Groundwater System
As crucial as groundwater is, the resource is vulnerable and hence needs to be protected and managed. Groundwater is poorly understood and managed in many parts of the world despite its pivotal role in the sustenance of ecosystems and the provision of water supply. Understanding groundwater and its linkage with surface water and other members of the ecosystem is paramount to proper management of these resources. Studies have shown that some groundwater resources might have been accumulated eons ago and whereas there is increasing demand as a result of increasing population, chances are that these resources are no longer replenished.
A system can be defined as a group of interrelated entities that form a unified whole. Groundwater systems can be defined as a system comprising the sub-surface water, geologic media containing the water, flow boundaries, sources, and sinks. These systems necessitate the storage and flow of groundwater within them. Groundwater systems are distinct and unique in different parts of the world. Its management would be simple if the groundwater system were all the same.
Understanding how groundwater systems work is important in the management of groundwater. Hydrogeologists should understand the types of rocks that make up good aquifers; the mode of recharge of aquifers and why some are naturally recharged while others are not; sustainable yields; and the pollution risks of some aquifers.
Groundwater is part of a larger water cycle. The entire system is linked to the surface water in that it sustains the streams, rivers, and wetlands all year round.
- Components of the groundwater system
- Groundwater storage Components
By definition, groundwater storage refers to the difference between recharge and discharge over the time that these events happen, which can range from days to years. Groundwater storage components comprise all the media that store underground water. They are responsible for ensuring that the water percolating into the subsurface is stored. The majority of groundwater is stored within the pore spaces of geologic media. These media are known as aquifers and their ability to store and transmit water will be determined by the following characteristics; specific retention, porosity, and storativity. An aquifer system is a group of formations that contain sufficient saturated permeable material to yield economic quantities of water to boreholes and springs. The aquifers and confining beds in an area comprise the groundwater system of that particular area. Aquifer systems are the storage medium from which groundwater is abstracted. Aquifers can be of igneous, metamorphic, or sedimentary origin. Sedimentary rock s make the best aquifers as their porosity is usually high. The porosity of these rocks however decreases as the finer particles increase. There are cases where the porosity of sedimentary rocks is less than 10%, especially when the sedimentary rock is highly consolidated. The presence of fractures in igneous and metamorphic increases their chances of holding water. Their porosity however rarely exceeds 1%, and in unweathered crystalline rocks, groundwater will only be found in fractured areas. About 38% of the land area in Africa is covered with these types of rocks. Deeper into the Earth, rocks are under compression hence reduced porosity and closure of fractures within them. The groundwater system, hydraulically, serves two main functions; storage and transmission. More information about types of aquifers can be obtained here https://app.site123.com/the-hydrogeologist/types-of-aquifers?w=4472136. The sensitivity of groundwater storage to climate will increase if it is renewable.
- Recharge
Water enters the groundwater system via the recharge zones. Precipitation which can be in form of rainfall and snowmelt is the main mode of recharge of groundwater resources. However, in some other instances, the recharge can take place through seepage from rivers, lakes, streams. That portion of rainfall that recharges the groundwater is usually determined by the characteristics of soils and vegetation. Confined aquifers have longer recharge time as compared to unconfined aquifer recharges. The rates of recharge vary annually and this depends on the precipitation amount, seasonal distribution, land use, and air temperature.
The confining beds through which groundwater flows, hydraulic conductivity, and hydraulic gradients determine the rate at which water moves the sinks to the discharges. Recharge areas are invariably larger than the discharge areas. At the recharge zones, the movement of water is in the vertical direction and the hydraulic conductivity is lowest. The direction of movement at the discharge zone is horizontal- where the hydraulic conductivity is largest.
- Groundwater
Vertical distribution of subsurface water
The subsurface occurrence of groundwater may be divided into two zones: Zone of aeration and
Zone of saturation (phreatic zone/groundwater zone).
Unsaturated zone
This zone is also known as the zone of aeration. It has interstices occupied partially by water and partially by air. Vadose water occurs in this zone.
It consists of the following: soil-water zone, which extends from the ground surface down through the major root zone.
The thickness of this zone varies with the soil type and vegetation. Intermediate Vadose zone extends from the lower edge of the soil-water zone to the upper limit of the capillary zone. Thickness may vary from zero to more than 100 meters under deep water table conditions. This zone serves primarily as a region connecting the zone near the ground surface with that near the water table through which water moving vertically downward must pass. Non-moving Vadose water is held in place by hygroscopic and capillary forces.
Temporary excesses of water migrate downward as gravitational water. The capillary zone is also known as the capillary fringe extends from the water table up to the limit of the capillary rise of water.
Saturated zone
It is also known as groundwater zone or phreatic zone.
All the pore spaces in this zone are filled with water hence the effective porosity provides a direct measure of the water contained per unit volume. The local geology, availability of openings in the geologic media, recharge of groundwater, and the movement of groundwater from recharge toward discharge are some of the factors that affect the thickness of groundwater.
- Sinks
These are areas where groundwater is hydraulically discharged from the aquifer. It is the point where the subsurface journey of groundwater is brought to a halt. They include zones of inter-aquifer transfers, springs, seepages, groundwater lakes & marshes, and wells. Those areas that exhibit low potential in the groundwater system, act as the groundwater discharge zones, and they are usually low-lying. A large body of water will form from a spring if it is protected from excessive evaporation. Hydraulic discharge of groundwater is usually actuated by the following attributes; a positive fluid-potential gradient; a low relative topographic position; an allochthonous water quality and an allochthonous water temperature.
Spring is a phenomenon where an observable flow of water jets out via a natural opening in the formation. For a spring to form, there have to be one or more openings, a discharge rate that is more than the local rate of evapotranspiration loss, steep hydraulic gradients, and a high rate of precipitation that ensures a continuous supply of water to the spring.
The development of a seepage requires the same conditions as that of spring except that no isolated individual opening is required at the outlet.
- Flow boundaries
Boundary conditions in problems of groundwater flow describe the specific conditions that are to be imposed at the boundaries of the flow region. These boundaries are not necessarily impervious layers or walls confining the groundwater. Rather, they are geometrical surfaces were, at all points, we know either the flow velocity of the groundwater, or an equipotential line, or a given function of both. Some characteristic boundary conditions include:
Impervious Layers
The boundary of an impervious layer can be regarded as a streamline because there is no flow across it. The flow velocity component normal to such a boundary therefore vanishes.
Free Water Surface
The surface where water pressure equals atmospheric pressure is known as a free water surface. At this point, the assumption is that the free water surface limits the groundwater flow region (there is no groundwater flow occurring above this surface). This assumption is not true in most instances of flow through soils, but it is useful when we are analyzing flow through a layer whose capillary fringe is thin in comparison with its thickness. The pressure component of the total head equals zero, making the elevation component equal to the hydraulic head, at the surface of the water i.e. h=z.
The development of a seepage requires the same conditions as that of spring except that no isolated individual opening is required at the outlet.
- Flow boundaries
Boundary conditions in problems of groundwater flow describe the specific conditions that are to be imposed at the boundaries of the flow region. These boundaries are not necessarily impervious layers or walls confining the groundwater. Rather, they are geometrical surfaces were, at all points, we know either the flow velocity of the groundwater, or an equipotential line, or a given function of both. Some characteristic boundary conditions include:
Impervious Layers
The boundary of an impervious layer can be regarded as a streamline because there is no flow across it. The flow velocity component normal to such a boundary therefore vanishes.
Free Water Surface
The surface where water pressure equals atmospheric pressure is known as a free water surface. At this point, the assumption is that the free water surface limits the groundwater flow region (there is no groundwater flow occurring above this surface). This assumption is not true in most instances of flow through soils, but it is useful when we are analyzing flow through a layer whose capillary fringe is thin in comparison with its thickness. The pressure component of the total head equals zero, making the elevation component equal to the hydraulic head, at the surface of the water i.e. h=z.
Materials
Heath, R. C. (1980). Basic elements of ground-water hydrology with reference to conditions in North Carolina: US Geological Survey Water-Resources Investigations Open-File Report 80-44, 86 p. 1984. Ground-water regions of the United States: US Geological Survey Water-Supply Paper, 2242(78), 1989.
Toth, J. (1971). Groundwater discharge: a common generator of diverse geologic and morphologic phenomena. Hydrological Sciences Journal, 16(1), 7-24.
