A hydrogeological and hydrogeochemical study of the evolution of ground water in a fractured granite, Holyrood Newfoundland

Sargent, Nicolas J. (1994) A hydrogeological and hydrogeochemical study of the evolution of ground water in a fractured granite, Holyrood Newfoundland. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (12MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

An extensive set of hydrogeological and geochemical data has been collected from a 150 m deep borehole in the near coastal discharge area of the Holyrood Granite, Newfoundland. Because of the selected location of the borehole, over the underlying saltwater wedge, it was possible to intersect flow paths that are believed to have extended considerably deeper within the aquifer than the depth of the borehole. -- The physical data indicate that the geometric average of the hydraulic conductivity over the length of the borehole is approximately 4 x 10⁻⁹ m/s and that the open fractures, controlling most of the flow in the aquifer are approximately vertical and parallel to the coast. Fractures of this nature have been postulated by others to have resulted from isostatic post glacial rebound. The hydraulic conductivity (measured at approximately 2 m intervals along the entire length of the borehole) shows a decrease of approximately three orders of magnitude with depth. Based on the hydraulic conductivity measurements made in the study borehole, known hydraulic gradients and probable flow-path length, it is believed that water samples collected may have had residence times of the order of 1000 a. -- Inspection of aqueous chemical data reveals that some parameters have a strong correlation with depth while other analytes have a strong inverse correlation with hydraulic conductivity. In general the water quality reflects the effect of low-temperature weathering of an alumino-silicate rich granite. However, the water samples collected were all relatively rich in chloride. Though there is no direct evidence of the source or sources of chloride in the study area, evidence from other granitic terrains indicate that the likely sources of chloride in the groundwater are from the rock mass (possibly from fluid inclusions) and from seawater. -- Study and analysis of fracture plane mineralogy using X-ray diffraction (XRD) and scanning electron microscopy (SEM) has provided evidence for a suite of minerals which may control the groundwater chemistry. Thermodynamic speciation calculations using the water analysis data indicates minerals including calcite, amorphous silica, kaolinite, and some varieties of feldspar may be precipitating. Subsequent mass balance modelling using the groundwater analyses could not identity a groundwater evolutionary scheme that was consistent with the speciation calculations unless both a seawater source of chloride and a rock source of chloride were invoked. However, the results indicated that the percentage of seawater mixed with the groundwater decreased with depth, while the fraction of chloride added from the hypothetical rock source increased with depth. This, together with the inverse correlation of dissolved silica with hydraulic conductivity, probably reflects the increasing importance of rock-water interaction with depth, in-turn reflecting increasing aquifer residence times at increasing depths. -- Oxygen and hydrogen isotope data collected during the study indicate that both isotope systems show a strong correlation with depth and both are increasingly enriched in their light isotopes with depth. The decrease in ¹⁸O and ²H abundance with depth is consistent with the expected differences in altitudes of recharge between the deepest groundwater samples collected (believed to have been recharged at approximately 170 m above sea level), and the shallowest groundwater samples collected (believed to have been recharged at 60 m above sea level). This evidence indicates that flow through the aquifer (although it occurs in discontinuous fractures) is, on a large scale, roughly equivalent to well ordered porous granular flow. -- The interpretation of the isotope data together with the results of mass balance modelling suggests an internally consistent evolutionary scenario. It is proposed that the modelled decline of the saltwater component with depth reflects the addition of marine aerosols that provide chloride to recharging waters in quantities that decrease with increasing distance inland. -- The increase in the modelled rock-derived chloride source with depth is consistent with the increasing degree of low-temperature rock-water mass exchange with increasing depth, itself resulting from increased aquifer residence time with depth.

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