Water Remediation Wetland Study
Oregon State University
North Willamette Research and Extension Center
By Jackson Kowalski
Introduction
The Pacific Northwestern region of the United States possesses a bustling nursery
industry. With so many nurseries scattered about, it is important that these nurseries are not
polluting the surrounding environment. A primary source of environmental contamination is
through irrigation runoff—the extra water leached through container plants after an irrigation
cycle—spreading into the surrounding areas. Many nurseries utilize a holding pond to collect
their runoff, but the water is often contaminated with higher than desired levels of nutrients
derived from the applied fertilizers.
Objectives
Determine the nitrate and phosphorus removal capabilities of various different wetland
vegetation zones
More specifically, determine these qualities for the following species: Lolium spp, Caltha
palustris and Juncus effuses used in combination, and Eichhornia crassipes
Provide the Pacific Northwest nursery industry with information pertaining to
remediation of irrigation runoff by using an installed wetland system to cleanse their
runoff of excess nutrients
Materials and Methods
Overview
A total of three containment boxes were created, one for each vegetation zone. Each box
was divided into four sections (cells) to allow for three replications—titled as the experimental
cells—with the final cell being left as a control. Box A contained Lolium (Ryegrass); box B
contained a mix of Caltha palustris (Yellow Marsh Marigold) and Juncus effusus (Soft Rush);
box C contained Eichhornia crassipes (Water Hyacinth). The boxes were elevated in such a
manner as to emulate the declining elevation as approaching a pond (figure 1); this was done so
that eventually each cell could be connected to in a linear manner, moving from box A to B and
finally to C, thereby created a series of linked cells in which water is only applied to box A,
runoff then flows into box B and again into box C. This would simulate the actual pathway of
irrigation water more closely than analyzing each box in isolation; however, we first wanted to
know the nutrient removal capacity of each vegetation zone in a closed system.
Figure 1. Elevation differences between each box. Box C sat at ground level, box B was elevated just high enough
so the drainage holes at the bottom of each cell were above the water level in box C, while box A was elevated so
that its drainage holes were above the water level in box B. Boxes A and B were set on approximately a 1° slope in
order to cause the water to flow toward the drainage end. The elevation was achieved via gravel mounds.
Wooden Frame
The containment cells were created using ¾” plywood. Each cell measured 16” wide, 46”
long and 16” deep. A total of twelve cells were created among the three boxes, producing an
outer dimension of 46” wide, 64” long and 16” deep per box. In preparation for the heavy weight
each cell was to be subjected to, the perimeter of each box was reinforced using 2x4” wooden
beams to increase structural integrity. The interior of each cell was lined with Firestone EPDM
40 mil thickness synthetic rubber pond liner. In box A, the liner only covered to a depth of 8”, as
opposed to covering the entire 16” depth as was the case for boxes B and C.
Drainage
All four cells within each box were equipped with a bulkhead fitting centered two inches
above ground level, thus creating a one-inch diameter, water-tight threaded hole. For box A, oneinch PVC pipe was cut into four 12-inch sections; these segments were drilled repeatedly to
create a permeable pipe, upon which a mesh screen was zip-tied around each pipe to prevent clay
particles from entering, yet still allowing water to enter. The pipe sections were connected to the
inside of the bulkhead fitting. The outside of the bulkhead fitting was equipped with a ¾” barb
thread connected to a short piece of ¾” polyvinyl tubing that led into a greenback valve used for
controlling the water level within each cell (figure 2).
Figure 2. Drainage system for box A, the rye grass. Perforated PVC pipe was placed horizontally near the bottom of
each cell and connected to a bulkhead fitting and later into a greenback valve. The valve was adjusted to control for
the amount of water released and therefore to set the water level within the cell.
The drainage for boxes B and C had the same mechanism; for each cell, the inside of the
bulkhead fitting was connected to a one-inch PVC pipe segment just long enough to fit a 90°
elbow. Rising vertically from the elbow, a 10” segment of one-inch PVC was placed to establish
the water level within the cell. Surrounding the PVC apparatus, 6-inch perforated pipe was cut to
match the depth of the cell (16”). A hole was drilled near the bottom of the pipe to allow for it so
slip over the PVC apparatus, thus creating a well of water that was isolated from the rest of the
cell (figure 3).
Figure 3. Drainage system for boxes B and C. Each cell contained a segment of 6-inch perforated pipe to create an
empty cylinder inside the clay, yet still allowed for water to pass through. A 90° PVC elbow was secured to the
inside of the bulkhead fitting; from it, a length of PVC extended vertically and acted to set the water level within
each cell. Once the water rose high enough, the water flowed into the pipe and out the bulkhead fitting at the bottom
of the cell.
Establishment
For boxes A and B, each cell was filled with Terra-Green absorbent clay granules by
mixing both fine and coarse bags in a 1:1 ratio. The resulting blend contained a maximum
particle size of approximately five mm in length, with particles ranging down to fine powder.
Pre-grown layers of ryegrass turf were cut to size and placed onto experimental cells from box A
on 7-6-09. Six plants of each Marsh Marigold and Soft Rush were planted on 7-9-09 in the
experimental cells of box B while implementing an approximately six-inch center spacing
scheme and alternating species in a grid-like fashion. Box C was filled with only water and
eleven Water Hyacinth plants were placed into the experimental cells on 7-9-09 (figure 4).
Figure 4. Establishing vegetation. Box A (upper right) had rolls of rye grass turf rolled on top of the clay; in box B
(left), six marsh marigold and six soft rush plants were transplanted into each cell in an alternation pattern leaving
approximately six inches space between each plant; box C (lower right) was filled with water hyacinth until each
cell had nearly equal surface area coverage—around 11 plants per cell. For all boxes, the fourth cell was left absent
of vegetation to serve as the control.
Irrigation
A ¾” PVC irrigation system was constructed to provide an overhead water source at the
start of each cell. A single line of PVC pipe extended across the beginning side of each box.
Holes were drilled into the PVC to allow for water to be applied. The application of fertilizer was
regulated by a MiniDosTM Professional injector that was calibrated to result in producing a 20
ppm nitrogen solution from a stock solution of liquid fertilizer—the fertilizer used was Plant
Marvel Nutricote 18-6-18 Mag-Iron Special. To create the stock solution of fertilizer, batches
were mixed using 118.9 g of fertilizer dissolved into 10 L of water. If necessary, flow to each
individual box was regulated by a valve placed on each PVC line (figure 5). The irrigation ran
for one minute each day, resulting in approximately four liters of diluted fertilizer solution
applied per cell per irrigation cycle.
Figure 5. Irrigation system. Liquid fertilizer was diluted to create approximately a 20 ppm nitrogen solution using a
MiniDosTM injector (top left); the diluted fertilizer solution ran through a PVC system which extended to each box,
where the flow to each box could be regulated by a valve placed on each line (top right); from here, the fertilizer
solution was applied to each cell via holes drilled into the PVC system (bottom).
Upkeep
Throughout the duration of the project, minimal upkeep was required. Summit brand
MosquitoDunks® were placed into the water wells of box B and into each cell of box C in order
to help kill off mosquito larvae. On particularly hot days, the rye grass was dampened with a
garden hose in order to sustain the moisture inside the cell—this was especially important for the
drainage end since it tended to dry out quicker. The greenback valves on box A were set to allow
for drops to be slowly released with the intention that this helped to retain the newly irrigated
water for longer, thereby allowing the rye grass more time to utilize the nutrients—additionally,
the gradual drainage of water prevented the cells from becoming flooded.
Starting the week of July 27th, the irrigation was doubled in order to compensate for
consistently lower water levels. The irrigation came on for one minute in the morning as well as
one minute in the afternoon, meaning that now a total of approximately eight liters of fertilizer
solution was being applied per cell per day.
Sampling
Water samples were collected approximately every five days, starting on 7-23-09. For
boxes B and C, samples were collected by submerging a vial upside down inside the water well
and flipping it upright near the bottom of the depth in order to avoid the surface water. For box
A, samples were collected by placing a vial under each greenback valve to collect the dripping
water. Samples for boxes B and C were taken right before the afternoon irrigation cycle—which
came on at 2:30pm—by collecting just before irrigation, this limited the affect of recently
applied fertilizer solution in contaminating the water samples. However, samples for box A were
taken just after irrigation because the added water was necessary for the dripping to occur. In
order to obtain the exact levels of nitrate of phosphate being applied, additional samples were
collected straight from the PVC irrigation pipe while the irrigation was running.
Upon collecting, the samples were placed into a freezer for storage. Preparation before
analysis included thawing the samples and purifying them via syringe filters—20 mL luer-lock
syringes were connected to 45 μm Millex® PVDF filters. The samples were measured for
concentrations of nitrate and phosphate.
Sample Analysis
Samples for anion analyses were filtered through 0.45 µm polytetrafluoroethylene
(PTFE) membrane filters into 1.5 mL ion chromatography (IC) vials and stored at 4 ºC until
analysis with a Dionex AS50 IC with AS50 auto-sampler (Dionex Corp., Sunnyvale, CA).
Non-purgeable organic carbon (NPOC) and total N (TN) were analyzed using a
Shimadzu TOC-V CPH total organic carbon analyzer with TNM-1 total nitrogen measuring unit
(Shimadzu Scientific Instruments, Kyoto, Japan). The inorganic (carbonate, bicarbonate and
dissolved carbon dioxide or carbon from non-living sources) and purgeable (hydrocarbons,
ketones, aldehydes, and halogenated hydrocarbons) fractions were removed by sparging carbonfree, ultra-pure air through the sample for a set time period. The non-purgeable organic carbon is
described as the fraction of dissolved organic carbon left behind after a sample is acidified.
Results
Data from between July 23rd and August 10th indicate that nutrients are being removed by
the system. Phosphorus levels were effectively lowered on a regular basis, typically by at least
90% for each vegetation type. Water hyacinth seemed to by slightly more efficient than other
vegetation, while the soft rush and marsh marigold mix was generally slightly more efficient
than rye grass—the least efficient (figure 6).
When looking at nitrate removal percentages, it is clear that water hyacinth is very
efficient; it removal nearly 99% of nitrate from the water for every sample collected. Rye grass
showed an increase in efficiency over time, suggesting that as the grass began to take root and
grow, the system became more efficient. Soft rush and marsh marigold started out with a
relatively high efficiency (77%) then dropped to 29% and slowly began to rise in efficiency,
reaching 99% by the last collection. The drop could be due to intensely hot weather weakening
the plants around July 26th to 31st (R2 = .5752), while the steady increase might be attributed to
expanding root and plant mass, a similar hypothesis to the trend seen in rye grass (figure 7).
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Figure 6. Phosphorus removal efficiency of each vegetation zone over a collection period. Water hyacinth (C,
floating aquatic) showed the highest efficiency, followed by the combination of soft rush and marsh marigold (B,
wetland) and lastly rye grass (A, grass); however, even rye grass averaged over a 90% removal rate. Overall, each
vegetation zone leaves less than 10% of phosphorus to remain in the water.
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Figure 7. Nitrate removal efficiency of each vegetation zone over a collection period. Water hyacinth (C, floating
aquatic) consistently removed over 99% of nitrate from the water. Rye grass (A, grass) showed increasing efficiency
over time, likely due to the grass’s root systems becoming more established as time progressed. The soft rush and
marsh marigold combination (B, wetland) displayed variable results; nitrate removal efficiency started relatively
high initially, then dropped sharply, followed by a steady increase up to over 99% efficiency. There are many
reasons for this, one of which could be the combination of high temperatures acting to decrease efficiency by
weakening the plants and plant/root growth acting to increase efficiency—once the heat spell ended, the plants
began to flourish again.
For the time being, the study will remain untouched and left to endure the winter. The
water hyacinth should die off as the weather cools. Next spring, the marsh marigold and soft rush
combination could be much different than the current situation. One of the two species may have
overpowered the other, resulting in primarily only one species for the wetland zone, or they may
still both be thriving. A new floating aquatic species could be tested in place of water hyacinth,
perhaps water lettuce (Pistia stratiotes) because grows to be smaller than the water hyacinth, yet
is still large enough to be easily removed from the surface of the water. This might be more
appealing to nurseries who are concerned about the intense growth of water hyacinth.
Irrigation will likely be left at the current rate of two, 1-minute sessions per day. If the
rainfall becomes too much, perhaps irrigation will be cut down to one minute total per day. If
this is the case, it might be necessary to double the concentration of the applied fertilizer in order
to keep a constant rate of fertilizer applied each day.
In the spring, another month’s data should be collected (after the new aquatic species is
introduced) with collection dates every five days. Upon completion, the cells will be connected
in a linear fashion causing A1 to flow into B1 and again into C1, and so on for A2, etc. At this
point, irrigation will only be applied to the rye grass box and water samples will only be taken
from the floating aquatic box. This will serve to show how nutrients are removed as an additive
system. If desired additional samples can be collected from both the rye grass and marsh
marigold/soft rush boxes in order to see how much nutrients are being removed by each cell, as
well as what is being applied to the next cell.