The Waimakariri-Avon River system is an important component of the Christchurch aquifer
system and has been identified as one of, if not the, primary groundwater flow path. The
Waimakariri-Avon River system is ideally suited to geochemical tracing of surface water-
groundwater interaction and while many past studies have been undertaken to characterise
this system, in terms of its geochemistry and physical hydrogeological components, there is
still a large amount of uncertainty as to how long it takes for groundwater to flow from the
Waimakariri River, through the Waimakariri-Avon River groundwater system, to the springs
that feed the Avon River. The primary goals of this thesis were to;
1) Constrain the residence time of groundwater connecting the Waimakariri-Avon River
groundwater system using stable oxygen and hydrogen isotopes and analysis of anionic
concentrations of: chloride, fluoride, nitrate, nitrite, bromide and sulfate,
2) Provide additional evidence of a hydrological connection between the Waimakariri River
and the Avon River systems,
3) Present observations of the stable isotopic and anionic response of surface water to
rainfall events,
4) Identify stable isotopic and anionic surface water variation along the Waimakariri-Avon
River system, and establish the reasons for these.
This study presents the use of natural isotopic and anionic tracers to characterise the
residence time of the groundwater that flows between the Waimakariri and Avon Rivers, by
sampling surface water and meteoric water at sites that are part of the Waimakariri-Avon
River system. 375 samples were collected from 10 surface water and 4 rainwater sites
distributed across the Waimakariri-Avon River surface water-groundwater flow path
between March 5th and August, 2014. Additionally the study provides further stable isotopic
evidence of the connection between the Waimakariri and Avon Rivers, as well as presents
the variability of surface water chemistry in response to rainfall events. Identification of isotopic and anionic variation along the Waimakariri-Avon River system, by surface water
sampling, was also conducted to establish the probable causes of observed variations.
This study found that the use of large rainfall events, as natural tracers, was not conclusive
in establishing the groundwater residence time of the Waimakariri-Avon River system within
the 4.5 month sampling period available. Surface water sampling provided further evidence
in support of past studies that have determined an isotopic connection between the
Waimakariri River and the Avon River with mean stable isotopic values for the Waimakariri
River (-8.85‰ δ18O and-60.65 δD) and Avon River (-8.53‰ δ18O and -58.72 δD) being more
similar than those of locally derived meteoric water (-5.48‰ δ18O and -35.13 δD).
Observations of surface water chemistry variations thorough time determined and
identified clear responses to rainfall events as deviations from baseline values, coinciding
with rainfall events. Isotopic variation along the Waimakariri-Avon River system was shown
to reflect Waimakariri River derived shallow groundwater with the contributions from
rainwater increasing with increased proximity to the Avon River mouth.
Anionic profiling of the Waimakariri-Avon River system identified increasing concentrations
of chloride, nitrate, sulfate, nitrite and bromide, relative to the Waimakariri River, with
increased proximity to the Avon River mouth. Fluoride concentrations were identified in
lower concentration, relative to the Waimakariri River, with increased proximity to the Avon
River mouth. Fluoride and nitrite concentrations were attributed predominantly, if not
entirely, to an atmospheric source as mean concentrations were greater in meteoric waters
by a factor of at least 2, compared to surface water samples. Chloride and bromide have
been attributed to possible salt water mixing as a result of the interaction of upwelling
deeper groundwater with the marine and estuarine sands beneath the upper unconfined
aquifer, that act as a confining layer within the Christchurch aquifer system. Nitrate and
sulfate concentrations have been attributed to potential fertilizer usage and past land-use
impacts.
A significant step-change increase in chloride, bromide, nitrate and sulfate concentrations
was observed between the surface water sample sites at Avonhead Park and the University
of Canterbury. The step-change coincides with the boundary of the upper confining layer
within the Christchurch aquifer system, and explains the increases in chloride and bromide concentrations. It also suggests a widely distributed source area as concentrations do not
become diluted at the Avon River site, at Hagley Park, , which would be expected from the
addition of other tributaries, if they did not have similarly high chloride and bromide
concentrations. The area between these two sites has also been identified by Environment
Canterbury as potentially impacted by past agricultural land-use practices and may explain
the increases in nitrate and sulfate concentrations.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/11034 |
| Date | January 2015 |
| Creators | Tutbury, Ryan William Owen |
| Publisher | University of Canterbury. Geology |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Ryan William Owen Tutbury, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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