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The
Filtration Effects of Palo Verde National Park Wetlands on Rice Field
Agricultural Run Off.
Sue Lloyd
Jewel Thornton
Tom Bogard
Rose Davidson
Ron Rushing
Abstract
As the need for food production increases, the necessity to closely
monitor the impact of agricultural chemicals upon non-targeted areas also
increases. We conducted research in the Palo Verde National Park,
Guanacaste Province, Costa Rica to see if applied nitrogen could be detected
within rice fields and wetlands within the same watershed. The intention of the
study was to see if wetlands act as a filter to remove excessive nitrogen
originating from agricultural production. A total of 12 samples were analyzed
for the presence of nitrates and due to the limitations of the testing method,
no detectable levels of nitrate nitrogen were found.
Introduction
Rice produced in Guanacaste is an important commodity. Rice is served at almost every meal. It is
an important staple of the Costa Rican diet.
The majority of rice produced in Costa Rica is used within the country. In the Guanacaste area near Palo Verde
National Park the rice is grown in the wet or paddy style of farming. Ten
percent of the rice in Costa Rica is grown in this manner. According to an area
rice farmer, Antonio Latamarillo, extensive use of fertilizer and pesticides
takes place on these rice paddies. Fertilizers are applied by hand to the rice
at two times during the growing season; 15 days after planting and just prior
to flowering and seeding out of the rice plant. A 10-30-10 fertilizer
containing nitrogen, phosphorous and potassium. A combination fungicide and
insecticide is used on the rice fields. Rice production started in the
Guanacaste area in 1982 after the construction of a nationally funded major
irrigation canal system throughout the area. The rice grown in this area is
11-30, the growing season is 62 days from planting to harvest. By showing that applied nitrogen runoff is
absorbed by wetland ecosystems, monitoring inputs on adjoining agricultural
lands would be justified.
The global nitrogen cycle has reached the stage where more nitrogen is
fixed by human driven means than natural processes annually. Fifty percent of
all industrial nitrogen applied throughout human history has been applied since
1982.[i] While necessary for rice production these
practices might have adverse affects on neighboring wetland ecosystems. Excess
nitrogen fertilizers can increase the plant biomass in the wetland ecosystem.
Some plant species respond more positively to high nitrogen content, resulting
in a change in plant distribution in these altered areas. Nitrogen demanding species,
especially grasses begin to dominate.
In addition to directly affecting plant species, elevated nitrogen
applications has been shown to affect populations of consumers[ii],
predators and parasites[iii],
and mycorrhizal fungi[iv].
Taking into consideration our time and equipment limitations, our group developed a process to test the presence of nitrates within the watershed that has rice production draining into a natural wetland. We will investigate how nitrate concentrations in agricultural run off from rice cultivation is mitigated by natural wetlands.
Materials and Methods
Our group began our investigation by interviewing the Palo Verde Station
manager, Ullisses Chavarria Garcia, and studying a topographic map of the area
to determine the flow of the watershed. We used this information to determine
areas for our water sampling. Samples were taken as water flowed from rice
irrigation canals, in rice fields, and through wetland areas. Samples were taken from locations that
could be easily accessed from the road.
Samples were obtained approximately three feet from footpaths, labeled
and stored analysis in the OTS Lab. Spacing between samples was approximately
200-300 meters apart and was taken within the top three inches of water. The
wetland samples were clustered around the Cantalina area all within an
approximate 500 meters of each other at the GPS reading of latitude N 10020’ and longitude
85017’. These samples were
tested in the lab for nitrates, phosphates, and pH using the LaMotte Water
Pollution Test Kit. Total sampling came from: two samples within irrigation
canals, five samples within rice fields (pre-planting and pre-harvesting), and
five samples within the Palo Verde wetland in the Catalina area. GPS readings
were taken at each site to verify location.
Results
|
Nitrate
levels were all less than 2.00 ppm, phosphate levels were all less than 1.0
ppm. These levels were both the
lowest detectable by the LaMotte Water Pollution test kit. The range of pH was from 7.0 to 8.0. |
|
Rice Fields |
|
|
|
|
|
|
|
Sample 1 |
Sample 2 |
Sample 3 |
Sample 4 |
Sample 5 |
|
Nitrate (ppm) |
>2 |
>2 |
>2 |
>2 |
>2 |
|
pH |
7.0 |
8.0 |
8.0 |
7.0 |
7.0 |
|
Phosphate (ppm) |
>1.0 |
>1.0 |
>1.0 |
>1.0 |
>1.0 |
|
|
|
|
|
|
|
Wetland
|
|
|
|
|
|
|
|
Sample 1 |
Sample 2 |
Sample 3 |
Sample 4 |
Sample 5 |
|
Nitrate (ppm) |
>2 |
>2 |
>2 |
>2 |
>2 |
|
pH |
7.5 |
8.0 |
7.5 |
8.0 |
8.0 |
|
Phosphate (ppm) |
>1.0 |
>1.0 |
>1.0 |
>1.0 |
>1.0 |
|
|
|
|
|
|
|
Irrigation
|
|
|
|
||
|
|
Sample 1 [v] |
Sample 2[vi] |
|
||
|
Nitrate (ppm) |
>2 |
>2 |
|
||
|
pH |
7.0 |
8.0 |
|
||
|
Phosphate (ppm) |
>1.0 |
>1.0 |
|
||
Collecting Site Locations[vii]
|
Rice Fields |
|
|
|
|
Latitude (N) |
Longitude (W) |
|
Sample 1 |
10° 25’ |
85°17’ |
|
Sample 2 |
10° 24’ |
85°17’ |
|
Sample 3 |
10° 24’ |
85°18’ |
|
Sample 4 |
10° 24’ |
85°18’ |
|
Sample 5 |
10° 23’ |
85°18’ |
|
Irrigation |
|
|
|
|
Latitude (N) |
Longitude (W) |
|
Sample 1 |
10° 25’ |
85°17’ |
|
Sample 2 |
10° 24’ |
85°17’ |
Discussion
Our investigation has found that there is no detectable nitrate or phosphate detectable in any of the sample sites. Due to limitations of the testing kit, the nitrate and phosphate levels were below the minimum detectable level of 2 ppm thereby limiting the possibility of adequate analysis. Research suggests that nitrate and phosphate should be detectable in agricultural systems. Aside from the aforementioned test kit limitations, other possible reasons could be from conservative use of fertilizers, the timing of testing not coinciding with fertilizer application or other unknown biochemical interactions. Our data were inconclusive with respect to our hypothesis that nitrogen levels from rice field run off are reduced by passing through wetlands.
Applications
Suggested Projects for implementation in educational settings:
1.Test streams, ponds, lakes, rivers in areas of agriculture for the presence of nitrates and other agricultural pollutants using appropriate water testing kits. Also do inventory of flora and fauna.
2. Have students learn to use topographical maps to trace the drainage area of the local watershed. Compare with SCS maps for comparison.
3. Have students draw maps of the terrain and the watershed for their area. Use math/mapping skills.
4. Have your students set up a wetland rehabilitation area. Involve local, state and national agencies as advisors.
5. Involve your classes in area wetland monitoring. Take seasonal year round data.
6. Use GPS devices to add areas of agriculture to maps developed of watersheds.
Endnotes