Water Quality Practice Test — Medium Level

A free 10-question Water Quality practice test for Science Olympiad, written at regional and state 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 marine environment with uniform salinity around 35 ppt and no freshwater influence
B.B) A partially enclosed coastal body of water where freshwater from rivers mixes with seawater to create brackish conditions
C.C) A deep offshore trench located beyond the continental shelf with permanently low light and low salinity
D.D) A man‑made freshwater reservoir used for drinking water storage and hydroelectric power

2. A student group is monitoring a temperate estuary in summer. At mid-tide they measure surface salinity of 10 ppt near the river mouth and 22 ppt near the ocean inlet. Dissolved oxygen (DO) is 6.5 mg/L at the river mouth and 4.2 mg/L near the ocean inlet. Which explanation best fits these observations?

A.A) Near the river mouth, fresher, cooler water and more photosynthesis from river phytoplankton keep DO higher than in the more saline, slightly warmer inlet water.
B.B) Near the river mouth, higher salinity from salt wedges displaces oxygen, reducing DO compared to the well-mixed, oxygen-rich ocean inlet.
C.C) Near the ocean inlet, high nutrient runoff from upstream farms boosts primary productivity, increasing DO compared to the relatively nutrient-poor river mouth.
D.D) Near the ocean inlet, upwelling of cold bottom water consistently raises DO above that of the stratified, fresher river mouth water.

3. A Division C team is measuring Biochemical Oxygen Demand (BOD₅) near a marine wastewater outfall that discharges into a coastal estuary. They prepare a standard 5-day BOD test by diluting 1 part estuary water with 4 parts oxygen-saturated dilution water (total dilution factor = 5). The initial DO of the bottle is 9.0 mg/L, and after 5 days of incubation at 20°C the DO has dropped to 2.5 mg/L. Based on these data, which answer best describes both the approximate BOD₅ of the sample and the likely water quality condition near the outfall?

A.A) BOD₅ ≈ 6.5 mg/L, indicating low organic loading and generally healthy oxygen levels
B.B) BOD₅ ≈ 13 mg/L, indicating mild organic pollution with limited oxygen stress
C.C) BOD₅ ≈ 33 mg/L, indicating high organic pollution and a strong risk of hypoxia for marine life
D.D) BOD₅ ≈ 45 mg/L, indicating only slightly elevated organic matter with little effect on DO

4. A student collects macroinvertebrates from a tidal creek in a salt marsh and finds many gilled snails, mayfly larvae, and riffle beetles, with very few tubifex worms or blood midges. The creek water is slightly brackish (about 5 ppt). What is the best interpretation of these results?

A.A) The site likely has good water quality because it contains mostly pollution-intolerant taxa despite being mildly brackish
B.B) The site must be severely polluted because tubifex worms and blood midges are rare in brackish water
C.C) The site is clearly freshwater because pollution-intolerant taxa cannot survive in any brackish conditions
D.D) The site has poor water quality because high salinity always favors pollution-tolerant macroinvertebrates

5. You are designing a study to test whether a small wastewater treatment plant is impacting an adjacent estuary. Which sampling plan would give the strongest evidence about its effect on water quality?

A.A) Measure DO, BOD, and nutrients once at a single site directly in front of the discharge pipe during low tide.
B.B) Measure DO, BOD, and nutrients at multiple sites upstream of the plant’s discharge point in the river and multiple sites downstream in the estuary, with replicates on several different days and tidal stages.
C.C) Measure only pH and temperature at one upstream site in the river and one downstream site in the estuary during a single high-tide event.
D.D) Measure only turbidity and salinity at several random shoreline locations throughout the estuary once per year.

6. During a spring tide, ocean water and river water mix in a small bar-built estuary. For one hour, the river contributes 500 m³ of freshwater (0 ppt), while the incoming tide contributes 250 m³ of seawater at 35 ppt. Assume the estuary is well mixed during that hour and has negligible prior volume compared to these inputs. What is the resulting salinity of the estuary water after mixing, and how should this water be classified?

A.A) About 7 ppt; classified as nearly fresh water, outside the typical estuary range
B.B) About 18 ppt; classified as nearly marine water, similar to open-ocean conditions
C.C) About 12 ppt; classified as brackish estuarine water typical of many nursery habitats
D.D) About 23 ppt; classified as fully marine water with minimal river influence

7. Why are oyster reefs considered especially important for maintaining water quality in estuaries?

A.A) Oysters are top predators that directly reduce populations of pollution-tolerant fish species
B.B) Oysters are filter feeders that remove suspended particles, phytoplankton, and some contaminants from the water column
C.C) Oysters increase water temperature, which improves dissolved oxygen solubility and reduces eutrophication
D.D) Oysters are primary producers that add oxygen by photosynthesis and absorb excess nitrates from the air

8. A coastal restoration project adds a large oyster reef to a turbid estuary that receives nutrient-rich runoff. Six months later, volunteers record lower turbidity, slightly higher average DO, and fewer harmful algal blooms near the reef. Which mechanism is the most direct way oysters are improving local water quality?

A.A) Oysters increase water temperature, which raises DO and kills algae.
B.B) Oysters are filter feeders that remove suspended particles and some phytoplankton, reducing turbidity and limiting algal bloom intensity.
C.C) Oysters excrete large amounts of nitrate and phosphate, which dilute existing nutrients and prevent blooms.
D.D) Oysters consume zooplankton, which directly decreases BOD and eliminates algal cells from the water column.

9. A scientist is studying plankton in sunlit surface waters above the continental shelf, just offshore from an estuary mouth. Which combination of marine zones best describes where the samples are being collected?

A.A) Photic, pelagic, neritic
B.B) Aphotic, benthic, oceanic
C.C) Aphotic, pelagic, abyssal
D.D) Photic, benthic, oceanic

10. A team calculates a Water Quality Index (WQI) for an estuarine site in early spring and again in late summer. Between the two visits, nitrate and phosphate concentrations increase moderately, DO decreases from 7.0 mg/L to 3.5 mg/L, and turbidity rises sharply after several storms. Which change is most likely to cause the largest drop in the WQI score for this site?

A.A) The decrease in DO from 7.0 mg/L to 3.5 mg/L, because low oxygen directly stresses or kills many estuarine organisms.
B.B) The moderate increase in nitrate and phosphate, because nutrients are always beneficial and therefore raise the WQI.
C.C) The rise in turbidity alone, because turbidity is the only parameter that affects the WQI in estuaries.
D.D) The seasonal change from spring to summer, because WQI is based only on time of year, not measured parameters.