MARKD. STEELE MDC Systems, Inc. Berwyn, Pennsylvania
INTRODUCTION AND DEFINITIONS
Contrary to the popular image of caverns and underground rivers, ground water is the water that fills or saturates the soil in a specific location. Water level drawdown describes the effect on the actual or potential water surface of removing water from a well, trench, or sump. The water table is the interface between this saturated water/soil zone (or aquifer) and an unsaturated water/soil zone. The
WATER LEVEL DRAWDOWN 103 concept of an aquifer can be easily seen by filling a jar
with rocks. Water can then be poured into the jar to fill up the empty spaces between the rocks. The level of the water in the jar is analogous to the subsurface water table, and the volume saturated with water is the aquifer. If one then placed a straw in the middle of the jar and began to pull out water, the water table would become depressed in a cone shape around the straw similar to the depression in the water level of a sink as the water goes down the drain. This depression in the water table is known as the cone of depression. The vertical difference, at any given point, between the original (or static) water level and the water surface formed by the cone of depression is called the drawdown.
The goal of this article is to provide an understanding of the key terms and concepts associated with drawdown and an introduction to the uses of drawdown in groundwater hydrology. The meaning of drawdown varies slightly between unconfined and confined (artesian) aquifers.
DRAWDOWN IN UNCONFINED AND CONFINED AQUIFERS
Depending upon whether an aquifer is unconfined or con- fined, drawdown is discussed in terms of either the water table or the piezometric surface, respectively (Fig. 1).
Unconfined aquifers have no impermeable layer between the water table and the ground surface. They can also be called ‘‘free’’ or ‘‘water table’’ aquifers. The water table in these aquifers is in direct contact with the atmosphere through the pores of the unsaturated soil above, and the water table is in balance with the air pressure. These aquifers are influenced by the infiltration of storm water and are also more easily contaminated through the infiltration of chemicals from surface spills.
For unconfined aquifers, the static water level is the height of the water in the well before pumping (Fig. 2). This is often calculated by measuring the depth to the water
Surface elevation Pumping well
Static water level
Drawdown at the well
s0 = H−h0
Drawdown at any radius, r1
s1 = H−h1
Pumping water level
Lower confining layer
r1 h1
h0
H
Figure 1. Drawdown in an unconfined aquifer.
Surface elevation Pumping well
Static piezometric surface
Drawdown at the well
s0= H−h0
Drawdown at any radius, r1 s1 = H−h1
Pumping water level
Lower confining layer
r1
h0
H
Upper confining layer
h1 Piezometric pressure
at radius, r1, from the well
104 WATER LEVEL DRAWDOWN
from the surface elevation. It can be given as ‘‘depth- to-water’’ or, sometimes, can be given as an elevation compared to mean sea level (MSL). In groundwater equations, it is expressed by a capital ‘‘H’’ and is measured, not from the surface down, but from the lower confining layer up. It is a measurement of pressure or head. So, if the depth to water is 26 feet and the surface elevation is 686 feet above MSL, the static water level is at 660 feet above MSL. If boring logs show that the lower confining level is at an elevation of 585 feet, then H= 75 feet (660− 585 = 75).
Confined (or artesian) aquifers have a confining layer, an impermeable bed, between the aquifer and the ground surface. As a result, the water in confined aquifers is often under pressure. Artesian springs are one result of this phenomenon. If a well is drilled into a confined aquifer, the water level in the well will rise to the height of the pressure in the aquifer. Artesian springs occur when the water pressure is such that the water rises to the surface through a fracture in the confining layer. This height, or head, is the piezometric level at the well. A combination of all piezometric or potentiometric heights throughout a confined aquifer is the piezometric surface. The piezometric surface is what the water table would be if there were no upper confining layer.
In confined aquifers, the static piezometric surface is the piezometric surface measured before pumping begins. It is expressed in the same terms as the static water level and is also measured as a pressure, or head, from the lower confining layer.
The pumping water level in a well is a water level in the well during pumping. Like the static water level or static piezometric surface, the pumping water level is measured from the lower confining bed and is a pressure or head in feet or meters. It is usually designated by a lower case ‘‘h.’’ The difference between the static level and pumping level at the well is the drawdown at the well. It is usually designated by s0. Observation wells can be placed
at various distances or radii from the pumping well to observe the drawdown at these locations. The difference between the static level and the pumping level at any radius, r1, from the well is the drawdown, s1, and it is the
difference between H and the pumping water level, h1, at
that radius.
If all the pumping levels could be viewed in cross section, they would form a curve, the drawdown curve similar to these shown in the two figures. Drawdown in three dimensions is an inverted cone known as the cone of depression. The cone of depression defines the distance at which drawdown occurs as a result of the pumping of the well in question. This distance is known as the radius of influence of the well, and the area circumscribed by this radius is known as the area of influence.
IMPORTANCE AND USE OF DRAWDOWN INFORMATION The measurement of drawdown is very useful to the study of the hydraulic characteristics of an aquifer. To understand this importance, it is first necessary to review the basic equilibrium equations for both unconfined
and confined aquifers. The equation for an unconfined aquifer is Q= π K(H 2− h2) ln ri rw (1) where Q= pumping rate K= coefficient of permeability
H= saturated thickness at the radius of influence (static water level)
h= saturated thickness at the well ri= radius of influence
rw= radius of the well
The equation for a confined aquifer is Q= 2π KM(H− h) ln ri rw (2) where
M= thickness of confined aquifer
H= piezometric pressure at the radius of influence (static piezometric)
h= piezometric pressure at the well ri= radius of influence
rw= radius of the well
When these equations are adjusted to solve for K at any two radii, r1and r2, using drawdown at those two radii, s1
and s2, then the equations become
for an unconfined aquifer
K= Q ln r2 r1 π(s1− s2) (3)
and for a confined aquifer
K= Q ln r2 r1 2π M(h22− h12) (4)
These equations can be used as part of a pump test to determine the permeability of an aquifer. The distances to the various confining layers and the thickness of the aquifers (as well as the static levels) should have been determined by soil borings. So, H and M are known. A pumping well is drilled along with one or more observation wells set at known radii from the pumping well. The well is then pumped at a known rate, Q, and time–drawdown data are recorded for each of the wells. From this information, the coefficient of permeability for the aquifer can be developed. This coefficient can then be applied to design other wells in this aquifer or for modeling the aquifer.
READING LIST
Cedergran, H. (1989). Seepage, Drainage and Flow Nets, 3rd Edn. John Wiley & Sons, New York.
Corbitt, R.A. (1999). Standard Handbook of Environmental Engineering, 2nd Edn. McGraw-Hill, New York, pp. 5.32–5.56.