THE HYDROLOGIC CYCLE 2: GROUNDWATER

The Hydrologic Cycle - Fresh Water Storage Reservoir % of Total Fresh Water Glaciers (Frozen)76% Groundwater22% Rivers & Lakes < 2% Groundwater – largest liquid fresh water storage reservoir.

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 = (h 1 - h 2 )/L Where: h 1 and h 2 are the elevations of the water table at two locations, and L is the horizontal distance between them h2h2 h1h1

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 Clay Shale Solid Limestone Sandstone Fractured Limestone Gravels & 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 (h 1 - h 2 )/L but we need to account for permeability 2)V = k (h 1 - h 2 )/L where k is a permeability variable, and 3)Q (discharge) = Ak (h 1 - h 2 )/L where A is a measure of cross sectional area

Darcy’ Law Q (discharge) = Ak (h 1 - h 2 )/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 = (h 1 - h 2 )/L = 500’ – 300’/2000’ = 0.1 2) Velocity = k (h 1 - h 2 )/L = 0.01ft/day x 0.1 =.001ft/day 3) Q = Ak (h 1 - h 2 )/L = 40’ x 40’ x.05 x.001 =.08 ft 3 /day

2) Groundwater Contamination Human activity Produces Many Sources of Waste!

Contaminant Plume

Contaminant Plume in Map View