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Factors controlling water quality and vertical accretion on experimental
constructed wetlands (Ebro Delta, Spain)
Juan Calvo-Cubero1, Carles Ibáñez2, Albert Rovira2, Peter J. Sharpe3, Carles Alcaraz2, Enrique Reyes1
1. Dept. of Biology, East Carolina University, Greenville, NC, USA. 2. Aquatic Ecosystems Program, IRTA, Catalonia, Spain. 3. Parks and Wildlife Service, National Park Service Natural Resources and Science, Fredericksburg, VA, USA.
Objective
Our goal was to asses the wetland nutrient uptake and sediment response to two
experimental treatments: water source and depth. Evaluating wetland response to
the use of available water for restoration purposes might provide ecological
guidelines for ecosystem managing in future wetland restoration of Ebro Delta
(Spain).
Introduction
It has been shown that constructed wetlands are an effective and economical way to treat
excess nitrogen and phosphorus concentrations from water effluents discharge. Constructed
wetlands are densely vegetated by a variety of submerged, emergent and floating plants and
typically have mean water depths of 0.3 m. Open water areas maybe incorporated in the design
to provide for optimization of hydraulics and for wildlife habitat enhancement. Constructed
wetlands could also serve as a restoration tool in coastal wetlands impacted by high rates of
relative sea level rise, since it has been demonstrated that wetlands persist under these
conditions when vertical accretion and elevation gain equals or exceeds the rate of water level.
Materials and Methods
The constructed wetland is a 1.3 ha plot located at the Riet Vell organic rice farm (hereafter
Riet Vell) and is located on the eastern tip of the Ebro River Delta (Lat 40039'90" N, Long
0046'22"E). The wetland is adjacent to active rice agricultural parcels and a previously
restored emergent wetland dominated by Phragmites australis and Typha spp. (Figure 1). The
experimental wetland was built with a total of 72 experimental units (EUs), each 100 m2,
using wooden walls to separate the EU from each other, reinforced with two layers of plastic
sheeting to prevent/limit water loss between plots (Figure 2).
Abstract
The Delta of the Ebro River (Catalonia, Spain) is among the most important wetlands in the
western Mediterranean, highly valuable both economically and ecologically. Its natural
hydrological and sedimentary regimes have been heavily modified by dams and land
reclamation leading to coastal erosion and surface elevation loss. The historic and current
agricultural practices have transformed large areas of wetlands and lagoons into rice fields,
which now occupy up to 60% of the deltaic plain. Agricultural runoff carries nutrients,
heavy metals and organic pollutants to natural habitats due to pesticide and fertilizer use.
To reduce these ecological impacts, several wetland restoration efforts have been initiated
seeking to increase vertical accretion, improve water quality and wildlife habitat. However,
few studies have focused on the factors controlling water quality and vertical accretion for
these wetland restoration initiatives. This study measured how the chemical and
hydrological factors controlled water quality and vertical accretion in a experimental
constructed wetland under 2 treatments: local inflows of drainage water and rice
irrigation water and variations on water depth. The specific objectives were (1) to quantify
nutrient uptake as a function of the type of water input and the water level; (2) to measure
the impact of these different water inputs and levels on sediment vertical accretion.
Results
Elevation change and vertical accretion
Mean elevation change for each EUs showed great variability between different water type and
water level treatments (Figure 4). About 83 % of the EUs treatments showed a positive
elevation change except in the EUs subject to irrigation water and 30 cm water depth. Positive
mean elevation changes ranges between 0.50 and 1.25 cm/year.
Marker horizons (MHs) on every EU and Surface Elevation Table (SETs) were used to measure
annual vertical accretion and elevation change rates, respectively. A total of 24 SETs were
randomly installed. The MHs and SET stations were established in August 2009. Initial SET
readings were collected in September 2009 to March 2010, whereas MHs were collected only
in March 2010.
Nutrients dynamics
Mean nutrient concentrations for each of the EUs outflows showed that 80 % of them removed
most of the nutrients (Figure 5). About 50 % of the EUs treatment didn’t remove PO4.
Substantial differences were found for inorganic nutrients between irrigation and drainage
treatments. However, no differences were found among treatments with different water levels.
Similar results were found for both study years, except for nitrate concentration, which showed
higher concentrations in 2009.
Annual inflow water nutrient concentration for nitrate, ammonia and total dissolved nitrogen
(TN) presented a seasonal variation mirrored by the nutrient concentration of the outflow for
both 2009 and 2010. For ammonia, the peak concentrations were found in July 2009 and June
2010, and for TN peak concentrations were found in August 2009 and July 2010.
Figure 1. Location and Planning of Riet Vell Experiment
Ecological
rice land
Restored Wetland
Irrigation water
treatment
Drainage water
treatment
Figure 2. Building process for the wetland experiment
Study area
The Ebro River is the largest river in
Spain (flow ca. 400 m3s-1). It flows from
the Northeast of Spain to the
Mediterranean Sea (ca. 930 km length),
forming a delta of 330 km2 (Figure 1).
The Ebro River has a total watershed area
of ca. 85,820 km2 and nearly 3.25 million
people live within the watershed. Thus
entire basin receives a load of pollution
coming from various anthropogenic
activities.
The current and historic water and land uses transformation in the Ebro River and its Delta
have converted large areas of coastal wetlands and lagoons to rice fields, which occupy 60% of
the deltaic plain. These in combination with heavy anthropogenic transformations have caused
that a significant portion of the delta plain is near or below mean high sea level.
The experimental design included 2 water
inflow treatments: one where 36 plots received
Ebro River Water (Irrigation Water); and
another comprised of 36 plots receiving
drainage water from adjacent agricultural lands
(Drainage Water). The design of both the
Irrigation and Drainage treatments was a
randomized complete block design (RCBD).
Both treatments included a second treatment
where water level was regulated to 3 depths
(10, 20, and 30 cm; Figure 3).
For further information contact: Juan Calvo-Cubero at calvocuberoj11@ecu.edu
ACKNOWLEDGMENTS
This study has been funded by the Government of Spain (Ministerio de Medio Ambiente, Research Project
056/RN08/04.3, Development of techniques to compensate subsidence and sea-level rise in coasts and wetlands of the
Ebro Delta, 2009-2010).
Figure 3. Wetland experiment diagram
Figure 5. Mean inflow and outflow nutrients water concentration
Figure 4. Mean elevation change and vertical accretion
Several chemical and sedimentological response variables were measured. A total of 24
shallow wells (1.5 meters height) were randomly installed (12 wells for each inflow treatment).
The wells were sampled using a YSI portable multi-meter at the surface and 0.5 m below the
ground once a month during the flooding season. We collected, for each EU, 3 monthly samples
during 2009 and 6 monthly samples during 2010 to quantify nutrients dynamics. Additionally
3 samples were collected on both irrigation and drainage inflow water. Furthermore, samples
were also collected at every outflow for each EU. Samples were analyzed for nitrate and nitrite,
phosphate, ammonium, silicate and total dissolved phosphorus and total dissolved nitrogen.
The Drainage and Irrigation treatments consisted of 3 blocks with 12 replicated EUs each, 4 for
each water level. The experiment was done during the flooding season of rice cultivation, from
June to December, in two consecutive years. During the first year, all EUs were fully flooded
from mid-July until December 2009. For 2010, the flood occurred from June to November 2010.
Water levels in all EUs were maintained using an average water in-flow rate of 4.5 L/s.
Mean vertical accretion showed
similar values between 1.5 and 2
cm/year although the EUs for
each treatment showed greater
variability. Substantial differences
in vertical accretion were found
between irrigation and drainage
treatments although they didn’t
appeared between water level
treatments.
Discussion
The experimental units subject to the irrigation water inflow treatment presented substantially
higher vertical accretion than those receiving drainage treatment. This differential soil
formation may be caused by the irrigation water higher inorganic nutrients than the drainage
inflow water for most of the year. Nutrients inputs might enhance soil formation through
organic matter production and accumulation.
Higher inorganic nutrients on the irrigation inflow water than on the drainage inflow water
treatment suggests that rice fields work as a nutrients sink. However, the EUs with drainage
treatment received higher ammonium concentrations during the first months (Figure 5). This
might be due to fertilizers (ammonia and ammonium sulphate) used for the rice production on
the Ebro Delta. The lack of phosphate removal in most of the months suggests that phosphate
concentrations are not the limiting factor for vegetation growth.
TN
TN