Two isotopes often used for groundwater dating are:

• A. Uranium-238 and Thorium-232
• B. Lead-210 and Potassium-40
• C. Oxygen-18 and Deuterium
• D. Tritium and Carbon

Answer: (D)

Groundwater dating relies on isotopes that decay at known rates to reveal how long water has spent underground. A common pair is Tritium and Carbon-14. Tritium is a radioactive form of hydrogen with a half-life of about 12 years, so its presence points to relatively recent recharge. Because much atmospheric tritium came from mid-20th-century nuclear testing, elevated tritium in groundwater signals recharge after that period, and measuring its decay helps estimate ages on the scale of decades. Carbon-14 has a half-life of about 5730 years, so it is suited to older waters. By measuring the dissolved inorganic carbon and applying corrections for exchange with soils and minerals, the remaining 14C indicates ages from thousands to tens of thousands of years. Using both isotopes lets scientists bracket groundwater ages across a wide range—from recent to ancient. The other isotopes listed are generally used for dating rocks, sediments, or tracing sources rather than providing the typical groundwater age ranges.