Controls on the long‐term downward transport of δ²H of water in a regionally extensive, two‐layered aquitard system

Abstract

[1] Seven high-resolution (0.3-0.6 m depth intervals), 1-D vertical profiles of the d 2 H of pore water were collected across a 300 km 2 study area in southern Saskatchewan, Canada, to define the vertical controls on solute transport in a >120 m thick, two-layered aquitard system. The 1-D profiles were augmented with an existing d 2 H profile collected from a previous study. The surficial aquitard in the area consists of Quaternary deposits (either glacial till or lacustrine deposits; 13 to 128 m thick) underlain by an upper Cretaceous claystone aquitard (80-110 m thick). The shape of the individual d 2 H profiles and associated 1-D transport modeling suggest diffusion is the regionally dominant vertical transport mechanism across the aquitards. The profile shape is controlled by the thickness of the Quaternary deposit and the d 2 H value at the upper boundary, which coincides with the depth of the water table. The upper boundary d 2 H value varies considerably across the area (À149% to À101%), perhaps due to differences in local hydrological conditions (e.g., slope, aspect, infiltration) across the landscape. Modeling of all profiles shows the timing for till deposition and the timing of climate change during the Holocene are consistent across the area ($30 ka and 7-10 ka before the present, respectively), corroborating other studies. This study provides insights into the hydrogeologic controls on solute transport in an aquitard system and associated geologic and climatic changes for a prairie region over the past 30 ka, and improves our understanding of initial and time-dependent transport boundary conditions for the study of aquitards.

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

“…Sample exposure to air following core opening was limited to a few minutes. Coring, sampling, and spiking of drill fluids used in these studies are described in Hendry et al (2011a).…”

“…Auger drilling and hand sample collection methods have major depth and media limitations. Because it is often difficult using auger drilling to collect solid samples at depths > 30 m or in highly consolidated geologic media, rotary drill methods that employ drilling fluids and split spoon core barrels are required to obtain core samples from deeper (< 500 m BG) formations (Hendry et al, 2011aSchmeling, 2014). To evaluate the impact of the required drilling fluids and potential to contaminate the core pore water, a series of techniques were employed using drilling fluids spiked with D 2 O tracer.…”

“…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.…”

Characterization of deep aquifer dynamics using principal component analysis of sequential multilevel data

“…Depth-dependent samples from deep aquifers are usually obtained during drilling (Williams and Chou, 2007;Hendry et al, 2011); hence interpretation of aquifer dynamics from these single-time data sets is limited. In the present study we explored deep aquifer dynamics using a unique set of observations from two sequential MLS campaigns.…”

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

“…The solute transport mechanism could be inferred to be advection if solute concentrations had no obvious vertical change; otherwise, curvilinear concentration profiles were attributed to molar diffusion transport [57,58]. In order to detect the influence of sand layers on Cl − transport, vertical 1D diffusion transport (see (1)) was postulated to be the dominant migration mechanism throughout the aquitard-aquifer system, because obvious vertical distributions appeared in the Cl − concentration profiles ( Figure 4).…”