DISSOLVED ORGANIC MATTER ACCUMULATION, REACTIVITY, AND REDOX STATE IN GROUND WATER OF A RECHARGE WETLAND Natalie Mladenov 1,2 , Philippa Huntsman-Mapila 3 , Piotr Wolski 3 , Wellington R. L. Masamba 3 , and Diane M. McKnight 1 1 INSTAAR, University of Colorado 450 UCB Boulder, Colorado, USA 80309 E-mail: [email protected]2 Departamento de Ecologı ´a, Universidad de Granada 18071 Granada, Spain 3 Harry Oppenheimer Okavango Research Centre, University of Botswana Private Bag 285 Maun, Botswana Abstract: Ground water beneath the seasonal swamp of the Okavango Delta, a recharge wetland in northwestern Botswana, is known to be a sink for solutes. In this study, measurements of organic carbon and inorganic ion concentrations, as well as UV-visible and fluorescence spectroscopy, were used to examine dissolved organic matter (DOM) storage and redox state of fulvic acids in ground water beneath an island and riparian woodland. Increasing dissolved organic carbon (DOC) concentrations along the ground-water flowpath suggests an accumulation of DOM in ground water, especially beneath island centers. However, the increase in DOC concentration was relatively less than the increase in chloride and sulfate concentrations, indicating non-conservative behavior of DOM in ground water beneath wetland islands. In combination with a decrease in fulvic acid content and specific UV absorbance, this result suggests that preferential sorption or destabilization of more aromatic organic compounds may be occurring under conditions of high pH and salinity. Finally, the increase in reduced fluorescence components (semiquinone- and hydroquinone-like components) along the ground-water flowpath strongly supports the transition to reduced fulvic acids in ground water of island centers. The reactivity and potential electron-shuttling function of fulvic acids may play an important role in the dissolution of metal oxides and associated DOM-iron-arsenic interactions in ground water of this recharge wetland. Key Words: EEM, fluorescence index, humic substances, Okavango Delta, PARAFAC, SUVA INTRODUCTION In aquatic ecosystems, dissolved organic matter (DOM) represents the major pool of organic carbon and is a substrate for heterotrophic microorganisms. DOM originating from plant/soil and microbial sources can be transported to ground water by infiltration. DOM can also be produced within the ground-water system as extracellular microbial products. DOM can be removed via bacterial degradation, as some organic compounds are substrates that support microbial growth. Some DOM fractions can adsorb to clay and oxide surfaces, whereas these and other fractions can also influence metal cycling. Fulvic acids are humic substances that are soluble across the pH range and have high electron accepting capacity (Scott et al. 1998). Fulvic acids, in particular, are known to be involved in strong metal binding (McKnight et al. 1992) and, in laboratory experiments, their role as electron shuttles between iron (Fe)-reducing bacteria and Fe-oxides has been shown to enhance Fe (III) reduction (Lovley et al. 1996). The concentration of DOC in ground water is generally lower than in many surface waters, reflecting the chemical and biotic processing of DOM in the subsurface (Thurman 1985). However, dissolved organic carbon (DOC) concentrations in ground water can be much higher if recharged by wetlands. In the Okavango Delta, a large wetland in northwestern Botswana, high DOC concentrations (from 13 to 25 mg C L 21 ) were measured in ground water of the seasonal swamp region (Figure 1) adjacent to channels and floodplains (Mladenov 2004, Bauer-Gottwein et al. 2007). Mladenov et al. WETLANDS, Vol. 28, No. 3, September 2008, pp. 747–759 ’ 2008, The Society of Wetland Scientists 747
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DISSOLVED ORGANIC MATTER ACCUMULATION, REACTIVITY, AND REDOXSTATE IN GROUND WATER OF A RECHARGE WETLAND
Natalie Mladenov1,2, Philippa Huntsman-Mapila3, Piotr Wolski3, Wellington R. L. Masamba3, and
*Two-sample student t-test (unequal variances) indicates significant difference from SW, with p , 0.01.**Two-sample student t-test (unequal variances) indicates significant difference from SW, with p , 0.001.a For comparison, SUVA of the terrestrial (SR) and microbial (LF) end-members are 2.5 and 1.3, respectively.b For comparison, FI of the terrestrial (SR) and microbial (LF) end-members are 1.24 and 1.74, respectively.
Mladenov et al., CHARACTERIZATION OF DOM IN WETLAND GROUND WATER 753
shown for island fringe ground water, island center
ground water, woodland ground water, and the
corresponding channel and floodplain surface water
samples and are grouped according to their molec-
ular association (Table 2). Of the 13 components,
C2, C3, C6, C7, C8, C9, and C13 are components
that have been associated with microbial sources
(Cory et al. 2007). Using the entire dataset of island
and woodland transect samples, the sum of micro-
bial fluorescent components was significantly related
with FI (Figure 6A). Two of the quinone-like
components (C2 and C11), the hydroquinone (C4),
and the unknown component (C6), were the most
prevalent in the island fringe and woodland samples.
At the island center, reduced quinone content
(specifically the hydroquinone C4 and the terrestrial
semi-quinone C5) was higher than in island fringe
ground water, whereas the content of oxidized
quinones (C2, C11, and C12), tryptophan-like
component (C8), and most microbially associated
components were lower (Table 2). In all ground
water and surface water samples, the total oxidized
and reduced quinone content were significantly
inversely related (R2 5 0.97, p , 0.01, Figure 6B).
Differences in Subsurface Chemistry between Island
and Woodland Transects
While most ground-water properties were similar
at the woodland and island fringe transects, student
t-tests indicated that DOC concentrations of wood-
land ground water were significantly higher (p 5
0.011, n 5 12) and SUVA and FI values were
significantly lower than those measured at the island
fringe transect (p , 0.001 for both). The only
significant difference between shallow (approximate-
ly 3.3 to 4.5 m below surface) and deep (approxi-
mately 5 to 6 m below surface) piezometers was
observed in ground-water conductivity measured in
both the island and woodland transects (p 5 0.008,
n 5 11), with higher conductivity occurring in
shallower piezometers (Figure 3).
DISCUSSION
The critical role that islands play in maintaining
the Delta as a freshwater system by acting as sinks
for inorganic solutes (including dissolved inorganic
carbon (DIC)) has been documented (Gieske 1997,
McCarthy et al. 2006, Ramberg and Wolski 2007).
The storage of organic matter in ground water may
also have an important role in terms of influencing
the biogeochemistry of the Delta. Our results,
showing substantially higher DOC concentrations,
conductivity, and alkalinity in island and woodland
ground water than in adjacent surface water,
confirm an enrichment of both dissolved organic
and inorganic ions. These results are consistent with
the known ground-water flowpath toward island
interiors, maintained by surface water recharge
(Wolski and Savenije 2006). In our study, DOC
concentrations measured in island center ground
water were an order of magnitude higher than those
measured in island fringe ground water. Yet our
island center measurements were an order of
magnitude lower than those measured in the same
piezometers (‘‘ORC island transect’’) in a previous
Figure 5. Representative EEMs of A) island surface
water, B) island fringe ground water at 50 m, C) island
center ground water at 240 m, D) woodland surface water,
and woodland ground water at E) 50 m and F) 200 m
collected on September 21 and December 2, 2005.
Positions of the region A peak are shown below each
EEM. Normalized intensities of the region A peak (in
parentheses) are shown only for samples for which fulvic
acid concentrations were known. Approximate locations
of region A peak and region C shoulder are labeled with
capital letters A and C, respectively, in Graph A.
Figure 6. Relationship between A) fluorescence index
and total microbial components (as defined in Table 2)
and B) between total oxidized (C2, C11, and C12) and
total reduced (C4, C5, C7, and C9) quinone-like
components. Dataset includes all ground-water samples
(n 5 23) collected September 21 and December 2, 2005.
Regression lines, equations and level of significance are
shown. **p , 0.01.
754 WETLANDS, Volume 28, No. 3, 2008
study (Bauer-Gottwein et al. 2007). This may be
due, in part, to analytical differences between the
two studies. Island center ground water is known to
have high salt and inorganic C concentrations
(McCarthy and Ellery 1994, Wolski et al. 2005,
Wolski and Savenije 2005, Bauer-Gottwein et al.
2007, Wolski and Ramberg 2008), which present
inherent analytical challenges in the measurement of
DOC concentrations. For example, if inorganic C is
not completely removed, measurements of DOC
concentration can be overestimated (Potter and
Wimsatt 2005). Also, different sample preservation
techniques may influence DOC concentration mea-
surements.
Nevertheless, accumulation of DOM in ground
water of the island center suggests that these zones
serve as sinks for OM and is consistent with the
model results of Mladenov et al. (2007b), which
showed substantial infiltration of DOM (between
24% and 62% of total DOM removal in 2001–2002).
The potential for a large carbon sink beneath other
recharge wetlands, such as the Hadejia-Nguru
wetlands in Nigeria (Goes 1999), the tree islands of
the Everglades, Florida, USA (Gann and Childers
2006), and the River Murray floodplains in Aus-
tralia (Holland et al. 2006), could have important
implications for regional and global C budgets. The
ultimate fate of organic C stored beneath wetland
islands, however, merits further research.
Another important finding of this study is that the
chemical and spectroscopic properties of DOM from
surface water and the adjacent ground water were
very similar, whereas the properties of DOM
beneath island centers were clearly distinct from
those of the island fringe, woodland transect, and
adjacent surface waters. These patterns may be
related to both hydrologic and biogeochemical
processes. During the annual flood, ground-water
table elevations in the island fringe and woodland
can rise over 1 m, while ground-water table fluctu-
ations in the island center are fairly low (between
0.10 m and 0.25 m), reflecting the evapotranspira-
tive uptake of water along the flowpath (Wolski and
Savenije 2006). More active ground-water recharge
may explain, in part, the greater similarities in
chemical and spectroscopic properties of surface
water and adjacent (island fringe and woodland)
ground water. In particular, the similar FI and
SUVA values of surface water and island fringe and
woodland groundwater suggests that ground-water
DOM originates in DOM-rich channels and flood-
plains of the Okavango Delta. These similarities also
reflect a dynamic hyporheic connection between
surface water and ground water in the island fringe.
Similar patterns between FI and SUVA at the
woodland transect suggest that a dynamic hydro-
logic connection between surface water and adjacent
ground water is also present at this site.
Table 2. Distribution of PARAFAC components in whole waters of island and woodland surface water (SW) and
ground water (GW) shown as percent contribution of each component to the total modeled EEM. Standard deviations and
C10 (T) 2.5 1.9 6 0.4 2.7 6 0.2 2.5 2.4 6 0.1a Components are labeled and identified according to Cory et al. (2007). M 5 components associated with microbially derived organicmatter; T 5 components associated with terrestrially derived organic matter; Q 5 quinone; HQ 5 hydroquinone; SQ 5 semi-quinone; Trp5 tryptophan; Tyr 5 tyrosine.
Mladenov et al., CHARACTERIZATION OF DOM IN WETLAND GROUND WATER 755
In comparison, the slow, 1–5 month long response
to recharge by the annual flood in ground water
beneath island centers (Wolski and Savenije 2006)
results in long water travel times and may promote
biogeochemical transformation of DOM in the
subsurface. At the island transect, the conductivity
and concentrations of conservative ions (chloride
and sulfate) increased by about 500 fold along the
ground-water flowpath from the channel surface
water to the ground water at 240 m. In contrast,
DOC concentrations increased only 10-fold over the
same distance. This non-conservative behavior of
DOM suggests that DOM evapoconcentration in
the subsurface is offset by DOM removal processes
such as coagulation and settling, sorption, and
possibly microbial uptake (preferential) along the
flowpath. High salt concentrations, such as those
measured at this site, have been shown to result in
destabilization of humic substances and subsequent
coagulation in estuary waters (Sholkovitz 1976).
Also, conditions of high pH (. 8.5) and high
calcium concentration have been shown to induce a
swelling/condensation transition of DOM to partic-
ulate organic matter (POM) microgels that can
result in POM settling (Chin et al. 1998). Both of the
former processes, previously observed in marine
systems, warrant consideration in this ground-water
setting. Furthermore, there may be preferential
losses of aromatic DOM via sorption to sediments
that would explain the decrease in SUVA from the
island fringe (mean of 2.3 L mg C21) to the island
center (less than 1 L mg C21 at 240 m). McKnight
et al. (2002) observed a 50% reduction in SUVA in
an alpine stream when abundant iron (Fe) oxyhydr-
oxides were present on the streambed. This was
attributed to surface complexation of strongly
binding aromatic compounds with Fe oxides
(McKnight et al. 2002). Additionally, nitrogen (N)
and sulfur (S) groups in fulvic acid are known to be
involved in strong metal binding (McKnight et al.
1992). In our study, a decrease in SUVA along the
flowpath by this mechanism is consistent with the
known high N and S content of infiltrating surface
water (Mladenov et al. 2007) and the presence of Fe
in Okavango sediments (Huntsman-Mapila et al.
2006). A corresponding shift to lower FA content,
from 70%–80% in island fringe ground water to
50%–60% in island center ground water, may
further reflect preferential sorption of the more
hydrophobic organic acids, resulting in an increase
in the non-humic fraction of DOM.
Although low SUVA values have been associated
with the presence of microbially derived DOM
(Hood et al. 2003), the low FI values of island
center groundwater suggest that the correspondingly
low SUVA is not likely to be related to increased
microbial DOM sources. In other ground-water
systems in which microbial DOM sources dominate,
high FI values (approaching 1.90) have been
reported (McKnight et al. 2001), but this is not the
case in either of the ground-water transects of this
study. In fact, the decrease in FI in island center
ground water can be interpreted as indicating a loss
of microbial precursor material in ground-water
DOM, a finding supported by the highly significant
relationship between FI and the sum of microbial
fluorescent components (Figure 6A). Additionally,
amounts of fluorescent components associated with
microbial sources (Cory et al. 2007; Table 2),
including component C8 that represents trypto-
phan-like fluorescence known to be associated with
bacteria (Cammack et al. 2004), were lower in island
center samples than in island fringe samples. These
results further suggest that microbially derived
moieties were also preferentially removed along the
flowpath. Taken together with the removal of
reactive fulvic acids (by sorption to sediments or
coagulation in the saline ground-water environ-
ment), the loss of microbially derived fluorescent
components along the flowpath means that a highly
altered DOM, deficient in both aromatic moieties
and microbial-type fluorophores, is transported to
ground water beneath island centers.
The preferential removal of reactive fulvic acids
along the ground-water flowpath is likely responsi-
ble for the non-conservative behavior of DOM and
lower DOC concentrations in island center ground
water. These new findings of lower DOC concen-
trations than those measured by Bauer-Gottwein et
al. (2007) and the accompanied lower fulvic acid
content may help to resolve the contradictory
findings (e.g., steady state composition of ground
water was found to be sodium chloride dominated
rather than sodium bicarbonate dominated) ob-
tained when humics substances were included in
model simulations (Bauer-Gottwein et al. 2007).
Therefore, our findings support the model results of
Bauer-Gottwein et al. (2007) that dissolved humic
substances concentrations in island centers are not
high enough to trigger CO2 degassing and delay the
onset of density-driven flow. However, taking into
account the potential sorption and coagulation of
humics that may occur along the ground-water
flowpath, the net influence of humic substances on
the geochemistry of islands is likely to be substantial.
Differences in redox state between island center
and island fringe ground water also demonstrate the
importance of humic substances in ground-water
biogeochemistry. The presence of reduced fulvic
acids (quantified using the RI) was more pro-
756 WETLANDS, Volume 28, No. 3, 2008
nounced at island centers, where the highest DOM
and Fe accumulation occurs, than at the island fringe
or woodland sites, where lower DOC and Fe
concentrations were also measured. Additionally,
the highly significant correlation between reduced
and oxidized quinone-like fluorescent components in
ground water of the island and woodland transects
(Figure 6B) indicates that the increase in reduced
components is directly related to the loss of oxidized
components and not other fluorophores. Further,
the lower intensity and red-shifting (to higher
emissions wavelengths) of the region A peak in
island center ground water indicates more reducing
conditions and is consistent with other studies
(Klapper et al. 2002, Fulton et al. 2004) that
attributed lower peak intensities to microbial reduc-
tion of fulvic acids. The redox state of fulvic acids in
island center ground water is significant when
considering the solubility of metals in the subsurface.
The solubility and reactivity of Fe and manganese
(Mn) has been linked to the electron-shuttling role of
fulvic acids (Lovley et al. 1996, Klapper et al. 2002,
Nevin and Lovley 2002, Fulton et al. 2005), and this
role has been attributed specifically to quinone
moieties (Cory and McKnight 2005). Quinones can
shuttle electrons to facilitate metal reduction if Fe-
reducing or other metal-reducing bacteria are present
and if DOM (substrate, electron donor) and metals
(electron acceptors) are present in sufficient concen-
tration (Nevin and Lovley 2002, Klapper et al. 2002).
In island center ground water, the dominance of
reduced (over oxidized) quinone moieties in combi-
nation with high total Fe concentrations (reaching
8 ppm) suggests that an electron shuttling cascade
may be underway that can promote metal dissolu-
tion in the subsurface. In ground water near the
Okavango Delta, Huntsman-Mapila et al. (2006)
found a positive correlation between specific UV
absorbance and high dissolved arsenic concentra-
tions and invoked a hypothesis of arsenic liberation
through iron dissolution. Given the importance of
this finding and its potential relationship to DOM
cycling, a better understanding of DOM-redox-metal
interactions is needed specifically for this system.
Additionally, given the preferential removal of
reactive fulvic acids along the flowpath in this study,
the potential role of competitive sorption by DOM in
promoting arsenic liberation should be evaluated.
Finally, our results show that ground water
beneath between bare island centers (lacking vege-
tation other than salt tolerant grasses) contains not
only accumulated inorganic ions (McCarthy et al.
1993, McCarthy and Ellery 1995, Ramberg and
Wolski 2007) but also DOM containing reduced
fulvic acids. Whether the appearance of bare island
centers and salt crusts elsewhere in the Okavango
Delta corresponds to similar ground-water organic
geochemistry is a question for future research. The
changes in chemistry and spectroscopic properties of
DOM along the woodland transect flowpath were
similar to those observed at the island fringe, but
from this study alone it is not possible to determine
whether greater accumulation (as occurs at the
island center) also occurs with greater distance
inland in woodland areas adjacent to seasonal
floodplains or whether sustained recharge of DOM
by a permanent water supply is needed to facilitate
this condition.
CONCLUSION
The biogeochemical significance of islands in
global wetlands is just beginning to be understood.
Specifically, the chemical character of ground-water
DOM may have an important influence on the
biogeochemistry of ground water beneath wetland
islands. Our findings provide chemical evidence for
the non-conservative behavior of DOM in the
subsurface and indicate that ground water beneath
island centers has undergone a greater degree of
biogeochemical processing. The potential removal of
reactive humic substances with distance along a
flowpath may explain why Bauer-Gottwein et al.
(2007) concluded that humic substances are not
found in high enough concentrations to drive
degassing of CO2 in island center ground water.
However, the interactions of humic substances in
ground water may be extremely important in terms
of biogeochemical processes, such as metal-DOM
interactions, electron shuttling, sorption, and/or
coagulation. Therefore, we conclude that humic
substances probably exert a significant influence on
the geochemistry of ground water beneath islands.
The significant differences we observed in chem-
ical and spectroscopic properties between ground
water of the island fringe and island center provide
evidence for surface water sources of ground-water
DOM, accumulation of DOM in the subsurface, and
the occurrence of important redox processes in the
ground water beneath island centers of the Oka-
vango Delta. The reducing conditions in the ground
water of island interiors may be linked to microbial
reduction of metals using the DOM as substrate and
fulvic acids as electron shuttles. Our findings have
important implications for the Okavango Delta and
other net recharge wetlands in regards to estimating
carbon budgets, understanding redox processes, and
evaluating the influence of DOM and humic
substances on ground-water geochemistry.
Mladenov et al., CHARACTERIZATION OF DOM IN WETLAND GROUND WATER 757
ACKNOWLEDGMENTS
We are grateful to I. Mosie, B. Mogojwa, K.
Mohembo for field expertise and assistance, K.
Mohembo, F. Luiszer, M. P. Miller, and M. Norris
for assistance with chemical analyses, R. D.
McGrath and L. Ries for assistance with sample
transport, and M. P. Miller and anonymous
reviewers for helpful comments on the manuscript.
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