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Origin and Distribution of Sulfate in a Fractured Till in Southern Alberta, Canada
Author(s) -
Hendry M. J.,
Cherry J. A.,
Wallick E. I.
Publication year - 1986
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/wr022i001p00045
Subject(s) - sulfate , geology , carbonate , groundwater , sulfur , bedrock , weathering , water table , geochemistry , mineralogy , hydrology (agriculture) , geomorphology , chemistry , geotechnical engineering , organic chemistry
Glacial till in a 22‐km 2 area of the Interior Plains Region of southern Alberta, Canada consists of an upper brown gypsiferous weathered zone (up to 20 m thick) and a lower discontinuous grey nonweathered zone (up to 25 m thick). Although the total sulfur content of the two till zones is the same (0.34%), the mean total sulfate content of the weathered till (4.0 mg g −1 ) exceeds that of the nonweathered till (0.8 mg g −1 ). Similarities suggest that all SO 4 2− in the nonweathered till and 20% of the sulfate in the weathered till was derived from sulfate‐rich bedrock fragments incorporated in the till during deposition. Laboratory experiments and δ 18 O and δ 34 S analyses of total SO 4 2− and the sulfur forms in the two zones show that the majority of the sulfate in the weathered till was derived from the oxidation of organic S at some time after the till was deposited. Mass balance and flux calculations indicate that this oxidation occurred under partially saturated conditions. Groundwater flow simulations suggest that the water table was much lower and that a thick partially saturated zone was possible during the drier Altithermal period (11,000 to 3000 years B.P.). The following chemical processes account for the major ions that are found in the groundwater but were generated when the partially saturated zone existed. The oxidation of reduced S produced SO 4 2− and H + . The H + reacted with carbonate minerals under elevated P CO 2 , with Ca 2+ loss by exchange for Na + and gypsum precipitation. Volume shrinkage in the weathered till resulting from these geochemical processes may have enhanced the development of fractures and caused the associated increased hydraulic conductivity.