lecture 22: groundwater: pumping wells and seawatergeology.wwu.edu/rjmitch/l22_seawater.pdf ·...

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