Ground water/ Hydrogeology Learning Objectives
•Ground water is a noun; groundwater is an adjective
•Understand how soil properties affect hydraulic conductivity
•Understand the difference between pressure head, elevation
head, velocity head and total head
•Understand Darcy’s Law
•Be able to describe the differences among unconfined, perched,
confined and artesian aquifers
•Be able to define relevant groundwater terms including but not
limited to water table, capillary fringe, vadose zone, storativity,
and transmissivity
•Understand the spatial and temporal properties of aquifers and
ground water
•Understand the effects of human activities on groundwater
resources
Ground water factoids
•
Scientists estimate groundwater accounts for more than 95% of
all fresh water available for use.
•
Approximately 50% of Americans obtain all or part of their
drinking water from groundwater.
•
Nearly 95% of rural residents rely on groundwater for their
drinking supply.
•
About half of irrigated cropland uses groundwater.
•
Approximately one third of industrial water needs are fulfilled by
using groundwater.
•
About 40% of river flow nationwide (on average) depends on
groundwater.
Hydrologic Cycle
Zones of Subsurface Water
Zone of Aeration
• pores filled with
both air and
water
• Water held
against gravity by
surface tension
• Soil water/ soil
moisture
• Zone of Saturation
– pores filled only
with water
– Water drained
through soil
under influence
of gravity.
– Ground Water
Essential
components of
groundwater
The rate of infiltration is a
function of soil type, rock
type, antecedent water,
and time.
The vadose zone includes all the material between the Earth’s surface and
the zone of saturation. The upper boundary of the zone of saturation is called
the water table. The capillary fringe is a layer of variable thickness that
directly overlies the water table. Water is drawn up into this layer by capillary
action.
S. Hughes, 2003
How does the geology affect
the existence of ground water?
• What is an aquifer?
A permeable, water-containing unit.
- Water enters from recharge.
- Temporarily stored.
- Leaves by flow to streams (baseflow)
or springs, or to wells
- Has sufficient water to be usable
What is an unconfined
aquifer?
• They are not sealed off at any
point.
• Recharge can occur anywhere.
• Water at water table under
atmospheric pressure.
• Must pump to remove water
Unconfined Aquifer
What is a confined (or
artesian) aquifer?
• Sealed off
• Transmits water down from
recharge area
• Water confined in aquifer unless
drilled.
- Water under hydrostatic
pressure.
- Water rises; well may flow.
Artesian well spouts water above land surface in
South Dakota, early 1900s. Heavy use of this aquifer
has reduced water pressure so much that spouts do
not occur today
Betsy Conklin for Dr. Isiorho
Unconfined and Perched Aquifers
discharge=2000 ft3/s
No tributaries
here
discharge=4000 ft3/s
How is this possible?
Ground Water
and Surface Water
• These are almost always connected
• If a stream contributes water to the
aquifer it’s called a “losing stream”
• If a stream receives water from the
aquifer it’s called a “gaining stream”
• Same stream can be both at different
places or at different times
Perennial
Stream
(effluent)
(from Keller, 2000,
Figure 10.5a)
• Humid climate
• Flows all year -- fed by groundwater base flow (1)
• Discharges groundwater
S. Hughes, 2003
Ephemeral
Stream
(influent)
(from Keller, 2000,
Figure 10.5b)
• Semiarid or arid climate
• Flows only during wet periods (flashy runoff)
• Recharges groundwater
S. Hughes, 2003
Groundwater flow patterns are
controlled by several
factors:
1. Elevation and location of
recharge and discharge
areas.
2. Heterogeneity of geologic
material
3. Thickness of material
4. Current or historic land use
practices
•Presence of streams,
lakes, or springs can
also greatly influence
groundwater patterns.
(Fetter 2001)
How does ground water move?
• Porosity: % by volume
of an earth material
that is pore space.
• Primary porosity
depends upon:
- shape of grains
- arrangement of grains
- size distribution
- compaction/cement’n
• Permeability: ability of
an earth material to
transmit water
• Depends upon
- porosity
- degree and size of
interconnecting pores
between larger pores
What are some typical values
of porosity and permeability?
• Porosity
clay
45-55 %
sand
30-40
sandstone
10-20
shale
1-2
limestone
1-10 (or larger)
• Permeability: varies over several orders of
magnitude. Expressed as a rate, e.g. ft/day
Darcy’s Law
• formulated by Henry Darcy based on the
results of 1855 and 1856 experiments
• stating that the flow rate of water through
porous materials is proportional to the
hydraulic head drop and the distance
(hydraulic gradient )
• The law holds only in laminar flows
laminar flow
• smooth, viscosity dominated flow. the direction of motion at
any point remaining constant as if the fluid were moving in a
series of layers sliding over one another without mixing
• Reynold's number for groundwater
Re = d * V * ρ / μ
where d= grain diameter, (usually d30),
V = velocity
ρ = density of water
μ = viscosity of water
• Laminar flow
Normally Re < 1
Never > 10
Bernoulli's equation
– describes the behavior of a fluid moving along a
streamline
v2 + z + p
2g
g
Datum
Velocity Head + Elevation Head + Pressure
Head
In ground water v is so small that can ignore
Darcys’ velocity v = k i
i = h / L
h = (p1 + z1) - (p2 + z2)
h is the change in hydraulic
gradient between point 1 and
point 2
p1
p2
z1
z2
L
Datum
Darcy’s Law
• V=k*i
– V : Flow Velocity
– k : Hydraulic Conductivity
(The rate at which a soil allows water to move
through it )
– i : Hydraulic Gradient; i = h / L
(Change in hydraulic head per unit of
horizontal distance )
Darcy’s Law
• Q=V*A
= k * i * A = Darcy’s Law
– A : Cross-sectional area of the Soil
– k = saturated hydraulic conductivity
– i = the change in hydraulic gradient
divided by the distance over which the
gradient changed
Hydraulic Conductivity, k
• Soil grain size
• Structure of the soil matrix
– pore size distribution
– pore shape
– porosity
• Type of soil fluid
– fluid density
– fluid viscosity
• Saturation
How to get k (lab)
• Constant-head conductivity test
Q = K i A (remember Q= VA and V = Ki)
i = h / L so Q = K (h / L) A
h
K = Q * (L / Ah)
Q : Discharge
A : Cross-sectional area
L
A
How to get k (field)
• Pumping methods / Tracking methods
• Auger-hole method
– Saturated soil materials near the ground
surface in the presence of a shallow water
table.
– pumping the water out of an auger-hole
extending below the water table and then
measuring the rate of the rise of the water in
the hole
• Tracking with dyes, chemicals, radiation,
or electrical conductivity
(Ward and Trimble 2004)
•Elevation head is the position of a particle relative to some standard
measurement plane, sea level, etc.
•Pressure head is the height of a column of fluid that will produce a
given pressure.
(Ward and Trimble 2004)
•Groundwater contours at streams form a V, they point upstream when they
cross a gaining stream and pointing downstream when they cross a losing
stream.
•When a stream has a greater level than the water in a aquifer the water in
the stream recharges the groundwater. This is common in the western U.S.
where stream begin high in the mountains and flow in alluvial fans formed at
the base.
Monitoring Wells and
Piezometers
Water levels within wetlands
and adjacent to streams
involves investigating
shallow groundwater
regimes.
Monitoring wells: have
perforation extending from
just below the ground
surface to the bottom of
the pipe.
Piezometers: are
perforated only at the
bottom of the pipe.
(WRP TN HY-IA-3.1)
(WRP TN HY-IA-3.1)
•Monitoring Well: Water levels in pipe is the result of intergraded water
pressure along the entire length of pipe.
•Piezometer: Water level inside the pipe is the result from water pressure
over a narrow zone of the pipe.
Instrument Selection
Monitoring Wells
•Determining timing, duration, and frequency of the water
table
•Collected data from various wells used to construct
groundwater contour maps or potentiometric surface maps
Piezometers
•Determining recharge or discharge
•Determining ground water flow direction
•Determine if perching layers are present
Ground water management issues
• Vegetation removal can decrease
interception and ET losses
– Leads to increase soil moisture- higher water
tables
• Ground water extraction exceeding
recharge
– Leads to higher pumping costs
– Land subsidence
– Sea water intrusion if coastal
Ground water management issues
• Urbanization can decrease recharge
opportunity
– Decrease soil storage, decrease low flows
• Ground water contamination
– Improper dumping of contaminants
– Difficult to clean up
Well drawdown
Problems associated with
extreme groundwater withdrawal
• Dry Wells – cone of depression already seen
• Saltwater contamination near coast
• Groundwater withdrawal causes saltwater to
be drawn into coastal wells, contaminating
supply
• Contamination of Wells with sewage
• Formation of Collapse Sinkholes
Saltwater contamination due
to excessive well pumping
Sinkholes in Urban Settings
• What happens when a new well here is
heavily pumped?
Flow
direction can
change
Groundwater Overdraft
Overpumping will have two effects:
1. Changes the groundwater flow
direction.
2. Lowers the water table, making it
necessary to dig a deeper well.
• This is a leading factor in desertification in
some areas.
• Original land users and land owners often
spend lots of money to drill new, deeper
wells.
• Streams become permanently dry.
S. Hughes, 2003
Groundwater Overdraft
• Almost half the U.S. population uses groundwater as a
primary source for drinking water.
• Groundwater accounts for ~20% of all water withdrawn for
consumption.
• In many locations groundwater withdrawal exceeds natural
recharge rates. This is known as overdraft.
•
In such areas, the water table is drawn down
"permanently"; therefore, groundwater is considered a
nonrenewable resource.
• The Ogallala aquifer underlies Midwestern states, including
Texas, Oklahoma, and New Mexico, while California, Arizona
and Nevada use the Colorado River as their primary water
source. All show serious groundwater overdraft.
S. Hughes, 2003
Groundwater Overdraft in the Conterminous U.S.
(from Keller, 2000, Figure 10.13a)
S. Hughes, 2003
where current ground moisture is significantly
lower than the long-term average,
http://www.circleofblue.org/waternews /wp-content/uploads/2011/12/GRACE_GWS.png
Groundwater Overdraft
• Water-level changes in the Texas-
-Oklahoma-High Plains area.
• The Ogallala aquifer -- composed
of water-bearing sands and gravel
that underlie about 400,000 km2.
• Water is being used for irrigation at
a rate up to 20 times more than
natural recharge by infiltration.
• Water level (water table) in many
parts has declined and the resource
eventually may be used up.
(from Keller, 2000, Figure 10.13b)
S. Hughes, 2003
Pollution of Ground Water
• pesticides, herbicides, fertilizers: chemicals
that are applied to agricultural crops that can
find their way into ground water when rain or
irrigation water leaches the poisons
downward into the soil
• rain can also leach pollutants from city dumps
into ground-water supplies
• Heavy metals such as mercury, lead,
chromium, copper, and cadmium, together
with household chemicals and poisons, can
all be concentrated in ground-water supplies
beneath dumps
www.geology.iupui.edu/Academics/.../G110-10-Ground_Water.ppt
Pollution of Ground Water
• liquid and solid wastes from septic tanks, sewage
plants, and animal feedlots and slaughterhouses
may contain bacteria, viruses, and parasites that can
contaminate ground water
• acid mine drainage from coal and metal mines can
contaminate both surface and ground water
• radioactive waste can cause the pollution of ground
water due to the shallow burial of low-level solid and
liquid radioactive wastes from the nuclear power
industry
www.geology.iupui.edu/Academics/.../G110-10-Ground_Water.ppt
Pollution of Ground Water
(cont.)
• pumping wells can cause or aggravate
ground-water pollution
Water table steepens near a dump, increasing the velocity
Water-table slope is reversed by pumping, changing
of ground-water flow and drawing pollutants into a well direction of the ground-water flow, and polluting the well
www.geology.iupui.edu/Academics/.../G110-10-Ground_Water.ppt
Balancing Withdrawal and
Recharge
• a local supply of groundwater will last
indefinitely if it is withdrawn for use at a
rate equal to or less than the rate of
recharge to the aquifer
• if ground water is withdrawn faster than it
is being recharged, however, the supply is
being reduced and will one day be gone
www.geology.iupui.edu/Academics/.../G110-10-Ground_Water.ppt
Balancing Withdrawal and Recharge
• heavy use of ground water can result in:
• a regional water table dropping
• deepening of a well which means more electricity
is needed to pump the water to the surface
• the ground surface settling because the water no
longer supports the rock and sediment
1925
1955
Subsidence of the land surface caused by the extraction
of ground water, near Mendota, San Joaquin Valley, CA.
Signs on the pole indicate the positions of the land
surface in 1925, 1955, and 1977.
The land sank 30 feet in 52 years.
www.geology.iupui.edu/Academics/.../G110-10-Ground_Water.ppt
Ground Water in Space and Time
Groundwater Hydrograph
•Seasonal water level fluctuations reflect variations in
precipitation, evaporation, and transpiration.
•Periods of rising water levels indicate recharge
Thurston County
Mean
Balancing Withdrawal
and Recharge
• to avoid the problems of falling water
tables, subsidence, and compaction, many
towns use artificial recharge to increase
recharge; natural floodwaters or treated
industrial or domestic wastewaters are
stored in infiltration ponds in the surface to
increase the rate of water percolation into
the ground
www.geology.iupui.edu/Academics/.../G110-10-Ground_Water.ppt
Ground water chemistry
• One of the most important natural changes
in groundwater chemistry occurs in the
soil. Soils contain high concentrations of
carbon dioxide which dissolves in the
groundwater, creating a weak acid
capable of dissolving many silicate
minerals
Ground Water vs
Surface Water Quality
• GW quality, temperature and other parameters are less
variable over the course of time than SW
• Range of groundwater parameters encountered is much
larger than for surface water, e.g., total dissolved solids
can range from 25 mg/L in some places in the Canadian
Shield to 300 000 mg/L in some deep saline waters in
the Interior Plains.
• At any given location, groundwater tends to be harder
and more saline than surface water.
• It is also generally the case that groundwater becomes
more saline with increasing depth, but again, there are
many exceptions.
Ground water and geology
• Ground water is also important quite apart from its value
as a resource or its close connection with surface water
supplies.
• Ignoring the effect of ground water on slope stability can
be both costly and dangerous.
• The fluid pressures exerted by ground water, for
example, play an important role in the occurrence of
earthquakes.
• Geologists also know that the movement of water
through underground geologic formations controls the
migration and the accumulation of petroleum and the
formation of some ore deposits.
Ground water as energy source
• Ground water may be used as a source of heat. Ground
source heat pumps are receiving increased attention as
energy efficient commercial and residential heating/cooling
systems. Although initial costs are higher than air source
systems – due to the additional costs of the underground
installations – the much greater energy efficiency of ground
source systems makes them increasingly attractive.
• Research into the use of geothermal water has been carried
out in a number of institutions across Canada. The City of
Moose Jaw has developed a geothermal heating system for a
public swimming pool and recreational facility. Carleton
University in Ottawa already uses groundwater to heat and
cool its buildings. The Health Centre complex in Sussex, New
Brunswick has been utilizing an aquifer for thermal energy
storage since 1995.
Hydraulic fracking
• 2 million to 4 million gallons of water per well, as well as
a variety of chemicals—some of them toxic—to reduce
friction, prevent corrosion, and kill bacteria in the well.
• 2 million to 4 million gallons of water, as well as a variety
of chemicals—some of them toxic—to reduce friction,
prevent corrosion, and kill bacteria in the well.
• After tapping the well, fracking chemicals are pumped
out along with any naturally occurring water. This “flowback” water is often temporarily stored in open-air pits
that, while lined, can leak or overflow during heavy rains.
GAS WELL
• Shale gas deposits are far deeper than
freshwater aquifers, reducing the
potential for groundwater contamination
from fracking chemicals, there have
been incidents of aboveground chemical
spills and gas leaks into well water
• One NAS study in PA and NY found
higher concentrations of methane in
water wells within 1 km of gas wells.
• No evidence of fracking fluids
• Few studies, much contention
• Uses large amounts of water to inject
chemicals
Washington urls
• http://dnr.metrokc.gov/wlr/wq/groundwater.
htm King County Groundwater
• http://www.ecy.wa.gov/programs/eap/gr
oundwater/resources.html WA DOEGW assessment
• http://apps.ecy.wa.gov/welllog/
WA DOE- well logs