HYDROCHEMISTRY AS AN INDEPENDENT GROUNDWATER AGE TRACER
– CASE STUDY: THE LOWER HUTT GROUNDWATER ZONE
Beyer, M.1,2 Jackson, B.1 Morgenstern, U.2 Daughney, C.2 Norton, K.1
1
2
Victoria University of Wellington
GNS Science
Introduction and Aims
Groundwater age or residence time is the time water has resided in the subsurface since
recharge. The determination of groundwater age can aid understanding and
characterization of groundwater resources, because it can provide information on
groundwater mixing and flow, and volumes of groundwater and recharge, etc.
Groundwater age can be inferred from environmental tracers, such as SF 6 and tritium.
Multiple tracers are often applied complementarily in order to increase the robustness of
age interpretations. To this end, it is desirable to develop cost-effective and easily
applicable age tracers/techniques to supplement the existing ones.
Hydrochemistry data are spatially and temporally widely available due to national and
regional groundwater monitoring programmes. Their determination is cost-effective and
relatively simple compared to the determination of existing age tracers. Hydrochemistry
has been used as an age proxy and has been suggested as a potential age tracer.
However, to date, the use of hydrochemistry as an independent age tracer has only been
demonstrated for water recharged weeks to months ago, by relying on seasonal changes.
This study assesses whether and under what conditions hydrochemistry can be used as
an independent groundwater age tracer over a wider age range (and as an age proxy and
indicator for recharge sources and weathering processes) in the Lower Hutt Groundwater
Zone, New Zealand. Comparison and combination of age information inferred from
hydrochemistry and tritium in our study allowed for demonstration of hydrochemistry for
use as an age tracer for groundwater recharged days to ~100 years ago.
Methods
The Lower Hutt Groundwater Zone (LHGWZ), located in a sedimentary basin in the
Wellington Region in New Zealand, appeared to be a suitable case study to assess
whether hydrochemistry can be used as an independent age tracer. Firstly, the LHGWZ
was believed to have a relatively simple/homogenous aquifer structure which was thought
to indicate relatively consistent, homogenous hydrochemistry-governing processes
throughout the aquifer. Secondly, hydrochemistry and age tracer data were available in
15 locations across the LHGWZ. Mean residence times (MRTs) inferred with
environmental tracers ranged from days to approximately 75 years. This rich dataset
allowed for comparison of hydrochemistry-inferred and tracer-inferred age information and
assessment of the complementary use of hydrochemistry and established age tracers for
groundwater dating.
To assess whether hydrochemistry can be used as a stand-alone or complementary
groundwater age tracer, the methodology illustrated in Fig. 1 was used. Firstly,
hydrochemistry data were collected from sites for which hydrochemistry-age relationships
were likely to exist (e.g. from sites located in one aquifer). The data were then assessed
for apparent internal consistency with regard to potential drivers and controlling factors
and to identify (potential) sub-datasets within the dataset (referred to as the data analysis
step in Fig. 1). This is necessary because there are various processes and conditions that
may affect the chemical composition of groundwater, aside from residence time and
groundwater mixing. This may hinder establishment of strong hydrochemistry-age
relationships and inference of age information from hydrochemistry, even if the sites were
all located in one aquifer.
After identification of potential sub-datasets, hydrochemistry-age relationships and age
information were inferred for each sub-dataset. For that a probabilistic approach illustrated
in Fig. 1 was used. Groundwater mixing was conceptualized by simplified LPMs (lumped
parameter models) from which the age distribution was determined. The following three
scenarios with regard to prior age information were used:
A) To assess whether groundwater age could be determined purely using hydrochemistry
and whether hydrochemistry could be used to confirm the mixing model, age
information at each site was not constrained a priori; a selection of LPM types (namely
the EPM, DM and PEM) were used.
B) To assess whether the often ambiguous tracer-inferred age information could be further
constrained with the aid of hydrochemistry, tracer-inferred age information were used
to establish hydrochemistry-age relationships.
C) To assess whether robustly pre-determined age information at one site would allow for
inferring of age information at the remaining sites purely based on hydrochemistry, the
age information was not constrained a priori except at one site for which tracer-derived
age information was used.
1) Data analysis
Initial dataset:
Assess internal consistency of dataset with
regard to potential drivers and controlling
factors and identify sub-datasets
Observed
hydrochemistry
Rnd generation
2) Inferring hydrochemistry-RT relationships
and age information
Model input
-mixing model- parameters
-hydrochemistry in recharge
-hydrochemistry alteration
with residence time
Mixing
Model
(LPM)
Simulated
hydrochemistry
Obj.
funct.
Behavioural
hydrochemistry
Model output
Figure 1: schema of 2-stepped modelling approach
Results
The main findings at our study site (the LHGWZ) include:
•
The hydrochemistry parameters Si, Na, Ca and TDS were successfully used as
complementary ‘age tracers’ to reduce uncertainty in age information inferred from
established age tracers and to resolve ambiguous tracer-derived MRTs (at 50% of the
affected sites).
•
When established age tracer data was unavailable, the parameters Si, Na, Ca and
TDS could act as groundwater age proxies to estimate the relative age of groundwater
and to distinguish older from younger water. This finding is in line with previous studies,
which have also demonstrated the use of hydrochemistry as an age proxy.
•
Although it was hoped to infer more information on the age distribution purely
based on hydrochemistry, this turned out difficult. No clear preference for any LPM type
was found. This indicated that the LPM type (or mixing model) could not be identified
purely using hydrochemistry in this study. It was speculated that it may be possible to
identify the mixing model if groundwater underwent more complex mixing than observed
in this study. Further study is needed to confirm this supposition.
•
The use of a reference age (i.e. tracer-inferred age information at one site) aided
the use of hydrochemistry as an ‘independent’ age tracer at the remaining sites. This
finding highlights the significant potential of using hydrochemistry to supplement age
information inferred from established techniques. The ‘complementary’ use of
hydrochemistry and other groundwater dating techniques appears to be a much more
cost-effective way to gain multiple age estimates than the sole use of expensive age dating
techniques.
Acknowledgement Thanks to Greater Wellington Regional Council for provision of
hydrochemistry data and financial support by SAC (smart aquifer characterization) project.