Movement of Ground water

The groundwater moves very slow and percolates through pores, spaces, and other openings in the rocks. The movement of groundwater is measured in meters or centimeters per day. Although rates of tens of meters per day have been measured occasionally, while a rate of 12-15 meters per day is considered high. The movement of the groundwater is dependant on the following features.

 1. Openings in the Rocks:

The open spaces contained in the rocks are of fundamental importance because groundwater stays in them and moves through them. The openings in the rocks are of two types; (i) Primary openings, (ii) secondary openings. The primary openings are those which existed when the rock was formed. The intergranular pores in sedimentary rocks and gas cavities in lavas are examples of primary openings The secondary openings are those which develop after the formation of rock. The joints, fractures, and solution channels belong to this category. The secondary openings are found in most igneous and metamorphic rocks, and also in some sedimentary rocks.

2. Porosity:

The quantity of groundwater that can be stored in a rock depends on its porosity. Porosity is a measure of pore spaces existing in a given volume of rock. Rocks vary widely in porosity. The loose sand and gravel may have porosity up to 50%, Sandstone up to 15%, while the porosity of the igneous rocks is of the order of 2% to 3%.

3. Permeability:

The permeability of a rock is its ability to transmit water. It is measured by the quantity of water passing through a unit cross-section of an aquifer in a unit of time under a 100% hydraulic gradient. The permeability in which full depth and unit width of the aquifer is considered is called ”transmissibility”. All porous rocks are not equally permeable. The permeability is closely related to the size of pore spaces and the degree to which they are interconnected. If the pores are small, the rock will transmit water very slowly. If they are large and interconnected, the rock will transmit water readily. Some clays have high porosities but they yield little or no water. The reason is that the openings in clays are very small and water in them is held by molecular attraction and therefore, it is not free to move. The absorptive value of the clays is also low. Many rocks with low porosity may be fractures and thus have very good permeability.

4. Darcy’s Law of Groundwater Motion:

From the observation of the height of water level in wells, Darcy proposed that for a given aquifer, the rate of water flow is directly proportional to the hydraulic gradient. The hydraulic gradient is the ratio between the difference in levels (h) of two points on the slope of the water table and the distance (l) between them. Mathematically, Darcy’s law can be expressed as follows:

                                                      Q=Kh/l

Where Q is the discharge per unit area and K is permeability. The hydraulic gradient (h/l) provides the driving force that keeps the groundwater in motion against internal friction.

5. Specific Yield and Specific Retention:

All the water that exists in an aquifer can not be recovered. A part, which is held thereby molecular attraction is not free to move. The terms ”specific yield” and ”specific retention” have been used to designate respectively the water which can be drained out freely under gravity and the water which is held in the aquifer. The specific yield may be defined as the ratio of the volume of water that can be drained and the total volume of the aquifer. The specific retention is the ratio of the volume of water, which is retained and the total volume of the aquifer.

Specific Yield = Volume of Water Drained ÷ Total volume of aquifer  ✕ 100

Specific Retention = Volume of Water Retained ÷ Total volume of aquifer  ✕ 100