Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
Lecture 22: Groundwater: Pumping Wells and Seawater
Key Questions 1. How does a groundwater well work?
2. What is a cone of depression and what controls it’s size and shape?
3. What problems can occur due to over pumping of groundwater?
4. How does groundwater influence streamflow?
5. What is the Gyben-Herzberg Relation (z = 40h)
6. Why are islands susceptible to sea-water intrusion?
Niigata Japan, 1964 liquefaction
http://www.uwsp.edu/geo/faculty/ozsvath/images/cone_of_depression.htm
Pumping wells create a cone-of-depression in the water table
well screen
the pump sits inside the well screen
water flow
sand gravelWell A Well B
gravelsand
QA QB
cone of depression
QA = QB = pumping rate
gravel
QB
The slope of the cone of depression is determined by the permeability
sand
QA
steep slopehigh permeability
shallow slope
low permeability
QA = QB = pumping rate
The depth of the cone of depression is determined by the pumping rate
sand
QA
low permeability
sand
QBQA < QB
low permeability low permeability
The radius of the cone of depression is determined by the pumping duration
sand
QA
sand
QB
low permeability
QB = QA
rA rB
rA rB<
Study Area
~ 6 sq miles
BC WA
Water table is just below the ground surface
Coarse gravels and sands
Pumping wells can influence neighboring wells
Irrigation Well
irrigation well
Over pumping of irrigation well lowers the water table below the domestic well (water rights)
Groundwater surface water interactions
A pumping well can influence streamflow
Irrigation Wells in the study area
Sate law highly restricts the water rights of farmers
However, State law allows Exempt wells (low use wells). So, farmers could sell there land for subdivisions and many homes can drill a well without needing a water right. This is a problem.
Groundwater “overdraft” mainly due to agricultural irrigation
Central Valley
Over pumping can cause land subsidence
Groundwater “overdraft” mainly due to agricultural irrigation
Ogallala or High Plains aquifer
Ogallala or High Plains aquifer
withdrawal exceeds recharge
Houston region in the state of Texas
Gyben-Herzberg Relation (sea water intrusion)
Δ
=
Δ
=
In a ‘U’ - tube, water seeks its own level
Δ
=
Hydraulic equilibrium between two fluids with contrasting densities
Δ
=
fresh water
sea water
interface between the fresh water and sea water
Δ
=
Δ
=
fresh water
sea water
interface between the fresh water and sea water
Sea water is denser so it displaces the fresh water upward
Δ
=
Δ
=
fresh water
sea water
interface between the fresh water and sea water
h
z
Δ
=
Δ
=
h
z
PB = ρf g (z + h)
ρf = fresh water density ρs= sea water density
ρs ρf
PA = ρs g z
z + h
A B
fluid pressure
Δ
=
Δ
=
h
z
PB = ρf g (z + h)
ρf = fresh water density ρs= sea water density
ρs ρf
PA = ρs g z
z + h
A B
fluid pressure
Δ
=
Δ
=
h
z
PB = ρf g (z + h)
ρs ρf
PA = ρs g z
z + h
A B
PA = PB
hydraulic equilibrium
Δ
=
Δ
=
h
z ρs g z = ρf g (z + h)
ρf = fresh water density
ρs= sea water density
z = ρf
ρf ρs -
solving for “z” yields
h
ρs ρf
PA = PB
if
then
hydraulic equilibrium
Δ
=
Δ
=
h
z ρs= 1.025 g/cm3
z = ρf
ρf ρs -h
ρf = 1.000 g/cm3
then
ρs ρf
z = 40h
Ghyben – Herzberg relation
if
h
z
sea levelwater table
z = 40 h
Ghyben – Herzberg relation
The depth to the freshwater/seawater interface (z) is about 40 times the height of the freshwater above sea level (h).
seawater freshwater
h
z = 40 h
sea levelwater table
sea level
drop in the water table
z = 40 h
rise in the interface
Gyben-Herzberg Relation (sea water intrusion)
Lopez Island
Chloride Concentrations are an indication of seawater intrusion
Greater than 100 mg/L means intruded
Greater than 250 mg/L means above drinking-water standard
Over pumping of coastal wells can cause sea water intrusion
Whidbey Island
Lummi Island