An Oxygen Sag in a Mid-Hill Lake Receiving Sewage
A community forest users group monitors a mid-hill lake that receives untreated sewage from a growing bazaar. They sample three stations on the same morning in April: Station U (400 m upstream of the discharge), Station D (at the discharge point), and Station R (2 km downstream, after a shallow riffle). Results: dissolved oxygen 8.4, 2.9, and 6.1 mg/L; BOD5 1.2, 22, and 4.8 mg/L; nitrate-N 0.1, 4.2, and 1.8 mg/L; chlorophyll a 2, 52, and 18 micrograms/L; macroinvertebrate families 26, 5, and 14. Sludge worms (Tubifex) dominate at D; mayfly nymphs are absent at D but present at U and R. The lake is also used for drinking water downstream and for religious bathing. The users group has a small budget and must choose one intervention: (a) install a sewage treatment wetland at the outfall, (b) aerate the lake at station D, or (c) ban fishing for one year. They ask for a scientific recommendation.
1. Interpret the data. Identify the most likely pollution type and explain the oxygen profile from U to D to R using BOD, decomposition, and photosynthesis.
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Station U is the reference: high DO (8.4), low BOD (1.2), low nitrate (0.1), low chlorophyll (2), 26 invertebrate families, and mayflies present. This indicates a clean, oxygenated stream. Station D receives untreated sewage. BOD5 rises to 22 mg/L because sewage supplies labile organic matter. Aerobic bacteria decompose it, consuming dissolved oxygen faster than reaeration can replace it, so DO falls to 2.9 mg/L. Nitrate rises to 4.2 mg/L from ammonification and nitrification of sewage nitrogen. Chlorophyll a rises to 52 micrograms/L because nutrients stimulate algae, but algal photosynthesis is not enough to offset bacterial oxygen demand, especially at night. Tubifex, which tolerates low oxygen and organic enrichment, dominates; mayflies, which need high oxygen, disappear. This is the classic oxygen sag. Station R, 2 km downstream after a shallow riffle, shows recovery: BOD falls to 4.8, DO rises to 6.1, nitrate falls to 1.8, chlorophyll is 18, and 14 families return. Reaeration at the riffle and continued decomposition and settling reduce the organic load. The pattern is consistent with organic sewage pollution, not with a toxic spill, which would not produce the BOD, nitrate, and algal signature.
2. Compare the three proposed interventions using ecological reasoning and choose one. Justify your choice under a small budget.
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(a) Constructed wetland at the outfall: treats the cause. It reduces BOD, suspended solids, and nutrients before they enter the lake; it is low-energy, can use local materials, and can be maintained by the users group. It needs land, a fall in water level, and regular harvesting of vegetation; performance drops in winter, and it may not remove all nitrogen. (b) Aeration at D: treats the symptom. It raises DO locally but does not reduce BOD or nutrients. It is energy-dependent, expensive to run, and may only shift the oxygen sag downstream. It does not protect drinking water from pathogens. (c) Fishing ban: irrelevant to the sewage load; it may protect fish temporarily but does not address the cause. Choice: wetland at the outfall, combined with source control (septic tanks, no direct discharge) and monitoring. Aeration could be a short-term emergency only if DO falls below 2 mg/L and fish kills threaten. The wetland gives the largest sustained reduction in BOD and nutrients per rupee.
3. Name two indicators from the data most reliable for long-term monitoring and explain why chlorophyll a alone can mislead.
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Reliable indicators: BOD5, which integrates organic load and predicts oxygen demand; and macroinvertebrate family richness (or mayfly/Tubifex ratios), which integrates chronic conditions over weeks and reflects ecosystem health. DO is useful but fluctuates daily, especially with algal photosynthesis. Chlorophyll a alone can mislead because it measures algal biomass, not pollution source or oxygen demand. Nutrients can produce high chlorophyll with high daytime oxygen, but night-time respiration can cause hypoxia; toxic or organic pollution may kill invertebrates without raising chlorophyll; and chlorophyll responds to agricultural runoff as well as sewage. It also varies seasonally. Use chlorophyll together with BOD, nutrients, and invertebrates.