How Deep is a Borehole in kenya: Hydrogeology and Geology

The average depth of a borehole refers to the typical depth at which a borehole is drilled into the ground to reach water. It represents the average distance from the surface to the bottom of the borehole.

When trying to estimate the average depth of boreholes in Kenya, It’s important to look at the hydrogeology and geology of different parts of Kenya. The hydrogeology reveals important insights into the country’s groundwater resources and their accessibility.

The average depth of boreholes in Kenya varies across different regions and geological formations, reflecting the diverse hydrogeological characteristics of the country. It can range from a few meters to several hundred meters. This variation is primarily influenced by factors such as the geological composition, rainfall patterns, and the specific purpose of the borehole.

According to a study published in the Science of the Total Environment journal, Named “Estimating groundwater use and demand in arid Kenya through assimilation of satellite data and in-situ sensors with machine learning toward drought early action” Coastal areas of Kenya is characterized by sedimentary rock formations, often have shallower boreholes due to the presence of relatively more accessible aquifers. The average depth of boreholes in these regions typically ranges from 30 to 100 meters.

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Also according to the same study, Inland regions of Kenya, including parts of the Rift Valley, highlands, and arid and semi-arid areas, often necessitate drilling deeper boreholes to reach suitable aquifers. The average depth of boreholes in these areas can exceed 100 meters and can go as deep as several hundred meters. The geological formations in these regions, such as volcanic rocks, basalt flows, and fractured aquifers, require specialized drilling techniques and drilling equipments to access groundwater resources at these depths.

Hydrogeology of Kenya

Hydrology refers to the scientific study of water in the Earth’s hydrological cycle, including its occurrence, distribution, movement, and properties on and below the Earth’s surface. The hydrology of Kenya focuses specifically on the water resources and water-related phenomena within the country’s boundaries.

According to “The Water Resources Management Authority (WRMA)” Kenya aquifers can be classied as shown in the following image.

Strategic Aquifer

A strategic aquifer is a subsurface water-bearing formation that serves as a primary source of water supply in a specific area, particularly when there are no readily available alternative resources or when developing such resources would require considerable time and financial investment. Example of Strategic aquifer in kenya include the following: Lotikipi, Tiwi, Nairobi, central Merti, Sabaki, Nakuru, Kabatini, Lake Naivasha Lamu Island.

Major aquifer

According to WRMA, major aquifers are High-yield aquifer systems are characterized by their ability to provide substantial amounts of water, and they are known for containing water of excellent quality. Examples of major aquifers in kenya are: Daua and Elgon volcanic rock aquifers.

Minor aquifer

According to WRMA, Minor aquifers are Aquifer systems with a moderate yield are capable of supplying a moderate amount of water. However, the quality of water within these systems can vary. Examples of minor aquifers in kenya include: Mandera – Jurassic (Mesozoic-Palaeozoic).

Poor aquifer

Aquifer systems with low to negligible yield provide minimal amounts of water, and the water quality within these systems ranges from moderate to poor. Example of poor aquifer in kenya is the Basement System.

Special aquifers

Aquifer systems designated as such by WRMA. Example of special aquifer include: Isinya aquifer.

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Goelogy of Kenya

The geology of Kenya refers to the study of the rocks, minerals, and landforms that make up the country’s geological framework. Kenya is located in East Africa and is bordered by the Indian Ocean to the southeast. The country’s geology is diverse and encompasses a wide range of geological features, including mountains, rift valleys, volcanic formations, sedimentary basins, and coastal plains.

In this section, we present a concise overview of the geology in Kenya as shown in the following map. 

Sedimentary Deposits- unconsolidated

Sedimentary deposits, which include a variety of soil types, alluvial sands, coastal beach sands, dune sands, evaporates, fossil coral reef and lagoonal formations, coastal sandstones, as well as alluvial and lacustrine sediments, are commonly found in different regions of Kenya. These deposits are often located at the base of volcanic successions or intercalated with them and are also deposited in tectonic troughs. The Rift Valley floor, which has experienced repeated faulting and numerous volcanic eruptions, has resulted in the formation of short-lived basins with internal drainage.

Igneous Volcanic

The central parts of the country in Kenya are predominantly covered by volcanic rocks, stretching from the south to the north. These volcanic formations can be observed both in the floor of the Rift Valley and on the peneplains situated to the west and east of the valley.

Sedimentary – Mesozoic-Palaeozoic

Palaeozoic and Mesozoic formations can be found in Kenya, primarily near the coast and in the northeast region. Among these formations, the oldest ones are the Taru, Maji-ya-Chumvi, Mariakani, and Mazeras formations, which consist mainly of sandstones, shales, and siltstones. Together, they form the Duruma series, equivalent to the Karroo system found in southern Africa. Stretching approximately 100 km from Taru to Mazeras, west of Mombasa, these formations exhibit their unique geological characteristics.

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