Updated: November 1, pm. Scientists estimate that Earth is approximately 4. Several different techniques were used to arrive at that number, but an important method for dating old rocks is radiometric dating.
This document is also available in pdf format: fs Information about the age of ground water can be used to define recharge rates, refine hydrologic models of ground-water systems, predict contamination potential, and estimate the time needed to flush contaminants from ground-water systems. CFCs also can be used to trace seepage from rivers into ground-water systems, provide diagnostic tools for detection and early warning of leakage from landfills and septic tanks, and to assess susceptibility of water-supply wells to contamination from near-surface sources. During the past 50 years, human activities have released an array of chemical and isotopic substances to the atmosphere.