How Does Saltwater Mix in an Estuary

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To determine the average and individual concentrations of salt found in water from various points along an estuary by analyzing the color, density, and residue of the water.

Materials Needed

  • 2-quart (-liter) samples of estuary water from seven locations along an estuary
  • 250-ml beaker
  • 14 test tubes
  • 4-quart (-liter) sample of fresh river water (or substitute distilled water)
  • 4-quart (-liter) sample of saltwater
  • Metric graduated cylinder
  • 15 plastic cups
  • Balance scale


Three different tests will be used to analyze estuary water. The first test will focus on color variations to distinguish the saltwater from the river water. The second will measure the density of the estuary water to determine the amount of saltwa­ter from the sea that has been mixed in. The third test will measure the residual percentage of salt after the water has evaporated.


Test A—To determine visually the average amount of saltwater present within samples from various locations along an estuary.

  1. Boil the seven 2-quart (-liter) samples of estuary water separately until each is reduced to 250 ml. Then fill seven test tubes halfway with each of the concentrated samples.
  2. Next, make seven saltwater/fresh river water color reference samples in the other seven test tubes with which to compare the concentrated estuary samples. Fill the first test tube halfway with 100% fresh river water, the second with 80% fresh river water and 20% saltwater, the third with 60% fresh river water and 40% saltwater, the fourth with 50% fresh river water and 50% saltwater, the fifth with 40% fresh river water and 60% saltwater, the sixth with 20% fresh river water and 80% saltwater, and the seventh with 100% saltwater (see diagram).
  3. Compare the colors of the samples of concentrated estuary water with the colors of the reference samples. For each estuary sample, record the reference sample that most closely resembles it in color.

Test B—To determine the actual amount of saltwater mixed in by calculating the densities of the seven samples.

[(volume of x) (density of x)] + [(volume of y) (density of y)] = total volume × density

  1. Using the graduated cylinder, measure 175 cc of each estuary sample into a plastic cup and weigh each cup separately on the balance scale. Then subtract the cup's weight to obtain the mass of each sample.
  2. Find the density of each sample by dividing the mass by the volume. Then calculate the average density for all the samples.
  3. Next, find the actual percentage of saltwater for each sample. Once you have determined the separate and combined densities of the fresh water and saltwater in your sample and know the sample's total volume, the percentage of fresh river water (x) and saltwater (y) can be calculated as follows:

Test C—To measure the residual percentage of salt in each sample.

  1. Pour the remaining estuary samples each into a separate plastic cup. Place another cup alongside and fill it with distilled water until its volume is the same as each of the sample cups.
  2. Weigh the cup containing the distilled water on the balance scale and record its mass. Then, weigh both cups on the balance scale and subtract the mass of the distilled water from the mass of the sample water. Repeat for each of the samples and record your results.


  1. What were the levels of salt concentration among the samples? Were your color reference samples accurate as to the salt concentrations in each sample?
  2. Were the results from each of the three tests consistent for each water sample?
  3. Were the salinity levels consistent for all seven locations? If not, which location of the estuary had the highest level of salinity? The least?
  4. Experiment with different estuaries. Are the salinity level distributions that you found in the original estuary comparable to those of other estuaries?
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