Hydrochemical study of the desalinisation of quick clays in the Holocene at Tiller-Flotten, Norway

Abstract

The development of quick clays has been studied for over 50 years in Norway to evaluate and mitigate risk of quick clay landslides. One of the main remaining challenges is qualifying the spatial distribution of quick clays and understanding the hydrological processes driving the desalinisation of quick clays, which determines their susceptibility to landslides. Quick clays are susceptible to landslides because of their loosely bound structure, so called “house of cards” structure, which has been weakened by the leaching of the salts from the clays, which were originally deposited in a marine environment. Recent advances in analytical techniques present an opportunity to enhance our understanding of quick clay hydrochemistry. The objective of this study is to provide insight into the desalinisation process and determine which type of freshwater acts as the primary driver of the desalinization of the originally marine pore water in quick clays.

To achieve this objective, the pore water chemistry of a large set of samples with a dense spacing (>1-2 m) was studied at the Tiller-Flotten quick clay research site near Trondheim. Pore water samples were extracted from depths of 4 to 19 m and hydrochemistry analysed using ion chromatography (IC) and inductively coupled plasma optical emission spectrometry (ICP-OES). After the hydrochemical analysis, ion composition and ionic ratios of the pore water were compared to rainwater, seawater and groundwater from neighbouring bedrock and quaternary aquifers. The results indicate a freshening of the quick clay pore water, though all samples remain relatively more saline than quaternary groundwater, bedrock groundwater or rainwater. The studied clay deposits appear to be influenced by slow groundwater flow, with cation composition stabilizing below a depth 8 meters. However, the upper part of the clay deposits appears to be chemically weathered, which has likely been driven by infiltration of rainwater. These findings are significant as they confirm that locally infiltrated rainwater and groundwater flow play a critical role in the desalinization process and the overall development of quick clays, impacting their stability and susceptibility to landslides.