Groundwater Age

Question 0

Limitations to using CFC levels: (4)

Answer 0

Only useful for water less than 50 years old.
May be a time lag of CFC diffusion through the unsaturated zone.
Degradation of CFC where oxygen levels are low.
CFC may absorb into aquifer material.

Question 1

Investigate CFC levels: CFC concentration is

Answer 1

Directly proportional to the atmosphere concentrations at that time.

Question 2

Measuring isotopes: Time is constant for

Answer 2

A particular element to decay to its stable isotope.

Question 3

Measuring isotopes: most commonly used isotopes are

Answer 3

Carbon-14 - λ 5700 years

Hydrogen-3 (tritium) - λ 12.4 years

Question 4

Measuring isotopes: D/P = 2n -1

Answer 4

Value of D/P depends on number of half lives (λ) since the isotope entered the water.

n = number of half lives (λ)

Question 5

Chemical composition variation: Rock type (3)

Answer 5

Ca2+ and Mg2+ are absorbed onto clay surface and Na+ and K+ are released.
CO2 dissolves in water to generate hydrogen ions.
If a rock contains 1% halite, gypsum or calcite, their dissolution defines water chemistry.

Question 6

Chemical composition variation: Age (3)

Answer 6

Increases with depth.

Low oxygen levels = low nitrate and sulfate levels : nitrate is reduced to nitrite / nitrogen, sulfate is converted to hydrogen sufide.

In a deep aquifer, CO2 may be reduced to CH4 gas.

Question 7

Chemical composition variation: Age; in a confined aquifer,

Answer 7

Oxygen levels may decrease and cannot be replenished by the atmosphere as the water is no longer in contact with the atmosphere.