An aquifer is an aggregation of saturated rock that water passes through quite easily. They are both porous and permeable to water. They include rocks such as conglomerate, sandstone, fractured limestone and unconsolidated sand and gravel. Also, volcanic rocks that are fractured such as columnar basalts are good aquifers. In essence, some rocks with low porosity are poor aquifers. Examples of such rocks with low porosity include schist and granite. However, when schist and granite become fractured they become excellent aquifers. The boundaries of aquifers are usually graded into other aquifers, in essence, an aquifer could be a part of an aquifer system. Aquifers are filled with moving water and the quantity of water stored in the aquifer varies from season to season and also from year to year. Also each aquifer possesses a recharge zone or recharge zones with a discharge zone or zones. Types of aquifers include perched aquifer, confined aquifer and unconfined aquifers. Since aquifers are aggregation of rocks which allows storage and movement of water through them, then potential aquifers abound everywhere but they all occur at different depths. This is so because all pieces of land have a base rock and water abound under the rock. In studying the geology of an area, a very important factor is the presence or absence of a river around. Presence of one implies that an aquifer will surely be located in the region. This is so because the rivers supply the aquifer with water and also the other way round with aquifers supplying the river with water known as percolation. Also, another factor to be considered is the presence or absence of gravel and also presence or absence of volcanic ash.
Areas that are polluted are to be avoided because the water can be infiltrated with pesticides and dirt which makes it an unsafe source of potable water.