Transport and geochemical controls on the distribution of solutes and stable isotopes in a thick clay-rich till aquitard, Canada

Abstract

A comprehensive understanding of the transport and geochemical processes controlling solutes in clay-rich aquitard confining units is needed to accurately predict the long-term migration of contaminants into the subsurface. To this end, the geochemical and stable isotopic composition of porewaters in the upper 22 m of a thick, unoxidized and nonfractured clay-rich, till aquitard (Sutherland Group) was examined in detail. The aquitard is overlain by about 8 m of oxidized and fractured till (Floral Fm). Concentrations of TDS, SO₄ ²⁻, HCO₃ ⁻, Cl⁻, Na⁺, Mg²⁺, Ca²⁺ and porewater deuterium were greater in the Floral Fm and decreased with depth through the aquitard. The elevated and seasonably variable solute concentrations in the oxidized Floral Fm were attributed to geochemical weathering and dynamic water movement through fractures. Good fits between measured δ²H, TDS, SO₄ ²⁻, Cl⁻ and HCO₃ ⁻ profiles through the aquitard and simulated solute transport profiles were obtained by diffusion (without advection) with transport times of 4–6 ka. The deficiency of geochemical reactions affecting HCO₃ ⁻ and SO₄ ²⁻ in the aquitard was supported by δ¹³C_DIC and δ³⁴S_SO4 analyses. Geochemical and isotope mass balance modelling (NETPATH) indicated that diffusive mixing with minor calcite dissolution and ion exchange could account for the distribution of Na⁺, Ca²⁺ and Mg²⁺ in the aquitard. Results of this study further suggested that microbiological activity in the aquitard was limited. With minor exceptions, the solute and isotopic profiles, their transport and the controlling geochemical reactions in the Sutherland are similar to those determined at another clay-rich till aquitard, 160 km south of this site, suggesting that geochemical and biological processes in some clay-rich aquitards may have a minimal effect on the migration of dissolved constituents.

Keywords:

• Aquitards
• Carbon 13
• Chemical evolution
• Deuterium
• Groundwater
• Sulfur 34
• Transport

Both authors are indebted to Professor Peter Fritz. He introduced both of us to the world of stable and radiogenic isotopes and their application to solving environmental and hydrogeologic questions during our Ph.D. tenure at the University of Waterloo, Canada. As for this paper, we thank G. van der Kamp who provided insight into the hydrogeology of the site; J. Cherry who provided comments on an early version of the manuscript, and K. Keller and V. Remenda who installed the instrumentation at the Warman site and conducted much of the early research upon which this paper was based. The funding for this study was provided by the National Water Research Institute, Environment Canada and Natural Sciences and Engineering Council of Canada.