Aquatic and Terrestrial Pollution

Pollution sources are point or nonpoint. A point source 点源 comes from a single, identifiable place (a factory pipe, a sewage outfall) – easy to locate and regulate. A nonpoint source 非点源 is spread out (farm runoff, urban street runoff, car exhaust) – much harder to control because it comes from everywhere.

Human activities – pollution, habitat destruction, overharvesting, introducing species – degrade ecosystems and reduce biodiversity. Because species are interconnected, harming one part can ripple through the whole system.

Endocrine disruptors 内分泌干扰物 are chemicals (some pesticides, plasticizers like BPA) that mimic or block hormones, disrupting growth, reproduction, and development in wildlife and humans even at very low doses.

Wetlands 湿地 and mangroves 红树林 filter pollutants, store floodwater, and nurse young fish, but they are drained and cleared for farms, shrimp ponds, and development. Losing them removes valuable ecosystem services and coastal protection.

Mangrove roots: this tangle traps sediment, calms waves, and shelters young fish, all services lost when mangroves are cleared

Eutrophication 富营养化 happens when excess nutrients (nitrogen, phosphorus from fertilizer and sewage) trigger algal blooms 藻华. When the algae die, decomposers use up the oxygen, creating hypoxic 缺氧 "dead zones" that suffocate fish.

A pond choked by an algal bloom: extra nutrients let algae cover the whole surface. When they die and rot, the water loses its oxygen

Extra fertiliser makes algae bloom, then the water loses its oxygen

Thermal pollution 热污染 is heat added to water, usually from power-plant cooling. Warm water holds less dissolved oxygen and stresses aquatic life adapted to cooler temperatures.

Persistent organic pollutants 持久性有机污染物 (POPs, like DDT and PCBs) resist breakdown, so they last for decades, travel far, and are fat-soluble, so they build up in organisms. This sets up bioaccumulation and biomagnification.

Bioaccumulation 生物累积 is the build-up of a toxin within one organism over its life. Biomagnification 生物放大 is the increase in concentration up the food chain – top predators end up with the highest levels, which is why large predatory fish carry the most mercury.

Worked example. Suppose lake water holds DDT at just $0.01\ \text{ppm}$. Plankton concentrate it to $0.5\ \text{ppm}$, small fish to $2\ \text{ppm}$, large fish to $10\ \text{ppm}$, and a fish-eating eagle to $25\ \text{ppm}$. From water to eagle the concentration has multiplied about $\dfrac{25}{0.01}=2500$-fold – which is why a top predator can be poisoned even when the water itself looks only lightly polluted.

Solid waste is any discarded material that is not a liquid or gas, generated by homes, industry, business, and agriculture. It is buried in sanitary landfills 卫生填埋场 (lined to limit leachate) or burned in incinerators 焚烧炉 (reduces volume but emits pollutants). Landfills take space and can leak; both have environmental costs, so reducing waste is preferred.

Two categories the exam names specifically: e-waste 电子垃圾 (discarded electronics - phones, computers, televisions - which contain toxic metals), and ocean dumping, where plastic and other waste collect into vast floating garbage patches 垃圾带 far out at sea.

A landfill: solid waste is buried here in lined cells. It takes up land and can leak, which is why reducing waste comes first

The waste hierarchy is reduce, reuse, recycle 减量、再利用、回收, plus composting 堆肥 for organic waste. Reducing consumption is best (no waste to manage); recycling and composting recover materials and cut landfill use.

Sewage treatment 污水处理 cleans wastewater in stages: primary (settling out solids), secondary (bacteria break down organic matter), and sometimes tertiary (removing nutrients and disinfecting). This protects waterways from pathogens and nutrient pollution.

Treating water to make it safe: sedimentation, filtration, then disinfection

The LD50 半数致死量 is the dose of a substance that kills 50% of a test population. A lower LD50 means a more toxic substance (less is needed to be deadly). It is a standard way to compare toxicity.

Worked example. Chemical A has an LD50 of $5\ \text{mg}$ per kg of body weight; chemical B, $500\ \text{mg/kg}$. Because A's value is $100$ times smaller, A is about $100\times$ more toxic. For a $70\ \text{kg}$ adult, the dose of A expected to be lethal to half of people is $5\ \text{mg/kg}\times70\ \text{kg}=350\ \text{mg}$ – a tiny amount, whereas B would take $500\times70=35{,}000\ \text{mg}$ ($35\ \text{g}$).

A dose–response curve 剂量反应曲线 plots the effect against the dose. Generally, a higher dose causes a greater effect. Some chemicals have a threshold 阈值 (no effect below it); for others any exposure carries risk. These curves guide safety limits.

Pollution harms health directly (respiratory disease from air pollution, poisoning from contaminated water) and indirectly. Dose, duration, and individual sensitivity all affect the outcome. The poor are often most exposed and least protected.

Pathogens 病原体 (bacteria, viruses, parasites) spread through contaminated water and food. The exam names specific diseases and expects you to know how each one is transmitted, because the transmission route decides the defence:

Two big-picture ideas carry marks. Poverty deepens the problem - low-income areas often lack sanitary waste disposal and clean water, giving water-borne diseases a haven. And climate change spreads disease: as warm equatorial climate zones expand toward the poles, mosquitoes and other vectors 传播媒介 - and the diseases they carry - move into temperate regions that never had them before. Clean water, sanitation, and vector control remain the main defences.

• Trace eutrophication step by step: nutrient runoff → algal bloom → decay → oxygen depletion → fish die.
• Bioaccumulation builds up in one organism; biomagnification increases up the food chain, so top predators are hit hardest.
• Read an LD50: a lower value means more toxic; multiply by body mass for a lethal dose.
• Order the waste hierarchy reduce > reuse > recycle, and the stages of sewage treatment.
• Link pathogens to contaminated water and to clean-water/sanitation defences.