HYDROGEOLOGY
Hydrogeology is the study of distribution, movement, and properties
of water in and on Earth's surface.
Whenever precipitation reaches the surface, it will either evaporate
immediately back into the atmosphere, or percolate beneath the surface
into the ground water system.
Any excess water runs off the surface into streams that
carry it into seas and lakes. The volume of run-off which passes through
a stream in a given period of time is called its discharge.
STREAMS
- Streams merge into drainage systems consisting of a main
stream and its tributaries.
- Every drainage system collects runoff from a drainage basin.
- Drainage basins are separated by drainage divides.
- The principal components of all streams are their channels and
overbank floodplains.
Channels are described on the basis of their:
- gradient: slope of channel bottom (measured from a longitudinal
profile)
- cross-sectional area: equals width x
depth, and increases with increasing discharge
- cross-sectional shape: varies from shallow but wide (typical
of streams with gravelly or sandy banks) to deep but narrow (muddy banks)
- sinuousity: degree of curvature, varying from straight
(S = 1.0 to 1.5) to meandering (S > 1.5)
Floodplains:
- broad, low relief areas adjacent to channels
- flooded when the discharge of a stream exceeds the capacity of its
channels, and the channel overflows its banks
- moderate floods occur nearly every 10 years; major floods occur nearly
every 100 years
- overbank flow deposits a sandy and silty ridge (levee) next
to the channel and a fertile blanket of clay and organic sediment across
floodplain
GROUNDWATER
There is a vast body of ground water in subsurface rock bodies
called aquifers.
Aquifers are rock bodies that act as reservoirs of ground water
and conduits for its movement in the subsurface.
They can function in this capacity because they are:
- porous: they contain open void space (pores)
- permeable: the pores are connected
The permeability of an aquifer to the flow of water is measured by
the hydraulic conductivity.
The best aquifers (i.e. the highest porosities and conductivities)
are comprised of well-sorted gravels and sands, fractured basalts, and
limestones.
The worst aquifers (aquicludes) are clays, mudstones and shales,
and massive crystalline rocks.
An aquifer is divided into two zones:
- lower zone of saturation in which the pores are filled with
water
- upper zone of aeration in which the pores are filled with water
and air
The boundary between these zones is the water table.
The position of the water table is evident from:
- the elevation of water surface in wells
- the elevations of springs, lakes, and streams (where the water table
intersects the land surface
The volume of water and the elevation of the water table in an aquifer
rises and falls over time, due to changes in the balance between:
- the rate of recharge: the rate of water input into the aquifer
from its recharge area
- the rate of discharge: the rate of water output thru springs,
water wells, etc.
When recharge > discharge, the water table rises.
When discharge > recharge, the water table falls.
The water table has a shape that generally mimics the topography of
the land surface.
The shape of the water table can be represented by a contour map that
shows:
- its elevation above sea level
- its relief
- its slope or hydraulic gradient
The relief of the water table creates a downward pressure or head
that drives the flow of ground water.
The flow direction is down the hydraulic gradient of the water
table: that is, from high points to low points and perpendicular to contour
lines of equal water elevation*.
The flow velocity (V) is determined by Darcy's Law:
V = K x G
where:
- K = hydraulic conductivity
- G = hydraulic gradient
Aquifers can be abused by:
- depletion: when water is discharged through wells faster than
it is recharged, the volume of water in an aquifer decreases
- subsidence: of the land surface caused by the withdrawal of
water from a shallow aquifer
- contamination: results from pollution in the recharge area:
pollutants are washed into the ground water by downward-percolating water
KARST
Karstification:
- large-scale dissolution of bedrock by slightly acidic surface and ground
waters
- most common in carbonate rocks, and aided by the presence of permeable
pathways (fractures, faults, bedding planes) in the bedrock
- the features of karst terrains include:
- low stream drainage densities
- poorly integrated drainage systems
- sinking creeks and blind valleys
- sinkholes and sinkhole ponds
- subsurface caverns and tunnels
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