Water Pollution

Question

Step-by-step solution

1. Step 1

   Nitrates and phosphates (nutrients) — Source: runoff from agricultural land (over-application of fertilisers). Effect: causes eutrophication — algal blooms deplete oxygen and kill aquatic life.

2. Step 2

   Sewage / pathogens — Source: untreated sewage discharged from homes and informal settlements directly into rivers or coastal waters. Effect: waterborne diseases (cholera, typhoid, dysentery); high biochemical oxygen demand (BOD) from decomposition depletes dissolved oxygen.

3. Step 3

   Heavy metals (e.g. lead, mercury, cadmium) — Source: mining and industrial waste, old lead water pipes. Effect: bioaccumulate in food chains (biomagnification); cause nervous system damage, kidney failure, and cancer in humans; toxic to fish and invertebrates.

4. Step 4

   Pesticides (e.g. DDT) — Source: agricultural runoff; direct spraying near water bodies. Effect: toxic to aquatic invertebrates and fish; bioaccumulate in fat tissue up the food chain — top predators (e.g. raptors, seals) accumulate lethal concentrations.

Question

Step-by-step solution

1. Step 1

   Nutrient input: excess nitrates and phosphates from fertilisers, sewage or detergents enter the water body through runoff or direct discharge.

2. Step 2

   Algal bloom: the sudden excess of nutrients causes rapid growth and reproduction of algae and cyanobacteria (blue-green algae) on the water surface — a dense green 'bloom'.

3. Step 3

   Light blocked: the dense algal mat blocks sunlight from reaching submerged aquatic plants. These plants are unable to photosynthesise and die.

4. Step 4

   Decomposition and oxygen depletion: aerobic bacteria decompose the dead algae and plants. The bacteria rapidly multiply and consume dissolved oxygen in the water. Oxygen levels fall sharply — this is called deoxygenation or oxygen depletion.

5. Step 5

   Death of aquatic life: fish, invertebrates and other aerobic organisms suffocate as dissolved oxygen levels fall below survival thresholds. The water becomes anoxic (oxygen-free) and is dominated by anaerobic bacteria producing foul-smelling gases (hydrogen sulphide, methane).

Question

Step-by-step solution

1. Step 1

   Sewage treatment plants — sewage is treated in stages (screening, settlement, biological filtration, chlorination) before discharge. Advantage: removes pathogens and most nutrient loads, preventing disease and eutrophication. Disadvantage: expensive to build and operate; many LICs lack the infrastructure; treatment is not 100% effective — some nutrients still pass through.

2. Step 2

   Legislation and regulation — governments set legal limits on pollutant discharge (e.g. EU Water Framework Directive; national effluent standards). Fines and prosecution deter illegal dumping. Advantage: legally binding with real penalties. Disadvantage: enforcement is difficult and expensive; corruption in some countries allows violations; difficult to police diffuse agricultural pollution.

3. Step 3

   Riparian buffer zones — strips of vegetation (trees, shrubs, grass) planted along river banks. Plant roots and soil absorb and filter nutrients from agricultural runoff before they reach the river. Advantage: low-cost, natural solution that also provides habitat and prevents bank erosion. Disadvantage: requires farmers to sacrifice productive land; vegetation takes time to establish; not effective for large pollutant loads.

Question

Step-by-step solution

1. Step 1

   Most likely source — agricultural fertiliser runoff: The data states that there is no direct sewage discharge, ruling out sewage as the primary source. The river runs through intensive farmland. The nitrate level of 45 mg/L is very high — natural river nitrate concentrations are typically <2 mg/L. This strongly indicates agricultural fertiliser runoff as the source. During heavy summer rainfall, excess nitrogen fertiliser applied to fields leaches through the soil into the river. Summer is also the period of fastest algal growth (long daylight hours, high temperatures) — consistent with the observed oxygen depletion occurring in summer.

2. Step 2

   Complete mechanism from source to fish death: (1) Intensive farms apply excess nitrogen fertiliser to fields. (2) Rain washes nitrates (NO₃⁻) and phosphates from fields into the river via surface runoff and leaching. (3) The elevated nitrate level (45 mg/L) stimulates rapid growth of algae and cyanobacteria on the river surface — an algal bloom. (4) The dense algal mat blocks sunlight from reaching submerged aquatic plants, which are unable to photosynthesise and die. (5) Aerobic bacteria multiply rapidly to decompose the dead algae and aquatic plants. These bacteria consume dissolved oxygen as they respire aerobically. (6) Dissolved oxygen falls to <2 mg/L (recorded in the data). Most fish species require >5 mg/L — they suffocate and die at <2 mg/L. (7) The river is now dominated by anaerobic decomposition, producing foul-smelling gases.

3. Step 3

   Strategy 1 — Establish riparian buffer strips: Plant 20–50 m wide strips of vegetation (grasses, shrubs, trees) along both riverbanks. Plant roots and soil filter nitrates out of runoff before it reaches the river; the vegetation absorbs nitrates as nutrients. Advantage: natural, low-cost, provides habitat for wildlife and stabilises banks. Disadvantage: requires farmers to sacrifice productive farmland (lost income); takes several growing seasons to become fully effective; cannot prevent nitrate leaching from subsurface drainage — only surface runoff.

4. Step 4

   Strategy 2 — Introduce nitrate application regulations: The government mandates a maximum fertiliser application rate and a closed period (winter months when crops are not growing and rainfall is highest) when fertiliser cannot be applied. Farmers must keep application records for inspection. Advantage: addresses the root cause directly — less nitrogen applied means less leaching. Evidence from EU Nitrates Directive shows significant river nitrate reductions in compliant regions. Disadvantage: requires strong enforcement infrastructure; farmers may under-report applications; short-term yield reduction may create economic hardship and political resistance; diffuse source pollution is inherently difficult to police.

Question

Step-by-step solution

1. Step 1

   Biomagnification factor calculation: Factor = concentration in large fish / concentration in water = 5 mg/kg ÷ 0.0001 mg/kg = 50,000×. Mercury has concentrated 50,000 times from the water to the large predatory fish. Even from small fish to large fish: 5 ÷ 0.5 = 10× in one trophic level.

2. Step 2

   Mechanism of biomagnification: Methylmercury is fat-soluble and does not break down or excrete readily from animal bodies — it accumulates in fatty tissues and organs over the organism's lifetime. At each trophic level, a predator must consume many prey organisms to obtain sufficient energy. Each prey organism already contains accumulated mercury, so the predator ingests the combined mercury from all its prey. Since only ~10% of energy (and none of the mercury) passes between trophic levels, mercury accumulates disproportionately. A large predatory fish at the top of a marine food chain may consume thousands of small fish over its lifespan, each containing accumulated mercury from thousands of phytoplankton — resulting in concentrations thousands of times higher than the surrounding water.

3. Step 3

   Options to protect the community's health:

   (1) Stop the industrial discharge immediately (source control): The most fundamental solution — preventing further mercury entering the bay. Advantage: stops the contamination at source; mercury in the bay will gradually reduce over time through dilution, sedimentation and natural chemical transformations. Disadvantage: industrial closure costs jobs and economic output; legal challenges from the company; even after closure, mercury already in sediments continues to enter the food chain for years to decades (Minamata Bay, Japan took decades to partially recover after mercury discharge ceased).

   (2) Issue fish consumption advisories and provide alternative protein: While source control is pursued, advise the community to reduce fish consumption (especially large predatory fish with highest mercury), particularly pregnant women and children (most vulnerable to methylmercury's neurotoxic effects). Provide alternative protein sources (beans, farmed fish from uncontaminated waters). Advantage: immediately reduces exposure without waiting for the bay to recover. Disadvantage: if fish is the only affordable protein source, this may cause nutritional deficiency; ignores the cultural and economic importance of fishing; advisory compliance depends on community trust and communication.

Question

Step-by-step solution

1. Step 1

   Why a catchment-wide approach is necessary: Most water pollution comes from diffuse (non-point) sources — agricultural runoff, urban stormwater, atmospheric deposition — spread across the entire catchment rather than from single identifiable pipes. Regulating individual discharge pipes ('point-source control') cannot address diffuse pollution. Additionally, land use decisions made anywhere in a catchment affect downstream water quality: deforesting headwaters increases erosion and nutrient runoff; building a suburb upstream adds stormwater inputs; changing crop types across hundreds of farms collectively shifts the nutrient load. A river is the integrating endpoint for everything that happens in its catchment — managing only the endpoint misses the vast majority of causes.

2. Step 2

   Advantages of ICM over point-source regulation: (1) ICM addresses both point and diffuse sources simultaneously, covering the full spectrum of pollution inputs. In Europe, point-source regulation (EU Urban Wastewater Directive) significantly cleaned rivers in the 1980s–90s, but diffuse agricultural pollution then became the dominant problem — only ICM approaches can tackle this. (2) ICM coordinates multiple stakeholders (farmers, planners, water utilities) to create coherent policies rather than isolated actions that may conflict. For example, a buffer strip requirement for farmers is more effective if water authorities also invest in wetland restoration downstream. (3) ICM can achieve cost savings through 'catchment payments' — paying farmers to manage land sustainably is often cheaper than building additional water treatment capacity downstream. (4) ICM supports biodiversity recovery across the whole river system, not just improved water chemistry at discharge points.

3. Step 3

   Limitations of ICM: (1) Governance complexity: coordinating multiple government agencies, local authorities, water companies, and thousands of individual farmers requires extensive institutional capacity and political will — difficult to achieve in countries with fragmented governance. (2) Long timeframes: catchment-scale restoration takes decades; political cycles are 4–5 years. Politicians may not prioritise investments with benefits beyond their term. (3) Monitoring cost: assessing the effect of diffuse source interventions requires dense monitoring networks across the catchment — expensive to install and maintain. (4) Farmer compliance: voluntary agri-environment schemes (which pay farmers for environmental services) have incomplete uptake — mandatory approaches face legal and political resistance.