Water pollution

A point source is pollution from a single, identifiable location such as a pipe, drain, or factory outlet.

Point sources come from one identifiable outlet; non-point sources are diffuse runoff from many places.

A non-point source is diffuse pollution spread across a wide area, such as agricultural runoff or urban stormwater, with no single discharge point.

Point-source pollution is usually easier to regulate because the discharge location is identifiable and can be treated at source.

Because pollution is spread across many locations and varies with rainfall and land use, making monitoring and regulation difficult.

Nutrients (nitrates/phosphates), pesticides, and sediment from soil erosion (also pathogens from livestock waste).

Fertilizers and animal waste contain nitrogen and phosphorus that can wash into rivers and lakes during rain, especially from large catchments.

Examples include nutrients (nitrates/phosphates), pathogens, heavy metals, and plastics (also organic matter, pesticides, thermal pollution, sediment).

Excess nitrates and phosphates can cause eutrophication, leading to algal blooms and oxygen depletion.

State it is non-point source from a wide area, monitoring/enforcement is difficult, and impacts can occur far downstream from sources.

Eutrophication is the process where excess nutrients (especially nitrogen and phosphorus) cause rapid algal growth, leading to oxygen depletion and ecosystem damage.

Excess nutrients → algal bloom → light blocked → plant death → decomposition → oxygen depletion (hypoxia) → fish kills/dead zone.

Nitrogen and phosphorus.

Nitrogen (often nitrates) and phosphorus (often phosphates).

Examples include the Gulf of Mexico and the Baltic Sea (also Lake Erie and Chesapeake Bay).

When algae and plants die, decomposers break them down and use up dissolved oxygen during respiration, causing hypoxia.

A dead zone is an area of water with oxygen levels too low to support most aquatic life, often caused by eutrophication.

Hypoxia and dead zones reduce fish and shellfish survival, forcing fish to migrate or die, lowering catches and damaging fisheries.

A clear cause-and-effect sequence linked to the question context (for example fisheries, ecosystem services, or biodiversity).

Agricultural fertilizers, sewage/wastewater, and animal waste (also urban runoff and atmospheric deposition).

More atmospheric CO2 dissolving into the ocean, forming carbonic acid and lowering pH.

Ocean acidification is the decrease in ocean pH caused by absorption of atmospheric CO2, forming carbonic acid in seawater.

CO2 dissolves in seawater and forms carbonic acid, which releases H+ ions, lowering pH.

It reduces shell/skeleton formation in corals and molluscs and disrupts food webs starting with plankton.

Lower pH reduces carbonate availability and makes it harder to build calcium carbonate shells/skeletons, weakening growth and survival.

It threatens fisheries/food security and reduces income/jobs in fishing and tourism sectors.

It can reduce fisheries and food security, harm jobs in fishing communities, and reduce tourism where coral reefs degrade.

Cover BOTH environmental systems and societies, with multiple distinct points on each side, not just chemistry.

Because it is driven by atmospheric CO2 levels; reducing emissions is the key solution, and local cleanup cannot remove the underlying CO2 cause.

Because pH is logarithmic, so a small numerical decrease represents a large increase in acidity.

Bioaccumulation is the build-up of a substance in an organism over time, faster than it can be broken down or excreted.

A seasonal dead zone is an area of water where dissolved oxygen becomes very low during certain months (often summer), so many organisms die or move away.

Warm water holds less dissolved oxygen, and summer conditions can intensify algal blooms and decomposition, increasing hypoxia.

Plankton absorb a toxin → small fish eat many plankton → larger fish eat many small fish → top predators accumulate the highest toxin concentration.

Biomagnification is the increase in concentration of a substance at higher trophic levels in a food chain.

Hypoxia is commonly defined as dissolved oxygen below about 2 mg/L.

Humans can be top consumers in marine food webs, so toxins such as mercury and POPs can reach high concentrations in seafood and then in people.

Top predators (including humans) receive the highest concentrations because toxins accumulate up the food chain.

Some pollutants are chemically stable and not easily degraded, so they remain in water/sediments and in organisms for long periods.

Heavy metals (for example mercury), persistent organic pollutants (POPs such as DDT/PCBs), and microplastics that can carry absorbed toxins.

Decomposers respire as they break down organic matter, using dissolved oxygen and lowering oxygen levels in the water.

Define the term clearly and apply it to a food-chain example, explaining why concentration is highest at the top.

They are not easily broken down or excreted and can be stored in body fat, so they remain in organisms and increase in concentration as predators eat many contaminated prey.

Fish and benthic organisms die or leave the area, reducing prey for higher trophic levels and disrupting the food web.

Prevention (stop at source), treatment (remove pollutants), and restoration (repair damaged ecosystems).

Prevention is usually most effective and lowest-cost, because stopping pollution at source avoids widespread damage.

A riparian buffer zone is a vegetated strip along a waterway that traps sediment and absorbs nutrients before they reach rivers or lakes.

Riparian buffer zones and cover crops (also precision agriculture and constructed wetlands).

Examples include precision agriculture, improved wastewater treatment to remove N and P, and constructed wetlands (also buffer zones and cover crops).

Legislation (pollution limits) and economic tools such as fines/penalties or subsidies (also education).

The polluter pays principle means those who cause pollution should cover the costs of preventing, controlling, and repairing environmental damage.

Because once pollutants spread through water bodies and food webs, removal is difficult and ecosystems may take years to recover, so stopping pollution earlier avoids larger costs.

Briefly explaining how each strategy reduces pollution and linking it to improved water quality/ecosystem protection earns higher marks than listing strategies only.

Because it comes from many small sources across a landscape, so it needs catchment-wide solutions like land management changes, incentives, and monitoring rather than a single treatment point.