ArticlePDF Available

Abstract and Figures

Arsenic contamination in groundwater affects tens of millions of people worldwide and is regarded as a major groundwater issue in New Zealand. This study was conducted to evaluate the connection between elevated groundwater arsenic concentrations and inferred redox state of aquifers across the Canterbury region. In total, 2,428 samples from 1,172 wells were analysed for arsenic concentrations, and groundwater redox state was inferred using geochemical indicators for each well. Most groundwater (~88% of wells; N = 1,031) had arsenic concentrations below laboratory detection limits, ~9.5% (N = 111 wells) had detectable arsenic concentrations below the New Zealand drinking-water Maximum Acceptable Value of 0.01 mg/L, and ~2.6% (N = 30 wells) had concentrations exceeding 0.01 mg/L. Arsenic concentrations were strongly associated with reducing groundwater; 47 (~43%) of 109 wells with reducing groundwater had detectable concentrations of arsenic, including 19 wells (~17%) with arsenic greater than 0.01 mg/L. Conversely, 840 wells had water classified as oxic, and only one of those wells exceeded the 0.01 mg/L Maximum Acceptable Value for arsenic in drinking-water. Arsenic concentrations were higher in coastal areas north of Christchurch, where confined aquifers are overlain and underlain by layers of peat and low-permeability sediment. Biogeochemical processes driving release are the same as those that cause arsenic contamination in groundwater elsewhere in the world. However, differences in regional geology and a relatively lesser availability of labile organic carbon appear to constrain the concentrations and spatial prevalence of groundwater arsenic in Canterbury compared to other arsenic-impacted regions (e.g., Southeast Asia).
Content may be subject to copyright.
137
Journal of Hydrology (NZ) 61 (2): 137-150
© New Zealand Hydrological Society (2022)
Arsenic in groundwater in the Waitaha/Canterbury region
Andrew R. Pearson,1,2 Lisa Scott,1 Scott R. Wilson3 and
Michael S. Massey1
1 Environment Canterbury, 200 Tuam St., Christchurch 8011, Canterbury,
New Zealand.
2 Institute of Environmental Science and Research (ESR) 8041, Christchurch,
Canterbury, New Zealand.
3 Lincoln Agritech Ltd, PO Box 69-133, Lincoln 7640, Canterbury, New Zealand.
* Corresponding author: Andrew.Pearson@esr.cri.nz
Abstract
Arsenic contamination in groundwater
affects tens of millions of people worldwide
and is regarded as a major groundwater issue
in New Zealand. This study was conducted
to evaluate the connection between elevated
groundwater arsenic concentrations and
inferred redox state of aquifers across the
Canterbury region. In total, 2,428 samples
from 1,172 wells were analysed for arsenic
concentrations, and groundwater redox state
was inferred using geochemical indicators for
each well.
Most groundwater (~88% of wells;
N = 1,031) had arsenic concentrations below
laboratory detection limits, ~9.5% (N = 111
wells) had detectable arsenic concentrations
below the New Zealand drinking-water
Maximum Acceptable Value of 0.01 mg/L,
and ~2.6% (N = 30 wells) had concentrations
exceeding 0.01 mg/L.
Arsenic concentrations were strongly
associated with reducing groundwater;
47 (~43%) of 109 wells with reducing
groundwater had detectable concentrations
of arsenic, including 19 wells (~17%) with
arsenic greater than 0.01 mg/L. Conversely,
840 wells had water classified as oxic,
and only one of those wells exceeded the
0.01 mg/L Maximum Acceptable Value
for arsenic in drinking-water. Arsenic
concentrations were higher in coastal areas
north of Christchurch, where confined
aquifers are overlain and underlain by layers
of peat and low-permeability sediment.
Biogeochemical processes driving release
are the same as those that cause arsenic
contamination in groundwater elsewhere in
the world. However, differences in regional
geology and a relatively lesser availability of
labile organic carbon appear to constrain
the concentrations and spatial prevalence
of groundwater arsenic in Canterbury
compared to other arsenic-impacted regions
(e.g., Southeast Asia).
Keywords
arsenic, redox, groundwater, drinking water,
aquifer
Introduction
Arsenic (As) is a toxicant and carcinogen that
is found in soil, minerals, and groundwater
worldwide (Nordstrom, 2002; Podgorski and
Berg, 2020; Smedley and Kinniburgh, 2002).
Groundwater is the world’s largest source of
fresh water (McDonough et al., 2020), and
globally, consumption of naturally occurring
138
arsenic in groundwater affects 94 million
to 220 million people (Podgorski and Berg,
2020). The World Health Organization
(WHO) standard concentration for arsenic
in drinking-water is 0.01 mg/L (Gorchev and
Ozolins, 1984), which has been adopted by
New Zealand as the ‘Maximum Acceptable
Value’ (MAV) in drinking-water (Ministry of
Health, 2018).
Globally, most arsenic contamination
in groundwater is associated with arsenic
mobilisation due to natural processes
rather than human influence (Smedley
and Kinniburgh, 2002), although human
activities, such as mining, timber treatment
and pesticide use, have contributed to
arsenic pollution, including in New
Zealand (Kerr and Craw, 2021; Safa et
al., 2020; Robinson et al., 2004). Arsenic
(predominantly arsenate) is present in or
on aquifer minerals, including iron and
manganese oxide and hydroxide minerals,
which can undergo microbially-mediated
reductive dissolution when groundwater
is in a reducing redox state (Borch et al.,
2010; Postma et al., 2016). Thus, reductive
dissolution of arsenic-bearing minerals is
the primary trigger of arsenic release to
groundwater (McMahon and Chapelle,
2008; Smedley and Kinniburgh, 2002)
and is associated with elevated arsenic
concentrations in Argentina (Litter et al.,
2019), Bangladesh (Ying et al., 2017),
China (Zhang et al., 2020), India (Bindal
and Singh, 2019), Italy (Rotiroti et al.,
2015), the United States (Welch et al.,
2000) and Vietnam (Wallis et al., 2020).
Although reductive dissolution of arsenic-
bearing minerals is the primary driver of
arsenic release, groundwater redox state
can also influence the toxicity of arsenic;
under reducing conditions, arsenate can
be reduced to arsenite, which is more toxic
(Singh et al., 2011). After desorption from
aquifer minerals, arsenic concentrations are
dependent on hydrological factors, including
dilution and flushing rates (Fendorf et al.,
2010; Smedley and Kinniburgh, 2002).
Arsenic is widespread in New Zealand’s
aquatic and terrestrial environments
(Robinson et al., 2004), and arsenic
contamination is regarded as a major
groundwater issue (Daughney and Wall,
2007). Concentrations above the drinking-
water MAV have been observed in Waikato
(Piper and Kim, 2006), Otago (Levy et al.,
2021) and Canterbury (Scott et al., 2016).
In Canterbury, arsenic contamination has
been observed in reducing redox-state
groundwater in some coastal areas (Scott et
al., 2016), but this study represents the first
regional-scale analysis of the relationship
between groundwater redox state and arsenic
concentrations. To evaluate the relationship
between groundwater redox state and
arsenic contamination, we assessed arsenic
concentration data from 1,172 wells, and
used geochemical indicators to estimate
groundwater redox state for each well,
following the approach used by Close et al.
(2016) and Wilson et al. (2020).
The Canterbury region
The Canterbury (or Waitaha) region is
located on the east of Aotearoa New Zealand’s
South Island and is bounded by the Southern
Alps to the west and the Pacific Ocean to the
east (Fig. 1). Canterbury is New Zealand’s
largest geographic region (approximately
50,000 km2), occupying approximately 17%
of New Zealand’s land area (Painter, 2018).
In Canterbury, most groundwater is found
in alluvial sediments (predominantly gravels)
underlying the Canterbury Plains, which
are one of the world’s major alluvial plains
(Leckie, 2003). The groundwater contained
in Canterbury aquifers represents 73% of
New Zealand’s total groundwater by volume
(Stats NZ, 2017), is the primary source
of drinking-water to a population of over
600,000 (including the city of Christchurch)
and provides water for 70% of New Zealand’s
irrigated land (Painter, 2018).
139
Canterbury has a temperate maritime
climate (Macara, 2016). Groundwater
recharge is heavily influenced by interactions
between the Southern Alps and westerly
winds; orographic rainfall in the Alps is a
significant source of recharge for rivers and
groundwater. From their sources in the
Southern Alps, groundwater and braided
rivers flow dominantly eastward across the
Canterbury Plains before draining into
the South Pacific Ocean. The Southern
Alps are primarily composed of greywacke,
tectonically uplifted by the collision of the
Australian and Pacific plates (Browne and
Naish, 2003). In some areas of the Southern
Alps, arsenic concentrations are elevated
in greywacke and argillite (Becker et al.,
2000; Horton et al., 2001) and present in
other minerals (e.g., calcite, quartz, and
sulphides) (Horton et al., 2001; Craw et
al., 2002). For example, in the Wilberforce
valley, arsenic concentrations have been
measured at 200 mg/kg (compared to a
background concentration of 10 mg/kg)
in some greywackes and argillites (Becker
et al., 2000). Weathering of the Southern
Alps, followed by the erosion and eastward
transport of sediment toward the coast, led
to the formation of a thick assemblage of
overlapping alluvial fans, which formed the
Canterbury Plains (Ballance, 2017). This
process has been occurring for at least the
last five million years, with increased erosion
and deposition during glacial periods
(Ballance, 2017). As a result of this erosion
and transport of arsenic-bearing bedrock
material, geogenic sources of arsenic may
be widespread in the alluvial gravels, with
arsenic generally present in source rocks
and associated sedimentary deposits.
The mostly gravel aquifers of the upper
plains and river valleys of Canterbury are
dominated by oxic groundwater conditions
(Wilson et al., 2020).
In some coastal areas east of the plains
there are series of confined aquifers (Fig. 1),
which typically sit in glacio-fluvial gravels
interbedded between aquitards formed from
silts, clays, sands, and buried layers of peat
extending to a depth of over 200 m (Brown
et al., 1988). The silts, clays, and sands were
deposited in low-energy marine or estuarine
environments that occurred during marine
high stands during interglacial periods
through the Quaternary (Browne and Naish,
2003). Groundwater is also found in some
inland basins, which were formed by glacial
activity and tectonic movement and partially
infilled with glacio-fluvial sediments. In
some parts of south Canterbury, known as
the ‘downlands’, groundwater is overlain
by layers of loess (of varying thickness),
which were deposited during the Pleistocene
(Schmidt et al., 2005) (Fig. 1). In some
coastal areas, particularly to the north of
Christchurch and in some parts of southern
Canterbury, reducing redox state conditions
can be prevalent in groundwater (Close et al.,
2016; Wilson et al., 2020).
Methods
Arsenic data analysis
Our data set includes 2,185 groundwater
arsenic measurements, sampled from 1,172
wells (tested from 1 to 25 times, though most
wells were only sampled once) from 1989 to
2021. The wells used in this study cover all
the main aquifer types found in Canterbury
(Fig. 1), and well depths ranged from 1.1 m to
433 m below ground level (mean depth of 33
m; median depth of 20 m). For wells sampled
more than once, mean concentrations of
arsenic were calculated for each well.
Samples were collected for various
reasons, as part of long-term monitoring
programmes or for specific investigations.
Due to changing laboratory service providers
and improving analytical technology over the
course of 30+ years, analytical methods used
for arsenic measurements changed through
the period of sample collection. In summary,
140
arsenic concentrations were predominantly
measured by hydride generation atomic
absorption spectroscopy (AAS) from 1989
to 1995; graphite furnace atomic absorption
spectroscopy (GFAAS) from 1995 to 2000,
and inductively coupled plasma mass
spectrometry (ICPMS) from 2000 to 2021.
Laboratory measurements were undertaken
at IANZ accredited laboratories. Detection
limits ranged from 0.0005 to 0.01 mg/L,
though reported ‘non-detections’ in samples
with detection limits of 0.01 mg/L (i.e., the
Figure 1 – Main aquifer types of Canterbury region (inset figure shows New
Zealand) include coastal confined aquifers, plains aquifers dominated by gravels,
downlands aquifers overlain by loess, fluvial aquifers associated with river systems,
and inland basins. Broadly, groundwater flows from the Southern Alps toward the
east coast, before draining into the South Pacific Ocean.
drinking-water MAV) were excluded from
this study.
Groundwater samples were collected
by trained sampling technicians following
groundwater sampling procedures based
on international best practice. From 2006,
sample collection followed a national protocol
for state of the environment groundwater
sampling (Ministry for the Environment,
2006) later replaced by the National
Environmental Monitoring Standard for
discrete groundwater quality sampling
141
(NEMS, 2019). Over time, Environment
Canterbury’s approach to measuring
groundwater arsenic concentrations varied
between measurements of dissolved arsenic
and total arsenic in groundwater, depending
on whether the technician filtered the
samples before analysis. In New Zealand, the
MAV refers to arsenic concentration (i.e., it
does not distinguish between dissolved and
total arsenic). Measurements of total arsenic
may capture a more complete picture of the
actual amount of arsenic, so we have used
‘total’ arsenic concentrations if available
and dissolved arsenic concentrations
otherwise. This study aims to assess arsenic
concentrations relative to the drinking-water
MAV (Ministry of Health, 2018); therefore,
we have not distinguished ‘dissolved’ from
‘total’ arsenic in this study.
Redox indicators
Redox state for each well was estimated using
a total of 19,687 groundwater samples. After
Close et al. (2016) and Wilson et al. (2020)
we broadly classified groundwater redox state
into three categories: ‘oxic’ (or ‘oxidising’),
‘mixed’, and ‘reducing’. Redox state can be
estimated by measuring concentrations of
redox-sensitive parameters, such as dissolved
O2 (DO), nitrate (NO3), manganese (Mn),
and iron (Fe) (Close et al., 2016). For example,
under reducing conditions DO and NO3
will have low concentrations (i.e., < 1.0 mg/L
for DO, < 0.5 mg/L for nitrate-N), whilst Fe
and Mn might have elevated concentrations
(i.e., > 0.1 mg/L for Fe; > 0.05 mg/L for Mn)
due to reductive dissolution and release of
iron and manganese from aquifer sediments.
Following Close et al. (2016) groundwater
was classified as ‘mixed’ when nitrate-N,
Fe, Mn, and DO concentrations provided
an inconsistent indication of redox state.
Inconsistent indicators may be observed
because the sample was taken from a well that
samples multiple redox zones, the sample
was in disequilibrium, or because of the
thresholds applied to estimate redox state.
Our study used the same redox parameters
as Wilson et al. (2020), but rather than
mapping redox state, arsenic concentrations
in individual wells were compared against
redox state estimated from parameters in the
same well. This approach enabled a direct
comparison of groundwater redox state
against arsenic concentrations in individual
wells. We compared the distribution of the
redox state classes for wells with no arsenic
detections, wells with detectable arsenic
concentrations, and wells with arsenic
concentrations exceeding the drinking-
water MAV of 0.01 mg/L for arsenic. Due
to sporadic measurements of individual
geochemical indicators through time, the
redox estimates were based on the mean of
each redox indicator measurement as the
best estimate of long-term status of a given
well. Therefore, redox estimates were not
exclusively linked to the same samples used
for arsenic measurements. One confounding
factor resulting from the use of mean values
over time is that temporal variations in redox
state were not considered in this work.
Results
Arsenic and redox data
Of the 1,172 wells sampled for arsenic, most
wells (88%) had arsenic concentrations
below detection limits, 9.5% (N = 111) had
detectable values below the drinking-water
MAV (0.01 mg/L), and 2.6% (N = 30)
had arsenic values above the drinking-water
MAV (Table 1). Most wells contained oxic
groundwater (N = 840; 71.7%), groundwater
was mixed in 223 wells (19.0%), and
reducing groundwater was observed in 109
(9.3%) wells (Table 1).
Arsenic presence and exceedances of the
drinking-water MAV were strongly influenced
by groundwater redox state (Table 1).
In oxic groundwater, 94.9% of wells did not
have detectable concentrations of arsenic and
5% of wells had arsenic detected but below
142
the MAV; only one well exceeded the arsenic
drinking-water MAV (Table 1). In mixed
groundwater, 4.5% of wells had arsenic
concentrations above the drinking-water
MAV and 18.4% of wells had detectable
concentrations below the MAV. Still, in
mixed groundwater, most wells (77.1%) did
not have detectable concentrations of arsenic.
By contrast, in reducing groundwater,
Table 1 – Exceedances of the arsenic drinking-water Maximum Acceptable Value (MAV)
(0.01 mg/L), arsenic detections, and arsenic concentrations below the laboratory
detection limit in relation to predicted groundwater redox state.
Arsenic
concentration
category
Number of wells
(% of total wells)
Count of wells per redox classification
(% of total wells in each redox state class)
Oxic Mixed Reducing
MAV 30 (2.6%) 1 (0.1%) 10 (4.5%) 19 (17.4%)
Detected below MAV 111 (9.5%) 42 (5.0%) 41 (18.4%) 28 (25.7%)
Not detected 1031 (87.9%) 797 (94.9%) 172 (77.1%) 62 (56.9%)
Total wells: 1172 840 223 109
17.4% of wells had concentrations of arsenic
exceeding 0.01 mg/L, whilst a further
25.7% had detectable concentrations of
arsenic below the MAV. Most wells with
reducing groundwater (56.9%) did not have
detectable concentrations of arsenic (Table 1),
but the differences between oxic and
reducing groundwater with respect to arsenic
concentrations were nonetheless evident.
Figure 2(a) Average groundwater arsenic concentrations in individual wells (N = 1,172) in the
Canterbury region, compared to the MAV (0.01 mg/L); (b) Redox state per individual well in the
Canterbury region. Redox state was estimated for each well using the same parameters (dissolved
oxygen, nitrate, iron, and manganese) and approach as Wilson et al. (2020). Canterbury’s main
aquifer types are also shown in each map.
143
Arsenic and redox state across Canterbury
In the inland Canterbury Plains, the
groundwater is dominated by oxic conditions
(Fig. 2b), with almost no exceedances of the
arsenic drinking-water MAV of 0.01 mg/L.
Although a geogenic arsenic source is present,
groundwater redox conditions do not enable
arsenic mobilisation to occur. Similarly, in
the inland basin aquifers, oxic conditions
are prevalent (Fig. 2b), with arsenic
concentrations generally below laboratory
detection limits (Fig. 2a).
By comparison, arsenic in groundwater
is more commonly found in low-lying
coastal areas (Fig. 2a), which in Canterbury
are typically underlain by confined aquifer
systems. Measured arsenic concentrations
in coastal aquifers ranged from values below
detection to a maximum concentration
of 0.92 mg/L (i.e., 92 times the MAV for
drinking-water). In the aquifers underlying
the South Canterbury downlands, reducing
conditions are more common, with some
exceedances of the drinking-water MAV for
arsenic, though arsenic was not detected in
most wells.
The spatial scale of elevated arsenic
concentrations (Figs. 2a and 3) indicate that
groundwater arsenic is diffusely distributed in
some coastal areas of Canterbury. However,
there is also spatial variability, particularly
within the reducing coastal aquifers. For
example, north of Christchurch there are
wells where arsenic has not been detected in
relative proximity (i.e., within hundreds of
metres) to wells where arsenic concentrations
exceeded the drinking-water MAV (Fig. 3).
Figure 3 – Arsenic concentrations relative to the MAV (0.01 mg/L) in an area
of North Canterbury (black rectangle in inset map shows location within
Canterbury). Groundwater arsenic concentrations are generally higher in
reducing aquifers, whilst inland oxic aquifers have lower concentrations and
fewer occurrences of arsenic. *Aquifer redox map reproduced from Wilson et al.
(2020) using data kindly supplied by the authors.
144
Discussion
Relatively higher risk of arsenic contamination
in coastal aquifers
Globally, reducing conditions tend to exist
within geologically young aquifers in low-
lying areas with low flushing rates (Smedley
and Kinniburgh, 2002), such as the coastal
confined aquifers of Canterbury. Elevated
arsenic concentrations are very strongly
associated with mixed or reducing redox
state groundwater in Canterbury (Table 1),
which is found primarily in the coastal
confined aquifer systems and the South
Canterbury downlands (Fig. 2b). In reducing
groundwater systems, the drinking-water
MAV for arsenic was exceeded in ~17% of
wells, with a further ~26% of wells having
detectable concentrations below the MAV.
Further, arsenic was also present in mixed
redox state groundwater, with 4.5% of wells
exceeding drinking-water MAV and a further
~18% of wells having detectable arsenic
concentrations below the MAV (Table 1).
These data show that arsenic is a potential
indicator of reducing conditions but reducing
conditions do not necessarily indicate the
presence of arsenic.
The prevalence of reducing conditions
in coastal areas is due to the presence of
in situ organic matter and confinement
of aquifers by low-permeability material
deposited by marine transgressions during
the Quaternary (Brown and Weeber, 1992).
Low-permeability material inhibits transport
of oxygen from the atmosphere, recharge,
and flow, potentially leading to a build-
up of arsenic through time (Fendorf et al.,
2010). Further, layers of subsurface peat
that overlie or underlie the coastal confined
aquifers (Brown et al., 1988) are a source of
labile organic matter, which is often a critical,
limiting component of reductive dissolution
of arsenic-bearing minerals in groundwater
(deLemos et al., 2006; Fendorf et al., 2010;
Islam et al., 2004). Organic matter release
is known to fuel microbially-mediated
reduction processes that release arsenic into
groundwater in countries where arsenic
contamination in groundwater is much more
common and has been extensively studied,
such as Bangladesh, Cambodia, and Vietnam
(Erban et al., 2013; Fendorf et al., 2010;
Harvey et al., 2002; Stuckey et al., 2016).
The presence of organic matter (which drives
microbial respiration) depletes dissolved
oxygen in groundwater, causing groundwater
to become suboxic or anoxic, which from
a chemical perspective is a ‘reducing’
environment. Under reducing conditions,
other favourable available electron acceptors
can be used in microbial respiration, such
as nitrate, manganese, iron, sulphate, and
carbon dioxide, and many trace metals and
metalloids, such as arsenic, chromium, and
uranium (Borch et al., 2010; McMahon and
Chapelle, 2008). Since arsenic commonly
adsorbs to the surface of iron (hydr)oxide
minerals or can be found as trace contaminants
within these oxides, desorption and release of
arsenic can occur when reduction of iron and
manganese oxides is favourable. Thus, the
occurrence of arsenic release in aquifers with
in situ sources of organic matter and reducing
groundwater aligns with expectations based
on the biogeochemical behaviour of arsenic
observed elsewhere in the world.
Local-scale variability is common in arsenic-
impacted groundwater (Jakobsen et al., 2018;
van Geen et al., 2003), and our sample data
also reveal complex patterns across small
spatial scales in some areas of Canterbury.
For example, in the reducing coastal confined
aquifers north of Christchurch there are wells
in which arsenic was not detected, less than
one hundred metres from wells where arsenic
concentrations exceeded the MAV. Although
redox state influences arsenic release,
variations in the source of water and the three-
dimensional patterns of groundwater flow
influence patterns of both redox state and
145
arsenic contamination, which are typically
complex in arsenic-impacted areas (Neumann
et al., 2010). For example, in some areas
of the coastal confined aquifers, mixed and
oxic groundwater is predicted (Wilson et al.,
2020), possibly due to preferential upwards
leakage though aquitards, whilst seepage
from the Waimakariri River (Fig. 3) supplies
groundwater flow towards Christchurch
(Stewart and van der Raaij, 2022), potentially
diluting arsenic and supplying oxygen-rich
water. Thus, in Canterbury’s coastal aquifers,
a combination of local-scale geochemical and
hydrological variability means that arsenic-
contaminated groundwater is not uniformly
observed.
Oxic groundwater and low arsenic
concentrations in the Canterbury Plains
In Canterbury’s oxic groundwater, 95% of
wells had arsenic values below detection
limits, with 5% of wells having arsenic
detections and one well (0.1% of oxic wells)
exceeding the drinking-water MAV of
0.01 mg/L (Table 1). Most of the groundwater
in the unconfined aquifers of the Canterbury
Plains and inland basins is oxic (Close et al.,
2016; Wilson et al., 2020) because the open-
framework gravels that form the unconfined
aquifers, and the thin soils that overlie
them, allow land-surface recharge (i.e., the
sum of irrigation and rainfall recharge) and
transport of oxygen from the atmosphere to
groundwater. Further, in some areas, recharge
is dominated by seepage from the gravel-bed
braided rivers, which transport oxygen-rich
water from the foothills and Southern Alps.
The lack of detectable concentrations of
arsenic in oxic zones of the Plains (despite
arsenic being present in the greywacke, rocks,
and minerals of the Alps that were eroded to
form them) implies that arsenic may be stored
in oxic aquifer sediments, or when arsenic is
released, it might be diluted to concentrations
below laboratory detection limits. Further,
significant parts of the Canterbury Plains
are dominated by agricultural activity
(Stewart and Aitchison-Earl, 2020), causing
unconfined gravel aquifers to have excessive
nitrate concentrations. Nitrate is a powerful
oxidant (Galloway et al., 2003), and the
diffuse addition of anthropogenic nitrate,
a more favourable electron acceptor than
manganese or iron (McMahon and Chapelle,
2008), might further limit reduction of
arsenic-bearing iron and manganese minerals
in the aquifers of the Canterbury Plains.
In the aquifers of the Canterbury Plains,
arsenic concentrations could be low because
of a higher rate of sorption rather than
desorption, which is influenced by redox
state, although dilution could also be a factor
due to high flow rates (Postma et al., 2017;
et al., 2018). For example, the much
larger and fast-flowing Plains’ oxic aquifers
can dilute dissolved organic carbon and
competing adsorbates (e.g., phosphate) whilst
transporting higher concentrations (and total
loads) of oxygen and nitrate, moderating
arsenic release from aquifer minerals (Sø et
al., 2018). Thus, in aquifers on the Plains,
mass loads of arsenic could be high but very
difficult to spot in groundwater: rates of
sorption to aquifer minerals are likely to be
higher than the rate of arsenic release (owing
to the oxic redox state), whilst the relatively
high flow rates mean that any desorbed
arsenic may be diluted to concentrations
below detection limits. For example, Burbery
et al. (2020) installed a denitrifying bioreactor
in a fast-flowing (hydraulic conductivity of
1332 m/d) shallow unconfined gravel aquifer,
in which woodchips within the bioreactor
released labile organic carbon. The release
of labile organic carbon drove ‘pollution
swapping’, whereby denitrification took place
under reducing conditions, but manganese,
iron, and arsenic were released from aquifer
minerals (Burbery et al., 2022). Notably,
although concentrations of arsenic reached
a maximum of 0.022 mg/L (i.e., more than
double the drinking-water MAV), detectable
146
concentrations of arsenic travelled less
than 60 m down-gradient before dilution
or attenuation by mechanisms such as
sorption (Burbery et al., 2022). Thus, in
Canterbury, arsenic release appears to be
heavily influenced by redox state, yet on
the Plains, high concentrations of geogenic
arsenic in groundwater are likely only occur
on local scales, where there is a source of
organic carbon. In most of Canterbury, high
arsenic concentrations are likely to be diluted
or attenuated to concentrations below
laboratory detection limits within a relatively
short distance (e.g., Burbery et al., 2022).
Future research
More frequent monitoring would improve
understanding of arsenic behaviour in
Canterbury’s groundwater. For example,
most wells have only been sampled once,
yet regular sampling of arsenic-affected wells
would allow assessment of seasonal and
long-term (i.e., decadal) variability and any
potential trends. Analysis of arsenic bound
to aquifer sediments could shed light on
mechanisms of arsenic retention and release,
though these mechanisms are expected to be
similar to those observed elsewhere the world.
In the coastal confined aquifers, labile organic
carbon is likely to be an important influence
on the spatial variability observed in the
coastal confined aquifers, thus measurements
of dissolved organic carbon could be of value.
From a monitoring perspective, the spatial
variability of arsenic concentrations in some
coastal areas means that it is likely to be
difficult to reliably predict the concentration
of arsenic in a particular well based on the
results from neighbouring wells. Analysis of
arsenic speciation in groundwater could also
be valuable because, although the drinking-
water MAV does not discriminate between
arsenic species, arsenite is much more toxic
than arsenate (Singh et al., 2011).
Ongoing and future human-induced
changes could impact arsenic contamination
on local scales, even in the oxic aquifers of
the Plains. Although arsenic contamination
is largely of geogenic origin, there are
numerous anthropogenic arsenic sources
(e.g., timber treatment plants, landfills,
sheep dip sites) in Canterbury, whilst human
activities can also alter groundwater redox
state and flow behaviour. For example,
irrigation pumping and return flow can
alter recharge rates, mixing, and natural flow
patterns, which can influence geochemical
conditions. Further, the release of organic
matter (e.g., from manure spreading,
constructed ponds, septic tanks, or landfill
sites) can cause groundwater redox state to
become reducing, whilst phosphate (released
from fertiliser) can compete with arsenate for
binding sites on aquifer minerals (Fendorf
et al., 2010; Lin et al., 2016; Smedley and
Kinniburgh, 2002), though the impact of
these activities on arsenic release is not well
studied in Canterbury.
Conclusions
In Canterbury, arsenic in groundwater
originates from greywacke and fault-fluid
minerals in the Southern Alps, before being
fluvially transported coastward by way of
the Canterbury Plains. Region-wide, most
wells (88%) had groundwater with arsenic
concentrations below detection limits, even in
wells with reducing redox state groundwater
(56.9%). Arsenic contamination was very
rare in the mostly oxic groundwater of the
inland basins and the Canterbury Plains,
because of dilution and oxic conditions.
Conversely, in some coastal areas, there
are diffuse sources of arsenic and conditions
favourable to arsenic release to groundwater.
These processes contributed to exceedances
of the drinking-water MAV for arsenic in
2.6% of wells sampled across the region in
the period from 1989 to 2021. In reducing
groundwater, which is relatively common
in the coastal confined aquifers, arsenic
147
concentrations were highly variable at
relatively small spatial scales. The observed
variability is likely due to a combination of
the presence of arsenic-bearing iron oxides,
the availability of labile organic carbon,
differences in local-scale groundwater
residence times, and sluggish flow rates.
Thus, arsenic concentrations in Canterbury
groundwater are dependent on the presence
of arsenic sources, groundwater redox state,
and the influence of physical hydrological
factors such as mixing and dilution.
Acknowledgements
Andrew Pearson was affiliated with
Environment Canterbury at the time most
of this work was undertaken; however, this
manuscript was completed after he became
affiliated with the Institute of Environmental
Science and Research (ESR). The authors
wish to thank Environment Canterbury’s
Groundwater Science field team for sample
collection, and Philippa Aitchison-Earl
(Environment Canterbury) for providing
the map of aquifer types. Gratitude is also
due to Jennifer Tregurtha for advice with
map production and to Kurt Van Ness and
Ben Wilkins (Environment Canterbury)
for assistance preparing the dataset. The
authors also wish to thank Murray Close of
ESR for sharing the methods for estimating
groundwater redox state for individual wells.
References
Ballance, P.F. 2017: New Zealand geology: an
illustrated guide. GSNZ Miscellaneous
Publication 148, version 2. Geoscience Society
of New Zealand.
Becker, J.A.; Craw, D.; Horton, T.; Chamberlain,
C.P. 2000: Gold mineralisation near the main
divide, upper Wilberforce valley, Southern
Alps, New Zealand. New Zealand Journal
of Geology and Geophysics 43(2): 199–215.
https://doi.org/10.1080/00288306.2000.951
4881
Bindal, S.; Singh, C.K. 2019: Predicting
groundwater arsenic contamination:
Regions at risk in highest populated state of
India. Water Research 159: 65–76. https://doi.
org/10.1016/j.watres.2019.04.054
Borch, T.; Kretzschmar, R.; Kappler, A.;
Cappellen, P.V.; Ginder-Vogel, M.; Voegelin,
A.; Campbell, K. 2010: Biogeochemical
redox processes and their impact on
contaminant dynamics. Environmental Science
& Technology 44(1): 15–23. https://doi.
org/10.1021/es9026248
Brown, L.J.; Weeber, J.H. 1992: Geology of the
Christchurch urban area: Institute of Geological
and Nuclear Sciences Geological Map 1. Institute
of Geological and Nuclear Sciences Limited,
Lower Hutt, New Zealand.
Brown, L.J.; Wilson, D.D.; Moar, N.T.;
Mildenhall, D.C. 1988: Stratigraphy of the
late Quaternary deposits of the northern
Canterbury Plains, New Zealand. New Zealand
Journal of Geology and Geophysics 31(3):
305–335.
Browne, G.H.; Naish, T.R. 2003: Facies
development and sequence architecture of
a late Quaternary fluvial marine transition,
Canterbury Plains and shelf, New Zealand:
implications for forced regressive deposits.
Sedimentary Geology 158(1-2): 57–86. https://
doi.org/10.1016/S0037-0738(02)00258-0
Burbery, L.; Abraham, P.; Sutton, R.; Close,
M. 2022: Evaluation of pollution swapping
phenomena from a woodchip denitrification
wall targeting removal of nitrate in a shallow
gravel aquifer. Science of The Total Environment
820: 153194. https://doi.org/10.1016/j.
scitotenv.2022.153194
Burbery, L.; Sarris, T.; Mellis, R.; Abraham,
P.; Sutton, R.; Finnemore, M.; Close,
M. 2020: Woodchip denitrification wall
technology trialled in a shallow alluvial
gravel aquifer. Ecological Engineering 157:
105996. https://doi.org/10.1016/j.
ecoleng.2020.105996
Close, M.E.; Abraham, P.; Humphries, B.;
Lilburne, L.; Cuthill, T.; Wilson, S. 2016:
Predicting groundwater redox status on
a regional scale using linear discriminant
analysis. Journal of Contaminant Hydrology 191:
19–32. https://doi.org/10.1016/j.
jconhyd.2016.04.006
148
Craw, D.; Koons, P.O.; Horton, T.; Chamberlain,
C.P. 2002: Tectonically driven fluid flow and
gold mineralisation in active collisional orogenic
belts: comparison between New Zealand and
western Himalaya. Tectonophysics 348(1-3):
135–153. https://doi.org/10.1016/S0040-
1951(01)00253-0
Daughney, C.J.; Wall, M. 2007: Groundwater
quality in New Zealand: State and trends 1995-
2006. GNS Science Consultancy Report
2007/23.
deLemos, J.L.; Bostick, B.C.; Renshaw, C.E.;
Stürup, S.; Feng, X. 2006: Landfill-stimulated
iron reduction and arsenic release at the
Coakley Superfund Site (NH). Environmental
Science & Technology 40(1): 67–73. https://doi.
org/10.1021/es051054h
Erban, L.E.; Gorelick, S.M.; Zebker, H.A.;
Fendorf, S. 2013: Release of arsenic to
deep groundwater in the Mekong Delta,
Vietnam, linked to pumping-induced land
subsidence. Proceedings of the National Academy
of Sciences 110(34): 13751–13756.
Fendorf, S.; Michael, H.A.; van Geen, A.
2010: Spatial and temporal variations of
groundwater arsenic in South and Southeast
Asia. Science 328(5982): 1123–1127. https://
doi.org/10.1126/science.1172974
Galloway, J.N.; Aber, J.D.; Erisman, J.W.;
Seitzinger, S.P.; Howarth, R.W.; Cowling,
E. B.; Cosby, B.J. 2003: The nitrogen
cascade. Bioscience 53(4): 341–356.
Gorchev, H.G.; Ozolins, G. 1984: WHO
guidelines for drinking-water quality. WHO
Chronicle 38(3): 104–108.
Harvey, C.F.; Swartz, C.H.; Badruzzaman,
A.B.M.; Keon-Blute, N.; Yu, W.; Ali, M.A.; Jay,
J.; Beckie, R.; Niedan, V.; Brabander, D.; Oates,
P.M. 2002: Arsenic mobility and groundwater
extraction in Bangladesh. Science 298(5598):
1602–1606. https://doi.org/10.1126/
science.1076978
Horton, T.W.; Becker, J.A.; Craw, D.; Koons,
P.O.; Chamberlain, C.P. 2001: Hydrothermal
arsenic enrichment in an active mountain belt:
Southern Alps, New Zealand. Chemical Geology
177(3-4): 323–339. https://doi.org/10.1016/
S0009-2541(00)00416-2
Islam, F.S.; Gault, A.G.; Boothman, C.; Polya,
D.A.; Charnock, J.M.; Chatterjee, D.;
Lloyd, J.R. 2004: Role of metal-reducing
bacteria in arsenic release from Bengal delta
sediments. Nature 430(6995): 68–71. https://
doi.org/10.1038/nature02638
Jakobsen, R.; Kazmierczak, J.; Sø, H.U.; Postma,
D. 2018: Spatial variability of groundwater
arsenic concentration as controlled by
hydrogeology: Conceptual analysis using
2-D reactive transport modeling. Water
Resources Research 54(12): 10–254. https://doi.
org/10.1029/2018WR023685
Kerr, G.; Craw, D. 2021: Arsenic residues
from historic gold extraction, Snowy River,
Westland, New Zealand. New Zealand Journal
of Geology and Geophysics 64(1): 107–119.
https://doi.org/10.1080/00288306.2020.175
7471
Leckie, D.A. 2003: Modern environments of
the Canterbury Plains and adjacent offshore
areas, New Zealand—an analog for ancient
conglomeratic depositional systems in
nonmarine and coastal zone settings. Bulletin
of Canadian Petroleum Geology 51(4): 389–
425. https://doi.org/10.2113/51.4.389
Levy, A.; Ettema, M.; Lu, X. 2021: State of the
Environment- Groundwater Quality in Otago.
Otago Regional Council Report. ISBN: 978-
0-908324-68-2
Lin, T.Y.; Wei, C.C.; Huang, C.W.; Chang,
C.H.; Hsu, F.L.; Liao, V.H.C. 2016: Both
phosphorus fertilizers and indigenous bacteria
enhance arsenic release into groundwater
in arsenic-contaminated aquifers. Journal of
Agricultural and Food Chemistry 64(11): 2214-
2222.
Litter, M.I.; Ingallinella, A.M.; Olmos, V.;
Savio, M.; Difeo, G.; Botto, L.; Torres,
E.M.F.; Taylor, S.; Frangie, S.; Herkovits, J.;
Schalamuk, I. 2019: Arsenic in Argentina:
Occurrence, human health, legislation
and determination. Science of the Total
Environment 676: 756–766. https://doi.
org/10.1016/j.scitotenv.2019.04.262
Macara, G.R. 2016: The climate and weather of
Canterbury. NIWA Science and Technology
Series 68.
149
McDonough, L.K.; Santos, I.R.; Andersen, M.S.;
O’Carroll, D.M.; Rutlidge, H.; Meredith, K.;
Oudone, P.; Bridgeman, J.; Gooddy, D.C.;
Sorensen, J.P.; Lapworth, D.J. 2020: Changes
in global groundwater organic carbon driven
by climate change and urbanization. Nature
Communications 11(1): 1–10. https://doi.
org/10.1038/s41467-020-14946-1
McMahon, P.B.; Chapelle, F.H. 2008: Redox
processes and water quality of selected
principal aquifer systems. Groundwater 46(2):
259–271. https://doi.org/10.1111/j.1745-
6584.2007.00385.x
Ministry of Health. 2018: Drinking-water
Standards for New Zealand 2005 (Revised
2018). Ministry of Health, Wellington.
National Environmental Monitoring Standards
(NEMS). 2019: Water Quality- Part 1 of 4:
Sampling, Measuring, Processing and Archiving
of Discrete Groundwater Quality Data.
Neumann, R.B.; Ashfaque, K.N.; Badruzzaman,
A.B.M.; Ali, M.A.; Shoemaker, J.K.; Harvey,
C.F. 2010: Anthropogenic influences on
groundwater arsenic concentrations in
Bangladesh. Nature Geoscience 3(1): 46–52.
https://doi.org/10.1038/ngeo685
Nordstrom, D.K. 2002: Worldwide occurrences
of arsenic in ground water. Science 296(5576):
2143–2145.
Painter, B. 2018: Protection of groundwater
dependent ecosystems in Canterbury, New
Zealand: the Targeted Stream Augmentation
Project. Sustainable Water Resources
Management 4(2): 291–300. https://doi.
org/10.1007/s40899-017-0188-2
Piper, J.; Kim, N. 2006: Arsenic in groundwater
of the Waikato region. Environment Waikato
Regional Council, report TR2006/14.
Podgorski, J.; Berg, M. 2020: Global threat of
arsenic in groundwater. Science 368(6493):
845–850. https://doi.org/10.1126/science.
aba1510
Postma, D.; Mai, N.T.H.; Lan, V.M.; Trang,
P.T.K.; Sø, H.U.; Nhan, P.; Larsen, F.; Viet,
P.H.; Jakobsen, R. 2017: Fate of arsenic during
Red River water infiltration into aquifers
beneath Hanoi, Vietnam. Environmental
Science & Technology 51(2): 838–845. https://
doi.org/10.1021/acs.est.6b05065
Postma, D.; Pham, T.K.T.; Sø, H.U.; Vi,
M.L.; Nguyen, T.T.; Larsen, F.; Pham,
H.V.; Jakobsen, R. 2016: A model for the
evolution in water chemistry of an arsenic
contaminated aquifer over the last 6000 years,
Red River floodplain, Vietnam. Geochimica et
Cosmochimica Acta 195: 277–292. https://doi.
org/10.1016/j.gca.2016.09.014
Robinson, B.; Clothier, B.; Bolan, N.S.;
Mahimairaja, S.; Greven, M.; Moni, C.;
Marchetti, M.; Van den Dijssel, C.; Milne, G.
2004: Arsenic in the New Zealand environment.
Paper presented at the 3rd Australian New
Zealand Soils Conference, 5–9 December
2004.
Rotiroti, M.; Jakobsen, R.; Fumagalli, L.; Bonomi,
T. 2015: Arsenic release and attenuation in a
multilayer aquifer in the Po Plain (northern
Italy): reactive transport modelling. Applied
Geochemistry 63: 599–609. https://doi.
org/10.1016/j.apgeochem.2015.07.001
Safa, M.; O’Carroll, D.; Mansouri, N.; Robinson,
B.; Curline, G. 2020: Investigating arsenic
impact of ACC treated timbers in compost
production (A case study in Christchurch,
New Zealand). Environmental Pollution 262:
114218. https://doi.org/10.1016/j.
envpol.2020.114218
Schmidt, J.; Almond, P.C.; Basher, L.; Carrick,
S.; Hewitt, A.E.; Lynn, I.H.; Webb, T.H.
2005: Modelling loess landscapes for the
South Island, New Zealand, based on expert
knowledge. New Zealand Journal of Geology
and Geophysics 48(1): 117–133. https://doi.or
g/10.1080/00288306.2005.9515103
Scott, L.; Wong, R.; Koh, S. 2016: The Current
State of Groundwater Quality in the Waimakariri
CWMS zone. Canterbury Regional Council
Report No. R16/48.
Singh, A.P.; Goel, R.K.; Kaur, T. 2011:
Mechanisms pertaining to arsenic
toxicity. Toxicology International 18(2): 87–93.
https://doi.org/10.4103/0971-6580.84258
Smedley, P.L.; Kinniburgh, D.G. 2002: A review
of the source, behaviour and distribution
of arsenic in natural waters. Applied
Geochemistry 17(5): 517–568. https://doi.
org/10.1016/S0883-2927(02)00018-5
150
Sø, H.U.; Postma, D.; Vi, M.L.; Pham, T.K.T.;
Kazmierczak, J.; Dao, V.N.; Pi, K.; Koch,
C.B.; Pham, H.V.; Jakobsen, R. 2018:
Arsenic in Holocene aquifers of the Red River
floodplain, Vietnam: effects of sediment-
water interactions, sediment burial age and
groundwater residence time. Geochimica et
Cosmochimica Acta 225: 192–209. https://doi.
org/10.1016/j.gca.2018.01.010
Stats NZ. 2017: Groundwater Physical Stocks.
Retrieved 1 August 2021 from https://www.
stats.govt.nz/indicators/groundwater-physical-
stocks
Stewart, M.K.; Aitchison-Earl, P.L. 2020:
Irrigation return flow causing a nitrate hotspot
and denitrification imprints in groundwater at
Tinwald, New Zealand. Hydrology and Earth
System Sciences 24(7): 3583–3601. https://doi.
org/10.5194/hess-24-3583-2020
Stewart, M.K.; van der Raaij, R.W. 2022:
Response of the Christchurch groundwater
system to exploitation: Carbon-14 and
tritium study revisited. Science of The
Total Environment: 152730. https://doi.
org/10.1016/j.scitotenv.2021.152730
Stuckey, J.W.; Schaefer, M.V.; Kocar, B.D.;
Benner, S.G.; Fendorf, S. 2016: Arsenic
release metabolically limited to permanently
water-saturated soil in Mekong Delta. Nature
Geoscience 9(1): 70–76. https://doi.
org/10.1038/ngeo2589
van Geen, A.; Zheng, Y.J.; Versteeg, R.; Stute,
M.; Horneman, A.; Dhar, R.; Steckler, M.;
Gelman, A.; Small, C.; Ahsan, H.; Graziano,
J.H. 2003: Spatial variability of arsenic in 6000
tube wells in a 25 km2 area of Bangladesh.
Water Resources Research 39(5): 1140. https://
doi.org/10.1029/2002WR001617
Wallis, I.; Prommer, H.; Berg, M.; Siade,
A.J.; Sun, J.; Kipfer, R. 2020: The river–
groundwater interface as a hotspot for arsenic
release. Nature Geoscience 13(4): 288–295.
https://doi.org/10.1038/s41561-020-0557-6
Welch, A.H.; Westjohn, D.B.; Helsel, D.R.;
Wanty, R.B. 2000: Arsenic in ground
water of the United States: Occurrence and
Geochemistry. Groundwater 38(4): 589–604.
https://doi.org/10.1111/j.1745-6584.2000.
tb00251.x
Wilson, S.R.; Close, M.E.; Abraham, P.; Sarris,
T.S.; Banasiak, L.; Stenger, R.; Hadfield, J. 2020:
Achieving unbiased predictions of national-
scale groundwater redox conditions via data
oversampling and statistical learning. Science of
The Total Environment 705: 135877. https://
doi.org/10.1016/j.scitotenv.2019.135877
Ying, S.C.; Schaefer, M.V.; Cock-Esteb, A.; Li,
J.; Fendorf, S. 2017: Depth stratification leads
to distinct zones of manganese and arsenic
contaminated groundwater. Environmental
Science & Technology 51(16): 8926–8932.
https://doi.org/10.1021/acs.est.7b01121
Zhang, Z.; Xiao, C.; Adeyeye, O.; Yang, W.;
Liang, X. 2020: Source and mobilization
mechanism of iron, manganese, and arsenic
in groundwater of Shuangliao City, Northeast
China. Water 12(2): 534. https://doi.
org/10.3390/w12020534
Manuscript received 16 February 2022; accepted for 19 April 2022
... Yet, the response of DOC export to rising global temperatures and changes in hydroclimate also has implications for freshwater and marine ecosystems. DOC increases in aquatic systems have significant impacts, including eutrophication 7 , reduced surface water clarity 9,13 , and enhanced contaminant transport 8 , as well as influencing groundwater pH 14 and fuelling redox transformations in the subsurface [15][16][17] . Generally, aquatic DOC concentrations are positively related to SOC stocks in developed soils, and vegetation cover 18 , but can also be influenced by the presence of peat, which can export up to 10 times more DOC than forest soils 19 . ...
... But, the analysis of climate-DOC feedbacks is confounded by human perturbation of landscapes (e.g., agricultural landuse and urbanisation) 21 and atmospheric chemistry (e.g., increased N and S deposition associated with fossil fuel combustion) over this time interval [22][23][24] . While some studies report links between aquatic DOC concentration and recovery from soil acidification on continental scales [14][15][16] , others show positive links between atmospheric temperature and DOC export [25][26][27] . For example, an assessment of 315 records (≥10 years) of riverine DOC across Great Britain showed that increasing concentrations were correlated with increasing air temperature and atmospheric CO 2 28 . ...
Article
Full-text available
Despite decades of research, the influence of climate on the export of dissolved organic carbon (DOC) from soil remains poorly constrained, adding uncertainty to global carbon models. The limited temporal range of contemporary monitoring data, ongoing climate reorganisation and confounding anthropogenic activities muddy the waters further. Here, we reconstruct DOC leaching over the last ~14,000 years using alpine environmental archives (two speleothems and one lake sediment core) across 4° of latitude from Te Waipounamu/South Island of Aotearoa New Zealand. We selected broadly comparable palaeoenvironmental archives in mountainous catchments, free of anthropogenically-induced landscape changes prior to ~1200 C.E. We show that warmer temperatures resulted in increased allochthonous DOC export through the Holocene, most notably during the Holocene Climatic Optimum (HCO), which was some 1.5–2.5 °C warmer than the late pre-industrial period—then decreased during the cooler mid-Holocene. We propose that temperature exerted the key control on the observed doubling to tripling of soil DOC export during the HCO, presumably via temperature-mediated changes in vegetative soil C inputs and microbial degradation rates. Future warming may accelerate DOC export from mountainous catchments, with implications for the global carbon cycle and water quality.
Article
Full-text available
High levels of arsenic in well water are causing widespread poisoning in Bangladesh. In a typical aquifer in southern Bangladesh, chemical data imply that arsenic mobilization is associated with recent inflow of carbon. High concentrations of radiocarbon-young methane indicate that young carbon has driven recent biogeochemical processes, and irrigation pumping is sufficient to have drawn water to the depth where dissolved arsenic is at a maximum. The results of field injection of molasses, nitrate, and low-arsenic water show that organic carbon or its degradation products may quickly mobilize arsenic, oxidants may lower arsenic concentrations, and sorption of arsenic is limited by saturation of aquifer materials.
Article
Full-text available
Nitrate concentrations in groundwater have been historically high (N≥11.3 mg L-1) in an area surrounding Tinwald, Ashburton, since at least the mid-1980s. The local community is interested in methods to remediate the high nitrate in groundwater. To do this, they need to know where the nitrate is coming from. Tinwald groundwater exhibits two features stemming from irrigation with local groundwater (i.e. irrigation return flow). The first feature is increased concentrations of nitrate (and other chemicals and stable isotopes) in a “hotspot” around Tinwald. The chemical concentrations of the groundwater are increased by recirculation of water already relatively high in chemicals. The irrigation return flow coefficient C (irrigation return flow divided by irrigation flow) is found to be consistent with the chemical enrichments. The stable isotopes of the groundwater show a similar pattern of enrichment by irrigation return flow of up to 40 % and are also enriched by evaporation (causing a loss of about 5 % of the original water mass). Management implications are that irrigation return flow needs to be taken into account in modelling of nitrate transport through soil–groundwater systems and in avoiding overuse of nitrate fertiliser leading to greater leaching of nitrate to the groundwater and unnecessary economic cost. The second feature is the presence of “denitrification imprints” (shown by enrichment of the δ15N and δ18ONO3 values of nitrate) in even relatively oxic groundwaters. The denitrification imprints can be clearly seen because (apart from denitrification) the nitrate has a blended isotopic composition due to irrigation return flow and N being retained in the soil–plant system as organic N. The nitrate concentration and isotopic compositions of nitrate are found to be correlated with the dissolved oxygen (DO) concentration. This denitrification imprint is attributed to localised denitrification in fine pores or small-scale physical heterogeneity where conditions are reducing. The implication is that denitrification could be occurring where it is not expected because groundwater DO concentrations are not low.
Article
Full-text available
Geogenic groundwater arsenic (As) contamination is pervasive in many aquifers in south and southeast Asia. It is feared that recent increases in groundwater abstractions could induce the migration of high-As groundwaters into previously As-safe aquifers. Here we study an As-contaminated aquifer in Van Phuc, Vietnam, located ~10 km southeast of Hanoi on the banks of the Red River, which is affected by large-scale groundwater abstraction. We used numerical model simulations to integrate the groundwater flow and biogeochemical reaction processes at the aquifer scale, constrained by detailed hydraulic, environmental tracer, hydrochemical and mineralogical data. Our simulations provide a mechanistic reconstruction of the anthropogenically induced spatiotemporal variations in groundwater flow and biogeochemical dynamics and determine the evolution of the migration rate and mass balance of As over several decades. We found that the riverbed–aquifer interface constitutes a biogeochemical reaction hotspot that acts as the main source of elevated As concentrations. We show that a sustained As release relies on regular replenishment of river muds rich in labile organic matter and reactive iron oxides and that pumping-induced groundwater flow may facilitate As migration over distances of several kilometres into adjacent aquifers.
Article
Full-text available
Climate change and urbanization can increase pressures on groundwater resources, but little is known about how groundwater quality will change. Here, we use a global synthesis (n = 9,404) to reveal the drivers of dissolved organic carbon (DOC), which is an important component of water chemistry and substrate for microorganisms that control biogeochemical reactions. Dissolved inorganic chemistry, local climate and land use explained ~ 31% of observed variability in groundwater DOC, whilst aquifer age explained an additional 16%. We identify a 19% increase in DOC associated with urban land cover. We predict major groundwater DOC increases following changes in precipitation and temperature in key areas relying on groundwater. Climate change and conversion of natural or agricultural areas to urban areas will decrease groundwater quality and increase water treatment costs, compounding existing constraints on groundwater resources.
Article
Full-text available
Excessive levels of Fe, Mn and As are the main factors affecting groundwater quality in Songliao plain, northeast China. However, there are few studies on the source and mobilization mechanisms of Fe, Mn and As in the groundwater of Northeastern China. This study takes Shuangliao city in the middle of Songliao plain as an example, where the source and mobilization mechanisms of iron, manganese and arsenic in groundwater in the study area were analyzed by statistical methods and spatial analysis. The results show that the source of Fe and Mn in the groundwater of the platform is the iron and manganese nodules in the clay layer, while, in the river valley plain, it originates from the soil and the whole aquifer. The TDS, fluctuation in groundwater levels and the residence time are the important factors affecting the content of Fe and Mn in groundwater. The dissolution of iron and manganese minerals causes arsenic adsorbed on them to be released into groundwater. This study provides a basis for the rational utilization of groundwater and protection of people’s health in areas with high iron, manganese and arsenic contents.
Article
Woodchip denitrification walls offer a potentially useful way for passive in situ remediation of groundwater nitrate pollution, yet because of the low redox state they induce on the subsurface environment there is an inherent risk they can promote pollution-swapping phenomena. We evaluated pollution-swapping phenomena associated with the first two operational years of a woodchip denitrification wall that is being trialled in a fast-flowing shallow gravel aquifer of quartzo-feldspathic mineralogy. Following burial of woodchip below the water table there was immediate export of dissolved organic carbon (DOC), phosphorus and ammonium into the groundwater. Under the low redox state sustained by labile DOC, the wall initially provided 100% nitrate removal at the expense of acute and localised pollution that occurred in the form of a plume of dissolved iron, manganese and arsenic that were mobilised from the aquifer sediments, in conjunction with methane gas emission. Within one year however, the reactivity of the woodchip wall subsided to support a steady state condition in which nitrate reduction was the terminal electron acceptor process with no measurable methane emission. Having initially functioned as a sink for the potent greenhouse gas nitrous oxide (N2O), evidence is that the woodchip wall is now exporting N2O, albeit at rates less than those associated with productive agricultural land.
Article
The Christchurch groundwater system is an exceptional water resource with very high drinking water quality supplying all the water requirements of the city. The groundwater system has changed over the years because of rising groundwater abstraction due to increasing population and development. The present (2017) data revealed slightly older ¹⁴C ages and increasingly steep west-east age gradients compared to the earlier work from 1976 to 2006, showing continued upflow of deep water into the exploited aquifers which is much older on the east (coastal) side than on the west (inland) side. In addition, the ³H ages for wells on the west side of the system are often much younger than their ¹⁴C ages showing that there is input of young shallow water to the wells in addition to the deep water input. Application of a binary model identifies the ages and amounts of the two components, showing that the young component is becoming younger although smaller as a proportion of the flow, and the old component from depth is becoming larger. Newly completed wells near the Waimakariri River have allowed identification of the young component, which is almost entirely composed of very young Waimakariri River seepage at all depths and therefore has very little chemical loading. Instead any chemical input (e.g. chloride, nitrate) to the Christchurch aquifers is being brought in by the old deep component which on the western side of the Christchurch system is derived from rainfall recharge on the developing Ashley-Waimakariri Plains area (plus river seepage). Chemical traces of this deep input from the northwest are at present very subtle, although more appreciable signals are seen in some wells further to the north of Christchurch. In the future, slowly increasing chemical input to the Christchurch aquifers on the west side of the system is to be expected as abstraction increases.
Article
Woodchip denitrification walls are a tried and tested groundwater nitrate remediation concept in shallow sandy aquifer conditions. There are however no published case studies of them having been applied in heterogeneous, fast-flowing gravel aquifers. Such a pilot study is being made in a shallow alluvial gravel aquifer on the Canterbury Plains, New Zealand, as part of an assessment of whether denitrification walls represent a viable edge-of-field nitrate mitigation option for the New Zealand hydrological landscape. Hydrogeological conditions at the field study site were characterised using a suite of investigative methods, the results from which informed design and placement of an experimental woodchip denitrification wall that was installed in November 2018. The average specific flux in the target gravel aquifer is estimated at 2.7 m/d, and 3.1 m/d through the woodchip wall itself, owing to its hydraulic efficiency. These groundwater fluxes are significantly higher than conditions reported for pre-existing denitrification wall case-studies. Monitoring of the groundwater chemistry over the first year of the denitrification woodchip wall's operational life has shown how the woodchip initially leached labile dissolved organic carbon and created a redox plume in which methanogenic conditions existed. Even though dissolved organic carbon concentrations have restored to background levels, the woodchip wall remains effective at nitrate reduction. The measured nitrate removal rate of between 4.2 and 5.4 g N removed/m³ wall/d is higher than what had previously been predicted from controlled lab-scale studies of the wall media and ranks towards the higher end of published removal rates for denitrification walls. Whilst there is direct evidence that heterotrophic denitrification is contributing to the observed nitrate removal, on the basis of chemical indicators, it is assumed other reactive process, such as dissimilatory reduction to ammonia, anammox, and possibly nitrate-reducing Fe(II)-oxidising reduction reactions may also be contributing to the overall removal of nitrogen in the system. Indications are the woodchip wall is enhancing emission of methane gas, albeit at rates less than what is typically reported for constructed wetlands that are an alternative nitrate-remediation option. Emission of the more potent greenhouse gas nitrous oxide from the woodchip denitrification wall has so far been immeasurably low. Longer-term study of the woodchip denitrification wall is continuing.
Article
Dowsing for danger Arsenic is a metabolic poison that is present in minute quantities in most rock materials and, under certain natural conditions, can accumulate in aquifers and cause adverse health effects. Podgorski and Berg used measurements of arsenic in groundwater from ∼80 previous studies to train a machine-learning model with globally continuous predictor variables, including climate, soil, and topography (see the Perspective by Zheng). The output global map reveals the potential for hazard from arsenic contamination in groundwater, even in many places where there are sparse or no reported measurements. The highest-risk regions include areas of southern and central Asia and South America. Understanding arsenic hazard is especially essential in areas facing current or future water insecurity. Science , this issue p. 845 ; see also p. 818
Article
The arsenic concentration is an important issue in compost production. The main inputs of a compost factory, including kerbsides, green wastes, food industry wastes, and river weeds are investigated in this study. Also, this study investigated how treated timbers, ashes, and other contamination can impact arsenic concentration in compost production. The results showed that most treated timbers and all ashes of treated and untreated timbers contained significant amounts of arsenic. These results revealed that the presence of a small amount of treated timber ashes can significantly increase the arsenic concentration in composts. The results of the study show the arsenic concentration in compost increase during cold months, and it dropped during summer, which would be mostly because of high arsenic concentration in ashes of log burners. This study shows ashes of burning timbers can impact arsenic contamination mostly because of using Copper-Chrome-Arsenic wood preservatives (CCA). Also, the lab results show the arsenic level even in ashes of untreated timber is around 96 ppm. The ashes of H3, H4, and H5 treated timbers contain approximately 133,000, 155,000, and 179,000 ppm of arsenic, which one kg of them can increase arsenic concentration around 10 ppm in 13.3, 15.5 and 17.9 tons of dry compost products. The main problem is many people look at ashes and treated timber as organic materials; however, ashes of treated and untreated timbers contained high concentrations of arsenic. Therefore, it was necessary to warn people about the dangers of putting any ashes in organic waste bins.