The significance of an identified species
Why this section exists
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The standard requires you to describe (Achieved), explain (Merit) or discuss (Excellence) the significance of an identified chemical species for people and/or the environment.
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This is not a decoration on the end of the chemistry. It is a graded criterion at every level, and it is where a lot of otherwise strong reports lose their grade.
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The clarification adds a crucial condition: the significance must relate to your investigation's stated purpose and context. A general fact about lead is not the same as an assessment of the lead in your sample.
What each verb requires
| Grade | Verb | What that means |
|---|---|---|
| Achieved | Describe | Say what the species does, or where it comes from, or why it matters |
| Merit | Explain | Say why — the mechanism, the chemistry behind the effect, the causal chain |
| Excellence | Discuss | Weigh it up — more than one perspective, the size of the effect, what is uncertain, what should be done |
- The jump from Merit to Excellence is the same as everywhere in this standard: not more facts, but more than one angle on the same facts.
Building a significance answer
Work through these four questions and you will have covered what each grade needs:
- Where does the species come from? Natural sources, industrial sources, agricultural sources.
- What does it do? Its effect on human health, on organisms, on the ecosystem, or on the material it is in.
- How does it do it? The chemistry — dissolving, reacting, binding, accumulating.
- What follows? Consequences, thresholds, regulation, what would reduce it, and what is still uncertain.
New Zealand contexts worth knowing
Using a real, local context makes the significance section much easier to write with substance.
| Species | NZ context | Significance |
|---|---|---|
| Zn2+ | runoff from galvanised iron roofs into urban streams | zinc is toxic to aquatic invertebrates and fish at low concentrations; a major contributor to urban stream degradation in Auckland and Wellington |
| NO3− | groundwater in Canterbury, from dairy intensification | nitrate leaching raises drinking-water nitrate; drives eutrophication in lowland waterways |
| Pb2+ | old lead-based paint and lead flashings, roof-collected drinking water | lead is a cumulative neurotoxin, most damaging to children; no safe level identified |
| Cu2+ | copper in brake pads and antifouling paints, marine sediments | highly toxic to aquatic life and shellfish larvae even at low concentration |
| Fe3+ / Fe2+ | iron in bore water, common in rural NZ | not a health risk at typical levels, but stains fixtures and gives an unpleasant taste; iron bacteria clog pipes |
| Ca2+ / Mg2+ | hard water in limestone regions such as Waikato and North Otago | scale in kettles and pipes; reduces soap lathering; not a health concern |
| SO42− | geothermal areas — Rotorua, Taupō | naturally high sulfate and hydrogen sulfide; affects taste and corrodes concrete |
| Cl− | coastal groundwater, saltwater intrusion | rising chloride indicates over-extraction of aquifers; corrodes reinforcing steel |
Writing it at each level
Take zinc in stormwater as the example.
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Achieved (describe): "Zinc is toxic to aquatic organisms. It enters urban streams from galvanised roofs, and high concentrations reduce the number of aquatic invertebrates."
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Merit (explain): "Zinc enters streams because rainwater is slightly acidic, so it slowly dissolves the zinc coating on galvanised roofing as Zn2+ ions, which are carried into the stormwater system. Zn2+ is toxic to aquatic invertebrates because it binds to proteins in the gills and disrupts ion regulation, so sensitive species such as mayfly larvae die and the stream's biodiversity falls."
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Excellence (discuss): the Merit answer, plus a weighing-up — how the concentration found compares with a guideline value, what the uncertainty is, what could be done, and what the trade-offs are.
Worked ExampleDiscussing significance at Excellence level
A student investigating water collected from a stormwater drain below a galvanised iron roof identifies Zn2+ in the sample. Discuss the significance of this species for the environment.
Step 1 — Establish the source, with chemistry
Galvanised roofing is steel coated with metallic zinc. Rainwater in New Zealand is naturally slightly acidic — carbon dioxide dissolves to form carbonic acid, giving a pH around 5.6 — and in urban areas it may be more acidic still. This acid slowly attacks the zinc coating:
The Zn2+ ions produced are soluble, so they are carried straight off the roof in the runoff. This is consistent with the solubility rules, which show almost all common zinc salts to be soluble — meaning zinc, once dissolved, stays dissolved rather than precipitating out on the way to the stream.
Step 2 — Explain the effect, and how it works
Dissolved zinc is toxic to aquatic organisms, particularly invertebrates. Zn2+ binds to proteins on the gill surfaces of aquatic animals and disrupts the ion-regulation those gills perform, so the organism cannot maintain its internal salt balance. Sensitive taxa such as mayfly larvae are affected first, which is why they are used as indicator species in stream health monitoring.
Because the effect scales with the dissolved concentration rather than the total zinc present, the fact that zinc remains in solution rather than precipitating is what makes it biologically available and therefore harmful.
Step 3 — Weigh it against a threshold
New Zealand water quality guidelines for the protection of aquatic ecosystems set trigger values for dissolved zinc in the range of around 8 µg L−1 for 95% species protection in fresh water. Urban stormwater in New Zealand cities routinely exceeds this, sometimes by an order of magnitude, during the first flush of a rain event.
A qualitative test cannot give a concentration, so this investigation establishes that zinc is present at a detectable level but not how far above or below the guideline it is. That is an honest limitation of the method, and it points directly at what should be done next: a quantitative determination, by titration or by an instrumental method, to compare against the guideline.
Step 4 — Consider more than one perspective
- Against the roofing: galvanised steel is the dominant roofing material in New Zealand, it is durable and cheap, and the zinc coating is precisely what protects the steel from corroding. Removing it is not a simple win.
- The scale of the source: roof runoff is a diffuse source, spread across every building in a catchment, which makes it much harder to manage than a single industrial discharge. There is no pipe to regulate.
- What can be done: rain gardens and constructed wetlands trap dissolved metals before they reach streams; alternative roofing materials such as coated steel with a factory-applied polymer finish substantially reduce zinc release; and treating the first flush captures the highest-concentration runoff.
- What is uncertain: toxicity depends strongly on water hardness and pH, since calcium and magnesium ions compete with zinc for binding sites on gills. The same zinc concentration is considerably less harmful in hard water than in soft, so a concentration alone does not determine the risk.
Step 5 — Conclude
Answer: identifying Zn2+ in this runoff is significant because it confirms a diffuse, continuous source of a metal that is toxic to stream invertebrates at very low concentrations, arising from the acid dissolution of the zinc coating on the roof. The qualitative result establishes presence but not concentration, so it justifies rather than replaces a quantitative follow-up; and because the harm depends on hardness and pH as well as concentration, an assessment of actual risk would need those measured too.