Portal de Cartagena
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Constructing small wetlands in strategic locations of the Campo de Cartagena, using soils and plants in the area, is an effective strategy to purify eutrophic waters and reduce nutrient input to the Mar Menor.
This is one of the proposals in the doctoral thesis of the agronomist María del Carmen Tercero Gómez, defended at the Technical Higher School of Agricultural Engineering (ETSIA) of the Universidad Politécnica de Cartagena (UPCT).
The thesis is entitled 'Biogeochemical processes and removal of nitrogen and phosphorus from eutrophic waters in wetlands around the Mar Menor: experimental results in mesocosmos' and was directed by María Nazaret González Alcaraz, researcher of the European Marie SkÅ‚odowska-Curie Program, and The professor of the Polytechnic José Álvarez Rogel.
The work was carried out in the Research Group Environmental Soil Science, Chemistry and Agricultural Technology (http://suelos.upct.es/es) with funding from the Ministry of Economy and Competitiveness and FEDER funds (Project (CGL2010-20214).
The purification treatments tested were highly effective in eliminating nitrate and phosphate from eutrophic water.
The mechanisms for the purification of both compounds were different, but in both cases they were favored by the biological and physicochemical characteristics of the soils used.
The nitrate was eliminated fundamentally by the activity of the microorganisms of the soil, when turning it into gaseous nitrogen through the denitrification process.
In just five days, more than 90% of the nitrate added was removed.
"In order to carry out this process, it is key that microorganisms have organic carbon as a source of matter and energy," explains Álvarez Rogel.
In the experiments, soluble carbon was added to the eutrophic water, but it was shown that the carbon was consumed in the first hours after adding it and that for most of the time it was the organic carbon of the soil itself that kept the microorganisms active.
"The possibility that native soil carbon supports the activity of microorganisms makes it possible to optimize the use of this type of systems for the purification of many of the agricultural effluents of the Campo de Cartagena, Brackish water with high nitrate content but no dissolved organic carbon, "adds Rogel.
"Although the nitrate uptake by the reed had a secondary role in the removal of this compound from eutrophic water, the presence of plants was important as it facilitated the activity of the microorganisms that carried out the denitrification process in the environment of its roots.
Therefore, most of the nitrate in the water was not retained in the soil-plant system, but was emitted into the atmosphere in the form of gaseous nitrogen, "it is abundant.
Phosphate, unlike nitrogen, was retained in the system, primarily by binding to soil particles.
At 24 hours of adding eutrophic water more than 90% of the phosphate had been retained and the efficiency of the wetlands after three weeks flooded was 98-99%.
"Most of the phosphate was retained by binding to calcium compounds, the content of which in most of the soils of the Campo de Cartagena is abundant," he recalls.
2,500 water samples and 300 soil samples
For the experimental study small-scale wetlands were built, with soils and plants collected from a coastal wetland of the Mar Menor.
They were vertical flow wetlands, which were flooded by introducing the water from the surface until the soil was completely saturated and submerged about 5 cm, remaining so for 3-4 weeks.
Eutrophic water of high nitrate content (up to 200 mg / L) and phosphate (up to 60 mg / L) was used for this purpose.
After the weeks of flooding, they were allowed to dry for another 3-4 weeks.
This cycle was repeated six times for one year.
Treatments with reed plants (Phragmites australis) and without vegetation were tested.
During the experiment, a thorough monitoring of experimental wetlands was carried out, collecting and analyzing more than 2,500 water samples and more than 300 soil and plant samples, and taking more than 4,000 physicochemical, biological and physiological data.
The results obtained in this thesis are scientifically endorsed since they have been published in international journals of high impact.
In addition, they corroborate the field data obtained in previous projects of this research group, which demonstrated that the coastal waters of the Mar Menor purify the eutrophic waters that cross them, contributing to preserve the water quality of the lagoon.
In the coming months the new doctor, María del Carmen Tercero, will continue working in the research group where she has done her thesis, in projects related to the characterization of pollution and eutrophication in the Mar Menor and its surroundings and the development of corrective measures .
Source: UPCT
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