NAEP Science Grade 12

Category - Earth Science

A hydrologist is collecting water samples to determine the dissolved oxygen content of a pond that has become choked with weeds. To most closely reflect the actual dissolved oxygen content of the pond water, which of the following methods should the hydrologist use to collect the water samples?
  1. The water should be collected from below the pond's surface in various locations, filling and then sealing the containers rapidly and recording the locations.
  2. The samples should be collected from the pond's surface near the outlet where the water is moving rapidly and few weeds are growing.
  3. The water should be collected from near the bottom, making sure to leave some air space in the container and including some of the organic matter found in the pond.
  4. The samples should be collected near the pond's inlet and should avoid including any organisms or debris that may be in the water.
Explanation
Correct Response: A. To ensure the dissolved oxygen in the water samples is representative of the pond as a whole, several samples should be collected from different locations below the pond's surface, filled completely, and then sealed. This avoids collecting the most oxygenated water at the pond surface and it prevents oxygen from dissolving into the sample from air in the sample jar. Fast moving water from the surface that is near the pond's outlet (B) or inlet (D) will not be representative of the pond's water as the water samples will likely contain more dissolved oxygen. Similarly, the dissolved oxygen content of water samples that do not fill the jar and include organic matter from the bottom (C) may change over time and will likely not provide representative samples. 
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