The terrestrial subsurface is the largest source of
freshwater globally. The organic carbon contained within it and processes
controlling its concentration remain largely unknown. The global median
concentration of dissolved organic carbon (DOC) in groundwater is low
compared to surface waters, suggesting significant processing in the
subsurface. Yet the processes that remove this DOC in groundwater are not
fully understood. The purpose of this study was to investigate the different
sources and processes influencing DOC in a shallow anoxic coastal aquifer.
Uniquely, this study combines liquid chromatography organic carbon detection
with organic (δ13CDOC) carbon isotope geochemical
analyses to fingerprint the various DOC sources that influence the
concentration, carbon isotopic composition, and character with respect to distance from
surface water sources, depth below surface, and inferred groundwater
residence time (using 3H activities) in groundwater. It was found that
the average groundwater DOC concentration was 5 times higher
(5 mg L-1) than the global median concentration and that the concentration doubled
with depth at our site, but the chromatographic character did not change
significantly. The anoxic saturated conditions of the aquifer limited the
rate of organic matter processing, leading to enhanced preservation and
storage of the DOC sources from peats and palaeosols contained within the
aquifer. All groundwater samples were more aromatic for their molecular
weight in comparison to other lakes, rivers and surface marine samples
studied. The destabilization or changes in hydrology, whether by
anthropogenic or natural processes, could lead to the flux of up to 10 times
more unreacted organic carbon from this coastal aquifer compared to
deeper inland aquifers.
Introduction
Organic matter (OM) in aquatic systems forms a fundamental part of the
global carbon cycle. The soil and unsaturated zone has the potential to
store at least 3 times more organic carbon than in the atmosphere or in
living plants (Schmidt et al., 2011; Fontaine et al., 2007) and represents
the largest source of carbon within terrestrial ecosystems (Keiluweit et
al., 2017). The below-ground environment forms the largest source of
freshwater, and the organic carbon within these systems remains largely
unaccounted for in the global carbon budget. Groundwater replenishment can
occur either as diffuse recharge through the OM-rich soil zone or as indirect recharge originating from streams and wetlands that have the
potential to contain high concentrations of OM. Despite the many sources of
OM, groundwater dissolved organic carbon (DOC) concentrations are typically low (∼1 mg L-1 for the global median DOC concentration; McDonough et al., 2020a),
suggesting significant processing in the subsurface.
Very few studies have investigated why concentrations of DOC in groundwater
are low. Recent research provided evidence of the attenuation of
chromophoric dissolved organic matter (DOM) at a scale of tens of metres
along a flow path with measurements of increased optical clarity of
groundwater (Chapelle et al., 2016). This attenuation was proposed to be due
to the combined effects of biodegradation and sorption. Chapelle et al. (2016) also showed that the presence of a hyperbolic relationship between
DOC concentrations and dissolved oxygen provided indirect evidence for
groundwater DOC that is bioavailable to microbes. Direct monitoring of
groundwater DOC within a fractured rock aquifer in South Carolina, USA,
which included measurements of both bioavailable and chromophoric DOM, was
performed by Shen et al. (2015). They demonstrated a decrease in
lignin-derived phenols in groundwater compared to surface inputs and
prevalent amino acids in groundwater. At this site, it was proposed that a
small fraction (8%±4 %) of groundwater DOC was bioavailable and that
a substantial fraction of groundwater DOC was of bacterial origin
(15 %–34 %). Shen et al. (2015) suggested that DOM mobility could be
described by a regional chromatography model (Hedges et al., 1986, 1994)
as it moved through the soil column to the groundwater. It was shown that
the molecular size, polarity, charge and bioavailability determined the
observed decrease in hydrophobicity of DOM along the flow path due to their
controls on sorption, desorption, biodegradation and biosynthesis (Shen et
al., 2015).
Carbon isotope ratios were first used in the 1960s to distinguish sources of
OM in the coastal zone in order to identify the difference between salt-marsh
sediments and freshwater peats (Emery et al., 1967). The use of carbon
isotopes as tracers has been instrumental in providing greater understanding
of the sources of carbon in coastal, terrestrial and marine environments
(Benner et al., 1984; Lamb et al., 2006). Very few studies that used DOM
chromatographic techniques also employed isotopic techniques to understand
the source of OM in groundwater and vice versa. The use of multiple DOM
characterization techniques to improve our understanding of the role of
different OM sources in contributing to the concentration, character, and
its subsequent processing along groundwater flow paths is therefore in its
early stages of research.
Further work is clearly warranted to improve our understanding of the role
of different OM sources and the physicochemical properties of the aquifer on
groundwater DOM. To this end, we present groundwater DOC concentration,
stable carbon isotopic signatures (δ13C) and organic matter
character data from a shallow (less than 20 m in depth) anoxic coastal
aquifer. Within this hydrogeologically well characterized system, it is
anticipated that multiple sources of OM exist including wetland, soil, peat
and palaeosols. To better understand and test the regional chromatography
model, we utilize a chromatographic technique (liquid chromatography organic
carbon detection – LC-OCD; Huber et al., 2011). Uniquely, this study
combines the chromatographic technique with organic isotope geochemical
analyses to fingerprint the various DOC sources that influence
concentration, δ13CDOC and character with respect to distance, depth
and inferred groundwater residence time (using 3H activities). This was
done to identify controls on groundwater DOC sources and processing in a
coastal groundwater system.
Environmental setting
The coastal environment of Samurai Beach is located northeast of Anna Bay,
New South Wales, Australia (Fig. 1). The Holocene sand dunes at the site
rise 30 m above sea level and extend up to 800 m inland. A freshwater
wetland and forest lie in the northwest corner of the sand dunes near Site 1
(S1), and the aquifer is part of the Tomaree groundwater source
supplying towns in the Hunter Valley with potable water. The local geology
of the field site was investigated during borehole construction and using
hydraulic profiling tools to produce the lithology cross section depicted in
Fig. 2 (Maric, 2013; Howley, 2014). The lithological cross section is
parallel with the main groundwater flow direction from the wetland to the
coast.
Location of the study area and groundwater monitoring wells in
relation to coastal quaternary geology (adapted from Hashimoto and Toedson,
2008).
The upper 15 m of the aquifer contained a combination of barrier and back
barrier sand deposits, capped by aeolian dune deposits. Site 1 was
located in the northwest corner closest to the freshwater wetland, while
Site 5 (S5) was closest to the Pacific Ocean. The lithology of the aquifer
differs with distance along the transect with a peat layer identified at
∼5 m below ground level at sites 1 and 2 and a clay unit at
Site 5 (Fig. 2). A shallower organic-rich layer was also identified at about
1 m b.g.l. (below ground level) at S1.
Five sites were drilled and wells installed to three different depths:
shallow (S_3.4 to 5.0 m b.g.l.), medium (M_9.8 to
12.5 m) and deep (D_12.7 to 17.5 m), which formed a
∼500 m long transect. The boreholes were drilled by hollow
stem auger using a Geoprobe rig, and wells were constructed out of 50 mm
diameter PVC with screened intervals of 1 m located at the bottom.
Lithology and potentiometric contours (in metres) in the aquifer
in February 2014 (adapted from Maric, 2013; Howley, 2014). Open black
rectangles represent groundwater wells, filled rectangles are the
location of the well-screen intervals within the aquifer and arrows indicate
groundwater flow direction.
Three different sources of groundwater were identified based on
hydrochemistry and the evaluation of groundwater flow direction (Maric, 2013; Howley, 2014). These groundwater sources included (1) direct rainfall
recharge through the dunes, (2) indirect wetland infiltration and (3) deeper regional groundwater (Fig. 2). Multiple sources of OM may be present
at the site from (1) surface vegetation, (2) transported particulate organic
matter (POM) and/or (3) in situ sedimentary sources such as peat, palaeosol
or finely disseminated POM.
Methods
From 17 to 22 February 2014, 15 groundwater samples
were collected from the five nested sites (Fig. 1). Surface water samples
were also collected from the adjacent wetland. Waters were measured for
major ion chemistry, carbon isotopes (δ13CDIC, where DIC represents dissolved inorganic carbon, and δ13CDOC), tritium (3H) and DOM character.
Groundwater samples were generally collected using a submersible centrifugal
pump (Monsoon). A Hach multimeter (HQ40d) and probes (LDO probe) were used for dissolved
oxygen and pH measurements in an inline
flow cell (Waterra)
isolated from the atmosphere. Subsamples for laboratory analysis were
collected through an inline, 0.45 µm filter (bypassing the flow cell)
with δ13CDIC and δ13CDOC samples
further filtered through a 0.22 µm filter. Total alkalinity was determined in
the field by Gran titration (Stumm and Morgan, 1996) using a Hach digital
titrator (model 16900) and 0.16 N H2SO4. Samples for anions were
collected in 50 mL polyethylene centrifuge bottles with no further
treatment, but they were stored cool (∼5∘C) and dark.
Samples for cations were collected in 20 mL high-density polyethylene
acid-washed bottles and acidified with 1 % of concentrated nitric acid
(HNO3). The δ13CDIC samples were collected in 12 mL
pre-combusted glass vials (Exetainers) with no head space. The DOC and
δ13CDOC were collected in 60 mL acid-washed HDPE bottles
and frozen within 12 h of collection. Samples for 3H analysis were
collected in 1 L Nalgene HDPE bottles and were sealed with tape to eliminate
atmospheric exchange during storage. Major and minor cations were determined
using a Perkin Elmer NexION 300D ICP-MS and a Perkin Elmer Optima 7300 ICP-OES.
Anions (for Cl and SO4) were analysed using a Dionex IC1000 ion
chromatography system. Cations and anions were assessed for accuracy by
evaluating the charge balance error percentage (CBE %; Table 2). Samples
fell within the acceptable ±5 % range.
The δ13CDIC isotopic ratios of waters were analysed with an
isotope ratio mass spectrometer (IRMS), and the results were reported as per mil (‰) deviations from the international carbonate
standard, NBS 19, with a precision of ±0.1 ‰,
according to the methods reported in Meredith et al. (2016). The DOC
concentration and δ13CDOC were analysed using a total
organic carbon analyser interfaced with a PDZ Europa 20-20 IRMS utilizing a
GD100 gas trap interface. Results were reported as ‰ deviations from the NIST standard reference material
with an analytical precision of ±0.6 ‰. The
3H activities were expressed in tritium units (TU) with an uncertainty
of ±0.1 TU and a quantification limit of 0.3 TU. Samples were analysed
by liquid scintillation counting. Extended methods for 3H activities
can be found in Meredith et al. (2012). Tritium activities were used as an
indication of groundwater recharge occurrence by rainfall, and groundwater
ages were not calculated due to a lack of time series data collected for
this study. Bryan et al. (2020) show the importance of collecting 3H
data and then calculating a groundwater age using a lumped parameter model
in a shallow unconfined aquifer.
The DOM character was determined using liquid chromatography organic
carbon detection (LC-OCD). The LC-OCD technique is a
size-exclusion chromatographic technique that allows for the
characterization of DOC, based on molecular weight, into six fractions. The
fractions obtained are biopolymers (> 20 kDa), humic substances
(∼1000 Da), building blocks (300–500 Da), low molecular
weight acids (< 350 Da), low molecular weight neutrals
(< 350 Da) and a hydrophobic fraction (fraction of DOC that remains
in the column and is determined by the difference between total DOC and the
total of the other fractions); for full details see Huber et al. (2011).
The humic substances fraction is further characterized for its molecular
weight (based on the retention time of the humic substances peak) and
aromaticity (the specific UV absorption at 254 nm of the humic substances
peak). Calibration is based on the chromatograms of the Suwanee River humic and fulvic acid standards of the International Humic
Substances Society (IHSS).
Principal component analysis (PCA) was performed in R. Parameters which
were consistently above their limit of detection were investigated by PCA.
The data were centred to the mean of the variable and then scaled using the
variable standard deviations in R and the prcomp function: https://stat.ethz.ch/R-manual/R-devel/library/stats/html/prcomp.html (last access: 18 February 2020).
Water chemistry data for the site. Elevation of midpoint of the
screen (m AHD), EC (electrical conductivity; µS cm-1), DO (dissolved oxygen; mg L-1), T (temperature; ∘C), Eh (redox potential; mV), cations and anions (mg L-1), and n.a. (not
detected).
The relationship between (a) chloride concentration (mmol L-1) with the dashed rectangle representing rainfall values and (b) tritium (TU) content with distance from the wetland with the dotted line
representing recently recharged groundwaters (i.e. between 1 to 5 years
old).
Results
Although common at coastal sites, a fresh–saline water interface was not
identified in the wells, even though Site 5 is located ∼100 m
from the ocean and there was no evidence of seawater infiltration after
storm events based on the hydrochemical data. Groundwater had low salinities
with Cl concentrations ranging from 0.5–1.2 mmol L-1. The highest Cl
concentration occurred in the deeper groundwater at sites 1 and 2 (Table 1
and Fig. 3a). Water level data suggested that under most conditions
groundwater followed a west to east direction towards the coastline (Fig. 2). Notably, groundwaters were anoxic with dissolved oxygen below 0.2 mg L-1 (Table 1). Water level patterns suggested that a buried-peat layer
identified at sites 1 and 2 was restricting the vertical flow of rainfall-derived recharge into the deeper sections of the aquifer at these two sites.
The presence of detectable 3H in all samples (> 0.7 TU)
indicates that the groundwater had a component of water that had been in contact with the atmosphere during the past decade. Variations in 3H contents were
seen with depth and distance from the wetland, suggesting groundwaters had
varied water residence times (Fig. 3b). The wetland had the highest 3H
content (1.7 TU) and represented a rainfall value for the region (Tadros et
al., 2014). All wells located between 3.4 to 12.5 m b.g.l. had high 3H
contents greater than 1.2 TU. Groundwaters from Site 3 had consistent values
around 1.5 TU, suggesting a similar source of water and that the aquifer is
hydraulically connected at this site. Deeper groundwaters (other than Site
3) had lower 3H values (less than 1 TU), indicating slightly older
groundwater at depth (Table 2).
Environmental isotope data for the site. DOC (dissolved organic
carbon), uncert (tritium uncertainty), QL (tritium quantification limit),
CBE (charge balance error), SIcc (saturation index for calcite), and
PCO2 (partial pressure of carbon dioxide).
The relationship between (a) DOC concentration compared to average
DOC for Anna Bay (solid black line) and global median DOC concentration for
groundwater (dotted line) (McDonough et al., 2020a) and (b)δ13CDOC values for groundwaters from Anna Bay with distance from
the wetland and compared to the terrestrial and marine OM ranges from Lamb
et al. (2006).
The average DOC concentration in groundwater for this site was high (5.0 mg L-1; n=15) compared to the ∼1 mg L-1 for the
global median DOC concentration in groundwater (McDonough et al., 2020a). The
carbon isotopic signature of the groundwater DOC represented a C3
vegetation signature (average δ13CDOC value of -27.4‰; n=15). Significant variation in DOC
concentration and δ13CDOC values occurred within the
aquifer (Fig. 4). The increase in DOC concentration in deeper groundwaters
compared to shallow groundwaters (except Site 4) suggested a source(s) of DOC within the
aquifer, which is generally not seen in groundwater environments and will be
discussed below.
The LC-OCD results did not show significant differences in DOM character in
the groundwater (Fig. 5). The fractions of humic substances were generally
between 40 % and 60 % of the total DOC (Fig. 5a). Minor exceptions existed in
the shallow samples at sites 2 and 3 for humic substances and at sites 2, 3
and 5 for the low molecular weight neutrals (LMW-Ns). The LMW-Ns were low,
representing approximately 10 % of the total DOM, and they did not change
significantly along the groundwater flow path (i.e. with distance from the
wetland) (Fig. 5b).
The relationship between (a) LC-OCD results with the total
percentage of humic substances and (b) LC-OCD total percentage of low molecular weight neutrals with distance from the wetland.
Principle component analysis for Anna Bay waters. The blue oval
represents the shallow waters, the green oval represents the medium waters and the red oval
represents the deeper waters. The wetland sample plots at the bottom of the
graph with a distinct chemistry compared to the groundwater samples.
Principal component analysis (Fig. 6) using water quality parameters,
isotopes and DOC variables included LC-OCD fractions (%),
δ13CDOC (‰), δ13CDIC
(‰), 3H, pH, Na, pCO2, Cl, Ca, Sr, DO,
SO4, NH4 and DOC concentration and confirmed the presence of
different groundwater sources. The variables contributing to PC1 in order
of importance were pH, Na, pCO2, Cl, DOC, Ca, humics, LMW-N, Sr and
3H. The variables contributing to PC2 in order of importance were
δ13CDOC, DO, biopolymers, HS aromaticity, δ13CDIC, HS
molecular weight, Cl, DOC and hydrophobic DOM. PC1 mainly explained the
variations with sample depth in the aquifer. Samples S1_D,
S2_S, S2_D and S3_S were the
samples that most strongly influenced PC1. The deep samples likely to have
originated from a deeper regional source of water (S1_D and
S2_D) influenced the right-hand side of the PCA with high
pCO2, humic substances, Na and Cl. These samples were also
characterized by low Ca, 3H and Sr. The shallow samples
(S1_S, S2_S and S3_S)
influenced the left-hand side of the PCA with low pCO2, humics, Na, and
Cl and had high Ca, 3H, and Sr, which is suggestive of rainfall recharge
waters. This analysis further highlighted the wetland sample that is not
related to the other samples (Fig. 6) and heavily influenced PC2 with low
δ13CDOC values, high DO, high biopolymers, low HS aromaticity,
low HS molecular weight and high DOC concentration.
DOC variation in groundwater
The wetland had the highest DOC (18 mg L-1) concentration and the
lowest carbon isotope value (δ13CDOC=-30.3 ‰) for the site. It also had a similar LC-OCD character
to groundwater. The closest groundwater sample to the wetland, located less
than 5 m downgradient at 3.5 m depth (S1_S), had a
significantly lower DOC concentration (5 mg L-1) and 1 ‰ higher carbon isotope value (δ13CDOC=-29.3 ‰). Interestingly, this
decrease in DOC was not reflected in the change in the mass fractions of the
LC-OCD results.
The deeper samples (> 17.0 m) at sites 1 and 2 had the highest
DOC concentrations (10±0.5 mg L-1) for groundwaters at the
site. The δ13CDOC values were higher in the deeper
groundwaters (-27.6±0.2 ‰) compared to the
shallow samples (-29.0±0.2 ‰). The humic
substances aromaticity and molecular weight showed there were two distinct
groups of DOM (Fig. 7). Samples that generally had lower values were from
sites 1 and 2 and the wetland. All samples were more aromatic for their
molecular weight in comparison to surface lakes, rivers and marine samples
(Huber et al., 2011).
Groundwaters from sites 3–5 had lower DOC concentrations (< 6 mg L-1) and a 1 ‰ higher average δ13CDOC
value (-26.6‰) than shallow and medium groundwater
samples from sites 1 and 2. Samples from sites 3, 4 and 5_S also
had higher humic substances aromaticity and humic substances molecular
weight than sites 1 and 2 (Fig. 7). The DOC did not show significant or
consistent trends in aromaticity or molecular weight, nor did it show a
trend of declining DOC concentration with depth or along a flow path.
The relationship between humic substances molecular weight
(Mn) and humic substances aromaticity (SAC / OC represents spectral absorption
coefficient / organic carbon). The curved lines show the area that samples
would plot if the humic substances aromaticity and molecular weight are
derived from the humic substances standard and water samples from Huber et
al. (2011). The figure also indicates that the humic substances
samples plotting at the top are interpreted as pedogenic in origin and that the fulvic acids at the bottom are aquagenic in origin. Note that humic acid (HA) and
fulvic acid (FA) are isolated from the humic substances standard of the IHSS
from Suwanee River (Huber et al., 2011). Surface water samples of humic-substances-aquagenic origin and humic-substances-pedogenic origin are also
indicated.
Schematic of OM sources and processes influencing DOC within the
coastal aquifer at Anna Bay based on concentration, character (isotopic and
chromatographic), tritium and hydrochemical data (Tables 1 and 2); marine and
freshwater DOM characterization from Lamb et al. (2006).
Discussion
Our results showed that groundwater DOC varied in concentration and isotopic
character along a 500 m groundwater flow path. The average groundwater DOC
concentration found in this coastal site was 5 times higher (5 mg L-1) than the global median DOC concentration for groundwaters
(McDonough et al., 2020a). The concentration of groundwater DOC doubled with
depth, but we did not see any consistent trends in the DOM chromatographic
character related to depth (Fig. 5). These results were in contrast to those
from a deeper fractured rock aquifer (Shen et al., 2015), where DOC
decreased with depth. Furthermore, despite observed differences in DOM
characteristics (e.g. higher humic substances aromaticity and molecular
weight in S3, S4 and S5_S), percentages of LC-OCD fractions
did not show any consistent trend along the groundwater flow path, contrary
to what was found in other studies where biodegradation, sorption,
desorption and biosynthesis controlled DOM (Chappelle et al., 2016; Shen et
al., 2015). The experimental work of Oudone et al. (2019) also confirmed
that sorption predominantly affects the humic substances fraction,
especially with high aromaticity. If biodegradation were occurring, we would
expect to see a decline in the biopolymer and low molecular weight neutral
fractions (Catalán et al., 2017); however, we do not make a clear
observation of this in our data. In contrast, Fig. 6 shows that the shallow
samples located further from the wetland (S4_S and
S5_S) have higher humic substances aromaticity and molecular
weight, which supports the conclusion that sorption is not the dominant
process determining DOM character from the wetland to the coastline.
Organic matter is more readily preserved under anoxic conditions (Bertrand
and Lallier-Verges, 1993), particularly in saturated environments. In such
systems, remineralization can be low, leading to enhanced preservation and
storage of OM (Schefuß et al., 2016). Marine sediments, wetlands and
peatlands have been suggested as having between 60 % and 95 % reduced
mineralization rates (Keiluweit et al., 2017). The OM itself is
thermodynamically unstable, but Schmidt et al. (2011) suggested it can
persist because of the physicochemical and potentially biological influence
of the surrounding environment that reduces the rate of decomposition. Our
hydrochemical data showed evidence to suggest OM degradation may have
occurred. Dissolved oxygen was less than 0.2 mg L-1 for all
groundwaters, which we inferred to be due to the respiration of microbes
that have adapted to access OM in relatively anoxic conditions. Additional
evidence for microbial activity included the presence of significant
concentrations of reduced redox-sensitive species such as ammonium (up to
0.9 mg L-1) and ferrous iron (up to 3.3 mg L-1) together with very
low nitrate concentration (< 0.3 mg L-1), the absence of
sulfate in the deeper groundwaters (Table 1) and the detection of H2S
odour. This fits with what was observed in anoxic subsurface environments
where microbes utilize nitrate, iron oxides and sulfate as electron acceptors
in the absence of oxygen, increasing the ammonia, ferrous iron and sulfide
concentrations (Berner, 1981; Appelo and Postma, 2005). A possible
explanation for the similar character of the DOM with the evolved inorganic
redox chemistry is that the rates of biodegradation are far lower than the
rate of DOM leaching into the groundwater. Furthermore, if there are
additional sources of DOM along the flow path, we would expect to the see
inconsistent variations in LC-OCD fractions along the flow path that we
observe in our data, rather than a consistent decline in any DOM fraction or
DOC concentration.
Sources of OM
Dissolved organic carbon in groundwater may originate from various sources.
The most abundant source is generally from decomposing OM in the soil zone,
and sedimentary OM from buried peat or palaeosols can also be a source
(Clark and Fritz, 1997). The wetland organic-rich sediments, such as organic
muds and silts associated with freshwater depressions, are characteristic of
coastal dune-slack systems which have formed since the maximum Holocene sea-level
transgression and have a global occurrence, including along the southeastern
coast of Australia (MacPhail, 1974). These units have the potential to form
peat. In suitable environments, the decay of OM under anaerobic conditions
is much slower (Benner et al., 1984), allowing a greater accumulation of
refractory OM, which may lead to peat accumulation (Lamb et al., 2006).
Three “peat” horizons were identified in a similar coastal environment to
Anna Bay (Fingal Bay, Central Coast, NSW) and ranged in age from 3.0 to 6.5 ka (MacPhail, 1974). The peat formation was suggested to be very rapid, with
metres of sediment accumulating over the past few thousand years.
The sand dunes at Anna Bay were likely to have formed around 7 ka (Sloss et
al., 2007; Jones, 1990; MacPhail, 1974), and the OM-rich layers contained
within them would have formed over this time. Organic matter can be
characterized by very long turnover times that increase with depth from 1 to
10 ka depending on the system (Schmidt et al., 2011). The radiocarbon age of
DOC for the deeper groundwaters at S1_D and S2_D was measured by McDonough et al. (2020b) for the Anna Bay site. The
results showed that DOC was ∼1 ka (88.3±0.4 pMC) in
the deeper groundwater samples, while the shallow water was modern
(> 100 pMC), suggesting an older OM source at depth. The
destabilization of these older OM-rich units, whether by anthropogenic or
natural processes, could result in an increase in the flux of older carbon
into the surface water environment as observed by Moore et al. (2013), and
the transport of this carbon has been shown to be dependent on the
hydrological response in coastal systems (Webb et al., 2018).
Wetland OM
Initially, it was thought that the wetland was the major source of OM for
the groundwater system because it contained elevated DOC concentrations (18 mg L-1) and appeared to be hydraulically connected to the aquifer.
Based on these assumptions, it would mean that approximately 70 % of the
OM from the wetland would be removed after groundwater recharge (i.e. 10 m downgradient at sample S1_S at 3.5 m depth). This
estimate seemed reasonable when considering that Shen et al. (2015) found that
about 90 % of surface-derived DOC was removed prior to reaching the
saturated zone.
Our data showed that the chromatographic character of the DOM in the wetland
was similar to that in the groundwater, suggesting that if the wetland was a significant
source of OM, the chemical composition and bioavailability of the wetland-derived DOM remained relatively unchanged during transport through the
aquifer. The higher relative contribution of DOC and evidence for the
limited transformation of the wetland OM compared to groundwater at this
site could also be explained by the limited sorption capacity of the
predominantly quartz sand aquifer.
If biological processing was influencing the wetland DOM during transport
into the groundwater, it would be expected that the δ13CDOC values would become heavier than the original C3
vegetation source – where the bacteria metabolize the isotopic light organics –
because the 12C–H bonds are easier to break, leaving the resultant OM
more enriched than the original source (Clark and Fritz, 1997). The carbon
isotope value in the wetland was 1 ‰ lighter compared to
the shallowest groundwater sample located near the wetland. This suggests that a
small amount of microbial processing may have occurred or that unprocessed
DOM inputs are very high at this site.
Alternatively, because we do not observe clear decreases in DOC
concentration or major changes in the LC-OCD fractions, the difference in
isotopic values could be explained by the DOM in the groundwater system
having a different source to the wetland and the limited interaction between the
surface water and groundwater at this site. The lack of interaction would also
explain the distinctly different water chemistry of the wetland sample
compared to the groundwater (Table 1 and Fig. 6). The wetland contained
elevated concentrations of DOC, but it is likely that the OM is being
mobilized and deposited into the hyporheic zone of the wetland (i.e. 1–2 m) by either sorption or abiotic transformation (Kerner et al., 2003).
This hypothesis is being investigated with further detailed water–sediment
investigations at this location.
In situ OM sources
Organic-rich sediments were present in the unconsolidated aquifer units, and
the physicochemical properties of the aquifer were governing the persistence
of the OM within the aquifer. We see several in situ DOM sources based on the
carbon isotopes and the humic substances aromaticity and molecular weight
data (Fig. 7). Two major groundwater groups were identified at sites 1 and 2 and
sites 3–5. The aromaticity of the humic substances in both groups was higher
than those previously reported for rivers and lakes for the corresponding
molecular weight, especially for sites 3–5.
Furthermore, we see groupings for the deeper samples from sites 1 and 2 that
corresponded with carbon isotopes that are 1 ‰ higher
(-27.6±0.2 ‰) than other groundwater at sites 1
and 2. These groundwaters contained less tritium, suggesting they were older
(< 0.8 TU), and the OM is likely to have originated from the deeper,
older palaeosol unit(s) contained within the coastal sediments (Fig. 8). The
shallow groundwaters had lower DOC concentrations and lower carbon isotope
values (-29.0±0.2‰), suggesting the OM
originating from these sources is also different from the deeper palaeosol.
The overlying peat units most likely formed in a similar environment to the
palaeosol located at depth but contained younger OM with lower source
δ13CDOC values.
Groundwater from sites 3–5 had lower DOC concentrations (< 6 mg L-1) and higher humic substances aromaticity and humic substances
molecular weights (Fig. 7) together with higher consistent δ13CDOC values (average δ13CDOC; -26.6‰). The exception was the shallow sample at Site 5,
which had the highest δ13CDOC value
(-25.0‰) and trended towards a marine OM value (Lamb et
al., 2006) with a DOC concentration (2.1 mg L-1) well below the average
for this site, which suggests a localized source of OM that may be derived
from marine sources due to its close proximity to the ocean.
Implications
Most studies investigate the character or carbon isotope signatures of DOC
in groundwater but rarely use both techniques. Our findings showed that
there are several sources of OM within this coastal aquifer, ranging from
buried peat units to palaeosols, and that if both techniques were not used, these
sources would not have been identified. It is important to understand the
sources to estimate the contribution of subsurface carbon to the global
carbon cycle. The combination of the low sorption capacity of the coastal
aquifer sediments and the presence of various sources of OM together with the associated anoxic aquifer conditions appears to have limited the sorption
and/or biodegradation that might be observed elsewhere in groundwater.
Therefore, this study further demonstrated that the persistence of OM was
found to be due to complex interactions between the OM and its environment
(Schmidt et al., 2011).
The groundwater that discharges from young unconsolidated coastal
environments such as these has much higher DOC concentrations than
groundwater from older, deeper aquifers that have undergone a greater degree
of OM processing (McDonough et al., 2020b). The degradative processes that
act on freshly produced DOM can produce greenhouse gases and produce less
reactive DOM that is exported (Davidson and Janssens, 2006; Zhou et al.,
2018). This means that the DOM transported from this coastal system has the
potential to impact the carbon budget. Moreover, sandy coastal aquifers
containing palaeosol horizons are globally widespread as they formed during
Holocene sea-level changes. These systems are sensitive to sea-level changes
and climate-change drivers that are likely to impact DOC export from coastal
systems because of changes in either the hydrology or ecosystem dynamics.
The OM sources found in this system are currently saturated and anoxic. If
the physicochemical conditions of the aquifer were altered, these coastal
groundwater systems would then have the potential to export a 1-order-of-magnitude-higher volume of unreacted carbon to the surface than are previously
realized based on the current global median DOC concentration of 1 mg L-1 for groundwaters (McDonough et al., 2020a). These coastal
groundwater systems form a significant OM source that is largely unaccounted
for in the global carbon budget. The magnitude and timing of the soil C
reservoir change as a result of climate and land-use changes are difficult
to quantify and is a large source of uncertainty in global C cycle models
(Kendall and McDonnell, 1998). Therefore, studies like this that identify the
sources of OM in aquifers will contribute to refining calculations in the
future.
Conclusions
The purpose of this study was to investigate the role of different OM
sources and the influence of the physicochemical properties of an aquifer on
groundwater DOM. This was done to identify the major controls on groundwater
DOC sources and processing in a coastal groundwater system. Our results
showed that groundwater DOC varied in concentration and isotopic character
along a 500 m groundwater flow path. The average groundwater DOC
concentration for this study was 5 times higher (5 mg L-1) than the
global median DOC concentration for groundwaters. The concentration of DOC
doubled with depth, reaching 10 mg L-1, but the DOM chromatographic
fractions did not change significantly with depth or along the groundwater
flow path. We note that the carbon isotopic composition did, however, vary
slightly.
Multiple sources of organic matter were identified, including those formed
during the maximum Holocene sea-level transgression such as wetland, soil,
peat and palaeosols. It was found that the DOC character at this site did
not follow the regional chromatography model due to OM processing with depth
and along a groundwater flow path. All samples were more aromatic for their
molecular weight in comparison to surface lakes, rivers and marine samples.
It was also found that the physicochemical properties of the aquifer and in
this case the anoxic-saturated conditions limited OM processes, leading to
enhanced preservation and storage of OM. The contradiction in DOM trends
when compared to the redox chemistry of the groundwater for this study
challenged our current understanding of groundwater DOC mobilization and
degradation in coastal aquifers. A possible explanation for not observing a
decrease and change in DOM character with evolved inorganic redox chemistry
is that DOM is being released from the OM sources at a higher rate than
biodegradation is occurring.
In situ sources such as peats and palaeosols are the main contributor of OM to the
groundwater DOC in this coastal system. The destabilization or changes in
hydrology, whether by anthropogenic or natural processes, could result in
increased fluxes of unreacted old carbon from deep within the peat column.
Significantly, the results of this study show that understudied anoxic
coastal groundwater systems have the potential to export up to 10 times
more unreacted carbon to the surface than previously realized,
considering the global median DOC concentration is 1 mg L-1 for
groundwaters. These coastal groundwater sources are largely unaccounted for
in the global carbon budget and are likely to play more of a role in carbon
transport in the future.
Data availability
All data are presented in the tables.
Author contributions
Field sampling was undertaken by MSA, AB, KTM, DMO, HR, EB and NSZ. KTM undertook the data analysis, wrote the paper and constructed the figures under the guidance of AB. LKM generated GIS files for Fig. 1 and did the PCA for Fig. 6. PO constructed the original Fig. 7, and HR provided the initial LC-OCD interpretation. All authors reviewed and provided comments on the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors would like to thank the NSW Office of Water for providing the
funding for drilling and monitoring the bore installation. Hamish Studholme, Sam McCulloch and Juan Carlos Castilla-Rho did the drilling and bore
installation. NSW National Parks provided the scientific license and
approval for the research to be conducted within the Tomaree National Park.
Nur Syahiza Zainuddin was supported by a Malaysian government PhD
scholarship. Thanks go to Ellen Howley for her help with the sampling. The
authors would also like to thank various ANSTO personnel such as Robert Chisari and Kelly Farrawell for analysis.
Financial support
This work has been supported by ANSTO, the Australian Research Council (ARC)/National Water Commission co-funded National Centre for Groundwater Research and Training (NCGRT), and the ARC project DP160101379.
Review statement
This paper was edited by Nandita Basu and reviewed by Hannelore Waska and one anonymous referee.
References
Appelo, C. A. J. and Postma, D.: Geochemistry, Groundwater, and Pollution, second
ed., A.A. Balkema, Rotterdam, 649 pp., 2005.
Benner, R., Newell, S. Y., Maccubbin, A. E., and Hodson, R. E.: Relative
contributions of bacteria and fungi to rates of degradation of
lignocellulosic detritus in salt-marsh
sediments, Appl. Environ. Microb., 48, 36–40, 1984.
Berner, R. A.: A new geochemical classification of sedimentary
environments, J. Sediment. Res., 51, 359–365, 1981.
Bertrand, P. and Lallier-Vergès, E.: Past sedimentary organic matter
accumulation and degradation controlled by productivity, Nature, 364, 786–788,
1993.Bryan, E., Meredith, K. T., Baker, A., Andersen, M. S., Post, V. E. A., and
Treble, P. C.: How water isotopes (18O, 2H, 3H) within an island
freshwater lens respond to changes in rainfall, Water Res., 170, 115301, 10.1016/j.watres.2019.115301, 2020.
Catalán, N., Casas-Ruiz, J. P., von Schiller, D., Proia, L., Obrador,
B., Zwirnmann, E., and Marcé, R.: Biodegradation kinetics of dissolved
organic matter chromatographic fractions in an intermittent
river, J. Geophys. Res.-Biogeo., 122, 131–144, 2017.
Chapelle, F. H., Shen, Y., Strom, E. W., and Benner, R.: The removal kinetics
of dissolved organic matter and the optical clarity of groundwater,
Hydrogeol. J., 24, 1413–1422, 2016.
Clark, I. D. and Fritz, P.: Environmental Isotopes in Hydrogeology, Taylor
& Francis, 1997.
Davidson, E. A. and Janssens, I. A.: Temperature sensitivity of soil carbon
decomposition and feedbacks to climate change, Nature, 440, 165–173, 2006.
Emery, K. O., Wigley, R. L., Bartlett, A. S., Rubin, M., and Barghoorn, E. S.:
Freshwater Peat on the Continental Shelf, Science, 158, 1301–1307, 1967.
Fontaine, S., Barot, S., Barré, P., Bdioui, N., Mary, B., and Rumpel, C.:
Stability of organic carbon in deep soil layers controlled by fresh carbon
supply, Nature, 450, 277–280, 2007.Hashimoto T. R. and Troedson A. L.: Nelson Bay 1:100000 and 1:25000, Coastal
Quaternary Geology Map Series. Geological Survey of New South Wales,
Maitland, available at: https://search.geoscience.nsw.gov.au/product/39 (last access: 28 April 2020), 2008.
Hedges, J. I., Ertel, J. R., Quay, P. D., Grootes, P. M., Richey, J. E., Devol,
A. H., Farwell, G. W., Schmidt, F. W., and Salati, E.: Organic carbon-14 in the
Amazon River system, Science, 231, 1129–1131, 1986.
Hedges, J. I., Cowie, G. L., Richey, J. E., Quay, P. D., Benner, R., Strom, M.,
and Forsberg, B. R.: Origins and processing of organic-matter in the Amazon
River as indicated by carbohydrates and amino-acid, Limnol. Oceanogr.,
39, 743–761, 1994.
Howley, E.: Lignin in a coastal groundwater aquifer: a useful tracer?
Bachelor of Engineering (Hons) Thesis, School of Civil and Environmental
Engineering UNSW, Sydney, 2014.
Huber, S. A., Balz, A., Abert, M., and Pronk, W.: Characterisation of aquatic
humic and non-humic matter with size-exclusion chromatography – organic
carbon detection – organic nitrogen detection (LC-OCD-OND), Water Res.,
45, 879–885, 2011.
Jones, R. L.: Late Holocene vegetational changes on the Illawarra coastal
plain, New South Wales, Australia, Rev. Palaeobot. Palynol., 65,
37–46, 1990.
Keiluweit, M., Wanzek, T., Kleber, M., Nico, P., and Fendorf, S.: Anaerobic
microsites have an unaccounted role in soil carbon stabilization, Nat.
Commun., 8, 1771, 2017.
Kendall, C. and McDonnell, J. J.: Isotope Tracers in Catchment Hydrology,
Elsevier Science, 1998.Kerner, M., Hohenberg, H., Ertl, S., Reckermann, M., and Spitzy, A.:
Self-organization of dissolved organic matter to micelle-like microparticles
in river water, Nature, 422, 150–154, 10.1038/nature01469, 2003.Lamb, A. L., Wilson, G. P., and Leng, M. J.: A review of coastal palaeoclimate
and relative sea-level reconstructions using δ13C and C/N ratios in
organic material, Earth-Sci. Rev., 75, 29–57, 2006.McDonough, L. K., Isaac, R. S., Andersen, M. S., O'Carroll, D. M., Rutlidge, H., Meredith, K., Oudone, P., Bridgeman, J., Gooddy, D. C., Sorensen, J. P. R., Lapworth, D. J., MacDonald, A. M., Ward, J., and Baker, A.: Changes in global groundwater organic carbon driven by climate change and urbanization, Nat. Commun., 11, 1279, 10.1038/s41467-020-14946-1, 2020a.McDonough, L. K., Rutlidge, H., O'Carroll, D. M., Andersen, M. S., Meredith,
K., Behnke, M. I., Spencer, R. G. M., McKenna, A. M., Marjo, C. E., Oudone, P.,
and Baker, A.: Characterisation of shallow groundwater dissolved organic
matter in aeolian, alluvial and fractured rock aquifers, Geochim.
Cosmochim. Ac., 273, 163–176, 10.1016/j.gca.2020.01.022, 2020b.
Macphail, M.: Pollen analysis of a buried organic deposit on the backshore
at Fingal Bay, Port Stephens, New South Wales, Proceedings of the
Linnean Society of New South Wales, 98, 222–233, 1974.
Maric, I.: Groundwater quality changes in a coastal aquifer due to
groundwater abstraction: A case study. Bachelor of Engineering (Hons) Thesis,
School of Civil and Environmental Engineering UNSW, Sydney, 2013.Meredith, K., Cendón, D.I., Pigois, J.-P., Hollins, S., and Jacobsen, G.:
Using 14C and 3H to delineate a recharge “window” into the Perth Basin
aquifers, North Gnangara groundwater system, Western Australia, Sci.
Total Environ., 414, 456–469, 2012.
Meredith, K. T., Han, L. F., Hollins, S. E., Cendón, D. I., Jacobsen, G. E.,
and Baker, A.: Evolution of chemical and isotopic composition of inorganic
carbon in a complex semi-arid zone environment: Consequences for groundwater
dating using radiocarbon, Geochim. Cosmochim. Ac., 188, 352–367,
2016.Moore, S., Evans, C. D., Page, S. E., Garnett, M. H., Jones, T. G., Freeman, C.,
Hooijer, A., Wiltshire, A. J., Limin, S. H., and Gauci, V.: Deep instability of
deforested tropical peatlands revealed by fluvial organic carbon fluxes,
Nature, 493, 660–663, 10.1038/nature11818, 2013.Oudone, P., Rutlidge, H., Andersen, M. S., O'Carroll, D., Cheong, S.,
Meredith, K., McDonough, L., Marjo, C., and Baker, A.: Characterisation and
controls on mineral-sorbed organic matter from a variety of groundwater
environments, EarthArXiv, https://eartharxiv.org/ue86w/, 10.31223/osf.io/ue86w, 2019.Schefuß, E., Eglinton, T. I., Spencer-Jones, C. L., Rullkötter, J., De
Pol-Holz, R., Talbot, H. M., Grootes, P. M., and Schneider, R. R.: Hydrologic
control of carbon cycling and aged carbon discharge in the Congo River
basin, Nat. Geosci., 9, 687–690, 10.1038/ngeo2778, 2016.Schmidt, M. W. I., Torn, M. S., Abiven, S., Dittmar, T., Guggenberger, G.,
Janssens, I. A., Kleber, M., Kögel-Knabner, I., Lehmann, J., Manning,
D. A. C., Nannipieri, P., Rasse, D. P., Weiner, S., and Trumbore, S. E.:
Persistence of soil organic matter as an ecosystem property, Nature, 478, 49–56, 10.1038/nature10386, 2011.
Shen, Y., Chapelle, F. H., Strom, E. W., and Benner, R.: Origins and
bioavailability of dissolved organic matter in groundwater, Biogeochemistry,
122, 61–78, 2015.
Sloss, C. R., Murray-Wallace, C. V., and Jones, B. G.: Holocene sea-level change
on the southeast coast of Australia: a review, Holocene, 17, 999–1014,
2007.
Stumm, W. and Morgan, J. J.: Aquatic chemistry, 3rd edition, Wiley and
Sons, New York, 1996.
Tadros, C. V., Hughes, C. E., Crawford, J., Hollins, S. E., and Chisari, R.:
Tritium in Australian precipitation: A 50 year record, J. Hydrol.,
513, 262–273, 2014.Webb, J. R., Santos, I. R., Maher, D. T., Tait, D. R., Cyronak, T., Sadat-Noori,
M., Macklin, P., and Jeffrey, L. C.: Groundwater as a source of dissolved
organic matter to coastal waters: Insights from radon and CDOM observations
in 12 shallow coastal systems, Limnol. Oceanogr., 64, 182–196, 10.1002/lno.11028, 2018.
Zhou, Y., Davidson, T. A., Yao, X., Zhang, Y., Jeppesen, E., de Souza, J. G.,
Wu, H., Shi, K., and Qin, B.: How autochthonous dissolved organic matter
responds to eutrophication and climate warming: Evidence from a
cross-continental data analysis and experiments, Earth-Sci. Rev., 185,
928–937, 2018.