Effectiveness of Man-Made Wetland Systems in
Filtering Contaminants from Urban Runoff in
Milledgeville, Georgia
Samuel Mutiti1*, Hannah Sadowski1, Christine Melvin1, Christine Mutiti1
ABSTRACT: A wetland system made up of linked basins was
investigated to determine its role in local flood control and contaminant
filtration. The study focused on a wetland basin that is dominated by the
Celtis laevigata plant and is underlain by clay, with a small sandy layer
approximately 1 m below surface. Field and laboratory data were
collected to understand the transport and filtration of phosphate, iron,
and nitrate. Field results showed the wetland to effectively reduce iron
and phosphates from runoff via groundwater flow. These results also
showed an increase in the phosphate concentration of surface water
while within the basin, resulting from agitation of wetland sediments.
Laboratory column experiments showed more than 90% reduction in
phosphorus and iron while nitrate concentrations increased above the
input concentration. Phosphate and iron were high in wetland water
immediately after a rain event. Nitrate concentrations increased as the
water filtered through the sediments due to desorption of previously
adsorbed nitrates. This wetland could potentially act as a temporal hot
spot and rain events as hot moments for these substances. Therefore, the
best flood control measure for this site would be to increase residence
time in the wetland. This would help to better manage/control the
concentration of phosphate, iron and nitrate pollution in surface waters.
Water Environ. Res., 87, 358 (2015).
KEYWORDS: filtration, wetland, flood control, urban pollutants,
sorption, nitrate, phosphate, nutrients.
doi:10.2175/106143015X14212658613758
Introduction
The Clean Water Action Plan lists urban runoff among the top
five contributors to water quality impairment in rivers, streams,
and estuaries (U.S. EPA/USDA, 1998). Urban and residential
runoff can introduce a multitude of anthropogenic chemical
pollutants, such as nutrients, organic compounds, and metals
into aquatic systems (Helmreich et al., 2010; Kayhanian et al.,
2007; Meland et al., 2010). Urban areas also increase the volume
of runoff resulting from increased impervious areas and reduced
infiltration. For example, the prominence of impermeable
surfaces in urban settings prevents rainwater from percolating
into the soils (Andoh and Declerck, 1997), which results in
significant overland flow (Leopold, 1968). Vast amounts of
runoff, therefore, rapidly move through cities collecting various
1
Department of Biological and Environmental Sciences, Georgia
College, Campus Box 81, Milledgeville, GA 31061.
*
Biological and Environmental Sciences, Georgia College and State
University, CBX 081, Milledgeville, GA 31061; e-mail address:
[email protected].
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pollutants from streets, garages, and parking lots. The effects of
fast-moving stormwater through a city and its watershed include
local flooding, erosion, sedimentation, and surface water
contamination (Andoh and Declerck, 1997; Hatt et al., 2004;
Thurston, 2006). Both short high-intensity storm events and
prolonged low-intensity rain events can produce these negative
effects and carry large volumes of water containing elevated
concentrations of pollutants that end up in rivers.
Excess nutrients (especially nitrates and phosphates) in rivers
can wreck havoc on aquatic systems by causing eutrophication
and the associated decrease in dissolved oxygen content.
Numerous studies have shown the adverse effects of increased
levels of nitrate and phosphate in aquatic systems (Aguiar, et al.,
2011; Li et al., 2011; Zhang et al., 2011). The problems associated
with elevated nutrients and severely low dissolved oxygen
include reduced biodiversity, fish kills, and foul smells.
Fortunately, there are a variety of natural and manmade
solutions to these problems. The natural solutions include
riparian zone vegetation and wetlands, which have been shown
to significantly reduce the amounts of nutrients and other
contaminants (Ackerman and Eagles-Smith, 2010; Faulwetter et
al., 2009; Lowrance et al., 1997) and sediments entering surface
waterbodies. Wetlands and riparian vegetation have also been
shown to reduce local flooding (Fisher and Acreman, 2004) as
they act as storage for excess water during high stage events.
These features of a landscape can act as buffer zones on the edge
of rivers and streams. They control the flow of water to prevent
flooding, remove chemicals, and can contribute to local
groundwater recharge (U.S. EPA/USDA, 1998). Wetlands are,
therefore, an important component of any landscape and need
to be protected.
Mitsch and Gosselink (2000) estimated that a temperate
landscape should be made up of 3 to 7% wetlands to maximize
its economic benefits. Unfortunately, the last century has seen a
significant loss of wetlands in these regions resulting from
human activities (Gibbs, 1999; Johnson, 1994). Wetlands,
whether natural or artificial, need to be functioning properly
to effectively help control flooding and reduce pollution.
Nutrients and other contaminants are removed in a wetland
when their input (into the wetland) exceeds the output (Dillaha
and Inamdar, 1996; U.S. EPA, 2008), thus preventing excess
nutrients from entering the river system. If wetland soils
accumulate pollutants, what then would prevent them from
acting as hot spots source of these same contaminants at some
point in the future? Vidon et al. (2010) discuss the concepts of
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Mutiti et al.
Figure 1—Site map, showing location of Baldwin County in Georgia, the City of Milledgeville, and location of the Oconee River
Greenway (ORG) in Milledgeville.
hot spots and hot moments in riparian and wetland environments. In their paper, they provide a review of literature on
riparian zones and wetlands acting as sources of contaminants
for streams.
The overall goal of this study was to understand the role of a
local wetland in removing contaminants from runoff and to
determine the best way of controlling local flooding. Specific
objectives included (1) investigating the main source of water in
the wetland system and quantifying the levels of iron, phosphate,
and nitrate in the inflows and outflows of the wetland; and (2)
evaluating the filtration capacities of the wetland soils to
determine their effectiveness in reducing nutrient contaminants
in runoff as well as infer the wetlands’ potential to act as hot
spots for some contaminants entering the Fishing Creek and the
Oconee River.
Materials and Methods
Site Description and Background. The site chosen for this
study is a structurally unique wetland located along the Oconee
River in Milledgeville, Georgia (Figure 1). The Oconee River
begins in the Piedmont physiographic province of Georgia and
flows south past the Fall Line into the Coastal Plains where it
joins with the Ocmulgee to form the Altamaha River. This river
is divided into the Upper and Lower Oconee River and the site
for this study is located in the latter. In the Lower Oconee
around Milledgeville, the main tributary to the Oconee River is
Fishing Creek. The two rivers exhibit meandering forms and
have a few meander cutoffs, including one at the study site
(Doran et al., 2010; Mangrum et al., 2010). The geology of the
area can be characterized as a mixture of piedmont crystalline
metamorphic rocks and coastal plain late Cretaceous/Eocene
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sands and clays (Cocker, 1995; GEPD, 1998; Gore and Witherspoon, 2013).
The wetland system has developed on a locally historic site,
the previous location of the McMillan Brick Works, at the
confluence of the Fishing Creek and the Oconee River. The
wetland system is made of five small hydrologically connected
basins at the Oconee River Greenway (ORG), a popular
recreational park southeast of downtown Milledgeville. The
basins are depressions left on the flood plain after clay was
excavated for brick making. The basins are arranged in series
toward the Oconee River and are mostly separated by sand and
clay, as well as leftover sediment-covered bricks in some places.
The soils in this area are generally classified as Inceptisols
belonging to the Wehadkee soil series (Soil Survey Staff, 2013).
These soils are mostly poorly drained fluvial deposits. In Alice
Basin, the recent soils are approximately 4 to 4.6 m thick lying
on metamorphic gneisses.
The current study investigated water flow through the entire
wetland systems but focused mostly on one of the basins, Alice
Basin. Alice is the third of the five basins that receives both
surface and subsurface water from two of the other basins. The
basin is dominated by trees that are commonly found in areas
like swamps, riverine forests, floodplains, and other types of
wetlands that experience flooding. The most dominant trees
are Celtis laevigata (hackberry), Platanus occidentalis (American sycamore), Acer negundo (box elder), Ulmus americana
(American elm), and Fraxinus pennsylvanica (green ash).
Other trees that are not common in the basin include Salix
nigra (black willow), Liquidambar styraciflua (sweetgum), and
Quercus nigra (water oak). There is complete canopy closure
and as a result, most of the ground has no cover except
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Mutiti et al.
Figure 2—The drainage system and topography of the area surrounding the study wetland. Elevation ranged from 65 to 70 m above sea
level (data used to make the maps were obtained from the Georgia College Research Commons).
Saururus cernuus and a few grasses (Phragmites spp) that are
generally confined to the edges of the basin where there is more
light availability. Water enters the basins mostly via overland
and groundwater flows. Subsurface flow is mainly through a
highly conductive sandy layer that is approximately 0.5 to 1 m
below the top clay layer and continues laterally to both Fishing
Creek and the Oconee River. A small proportion of water
enters Alice Basin through direct precipitation. Because these
basins, with the exception of the last one in the sequence, do
not have well-established outlets, it was hypothesized that the
basins mostly lose water through subsurface flow and
evapotranspiration. This wetland is seasonal and only has
standing water during the wet season (spring) of nondrought
years and periods of prolonged rainfall. During the dry season
or after prolonged drought, water disappears from the basin
almost instantaneously during and after rain events with little
to no accumulations in the basin. However, after a prolonged
rain event (during the wet season), the water accumulated in
the basin takes a long time to leave the wetland via subsurface
flow and evapotranspiration. In the wet season, overtopping of
the basin walls is also frequently experienced in Alice Basin.
The park frequently floods after heavy rains and the resulting
overtopping of the sidewalks makes it unusable to residents. The
ORG Park sits at a lower elevation relative to the city of
Milledgeville (Figure 2), allowing urban runoff to flow through it
on its way to the two rivers. A large football practice field owned
by Georgia Military College (GMC), Milledgeville city center, as
well as surrounding parking lots, and residential areas contribute
urban runoff directly into the ORG during rain events. In
addition, there is a sewer line with more than five manholes at
the greenway (with a couple located uphill of the study site) that
frequently overflow, adding pollutants into the wetland and river
360
system during rain events. This wetland system provides the only
opportunity for runoff to be filtered before entering the rivers on
this side of town.
Methodology
Field. A Sokkia SET-600Total Station (SOKKIA, Olathe,
Kansas) and a Trimble GeoXM Global Positioning System
receiver (Trimble Navigation Limited, Sunnyvale, California)
were used to collect geographic information systems (GIS) data
that were used to create a fine-resolution topographic grid and a
map of all low-lying areas (basins) and the ridges separating
them. A detailed analysis of the basins was conducted to
understand overtopping of sidewalks and also to determine the
best location for flood control measures. Potential sources of
water in these basins were determined using both GIS analyses
and field observations.
Data for understanding the hydrologic budget were also
collected from literature (evaporation data) and field observations. Evaporation and evapotranspiration were estimated in the
field using evaporation pans and maps by Kohler et al. (1959).
Ponded infiltration was estimated during the dry season using a
double ring infiltrometer in the wetland system, mostly within
Alice Basin. Subsurface water flow was investigated by
trenching, installing a series of polyvinyl chloride (PVC)
piezometers, analyzing soil cores, slug tests, and application of
Darcy’s Law to determine groundwater outflows. STELLA
Modeling software, version 9.1.4 (ISEE Systems Inc, Lebanon,
New Hampshire) was then used to analyze hydrologic connections. A basin water balance conceptual model was designed and
used to model water flow movement in Alice Basin (Figure 3).
The surface runoff component was estimated using the rational
methods (Fetter, 2001; Young et al., 2009):
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Mutiti et al.
Figure 3—Conceptual model used in STELLA and GIS to model water flow into and out of Alice Basin at the Oconee River Greenway
Park.
Q¼C*I *A
ð1Þ
where,
Q ¼ peak runoff ðft 3 =sÞ;
I ¼ average rainfall intensity ðin:=hrÞ;
and
A ¼ drainage areas ðacresÞ:
Because the drainage area was small (less than 200 acres), the
rational equation was a valid easy way of estimating the runoff
component for the STELLA model. The rational coefficients
were estimated from values published by the Joint Committee of
the American Society of Civil Engineers and the Water Pollution
Control Federation (1960). The drainage area for the runoff (24
000 m2) and infiltration area (3756 m2) were estimated in
ArcMap 10.0 (ESRI, Redlands, California) and Google Earth
(Google Inc., Mountain View, California) using satellite images.
The volume of Alice Basin was also estimated in ArcMap 10.0
using GPS and elevation data collected using the total station.
Twenty piezometers were installed in and around Alice basin
to map the water table and monitor the direction of the
groundwater movement. Piezometers were installed using an
open face auger, 0.75-in. to 2-in. PVC pipes with sand or gravel
packing. Depth below ground of the piezometers ranged from
0.3 to 4 m (to the gneiss bedrock). Water levels in the
piezometers were measured using water tapes and InSitu
(INSITU, Bingen, Washington) pressure transducers. Falling
head slug tests were conducted using the Bouwer and Rice
method (Bouwer, 1989; Bouwer and Rice, 1976) for determining
hydraulic conductivity (K).
Laboratory Experiments. Soil samples were collected from
the field for soil characterization and filtration experiments in
the laboratory. The samples were collected from various
locations within the basin using the open-faced auger. Some of
the soil samples were visually analyzed for soil structure while
another set was oven dried and used to determine grain size
distribution. After the soils had dried, they were mechanically
pulverized with a mortar and pestle, weighed, and then sieved.
The sieved samples were used to determine grain size
distribution. Filtration capacities were primarily investigated
using column experiments. Intact soil cores of approximately 6
in. in depth were collected from the wetland (from the top clay
and the lower sandy layers) during the dry season when there
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was easy access. The cores were collected using columns
constructed from clear plastic tubes with a height of 80 cm
and an internal diameter of 3.5 cm. Approximately 10 cm of soil
cores were collected from both the clay and sandy soils found in
Alice Basin. The cores were taken with great care to ensure that
the soil sediments remained intact and, therefore, retained the
same compaction and bulk density during the column
experiments. The cores were then brought back to the
laboratory and used in column experiments.
The experiments were conducted by initially flushing each
core with 500 mL of deionized water (DI; EMD Millipore Corp.,
Billerica, Massachusetts) followed by a 250 mL slug of a nutrient
standard. The standard concentrations used in the lab experiments were all within the range of values observed in the field.
The nitrate standard was made using a known mass of
ammonium nitrate dissolved in a known volume of DI water.
Nitrate standard concentrations used in the column experiments
ranged between 2 and 3 mg/L. The iron standard was made
using a ferric chloride (2.5 and 4 mg/L) and phosphate standard
using sodium phosphate (2.0 and 2.5 mg/L). The nutrient slugs
were immediately followed by 250 mL of DI water. The whole
experiment was conducted to simulate continuous flow (no
breaks in between the DI, nutrient solution, and DI sequence).
For each nutrient, approximately 15 samples of 50 mL each were
collected over time from each column. The 15 collected samples
were tested in triplets using the HACH DR 5000 UV-VIS
Spectrophotometer and the HACH DR/890 Portable Colorimeter (HACH Inc, Loveland, Colorado). The experiments were
repeated at least 3 times, but after each standard had been run
through the soil cores, the cores were allowed to dry overnight
before another test was run.
To investigate the temporal variability of infiltration rates
through the soils, column experiments were also conducted
under two different moisture conditions: initially when the soils
were wet and then when the soils had been dried over a period of
2 months. During these experiments the time it took for 750 mL
of solution to filter through the soils under a constant hydraulic
head was monitored.
Results
Water Flow and Flooding. Observations of storm flow
during rain events and the evidence left by moving water after
the events revealed that most of the water in the wetland systems
comes from overland flow, especially in the upper basins.
Subsurface flow continues to add water to the basins after rain
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Mutiti et al.
Figure 4—Topography and groundwater characteristics of Alice
Basin.
events have ceased (at a specific discharge of 3.2 m/d) but in
much smaller amounts than that added by direct runoff.
Groundwater flows from the forested upland areas, through
the basins and then into the river. Groundwater flowed out of
Alice Basin through a restricted section on the southern end of
the basin (Figure 4).
Average evaporation rates were estimated to be approximately
0.012 m/d. These evaporation results did not support the initial
hypothesis that evaporation played a key role in the water budget
of this wetland system. The water lost via evaporation was
insignificant and, therefore, could be ignored in the overall
wetland’s water budget. Transpiration was also not very
significant because the wet season coincided with leaf fall and
low temperatures.
Infiltration results in Alice Basin showed a spatial variability
that is controlled by topography (elevation) within the basin.
Low elevation areas had the lowest infiltrations rates (0.012 m/
d). The low spots fill up with water earlier, but drain out later
than the relatively higher spots with an infiltration rate of
approximately 0.72 m/d. This spatial variability in infiltration
rates was only observed during the wet season because in the dry
season all parts of the basins drained very rapidly. In fact, during
the dry season, water from rain events rarely accumulated and
when it did, drained out almost immediately. This is in contrast
to the wet season when water stayed in the basin for months on
end, even when there had been no rain for weeks.
Depth to water in the piezometer ranged from 0 to 4 m during
the study period. Hydraulic conductivity values ranged from
0.002 to 0.1 m/d with an average 0.03 m/d. Due to the low
hydraulic conductivity of the clay soils (which make up most of
the wetland), groundwater flow in wetland system was mainly
through the sandy layer. Topographic analysis in GIS and water
balance modeling in STELLA showed that during wet periods,
when infiltration rates were very low, it would only take medium
rain events (0.48 m/d for 0.25 days) to fill the basins (970 m3)
(Figure 5). A combination of field observations, topographic
analysis in GIS (Figure 4), and water balance modeling in
STELLA (Figure 5) showed that during wet periods, Alice Basin
would flood before the water volume reached the calculated
maximum volume of 970 m3. It would only take a medium rain
event (0.48 m/d for 0.25 days) to overtop the sidewalks. This
occurs at approximately half of the basin’s volume (480 m3)
Figure 5—STELLA modeling results showing that a 0.02 m/hr rain event would cause flooding in Alice Basin just after 6 hours.
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Mutiti et al.
Figure 6—Concentration of phosphate, iron, and nitrate in runoff measured in the field during and after a rain event.
Figure 7—Filtration of (a) phosphate standard in sandy soil
column, and (b) phosphate in clay soil column (with no DI water
preceding the standard solution).
April 2015
resulting from the geometry of the basin. The southern end of
the basin, where flooding occurs, is much shallower than the rest
of the basin.
Environmental Samples. Analysis of the phosphate and iron
concentrations in the runoff, basin-water, and groundwater
seepage to the stream indicated that the wetland was a reservoir
for these contaminants. The wetland could, therefore, potentially
act as a hotspot for some of these contaminants (Figure 6). The
concentrations of phosphate and iron coming into the system
via runoff from the GMC practice field were initially low but
then increased after entering Alice Basin during the storm event.
Once the water had moved through Alice to Fishing Creek via
seepage (a week later), there was a reduction in phosphate
concentration.
Iron also showed an increase in concentration as the water
moved from GMC into Alice Basin during the event and then a
decrease afterward. The concentration continued to decrease as
the water moved through the subsurface from the basin to
Fishing Creek (Figure 6). Nitrate concentrations were highest in
the runoff before entering the basin but decreased significantly
once it was in Alice Basin. There was a slight increase when
water entered Fishing Creek via groundwater.
Laboratory Experiments. Soil Characterization. Analysis of
the soils showed the soils within the basin to be mostly fine
loams, dominated by massive structured clay. In some parts,
macroaggregates, stabilized by organic matter, were also
observed. In the sandy layer the soils exhibited a granular
(single-grained) structure during both dry and wet conditions.
Most of the soils contained about 70 to 80% fine particles (less
than 2 mm in size).
Phosphate Filtration. Column experiment showed a sharp
decrease in phosphate concentration (up to 98% reduction,
Figure 5a) when the phosphate standard was run through the
sandy column. A reduction in phosphate concentration was also
observed in the clay column (Figure 7b), though not as sharply
as that achieved by the sandy column (Figure 7a). The maximum
reduction in clay soils was only 40%.
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Mutiti et al.
Figure 8—Filtration of iron in sandy soil column.
Iron Filtration. Running iron standard through the sandy soil
core showed an immediate decrease in iron concentration (up to
98% reduction) in the filtrate (Figure 8). The clay soil column
also showed a reduction in iron concentration as well as a
unique double sorption curve (Figure 9a). Additional runs using
the iron standard in the clay soil core were conducted and the
pattern was repeated (Figure 9b is a representative of the curves
obtained) with a 40% maximum reduction.
Nitrate. Results from the nitrate column experiments
consistently showed an increase in nitrate concentration (above
the standard concentration) as the standard passed through soil
columns (Figure 10a). The breakthrough curves were similar to
the iron curves in that they also showed the double-sorption
behavior. The nitrate concentration increased at the beginning
and then decreased slightly before increasing again as the nitrate
standard slug passed through the sandy soil column. This
behavior was also observed in the curves from both the sandy
and clay soil columns (Figure 10b is a representative of the
curves obtained). Repeated measurements, except in one case
showed the same behavior for the clay sediments.
Filtration Rates. When the first experiments were conducted
(right after collection of wet samples), it took about 84 minutes
for 750 mL to be filtered through the clay soils, 8.9 mL/min
(0.00024 m/d) flow rate. The soil columns were then allowed to
dry up (mimicking summer drying period) and then analyzed.
Cracks (macropores) were observed in the samples. Column
experiments conducted immediately after this drying period
exhibited relatively faster filtration times, 150 mL/min (0.0048
m/d). As the experiments were continued in the following
months, there was a steady decrease in filtration.
Summary of Field and Laboratory Results. For filtration
rates and contaminant removal, field and laboratory results were
in agreement as shown in Table 1.
Figure 9—(a) Filtration of iron in clay soil column, (b) replication
of filtration pattern.
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Discussion
The wetland basins are mostly fed by surface water runoff and
only had standing water in them during wet periods. The
presence of water is strongly dependent on weather and seasons.
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Mutiti et al.
Table 1—Field and laboratory filtration and contaminant removal
summary.
Parameter
Vertical infiltration
Horizontal hydraulic conductivity
Phosphate
Iron
Nitrate
Figure 10—Filtration of nitrate standard in (a) sandy soil column,
(b) clay soil.
The basins exhibited variable infiltration rates depending on the
location, season, and preceding rainfall (antecedent moisture
content). The spatial variability was controlled by the presence
of a layer of clay on the surface. It appears that the low point of
the basin had a layer of fine particles clogging all the surface
pores resulting in almost an impermeable layer on top. These
locations also are lacking in vegetation and roots that would
otherwise help with percolation. The observed temporal
(seasonal) variability in field infiltration rates is possibly a result
of the presence or absence of macroporosity. Macroporosity was
observed to develop during prolonged periods of dryness. The
same phenomenon was also observed in the laboratory column
experiments results. Water flowed faster through the soil
columns after prolonged drying and macropore development
than when the soils were wet.
Macropores would typically increase the porosity and
permeability of the soils, resulting in more groundwater flow
(Beven and Germann, 1982). Immediately after drying, the soil
allowed water to move through very rapidly but the flow rate
progressively got slower as the soils got wet and the macropores
closed again (clay particles swelled). The reduced infiltration
could also be a result of clogging of the surface pores when fine
particles suspended during a rain event settle out when runoff
ceases. Another potential contributing factor to reduced
April 2015
Field
Laboratory
0.012 to 0.72 m/d
0.002 to 0.1 m/d
Decreased
Decreased
Increased
0.0002 m/d
Not determined
Decreased
Decreased
Increased
infiltration is bacterial growth. Beach et al. (2005) reported a
temporal variability of hydraulic conductivity resulting from
biomat formation. It is possible that under field condition,
especially during wet conditions, a bacterial mat (biomat) could
grow and further reduce infiltration. A combination of both
clogging by fine particles and biomat growth would significantly
reduce infiltration. However, biomat formation cannot account
for the reduced infiltration in column experiments because there
was no sufficient time for the mats to form.
The observed increase in phosphate and iron concentration as
the runoff from GMC entered the wetland system is most likely
a result of sediment agitation and desorption during the storm
(Vidon et al., 2010). The reduction in concentration observed in
the groundwater seepage (from Alice Basin into Fishing Creek)
is a result of sorption onto sediments in the basin. The reduction
in both phosphate and iron concentration as the runoff filters
through the clay and sandy soils concurs with the reduction seen
in the laboratory results. This suggests that the potential for
maximum filtration is realized when the runoff infiltrates
through the soils or stays within the basin for a long time after
the rain event. This is in agreement with other studies that have
observed accumulation of iron and phosphate in wetlands and
detention pond sediments (Heal and Drain 2003; Mitsch, 1992;
Woltemade, 2000). If water moves too quickly as overland flow
through the basin, the wetland would actually act as source of
phosphate and iron instead of being a filter. This would happen
because wetland sediments have been shown to accumulate
contaminants and nutrients to toxic levels (Heal and Drain,
2003). Therefore, fast-moving surface runoff would agitate the
phosphorus-rich sediments and transport them to streams
(Dillah and Inamdar, 1997; Vidon et al., 2010) making the
wetland a temporal source. This behavior of the basin acting as a
temporal source of these nutrients is what Vidon et al. (2010)
described as hot moments.
Nitrate concentrations in the field exhibited the opposite
behavior to that shown by phosphate. The relatively higher
nitrate concentration in runoff could be explained by influences
from fertilizer on the GMC practice field and wastewater
overflow from the manholes. The runoff passes through two
smaller upland basins (and the walls separating them) before
entering Alice Basin. Sorption on the clay material as runoff
passes through the basins and separating walls could account for
the field-observed sharp decrease in concentration by the time
the runoff reaches Alice Basin. Wetland systems can act as net
sinks for nitrate (Jansson et al., 1994; Woltemade, 2000), through
both biotic and abiotic processes. For example, GonzalezAlcaraz et al. (2012), Powers et al. (2012), Shao et al. (2013), and
Sekadende et al. (2014) all investigated the role of different types
of wetlands in nutrient retention. Although this investigation did
not specifically look at nutrient levels among plants in the
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Mutiti et al.
Figure 11—Recommended solution for alleviating frequent flooding in Alice Basin while maintaining wetland functions as contaminant
filter.
basins, these studies showed that plants absorbed high amounts
of inorganic nitrogen, especially during the growing seasons.
According to Shao et al. (2013), rhizomatous plants such as
Phragmites species, some of which are found in this wetland
basin, translocate nutrients to belowground parts in winter
possibly as reserves for the next year’s growth. Saururus cernuus,
an obligate wetland plant that was common in this area, has
been shown to respond to high phosphorus levels by increasing
biomass and nutrient uptake (Moore and Kroger, 2011). Among
the common tree species found in the study site, Celtis laevigata,
Platanus occidentalis, and Acer negundo responded favorably to
high N levels, while Fraxinus pennyslvanica responded to P
(Aubrey et al., 2012; Douds and Chaney, 1986; Siemann and
Rogers, 2003). Therefore, some of the plant species in the
wetland might be directly involved in filtering nutrients by
increasing their uptake. Some of these studies have also
suggested that the ability of sediments to retain nutrients is
improved by the presence of plants. Faulwetter et al. (2009)
showed that microbial processes actively reduce nitrate concentrations; however this removal mechanism is beyond the scope
of this paper. The slight increase in nitrate as the water leaves
Alice Basin through subsurface flow could be a result of
desorption from the wetland sediments. Increases in nitrate
concentrations were also consistently observed in the laboratory
column experiments (all soils used in laboratory experiments
came from the field and, therefore, represented the field
condition in terms of substances adsorbed on them). It appears
that desorption of nitrate was the cause of this increase and,
therefore, this wetland can potentially act as a secondary source
when runoff gets into the river via subsurface flow.
If flooding were the only concern, a series of culverts would be
sufficient to reduce the frequency of flooding in Alice Basin.
Based on field and GIS analyses, the best location for a culvert
that would reduce the frequency of flooding in Alice Basin is
shown in Figure 11. However, flood control measures in wetland
systems should consider both the holding water capacity and
366
filtration capacity of the basins. For the Oconee River Greenway
system, a culvert that would connect directly from Alice Basin to
Fishing Creek was initially proposed. After analyzing laboratory
and field nutrient-transport results, this recommendation was
not pursued as it would lead to an increase in river phosphate
and iron levels. Because most streams in this area tend to be
phosphate limited, this would lead to undesired consequences.
The best solutions would be to increase the residence time of
water in the wetland system by creating small levees on the
southeast end of Alice Basin and increasing the connectivity to
the two lower basins (Figure 11). This will allow water to take
the longer and slower pathway to the river and increase
infiltration, which would result in increased phosphate sorption
and removal. Jansson et al. (1994) concluded that increased
residence time could also increase nitrogen removal from runoff.
This would be very beneficial to the Oconee River Greenway
Park, Fishing Creek, and the Oconee River. Increasing residence
time in the wetland system of this site would not only help keep
the stream clean but also save money as it would eliminate the
need for more culverts that rapidly channel runoff into the
rivers.
Conclusion
This study demonstrated the importance of solving environmental problems using a holistic approach, in this case flood
control. To date, flood control measures at the ORG have been
limited to culverts that quickly direct water from the park to the
Fishing Creek and the Oconee River with no consideration of
wetland functions. A different approach that considered wetland
functions was taken in this project. Factors such as spatial
location, type of wetland, source of water, and duration of wet
periods control wetland functions. In wetlands that are mostly
fed by surface water runoff, hydrologic properties are strongly
dependent on seasons and the duration of dry periods. In this
particular case, infiltration rates and water residence time are a
function of macropore development during dry periods, and
Water Environment Research, Volume 87, Number 4
Mutiti et al.
macropore closure and biomat formation during wet times. The
longer the dry period, the more water that infiltrates and the less
it accumulates in the wetland basins. Wetland sediments can act
as both filters and secondary sources of contaminants. Clay and
sandy soils acted as effective filters for both iron and phosphate,
which made them contain high concentrations of the adsorbed
phases. However, both soil types also acted as secondary sources
when the contaminants desorbed from the sediments after
agitation during rain events. These observations are crucial when
making recommendation for flood control measures. Increasing
residence time and subsurface flow was determined to be a
better solution than rapidly channeling the water through the
wetland system and lose the benefits of wetland functions. All
urban wetlands used for water filtration should be thoroughly
investigated to maximize wetland functions and ensure that the
wetlands are not acting as secondary sources of contaminants.
The results from this study also showed a double sorption
pattern for both iron and nitrate at the study site, which needs
further investigation. This behavior is probably a result of a tworegion flow system (faster through macropores and relatively
slower through granular porosity). Further investigations are
currently under way to determine the actual cause of the double
sorption peaks.
Acknowledgments
The authors would like to thank the Oconee River Greenway
Authority for access to the research site; Georgia College’s
Chemistry Department and Georgia College’s Department of
Biological and Environmental Sciences for supplies. The authors
would like to acknowledge Sarah Hazzard, Lori Berry, Chad
Hobson, Caralyn Zehnder, Doug Oetter, Catrena H. Lisse, Emily
Parish, Lacey Green, Houston Chandler, Evan Crowe, Charles
Lampkin, and Daniel Sitaras for field and laboratory assistance.
Submitted for publication December 22, 2013; revised
manuscript submitted September 28, 2014; accepted for
publication December 4, 2014.
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