GROUNDWATER HYDROLOGY
HYDROLOGY AND WATER RESOURCES RG744
December 18, 2015
Institute of Space Technology
Institute of
Space Technology
GROUNDWATER (GW)
• Groundwater: A component of Hydrologic Cycle
• Comprises more than 97% of all freshwater on the
earth (not considering water trapped in glaciers
and icecaps)
• Globally more than one-half of the world’s
population depends on groundwater
• Occurring in the saturated zone of a soil profile
Revisit: Some Important terms
• Zone of aeration: below the soil surface pores
contain both air and water. Water stored here is
called 'soil moisture' or 'vadose water'
• Zone of saturation: pores of the soil or rock
saturated with water through gravity drainage
• The top of zone of aeration is called the water
table
• Water stored in zone of aeration is groundwater
BASEFLOW
Groundwater Contribution to Stream
Gaining Stream
Losing Stream
• A stream that is gaining during low-flow
periods can temporarily become a losing
stream during flood stage
Baseflow recession
Q = Qoe-at
Q = flow at some time ‘t’ after the recession started
Qo= flow at the start of the recession
a = recession constant for the basin (1/T)
t = the time since the recession began
• Find the recession constant for the basin
Groundwater Flow
• Groundwater flow: through the rock and soil
layers of the earth until it discharges as a
spring, or as seepage into a pond, lake, stream,
river or ocean
• Similar to drainage basin for surface water, in
groundwater hydrology the concept of
groundwater basin is used
• It is surrounded by groundwater divides
GROUNDWATER (GW)
• Moves very slowly
– Typical average horizontal velocity = 100 meters per
year
– Typical average vertical velocity = one meter per
year (K. R. Rushton)
• Sometimes difficult to extract
• Problem associated with GW are: contamination
and deep pumping may become uneconomical
GROUNDWATER HYDROLOGY
• Groundwater hydrology is important in the field
of surface water flood hydrology
• Soil properties and rate of infiltration affect the
proportions of rainfall as surface runoff and
groundwater losses
• Thus soil properties
hydrologic designs
affect
surface
water
WATER MOVEMENT IN SOIL
• When soil pores filled with water - gravity
dominates
• When field capacity is exceeded water starts
flowing down
GW TERMS
• Aquifer
– Water bearing porous soil or rock strata that yields
significant amount of water to wells
• Aquiclude
– Water bearing soil or rock strata that are effectively
impermeable
– which can absorb water but can not transmit significant
amounts
– such as clay, shales, slates, etc.
• Aquitard
– Rocks that are poorly permeable (silt and mudstone)
GW TERMS
• Aquifuge
– A geologic formation with no interconnected pores
and hence can neither absorb nor transmit water
– Exemples are basalts, granites, etc.
• Water Table
– Undulating plane below the ground surface at which
GW water pressure is equal to atmospheric pressure
(also dividing line between saturated and
unsaturated zone)
CONFINED AND UNCONFINED AQUIFERS
• Aquifers that contain water and is in direct contact with the
atmosphere through porous material are called unconfined
aquifers
• A confined aquifer is separated from atmosphere by an
impermeable layer or aquiclude
• An unconfined aquifer can become a confined aquifer at
some distance from the recharge area
• Confined aquifers, also called artesian aquifers, contain water
under pressure
• Water pressure (P), or pressure potential, is a function of the
height of the water column at a point (hp), the density of
water (ρ), and the force of gravity (g)
P = ρ g hp =γ hp
CONFINED AND UNCONFINED AQUIFERS
• The potentiometric (or piezometric), surface of an artesian aquifer
describes the imaginary level of hydraulic head to which water will
rise in wells drilled into the confined aquifer
• The direction of groundwater flow, or flow lines, can be
determined by constructing lines perpendicular to the water table
contours from higher to lower elevation contours
• The potentiometric surface declines because of friction losses
between points
• When the land surface falls below the potentiometric surface,
water will flow from the well without pumping (artesian or flowing
well)
ARTESIAN WELL
SPRINGS
• Where GWT intersects the topography or
ground surface, springs are formed
AQUIFER CHARACTERISTICS
• The amount of water stored or released from a
water bearing strata depends on porosity, the
size of pore spaces, and the continuity of pores
• Therefore, mapping the GW Flow is not Easy!
AQUIFER CHARACTERISTICS
Porosity: percentage of rock or soil that is void of
material
Porosity (n)= 100Vv/Vt
Where:
Vv= volume of void space in a unit volume of
rock/soil
Vt= total volume of earth material including void
space
AQUIFER CHARACTERISTICS
Effective Porosity: ratio of the void space through
which water can flow to the total volume
• If pores are of sufficient size and interconnected
to allow water to move freely, the soil or rock is
permeable
Aquifer Characteristics
• It is not possible to measure GW velocities within an
aquifer
• Observation boreholes (piezometers) are constructed to
determine the elevation of the water level in piezometer
• The GW head in an aquifer is the height to which water
will rise in a piezometer
• GW head gradients can be used to estimate magnitude
and direction of GW velocities
• The amount of water discharged from an aquifer can be
approximated with Darcy’s law
Darcy Law: Flow through a Porous Medium
• Darcy law states:
“ specific discharge in a porous medium is in the direction of
decreasing head and directly proportional to the hydraulic
gradient”
• Darcy performed a series of experiments on water flow
through columns of sand
• He packed sand in iron pipes and systematically measured
the parameters that he expected to impact the flow
• Darcy found that the total discharge Q varies in direct
proportion to X-sectional area of the column, hydraulic head
difference at each end of the column, and inversely with
length of column
Darcy Equation
• Q α A (h1-h2)/L
• Q = KA (h1-h2)/L
K = hydraulic conductivity
Darcy Equation
• Darcy equation can be rewritten as:
Q/A = -K (h2-h1)/L
V= Q/A = -K (h2-h1)/(l2-l1)
• This can be written more generally as:
q = -K (dh/dl)
Where:
q = Q/A is the specific discharge
(dh/dl) = Hydraulic gradient
Negative sign indicates that positive specific discharge (indicating
direction of flow) correspond with a negative hydraulic gradient
• The Darcy velocity is an average discharge
velocity through the entire x-section of the
column, the actual flow is limited to the pore
channels only
• The seepage velocity Vs is equal to the Darcy
velocity divided by porosity
Vs = Q/nA
• Actual seepage velocities are therefore much
higher (by a factor of 3) than the Darcy velocities
Example Darcy’s Law
Determine the discharge of flow through a well
sorted gravel aquifer. The change in head is 1 m
over the distance of 1,000 m and the crosssectional area of the aquifer is 500 m2. find Q?
Q = K A dh/dL
Q = (0.01 cm/sec ) (500 m2) (0.001 m/m) (0.01 m/cm)
Q = 0.00005 m3/sec or 4.32 m3/d
AQUIFER CHARACTERISTICS
• Hydraulic Conductivity: velocity of flow
through a porous medium resulting from 1
unit of energy head (m/d) (ability of a porous
media to transmit water)
Examples of Hydraulic conductivity
• From Brooks
Material
Hydraulic Conductivity
(cm/sec)
Well sorted gravel
10-2 - 1
Well sorted sands, glacial outwash
10-3 – 10-2
Silty sands, fine sands
10-5 – 10-3
Silt, sandy silts
10-6 - 10-4
Clay
10-9 - 10-6
AQUIFER CHARACTERISTICS
• Transmissivity (m2/unit time): amount of water that
can flow horizontally through the entire saturated
thickness of the aquifer under the hydraulic
gradient of 1m/m
Tr = bkv
Where: Tr =transmissivity (m2/unit time)
b = saturated thickness (m)
kv= hydraulic conductivity of the aquifer (m/unit time)
AQUIFER CHARACTERISTICS
• Specific yield: ratio of
the volume of water that
can drain freely from the
saturated earth material
due to the force of
gravity to the total
volume of the earth
material
Specific Yield
Storativity of confined aquifer
AQUIFER CHARACTERISTICS
Specific retention: ratio of the volume of water
a rock can retain against gravity drainage to the
total volume of rock
– Increases with decreasing grain size
Porosity = Specific yield + Specific retention
Higher porosity not always ensures higher water yield!!!
Clay may have a porosity of 50% with specific
retention of 48%
Other factors that affect water transmission through soil pores
• Interconnectivity of pores
• size of the pores
• Specific yield can be determined in both lab
and field
• Field:
– Water wells are pumped, and the rate at which
the water level falls in nearby wells is measured
Groundwater Development
• Assessing groundwater potential for GW development requires knowledge of
the local geology and aquifers
• Surface features ordinarily does not indicate any sign related to the location,
depth, and extent of water bearing material or strata
• Geological maps can be used to help identify potentially productive water
bearing strata by examining the direction and degree of dipping strata,
locating faults and fracture zones, and determining the stratigraphy of rocks
with different water bearing and hydraulic characteristics
• As a rule, opportunities for GW development increase as one moves from
upland watersheds to lower basins and floodplains
• Extensive and high yielding aquifers occur in the most major river valleys and
alluvial plains
Well
• A vertical hole dug into the ground
• Many types of wells
• Well Point
– Lower end of pipe
• Cone of depression
– Created by pumping water from well that lowers the water table around the well
• Drawdown
– The difference between original water level and the water level after a period of
pumping
• Discharge rate is measured through flow meter attached to the discharge pipe
• Interference
– Locating wells too close together causing more lowering of a water table than
spacing them far apart
MANAGEMENT OF GROUNDWATER RESOURCES
• Means controlled use in accord with some plan
• Continued extraction of GW may create many problems
• Use of GW without consideration to its effects is unwise
• Good management is to minimize the adverse effects of
GW use with good knowledge of the probable effect
• Need to know local area GW conditions (including quality)
and basic research on recharge and movement of GW is
required
• GW can be managed using the concept of safe yield
MANAGEMENT OF GROUNDWATER RESOURCES
• Safe Yield: The rate of water that can be
extracted from an aquifer during anytime period
that do not produce undesirable effects
(excessive lowering of water table, saltwater
intrusion, high pumping cost)
• Water removed from an aquifer in excess of the
safe yield is termed overdraft
When water is extracted at a rate that exceeds the recharge of the
aquifer, the water table is lowered
GW MANAGEMENT
(Sustained withdrawal of GW)
• Water budget analysis to study quantitative aspects of
safe yield
I – O = ΔS
• Where I = inputs to GW (including GW recharge by
percolation of rainwater and snowmelt, artificial
recharge through wells, and seepage from lakes and
streams)
• O = output from GW (including pumping, seepage to
lakes and streams, springs, etc.
• Δ S = change in storage
GW Recharge
• Natural
• Artificial
– Induced infiltration, spreading, recharge wells
• Water moves through aquifer under the
influence of gravity, therefore the zone of
recharge should be higher than areas of
discharge
Water table or potentiometric maps
• Can be shown as contour maps with equal
elevation
Groundwater contours
• Surface water and topography features should be taken
into account
• The datum for water level in wells should be the same
as the datum for the surface topography
• Groundwater contours can never be higher than water
surface contours
• Typically the depth to groundwater will be greater
beneath hills than beneath valleys
• If a lake is present - the lake surface is flat as is the
water table beneath it. Hence the groundwater
contours should go around it
• The only exception is perched lake having surface
elevation above the main water table
• Groundwater contours form a 'V' pointing upstream
when they cross a effluent or gaining stream
• Groundwater contours bend downstream when they
cross a influent or losing stream
• The potentiometric surface of a confined aquifer is not
influenced by the surface topography and surface water
features as there is no direct hydraulic connection between
them
• Potentiometric surface contours can even be above the
land surface
• In areas with shallow water table or potentiometric surface
the groundwater contours will be spaced well apart
• If the gradient is steep, the groundwater
contours will be closer
• Groundwater will flow in the general direction
that the water table or potentiometric
surface is sloping
Gradient of Potentiometer Surface
• Manual contouring is practically always utilized in
GW studies (sometimes in conjunction with
computer-based methods)
• Complete reliance on software contouring may
lead to erroneous results missing interpretation
of
–
–
–
–
Geological boundaries
Varying porous media
Influence of surface water bodies
Principles of GW flow
Triangular Line Interpretation
Steps: Graphical method
Problems with computer-based method
WT Contour maps
Manually drawn
Inverse distance to power method
using super computer program
Recorded WT elevation in feet amsl
GW software
• MODFLOW
– http://water.usgs.gov/software/lists/groundwater/
• GMS
– http://www.aquaveo.com/gms?gclid=CJ-iYO40qYCFRIRfAodDhH4gA
– GMS is a comprehensive groundwater modeling
environment with GIS based graphical preprocessing tools to
automate and streamline the modeling process. GMS
seamlessly interfaces with MODFLOW and several other
famous groundwater models, and provides advanced
graphical features for viewing and calibrating model results.