Duncan Johannessen

The use of isotopes and suites of isotopes to aid in hydrogeological studies is increasing as analytical techniques and understanding of isotope processes improve. However, these techniques are rarely used commercially because they can be expensive and their usefulness has not been fully proven. This thesis was designed to demonstrate the use of a number of isotopes in gaining an understanding of the groundwater processes in the Maritimes. The study was carried out in the Avon Valley located about 55km northwest of Halifax and stretching roughly 30km along the #14 highway from Card Lake to Windsor, N.S. This valley was chosen because it straddles two typical Nova Scotian geographic and geologic settings; the granitic highlands and the sedimentary lowlands. Furthermore, the valley is known to host uranium deposits allowing radioisotopes to be a viable part of the study. Samples of lake, stream, and domestic well waters were taken in the summer of 1998 with some follow up sampling in 1999. The samples were analysed mainly for general chemistry as well as isotopes of oxygen, hydrogen, carbon, strontium, and uranium and the concentration of radon. Nitrogen and radium were not recovered in sufficient amounts to allow isotopic analysis. Most of the hydrogen analyses were not done due to technical laboratory problems.

Having considered various working hypotheses, the thesis proposes a preferred model that is most compatible with the results of these analyses. The processes below appear to strongly affect the waters of the Avon River basin.

The rainfall has a sea-salt signature and has low pH due to anthropogenic pollution. Some of this rain collects in the abundant lakes of the granitic highlands where it retains the slight saltiness and low pH but picks up minor dissolved components of the granites, including uranium and radiogenic strontium. The lakes are also strongly influenced by solid and dissolved organic matter, as is shown by the tea-brown colour of the water.

The lake water infiltrates into the ground where sudden and drastic changes take place. The study area is covered by glacial till, much of which contains mineral material from the Carboniferous sediments in the lowlands to the north. Any limestone and gypsum in these sediments are readily dissolved by these aggressive, low pH waters resulting in higher pH, alkalinity, and conductivity. By far the dominant component of these waters is calcium carbonate and its addition is reflected in a shift in the carbon and strontium isotopes as well as an increase in uranium concentration due to its higher solubility as a uranyl-carbonate complex. These more mature groundwaters also have the opportunity to mix with older water such that their oxygen isotope signature is closer to the yearly average while the surface waters reflect the current seasonal average. Thus the highlands act as a very short-term recharge area while the lowlands receive the discharge of mature and mixed waters.

This model suggests high flushing rates, which decrease the sensitivity of the system to natural and anthropogenic pollution. The model also suggests that the highland area is susceptible to groundwater shortage in the event of low rainfall conditions even if those conditions are on the scale of only a few years. The sensitivity of the lowland groundwater depends upon the reservoir capacity of the bedrock both in the highlands and the lowlands.

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Pages: 130
Supervisor: Dorothy Godfrey-Smith