Transcript Lectures 26, Marshak Ch. 17 “Streams and floods”

WATER
1. Where is it and how much is there?
2. What controls its movement?
3. What is the future availability of water
and the impact of climate change?
Water Cycle
Hydrologic cycle: the movement of water on, above,
and below the surface of the Earth
In addition to shallow ground water, the deep subsurface contains saline (salty)
brines that form from the reaction of water with rock and minerals
Water Cycle
Lots of water in the oceans…not so much freshwater
Reservoir
Volume of Water
(106 km3)
Percent
Oceans
1370
97.25
Ice
29
2.05
Groundwater
9.5
0.68
Lakes
0.125
0.01
Soil moisture
0.065
0.005
Atmosphere
0.013
0.001
Rivers
0.0017
0.0001
Biosphere
0.0006
0.00004
About 80% of river and groundwater use in USA is for irrigation and
agriculture
Temperature and the Water Cycle
Saturation vapor pressure: pressure of a vapor is in
equilibrium with non-vapor (liquid) phases
• Increases with temperature
• 7% increase in atmospheric water vapor with every 1C warming
•**Warmer Air can hold more water vapor: Cooling causes
condensation
•What will be the effect of global temperature increase?
Water Cycle
Cycling of water is linked to global circulation
• Intertropical convergence zone
• Subtropical deserts
WATER – WHERE IS IT AND HOW DOES IT MOVE?
Aquifers and Aquitards
POROSITY = PROPOSTION OF PORES RELATIVE TO VOLUME
PERMEABILITY = INTER-CONNECTEDNESS OF PORES
Porosities of various materials
Lake-bottom mud >40%
Loose sand
Sandstone
Shale
Marble
<3%
Obsidian
<1%
Limestone
Shale
Permeability
Marble
Poorly sorted
sandstone
Granite
When pore spaces
are connected,
fluids (blue)
can flow.
>25%
5 – 30%
<10%
UNSATURATED ZONE
Capillary
action above
saturation level
SATURATED ZONE
Water table head (“H”) is the elevation
of the water table above a base level;
h1 > h2. Because of gradient, water
always is flowing.
Recharge versus Discharge
--controlled by porosity and flow rates
Darcy’s law* measures the volume of water moving along in a given time interval. It is
measured in m3/sec. Darcy’s law states also that the velocity of this movement is
proportional to the hydraulic conductivity and HG (gradient) = Δh/d.
Q (volume) =A (area)*K (permeablilty)*HG (gradient)
where: K=Size of Pipe*Resistance to Flow*Drive for Flow
Tap water
Water towers
and artesian
wells function
by virtue of the
same physical
principles
(“head”).
Oases as an
expression of
aquifers
STRESS ON THE
FRESHWATER SYSTEM
Annual precipitation (above) is low in the higher great
plains and mountain west. Groundwater shortages
are therefore likely in view of the large needs of
agriculture (irrigation). The watershed of the
Colorado River (right) cannot provide enough for the
large metropolitan centers, such as Phoenix, Los
Angeles, or San Diego, in the very near future.
The Ogallala Aquifer
It’s likely to run dry in ~ 20 years
In S. Dakota, the aquifer is
confined.
Profile along A-A’ in Nebraska on map, shows the
unconfined nature of the aquifer.
Excess drawdown lowers
water table, and can cause
wells to go dry
Fresh-water is lighter than salty water.
Saltwater intrusion is caused by over-pumping of groundwater.
Previously
Today
Note the saltwater
intrusion (red arrows)
POLLUTION
Contaminant plumes in crosssection (above) and in map
view (right).
Pall Life Sciences troubles:
•They started in 1986 with the discovery of a dioxane leak
•They involve a possibly carcinogenic substance, dioxane (when
administered in large doses, it is lethal to animals),
but it should not be confused with the very toxic dioxin
•Dioxane spread in subsurface aquifers way beyond expectation
•Underground geological structures are of glacial origin and
difficult to map or predict
•Dioxane may reach the Huron River through the Honey Creek
tributary and can then contaminate all of Ann Arbor’s
drinking water
•Clean up is costing more than $5M per year, but the public is
not satisfied – a major P.R. disaster.
Pall Life Sciences in west Ann Arbor has a dioxane leak problem
in the subsurface that simply won’t go away.
1. Water is pumped from one of the purge wells scattered on Pall's property and surrounding
areas (see next slide).
2. Pre-treated water is stored in a lined lagoon, the Red Pond.
3. During treatment, sulfuric acid is added to the water, lowering the pH to about that of lemon
juice, to make treatment easier. Water is pumped into a machine that exposes it to
hydrogen peroxide and 4.9 seconds of ultraviolet light, destroying most of the dioxane.
After treatment, the pH is raised
back to normal levels with sodium
hydroxide.
Sodium bisulfite is added to help
destroy the excess hydrogen
peroxide.
4. Water goes into the Green Pond and is
retested.
5. Clean water is pumped into a tributary
of Honey Creek, which flows
into the Huron River.
Dioxane is a solvent that is
found in shampoo. It was
used by Pall (then Gelman)
to construct filters.
Freshwater and Climate Change
Freshwater stress is increasing
The hydrologic cycle is intensifying. This means more precipitation and
evaporation, more floods and droughts.
• Growing demand (population growth) and Overuse
• Saline intrusion
•Pollution
•Climate Change Effects
Precipitation
Precipitation distribution
and rate
• Change in distribution of
precipitation toward higher
latitudes
• More heavy precipitation
events (2-4% increase in
frequency of heavy precipitation
events)
Intensification of Water Cycle
Increasing global temperature will intensify the hydrologic cycle
Drought
Droughts have expanded and intensified
• Palmer drought index: measures severity of drought (0= none, -4 =
severe) and rainfall (+4 = extreme)
IPCC, 2007