00 · IN THREE MINUTES

The answer in three steps

  1. 1Groundwater age usually means time since recharge left contact with the atmosphere.
  2. 2Different isotopes and industrial gases cover different age ranges.
  3. 3A sample commonly contains a distribution of travel times rather than one birthday.

01 · RAIN STARTS THE CLOCK

Rain starts the clock

Water infiltrates soil and reaches an aquifer during recharge. Once isolated from modern air, dissolved gases and isotopes begin recording a subsurface history shaped by flow and reactions.

02 · YOUNG WATER CARRIES RECENT MARKERS

Young water carries recent markers

Tritium, chlorofluorocarbons and sulfur hexafluoride identify much groundwater recharged in recent decades because their atmospheric histories are known.

03 · LONG-LIVED ISOTOPES REACH DEEPER TIME

Long-lived isotopes reach deeper time

Carbon-14 covers millennial scales, while chlorine-36 and krypton-81 can probe waters hundreds of thousands of years old. Each clock needs corrections and an appropriate age window.

FIG. 02 · INTERACTIVEHow the evidence fits together

A conceptual model: the geometry shows relationships, not measured scale.

1

Rain starts the clock

Water infiltrates soil and reaches an aquifer during recharge. Once isolated from modern air, dissolved gases and isotopes begin recording a subsurface history shaped by flow and reactions.

recharge
A conceptual model: the geometry shows relationships, not measured scale.

04 · A WELL BLENDS PATHWAYS

A well blends pathways

Pumping draws water from fractures and pores with different velocities. The measured concentration is therefore often a mixture of ages that a flow model must interpret.

05 · OLD DOES NOT AUTOMATICALLY MEAN SAFE

Old does not automatically mean safe

Ancient water may have avoided recent pollution, yet it can contain naturally dissolved arsenic, salts or radionuclides. Age and quality are separate measurements.

06 · SOURCES AND EVIDENCE

Sources and evidence

The explanation follows the evidence chain below. Links open the original source.

  1. 01
    Groundwater Age

    Supports a specific claim or evidence boundary used in this article.

    INSTITUTIONAL EXPLAINER ↗
  2. 02
    Isotope Hydrology

    Supports a specific claim or evidence boundary used in this article.

    INSTITUTIONAL EXPLAINER ↗
  3. 03
    Radiokrypton Dating of Groundwater

    Supports a specific claim or evidence boundary used in this article.

    PEER-REVIEWED REVIEW ↗
CHANGE LOG28 Aug 2026 · First five-language edition; claims, evidence boundaries and visual model checked.