| Summary: | <p>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<sup>-4</sup> and 1.0 x 10<sup>-4</sup> m<sup>-1</sup>
,
respectively. Analysis of 20 single-well response tests suggested that the hydraulic
conductivity of the unoxidized till ranged from 4.4 x 10<sup>-10</sup> to 2.4 x 10<sup>-11</sup> 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<sup>-11</sup> m/s and 4.3 x 10<sup>-12</sup> m/s, respectively.
Analysis of δ<sup>18</sup>O distributions and simulated advection-dispersion profiles suggested that
bulk hydraulic conductivity of the till is < 9.6 x 10<sup>-10</sup> m/s. This result, with the mass
balance analysis, indicated that the bulk hydraulic conductivity of the bedrock-clay was
2.9 x 10<sup>-12</sup> 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.</p>
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