Paleohydrogeology of the Cretaceous sediments of the Williston Basin using stable isotopes of water

Abstract

[1] Hydraulic and isotopic data collected from aquifers are routinely used to characterize hydrogeological conditions within sedimentary basins, but similar data from confining units are generally not collected despite their ability to provide insights on important water/solute transport controls. We characterized paleogroundwater flow and solute transport mechanisms across 384 m of Cretaceous shale (aquitard) in the Williston Basin, Canada, using high-resolution depth profiles of water isotopes ( 18 O, 2 H). Water samples were also collected from wells installed in the underlying regional sandy aquifer (Mannville Fm; 93 m thick) and from seepage inflows into potash mine shafts (to 825 m below ground). The 1-D numerical transport modeling of 18 O profiles provided insight into large-scale/longterm solute transport in both Cretaceous sediments and the basin. Despite the potential for significant advective migration during glaciations, molecular diffusion appears to be the dominant solute transport mechanism through the aquitard. Simulations suggest average vertical groundwater velocities of <0.05 m/10 ka and an average excess hydraulic head of <10 m; these values are much less than anticipated by successive glaciations. The dominant paleoevent reflected in present-day profiles is introduction during the Pleistocene of glaciogenic meteoric water to the aquifer underlying the shale, likely along an aquifer outcrop area east of the site or through local vertical conduits. Simulations suggest these recharge events occurred during one or more glacial periods. The isotopic profile over the upper 25 m of Pleistocene till and shale is consistent with glacial deposition and transport processes within these units over the Holocene (past 10 ka).

Determining the stable isotope composition of pore water from saturated and unsaturated zone core: improvements to the direct vapour equilibration laser spectrometry method

“…The DVE-LS method has since been adopted and modified to obtain high-resolution isotopic profiles to define solute transport mechanisms, timing of hydrogeologic events, and sources of waters in near-surface environments such as soils and the unsaturated zone (< 15 m depths; Garvelmann et al, 2012;Hendry and Wassenaar, 2009;Klaus et al, 2013;Mueller et al, 2014;Stumpp and Hendry, 2012;Sprenger et al, 2015), to intermediate depths (< 100 m depths; Hendry et al, 2011a;Filippini et al, 2015), and on deep geological cores at basinal scales (< 500 m depths; Harrington et al, 2013;Hendry et al, 2013). Since the original DVE-LS publication , we have applied this technique to a number of field studies and made a number of important technical adaptations gained from the experience of thousands of pore water isotopic analyses.…”

“…Solid red squares represent the source water for rotary drilling, and solid and open stars represent isotopically spiked drill waters for core holes for samples trimmed in the field and laboratory, respectively. The local meteoric water line (LMWL) for Saskatoon, Saskatchewan, is also presented with associated 95 % confidence intervals (after Hendry et al, 2013). Gravimetric water content of all samples ranged from 7.8 to 31.3 % (n = 101).…”

Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments

“…Subsamples of each core (250-300 g) were placed under 50 MPa pressure in a Carver Series NE mechanical press with a 0.5-μm filter placed at the base of the squeezing chamber, which was placed within an Ar atmosphere to minimize oxidation. A syringe was attached to the base of the apparatus and 15 mL of filtered pore water were collected for analyses within 3.5 to 6.0 h (Hendry et al, 2013). 20…”

Cl− as a Chemical Fingerprint of Solute Transport in the Aquitard-Aquifer System of the North Jiangsu Coastal Plain, China

“…Many researches have been conducted on the thicker, nonfractured clay-rich and shale units with the bulk hydraulic conductivity (K) below 1 × 10 -10 m/s [3][4][5][6]. Under such conditions, solute transport in the aquitards is demonstrated to be dominated by molecular diffusion [7,8].…”

“…However, the water-bearing units (aquifers or sand streaks) with various thicknesses were often interspersed within aquitards [6,[10][11][12] and particularly marked in the late Quaternary sediments of coastal plains in China due to the transgression-regression effect [13]. Critically, these occurrences could cause perturbations in transport paths and partial advection solute transport in porewater profiles and lead to problems with the interpretation of the palaeohydrogeological information [12].…”