DISSOLVED NOBLE GASES REVEAL THREE-DIMENSIONAL FLOW PATTERNS OF VERY OLD GROUNDWATER IN SEDIMENTARY BASINS

Matching the results of reactive transport models to the distribution of ⁴He and ⁴⁰Ar in sedimentary basins reveals details of the three-dimensional patterns of basin groundwater flow. The isotopes are produced in the subsurface at constant rates by the radioactive decay of uranium and thorium both within and below basin strata, and accumulate in groundwater as it flows through basin strata.

Hydrologists have traditionally interpreted the distribution of these noble gas isotopes in terms of the piston flow model. This one-dimensional concept, however, is oversimplified because it accounts only for unidirectional advective transport, ignoring cross-formational flow, diffusion, and dispersion. At best, the piston flow model gives less information than is available from the isotope distributions; at worst, it provides misleading results.

We used the BASIN2 software, implemented within the USGS UCODE program, which optimized the fit between observed and predicted isotope concentrations, to model ⁴He and ⁴⁰Ar transport through basin strata. In the Great Artesian Basin of Australia, the distribution of ⁴He along the basal J aquifer follows an enigmatic pattern that resists interpretation using one-dimensional concepts. Modeling results show that the ⁴He concentration varies by about two orders of magnitude from the bottom of the aquifer to the top. Transport in the aquifer, therefore, differs sharply from the piston flow model.

The ⁴He concentration observed along the top of the aquifer is sensitive to cross-formational flow: where water recharges downward, concentration is depressed, but it increases sharply where water discharges upward. As such, the ⁴He distribution directly reflects the regional and subregional localization of recharge and discharge areas within the basin.

In the Paris basin, the distribution of noble gas isotopes again reflects not just transport along the aquifers, but advection and diffusion across stratigraphy in the deep basin. Upward discharge along fault zones carries the isotopes from depth into shallower strata. The discharge leaves a zone of slow flow in the deep basin, helping explain contradictory characterizations of groundwater in the basin as both flowing and stagnant.