GATE — Environmental Science and Engineering (ES) · India · IITs and IISc on rotation (2026: IIT Guwahati)
Original practice questions written to the published syllabus · not past papersHow farming pollutes water and how to stop it: nitrate leaching, phosphorus runoff, sediment, pesticides and pathogens, eutrophication and dead zones, and the best management practices and policies that work.
In the exams: GATE ES Section 5 (eutrophication, oxygen sag); ICAR AICE subject 71 Unit 4; UGC NET Unit 6
GATE -- Environmental Science and Engineering (Sections 2 and 5)
BOD kinetics, mass-balance mixing, Streeter-Phelps deficit and critical time, eutrophication and trophic state indices, lake stratification, nitrate and DO saturation; frequent numerical-answer items.
How our questions for GATE spread across the units of Water Quality & Agricultural Non-point Source Pollution. Difficulty: 12 easy, 11 medium, 3 hard.
| River pollution and the oxygen sag curve | 4 | |
| Eutrophication, algal blooms and hypoxic zones | 3 | |
| Nutrient indices and catchment modelling | 3 | |
| Point and non-point sources of water pollution | 2 | |
| Nitrate leaching and groundwater contamination | 2 | |
| Phosphorus loss: particulate and dissolved pathways | 2 | |
| Thermal stratification of lakes | 2 | |
| Pesticide and veterinary drug residues in water | 2 | |
| Pathogens from manure and wastewater | 2 | |
| Best management practices: buffer strips, constructed wetlands, cover crops | 2 | |
| Sediment and turbidity | 1 | |
| Water quality standards and regulation of diffuse pollution | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
What is the WHO drinking-water guideline for nitrate?
A. 50 mg/L as nitrate
50 mg/L NO3 (11.3 mg/L NO3-N). The US limit of 10 mg/L is expressed as nitrate-nitrogen.
Particulate phosphorus reaches streams mainly by:
A. Soil erosion in runoff
PP is attached to soil particles and moves with eroded sediment.
A lake has a Secchi depth of 2.0 m. Using Carlson's TSI(SD) = 60 - 14.41 ln(SD), what is its trophic state index?
B. 50.0
60 - 14.41 x ln 2 = 60 - 9.99 = 50.0. Using log10 gives 55.7; adding gives 70.0; omitting 60 gives 10.0.
A river (5 m3/s, BOD 2 mg/L) receives effluent (0.5 m3/s, BOD 150 mg/L). What is the BOD just downstream after complete mixing?
D. 15.5 mg/L
(5 x 2 + 0.5 x 150) / 5.5 = 15.5. A simple average ignores flows; dividing by river flow only overstates; 15.0 ignores river BOD.
What does BOD5 measure?
B. Oxygen consumed by microbes decomposing organic matter in 5 days at 20 degrees C
BOD is biochemical; COD uses dichromate; TOC uses combustion.
Which route carries pesticide droplets from a sprayer directly onto a nearby stream?
D. Spray drift
Drift moves droplets off-target during application; buffer zones and nozzle choice reduce it.
Bacteria in a stored manure die off by first-order decay with k = 0.5 per day. What is the time for a 90% reduction (T90)?
C. 4.6 days
T90 = ln(10)/k = 2.303/0.5 = 4.6. Using log10 gives 2.0; 0.9/k is linear; ln2/k is the half-life.
Riparian buffer strips are most effective at trapping:
B. Sediment and particulate P
Vegetated buffers filter surface runoff; deep groundwater nitrate bypasses them.
A nitrogen leaching index typically combines:
D. Soil leaching potential, excess water and residual nitrogen
Soil, climate or irrigation, and management determine leaching risk.
In India's CPCB classification by designated best use, class B water is designated for:
A. Outdoor bathing
A: drinking source without conventional treatment after disinfection; B: outdoor bathing; C: drinking after treatment; D: wildlife and fisheries; E: irrigation, cooling, controlled waste disposal.
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