Groundwater Transport in a Glaciomarine Aquitard: Paleosalinity and Landslide Implications

Leaching of marine salinity in the porewater of glaciomarine muds is one precursor to landslide hazard. In this study, groundwater modeling is used to quantify vertical groundwater flow, constrain paleosalinity, and characterize past and future progression of leaching with depth in Champlain Sea sediments. The Breckenridge Creek site, ~15 km northwest of Ottawa, Canada, was cored within a thick sequence (up to 98 m) of Champlain Sea muds that form a regional aquitard in the St. Lawrence Lowlands and Ottawa Valley. Porewater chloride concentrations ([Cl]), up to 12,250 mg/kg, and δ¹⁸O as high as -7.18‰, indicate remnant seawater. One-dimensional groundwater transport modeling simulates porewater [Cl] and δ¹⁸O with depth simultaneously and constrains specific discharge, q, from 2.40 to 2.51 mm/a. Groundwater transport modeling and three-component mixing of seawater, glacial meltwater and meteoric water constrain the range of initial [Cl] between 14,000 and 15,700 mg/kg (72-80% seawater) and initial δ¹⁸O between -5.99 and -5.61‰. The glacial meltwater component of Champlain Sea bottom waters at the Breckenridge site has a maximum δ¹⁸O value of -22.4‰. Downward leaching to the salinity threshold of <2 g/L for geotechnical sensitivity development reached a depth of 20.6 m. Modeling indicates the leaching front currently progresses at a rate of 2.5 m/1000 years, slower than advection of freshwater infiltration because of upward diffusion and dispersion of marine solutes. Notably for landslide hazard, the highest measurements of geotechnical sensitivity coincide with the leached zone.