37 Cl- 35 Cl variations in a diffusion-controlled groundwater system

Details 37 Cl- 35 Cl variations in a diffusion-controlled groundwater system 37 Cl- 35 Cl variations in a diffusion-controlled groundwater system

Aug 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.1757d&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 50, Issue 8, pp.1757-1764

Mathematics

12

Scientific paper

Groundwaters in the low-permeability Quaternary glacial deposits of southwestern Ontario commonly exhibit increasing Cl - concentrations with depth. Because these deposits were formed in a freshwater glacial environment and because Cl - is not derived from weathering of these deposits, previous investigations hypothesized that upward diffusion of saline formation waters from the shallow bedrock caused this Cl - trend. This hypothesis was tested by measuring 37 Cl / 35 Cl ratios in the groundwaters from two typical sites and comparing the results with theoretically derived isotopic distributions produced by diffusion. 37 Cl measurements, expressed relative to standard mean ocean chloride (SMOC), ranged from -0.8 to +2.2% and vertical profiles showed a gradual enrichment in 35 Cl relative to 37 Cl with increasing distance upward from the bedrock-till interface. 537 Cl distributions were simulated with a mathematical solute transport model incorporating the effects of kinetic isotopic partitioning. A sensitivity analysis indicated that the model results were affected by the groundwater velocity, the isotope enrichment factor and diffusion times. Agreement between the simulated and observed Cl - isotope profiles was obtained using a groundwater velocity of 0, an isotopic enrichment factor of 1.0012 and diffusion times ranging from 14,000 to 16,000 years. Based on these results, we conclude that the isotopic enrichment in 35 Cl was caused by upward diffusion over a period of about 15,000 years. This diffusion period is consistent with what is known about the Quaternary history of the area.

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