Basics of Hydrogeology
Definition of hydrogeology
“Hydrogeology is a natural science that studies the interactions between groundwater and its geological environment, as well as groundwater phenomena, particularly the influence of geological factors on the physical behavior and chemical composition of groundwater.” (Tieteen termipankki)
In short, hydrogeology is the study of groundwater. However, the field encompasses much more than just locating groundwater and extracting it for use.
Groundwaters
Groundwater is water located beneath the Earth’s surface, filling the pores in soil and the fractures in bedrock. It is one of the Earth’s most important natural resources. Together with its geological environments and associated phenomena, groundwater has gained increasing research attention since the 1950s. Groundwater has been utilized primarily from locations where it is most easily accessible. (Mälkki, 1999)
Forming of groundwater
Groundwater is formed when precipitation on land or surface water infiltrates the soil or flows into rock fractures. The soil must be coarse-grained and porous for the water to seep into it. Groundwater is typically very clean because it is filtered as it percolates through the soil layers.
Groundwater research
Groundwater research initially focused primarily on locating groundwater and determining its yield. However, the scope of tasks has diversified over time. Today, groundwater is studied to assess the total water reserves of aquifers, their usability, protection needs, and regulation. In addition, research focuses on groundwater quality, identifying causes of contamination, and exploring possibilities for artificial groundwater formation. (Mälkki, 2005)
The importance of sustainable groundwater use has become increasingly recognized and is now a major priority in groundwater management. Groundwater research integrates geological, geomorphological, hydrological, and hydrogeological methods.
Aquifers
Aquifers are layers of soil or bedrock capable of storing and transmitting groundwater. They act as groundwater reservoirs and are often used for water supply through wells and groundwater pumping.
Aquifers can be categorized based on how they store and transmit water:
Unconfined Aquifer
This is the most common type of aquifer, where the groundwater surface is in open connection with the ground surface. An unconfined aquifer typically has a saturated zone filled with water and an unsaturated zone containing both water and air. Water moves freely up and down, but the aquifer is more vulnerable to contamination due to exposure to surface pollutants.
Confined Aquifer
Confined aquifers are located between impermeable layers, such as clay or rock, which lie above and below the aquifer. These aquifers are under pressure, saturated with water throughout their thickness. When tapped by a well, water can rise on its own (artesian wells) due to the pressure. The natural isolation makes confined aquifers well-protected from surface contamination.
Semi-Confined Aquifer
A semi-confined aquifer, also known as a leaky aquifer, lies beneath an impermeable or semi-permeable layer. Unlike fully confined aquifers, some water can flow through the semi-permeable layer. These aquifers share characteristics of both confined and unconfined aquifers and may be exposed to some level of contamination, but not as directly as unconfined aquifers.
Bedrock Aquifer
Bedrock aquifers consist of dense, impermeable rock with water flowing through cracks and fractures. Their water retention and flow characteristics depend on the type of rock and its fracture network.
Water enters a bedrock aquifer through fractures, either directly from surface water or from other soil layers. Recharge can be slow, as water infiltrates only through cracks, joints, and fractures, making groundwater extraction more challenging.
Each type of aquifer plays a critical role in groundwater availability and management, requiring careful study to ensure sustainable use and protection from contamination.
Aquitards
Aquitards are soil or rock layers capable of storing and transmitting water, but at a much slower rate than aquifers. They act as barriers or retarders to water movement. Aquitards are typically composed of materials with low permeability, such as clays and silts.
Role of Aquitards
Aquitards play a crucial role in groundwater systems by slowing down water movement. This slow percolation through the layer also helps in purifying the water. The time it takes for water to pass through an aquitard depends on its density and material composition. In the case of thick aquitards, water filtration can take hundreds or even thousands of years.
This natural filtration process makes aquitards important for protecting deeper aquifers from contamination and regulating groundwater recharge over long timescales.
Fractures and discontinuities in bedrock
Fractures and discontinuities are key geological structures that significantly influence the storage and movement of groundwater in bedrock. These features occur in almost all types of rocks, and their characteristics determine how water flows through the rock mass.
Fractures
Fractures are breaks in the bedrock where the rock material has partially lost cohesion due to stress. They create permeable pathways for groundwater, enhancing the hydraulic conductivity of the bedrock. While the rock matrix itself may have low permeability, large fractures and continuous discontinuities can store and transmit substantial amounts of water.
Discontinuities
Discontinuities encompass all geological breaks in the rock, such as bedding planes, faults, foliation, and fractures, which disrupt the continuity of the bedrock. These structures can either facilitate water flow or act as barriers, depending on their properties. The orientation, density, and connectivity of discontinuities are critical factors in predicting water flow and managing groundwater resources.
Understanding the role of fractures and discontinuities is essential for hydrogeological studies, as they directly impact groundwater availability, flow dynamics, and the overall behavior of aquifers in fractured rock systems.
Water balance
Water balance is a calculation that describes changes in the amount of water within a specific area or water system over time. The calculation includes all natural and artificial water inputs and outputs, as well as changes in stored water over a given period.
By analyzing the water balance, it is possible to assess whether water resource use in a given area is sustainable. A positive water balance indicates that more water is entering the system than leaving it, while a negative water balance shows that more water is leaving than entering, which can lead to water scarcity over time.
Water Balance Equation
P = Q + E + ΔS
Where:
- P = Precipitation
- Q = Runoff
- E = Evaporation
- ΔS = Change in stored water
The water balance equation provides a framework for understanding the hydrological processes in a region, helping in water resource management and sustainability planning.
Groundwater quality
Water quality refers to the chemical, physical, biological, or radiological state of a surface or groundwater system. In hydrogeology, evaluating groundwater quality is a complex task. Both human activities and natural processes can adversely affect groundwater quality by increasing the concentrations of certain substances.
For instance, the dissolution of minerals into groundwater can naturally elevate metal concentrations. High levels of arsenic and uranium are commonly found in aquifers and groundwater systems in various regions around the world. These occurrences highlight the need for thorough monitoring and management to ensure safe and sustainable use of groundwater resources.
Groundwater contamination and pollutants
Groundwater contamination refers to the degradation of groundwater quality due to the intrusion of harmful substances. Any water containing toxic or disease-causing substances is classified as contaminated. This definition does not differentiate between the sources of contamination or the types of pollutants—any substance, whether of natural or synthetic origin, that is toxic to humans or poses health risks, is considered a groundwater pollutant.
For a detailed list of various contaminants and their concentration requirements, refer to the link below.
Sources
- Kresic, N. (2008). Groundwater resources: sustainability, management and restoration. McGraw Hill Professional.
- Leppäranta, M., Virta, J. & Huttula, T. (2017). Hydrologian perusteet. Helsingin yliopisto, Helsinki.
- Mälkki, E. (1999). Pohjavesi ja pohjaveden ympäristö. Tammi.
- Mälkki, E. (2005). Pohjavesitutkimusopas: Käytännön ohjeita. Suomen vesiyhdistys.
- Singhal, B.B.S. & Gupta, R.P. (2010). Applied Hydrogeology of Fractured Rocks (2nd edition). Springer Science+Business Media.
- Suomen YK-liitto. Puhdas vesi ja sanitaatio. Noudettu 23.6.2024 osoitteesta https://www.ykliitto.fi/puhdas-vesi-ja-sanitaatio.
- Tieteen termipankki 2.7.2024: Geologia:hydrogeologia.