Hydraulic and hydrochemical characteristics of the phreatic basement aquifers in parts of southwestern Nigeria

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Investigations of the hydraulic characteristics of shallow basement aquifers and the hydrochemical nature of the groundwater in parts of southwestern Nigeria, were conducted with the main aim of assessing the disposition of the phreatic aquifers to development. Pumping and recovery tests conducted during the dry season on 32 large diameter hand-dug wells show that the phreatic aquifers, in the regoliths, have low transmissivities which range from 12.27-60.74 m 2 /day for the pumping regime and 10.45-105.84 m2/day for the water level recovery regime. The specific yields between 0.092 and 0.961 are generally high, indicating a potential for a high storage or water holding capacity. Two major recharge areas are identifiable in the study area. One occurs on a groundwater divide in the central part, with groundwater flowing in opposite directions of the divide. The other is located in the northeastern part, with groundwater flowing in the southwestern direction, thereby making possible a mixing of the two water types in the central area. For wells with an average depth of 3.96 m, water level recovery generally appears appreciable with an average recovery of 65% for average recovery duration of 114 minutes and pumping duration of 26 minutes. Most of the wells stabilized within a pumping duration of 30 minutes. With adequate planning, the shallow aquifers can be comfortably exploited, not only for domestic purposes as is presently widespread, but also for agricultural purposes, especially in the dry season when farming virtually stops because of inadequate rainfall. The determined average nitrate and cadmium concentrations of 90 ppm and 0.015 ppm respectively indicate susceptibility of the shallow groundwater to pollution from domestic, agricultural and industrial wastes.