dissolved organic matter accumulation, reactivity, and

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

Diane M. McKnight1

1INSTAAR, University of Colorado

450 UCB

Boulder, Colorado, USA 80309

E-mail: [email protected]

2Departamento de Ecologı́a, Universidad de Granada

18071 Granada, Spain

3Harry 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 theground-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 rangeand 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 L21) 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

(2007b) hypothesized that high ground-water DOC

concentrations were directly related to the high DOC

concentrations in surface waters (from 8 to

30 mg C L21 during flood periods) and high infiltra-

tion rates. Mladenov et al. (2007b) modeled temporal

variations in DOC concentrations in a seasonal

floodplain of the Okavango Delta. The model results

indicated that approximately 28% of the DOM

brought to the floodplain during a high flood season

infiltrated into the subsurface. Because infiltrating

water follows lateral ground-water flowpaths driven

by evapotranspiration (McCarthy and Ellery 1994), it

was hypothesized that infiltration of large amounts of

DOM would result in substantial DOM storage in

ground water beneath islands and uplands adjacent to

surface water (Mladenov et al. 2007b).

Chemical speciation modeling of ground water

beneath three islands in the seasonal swamp of the

Okavango Delta (Bauer-Gottwein et al. 2007)

showed that density-driven flow transports solutes

downward into deeper ground-water layers beneath

these islands. However, both the onset of density-

driven flow and the accumulation of solutes could be

delayed if ground-water humic substances concen-

trations are high enough to induce CO2 degassing

and/or mineral precipitation. The results of Bauer-

Gottwein et al. (2007) suggested that humic sub-

stances concentrations were not high enough for

CO2 degassing to represent a major mass loss

mechanism, even though ground-water DOC con-

centrations were found to be very high (on the order

of 1,000–4,000 mg C L21) in ground water beneath

island centers. Although humic or fulvic acid

content was not directly measured, Bauer-Gottwein

et al. (2007) concluded that the influence of humic

substances on the geochemistry of ground water

beneath islands was minimal, based on the results of

geochemical modeling.

Figure 1. Features of the study site, including the Boro Channel, study floodplain, surface water sampling points, island

and woodland transects. C1 to C10 (approximately 1 m to 240 m from edge of water) and W1 to W8 (approximately 1 m

to 200 m from edge of water) correspond to the locations of island and woodland piezometers, respectively. Inset shows

Okavango Delta and Botswana, and the study site is indicated with an arrow.

748 WETLANDS, Volume 28, No. 3, 2008

The conclusions of Bauer-Gottwein et al. (2007)

diverge from the expected high reactivity of humics

in ground water of the seasonal swamp, based on the

known infiltration of surface water DOM with a

high degree of aromaticity and high N and S content

(Mladenov et al. 2007a). Isolation of humic sub-

stances by XAD-8 resin also revealed high fulvic acid

content (approximately 70% of DOC) in surface

waters in the seasonal swamp (Mladenov et al. 2005).

Because surface water recharges ground water in the

seasonal swamp of the Okavango Delta, these

findings suggest that humic substances exert an

important influence on biogeochemical processes,

such as nutrient immobilization (Wolski et al. 2005)

and metal cycling processes (Huntsman-Mapila et al.

2006) in ground water of the Okavango Delta region.

Recent advances in fluorescence spectroscopy

have made it possible to elicit information regarding

the sources (McKnight et al. 2001, Mladenov et al.

2005) and chemical character of DOM (Cory and

McKnight 2005, Mladenov et al. 2007a), as well as

the redox state of fulvic acids (Klapper et al. 2002,

Fulton et al. 2004, Cory and McKnight 2005, Miller

et al. 2006). Parallel factor analysis (PARAFAC)

has been applied to resolve the dominant fluorescent

components present in three-dimensional excitation

emission matrices (EEMs) of DOM from diverse

natural waters (Stedmon et al. 2003, Cory and

McKnight 2005). These fluorescent components

include oxidized and reduced quinone-like fluoro-

phores (Cory and McKnight 2005). The ratio of

reduced quinones to total quinones, known as the

redox index (RI) (Miller et al. 2006), is a useful tool

to identify the redox state of fulvic acids in ground

water, and has been used to examine transforma-

tions of humic substances along flowpaths in a

wetland hyporheic zone (Miller et al. 2006).

In order to explore relationships between hydro-

logic controls and the sources, chemical character,

and redox state of fulvic acids in wetland-recharged

ground water, this study investigates chemical

properties of DOM in ground water of the seasonal

swamp in the Okavango Delta. Also, this study

employs direct measurements of ground-water DOC

concentrations and fulvic acid content to evaluate

the hypothesis posed by Bauer-Gottwein et al.

(2007) that humics concentrations are not high

enough to induce geochemical reactions that influ-

ence density-driven flow of solutes in island ground

water. Finally, by examining basic water chemistry

and UV absorbance and fluorescence properties of

DOM along a hydrologic flowpath from surface

water to ground water in the island interior

(Figure 1), we evaluate the potential for both

biogeochemical processing and substantial storage

of DOM in ground water supplied by DOM-rich

surface water.

METHODS

Site Description and Field Sampling

Ground water was sampled at two locations

adjacent to surface water sources. The first transect

(‘‘island transect’’) terminates on the southeastern

end of an island that receives regular annual

flooding on its southwest side due to its proximity

to the Boro River channel (Figure 1). This island is

one of the larger islands in the Delta (approximately

400 m 3 1,500 m) and becomes completely sur-

rounded by water only during high flood years (as in

2001). The island transect is referred to as the ‘‘AB

transect’’ in Wolski and Savenije (2006) and Wolski

et al. (2005) and as ‘‘ORC Island’’ by Bauer-

Gottwein et al. (2007). The second location (‘‘wood-

land transect’’) crosses an area of dryland adjacent

to a seasonal floodplain, which is hydraulically

connected to the Boro River channel (Figure 1).

The annual flood occurs during the dry season

when floodwaters originating in the Angolan

Highlands inundate the Okavango Delta. During

the annual flood, surface water levels in the Boro

River increase between 0.8 and 1.5 m; and surface

water infiltrates and flows laterally toward island

centers. This lateral ground-water flow is driven by

evapotranspiration at the island centers and has

been documented by McCarthy and Ellery (1994)

and modeled by Gieske (1997) and Wolski and

Savenije (2006).

The island has a relatively wide (100 m) vegetated

fringe composed of riparian woodland species.

Ground-water piezometers sampled in this region

(C1 to C7) are referred to as being in the ‘‘island

fringe.’’ During the annual flood, the water table of

the island fringe is active and can rise 3 m (Wolski

and Savenije 2006). The island’s center is covered by

grassland and salt crusts and is underlain by a

relatively thick (more than 7 m) lens of deposits of

predominantly clayey texture. Such lenses are typical

of Okavango Delta islands and have been described

by McCarthy and Ellery (1994). The lenses are

composed of quartz parent sand and ground-water

precipitates comprising amorphous silica and car-

bonates. Ground-water piezometers sampled in this

region (piezometers C8 (at 140 m) to C10 (at

240 m)) are referred to as being in the ‘‘island

center.’’ During the annual flood, the water table of

the island center remains fairly constant, rising only

between 0.10 m and 0.25 m about 1–5 months after

the surface water peak (Wolski and Savenije 2006).

Mladenov et al., CHARACTERIZATION OF DOM IN WETLAND GROUND WATER 749

The dryland body adjacent to the floodplain at

the woodland transect is covered by a mixture of

dryland and riparian species, while further inland, a

typical dryland forest occurs. The floodplain is

covered by dryland grasses in dry periods, and by

aquatic sedges and grasses while inundated during

the annual flood. The substratum penetrated by

piezometers (W1 to W8; at 1 m to 200 m) is

uniformly sandy, apart from a small clayey lens in

the central part of the floodplain. No ground water

was sampled from the central part of the floodplain.

The floodplain is annually flooded, and the duration

and level of the floods vary between years. Annually

the ground-water table can rise up to 2.5 m (Wolski

and Savenije 2006).

Ground-water samples were collected from a

network of piezometers located along the island

and woodland transects (Figure 1) on June 12, 26,

and 28, 2001; on July 10, 12, and 25, 2001; on June

26, 2002; on September 21, 2005; on December 2,

2005; and on May 25, 2007. At locations W1-W8

and C1-C10 nested piezometers were installed,

comprising two or three pipes filtered at different

depths, usually at 2, 4, and 6 m below ground. Only

the two deepest piezometers were sampled for this

study (Figure 2).

Piezometers were pumped dry five times prior to

sample collection. All samples were filtered with GF/

C glass fiber filters with 1.2 mm nominal pore size.

Samples collected for organic chemistry and UV-vis

and fluorescence spectroscopy were acidified with

concentrated HCl to a pH of about 2. Conductivity

and temperature were measured in situ using a YSI-

30 Salinity-Conductivity-Temperature meter. Dis-

solved oxygen (DO) was measured with a YSI-55

DO meter and pH was measured with a Fisher

Scientific Accumet AP60 portable pH meter.

Laboratory Analyses

Alkalinity, Anions, and Total Fe. Alkalinity was

measured on September 21, 2005 and December 2,

2005 samples in the lab with a Mettler Toledo DL50

auto Titrator. Anions (chloride and sulfate) were

measured on the samples collected on September 21

and December 2, 2005 using a Dionex Series 4500I

Liquid Ion Chromatograph with a 50 ppb detection

limit. Fe was measured within one month of

collection on May 25, 2007 samples on a Finnigan

Element 2 ICP-MS.

DOC Concentrations. DOC concentrations were

measured on samples collected in 2001, 2002, and

2005 using a Shimadzu TOC-5050 Total Organic

Carbon Analyzer, within one to four months of

sample collection and were replicated within runs

and over time. The standard deviation of replicates

was , 5% for all samples. It has been shown that

high inorganic carbon (IC) concentrations can lead

to an overestimation of DOC concentrations when

the TOC analysis technique uses oxidation to drive

off the IC (Potter and Wimsatt 2005). In order to

accurately measure DOC concentrations in piezom-

eters C8–C10, samples were diluted (1:100) with

MilliQ water and re-acidified to pH 2 to drive off

high inorganic carbon content. CO2 degassing was

observed during acidification.

Absorbance. Absorbance at 280 nm was measured

on samples collected in 2001, 2002, and 2005 using

an Agilent 8453 UV-VIS spectrophotometer with

ChemStation software and a 1 cm path length cell.

Figure 2. Profiles of piezometer depth, pH, alkalinity

(alkalinity expressed as HCO3 is circled), DO, and

temperature in the ground water of island (left panels)

and woodland (right panels) transects. In order to

maintain figure clarity, all distances and alkalinity y-axis

are shown using a log scale. Mean values and standard

deviations are shown. Surface water results are shown

with a dashed line.


Each sample was measured three times and the

standard deviation of replicates was , 5% for all

samples. Specific UV absorbance (SUVA) was

calculated as the absorbance (measured at 280 nm)

normalized to the DOC concentration and reported

in units of m21 mg21 L. UV absorbance at 280 nm

or 254 nm have both been commonly used to

calculate SUVA (Chin et al. 1994, Weishaar et al.

2003). Because nitrate can absorb at the lower

wavelength, SUVA at 280 nm was chosen for this

study. For comparison, the SUVA of two end

member aquatic fulvic acids, Suwannee River

reference fulvic acid (SRFA, a standard of the

International Humic Substances Society) and Lake

Fryxell fulvic acid (LFFA, McKnight et al. 2001),

was measured to indicate plant/soil source and

microbial DOC sources, respectively.

Fulvic Acid Isolation. Hydrophobic organic acids

were isolated from approximately 150 mL of filtered

and acidified water samples collected in 2002 and

2005 using small volume (10 mL) columns filled

with XAD-8 resin, following the method of Thur-

man and Malcolm (1981). For consistency with

other studies (McKnight et al. 1997, Klapper et al.

2002, Hood et al. 2003), these hydrophobic organic

acids will be referred to as fulvic acids (FA).

Fluorescence. Fluorescence spectroscopy can pro-

vide insights into the chemical properties of DOM

by generation of EEMs, determination of the

fluorescence index (FI), and PARAFAC modeling.

EEMs are a 3-dimensional representation of fluo-

rescence intensities scanned over a range of excita-

tion/emissions (Ex/Em) wavelengths. Prominent

peaks in fulvic acids have been found at Ex/Em

wavelengths of approximately 240/450 nm (referred

to as region A) and 320/450 nm (referred to as

region C) (Coble 1995). EEMs were generated for

the 2005 whole water samples (measured using a JY-

Horiba/Spex Fluoromax-3 spectrophotometer) and

were scanned over an excitation range of 240 to

450 nm at 10 nm increments and an emission range

of 350 to 550 nm at 2-nm increments with DataMax

data acquisition software. To minimize quenching of

the fluorescence signal due to metal complexation by

iron, all samples were acidified to a pH of about 2.

MilliQ water blanks were subtracted to remove

Raman scattering and all samples were normalized

to the Raman area to account for lamp decay over

time. All samples were corrected for the inner-filter

effect (Mobed et al. 1996) using the correction

specified in McKnight et al (2001). High DOC

samples (with concentrations . 20 mg C L21) were

also diluted with MilliQ water so that absorbance

(measured at 300 nm) was below 0.02. Corrections

and generation of EEMs was performed using

MATLAB. Peaks in region A were identified and

the intensities of those peaks were normalized to

fulvic acid concentration for each sample, if

available.

To obtain the FI (McKnight et al. 2001), two-

dimensional spectra excited at 370 nm were gener-

ated and corrected for Raman scattering by blank

subtraction. Two-dimensional spectra were collected

on a Fluoromax-3 spectrofluorometer in 2005 and

on a Fluoromax-2 spectrofluorometer in 2001 and

2002 and the spectra run on the two spectrofluo-

rometers were corrected for instrument specific

response (Cory 2005). FI values (dimensionless)

were calculated from the ratio of intensities emitted

at 470:520 nm (Cory 2005) with a confidence

interval of 0.01. Among samples, collected over

time from the same system and analyzed with the

same instrument with appropriate corrections,

changes in FI of 0.05 have been found to indicate

shifts in dominant DOM source (Hood et al. 2003,

Mladenov et al. 2005).

Recently, PARAFAC has been used to decom-

pose EEMs into different classes of fluorophores,

referred to as components (Stedmon et al. 2003,

Cory and McKnight 2005). Using a dataset of 379

DOM samples from diverse aquatic environments,

Cory and McKnight (2005) developed a PARAFAC

model that identified 13 individual components

responsible for fluorescence and showed that qui-

none-like fluorophores accounted for about 50% of

the fluorescence of every sample analyzed. In our

study, EEMs of 23 ground-water samples collected

in 2005 were fit to the 13-component model of Cory

and McKnight (2005), and the relative amount

(percent of total) of each component was measured.

Model fit was considered suitable if intensities in the

residual EEM, generated by subtracting the PAR-

AFAC-modeled EEM from the measured EEM,

were within 10% of measured EEM intensities.

To assess redox state of aquatic fulvic acids, a

redox index, as defined in Miller et al. (2006) was

used. The RI is a ratio of reduced quinone

components to total quinone components (sum of

reduced and oxidized components) identified by the

PARAFAC model (Miller et al. 2006). Of the 13

components identified by PARAFAC, the three

quinone-like components (Q1, Q2, and Q3) repre-

sent oxidized components, and the semiquinone-

and hydroquinone-like components (SQ1, SQ2,

SQ3, and HQ) represent reduced components (Cory

and McKnight 2005). Components found to be

associated with microbial DOM sources (C2, C3,

C6, C7, C8, C9, C12, and C13; Cory et al. 2007)

were also examined.


Given that the handling and transport of ground-

water samples would have resulted in some atmo-

spheric exposure, samples for fluorescence analysis

were filtered and acidified immediately upon collec-

tion but not processed in an anoxic environment.

Studies have found that reduced quinones can

remain stable without precautions taken to limit

oxygen exposure (Scott et al. 1998, Klapper et al.

2004). Even in waters with high iron concentration,

the fluorescence spectra remained relatively unal-

tered in the presence of oxygen (Mladenov, unpub-

lished).

RESULTS

Changes in Subsurface Chemistry along a Flowpath

Island Transect. Along the island fringe (1–100 m),

ground-water pH, alkalinity, DO concentrations,

conductivity, and chloride and sulfate concentra-

tions remained constant (Figures 2 and 3). Similarly,

DOC concentrations were fairly constant along the

island fringe, ranging from 11.4 to 16.1 mg C L21

(Figure 4). FA content also remained fairly constant

in island fringe ground water (at about 70%), similar

to that of water in the adjacent Boro channel and

significantly higher than the estimate of 50% used in

Bauer-Gottwein et al. (2007). Only temperatureshowed a gradual increase along the flowpath from

surface water toward the island interior (Figure 2).

At the island center (100–240 m), where the island

vegetation structure is dominated by salt-tolerant

grasses, distinct increases in ground-water pH,

alkalinity, conductivity, chloride, sulfate, iron (Fe),

and DOC concentration were observed (Figures 2–

4). Conductivity ranged from 5,030 to18,860 mS cm21, the pH was basic, and total Fe

concentrations ranged from 4.0 to 7.8 ppm. Chlo-

Figure 3. Profiles of conductivity, chloride, sulfate, and

total dissolved iron (Fe) in the ground water of island (left

panels) and woodland (right panels) transects. In order to

maintain figure clarity, distances and the y-axes of all

chemical species are shown using a log scale. Mean values

and standard deviations are shown. Surface water results

are shown with a dashed line.Figure 4. Profiles of DOC concentration, fulvic acid

(FA) content, specific UV absorbance at 280 nm (SUVA),

fluorescence index (FI), and redox index (RI) in the

ground water of island (left panels) and woodland (right

panels) transects. In order to maintain figure clarity,

distances and DOC concentrations are shown using a log

scale. Mean values and standard deviations are shown. RI

data is presented only for samples collected on September

21 and December 2, 2005. Surface water means are shown

with a dashed line and standard deviations are reported in

Table 1.


ride, generally considered a conservative ion, in-

creased in concentration by about 3–4 orders of

magnitude (from 0.3 ppm at the channel to 470 ppm

at 240 m), while DOC concentrations increased by

only one order of magnitude (from almost

10 mg C L21 at the channel to . 170 mg C L21

at 240 m).

Significant differences were not found (using

Student t-tests) in any of the parameters listed in

Table 1 between ground-water samples collected

from the island fringe and adjacent channel surface

water. In contrast, ground-water samples collected

from the island center had significantly higher DOC

concentrations and conductivity than either island

fringe ground water or channel surface water

(Table 1).

Woodland Transect. DOC concentrations ranged

from 14.2 to 21.3 mg C L21 and FI ranged from

1.23 to 1.37 in ground water of the woodland

transect (Figure 4). Student t-tests indicated that all

measurements shown in Table 1, except FI, were not

significantly different between woodland ground

water and woodland surface water. Temperature

and alkalinity were the only variables that showed

an increase with distance toward the woodland

interior (Figure 2).

Changes in DOM Spectroscopic and Redox

Properties along a Flowpath

Along the island fringe, SUVA, FI, and RI values

were constant (Figure 4). SUVA and FI decreased

and RI increased in island center ground water.

Student t-tests indicated that these differences

between island center and island fringe ground

water were significant (p , 0.001 for all). High RI

values (. 0.5), indicative of a greater relative

abundance of reduced fulvic acids, observed at the

island center transect were similar to values found in

reduced shallow ground water of an alpine wetland

(Miller et al. 2006). Student t-tests indicated that FI

was significantly lower in island fringe and wood-

land ground water than in adjacent surface water.

Lower FI signifies greater contributions from plant/

soil-derived DOM than from microbially derived

DOM.

Ground water of the island fringe had higher

fluorescence intensities (normalized to fulvic acid

concentration) than surface water (representative

EEMs are shown in Figures 5A and 5B). EEMs

from the seven piezometers at the island fringe

transect contained a distinct shoulder in region C

and peak in region A. The 200 m and 240 m EEMs

(representative EEM is shown in Figure 5C) had

lower region A intensities than EEMs of island

fringe ground water and displayed a much broader

shoulder in region C. Also the region A peaks of

island center ground water were more red-shifted (to

higher emission wavelengths) than in island fringe

ground water.

Along the woodland transect, the shape of the

EEMs (representative EEMs are shown in Fig-

ures 5D through 5F) resembled those from the

island fringe transect with a distinct shoulder in

region C and peak in region A present in all samples.

PARAFAC modeled EEMs matched measured

EEMs with a residual of , 10% in all cases. In

island fringe samples and in all woodland transect

samples, the amounts of each component present in

the EEM changed very little or not at all with

distance along the flowpath. To illustrate the

differences between the large number of samples

more clearly, mean values of each component

(expressed as percentage of total components) are

Table 1. Mean values of measured parameters (dissolved organic carbon (DOC), conductivity, fulvic acid (FA) content,

specific UV absorbance (SUVA), fluorescence index (FI), and redox index (RI)) for surface water (SW) vs. adjacent

ground-water (GW) samples at island and woodland transects for all sampling periods. Standard deviations and number of

samples (in parentheses) are shown.

Island Transect Woodland Transect

SW Fringe GW Center GW SW GW

DOC (mg C L21) 13.3 6 2.6 (8) 13.8 6 3.1 (48) 170 6 76 (5)* 16.6 6 4.5 (9) 16.3 6 2.6 (17)

Conductivity (mS cm21) 101 6 26 (4) 250 6 114 (19) 15016 6 5882 (5)** 178 6 64 (5) 202 6 138 (15)

FA content (%) 68 6 4 (5) 70 6 4 (23) 58.1 6 5.7 (2) 65 6 1 (5) 70 6 4 (5)

SUVA (L mg21)a 2.30 6 0.10 (5) 2.32 6 0.40 (37) 1.59 6 0.37 (5)** 2.70 6 0.20 (6) 2.59 6 0.50 (15)

FIb 1.50 6 0.01 (8) 1.48 6 0.10 (18) 1.20 6 0.02 (4)** 1.45 6 0.06 (9) 1.31 6 0.10 (12)*

RI 0.44 0.41 6 0.01 (9) 0.58 6 0.01 (3)** 0.44 0.46 6 0.004 (8)

*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.


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.


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

number of samples (n) are also shown.

Molecular

association

PARAFAC

componenta

Island Transect Woodland Transect

SW (n 5 1) Fringe GW (n 5 9) Center GW (n 5 3) SW (n 5 1) GW (n 5 8)

Quinone

C2 (Q2, M) 20.4 21.8 6 0.3 16.5 6 0.3 20.6 20.7 6 0.8

C11 (Q1, T) 13.1 13.4 6 0.1 10.1 6 0.3 13.0 13.0 6 0.1

C12 (Q3, M) 8.2 9.3 6 0.5 5.6 6 0.4 8.4 7.7 6 0.4

Hydroquinone and semi-quinone

C4 (HQ, both) 21.2 18.2 6 0.4 29.9 6 1.0 21.3 22.9 6 0.2

C5 (SQ, T) 5.3 4.3 6 0.2 9.2 6 0.5 5.4 5.8 6 0.1

C7 (SQ, M) 4.4 5.6 6 0.3 3.1 6 0.4 3.9 4.0 6 0.4

C9 (SQ, M) 2.4 2.8 6 0.3 2.6 6 0.2 1.9 1.9 6 0.1

Amino-acid

C8 (Trp, M) 0.5 1.5 6 0.5 0.0 0.8 0.1 6 0.2

C13 (Tyr, M) 3.0 2.4 6 0.6 2.2 6 0.1 3.3 2.8 6 0.5

Unknown

C1 (T) 7.0 6.9 6 0.3 5.8 6 0.1 7.0 7.0 6 01

C3 (M) 3.9 4.9 6 0.2 2.4 6 0.3 3.4 3.0 6 0.1

C6 (M) 8.2 7.0 6 0.3 9.9 6 0.2 8.6 8.5 6 0.3

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.


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-


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.


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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