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Carbon 14 dating of groundwater in confined aquifers: Implications of aquitard diffusion
Author(s) -
Sudicky E. A.,
Frind E. O.
Publication year - 1981
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/wr017i004p01060
Subject(s) - aquifer , diffusion , groundwater , geology , magnitude (astronomy) , advection , soil science , hydrogeology , steady state (chemistry) , groundwater flow , hydrology (agriculture) , geotechnical engineering , thermodynamics , chemistry , physics , astronomy
The influence of diffusive losses into fine‐grained aquitards on the 14 C age interpretation of groundwater in long, thin confined aquifers is examined. An analytical model is developed by coupling two one‐dimensional steady state differential equations, one representing advective‐dispersive transport in the aquifer and the other representing diffusion into the aquitards. The results indicate that aquitard diffusion is a mechanism that can cause a significant reduction in 14 C concentrations in a confined aquifer. Groundwater velocities derived from 14 C measurements that are unadjusted for diffusion would therefore be underestimated. The error that ensues is highly sensitive to the aquifer thickness and the magnitude of the diffusion coefficient. In contrast, the steady‐state distribution of 14 C is shown to be relatively insensitive to the magnitude of the longitudinal dispersivity value of the aquifer. Dispersivities representative of the upper range of values that have been reported in the literature produced a negligible difference in steady state 14 C concentration compared to results obtained using small dispersivities. An equation is presented which permits the adjustment of 14 C data to account for diffusion, providing that the aquifer‐aquitard system is not stratigraphically complex and heterogeneous. The analysis can be extended to include age‐dating isotopes that have half‐lives different from that of 14 C. The results of this study also apply to age dating in fractured porous media.