables.

Derived quantities

For each station, the average of its matched years: one ammonium value and one index per station. Most stations were sampled in two to four of the years; averaging stops a station counting several times over.

How the numbers were made

Measured directly. Invertebrates are netted from the riverbed and identified in the laboratory, and the score comes from the kinds found. Ammonium comes from water samples analysed in the canton's own programme.

Controls

Pair: both values come from the same station in the same year before anything is averaged. Restrict: only the years both files cover, and only stations with a matched year.

Credit

Canton de Genève, SITG open data, with the extraction dates.

Treat your data

Run a statistical test, leading to figures you built and a statistical result.

Spearman's rank correlation, two-tailed, at p = 0.05.

Why this test

A relationship between two measurements on the same 145 stations. Ammonium is heavily skewed (most stations low, a few very high), so a rank test is safer than Pearson's.

Decided before running it

n = 145 stations. Critical value about 0.16 (most tables stop at 100; use the formula your table gives for larger samples). Tied values take the average of their ranks. Free calculators: Social Science Statistics, Geography Fieldwork.

Which way

More ammonium with a lower index is a negative correlation. Write down which sign you expect, and why, before you run it.