# Groundwater Transport in a Glaciomarine Aquitard: Paleosalinity and Landslide Implications

## TL;DR

This study uses groundwater modeling to understand leaching of ancient seawater in glaciomarine muds, which affects landslide risks in the Ottawa region.

## Contribution

The paper introduces a 1D transport model combining chloride and oxygen isotope data to quantify leaching and paleosalinity in glaciomarine sediments.

## Key findings

- Porewater chloride and δ18O data indicate remnant seawater with initial concentrations of 14,000–15,700 mg/kg Cl.
- Leaching front progresses at 2.5 m/1000 years, slower than freshwater advection due to upward diffusion of marine solutes.
- High geotechnical sensitivity, linked to landslide risk, coincides with the leached zone in the muds.

## Abstract

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.

Transient 1D modeling of both [Cl] and δ18O demonstrates that advection and diffusion contribute to solute transport in porewater of glaciomarine Champlain Sea muds. Samples with the highest sensitivity (>30) for landslide hazard occur in the current leached zone.