The use of 3H and tritiogenic 3He to determine CFC degradation and vertical mixing rates in Framvaren Fjord, Norway

Concentrations of CFC-11 and CFC-12, as well as tritium and helium isotopes were measured in the super-anoxic, sub-surface waters of Framvaren Fjord in Norway to assess the stability of CFCs in reducing environments. This fjord provides an ideal environment to study the degradation of CFCs because of its relatively simple dynamics and the narrow zone in which chemical reactions and biological activity, relevant to CFC degradation, occur in the water column. CFCs and tritium enter the fjord at the surface and are transported into the deep water primarily by turbulent vertical mixing. The turbulent vertical mixing rate was determined by fitting a 1D model to the vertical distributions of [3H] and [3H] + [3Hetrit], and found to be 0.006–0.008 cm2 s−1. Using these values, the degradation rates for CFC-11 and CFC-12 were obtained by comparing model simulations incorporating first-order degradation rates to the observed CFC profiles. CFC-11 is degraded at a rate of 6–9 yr−1 below 19 m depth where oxygen concentrations are zero. CFC-12 is found to have a maximum degradation rate of 0.01 to 0.03 yr−1.