In coastal areas the depth of the fresh water interface below sea level is approximately how many times the height of the potentiometric surface above sea level?

In coastal areas the depth of the fresh water interface below sea level is approximately how many times the height of the potentiometric surface above sea level?

• A. 2 times.
• B. 10 times.
• C. 40 times.
• D. 0.4 times.

Answer: (C)

This question tests the Ghyben-Herzberg relationship for coastal aquifers. Because seawater is denser than freshwater, the freshwater head that extends above sea level creates a much deeper saltwater interface offshore. The familiar rule of thumb is that the depth of this freshwater–saltwater interface below sea level is about 40 times the height of the potentiometric surface (freshwater head) above sea level. This 40:1 factor comes from the density difference between freshwater and seawater and is used to estimate how far the interface extends beneath the coast. So, the depth below sea level is approximately 40 times the height of the freshwater head above sea level. If the head is, for example, 1 meter above sea level, the interface would be about 40 meters below sea level. The other options don’t fit this established ratio: they imply much smaller or inverted relationships.

This question tests the Ghyben-Herzberg relationship for coastal aquifers. Because seawater is denser than freshwater, the freshwater head that extends above sea level creates a much deeper saltwater interface offshore. The familiar rule of thumb is that the depth of this freshwater–saltwater interface below sea level is about 40 times the height of the potentiometric surface (freshwater head) above sea level. This 40:1 factor comes from the density difference between freshwater and seawater and is used to estimate how far the interface extends beneath the coast.

So, the depth below sea level is approximately 40 times the height of the freshwater head above sea level. If the head is, for example, 1 meter above sea level, the interface would be about 40 meters below sea level. The other options don’t fit this established ratio: they imply much smaller or inverted relationships.