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Hydrol. Earth Syst. Sci., 19, 803–822, 2015
www.hydrol-earth-syst-sci.net/19/803/2015/
doi:10.5194/hess-19-803-2015
© Author(s) 2015. CC Attribution 3.0 License.
Using groundwater age and hydrochemistry to understand
sources and dynamics of nutrient contamination through
the catchment into Lake Rotorua, New Zealand
U. Morgenstern1, C. J. Daughney1, G. Leonard1, D. Gordon2, F. M. Donath3,4, and R. Reeves4
1GNS Science, P.O. Box 30368, Lower Hutt, New Zealand
2Hawke’s Bay Regional Council, Private Bag 6006, Napier, New Zealand
3Department of Applied Geology, Georg-August-Universität Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany
4GNS Science, Private Bag 2000, Taupo, New Zealand
Correspondence to: U. Morgenstern (u.morgenstern@gns.cri.nz)
Received: 18 July 2014 – Published in Hydrol. Earth Syst. Sci. Discuss.: 28 August 2014
Revised: 22 December 2014 – Accepted: 10 January 2015 – Published: 5 February 2015
Abstract. The water quality of Lake Rotorua has steadily de-
clined over the past 50 years despite mitigation efforts over
recent decades. Delayed response of the groundwater dis-
charges to historic land-use intensification 50 years ago was
the reason suggested by early tritium measurements, which
indicated large transit times through the groundwater system.
We use the isotopic and chemistry signature of the ground-
water for detailed understanding of the origin, fate, flow path-
ways, lag times and future loads of contaminants. A unique
set of high-quality tritium data over more than four decades,
encompassing the time when the tritium spike from nuclear
weapons testing moved through the groundwater system, al-
lows us to determine detailed age distribution parameters of
the water discharging into Lake Rotorua.
The Rotorua volcanic groundwater system is complicated
due to the highly complex geology that has evolved through
volcanic activity. Vertical and steeply inclined geological
contacts preclude a simple flow model. The extent of the
Lake Rotorua groundwater catchment is difficult to establish
due to the deep water table in large areas, combined with in-
homogeneous groundwater flow patterns.
Hierarchical cluster analysis of the water chemistry pa-
rameters provided evidence of the recharge source of the
large springs near the lake shore, with discharge from the
Mamaku ignimbrite through lake sediment layers. Ground-
water chemistry and age data show clearly the source of nu-
trients that cause lake eutrophication, nitrate from agricul-
tural activities and phosphate from geologic sources. With a
naturally high phosphate load reaching the lake continuously
via all streams, the only effective way to limit algae blooms
and improve lake water quality in such environments is by
limiting the nitrate load.
The groundwater in the Rotorua catchment, once it has
passed through the soil zone, shows no further decrease in
dissolved oxygen, indicating an absence of bioavailable elec-
tron donors along flow paths that could facilitate microbial
denitrification reactions. Nitrate from land-use activities that
leaches out of the root zone of agricultural land into the
deeper part of the groundwater system must be expected to
travel with the groundwater to the lake.
The old age and the highly mixed nature of the water
discharges imply a very slow and lagged response of the
streams and the lake to anthropogenic contaminants in the
catchment, such as nitrate. Using the age distribution as de-
duced from tritium time series data measured in the stream
discharges into the lake allows prediction of future nutri-
ent loads from historic land-use activities 50 years ago.
For Hamurana Stream, the largest stream to Lake Rotorua,
it takes more than a hundred years for the groundwater-
dominated stream discharge to adjust to changes in land-
use activities. About half of the currently discharging water
is still pristine old water, and after this old water is com-
pletely displaced by water affected by land use, the nitrogen
load of Hamurana Stream will approximately double. These
timescales apply to activities that cause contamination, but
also to remediation action.
Published by Copernicus Publications on behalf of the European Geosciences Union.
804 U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources
1 Introduction
Detailed information on groundwater age distribution is re-
quired for the Lake Rotorua catchment to understand the
agricultural contaminant loads that travel from land to the
lake with the groundwater and discharge via springs and
streams into the lake, with a large lag time. The water qual-
ity of Lake Rotorua has declined continuously over the past
50 years, despite cessation of direct-to-lake sewage discharge
in 1991 (Burger et al., 2011) and the fencing-off of streams
in grazing land in parts of the lake catchment.
Land use in the catchment has intensified significantly
over the past 60 years and is now predominantly forest
(39%), pasture (27%) and dairy (9%) (Burger et al., 2011;
Rutherford et al., 2009). Increasing nitrate concentrations
had been observed in virtually all of the major streams flow-
ing into the lake during the period 1968–2003 (Hoare, 1987;
Rutherford, 2003). We measured nitrate concentrations of 6–
10mgL−1NO3–N in three young groundwater samples un-
der dairy farms in the SE catchment. In the absence of sig-
nificant overland runoff, nutrients from land use are trans-
ported with the water through the groundwater system to
the lake. Early tritium measurements indicated large tran-
sit times through the groundwater system (the subject of
this study). With a time lag>50 years in the groundwa-
ter system, nitrate loads to the lake may be expected to in-
crease further in the future due to delayed arrival of nutri-
ents from historic land use as they ultimately discharge from
the groundwater system via the springs and streams into the
lake. This trend will be exacerbated by any further intensifi-
cation of land use within the catchment over recent decades,
as this recently recharged water has largely not yet reached
the streams (Morgenstern and Gordon, 2006).
Groundwater age is a crucial parameter for understanding
the dynamics of the groundwater and the contaminants that
travel with the water. Determining water age, and hence tran-
sit times, allows identification of delayed impacts of past and
present land-use practices on water quality (Böhlke and Den-
ver, 1995; Katz et al., 2001, 2004; McGuire et al., 2002; Mac-
Donald et al., 2003; Broers, 2004; Moore et al., 2006), and
for identification of anthropogenic versus geologic impacts
on groundwater quality (Morgenstern and Daughney, 2012).
Understanding the dynamics of groundwater is fundamen-
tal for most groundwater issues. Water age is defined by the
transit time of water through catchments and hence is vital
for conceptual understanding of catchment processes such as
response to rainfall, stream flow generation, recharge source
and rate (McGuire and McDonnell, 2006; Morgenstern et al.,
2010, 2012; Stewart et al., 2010; Cartwright and Morgen-
stern, 2012). Water age, being directly related to fluid flux, is
also very useful for calibrating numerical surface water and
groundwater transport models (Goode, 1996; Burton et al.,
2002; Molson and Frind, 2005; Bethke and Johnson, 2008).
Water age provides important information on vulnerability
to contamination and can therefore be used to assess the se-
curity of drinking water supplies, particularly from ground-
water bores (Darling et al., 2005; Morris et al., 2005; New
Zealand Ministry of Health, 2008). Water age measurements
can also be used to quantify rates of hydrochemical evolu-
tion resulting from water–rock interaction (Katz et al., 1995;
Burns et al., 2003; Glynn and Plummer, 2005; Daughney et
al., 2010; Beyer et al., 2014). These applications of water
dating cover the spectrum from applied water resource man-
agement to fundamental scientific research.
In all of the above-mentioned applications it is important
to constrain not only the mean age of water, but also the dis-
tribution of ages within a sample from the groundwater dis-
charge. Transit time determinations in catchment hydrology
typically identify a range of water ages contributing to stream
flow, and the time- and location-dependent distribution of
transit times provides insight into the processes that gener-
ate runoff (Maloszewski and Zuber, 1982; McGuire and Mc-
Donnell, 2006; Stewart et al., 2007; McDonnell et al., 2010).
Use of water age determinations for calibration of numerical
transport models must also account for the full distribution
of age and its variation in space and time (Goode, 1996; Cor-
naton et al., 2011; Cornaton, 2012). Assessment of the secu-
rity of drinking water from groundwater bores also requires
an understanding of the water’s age distribution (Eberts et
al., 2012; Morgenstern, 2004). For example, New Zealand
legislation states that a water supply bore is considered se-
cure (unlikely to have a risk of contamination by pathogenic
organisms) when less than 0.005% of the water has been
present in the aquifer for less than 1 year (New Zealand Min-
istry of Health, 2008).
For the Lake Rotorua catchment study, tritium is the tracer
of choice. Tritium dating can be applied to both river/stream
water and groundwater, whereas gas tracers are less suitable
for surface waters that are in contact with air. Tritium ages,
in contrast to gas tracer ages, include travel through the un-
saturated zone (Zoellmann et al., 2001; Cook and Solomon,
1995); travel times can be >40 years through the thick un-
saturated zones of the Rotorua catchment ignimbrite aquifers
(Morgenstern et al., 2004). Tritium is not subject to trans-
formation, degradation or retardation during water transport
through the catchment. Tritium dating is applicable to water
with mean residence times of up to about 200 years (Cook
and Solomon, 1997; Morgenstern and Daughney, 2012), as is
typical of New Zealand’s dynamic surface waters and shal-
low groundwaters. In addition, monitoring the movement
of the pulse-shaped bomb-tritium through groundwater sys-
tems is an excellent opportunity to obtain information about
the age distribution parameters of the groundwater. This is
particularly useful in groundwater systems, such as the Ro-
torua system, that have high uncertainties within flow mod-
els due to a deep water table and preferential flow paths. Fi-
nally, tritium is a particularly sensitive marker for study of
the timing of nitrate contamination in groundwater, because
the main anthropogenic nitrate contamination of groundwa-
ter systems started coincidentally with the bomb-tritium peak
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U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources 805
from the atmospheric nuclear weapons testing after WWII;
water recharged before this post-war upsurge in intensive
agriculture has low tritium and low nitrate concentrations.
For the Rotorua catchment we have an extensive data set
available over time and space. Tritium time series data for the
main lake inflows cover more than 4 decades, and data cover-
ing the last decade are available with an extremely high spa-
tial resolution of about 100 sites in the Lake Rotorua catch-
ment. Tritium concentration can be measured at GNS Sci-
ence with the required extremely high accuracy using 95-fold
electrolytic enrichment prior to ultralow-level liquid scin-
tillation spectrometry (Morgenstern and Taylor, 2009). Tri-
tium is highly applicable for groundwater dating in the post-
bomb low-tritium environment of the Southern Hemisphere,
as bomb tritium from atmospheric thermonuclear weapons
testing has now been washed out from the atmosphere for
20 years, as is described in detail in Morgenstern and Daugh-
ney (2012).
The objective of this study is to understand the origin,
fate, flow pathways, lag times, and future loads of contam-
inants that cause lake eutrophication in the Lake Rotorua
catchment, central North Island, New Zealand. This will as-
sist in mitigating the deterioration in lake water quality since
the 1960s (Rutherford et al., 1989) that threatens the lake’s
significant cultural and tourist value. Environmental hydro-
chemistry tracers and age tracers are used to identify the
recharge source of the main water discharges to Lake Ro-
torua, to identify the source of the contaminants (anthro-
pogenic versus geologic), and to evaluate the water age dis-
tributions in order to understand groundwater processes, lag
times, and the groundwater flow dynamics. The Rotorua
groundwater system is complicated due to the catchment’s
highly complex geology, which has evolved through volcanic
activity, and due to the deep water table of >50 m in large ar-
eas, which prevents detailed groundwater studies and intro-
duces uncertainty in catchment boundaries and flow patterns.
The complex geology leads to inhomogeneous groundwater
flow patterns, as indicated by large parts of the catchment
having particularly large positive or negative specific water
yields (White et al., 2004). The groundwater discharge in the
northern catchment is unusually large for the size of the sur-
face water catchment, probably due to preferential flow paths
that route groundwater towards the north across the surface
slope in this part of the catchment.
The tritium and age distribution data are currently being
used to calibrate a numeric groundwater transport model.
The use of this rich data set for groundwater transport model
calibration is part of a larger investigation that evaluates the
calibration of hydrological and hydrogeological models us-
ing hydrochemical data, including tracers of water age, with
the aim of using tracer-calibrated groundwater models for
nutrient transport and economic modelling (e.g. Lock and
Kerr, 2008; Rutherford et al., 2009), ultimately supporting
optimal and sustainable land and water management in catch-
ments. The broader findings from the Rotorua investigation
will be applied to the many other New Zealand catchments
for which time series age tracer data are available (Stew-
art and Morgenstern, 2001; Morgenstern, 2004; Stewart and
Thomas, 2008; Gusyev et al., 2014).
2 Hydrogeological setting
2.1 Geology
Lake Rotorua is located in a roughly circular caldera basin
in the central North Island, New Zealand (Fig. 1), situated
in the Taupo Volcanic Zone (TVZ), an area of silicic volcan-
ism with NW–SE extension and geothermal activity roughly
60km wide by 300km long that is related to subduction
of the Pacific plate beneath the Australian plate off New
Zealand’s east coast (Wilson et al., 1995; Spinks et al., 2005).
Figure 1 shows the surficial geology. Mesozoic greywacke,
which outcrops to the east and west of the TVZ, forms the
basement rocks in the area. Younger formations are predom-
inantly rhyolite ignimbrites, rhyolite and dacite lava domes,
and lacustrine and alluvial sediments derived from these vol-
canic lithologies. Note that “rhyolite” is sometimes used col-
loquially to refer to rhyolite lava, but the use of “rhyolite” is
here applied only as a formal compositional definition (vol-
canic rocks>69% SiO2). Deposits of rhyolite composition
may be pyroclastic (explosively formed, including airfall de-
posits and the pyroclastic flow deposit termed “ignimbrite”)
or they may be lavas (effusively erupted without explosion).
Rhyolite lavas are viscous and often push up into high lava
domes over the eruption vent. The geological formations and
processes of greatest relevance to the hydrology and hydro-
geology of the Lake Rotorua catchment area are shown in the
three-dimensional geological model of White et al. (2004)
(Fig. 2) and summarized in the following paragraphs.
From 2million to 240 thousand years ago (ka), a num-
ber of rhyolite lava domes were emplaced and volcanic ac-
tivity from TVZ calderas resulted in pyroclastic deposits
across the area, including the highly welded Waiotapu ign-
imbrite (ca. 710ka; “older ignimbrite” in Fig. 1) and a range
of variably welded, variably altered, sometimes-jointed ign-
imbrites of which the Matahina, Chimp and Pokai formations
(ca. 320–270ka) are most significant and mapped within
“undifferentiated rhyolite pyroclastics” in Fig. 1. These ig-
nimbrites are expected to be the main basal units for ground-
water aquifers in the study area (White et al., 2004).
The period from 240 to 200ka is defined by the erup-
tion that deposited the Mamaku Plateau Formation (240ka)
and formed the Rotorua Caldera. The caldera collapse down-
faulted parts of older lava domes positioned across the west-
ern and northern edge of the caldera. The Mamaku Plateau
Formation is predominantly composed of ignimbrite (here-
after “Mamaku ignimbrite”), which is variably welded, vari-
ably jointed and very permeable. Several rhyolite lava domes
(mainly Ngongotaha and its neighbours) began to develop
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806 U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources
Figure 1. Location and geology of the Lake Rotorua catchment, with sampling sites. The assumed groundwater catchment is from White and
Rutherford (2009). Surficial geology is based on the 1:250000 map of Leonard et al. (2010). For abbreviations of the names of the major
streams refer to Table 1. The approximate trace of the caldera is shown. Cross-section A0–A is shown on the cut-away face of Fig. 2.
soon after the caldera collapse. Also, soon after the eruption,
a lake began to form in the collapsed depression, leading to
the deposition of lacustrine fine ash and pumice, commonly
referred to as lacustrine sediments (Leonard et al., 2010);
note that these sediments have sometimes been referred to as
Huka or Huka Group sediments throughout the TVZ, but this
definition formally refers only to specific units near Taupo
City and the term Huka is avoided here.
From 200 to 61 ka, volcanic activity in the vicinity of Lake
Rotorua was relatively subdued. A number of eruptions from
the Okataina Volcanic Centre (OVC), located to the east of
the Rotorua caldera, produced widely dispersed but relatively
thin airfall deposits. These pyroclastic materials caused peri-
odic damming of drainage pathways and led to fluctuations
in lake level that in turn resulted in widespread and variably
thick sediments being deposited in the Rotorua Caldera. This
period of relatively quiet volcanic activity ended with the
Rotoiti and Earthquake Flat eruptions from the OVC, which
produced widespread pyroclastic deposits, including the non-
welded ignimbrites of the Rotoiti Formation and the Earth-
quake Flat Formation.
From 61ka to present, numerous eruptions from the OVC
(the most recent of which was in 1886) deposited airfall lay-
ers in the Lake Rotorua catchment area. Numerous rhyolite
lava units were also emplaced during this period. The pe-
riodic deposition of pyroclastic materials, along with activ-
ity on faults of the Taupo Rift (Leonard et al., 2010), pre-
sumably caused fluctuations in the lake level, with current
lake level being reached sometime within the last few thou-
sand years (White et al., 2004). Due to the decline of lake
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U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources 807
level, Holocene alluvial sand and gravel deposits are found
in stream channels and around the current lake shoreline.
The southern Rotorua basin hosts a vigorous geothermal
system producing many hot water, hot mud, steam and geyser
features, along with gas emission, between the southern edge
of the Lake and about the southern edge of the caldera
(Fig. 1). There is hot local groundwater flow in this area, gen-
erally flowing down-hill northwards into the lake. Beyond
this relatively confined area the groundwater system does not
appear to interact with fluids from this geothermal system.
2.2 Hydrology
Lake Rotorua has a surface area of 79km2and a mean depth
of 10.8m (Burger et al., 2011), with a total water volume of
0.85km3. The assumed total catchment area is ca. 475km2
(White and Rutherford, 2009) (Fig. 1).
Annual rainfall in the catchment is strongly affected by to-
pography and varies from more than 2200mm northwest of
the lake to less than 1400mm southeast of the lake (Hoare,
1980; White et al., 2007; Rutherford et al., 2008). Approxi-
mately 50% of rainfall infiltrates into the groundwater sys-
tem. This is based on two sources of information: (1) com-
parisons of rainfall and actual evapotranspiration that have
been made for various parts of the catchment (Hoare, 1980;
Dell, 1982; White et al., 2004, 2007; Rutherford et al., 2008);
and (2) data from paired lysimeters, a standard rain gauge
and a ground-level rain gauge installed at Kaharoa (White
et al., 2007) (Fig. 1). With 50% of rainfall recharge, to-
tal infiltration into the groundwater system is estimated to
be 14500Ls−1, based on the catchment shown in Fig. 1,
excluding rainfall inputs direct to the lake, and assuming
recharge is 50% of rainfall. This rainfall recharge supports
stream flow and potentially direct inputs of groundwater to
the lake.
There are nine major streams (Fig. 1, for abbreviations
refer to Table 1) and several minor streams that flow into
the lake; the remainder of the inflows are provided from
direct inputs of rainfall and lake-front features, and poten-
tially from groundwater seepage through the lake bed. The
major streams are baseflow-controlled and characterized by
very constant water flow (Hoare, 1980) and temperature, and
groundwater-derived baseflow accounts for approximately
90% of the average flow in the typical Rotorua stream
(Hoare, 1987). Baseflows in the nine major streams entering
Lake Rotorua cumulatively amount to 11800 Ls−1, and to-
tal inflows to the lake from minor streams and lake-front fea-
tures amounts to 350Ls−1(Hoare, 1980; White et al., 2007)
(Table 1).
With the lake water volume of 0.85km3, the lake water
turnover time via the groundwater-fed streams is 2.2 years.
The only surface outflow occurs through Ohau Channel via
Lake Rotoiti (Fig. 1). Water balance calculations suggest that
the total catchment area exceeds the surface water catch-
ment area (White et al., 2007); in other words, groundwater
Figure 2. Three-dimensional geological model of the Lake Rotorua
catchment area (from White et al., 2007). The location of the verti-
cal cut-away face A0–A is shown in Fig. 1. The vertical exaggeration
is 5 ×, with a 1 km vertical scale bar shown in the centre. The colour
scheme is similar to that in Fig. 1 (bottom to top): pre-Mamaku for-
mations (grey), Mamaku Plateau Formation (light blue), sediment
formation (dark yellow), lava domes (red) and Holocene alluvial
deposits (light yellow).
from outside the surface water catchment is flowing through
the aquifer system to Lake Rotorua (White and Rutherford,
2009).
2.3 Hydrogeology
Groundwater flow in the Lake Rotorua catchment is in-
fluenced by several fundamental geological characteristics.
First, the Mamaku ignimbrite, the dominant hydrogeological
feature in the catchment, is assumed to be up to 1 km thick in
the centre of the caldera depression, and from about 200 to
tens of metres thick outside of the caldera, decreasing gener-
ally with distance from the caldera (Fig. 2). Ignimbrite tends
to fill in pre-existing valleys and landforms, so its thickness
can be quite variable over horizontal distances of as little as
hundreds of metres. Transit times of groundwater through
such a thick aquifer may be lengthy compared to times in
the shallow alluvial aquifers used for water supply in many
other parts of the world and New Zealand. Second, the ign-
imbrites in the Lake Rotorua catchment are known to be vari-
ably welded, altered and jointed, with the potential for pref-
erential groundwater flow paths. Groundwater may be routed
from its recharge area along lengthy preferential flow paths
and discharge in neighbouring surface catchments, leading to
water ages that vary substantially even within a localized area
(the presence of such preferential flow paths can therefore
be demonstrated with age tracers). Third, the broadly circu-
lar collapse faults of the caldera constitute a major structural
feature that may influence the flow of groundwater within the
catchment.
The major water contribution to Lake Rotorua is from the
western catchment that drains the eastern flanks of the Ma-
maku Plateau (Fig. 1). The Mamaku ignimbrite formation
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808 U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources
Table 1. Estimates of baseflow from White et al. (2007). Abbrevia-
tions refer to Fig. 1.
Stream Abbreviation Baseflow
(Ls−1)
Hamurana Ham 2750
Awahou Awh 1700
Waiteti Wtt 1300
Ngongotaha Ngo 1700
Waiowhiro Wwh 370
Utuhina Utu 1600
Puarenga Pua 1700
Waingaehe Wgh 250
Waiohewa Whe 390
Minor streams n/a 350
serves as a major source of groundwater in the area (Gor-
don, 2001). A large area (∼250km2) is drained by several
major springs (>1000Ls−1) emerging from the ignimbrite
on the western side of Lake Rotorua. Given the large extent
and thickness of the ignimbrite aquifer, a large groundwater
reservoir exists, with long water residence times expected in
the aquifer. Taylor and Stewart (1987) estimated the mean
residence time of the water of some of the springs as 50–
100 years.
The post-240ka ignimbrites in this area (and some lava
domes) are extremely porous; they sustain hardly any over-
land water flow (Dell, 1982), with most of the stream beds
dry throughout most of the year except during heavy rain, and
they allow the infiltrated water to percolate down to an ex-
tremely deep groundwater table>50m (the ignimbrite for-
mations around Ngongotaha Dome are an exception).
Little is known about the hydrogeology of the groundwa-
ter system; borehole data collected by drillers is often not
of sufficient quality to identify and correlate aquifer units.
Rosen et al. (1998) developed a schematic model for the Ma-
maku ignimbrite, with a lower and upper ignimbrite aquifer
sheet considered permeable, and a middle sheet considered
impermeable but fractured and not acting as an aquitard for
the other two sheets. This is probably an over-simplification
in many areas (see Milner et al., 2003) but does point at hor-
izontally planar discontinuities within the formation that ap-
pear to influence groundwater flow. The water, after easy pas-
sage through the large aquifer, is forced to the surface only
1–2km before the lake shore via large groundwater springs,
feeding large streams that drain into Lake Rotorua.
The water-bearing lava dome formations that predate the
Mamaku ignimbrite are likely to have fracture flow, based
on spring discharge permeability analysis, varying depending
on fracture sizes and linkages. Faulting associated with the
Rotorua caldera has offset several of the rhyolite domes and
groundwater may flow through these faults.
The palaeo-lake sediments that post-date the Mamaku ig-
nimbrite and rhyolite lava formations comprise silt, sand and
gravel (ignimbrite, obsidian and rhyolite pumice) and are
considered permeable, with lenses of low permeability that
can also act as confining layers.
Overall, understanding of the Rotorua groundwater sys-
tem is complicated due to the highly complex geology that
has evolved through volcanic activity. Vertical and steeply
inclined geological contacts are common, precluding a sim-
ple horizontal-layer-based succession model throughout the
catchment usually applicable in sedimentary basins. Aquifers
have not been well determined due to insufficient bore log
data, and also the extent of the Lake Rotorua groundwater
catchment is difficult to establish, due to the deep water table
of>50m in large areas at the catchment boundaries, com-
bined with an inhomogeneous groundwater flow pattern, as
indicated by the groundwater discharge in the northern catch-
ment being too large compared to the size of the surface wa-
ter catchment.
3 Methods
3.1 Determination of water age
The age of the groundwater at the discharge point charac-
terizes the transit time of the water through a groundwater
system. For groundwater dating, we use tritium time series
(repeated sampling after several years), and the complemen-
tary tracers tritium, CFCs and SF6together where possible.
The method of dating young groundwater with mean ages of
less than 200 years for current New Zealand Southern Hemi-
spheric conditions is described in detail in Morgenstern and
Daughney (2012, Sects. 2.3 and 2.4). In short: tritium dating,
in previous decades problematic due to interference from the
artificial tritium produced by atmospheric nuclear weapons
testing in the early 1960s, has now become very efficient and
accurate due to the fading of the bomb-tritium.
For groundwater dating, one or more tracer substances are
measured that have a time-dependent input into the ground-
water system or a well-defined decay-term (e.g. radioactive
decay). The tracer concentration data are then fitted using
a lumped-parameter model (Maloszewski and Zuber, 1982;
Zuber et al., 2005). For dating young groundwater – i.e. water
less than about 100 years – the most commonly used tracers
are tritium, chlorofluorocarbons (CFCs) and sulfur hexafluo-
ride (SF6) (Cook and Solomon, 1997; Edmunds and Smed-
ley, 2000; Stewart and Morgenstern, 2001; Morgenstern et
al., 2010). The measured output tracer concentration in the
groundwater (Cout) is then compared to its tracer concentra-
tion at the time of rainfall input (Cin) using the convolution
integral
Cout(t ) =
∞
Z
0
Cin(t −τ )e−λτ g (τ )dτ, (1)
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U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources 809
where tis the time of observation, τis the transit time (age),
e−λτ is the decay term with λ=ln(2)/T1/2(e.g. radioactive
decay of tritium with a half-life T1/2of 12.32 years) and g(τ )
is the system response function (Cook and Herczeg, 1999;
Zuber et al., 2005).
The system response function accounts for the distribu-
tion of ages within the water sample, for example from mix-
ing of groundwater of different ages within the aquifer, or
at the well (Maloszewski and Zuber, 1982, 1991; Goode,
1996; Weissman et al., 2002; Zuber et al., 2005). The two
response functions most commonly used are the exponen-
tial piston flow model and the dispersion model (Zuber et
al., 2005). The exponential piston flow model combines the
piston flow model, assuming piston flow within a single flow
tube in which there is minimal mixing of water from different
flow lines at the discharge point (e.g. confined aquifer), and
the exponential model, assuming full mixing of water from
different flow paths with transit times at the groundwater dis-
charge point that are exponentially distributed (e.g. mixing
of stratified groundwater at an open well in an unconfined
aquifer). The response functions of the various models are
described in Maloszewski and Zuber (1982) and Cook and
Herczeg (1999). To interpret the ages of the Lake Rotorua
catchment data set, the exponential piston flow model was
used, given by
g=0 for τ < T (1−f ) (2)
g=1
Tf e(−τ
Tf +1
f−1)for τ≥T (1−f ), (3)
where Tis the mean residence time (MRT), fis the ratio of
the volume of exponential flow to the total flow volume at
the groundwater discharge point, and T(1−f) is the time it
takes the water to flow through the piston flow section of the
aquifer (Maloszewski and Zuber (1982) use the variable η;
η=1/f ). When f=0 the model becomes equivalent to the
piston flow model, and when f=1 it becomes equivalent to
the exponential model.
The two parameters of the response functions, the MRT
and the distribution of transit times (f), are determined by
convoluting the input (tritium concentration in rainfall mea-
sured over time) to model water passage through the hydro-
logical system in a way that matches the output (e.g. tritium
concentrations measured in wells or springs). Because of its
pulse-shaped input, tritium is a particularly sensitive tracer
for identifying both of these two parameters, which can be
deduced uniquely by comparing the delay and the disper-
sion of the bomb-pulse tritium in the groundwater to that
from tritium in the original rain input. This method is partic-
ularly useful for interpretation of ages of groundwater in the
Lake Rotorua catchment, where most of the groundwater dis-
charges lack any other information on mixing of groundwater
with varying flow path lengths and of different age, such as
ratio of confined to unconfined flow volume, or screen depth
for wells.
For tracer age interpretation, the integral (Eq. 1) was used
to convolute the historical rainfall tracer input to an output
that reflects mixing in a groundwater system, with the best
match of the simulated output to the measured output time
series data (Fig. 3). The TracerLPM workbook (Jurgens et
al., 2012) was used. The tritium input function is based on
concentrations of tritium in rainfall measured monthly since
the 1960s at Kaitoke, near Wellington, New Zealand (Mor-
genstern and Taylor, 2009). The Kaitoke rainfall input func-
tion is multiplied by a scaling factor of 0.87 to account for
variation in atmospheric tritium concentrations due to lati-
tude and orographic factors, as deduced from measurements
from rain at various locations in New Zealand (e.g. Morgen-
stern et al., 2010). For the prevailing New Zealand climatic
conditions there is no need for correction of the tritium input
for seasonal infiltration (Morgenstern et al., 2010).
The problem of ambiguity in tritium dating over the last
decades is demonstrated in Fig. 3. Hangarua Spring dis-
charges old water with a mean residence time of about
90 years (see below), but during the late 1980s its tritium
concentration was similar to that of very young water (rain
curve in Fig. 3). At that time, the tritium concentration in
Hangarua Spring would have been in agreement with both
very young water and old water with a mean residence time
of 90 years. Tritium data covering several decades, however,
clearly distinguish this old water (low tritium concentration)
from young rain water. Figure 3 also shows that due to the
fading of the bomb-tritium in recent decades (tritium decay
over four tritium half-lives since the bomb spike), in recent
years the tritium concentration of old water is clearly distin-
guishable (lower) from that of young water, without ambigu-
ity. The tritium time series data allow also for constraining
groundwater mixing models. Figure 3 shows the model out-
put curves that match the measured tritium data. Given suf-
ficient analytical accuracy, this is also possible for extremely
low tritium concentrations; the data for Hamurana water in-
take spring (blue in Fig. 3) are all below 0.4TU, which is
below the detection limit of many tritium laboratories (http://
www-naweb.iaea.org/napc/ih/IHS_programmeihltric.html).
The application of mixing models is described in Mor-
genstern and Daughney (2012, Sect. 2.6). Throughout New
Zealand, for springs and wells in almost all hydrogeolog-
ical situations, the exponential piston flow model, with its
age distribution, has produced good matches to most (about
a hundred) tritium time series data. It was not, however, pos-
sible to obtain adequate matches in the ignimbrite area of the
Rotorua catchment using such a simple exponential piston
flow model. Alternatively, using the dispersion model did not
improve the matches. The complex volcanic aquifers of the
Lake Rotorua catchment, which have evolved through vol-
canic activity, require a more complex system response func-
tion. A combination of two exponential piston flow models
was used.
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810 U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources
3.2 Sample collection and analysis
Samples were collected from 41 springs, from
31 groundwater-dominated stream flow sites, and from
26 groundwater wells. To obtain the residence times of
the water discharging into the lake after passage through
the entire groundwater system, sampling focused on the
naturally flowing groundwater discharges, the springs and
streams. Samples were collected at times of base flow
conditions.
All nine major streams were sampled multiple times near
the inflow into the lake, typically 3–4 times (Figs. 3 and 4).
Most of these tritium time series go back to the early 1970s
and encompass the passage of the “bomb” tritium peak
through the groundwater system, allowing determination of
detailed age distribution parameters for these major inflows
to the lake. These “historic” samples had been collected spo-
radically for various projects over the decades to study the
transfer of the bomb-tritium through the hydrologic cycle.
Over the recent decade, the streams have also been sampled
for tritium at various points upstream, at various main conflu-
ences, or at main springs to obtain a detailed spatial distribu-
tion of water ages. Springs and wells were also once sampled
for CFCs, SF6, argon and nitrogen, to obtain complementary
age information.
Sampling locations are shown in Fig. 1. Many of the sites
have no road access, with some of them in remote steep gul-
lies. A portable sampling system was required for the gas
samples to allow fresh water from the well or spring to be
pumped into the sample bottles from below the water sur-
face without air contact. We used a pneumatic Bennet pump,
powered from a cylinder of compressed air at the remote lo-
cations, and from a compressor powered by the car battery at
sites with car access. Sampling from streams (tritium only)
involved simply dipping the bottle under the water surface
and filling the bottle.
Sampling methods for hydrochemistry and nutrients were
according to Daughney et al. (2007). Age tracer samples
were collected without filtration or preservation. For tritium,
a 1L plastic bottle was filled to the top. For CFC samples,
two 125mL glass bottles with aluminium liner cap were
filled, rigorously excluding air contact by filling from the bot-
tom via a nylon tube and three times volume replacement be-
low the surface of the overflowing sample water. 1L bottles
were filled for SF6.
Analytical details for hydrochemistry are described in
Daugney et al. (2007). Details of the tritium analysis pro-
cedure are described in Morgenstern and Taylor (2009).
While the early tritium measurements in the 1970s were
performed with a detection limit of approximately 0.1 Tri-
tium Units (TU), we now achieve significantly lower de-
tection limit of 0.02 (TU) via tritium enrichment by a fac-
tor of 95 and reproducibility of tritium enrichment of 1%
via deuterium calibration. Analysis procedures for CFC-
11, CFC-12, and SF6are described in van der Raaij and
Figure 3. Tritium rain input for the Rotorua catchment, and mea-
sured tritium output at Hangarua Spring and at Hamurana water
intake spring. The input curve is based on monthly measurements
in Kaitoke near Wellington, New Zealand, scaled to the latitude of
Rotorua with a factor 0.87, and smoothed by an exponential pis-
ton flow model with 0.3 years mean residence time and 50% ex-
ponential flow within the total flow volume. One TU=one tritium
atom per 1018 hydrogen atoms. For the spring samples one-sigma
measurement errors are shown. Note the logarithmic scale of the
TU axis.
Beyer (2014). Detection limits are 3×10−15 mol kg−1for
CFCs and 2×10−17 molkg−1for SF6. Dissolved argon and
nitrogen concentrations were measured for estimating the
temperature at the time of recharge, and the excess air con-
centration, as described by Heaton and Vogel (1981), for cal-
culation of the atmospheric partial pressure (ppt) of CFCs
and SF6at the time of recharge.
4 Results and discussion
In the following section hydrochemistry cluster analysis and
hydrochemistry evolution are discussed to assess the geo-
graphic sources of groundwater and groundwater processes
in the aquifer. The nutrients nitrate, sulfate, potassium and
phosphate are discussed to evaluate their source (anthro-
pogenic versus geologic), lag time, fate and impact on lake
eutrophication. The age distributions of the groundwater dis-
charges to Lake Rotorua are discussed to understand the con-
ceptual groundwater flow pattern and the lag time in the
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U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources 811
Figure 4. Tritium time series data, together with their matching
lumped-parameter model outputs, for six major streams. Grey line
is tritium input via rain from Fig. 3. The locations of the streams are
shown in Fig. 1.
groundwater system. The ultimate goal of this project is the
use of the hydrochemistry and groundwater age parameters
for calibration of a groundwater transport model for im-
proved management of the nutrient loads to the lake – the
subject of follow-up papers.
4.1 Groundwater age interpretation
To obtain the unique solution for both parameters of the age
distribution for a specific model, time series data are required
(Sect. 3.1). Most of the large water inflows into Lake Ro-
torua have long time series data available (up to over four
decades), allowing for well constrained age distribution pa-
rameters for both MRT and the fraction fbetween different
flow models (or Péclet number for the dispersion model). The
tritium time series data, together with the matching lumped-
parameter model simulations, are shown in Fig. 3 for two of
the large springs, and in Fig. 4 for six of the large streams
(covering 2/3 of stream baseflow to Lake Rotorua). For the
sites with shorter time series data (subcatchment stream dis-
charges, groundwater wells), most of the sites have at least
sufficient time series or multi-tracer data for unambiguous
robust age interpretations. If fraction fcannot be established
Table 2. Age distribution parameters for the two binary exponential
piston flow models (EPMs) for the major stream discharges to Lake
Rotorua. Average MRT is the mean residence time between the two
EPMs.
Stream EPM1 Fraction EPM2 Average
MRT1 f1 of EPM1 MRT2 f2 MRT
[year]
Hamurana 185 0.82 0.65 12 0.77 125
Awahou 80 1.00 0.92 6 0.91 75
Waiteti 60 1.00 0.78 3 0.90 45
Ngongotaha 35 1.00 0.82 1 0.91 30
Waiowhiro 40 0.63 1.00 n/a n/a 40
Utuhina 85 0.60 0.70 1 1.00 60
Puarenga 44 1.00 0.95 2 1.00 40
Waingaehe 160 0.94 0.90 3 1.00 145
Waiohewa 55 1.00 0.75 1 1.00 40
uniquely from the tritium time series data, we applied mixing
models that matched long tritium time series data from other
sites with similar hydrogeologic settings to these sites. All
96 sites with tritium time series or tritium and complemen-
tary CFC and SF6data have unambiguous age interpretation.
For the tritium time series data shown in Figs. 3 and 4, the
lumped-parameter models, with their respective age distribu-
tion parameters that match the measured data, are listed in
Table 2.
Throughout New Zealand, and including all hydrogeo-
logic situations (but mainly groundwater wells), we have
measured approximately a hundred long tritium time se-
ries covering several decades. A simple lumped-parameter
model, the exponential piston flow model, usually can match
these time series data well (e.g. Morgenstern and Daugh-
ney, 2012). The long-term tritium data from most of the
large stream discharges shown in Fig. 4, however, cannot
be matched by a simple model such as the exponential pis-
ton flow or dispersion model and require a more complex
groundwater flow model combination. Using a binary mix-
ing model, with parallel contributions from two exponential
piston flow models, resulted in excellent matches. We justify
this binary mixing model by inferring two different flow con-
tributions in the catchment to stream and spring flow – from
deep old groundwater, as indicated by very deep groundwa-
ter tables in the area (generally >50m), and from younger
groundwater from shallow aquifers, as indicated by minor
stream flows maintained by shallow aquifers. In Table 2 are
also listed the average mean residence times between the
two parallel models, weighted by their fraction within the
total flow. For the MRTs, errors caused by our tritium mea-
surement error and uncertainty in tritium input are typically
±1 year for MRTs<5 years, ±2 years for MRTs between
5 and 10 years, ±3 years for MRTs between 10 and 50 years,
±5 years for MRTs between 50 and 100 years, and larger er-
rors for older water towards the detection limit.
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812 U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources
For convenience, the average MRTs are also listed in Fig. 4
next to their model output curve. It is obvious from Fig. 4
that all the main streams discharge very old water into Lake
Rotorua. The tritium response of the streams is clearly dis-
tinguishable from that of young rain water (grey line). The
youngest water discharge, Ngongotaha Stream (green), has
an average MRT of 30 years. All other main streams dis-
charge significantly older water, up to MRT of 145 years
for Waingaehe Stream (dark blue). Note that even though
bomb-tritium from atmospheric nuclear weapons testing in
the 1960s has decayed enough to no longer cause ambigu-
ous age interpretations, it is still possible to detect the tail of
the bomb-tritium for matching model parameters if tritium
analyses have sufficient accuracy; all data for the Hamurana
water intake spring (blue in Fig. 3) are below 0.4TU.
Most of the streams and springs discharge very old
groundwater into Lake Rotorua, with MRTs typically be-
tween 50–150 years, indicating discharge from a large
groundwater system with large water residence (turn-over)
times. Only a few small subcatchments with minor flow rates
discharge young water (MRT<20 years), indicating local
geologic units below the surface that do not allow water to
infiltrate into and flow through larger, deeper groundwater
systems.
Substantial fractions of that long residence time in the
groundwater system may occur during passage through the
thick unsaturated zones (50–100 m) as indicated by CFC and
SF6results measured at groundwater wells and springs (Mor-
genstern et al., 2004). CFCs and SF6in groundwater are
still exchanged with the atmosphere during passage through
the unsaturated zone, therefore CFC and SF6ages represent
travel time through the saturated zone only. Large observed
differences between CFC and SF6ages, compared to tritium
ages of up to 40 years and greater for the older waters, there-
fore indicate travel time of the groundwater through the un-
saturated zone of >40 years for the older groundwater dis-
charges.
The old age of the majority of the Lake Rotorua water in-
flows and the highly mixed nature of the water discharges
(note the high fractions of exponential flow, up to 100%,
in Table 2) implies a very slow and lagged response of the
streams and the lake to anthropogenic contaminants in the
catchment, such as nitrate. The majority of the nitrate load
currently discharging into the lake is thus from land-use ac-
tivities 30 and more years ago.
About a hundred stream and groundwater well samples
have been dated in the Lake Rotorua catchment. The ground-
water age distributions are used in the following sections to
identify hydrochemistry evolution, sources of contaminants,
and to predict future nitrate loads that will enter Lake Ro-
torua from the large contaminated groundwater system. In a
future paper, the conceptual groundwater flow model in the
Lake Rotorua catchment will be inferred from the groundwa-
ter age distribution data. The data will subsequently be used
for calibration of a groundwater transport model.
4.2 Hydrochemistry and recharge source
The hydrochemical composition of the groundwater and sur-
face waters in the Rotorua catchment have been investi-
gated by Morgenstern et al. (2004) and Donath et al. (2014);
the following section summarizes the results relevant to the
present study.
Hydrochemistry is driven by interaction between water
and the different major lithologies and can be used to track
the origin of the groundwater. Hydrochemistry reflects the
rhyolite ignimbrite and lava aquifer lithologies that dominate
the Lake Rotorua catchment, with much lower concentra-
tions of Ca, Mg and SO4and much higher concentrations
of F, PO4and SiO2, compared to groundwater in other parts
of New Zealand.
Several statistical and graphical techniques were applied
to characterize the variations in hydrochemistry across parts
of the catchment. Hierarchical cluster analysis (HCA) was
shown to be a useful technique to identify samples with sim-
ilar hydrochemical composition, and to relate the ground-
water samples to their origin from one of the main aquifer
lithologic units. HCA conducted with Ward’s linkage rule al-
lowed the samples to be partitioned into four hydrochemical
clusters. Three of the clusters, accounting for the majority
of the samples, were inferred to reflect water–rock interac-
tion with the dominant lithologies in the catchment, namely
Mamaku ignimbrite, lava or palaeo-lake sediments. Hydro-
chemistry inferred to indicate interaction between water and
the Mamaku ignimbrite had Na and HCO3as the dominant
cation and anion, respectively, and among the highest con-
centrations of Mg, PO4and SiO2and among the lowest con-
centrations of F, K and SO4observed. Hydrochemistry in-
ferred to indicate interaction between water and rhyolite lava
also had Na and HCO3as the dominant cation and anion, re-
spectively, but had relatively low concentrations of PO4and
among the highest concentrations of K. Hydrochemistry in-
ferred to indicate interaction between water and sediments
had Na–Ca–HCO3–Cl water type and relatively low concen-
trations of SiO2. The remaining cluster was inferred to rep-
resent geothermal influences on the hydrochemistry (e.g. el-
evated concentrations of Na, Cl, SO4, SiO2and NH4).
Figure 5 shows the hydrochemical clusters of the water
samples inferred to indicate interaction between water and
Mamaku ignimbrite (light blue), lava (red) or lacustrine sed-
iments (dark yellow). Note that samples assigned to the clus-
ter inferred to indicate geothermal origin are not displayed
in Fig. 5 or discussed further in the present study because
geothermal influence is not the subject of this study.
Samples with hydrochemistry indicative of interaction
with the Mamaku ignimbrite occur predominantly in the
north and northwest portion of the catchment (Fig. 5, blue).
All of the large springs discharging into Hamurana (Ham),
Awahou (Awh), and Utuhina (Utu) streams have a Ma-
maku ignimbrite hydrochemistry signature (blue circles).
The stream reaches in the Mamaku ignimbrite area upstream
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U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources 813
Figure 5. Distribution of hierarchical cluster analysis (HCA) clus-
ters in Lake Rotorua catchment, together with the geological units
(Leonard et al., 2010) and stream reaches. Stream reaches shown
as dotted lines are usually dry. HCA clusters relate to origin of
the groundwater from one of the three main geologic formations:
Mamaku ignimbrite (light blue), lava (red) and lacustrine sediment
(yellow).
from these large springs are usually dry. This, together with
the Mamaku ignimbrite hydrochemistry signature, implies
that these large springs drain the large Mamaku ignimbrite
areas upstream that have negligible surface runoff. In the
northwest in the Hamurana and Awahou catchments, these
springs emerge close to the lake shore within the sediment
area (Fig. 5, yellow), indicating that close to the lake shore,
due to the more impermeable nature of the intra-caldera sed-
iment, the deeper groundwater flow from the Mamaku ign-
imbrite is forced to the surface. All of these large springs
emerge within the slopes of the sediment formation where
sediment layers are thinner and weaker compared to the level
area closer to the present lake shore; we infer that the thin na-
ture of the sediments on these slopes allows the water from
the underlying ignimbrite to flow to the surface. No large
spring occurs in the level area closer to the lake, where sedi-
ments are thicker. Also the large Utuhina Spring in the south-
east emerges within the slopes of the sediment (Fig. 5), indi-
cating the more impermeable nature of the sediment forc-
ing the groundwater from the Mamaku ignimbrite to the sur-
face in the area of thin sediment layers. The Utuhina Spring
emerges below a small local lava dome feature, but the ig-
nimbrite signature of the water indicates that this spring is
the discharge from the large ignimbrite area southwest of the
lava dome feature. The small lava feature may be fractured,
discontinuous, or act as a water conduit, allowing water dis-
charge from the ignimbrite behind.
Shallow wells and streams that gain most of their recharge
and flow within the lacustrine sediments display a character-
istic hydrochemical signature (Fig. 5, yellow circles). Such
samples originate from the downstream parts of Waiteti (Wtt)
and Ngongotaha (Ngo) streams. The study by Donath et
al. (2014) also detected this characteristic hydrochemistry in
samples collected with higher spatial resolution in parts of
the Ngongotaha subcatchment that are not discussed here.
Hydrochemistry of the water draining the Ngongotaha lava
dome west of the lake (Fig. 5, red circles) is inferred to
indicate interaction with lava formations. The Ngongotaha
dome, similar to the Mamaku ignimbrite, has no drainage
via surface flow (stream beds are dry), indicating a highly
porous nature, likely due to fractures and pumiceous zones
within the dome. Only where the rhyolite dome intercepts
the palaeo-lake sediments is the groundwater flow from the
lava forced to the surface due to the low permeability of the
sediments.
The investigated water discharges from the eastern catch-
ment of Lake Rotorua entirely show geothermal influence
in their hydrochemistry composition (not the subject of this
study).
The above HCA results give, for the first time, consistent
evidence of the link between the main recharge areas in the
Mamaku ignimbrite, and the main groundwater discharges
into the lake.
4.3 Hydrochemistry evolution
In the following two sections hydrochemistry data versus
groundwater age is discussed for a better understanding of
groundwater processes and geologic versus anthropogenic
origin of contaminants.
The groundwater of the Rotorua rhyolite ignimbrite and
lava dome aquifers (Fig. 6a) displays high dissolved oxy-
gen (DO), between 5 and 11mg L−1(50–100 % of equilib-
rium with air). There is no trend of decreasing DO with in-
creasing age, indicating that microbial reduction reactions
are insignificant in this volcanic aquifer within timescales
of the water residence time in the aquifer. Microbial reduc-
tion reactions, facilitated by the presence of organic matter
or other electron donors (e.g. pyrite), would usually con-
sume the dissolved oxygen in the groundwater. Reduction
of oxygen is energetically the most favourable reaction that
micro-organisms use in a series of reactions, with the result
that other reduction reactions (e.g. denitrification) typically
do not occur until most of the dissolved oxygen has been
consumed. These reduction reactions take time, and if these
reactions are supported by the presence of electron donors
in the geologic formation, it is expected that old waters be-
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814 U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources
38
1398 Fig. 6. a) Dissolved Oxygen (DO) and b) pH versus mean residence time (MRT). The colour codes of 1399 the samples indicate water from the relevant geologic formation, as indicated by Hierarchical Cluster 1400 Analysis (HCA). 1401
1402
Figure 6. (a) Dissolved oxygen (DO) and (b) pH versus mean residence time (MRT). The colour codes of the samples indicate water from
the relevant geologic formation, as indicated by hierarchical cluster analysis.
come increasingly anoxic (e.g. Böhlke et al., 2002; Tesoriero
and Puckett, 2011). No trend of decreasing DO with increas-
ing groundwater age was observed, suggesting an absence of
significant amounts of electron donors such as organic mat-
ter or pyrite in this ignimbrite formation. This is supported
by its depositional history as a single large ignimbrite forma-
tion without any organic matter involved. Absence of oxygen
reduction indicates that there is no potential for significant
denitrification reactions in this aquifer system.
Absence of a trend of decreasing DO with increasing
groundwater age, but rather constant DO in very young and
old groundwater of between 50 and 100%, suggests that the
partial oxygen reduction that is observed has occurred in
the soil zone, which does contain organic matter, and that
once the water has passed the soil zone, no further oxy-
gen reduction processes occur. Only one groundwater sam-
ple in the Rotorua catchment was depleted in oxygen (below
1mgL−1). This is related to the palaeo-lake sediments, sug-
gesting localized deposits of reactive organic matter in these
sediments, as would be expected in lake sediments.
The pH of groundwater usually increases over time due
to ongoing hydrogeochemical reactions, resulting in an in-
creasing pH of groundwater with age. In New Zealand we
observed in groundwater an increase from about pH 6 for
very young groundwater (<1 year) to about pH 8 in very
old groundwater (>10000 years) (Morgenstern and Daugh-
ney, 2012). For the Rotorua catchment, the groundwater pH
data from lava formations (Fig. 6b) show a sharp increase
from pH 5.8 to 6.7 over the age range from 5 to 50 years,
with a power law fit of ln(pH)=0.073 ×ln(MRT)+1.62,
R2=0.98. For the groundwater from the Mamaku ign-
imbrite, the pH increases from just 6.3 to 6.6 over the
age range from 0 to 100 years, with a power law fit of
ln(pH)=0.025×ln(MRT)+1.76, R2=0.47. The ground-
water from the sediment formation shows no clear trend in
pH with groundwater age, but displays higher pH for rela-
tively young water, pH 6.5–7.2 for water with MRT between
1 and 60 years.
As groundwater becomes more evolved over time due to
water–rock interaction, concentrations of phosphorus, silica,
bicarbonate and fluoride typically increase due to dissolu-
tion of volcanic glass, silicate minerals, carbonates and fluo-
ride likely deposited from the volatile phases in magma ex-
solved during eruption (Morgenstern and Daughney, 2012).
With increasing groundwater age, ion concentrations are ex-
pected to increase up to a maximum equilibrium concentra-
tion. Groundwater from the sediment formation often fol-
lows different or unclear trends compared to the rhyolite ig-
nimbrite and lava formations.
Dissolved reactive phosphate (PO4–P) in groundwa-
ter from all three formations, the rhyolite lava and ig-
nimbrite aquifers, and the sediments originating from
the same formations, shows excellent correlation with
groundwater age (Fig. 7a, black curve), with ln(PO4–
P)=0.458×ln(MRT)−4.72, R2=0.94.
Silica (SiO2) also shows good correlation with ground-
water age for the rhyolite ignimbrite and lava for-
mations (Fig. 7b). The silica concentration of ground-
water in lava formations (red circles) increases faster
compared to ignimbrite (blue circles). The power fit
to the lava data is ln(SiO2)=0.310×ln(MRT)+2.96,
R2=0.88 (red curve), and to the ignimbrite data is
ln(SiO2)=0.238×ln(MRT)+3.05, R2=0.83 (blue curve).
The correlation between silica and groundwater age for the
lacustrine sediment aquifers (yellow circles) is rather er-
ratic; high silica concentration can also occur in very young
groundwater.
For bicarbonate (HCO3), only groundwater samples
from the Mamaku ignimbrite show a reasonable cor-
relation with groundwater age, with a power fit of
ln(HCO3)=0.206×ln(MRT)+2.58, R2=0.71 (Fig. 7c).
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U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources 815
39
1403 1404
1405 1406
1407 1408 Fig. 7. a) Dissolved reactive phosphate (PO4-P), b) silica (SiO2), c) bicarbonate (HCO3), d) sodium 1409 (Na), and e) fluoride (F) versus mean residence time (MRT). The sample colour code for all graphs is 1410 shown in graph a), and indicates water origin from the relevant geologic formation, as indicated by 1411 Hierarchical Cluster Analysis (HCA). 1412
Figure 7. (a) Dissolved reactive phosphate (PO4–P), (b) silica (SiO2), (c) bicarbonate (HCO3), (d) sodium (Na) and (e) fluoride (F) versus
mean residence time (MRT). The sample colour code for all panels is shown in (a), and indicates water origin from the relevant geologic
formation, as indicated by hierarchical cluster analysis.
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816 U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources
The high data point at 58mgL−1was considered an outlier
and not included in the fit.
Sodium (Na) in general also shows increasing concen-
tration with groundwater age because it is part of com-
mon minerals and leached from these. But the correlation
is poor (Fig. 7d). Note that elevated Na in young ground-
water can also be caused by land-use impacts, as observed
in other parts of New Zealand (Morgenstern and Daughney,
2012). Considering the high data point at 15.8mgL−1an
outlier (it is from the same site considered as an outlier for
HCO3), the correlation for the data from all three formations
is ln(Na)=0.07×ln(MRT)+1.9, R2=0.27.
Fluoride concentrations (F) show good correlations with
age for the rhyolite ignimbrite and lava formations (Fig. 7e),
even though the trends are masked by the fact that the con-
centrations are close to the detection limit. Concentrations in-
crease in lava formations significantly faster, with a power fit
of ln(F)=1.087×ln(MRT)−6.44, R2=0.97, compared to
ignimbrite with ln(F)=0.238×ln(MRT)−3.99, R2=0.58.
In groundwater of the Rotorua catchment (excluding
groundwater from the eastern catchment indicating geother-
mal influence, which is not the subject of this study), the
hydrochemistry parameters phosphate, silica, bicarbonate,
sodium and fluoride are purely of geologic origin, because
they do not display elevated concentrations in young water
that was recharged during the time of anthropogenic high in-
tensity land-use activities. The groundwater samples show,
for the rhyolite Mamaku ignimbrite and lava formations, ex-
cellent correlations across the western and northern Lake Ro-
torua catchment. The samples in each of these geologic units
follow similar trends of hydrochemistry concentration ver-
sus mean residence time, indicating the relatively homoge-
neous nature of these aquifers. Rather erratic trends in water
originating from the sediments suggest that these are not a
homogeneous formation but rather finely layered lensoidal
geologic deposits that vary spatially and support complex
or fragmented groundwater systems. Good trends of hydro-
chemistry versus groundwater age may be an indirect indica-
tion of robust age interpretations.
In rhyolite lava formations, geochemical reactions lead to
increased pH, Si, and F in groundwater significantly faster
than in ignimbrite, indicating higher reaction rates for dis-
solution of these elements from lava formations. While this
is important for understanding water–rock interaction, we do
not yet have sufficient information on the lithogeochemistry
to develop a mechanistic understanding of the reaction pro-
cesses.
4.4 Nutrients
Elevated nutrient levels in surface water cause poisonous al-
gal blooms and lake eutrophication. Presence of both phos-
phate and nitrate, above a threshold concentration, triggers
algae blooms in lakes. Limitation of one of these, P or N,
can limit algae blooms. In New Zealand, increasing nutrient
loads from high-intensity animal farming and fertilizers have
triggered lake eutrophication. In the absence of significant
overland runoff, nitrate travels from the land to the lake via
the groundwater, which eventually discharges into streams
and lakes.
Nutrient concentrations in New Zealand groundwaters
from agricultural sources have increased steadily after Eu-
ropean settlement in the early 19th century and with de-
velopment of the meat industry after 1880 (Morgenstern
and Daughney, 2012). In a national context, for ground-
water recharged before 1880 at pre-anthropogenic pristine
conditions, low nutrient concentrations prevailed (e.g. ni-
trate<0.2mgL−1NO3–N). In groundwater recharged be-
tween 1880 and 1955, nutrient concentrations are slightly
elevated due to low-intensity land use. In groundwater
recharged after 1955 a sharp increase of nutrient concentra-
tions is observed due to the impact of high-intensity land use
after World War II (Morgenstern and Daughney, 2012).
The main nutrients derived from land use in the Ro-
torua catchment, as indicated by elevated concentrations in
young groundwater, are nitrate (NO3), sulfate (SO4) and
potassium (K). These nutrient concentrations are shown in
Figs. 8 and 9a and b versus mean residence time, also corre-
lated to recharge year (upper x-axes). The majority of the
chemistry data of the Rotorua data set are from calendar
year 2003, therefore mean residence times of about 50 and
125 years correspond to mean groundwater recharge years
1955 and 1880, respectively. Homogeneous nitrate concen-
trations in discharges from within subcatchments of typi-
cally 0.7±0.2mgL−1NO3–N indicate that nutrient inputs
are derived from diffuse rather than a small number of point
sources, pointing to agricultural sources.
Figure 8 also includes data (labelled “other”) from the sites
in the Lake Rotorua catchment that could not be assigned to
one of the HCA clusters because these sites had not been
analysed for the full suite of hydrochemical parameters re-
quired for input into HCA. In several surveys only nitrate
was measured to obtain a higher spatial resolution of the ni-
trate distribution. The analysis of all hydrochemical parame-
ters, as required for HCA, was mainly undertaken at the large
discharges into the lake that contain old water, and only few
of these sites contain water young enough to show the im-
pact of recent land-use intensification. Therefore the “other”
samples were added to Fig. 8 to better represent younger wa-
ters. In addition, samples from the eastern catchment hav-
ing a geothermal signature are also included in the cluster
“other”. The geothermal influence is minimal and does not
affect the nitrate signature, and hence does not bias the dis-
play of results in Fig. 8.
Nitrate concentrations (Fig. 8) in oxic groundwaters with
MRT>125 years (recharged prior to 1880) in the Rotorua
catchment are higher, with up to about 0.7mg L−1NO3–N
(dotted line in Fig. 8) compared to other regions in New
Zealand with 0.2mgL−1NO3–N. The reason for elevated
nitrate in water despite a high mean residence time is the
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U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources 817
Figure 8. Nitrate (NO3) versus mean residence time (MRT). The
sample colour code indicates water origin from the relevant geo-
logic formation, as indicated by hierarchical cluster analysis.
high degree of mixing in the groundwater discharges from
the highly porous unconfined Rotorua ignimbrite aquifers
(see next section). In such aquifer conditions, groundwater
from short and long flow paths converge at the groundwater
discharges, causing a high degree of mixing of young and
old water. For example, groundwater with a MRT as high
as 170 years, using an exponential piston flow model with
95% exponential flow volume within the total flow volume
(see next section), contains over 20% of water recharged
after 1955. This post-1955 water can contribute significant
amounts of nitrate from high-intensity land use, raising the
nitrate concentration of the water mix considerably, despite
such a long MRT.
A significant increase in nitrate occurred only recently
(Fig. 8). Apart from one data point, an increase up to
1.5mgL−1NO3–N was observed only in water with MRTs
of less than 75 years, and a dramatic increase up to
14mgL−1NO3–N was observed in water with MRTs of less
than 50 years. Note the dramatic increase of nitrate in water
with MRT<20 years, reflecting the increased conversions to
dairy farming during the 1980s and 1990s (Rutherford et al.,
2011). As the majority of the water discharges into Lake Ro-
torua are significantly older than a few decades, with MRTs
of up to 145 years, the impact of the dairy conversions and
their nitrate loads over the recent decades has to a large ex-
tent not yet reached the lake. Increased nitrate loads to the
lake over the next decades must be expected as these nitrate
loads work their way through the large groundwater system
and eventually discharge into the streams and lake.
Sulfate and potassium are part of fertilizers and also show
elevated concentrations in young groundwater (Fig. 9). Note
that sulfate in groundwater in the eastern lake catchment
has much elevated concentrations, up to 40mg L−1SO4,
due to geothermal influence. Groundwater with indications
of geothermal influence is not discussed in this study. Also
note that sulfate can be biased due to anoxic SO4reduction.
The data shown are, however, not from anoxic groundwa-
ter environments. Sulfate and potassium show slightly ele-
vated concentrations, up to a factor of 3, only in water with
MRT<50 years, corresponding to water recharged after ap-
proximately 1950. Sulfate concentrations (in mgL−1SO4)
in the Rotorua volcanic aquifers are significantly lower com-
pared to a national survey: pre-anthropogenic concentration
of 2 versus 12, and high-intensity land-use concentrations of
up to 6 versus 94 for the Rotorua volcanic aquifers and the
national survey (Morgenstern and Daughney, 2012), respec-
tively.
Phosphate, in conjunction with nitrate the cause for
lake eutrophication, is not elevated in young groundwater
(Fig. 7a) despite its frequent application as super-phosphate
fertilizers. Absence of elevated PO4in young groundwater
implies that fertilizer phosphate from non-point sources has
not yet reached the saturated groundwater systems and is still
retained in the soil. This finding is consistent with the usu-
ally high P-retention scores for ashfall soils and thick unsat-
urated zones across this region, which are very efficient at
buffering P loss. P-retention in soils was also observed in the
New Zealand National Groundwater Monitoring Programme
across other soil types (Morgenstern and Daughney, 2012).
The presence of elevated PO4only in old groundwa-
ter indicates that its source is purely due to geological
factors, because these waters were recharged before land-
use intensification. PO4concentrations up to 0.1mgL−1
PO4–P are observed, due to phosphate leaching from
the rhyolite ignimbrite and lava formations. With most
groundwater discharging into Lake Rotorua being very old
(MRT>50 years), the water has naturally high PO4concen-
trations, well above the threshold for primary algae produc-
tion of ca. 0.03mgL−1total phosphate (Dodds, 2007).
The high phosphate load to the lake via groundwater is nat-
ural. As the turn-over time of the lake water is only 2.2 years
via the high PO4-bearing streams, there is a constantly high
PO4load reaching the lake via all streams. Therefore, the
only effective way to limit algae blooms and improve lake
water quality in such environments is by limiting the nitrate
load.
4.5 Prediction of future nitrate load
The water quality of Lake Rotorua has declined continuously
over the past 60 years, responding very slowly to histori-
cal agricultural and urban development in the catchment, and
large amounts of groundwater have insidiously become con-
taminated over the last 60 years because of the long travel
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818 U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources
41
1418 1419 Fig. 9. a) Sulphate (SO4) and b) potassium (K) versus mean residence time (MRT). The sample colour 1420 code indicates water origin from the relevant geologic formation, as indicated by Hierarchical Cluster 1421 Analysis (HCA).The upper axis indicates calendar year. 1422
1423
Figure 9. (a) Sulfate (SO4) and (b) potassium (K) versus mean residence time (MRT). The sample colour code indicates water origin from
the relevant geologic formation, as indicated by hierarchical cluster analysis. The upper axis indicates calendar year.
Figure 10. Age distribution for Hamurana Stream (at inflow into
Lake Rotorua). The red shaded area indicates the fraction of water
that was recharged after land-use intensification. EPM1 and EPM2
are exponential piston flow models.
times through the groundwater system of the Lake Rotorua
catchment. The response time of the groundwater system to
mitigation action will also be lengthy; it will take similar
time frames until the contaminated water is flushed out of the
aquifers. To improve lake water quality and define reduction
targets for nutrients that affect lake water quality, prediction
of future contaminant loads from current and historic activi-
ties in the Lake Rotorua catchment are required.
In the previous section we have shown that of the two main
contaminants that together cause lake eutrophication, phos-
phorus is naturally present in the volcanic lake environment,
but nitrate from anthropogenic sources has been leaching into
the groundwater since the onset of industrial agriculture, de-
livering increasing nitrate loads to the lake. Figure 8 shows
significantly elevated nitrate concentrations in groundwater
recharged after 1955.
Due to the large lag time in the groundwater system, these
younger groundwaters, with their higher nitrate load, have
not yet worked their way fully through the groundwater sys-
tem. Significant fractions of the groundwaters discharging to
the lake are older (Figs. 3 and 4), with MRT>50 years, and
were recharged before land-use intensification. Therefore the
water discharges into the lake are currently still diluted by
old pristine water. With the delayed arrival of nitrate from
historic land use, which ultimately will discharge from the
groundwater system via the springs and streams into the lake,
nitrate loads to the lake from historic land-use activities must
be expected to increase further in the future. No significant
denitrification can be expected in the Rotorua groundwater
system (Fig. 6a).
The age distributions functions derived from the tritium
time series data in the stream discharges to Lake Rotorua (Ta-
ble 2) can be used to project the future arrival to the lake of
water that was recharged since land-use development in the
catchment (Morgenstern and Gordon, 2006). The age distri-
bution function for Hamurana Stream, the largest stream (Ta-
ble 1), which discharges some of the oldest water to the lake
(Fig. 2), is shown in Fig. 10.
Figure 10 shows the two superimposed age distributions of
exponential piston flow models: EPM2 with younger water
of MRT=12 years and EPM1 with significantly older water
of MRT=185, together with the average MRT=125 years
between the two models (blue). Only the water younger than
55 years has been recharged after land-use intensification
(red shaded) and contains elevated nitrate. The cumulative
fraction of land-use impacted water is about 45%, implying
that more than half of the water is still pristine old water.
After this old water is completely displaced by land-use im-
pacted water, the nitrogen load of Hamurana Stream will ap-
proximately double. The projected increase in nitrogen load
over time, as derived from the age distribution, is shown in
Fig. 11.
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U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources 819
Figure 11. Projected increase over time of nitrogen load to Lake
Rotorua from Hamurana Stream.
Nitrate, as opposed to other nitrogen fractions, is clearly
the major component of nitrogen in the Rotorua groundwa-
ter system (Morgenstern et al., 2004). Concentrations of ni-
trate in the catchment were low (0.14mg L−1NO3–N) prior
to catchment development, as determined from old ground-
water (Morgenstern and Gordon, 2006). The prediction of
nitrogen load increase is calculated by scaling the nitrogen
load currently measured in the stream (full symbol, Fig. 11)
according to its fraction of land-use impacted water to the
various years over time, using the age distribution (Fig. 10).
Good agreement with average historic monitoring results
of nitrogen loads (Rutherford, 2003, hollow symbols in
Fig. 11) confirms that the assumptions regarding the base-
line concentration and timing of nitrogen input in the catch-
ment are reasonable. Using the age distributions derived for
all stream discharges to the lake, we also projected the to-
tal nitrogen load increase to Lake Rotorua (Morgenstern and
Gordon, 2006). In regard to phosphorus, there are no elevated
phosphorus concentrations in young groundwater (Fig. 7a)
and the phosphorus load to Lake Rotorua is projected to
stay constant, as long as fertilizer phosphate does not break
through the soil into the groundwater.
The timescales necessary for the Hamurana Stream to ad-
just to changes in land-use activities in the catchment are
long. Due to the long residence time of the water in the large
aquifer system, it takes more than a hundred years for the
groundwater discharge to the lake to adjust to changes in
land-use activities. These timescales apply to activities that
cause contamination, but also to remediation action.
This projection of nitrogen load via the stream is based on
actual nitrogen concentrations in the stream (combined with
the age of the water) and accounts only for the nitrogen from
land-use activities that leaches out of the root zone of agri-
cultural land into the deeper part of the groundwater system.
Any nitrogen uptake in the soil is already taken into account.
The above nitrogen prediction is based on constant ni-
trogen input since catchment development. This trend will,
however, be exacerbated by any further intensification of land
use within the catchment over recent decades, as this recently
recharged water has largely not yet reached the streams.
5 Conclusions
This study shows how the isotopic and chemistry signature
of groundwater can be used to help determine the sources
and the dynamics of groundwater and contaminants that
travel with it, in particular in complex groundwater systems
that are difficult to characterize using conventional hydro-
geologic methods, such as that of the Lake Rotorua catch-
ment. The isotopic and chemistry signatures of the major
groundwater-dominated stream discharges to the lake, after
passing through the large aquifer system of the catchment,
allow us to understand groundwater processes and lag time
on a catchment scale.
Tritium time series data and complementary age tracers
SF6and CFCs can be used to establish age distribution pa-
rameters, allowing for understanding of groundwater pro-
cesses and dynamics, and the timing of groundwater contam-
ination. This is particularly useful in catchments where little
information is available on historic land-use activities.
After long-standing controversies (e.g. White et al., 2004;
Rutherford et al., 2011), hierarchical cluster analysis of the
water chemistry parameters has provided evidence about the
recharge areas and hydraulic connections of the large springs
near the northern shore of Lake Rotorua. Streams and shal-
low wells that gain most of their flow and recharge within the
lacustrine sediments display a characteristic hydrochemical
signature. Hydrochemistry of the water draining the Ngongo-
taha lava dome also has a characteristic signature due to in-
teraction with lava formations. Only where the lava dome
intercepts the palaeo-lake sediments is the groundwater flow
from the lava formation forced to the surface due to the low
permeability of the sediments. The water from the ignimbrite
also displays a characteristic hydrochemical signature. Sim-
ilarly to the discharges from the lava formation, the water
from the ignimbrite discharges near the intercept of the ig-
nimbrite formation with the palaeo-lake sediments, indicat-
ing that the groundwater flow from the ignimbrite is forced
to the surface due to the low permeability of the sediments.
The largest springs, discharging in the northwest of the lake,
emerge close to the lake shore within the sediment area,
but the ignimbrite signature of these water discharges im-
plies that these springs drain the Mamaku ignimbrite plateau,
which has negligible surface runoff, through the lake sedi-
ment layers in slope areas where the sediments are thinner
and weaker.
Groundwater chemistry and age data show clearly the
source of nutrients that discharge with the groundwater into
the lake and cause lake eutrophication. Low nitrate concen-
tration in old oxic groundwater and high nitrate concentration
in young groundwater recharged after catchment develop-
ment in the 1950s implies an anthropogenic source of nitrate
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820 U. Morgenstern et al.: Using groundwater age and hydrochemistry to understand sources
from agricultural activities, while low phosphate (PO4) con-
centrations in young groundwater but high PO4concentra-
tions in old groundwater imply a geologic source. High PO4
is a natural constituent of the groundwater that discharges
via the streams into the lake, and with a turn-over time of the
lake water of only 2.2 years, there is a constantly high PO4
load reaching the lake via all streams. Therefore, the only
effective way to limit algae blooms and improve lake water
quality in such environments is by limiting the nitrate load.
The groundwater in the Rotorua catchment, once it has
passed through the soil zone, shows no further decrease in
dissolved oxygen over the full range of residence time of
the water in the aquifer, indicating an absence of significant
microbial reactions due to limitation of electron donors in
the aquifer (e.g. organic matter) that could facilitate micro-
bial denitrification reactions (Kendall and McDonnell, 1998;
Tesoriero et al., 2007). Nitrate from land-use activities that
leaches out of the root zone of agricultural land into the
deeper part of the groundwater system is unlikely to undergo
any significant degree of reduction through denitrification
and must be expected to travel with the groundwater to the
lake.
The old age of the water, with mean residence time of
>50 years for most water discharges to the lake, implies
that there is a large lag time for transmission of the ni-
trate through the groundwater system. Younger groundwa-
ters, with their higher nitrate load, have not yet worked their
way fully through the groundwater system. With increasing
arrival of this nitrate from historic land uses, a further in-
crease of the nitrate load to the lake must be expected in the
future.
The old age and the highly mixed nature of the water dis-
charges imply a very slow and lagged response of the streams
and the lake to anthropogenic contaminants in the catch-
ment, such as nitrate. Using the age distribution as deduced
from tritium time series data measured in the stream dis-
charges to the lake allows extrapolation of the nutrient load
from historic land-use activities into the future. For Hamu-
rana Stream, the largest stream to Lake Rotorua, it takes more
than a hundred years for the groundwater-dominated stream
discharge to adjust to changes in land-use activities. These
time scales apply to activities that cause contamination, but
also to remediation action.
Without age information on the groundwater-dominated
streams, it would be difficult to obtain such an understanding
of groundwater process, groundwater dynamics, and contam-
inant loads that travel with the groundwater.
Acknowledgements. We thank personnel from Bay of Plenty Re-
gional Council for assistance with sample collection and for provi-
sion of some information used in this study, and Eileen McSaveney
for editing the paper. This research was supported by funding from
the Bay of Plenty Regional Council and the Ministry of Science and
Innovation (Contract C05X1002).
Edited by: C. Harman
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