# Groundwater Movement

• Apuntes

### Diploma in Environmental Science - Groundwater Movement

• Revisión por tema
Liberia 09 abril 2017
Pewu s W.

Q=KAH/I is what formula?

Kenya 08 abril 2017
Kevin K.

derive the darcy`s law equation.

Colombia 08 abril 2017
Jean Carlos M.

Buena leccion

South Africa 07 abril 2017
Sefala niger M.

True groundwater movement is determined by slope from high areas to low areas.

South Africa 07 abril 2017
Sefala niger M.

What is hydraulic pressure?

South Africa 07 abril 2017
Makhehla D.

Groundwater movement follows a topography from high to low.Areas of high pressure to low areas.

• Text Version

### Diploma in Environmental Science - Groundwater Movement

Groundwater flows underground in response to elevation differences (downwards) and pressure differences (from areas of high pressure to areas of low pressure). Near the water table, this means that groundwater usually flows ‘downhill’, i.e. from a higher level to a lower level, just as it would on the surface.

The difference in energy between two points that are l metres apart horizontally on a sloping water table is determined by the difference in height (h) between them. This height is called the head of water. The slope of the water table is called the hydraulic gradient and is defined as h/l. The rate of groundwater movement (Q - the volume of water flowing in unit time, with units of m3 s-1) is related to the hydraulic gradient by Darcy's law:
In the equation, K is the hydraulic conductivity and is defined as the volume of water that will flow through a unit cross-sectional area of rock per unit time, under a unit hydraulic gradient and at a specified temperature.

A is the cross-sectional area at right angles to the flow path.

The units of hydraulic conductivity are metres per second (ms-1 ) or metres per day.
The hydraulic conductivity depends on the properties of the rock that allow water to flow through it (its permeability) and also on the properties of the water.
Unlike hydraulic conductivity, permeability is an intrinsic property of the rock, so it is the same whatever the nature of the fluid flowing through the rock - whether water, as in this instance, or oil or gas.

The hydraulic conductivity (K), however, depends on the density and viscosity of the fluid, so it will vary accordingly. When the fluid is water, the most important factor that affects the hydraulic conductivity is temperature. For example, an increase in water temperature from 5 °C to about 30 °C will double the hydraulic conductivity and, from Darcy's law, will therefore double the speed at which the groundwater flows.
Rocks can be divided into two broad categories - permeable and impermeable - on the basis of their hydraulic conductivity. Rocks generally regarded as permeable have hydraulic conductivities of 1 m per day or more.
Hydraulic conductivity is proportional to permeability. So from Darcy's law it can be deduced that in a rock of constant hydraulic conductivity (K), and hence of constant permeability for a given fluid, the rate (Q) at which the groundwater flows will increase as the hydraulic gradient (h/l, the slope of the water table) increases.
The flow of groundwater in the direction of the slope of the water table is only part of the picture, for groundwater is also in motion at greater depths, where it generally moves in a curved path rather than a straight line when seen in cross-section, towards a stream or river, a spring, or even a well. This path is the result of movement towards an area of discharge, such as the stream.
The direction of flow of groundwater at depth is not parallel to the water table; instead, water moves in a curved path, converging towards a point of discharge. In (Section a) the rock is uniformly permeable, and the water discharges into streams in the valleys - it may approach the stream from below. In (Section b) the hill is capped by a permeable rock which is underlain by an impermeable rock. The water is diverted laterally by the impermeable rock, and springs result where the boundary between the permeable and impermeable rocks intersects the ground surface.