• Subsurface Production of Chlorine-36 and Its Impact on Ground Water Dating

      Kuhn, Mark W.; Davis, Stanley N.; Zito, Richard; Bentley, Harold W.; University of Arizona, Tucson, Arizona (Arizona-Nevada Academy of Science, 1983-04-16)
      Chlorine-36 is an important radioisotope with which to date old ground water. The initial chlorine-36 in ground water originates in the atmosphere by cosmic ray spallation of argon-40. Following precipitation and infiltration processes, the natural decay of this radioisotope is then used to date ground water. One must consider, however, the production of chlorine-36 in the subsurface. The production reaction of most interest is ³⁵C1 + neutron → ³⁶C1 + gamma. Buildup of chlorine-36 in the subsurface can result from cosmic ray secondary neutrons near the surface and natural radioactivity produced neutrons below the surface. These production mechanisms, if not taken into consideration, will contribute to the error in chlorine-36 age determinations. To predict subsurface production rates, field measurements were made of thermal neutron fluxes for various geologic materials and depths below the surface. Thermal neutron fluxes were found to vary by more than three orders of magnitude. Theoretical calculations of neutron flux were compared to filed measurements. Estimates of chlorine-36 production rates were then calculated and compared to measured values of chlorine-36 in very old ground water, where decay rates have been hypothesized to be equal to production rates.