The first type of water resource is surface water.
Among the surface waters, we find the rivers. The rivers located in the vicinity of the project are the Miouze (closest river), the Sioulot and the Sioule. The daily flow of each of these rivers is measured at different points upstream and downstream of the project site. The five-year minimum flow of the Miouze upstream of the project is on average 470 l/s and that of the Sioule downstream of the project is 1040 l/s.
Other surface water types are ponds and wetlands. None are present within the project site right-of-way. Therefore, there will be no impact. It should be noted that the drinking water catchments supplying the municipalities concerned by the project are not located in the vicinity of the project and will not be impacted by the work.
The other water resources are groundwater. This water is stored in what are called underground aquifers. An aquifer is a volume of rock that allows the storage and circulation of groundwater. The quality of an aquifer depends on the amount of space available to store water in the rock and the ability of the rock to allow it to flow.
Aquifers are recharged by rainwater that infiltrates the ground to impermeable levels where it can no longer circulate. The aquifers feed in part the surrounding watercourses.
There are different types of aquifers that depend on the local geology. Among them, two are present in the environment of the project.
Volcanic aquifers are generally old lava flows. These are very good aquifers, which are found in Volvic. This type of aquifer is present in the project environment but not in the immediate vicinity of the drilling site. These are for example the puy Banson and puy Marmont flows. In addition to their remoteness, they are located higher in altitude than the project site. They will therefore not be impacted.
Fractured basement aquifers are present in rocks such as granites in a superficial manner. They are networks of sometimes connected fractures in which water can be stored. Fractured aquifers are present on the site but their dimension is very small and not continuous. The opinion of an independent hydrogeologist confirms the low stakes of this type of aquifer on the project site.
Moreover, there will be no interaction between these aquifers and the well since the latter will be perfectly watertight in its upper part. In fact, the well construction is composed of several alternating layers of cement and steel tubes that ensure its watertightness.
Impact during drilling
The first impact of the project on the water resource concerns the withdrawal of water for the needs of the drilling. Indeed, the drilling work requires water for the production of the drilling mud and the well tests. The total capacity required is 10,000 m3, which will be stored in watertight basins on the site. These sumps will be filled by pumping into the Miouze River.
To avoid any impact on the river flow and ecosystems, the pumping rate will be reduced over an extended period. For example, a pumping rate of 9 l/s will fill the ponds in approximately 13 days. This flow rate corresponds to 2% of the minimum 5-year flow of the Miouze River upstream of the project. The one-time pumping of the Miouze River will not alter the flow of the river.
No other withdrawals will be necessary and the work site will be completely self-sufficient in water for the duration of the work. The impact of the project via the withdrawal of water from the Miouze is therefore considered very low.
The other impact of the project on the water resource concerns runoff water. This water is composed solely of rainwater falling on the site. The water that does not infiltrate and run off the parcel is drained to a basin in the downstream part of the parcel to be stored.
In this pond, the water is treated to remove any sludge or oil that may have accumulated as it flows over the site. The water is then evacuated from the basin through a ditch that promotes slow infiltration and reduces the flow rate to the Miouze River. Runoff is therefore the only water discharge from the project and its impact is considered very low.
Impact during operation
During the operation phase of the plant, if confirmed by the results of the first drilling, no natural water will be withdrawn. The water storage ponds for the drilling will be dismantled and the area of land they cover restored. The stormwater and geothermal fluid storage sumps will be retained.
The geothermal fluid storage basin is necessary in case of pumping stop for maintenance operations. The small volumes that are stored there for a limited period of time are then reinjected into the well.
The impact during the operation phase of the geothermal power plant is therefore very low.