Recycled water from a geothermal well used to irrigate in Mont de Marsan
In the 1970s, a major company discovered a deposit of hot water in Mont de Marsan at a depth of 2200 metres while prospecting for oil. They didn’t know what to do with this water. They then restored the two boreholes discovered in Mont de Marsan, one to the north and the other to the east of the city, using this water to heat the public builadings: the air base, Sainte Anne’s hospital, the police station and an administrative building for handicapped people are heated in this way at a temperature of 62° C.
The only concern was that, after using the water, the city council was discharging the wastewater into the river while it was still hot (around 40°C). However such practices have been forbidden since 2006. (Discharged wastewater must not be hotter than 20°C). The municipal authorities then considered reinjecting it back into its original source. As the cost of such an operation would be very high (6 million euros), they were then left with the choice of either abandoning the well or looking for another option.
The Chamber of Agriculture then came up with the idea of using this water for agricultural purposes. It was Mr. Rabe, a water management consultant at the Mont de Marsan Chamber of Agriculture, who pursued the matter at the time. “We envisaged building a storage reservoir on the agricultural land, which was located at 3 kilometres from the well in the town of Mazerolles and subject to water-use restrictions in the summer”, explained Mr. Rabe.
In fact, there is a shortage of water in the Midou basin. Four farmers were pumping water from the Midou basin to irrigate their maize (corn) crops; Mr. Van Daele, one of the farmers who irrigate, explained: “Our right to extract water was always compromised by the restrictions. Each year, we were restricted from 14th July, which coincided with the critical flowering period.” And the effects of global warming have aggravated the situation even more over the last few years.
For technical and financial reasons the project took 10 years to be completed. “To build this very large water reservoir of 300,000 m3, eight hectares of crops had to be sacrificed”, explained Mr. Rabe. The chamber of agriculture purchased 8 hectares from one of the four farmers concerned. It was also necessary to dismantle the pivot located in the field in question.
The water is conveyed in winter (the period when heating is required) via an underground pipe and it is air-cooled in an enormous 300,000 m3 reservoir. A winter pumping station was planned for topping up the reservoir with water from the river, in the case of there being a mild winter. However, this has not yet been necessary during the two years that it has been operating. There was enough geothermal water from the well.
The total cost of the project was 2 million euros excluding taxes. For their part, the four farmers who formed an association found the funding for the communal irrigation station, transformer and distribution network in an amount of 474,000 Euros, including €222,750 in grants (regional and from FEADER/ European Agricultural Fund for Rural Development).
The water, pumped from deep underground, is naturally slightly radioactive. According to Mr. Rabe, this water is as radioactive as the water in Britany, so it does not affect the composition of the soil. As a precaution, the soil has been analysed every year for three years. However, according to Mr. Rabe, “this is just a formality”.
Each of the farmers who irrigate now has 2200 m3 of water per hectare available for withdrawal when required. For the first time in 2019, they have been able to irrigate the equivalent of 137 hectares. And Mr. Daele continues “The aim is not to irrigate more but to irrigate more efficiently. This would allow, for example, for irrigation to be stopped when it is windy. Previously, because of the restrictions, we would irrigate whenever we could.”
The 4 farmers have medium-sized farms of around 50 hectare (the largest is 100 hectares) and they essentially grow maize (corn), as well as sunflowers, French beans and soya. They are also poultry farmers. Mr. Van Daele has 70 hectares under cultivation, of which 25 are now irrigated with water from the reservoir.
He grows seed maize (seed corn), grain maize (grain corn), soya, asparagus and sorghum. He also has poultry and cattle grazing on the grassland. He mainly irrigates with the hose reels and a pivot, which he shares with another farmer.
“It has been a long haul with huge investment”, continues Mr. van Daele. “But it enables me to maintain crop yields and keep my seed production contracts.”
As a result of having this water pumped from the geothermal well and then cooled in the reservoir, the four pumping points located in the river Midou have been removed, restoring the quantitative balance of the water catchment area. And our 4 farmers are no longer affected by the water-use restrictions that were imposed each year. They are guaranteed to have water when required, thus maintaining crop yield.
by Fleur Martin, Irrigazette with the participation of Julien Rabe, Chambre d’agriculture des Landes