The groundwater level has a direct impact on determining the type and level of the foundation base. In addition, a map of aquifers is required when drilling artesian wells. Without it, it is difficult to predict the volume of work and operational characteristics of the future facility, the occurrence of flooding and dampness in the basement, and also to find out how many concrete rings will be required, for example, when constructing a mine well. Having in hand a diagram of the location of groundwater, you can determine the optimal location and direction of the slope of the drainage system and choose the best location for installing a water intake well.
What is an aquifer map?
Hydrogeological study of soils makes it possible to determine the types and characteristics of soil layers in a small area or a large area, as well as the level of groundwater. Based on the study and analysis of the results, a number of documents are compiled. As a rule, local archives of settlements have long had geological sections and maps of aquifers. But outside the city or on newly developed sites, it is necessary to excavate soil samples and determine the location of underground levels of water surfaces.An aquifer map is a diagram of the occurrence of all types of groundwater in a longitudinal geological section, indicating soil layers and aquitards, or a plan indicating the levels and directions of free flows.
Underground water behaves somewhat differently than in a container, where there is no doubt about the horizontalness of its level. In the thickness of the soil, the line of the water table can bend under the influence of many factors:
- terrain;
- shape and placement of waterproof layers;
- make-up and reset options;
- throughput and density of soil layers;
- proximity to bodies of water, etc.
When compiling maps, they use groundwater level measurements from available natural and artificial sources. These can be wells and workings, wells and pits, water bodies and water-measuring posts. To ensure the “purity” of the data obtained, measurements at points located close to each other are carried out on the same day due to the fact that the groundwater level can change significantly under the influence of external influences. In this regard, maps of aquifers must be dated.
If, when developing a site, groundwater can be detected during the construction of a pit, then when constructing a mine well or artesian well, specialists will need to look at the map of aquifers. Its absence in most cases leads to unforeseen situations. For example, during the process of lowering well rings, it may turn out that the water is much deeper than the expected level. The meaning in further work will disappear by itself, and the rings will most likely remain in the ground. In this case, it would be more profitable to immediately stop at constructing a well.
Experienced professionals recommend not to neglect familiarization with maps of aquifers or carrying out exploration drilling. By the way, you can determine the proximity of an aquifer using traditional methods, but this does not always lead to the detection of drinking water.
Types of cards
Measured groundwater levels are plotted on diagrams or graphs. The name of the documents depends on the information they contain. The most common cards are:
- hydroisogypsum;
- hydroisopyesis;
- changes in groundwater levels;
- fluctuations in water depth in wells;
- hydrogeological sections, etc.
Maps of hydroisohypses and hydroisopiesis are made according to available data. The piezometric surface is characterized by the pressure of the pressure water and the height of its horizon. The term itself does not have a specific meaning, and the conditional level of the water surface can be located both above and below the ground. In other words, this is the height to which water rises in opened artesian wells. This indicator affects the length of the casing pipes, the upper edge of which should rise above the piezometric surface.
For free-flow conditions, a hydroisohypsum map is created. They characterize a unified system of water movement in aquifers. By the location of the lines on graphic plans you can determine:
- features of the direction and slope of flows;
- level and nature of the arrangement of free surfaces;
- places of feeding of layers and sources of unloading;
- connection of groundwater with open reservoirs - flows are drained or fed by the river.
It should be noted that the upper level of free-flowing water remains almost horizontal. However, on the aquifer plan, several curved lines are drawn connecting the same groundwater table elevations.
Hydroisohypsum maps are often marked with hydroisobath lines, which are constructed based on interpolation.
Classification of groundwater
Groundwater is grouped according to its nature (hydrodynamics) and depth. First of all, they distinguish:
- Gravity waters - “rely” on the first aquifer from the surface of the earth. Their upper level is unstable and depends on the presence of precipitation, intense snow melting or drought in a certain period of time. The permeable layer is partially saturated with groundwater, and its surface remains free;
- pressure waters are located at greater depths, between two aquifers.
Based on the depth of placement in the soil, groundwater is divided into four types.
Verkhovodka – depth up to five meters. Recharge is made from atmospheric precipitation. For the construction of wells, perched water is considered far from the best option, since in dry periods the water may simply disappear, and in rainy periods it may not have time to filter out.
Groundwater – depth up to ten meters. Clay serves as a waterproof layer, so the source should not be used for drinking. In addition, if the thickness of the soil above the aquifer is less than six meters, sufficient filtration of water will not occur, but the risk of contamination by technical fluids will be too great.
Interformational waters- depth from 10 to 100 meters. As a rule, they are located between horizontally waterproof layers, although the overlying layer may also be permeable. Interstratal waters are considered the most optimal option for constructing wells. Sufficient depth ensures good filtration and uninterrupted water supply using household pumping equipment.
Artesian waters– the deepest location (more than a hundred meters underground). Water is naturally purified from contaminants as much as possible, so it does not require additional filtration. But the composition may contain an unacceptable concentration of mineral inclusions. An artesian well is drilled for collective use, since the volume of incoming water is incommensurate with the needs of one private household, not to mention the high cost of constructing a deep water intake.
It should be noted that for foundations the main factor is not the purity of groundwater, but its level. It is he who influences the decision on the design features of the foundation, as well as the list of measures for its waterproofing.
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