Geologic factors leading to borehole failure in Machakos county, Kenya

Abstract

Machakos County is found in Eastern province in Kenya. It has a high population of 1,098,584. Therefore the demand for water is very high in this county. In fact the demand for water exceeds the water resource available. The county experiences harsh climatic conditions and has very little vegetation cover. This has led to a decrease in volume of surficial water in the county. Therefore the residents depend mainly on underground water. The government and non-governmental orgarnisations have helped in construction of boreholes in the area. The boreholes being constructed in this county face one major problem, borehole failure. Some of them after being constructed, they fail to produce water or the water they produce is not viable. The county has 69 boreholes. The oldest borehole was constructed in 1939 and the youngest in 2010 according to the data obtained from the Ministry of Water and Irrigation. The county has a borehole failure rate of 20%. Borehole failure is caused by several factors. Geology of the area was one of the major factors. The dominant lithologies in the area are metamorphic and igneous rocks. These rocks are known to be compact and very dense and therefore they do not make good aquifers. A good aquifer is one that has a good network of joints, faults and fractures or is porous so as to allow water to flow. Metamorphic and igneous rocks have fewer joints, faults and fractures and they are not porous and this makes them poor aquifers. The few aquifers present in this region are found in those zones that are rich in joints or zones rich in weathering. Over-extraction is also another problem especially in the major towns like Machakos town. Over-extraction results in drops in the water table. The water table may drop to depths exceeding the depths of the boreholes in the area. Precipitation of minerals in the aquifer causes blockages and this leads to borehole failure. The conditions for precipitation of these minerals are optimum when the borehole is being pumped. Therefore to solve this problem the time for pumping water in the borehole has to be reduced. Climate is also another factor. The county experience prolonged droughts. This leads to fall in the water table. The data used in the project was sourced from the Ministry of Water and Irrigation. The data has an element of error especially in the location of the boreholes.