Exploring subsurface ground water and geochemical rock interactions during drainage of a surface water reservoir in Switzerland

This thesis examines the spatial and temporal controls on groundwater chemistry in a fractured granitic environment. The research was conducted at the Grimsel Test Site, Switzerland (GTS), where draining and refilling of a nearby surface water reservoir induced microseismicity In the surrounding rocks. The GTS is a network of closely monitored tunnels and boreholes approx 300-500m below ground surface.Characterisation of background geochemical conditions over a two-year period consisted of a time series of physiochemical, major and minor dissolved ion chemistry, stable isotope analysis and dissolved organics, for boreholes spanning the length of the GTS. Results show poor fracture connectivity; physiochemical and dissolved ion chemistry are dominated by water-rock reactions between infiltrating meteoric waters and spatially varying host rock lithology.A new technique is developed that compares the differing signatures of dissolved organic compounds (2D-gas chromatographs) found within surface soils, river sediments and the lake, to those found in groundwater samples from the GTS. Results show that organic signatures are well-preserved and that different groundwater samples can be traced to different surface infiltration sites. This organic fingerprinting technique has the potential to be a powerful new tool for determining groundwater origins. Analysing the groundwater data over time, identified no changes to major or minor ion chemistry, but repeated drops in groundwater pH (1-3 units) were observed during periods of reservoir drainage.These drops were concurrent with nearby shallow (<1 km below ground surface) microearthquakes -1.2 < ML <1. Experiments to crush granite from the GTS in the presence of equilibrated groundwater, and in the absence of oxygen, were able to reproduce similar pH drops with no changes to water chemistry. This is the first evidence that microseismic events cause substantial pH drops, these findings have significant implications for a wide variety of geological processes.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral-pg

Grantor dc:publisher.institution

University of Strathclyde

Year dc:date.issued

2020

Author dc:creator

• Stillings, Mark

Advisor dc:contributor.advisor

• Shipton, Zoe

DOI

10.48730/p0ev-aq45

Identifier

T15664

Author Identifier

201576348

OAI identifier oai:identifier

oai:strathclyde:sq87bt648