Fertilizer Runoff in a Village Pond
A village pond covers 0.5 hectare and has an average depth of 1.5 m. Upstream, the water is clear, with dissolved oxygen (DO) 7 mg/L and little algae. Three days after a farmer spreads 150 kg of urea (46% nitrogen) on a 2-hectare field beside the pond, heavy rain washes some fertilizer into the pond. The pond water turns bright green, algae form a surface scum, and early-morning DO falls to 3 mg/L. Fish are gulping at the surface. A villager proposes spraying a herbicide to kill the algae; another suggests pumping out the pond; a third recommends planting a 5 m-wide strip of tall grass between field and pond. The panchayat has a small budget and wants a plan that addresses both the immediate fish-kill risk and next year's monsoon. Assume 10% of the urea nitrogen reaches the pond and that the pond volume is 0.5 ha × 1.5 m.
1. Explain the chain of events that led from the fertilizer application to the low early-morning DO, and estimate the nitrogen load entering the pond if 10% of the urea nitrogen reaches it.
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Nitrogen in the urea = 150 kg × 0.46 = 69 kg N. If 10% reaches the pond, the load is 69 × 0.10 = 6.9 kg N.
The chain is eutrophication. The added nitrogen fertilises algae, producing the bright green water and surface scum. When algae die, bacteria decompose them and consume dissolved oxygen. At night, algae and other organisms respire without photosynthesising, so oxygen is used up and not replaced. Early morning is therefore the time of lowest DO, falling to 3 mg/L, well below the upstream 7 mg/L. Low DO stresses fish, which gulp at the surface where oxygen diffuses in. The fertilizer does not directly poison the fish; it triggers an oxygen crisis through excess plant growth and decay.
2. Evaluate the three proposals (herbicide, pumping out, grass buffer strip) for immediate risk and long-term prevention, and recommend a combined action.
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Herbicide is risky as an immediate action: killing the algae creates a large mass of dead organic matter, and bacterial decomposition could drive DO even lower, worsening the fish kill. It also does not remove the nutrient already in the pond.
Pumping out could remove nutrient-rich water, but it is expensive, may harm pond life, and does not stop the next runoff. It might be justified only as an emergency partial water exchange if clean water is available.
The grass buffer strip is the best long-term measure because it slows runoff and takes up nitrogen before it reaches the pond. It will not solve the immediate oxygen crisis.
A sensible combined plan is: stop further fertilizer input, monitor DO, aerate or add clean water if possible, remove dead algae, and plant the 5 m buffer strip before the next monsoon. The buffer addresses the source; emergency aeration addresses the immediate fish risk.
3. Explain why DO is lowest just before dawn and describe one simple monitoring plan the village could use to decide when to act.
Read the worked answer
DO is lowest just before dawn because photosynthesis has been stopped all night while algae and other pond organisms continue to respire and consume oxygen. During the day, photosynthesis produces oxygen faster than respiration uses it, so DO rises; at night, only consumption occurs. The longer the dark period, the more oxygen is used, so the minimum is reached near dawn.
A simple plan is to measure DO with a pond test kit at dawn and again in mid-afternoon at the same two or three spots, recording the values daily during the risk period. If dawn DO falls below about 3 mg/L, or fish are gulping at the surface, the village should act: stop fertilizer input, aerate or exchange water if possible, and remove decaying algae. Repeated records show whether the buffer strip and changed fertilizer timing are working.