Environments
Groundwater
Groundwater represents 96% of the volume of non-glaciated freshwater on Earth.
It constitutes a true “continental ocean” beneath the surface of the ground. While its socio-economic value is well established, its biological value is much less known.
Yet most groundwater has been colonized by animal organisms, a little-known richness that is explained in particular by difficult access and collection conditions.
Groundwater: water contained in rocks. Groundwater aquifers are neither underground lakes nor underground watercourses!
These “groundwater aquifers” consist of water contained in the pores or fissures of reservoir rocks saturated by rainwater that has infiltrated into the subsurface.
The diversity of reservoir rocks (or aquifers), combined with that of climates and topography, gives rise to a wide variety of groundwater aquifers, in terms of size, depth, and behavior.
Groundwater: the different deposits
Aquifer contained in crystalline rocks (granite, gneiss, etc.) and volcanic rocks (lava, ash).
Water is stored in fissures and sands (arenas) resulting from the weathering of the parent rock.
Aquifer contained in sedimentary rocks (limestone, sand, sandstone, chalk). These aquifers can be unconfined or confined.
Unconfined aquifers are generally shallow and communicate directly with the ground surface because no impermeable layer covers them.
Rainwater infiltrates down to the aquifer, whose level rises or falls depending on precipitation. It renews rapidly. Water table aquifers belong to this category.
Confined aquifers are often several hundred meters deep or more. They are located between two impermeable geological layers that confine the water under pressure (which can flow in artesian wells).
They renew more slowly and their recharge comes partly from outcrop zones of the aquifer.
Limestone massif within which there is an underground drainage network, part of whose voids, enlarged by dissolution, can reach the size of sinkholes and caverns.
Permanent or temporary emergence (natural outlet) of an active karst network: a type of spring specific to karst aquifers, whose water may come partly from losses of surface watercourses.
Aquifer contained in alluvium (sands and gravels, interspersed with fine silts) deposited by watercourses in their valleys and mainly resulting from the erosion of the Pyrenees or mountain ranges.
These aquifers in connection with surface waters (rivers and streams) often serve as relays for large unconfined aquifers that flow toward the low points that are the valleys.
Hyporheic (literally, “underflow”): qualifies the zone corresponding to the deep bed of surface watercourses.
The hyporheic zone consists of all water-saturated sediments located below and beside a watercourse, containing a certain proportion of surface water.
Source: ©BRGM – M. Villey
Typology of underground aquatic habitats of stygofauna
While the different types of unconfined groundwater aquifers that exist at the scale of a territory or region may constitute a first level of differentiation, at a smaller scale, four major types of underground aquatic habitats can be defined (based on the local hydrogeological context) as potential living environments for stygofauna:
- 1 Exsurgence: type of spring corresponding to the emergence into open air of groundwater from a karst network fed only by infiltration of rainwater (as opposed to resurgence)
- 2 Resurgence: type of spring corresponding to the emergence into open air of a surface watercourse that upstream has plunged into the fissures and losses of a karst network (as opposed to exsurgence)
- 3 Rheotic (or lotic): qualifies a body of water with a flow (typically spring head and underground river)
- 4 Lentic: qualifies a stagnant or very low-flow body of water (typically groundwater aquifer, pool, puddle)
- 5 Hyporheic (literally, “underflow”): qualifies the zone corresponding to the deep bed of surface watercourses
This categorization of habitats is the one classically adopted in underground hydrobiology, but it is by nature arbitrary; thus, for example, springs and underground rivers are only the continua of the same underground hydrosystem.
Selection of sites to survey
The sites surveyed in each of the departments of the two former Aquitaine and Limousin regions were sought in the following environments:
- in unconfined groundwater aquifers:
aquifers contained in sedimentary aquifers (alluvial aquifers or aquifers accompanying watercourses, unconfined parts of confined aquifers, etc.), particularly in karst aquifers, and aquifers contained in basement aquifers; - in hyporheic environment (underflow of watercourses):
Concerning this last type of habitat, the search for sectors (river sections) where aquifer/river exchanges are known or assumed has been prioritized.
In practice, these different environments are accessible via natural structures or structures developed by humans. The sites surveyed as part of this program thus correspond to:
- caves, cavities, underground galleries in which underground water circulation exists (active cavities in the sense of speleologists);
- underground quarries in which the aquifer is at the surface, mine galleries partly submerged or flooded;
- but also to outlets of unconfined aquifers: springs (karst or non-karst) and shallow structures (wells, boreholes, piezometers, drainage galleries or aqueducts);
- and to the deep bed of watercourses (or underflow of these watercourses).
