Transcript Groundwater

THE HYDROLOGIC CYCLE 2:
GROUNDWATER
The Hydrologic Cycle - Fresh Water Storage
Reservoir
% of Total Fresh Water
Glaciers (Frozen)
76%
Groundwater
22%
Rivers & Lakes
< 2%
Groundwater – largest liquid fresh water storage
reservoir.
The Groundwater System
The Groundwater System
Gaining Streams
Losing Streams
The Water Table Mirrors Surface Topography
Groundwater Flows From High Pressure To
Low Pressure Areas
Usually, this means Down Gradient
So, To Understand Groundwater
Flow We Need To Know:
1) Depth of the water table.
2) Gradient (slope) of the water
table.
3) Characteristics of the rocks
that the groundwater is
traveling through.
Depth and Gradient: Gradient determines the
direction of groundwater flow.
Gradient = (h1- h2)/L
Where: h1and h2 are the elevations of the water
table at two locations, and L is the
horizontal distance between them
h1
h2
Rock Characteristics:
1) Porosity – How much water the rock will hold.
2) Permeability – The ability of a rock to transmit
water. Depends on size and connectivity of pores.
Porosity
A) Sediments
Soil
55% by volume
Clay
45
Sand
35
Gravel 20
B) Rocks
Shale
5%
Limestone 5
Sandstone 15
Granite
1
Fractured
Limestone 20
Permeability
Low
Clay
Shale
Solid Limestone
Sandstone
Fractured Limestone
Gravels & Sands
High
The first person to study these characteristics of the
groundwater system was a geologist named Henri
Darcy.
Darcy noted that the velocity of groundwater flow was
proportional to the gradient of the water table.
1) V α (h1- h2)/L but we need to account for permeability
2) V = k (h1- h2)/L where k is a permeability variable, and
3) Q (discharge) = Ak (h1- h2)/L where A is a measure
of cross sectional area
Darcy’ Law
Q (discharge) = Ak (h1- h2)/L = AV
Pore
Remember, when calculating cross – sectional area the water is
only flowing through the open pores (unlike a river channel). So
A (area) = width X depth X porosity.
Problem
1) What is the gradient of the water table whose
elevation is 500’ at well 1, 300’ at well 2, and
the wells are 2000’ apart?
2) If the aquifer has a width of 40 feet, and a
depth of 40 feet, a porosity of 5%, and a k =
0.01 ft/day, what is the groundwater velocity
and discharge?
1) Gradient = (h1- h2)/L =
500’ – 300’/2000’ = 0.1
2) Velocity = k (h1- h2)/L =
0.01ft/day x 0.1 = .001ft/day
3) Q = Ak (h1- h2)/L =
40’ x 40’ x .05 x .001 =
.08 ft3/day
2) Groundwater Contamination
Human activity Produces Many Sources of Waste!
Contaminant Plume
Contaminant Plume in Map View