Mechanisms controlling the distribution and transport of C-14 in a clay-rich till aquitard

The distribution and transport of radiocarbon in dissolved inorganic carbon (DIC) in a thick clay-rich till aquitard was investigated. Radiocarbon act ivities decreased with depth from 109 percent modern carbon (pmC) in the su rficial oxidized and fractured zone (at 1.5 m BG) through the thick underly ing unoxidized and nonfractured aquitard, reaching minimum values of betwee n 3 and 5 pmC at between 29 and 37 m depth. Numerical simulations of diffus ive transport of radiocarbon from the oxidized zone into the unoxidized zon e during the Holocene closely approximated the measured values and showed t hat diffusive transport and radioactive decay fully described the transport of radiocarbon in the aquitard, The lack of geochemical reactions involvin g DIC permitted the age of DIC in the pore water deposited,vith the till (b etween 29 and 37 m BG) to be estimated at 25 to 31 ka BP. Results of this s tudy showed that the use of radiocarbon in the study of solute transport in aquitards provides age information that cannot be obtained from natural co nservative tracers such as delta(2)H and delta(18)O. Our results also showe d that the application of conventional radiocarbon age-dating models based on piston-flow transport assumptions, is inappropriate in diffusion-dominat ed environments and considerably overestimates the age of the ground water.