I. Groundwater (continued)

A. Movement of Groundwater

• one relatively common misconception regarding groundwater is that it occurs in underground rivers that resemble surface streams

• although subsurface streams do exist, they are not common

• rather, most groundwater must migrate through pore spaces in rock and sediment -- thus the movement of groundwater is exceedingly slow

• recall that the energy responsible for groundwater movement is provided by the force of gravity, which moves water form areas where the water table is high to zones where the water table is low -- that is, toward a stream channel, lake or spring

• although some water takes the most direct path down the slope of the water table, much of the water flows along curved paths toward the zone of discharge

• water percolates into a stream from all possible directions, with some of the paths turing upward, apparently against the force of gravity, and entering through the bottom of the channel

• such paths are followed because differences in the height of the water table create differences in the groundwater pressure at a particular height

• i.o.w., the water at any given height is under greater pressure beneath a hill than beneath a stream channel -- and water tends to migrate toward points of lower pressure

• thus, the looping curves followed by water in the saturated zone may be thought as a compromise between the downward pull of gravity and the tendency of water to move toward areas of reduced pressure

(1) Darcy's Law

• Henry Darcy, a French engineer (mid 19th century) formulated a law that now bears his name and is basic to an understanding of groundwater movement

• Darcy discovered that if permeability remains uniform, the velocity of groundwater will increase as the slope of the water table increases

• the water table slope is know as the "hydraulic gradient"

• the hydraulic gradient is determined by dividing the vertical difference between the recharge and discharge points (known as the "head") by the length of flow between these points

V = K (b/l)

• where, V represents velocity, h the head, l the length of flow, and K a coefficient that accounts for the material's permeability

• although the rate of groundwater movement if highly variable, a typical rate for many aquifers is about 15 meters per year (4 cm per day); velocities more than 15 times this figure have been measured
  

B. Springs

• a natural flow of water results when the water table intersects Earth's surface - which is called a "spring"

• springs can occur when an aquiclude blocks the downward movement of groundwater and forces it to move laterally -- where the permeable bed outcrops, a spring results

• springs can result when an aquiclude is situated above the main water table -- as water percolates downward, a portion of it is intercepted by the aquiclude, thereby creating a localized zone of saturatation and a "perched water table" -- where the perched water table intersects the side of the valley a spring results

• there are a wide variety of spring types because subsurface conditions vary from place to place

• even in areas underlain with impermeable crystalline rocks, permeable zones may exist in the form of fractures or solution channels -- if these openings fill with water and intersect the ground surface along a slope, a spring will result