Water Quality Practice Test — Easy Level

A free 10-question Water Quality practice test for Science Olympiad, written at invitational level. Every question and answer choice is listed below — correct answers and scoring are revealed when you take the timed test.

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1. Which description best defines an estuary in the context of marine and aquatic science?

A.A) A fully freshwater lake surrounded by wetlands with no connection to the ocean
B.B) A partially enclosed coastal body of water where river runoff mixes with seawater, creating brackish conditions
C.C) A deep offshore ocean basin located beyond the continental slope
D.D) A man-made reservoir used for storing drinking water and controlling floods

2. Students are investigating the effects of a wastewater treatment plant on an estuary. They collect water samples just upstream of the plant’s discharge and at a site 500 m downstream in the estuary. Downstream, they measure higher biochemical oxygen demand (BOD) and lower dissolved oxygen (DO) compared to upstream. What is the most reasonable conclusion?

A.A) The plant is adding large amounts of cold, clean water that increases oxygen consumption
B.B) The plant’s discharge contains organic matter that is being decomposed, lowering DO
C.C) The lower DO is caused only by tidal action and is unrelated to the plant’s discharge
D.D) The estuary is experiencing ocean acidification, which directly raises BOD

3. A simple estuary mixing model assumes that salinity is a conservative property (no salt is added or removed during mixing). A student mixes 300 mL of river water with a salinity of 0.5 ppt with 100 mL of seawater with a salinity of 35 ppt to simulate an estuary. What is the approximate salinity of the mixed sample, rounded to the nearest whole ppt?

A.A) 5 ppt
B.B) 9 ppt
C.C) 18 ppt
D.D) 26 ppt

4. A student sampling a coastal stream finds many stonefly nymphs, mayfly nymphs, and caddisfly larvae, with very few blood midge larvae or tubifex worms. What does this macroinvertebrate community most strongly suggest about the water quality?

A.A) The water is likely clean and well-oxygenated, indicating good water quality
B.B) The water is highly polluted with organic waste and very low in oxygen
C.C) The water is extremely acidic with a pH well below 4
D.D) The water has no measurable salinity and must be from a groundwater spring

5. A team measures dissolved oxygen (DO) in surface water at two nearby marine sites at sea level. Their data are shown below: Site 1: Temperature = 12°C, DO = 9.5 mg/L Site 2: Temperature = 26°C, DO = 6.5 mg/L Assuming no measurement error, which statement best explains why Site 1 has higher DO than Site 2?

A.A) Colder water at Site 1 can hold more dissolved oxygen than the warmer water at Site 2
B.B) Warmer water at Site 2 can hold more dissolved oxygen, but fish have used it up
C.C) Colder water at Site 1 can hold less oxygen, so the meter must be incorrect
D.D) Temperature does not affect oxygen solubility; the difference is due only to salinity

6. Students collect a seawater sample from an estuary near a seafood-processing plant and run a 5-day BOD test at constant temperature. The initial dissolved oxygen (DO) is 8.5 mg/L, and after 5 days of incubation the DO is 2.5 mg/L. What is the 5-day biochemical oxygen demand (BOD₅), and what does this most likely indicate about the water quality at this site?

A.A) BOD₅ = 2.5 mg/L, indicating low organic pollution and good water quality
B.B) BOD₅ = 6.0 mg/L, indicating high organic pollution and likely oxygen stress for aquatic life
C.C) BOD₅ = 8.5 mg/L, indicating extremely clean water with abundant dissolved oxygen
D.D) BOD₅ = 3.4 mg/L, indicating moderate organic pollution but no concern for oxygen levels

7. A team monitors an estuary before and after a heavy rainstorm. After the storm, which measured change would most directly indicate that large amounts of sediment-laden runoff have entered the estuary?

A.A) A sharp increase in turbidity measured with a Secchi disk
B.B) A slight decrease in salinity from 30 ppt to 28 ppt
C.C) A gradual decline in dissolved oxygen from 8 mg/L to 7 mg/L
D.D) A small increase in nitrate concentration from 0.5 mg/L to 0.7 mg/L

8. Students map salinity in an estuary on a calm day at high tide. They sample four locations: Site A: River mouth (where the river enters the estuary) Site B: Middle of the estuary Site C: Near the ocean inlet Site D: A tidal creek far upstream from the ocean All samples are taken at the surface. Which site is most likely to have the highest salinity?

A.A) Site A, because river water always has the highest salinity
B.B) Site B, because mixing always makes the middle of an estuary the saltiest
C.C) Site C, because it is closest to the open ocean source of salt water
D.D) Site D, because shallow tidal creeks concentrate salt as water flows in

9. Most of the ocean’s photosynthesis by phytoplankton occurs in which marine zone?

A.A) Photic zone, where enough sunlight penetrates for photosynthesis
B.B) Aphotic zone, where no sunlight reaches the water column
C.C) Abyssal zone, on the deep, dark ocean plains
D.D) Benthic zone, on the seafloor at all depths

10. A coastal town restores an oyster reef in a shallow estuary and wants to know if water clarity has improved after one year. Which student plan is the best simple experimental design to answer this question?

A.A) Measure turbidity once at the restored reef after one year and compare it to published turbidity values from other estuaries
B.B) Measure turbidity at the restored reef and at an untreated nearby site, both before restoration and one year after, using the same methods
C.C) Measure only dissolved oxygen at the restored reef before and after restoration, since DO and clarity are the same parameter
D.D) Take one water sample at low tide and one at high tide at the restored reef, then average the two turbidity values