A hydrogeological survey is an investigation of the hydrologic and geologic parameters at the subsurface level in a particular area. Hydrogeological maps may be formulated with the data gathered during such a study. It involves the detailed evaluation of the water-bearing levels of rocks and their capability for filtration. Moreover, the intrinsic ability of these rocks to either store or resist water is also assessed. The pressure, type and quality of the underground water is noted, delving into the intensity of the water flow through pores or fractures.
The Importance of Hydrogeological Surveying:
A hydrogeological survey is done to determine the underground water level. Hydrogeological surveying is conducted prior to drilling a borewell in order to ascertain the quality and quantity of water available at a particular location. It has been well established that underground water is a better source of drinking water than surface water. This is due to the following reasons:
- Lower chances of pollution, especially if it is at least twenty meters away from the sanitary work.
- Water becomes naturally purified as it flows through sand and stone.
- Borewells remain shut and therefore have a reduced chance of contamination.
- Water temperature at the underground level is always at desirable levels in spite of seasonal changes.
- Greater chances of water availability in summer as the water sources are situated at a deeper level.
Typically, the hydrogeological assessment may be basic or complete. A basic survey comprises of the following:
- Establish the needs of the end-user regarding the location and quantity of groundwater required
- Nearby boreholes
- Evaluation of the quality of groundwater
- Appraisal of the catchment area
- Borehole construction and design recommendations
- Assessment of risk to health or environment
- The amount of groundwater that is anticipated from the given location
A full hydrogeological assessment consists of a few additional features such as determining the availability of aquifers risk assessment of the catchment and analysis of the water source especially for possible pathways of contamination. The hydraulic properties of the aquifers, the direction of flow of groundwater and the rate of flow are also verified.
Types of Hydrogeological Survey:
Hydrogeological surveys may be conducted on a large, medium or small scale depending upon the purpose of the procedure. Firstly, researched data from government and private agencies that are already available is assessed, followed by extensive field work as needed.
A small-scale survey (1:1,000,000-1:500,000) is done in areas that have never been investigated previously, to gather preliminary information of the hydrogeological condition. These comprise details regarding the ability of the rocks in the area to retain water and overall underground water quality. Medium-scale surveys ((1:200,000-1:100,000) are more elaborate, conducted to collate information for hydrogeological mapping. Water-bearing complexes are mapped and a specific study is conducted regarding the retentive capacity of the rocks in the region.
Large-scale hydrogeological surveys (1:50,000and greater) are mostly initiated in order to deal with precise issues at the stage of engineering and operational planning. Areas, where water collection can be done, are identified and underground water reserves are examined through this type of survey. Medium and large-scale surveys also require drilling procedures, evaluation of the composition of the underground water and its quantity along with a proper measure of its flow. Construction of wells and pumping tests may be performed to understand the hydrogeological condition thoroughly.
Examples of Hydrogeological Survey:
Case I: Kainuk, Kenya by Red Cross Society
The Kenya Red Cross Society, through a contract with Everesta Water Ltd, conducted Hydrogeological surveys for two boreholes in Kainuk division, Turkana and Kesei division, West Pokot between 4th-7th March 2017. This particular area is suggested for the construction of shade nets houses. The currently available water source is at a large distance. The Turkwel river is 600 metres from the proposed site. Geological conditions were determined to be fair for the presence of good quality groundwater.
Horizontal electrical profiling was first conducted, followed by three vertical electrical soundings (VES). VES 2 was selected, which showed some fracturing of gneisses (metamorphic rocks), which are poor water bearers.
Taking all factors into consideration, a site for borehole drilling is recommended in Kainuk, at an elevation of 557 meters as per the following specifications:
- The depth of the borehole: 100 meters below ground level.
- Diameter: Open-hole diameter not less than 8 inches and production diameter: not less than 6 inches.
- Yield estimated: 4 to 8 cubic meter per hour.
- Quality of borewell water: Expected to be good, taking into account neighboring borehole water sources.
Case II: Osembe Area of Kisumu North District, Nyanza Province, Kenya
A hydrogeological investigation was conducted for the Osembe African Devine Church, Kisumu District, Kenya for the purpose of constructing a borehole or shallow well-drilling area. As per the survey, the site consists of volcanic rock, mainly basalt, andesite and rhyolite. The Resistivity Method was used for Geophysical fieldwork. Various Geophysical measurements were taken to determine the following:
- The thickness of subsurface layers
- The potential of these layers to function as aquifers
- The anticipated groundwater quality at the site
Three vertical electronic sounding (VES) were carried out at the site. The conclusion drawn by the hydrogeological study was that the groundwater was of good quality and quantity. Unconfined and semi-confined aquifers were formed by Recent and Pleistocene sediments. Tertiary volcanic rocks form confined aquifers.
The recommendation made was to drill for the borehole at a depth of 80 mbgl at the VES 3 site.
In the present world scenario where there is a paucity of pure, uncontaminated water, groundwater is a sustainable alternative that could possibly solve a water crisis in certain regions. However, it can be challenging to gauge the quality and quantity of available groundwater at the required site. Hydrogeological surveying techniques have the ability to identify groundwater sources and determine the anticipated yield and quality of the water as well. Analysis of the subsurface layer of rocks also aids in gathering important information regarding the future utility of the groundwater. Thus, hydrogeological surveying is a necessity to provide sound recommendations for borehole drilling that will help in procuring good quality groundwater for human use.