2. Introduction
• Hydrogeology (hydro- meaning water, and - geology meaning the study of the
Earth) is the area of geology that deals with the distribution and movement
of groundwater in the soil and rocks of the Earth's crust (commonly in
aquifers).
• The terms groundwater hydrology, geohydrology, and hydrogeology are often
used interchangeably.
4. The Hydrologic Cycle
• The hydrologic cycle explains the exchange of water between the atmosphere, lithosphere and the
hydrosphere.
• It depicts how part of the ocean water evaporates, the water vapour turns into fresh water precipitation
(rain, hail, snow) on the earth’s surface (seas, land), then flows over the land surface (glaciers, runoff,
streams) and partly infiltrates into the soil (soil water) to be used by the vegetation (evapotranspiration), or
to recharge the groundwater bodies.
• The water on the Earth's surface--surface water--occurs as streams, lakes, and wetlands, as well as bays and
oceans.
• Surface water also includes the solid forms of water-- snow and ice.
• The water below the surface of the Earth primarily is ground water, but it also includes soil water.
• The movement of water through the cycle annually replenishes our water supplies, thus making water a
renewable resource.
6. Components (parts)of the Hydrologic Cycle
• Precipitation - is water being released from clouds as rain, sleet, snow or hail.
• Runoff - is precipitation that reaches the surface of the Earth but does not
infiltrate the soil. Runoff can also come from melted snow and ice.
• Infiltration - when a portion of the precipitation that reaches the Earth's
surface seeps into the ground.
• Percolation - is the downward movement of water through soil and rock.
• Evapotranspiration (Evaporation + Transpiration) is water evaporating from the
ground and transpiration by plants.
• Condensation - is the process of water changing from a vapor to a liquid.
8. What is groundwater?
• Groundwater: the water that lies beneath the ground surface, filling the pore
space between grains in bodies of sediment and clastic sedimentary rock, and
filling cracks and crevices in all types of rock
• The rainfall that soaks into the ground and moves downwards into spaces and
cracks in the rocks below the ground surface becomes groundwater.
• Groundwater is a major economic resource
9. Division of sub surface water
• Zone of Aeration, also termed as Vadose zone or unsaturated zone, is that
portion of sub-soil in which pores are filled partially with air and partially with
water.
• Vadose zones extends from the Earth’s surface to the top of the saturated
zone.
• The soil and rock in the vadose zone are not fully saturated with water; that
is, the pores within them contain air as well as water.
• In some places the vadose zone is absent, as is common where there are
lakes and marshes, and in some places it is hundreds of meters thick, as is
common in arid regions.
10. Division of sub surface water
• Zone of Saturation, also termed as Pheratic zone, is the one in which all the
pore spaces in the soil/rocks are completely saturated and filled with water
under hydrostatic pressure.
• The upper limit of this zone of saturation is termed as the water table.
• The water table, is a very significant feature of the groundwater system.
• The water-table level is important in predicting the productivity of wells,
explaining the changes in the flow of springs and streams, and accounting for
fluctuations in the levels of lakes.
11. Where does GW occur, in what environment?
• An aquifer is a geological formation capable of yielding useful groundwater
supplies to wells and springs.
• All aquifers have two fundamental characteristics:
• A capacity for groundwater storage (Porosity ) and;
• A capacity for groundwater flow (Permeability).
• Porosity is defined as the percentage of a volume of rock (the all purpose
term hydrologists use for ‘earth materials’) that is empty space.
• Effective porosity: void spaces that are too small to admit water molecules
are of little interest to hydrologists. The amount of void space available for
fluid flow is the effective porosity.
• Permeability describes of how easily water is able to move through rock.
Permeability is related to the connectedness of the void spaces and to the
grain size of the rock.
12. GW Occurrence (Hydro-stratigraphic Units)
• An aquifer is a saturated formation of the earth. It not only stores the water but
also yields it in adequate quantity. Aquifers are highly permeable formations
and hence they are considered as main sources of groundwater applications.
• An aquitard is also a saturated formation. It permits the water through it but
does not yield water in sufficient quantity as much as aquifer does. It is because
of their partly permeable nature.
• An aquiclude is a geological formation which is impermeable to the flow of
water. It contains a large amount of water in it but it does not permit water
through it and also does not yield water. It is because of its high porosity. Clay is
an example of aquiclude.
• An aquifuge is an impermeable geological formation which is neither porous nor
permeable - which means it cannot store water in it and at the same time it
cannot permit water through it. Compact rock is an example of aquifuge.
14. Aquifer Types
• Aquifers are classified into two types based on their occurrence which are as
follows :
• Unconfined aquifer: An unconfined aquifer is an aquifer which has free
water surface - which means the water table exists for this type of aquifer. This
is also called as water table aquifer or free aquifer or phreatic aquifer.
• Confined Aquifer: A confined aquifer is an aquifer confined between two
impermeable beds such as aquifuge, aquiclude, etc. The water in the confined
aquifer will be under greater pressure which is greater than atmospheric
pressure. Hence, the water level shown by piezometer is always higher than
the top level of the confined aquifer. The recharge of confined aquifer occurs
at a place where it exposes to the ground surface.
16. Groundwater Observation and Extraction (Piezometer, Well)
Piezometer: An observation well only measures
depth to the water table. Piezometers are
slotted over only a short section below the
water table, within the aquifer.
17. Groundwater Observation and Extraction (Piezometer, Well)
Well: Well is a hole with screen for entire
length of the well casing that is drilled
into the ground to access water
contained in an aquifer.
22. Aquifer Properties
• Hydraulic conductivity is a measure of how easily water can pass through soil
or rock: high values indicate permeable material through which water can pass
easilyHydraulic conductivity is typically given the symbol k and has units of
velocity, for example m/s or m/d.
• Transmissivity: The amount of water that can be transmitted horizontally
through a unit width by the full saturated thickness of the aquifer under a
hydraulic gradient of 1.
• T = bK
T = transmissivity.
b = saturated thickness.
K = hydraulic conductivity.
• Specific storage S = amount of water per unit volume stored or expelled
owing to s compressibility of mineral skeleton and pore water per unit change
in head (1/L).
