Hydrogeology of a thick clay-rich till and Cretaceous bedrock clay sequence in Saskatchewan, Canada
Shaw, Randall Joel
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Aquitards composed of clay-rich glacial till and Cretaceous bedrock-clay are common throughout the Interior Plains of North America. Characterizing groundwater flow through these aquitards has implications for aquifer recharge rates, water quality and waste disposal. Groundwater flow through a two-tiered aquitard system consisting of 80 m of clay-rich till overlying 77 m of Bearpaw Formation marine bedrock-clay was investigated at the King Test Site, Saskatchewan, Canada. Preconsolidation pressures measured on 10 samples of till (400 to 600 kPa) suggested that this unit was Battleford Formation till deposited during the last glaciation (12 to 18 ka BP). Preconsolidation pressures measured on 8 samples of bedrock-clay ranged from 6,000 to 10,000 kPa and indicated that as much as 1000 m of Late Cretaceous and Tertiary overburden was removed by erosion. The specific storage of the till and bedrock-clay, calculated from the rebound compressibility in the oedometer, was 2.4 x 10⁻⁴ and 1.0 x 10⁻⁴ m⁻¹ respectively. Analysis of 20 single-well response tests suggested that the hydraulic conductivity of the unoxidized till ranged from 4.4 x 10⁻¹⁰ to 2.4 x 10⁻¹¹ m/s. Laboratory steady-state, quasi-steady-state and transient hydraulic conductivity tests measured from 30 samples cut from aquitard core indicated that matrix hydraulic conductivity of the unoxidized till and the bedrock-clay are 2.3 x 10⁻¹¹ m/s and 4.3 x 10⁻¹² m/s, respectively. Analysis of δ⁻¹⁸O distributions and simulated advection-dispersion profiles suggested that bulk hydraulic conductivity of the till is < 9.6 x 10⁻¹⁰ m/s. This result, with the mass balance analysis, indicated that the bulk hydraulic conductivity of the bedrock-clay was 2.9 x 10⁻¹² m/s. For the scales investigated (0.02 to 100m), these results implied that hydraulic conductivity is independent of scale for these relatively thick (~80 m) clay-rich till and bedrock-clay deposits. The present-day downward groundwater velocity through this aquitard system is 1.3 m per 10 ka. Transient groundwater flows in the aquitards caused by glaciation and erosion were shown to have dissipated gradually (~3 ka) and have had a minimal effect on the flow regime for the past 9 ka. These findings advance our understanding of the nature of groundwater flow in low permeable sediments and assist in the practice of environmental engineering and water management throughout the Interior Plains of North America. Note: This thesis contains a map that has been sized to fit the viewing area. Use the zoom in tool to view the map in detail or to enlarge the text.