Groundwater management

Groundwater is more than just a resource for mining and quarrying; it is also a crucial element in the architectural beauty of existing structures. In the underground world of mining and quarrying, where engineering meets nature, groundwater provides a dramatic and often unpredictable backdrop. From underground lakes to aqueducts, groundwater creates sublime landscapes that defy imagination.

Table des matières

Provenance des eaux souterraines

In quarries and mines, the formation of underground lakes is, on the one hand, the result of rainwater infiltration into the cavities and fractures of the bedrock. Rainwater seeps through the layers of soil and rock, reaching underground areas. The water then accumulates to form underground lakes, creating amazing aquatic landscapes. These lakes are very common in gypsum, because it is a very soluble rock, or in chalk because it is a very porous and not very permeable rock.

On the other hand, the formation of these lakes or flooded levels are the result of a phenomenon of rising water tables. This occurs when the water table rises and water flows into the underground cavities, filling the spaces and creating bodies of water. This phenomenon often results in the complete disappearance of underground networks or certain levels under water.

Water management

Groundwater in mining and quarrying can be both a constraint and an opportunity, depending on how it is managed. Initially, groundwater may be perceived as a major constraint for mining and quarrying operations. Constant water infiltration can compromise wall stability, increase the risk of collapse, and create hazardous working conditions for miners and quarry workers. In addition, the presence of groundwater can increase the pumping and drainage costs required to maintain a dry and safe working environment.

However, effective groundwater management can turn this constraint into a strength. By implementing groundwater management strategies such as pumping, drainage, wall sealing, and water reuse, it is possible to effectively control water levels and minimize risks to the safety and stability of mining and quarrying operations. Furthermore, by recovering and using groundwater for industrial uses or irrigation, it is possible to reduce dependence on external water resources and contribute to the environmental sustainability of operations.

Basins

Mainly supplied by rainwater from infiltration or wells (surface or water wells), basins, particularly in quarries, have various essential uses.

The lime basins

Often present in quarry workshops, these basins were used to slake quicklime in order to:

make the mortar for the masonry of the quarry consolidations;
in mushroom farms, cover the walls with lime to disinfect the space and prevent the proliferation of bacteria that can contaminate the crops;
absorb moisture from the air, which can help maintain optimal dry conditions and prevent condensation and mold in the quarries;
reinforce walls and structures, because it protects against water infiltration;
control unwanted odors, because it helps reduce musty, damp and other odors, thus improving working conditions (mushroom growers) or living conditions in the event of a population sheltering underground.

The slaking process was as follows. The upper basin, about 60 cm deep, was filled with lime and water simultaneously. By mixing them, the lime absorbed up to 3.5 times its weight in water. Once the mixture was made, it was vigorously worked with a plane, repeatedly, over several days. The mixture then flowed naturally into the second basin. After allowing the slaked lime to cool for a few days, it was ready to be used.

With the advances in consolidation techniques, these basins are now largely forgotten under Paris, but on an industrial scale, these techniques continue to be widely used.

Settling basins

Decantation is an ancient process for collecting clear water, but it was also widely used in chalk quarries for the production of Meudon white. After the grinding and diluting stage (dilution of the chalk in a large volume of water in order to prepare for decantation), the mixture was sent to a series of basins dug directly into the chalk, arranged in a cascade, one behind the other following the slope. The networks, sometimes complex, extended over several levels of the quarry.

These basins were interconnected by metal or pottery pipes. The decantation process had several stages. Initially, it allowed the chalk to be separated from the flint particles, the heaviest, which were deposited in the first basin. Then, the chalk particles were sorted according to their size. The heaviest were deposited first, while the lightest accumulated at the bottom of the last basin. The chalk deposits in each basin were treated differently to produce different derivatives.

In order to speed up the process, some basins were designed as “chutes”. By directing the flow of water laden with particles through a chute, the heavier particles tended to settle to the bottom more quickly, while the lighter particles could be carried further away before settling. This design allowed for more efficient separation of the different particle fractions, facilitating the chalk treatment process in the settling basins. In addition, the chutes often provided a practical solution for channelling the flow of water between basins, ensuring a smooth transition from one basin to another in the settling system.

Tanks

Often larger in size, their main function is to store a large quantity of water. This stored water has two major functions: supplying the population with water and agriculture. In addition, some are used to water underground crops of mushrooms or endives.

Roman Cistern: the example of the Berelle cave

Smaller reservoirs can be found specifically from the Roman period, called Roman cisterns. They were in use in the 5th century AD and then fell into oblivion. They were often located under public buildings, forums, baths and even private homes. Cisterns were designed to be strong and durable, with barrel or half-barrel vaults to support the weight of the earth and structures above them. Above them was a well to draw water.

Aqueduct

Water transport through the centuries

An underground aqueduct is a structure designed to transport water underground, often over long distances. Unlike above-ground aqueducts, which are structures visible on the surface, underground aqueducts are buried underground to protect the water from the elements and maintain its purity.

The usefulness of underground aqueducts has spanned the ages and civilizations. In ancient times, particularly among the Romans, underground aqueducts were essential for providing drinking water to cities, military forts, baths, and gardens. They allowed the development of large metropolises and contributed to the rise of civilization by ensuring a reliable supply of water for the population and economic activities.

Over time, underground aqueducts have remained important for water supply, but their use has also expanded to other areas such as agricultural irrigation, mining, hydroelectric power generation, and even stormwater and wastewater management.

Water always starts from a source and is then transported by penstock or free-flow.

Penstocks

Forced pipe routing is a more recent method that limits contact between water and the outside to reduce contamination, especially if the water has been treated beforehand. This solution is also considered when continuity of the masonry structure is not possible (e.g. motorway crossing).

The example of the Medicis aqueduct

The Medicis aqueduct was built at the beginning of the 17th century to meet several important needs of the city of Paris, including the supply of drinking water. It is composed of an underground vaulted gallery whose height varies from 2 m to 80 cm. It is masoned with uncut millstones. In the center of the gallery, there is a “cunette” in which the water circulates. This channel is bordered by two sidewalks called “benquettes”. The whole rests on a concrete screed called “radier”. On certain sections, the water flows directly onto the raft.

Access and works of the Medicis aqueduct

The layout of these galleries is punctuated by sumps and manholes. The sump (or air vent) is a well that provides access to the aqueduct from the top of the vault. When the aqueduct is deep, the sump takes the form of a small trapezoidal masonry well. In order to be able to descend into it, the walls are provided with notches allowing one to put one’s feet in. On the surface, the sumps are closed by a stone cover. Later, in the 19th century, some were closed by concrete or cast iron plates.

There are many more of them than the manholes, because they are much more modest structures. Their roles are multiple because they allow:

— Access to the aqueduct for its maintenance.
— Promote the ventilation of the gallery, particularly during the construction of the aqueduct.
— Allow the evacuation of spoil during construction. There are also notches in the vaults, formed by the traction of the blocks in the manhole by ropes.

The manholes are aedicules that allow reserved access to the underground gallery, via a staircase. At their level, the water passes through a basin whose purpose is to promote the oxygenation of the water and the deposition of impurities.

Access viewpoint to the aqueduct:

Interior of the aqueduct manholes:

Accès au regard n°3 de l’aqueduc de Médicis (regard de la Loge)

Virtual tour available

Drains

Unlike an aqueduct, which is used to transport water over long distances from a water source to areas where it is needed, a drain is used to remove excess water or to drain soils.

In many regions, the creation of drainage galleries is often necessary to ensure the stability of rock masses. This necessity arises from several geological and geotechnical factors.

Geologically complex regions, characterized by the presence of weathered or fractured rocks, make rock masses prone to water infiltration, which can lead to saturation of soils and rocks. Infiltrated water can also exert pressure on rock walls, increasing the risk of landslides or rockfalls. In addition, if these rocks are crossed by several streams and rivers, this contributes to increasing the potential for water infiltration into the surrounding rock masses. Periods of heavy rainfall can lead to a sudden increase in river flow, exacerbating infiltration and soil saturation problems.

To prevent these geotechnical risks and stabilize rock masses, the creation of drainage galleries is often considered. These galleries are designed to collect and evacuate water infiltrated from the rocks, thus reducing the hydrostatic pressure exerted on the rock faces. By eliminating excess water, drainage galleries help maintain the stability of rock masses and prevent landslides and rockfalls.

This is what was done in the city of Lyon.

Storm overflow

This is the final receptacle for this drainage water or rainwater more broadly. Overflows and storm drains collect excess water to reduce the load on treatment plants and networks by discharging this water into watercourses. These are often recent structures, made of concrete.

Pumping

One of the main purposes of pumping while the mine or quarry is still in operation is to maintain an optimum water level to allow mining or material extraction operations to continue without interruption due to water accumulation. This involves maintaining the water level below a specific threshold, usually below the working level of miners or below the extraction equipment.

Once mining has been completed, it is sometimes necessary to continue pumping in order to:

Prevent flooding: Abandoned excavations can fill naturally with water from precipitation or groundwater sources. Pumping is then necessary to remove this water and prevent flooding, which can damage surrounding infrastructure or cause safety hazards.

Maintain stability: Abandoned excavations can become unstable if filled with water, which can lead to collapses or landslides. Pumping helps keep the water level at a manageable level to avoid compromising the stability of underground structures and surrounding land.

Protecting the environment: Post-mining pumping may also be necessary to prevent contamination of groundwater or nearby waterways by toxic substances or residual chemicals from mining or extraction activities. This water therefore generally passes through treatment units before being released into the environment.

Very often, in order not to release this water anywhere and anyhow, throughout history, man has shown ingenuity in order to best exploit water as a source of energy.

Water use in the mining context

Hydrogeological engineers have invented a system to facilitate the lifting of ore by using the natural passage of water.

Toutes ces structures sont liées dans le but de maintenir le cycle de l’eau et d’améliorer la gestion de la ressource hydrique.

Mine drainage

Dewatering is a term used primarily in the field of mining and hydrology. It refers to the set of techniques and operations aimed at evacuating groundwater that invades the galleries and mine workings, in order to allow mining work to continue.

Groundwater pumping: Water can come from aquifers, underground rivers or from the infiltration of surface water. Powerful pumps and drainage systems are used to evacuate this water to the outside of the mine.
Safety and efficiency: Dewatering is essential to ensure the safety of miners and the efficiency of mining operations. A flood in a mine can not only endanger the lives of workers, but also damage the mine infrastructure and equipment.
Types of dewatering systems: Systems can be temporary or permanent, and include pumping wells, drainage galleries, and pipeline networks. The choice of system depends on the geology of the site and the volume of water to be evacuated.
Costs and environmental impacts: Dewatering can represent a significant cost for mining operations. In addition, the treatment of evacuated water is crucial to avoid pollution of surrounding ecosystems.

Water pollution

Water from mining operations can be polluting in several contexts, particularly due to the nature of the substances it may contain after having been in contact with mining materials and processes. Here are some specific examples of water pollution in the mining context:

Heavy Metals: Mine drainage water can contain heavy metals such as lead, mercury, cadmium, and arsenic. These metals can dissolve in water and contaminate drinking water sources, soils, and aquatic ecosystems.
Acidity: One of the major problems associated with mine water is acid mine drainage (AMD). This occurs when sulfide minerals, such as pyrite, react with water and oxygen to produce sulfuric acid. This acidic water can dissolve more toxic metals, further increasing the pollution.
Chemicals used in mining processes: Ore processing operations often use chemicals such as cyanide (in gold mining) or flotation reagents. If these substances contaminate mine water, they can pose significant risks to the environment and human health.
Sediments: Mining activities can increase soil erosion and cause excessive amounts of sediment to enter waterways. These sediments can clog aquatic habitats, affect water quality, and harm aquatic organisms.
Organic substances: Some mines, particularly those for resources such as coal, can generate water containing toxic organic compounds. These substances can come from waste from processing operations or from the decomposition of organic matter naturally present in ore deposits.

Environmental and health impacts:

Aquatic Ecosystems: Water contamination can kill aquatic wildlife, disrupt food chains, and alter natural habitats.
Human Health: Local populations that rely on these water sources for drinking, washing, or irrigating crops may be exposed to serious health risks, including chronic and acute diseases and cancers.
Soil and Vegetation: Water pollution can also contaminate soils, affecting plant growth and, by extension, the wildlife that depends on these plants.

In conclusion, water from mining operations can become polluting when it comes into contact with toxic substances or chemicals used in the extraction and treatment processes. Proper management and treatment of mine water is therefore essential to minimize environmental and health impacts.

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Provenance des eaux souterraines