Sand ponds

ACTION B4

Recovery and integration in the hydrological cycle of abandoned sand ponds for the generation of a lagoon system that enhances the filtering capacity of the river bed

PARTNERS RESPONSIBLE FOR THE ACTION

ACTIVITIES

The margins of the Channel of the Antela Lagoon are heavily anthropized and altered by the mining activity of sand extraction, which has led to the existence of numerous abandoned ponds disconnected from the river bed.
This action will lead to a demonstration experience consisting of the restoration and recovery of old abandoned ponds and their subsequent hydrological-hydraulic connection to the channel, generating a lagoon system that will increase water retention periods.
This will allow an assimilation of nutrients and organic matter present in them such as diatoms, microalgae, macrophytes and halophiles plants, typical of these ecosystems, generating new biomass available for fish and birds, while enhancing the protection figure of the SPA of A Limia.
The objective is to integrate them into the hydrological cycle and allow the development of biological processes that favour the reduction of nutrients present in the waters.

RESULTS

Connection works were made between both ponds and the canal, building a total of three steps in the road on the right bank of the canal with prefabricated frames: one gives way to the water from the canal to the ponds, another connection between both ponds and a third to release the circulating water.
The three steps were integrated environmentally with stone bed inside and with planting of willow cuttings in the nearby slopes, to cause the vegetation itself to colonize the slopes modified by the works.
The existing ecosystems have been improved by creating two platforms for progressive flooding, to promote their colonization by hygrophilic plant communities adapted to flood periods and specialists in the retention of water nutrients.

Three floating islands were installed, mounted on a rigid perimeter on floats, anchored to the bottom to prevent their displacement.
They were revegetated with macrophytic plant species (purification specialists) previously collected from the rafts adjacent to the channel of the Antela Lagoon. The inner part of the islands are lined with double coconut mesh with propagules of different species Typha latifolia, Glyceria declinata, Lythrum salicaria, Sparganium erectum subsp. Neglectum; Polygonum hidropiper and Apium nodiflorum to try to achieve islands with high biodiversity.
This action is in the SPA area of A Limia (ES010ZEOPAES0000436), which is why an exhaustive environmental monitoring has been carried out in the execution of the works to create the least possible impacts, as well as to carry out a monitoring of fauna in the ponds.

Guide for the recovery and integration in the hydrological cycle of ponds generated after the activity of the sand sector

Xunta de Galicia

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Evolution of water quality

The rates of reduction of pollutants are being monitored through the evolution of water quality, both physical and chemical parameters, as well as monitoring of the evolution of ecosystems taking into account birds, flora and herpetofauna, Below are the results detected so far:

For the monitoring of the actions, periodic samples are taken at the entrance and exit of the artificial lagoons, as well as at the same points in the Channel of the Antela Lagoon itself to compare the nutrient reduction rates realized by the lagoons versus what happens in the channel.
It has been proven that phosphorus compounds (phosphates and phosphorus) have a more effective reduction after passing through the lagoons due to the presence of aquatic flora that consume this type of compounds.
Regarding total ammonium, in recent months the behaviour is much more positive after passing through the ponds, which may be related to a greater oxidation of ammonium in the pond than in the channel.
Regarding other compounds such as organic matter (BOD5), nitrites and nitrates, no conclusive results have been obtained.

total ammonium

phosphates

phosphorus

nitrites

In terms of improving water quality through the analysis of species biodiversity, the results of the indicators of macrophytes and diatoms are positive, as well as the high value of the chlorophyll obtained; these results would be indicating the development of aquatic flora as a consequence of the contributions and the consumption of nutrients.
The results are encouraging and would allow demonstrating that part of the project's objectives consisting of favouring the retention of nutrients in a natural way are being achieved.

Evolution of the generated biotypes

The river bank vegetation is evolving correctly, and the current situation of the artificial islands installed is optimal, both in plant cover and in biodiversity of plant species.
As a result of the environmental monitoring work, the presence of bird species such as little grebes, grey herons, green sandpipers, which visit the lagoons and use the floating islands as resting areas has been proven, which is why they are considered to be fully naturalized in the environment. It has also been detected Eurasian teal, a species of Anatidae family whose nesting population in Galicia is included in the Galician Catalog of Threatened Species as “Endangered Species”. Otter also uses these lagoons as a feeding area, preying on other species considered invasive: red swamp crawfish and common carp. These data corroborate that although the lagoons have an artificial origin, the fauna has colonized its waters and different habitats.

Most of the actions are not relevant for the herpetofauna of the area because the great depth of the sandbars and the high slope of the slope do not favour the presence of amphibians and reptiles in the area. However, these humid zones are suitable for heliophytes species such as the Perez's frog (Pelophylax perezi) and the Moller’s tree frog (Hyla molleri). Nearby areas are suitable for reptile species such as water snakes, like the Iberian grass snake (Natrix astreptophora) and Viperine water snake (Natrix maura) or Iberian emerald lizard (Lacerta schreiberi) use these sunny wetlands more frequently. The floating islands will provide a new habitat for the most heliophytes species, specifically, Hyla molleri and Pelophylax perezi.

With the hydraulic connection and improvement of the lagoons with the canal, it is expected that the specific and interspecific biodiversity will continue to increase.

Results

Video