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

h2

h1

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.

PorosityA) SedimentsSoil 55% by volume

Clay 45Sand 35Gravel 20

B) RocksShale 5%Limestone 5Sandstone 15Granite 1FracturedLimestone 20

Permeability

ClayShaleSolid LimestoneSandstone Fractured LimestoneGravels & Sands

High

Low

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). SoA (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