Ground-Water Levels

Ground-water levels are dynamic, changing in response to additions and losess to the system. For example, ground-water levels change in response to pumping. To understand how they change, it is first necessary to understand how they behave under natural non-pumping conditions.

The surface that corresponds to the elevation of the top of the ground-water system is called the water table; it represents the top of the unconfined part of the flow system. This surface can be mapped by plotting the water levels in shallow wells that are open to the interval where the ground becomes fully saturated. It is also possible to map other surfaces - for example, the surface that corresponds to water levels in wells that are open to a specific formation or horizon deeper in the ground-water system. This deeper surface is called a potentiometric surface and often is chosen to correspond to a confined aquifer.

At the water table, ground water flows mostly horizontally from the high areas of the surface to the low areas following the shallow "hydraulic gradient" (an energy gradient that responds to the height of the water and the water pressure). The same is true deeper in the system - water flows mostly horizontally from the high areas of the potentiometric surface to the low areas. However, it is also possible for ground water to flow at an appreciable angle to the horizontal, trending vertically from a higher (or lower) point in one water-level surface to a lower (or higher) point in another water-level surface.

For example, if at a particular location the water-table elevation is at a higher elevation than the potentiometric surface for an underlying layer, then ground water will tend to flow downward along the vertical hydraulic gradient at that point. Elsewhere, the water table elevation might be lower than the deeper potentiometric surface and ground water will flow upward. This last condition is common in valleys or coastlines where rivers or lakes act as ground-water discharge areas for a region. The amount of vertical flow is controlled by the difference in the shallow and deep water level and the resistance posed by the intervening rock.

The next two figures show example surfaces corresponding to, first, the water table in an unconfined system and, second, the potentiometric surface in a deep aquifer. They are each represented in two ways: as 3D "hills and valleys" and as 2D contour lines. Flow is northeastward toward a large lake. The numbers on the contour lines are not important, but if you look closely you might notice that the flow is downward from the water table to the deep aquifer over most of the area, but that it is upward near the Lake:

Drawdown

Ground-water levels not only change laterally and vertically, but also in time. Short-term changes result from seasonal variations in precipitation and recharge. Long-term changes result from pumping. Well pumping at a constant rate causes water levels to drop, quickly at first, then more gradually. For a given amount of pumping, the long-term drop is often less for a well in a shallow, unconfined water-table aquifer than a deep confined aquifer because the shallow wells can obtain part of their water from streams and lakes, which causes water levels to stabilize. Sometimes, however, the drop can be less in the confined aquifer than the shallow aquifer when the former is thicker and can support more flow to the deep
well.

The next figure shows water levels in a deep aquifer after over 100 years of pumping. The bowl-shaped surface is typical of how water-levels drop around pumping centers where many water-supply wells are pumping:

Model output: 3D and 2D contour maps of YEAR 2000 potentiometric surface in St. Peter sandstone for southeastern Wisconsin (source: D.T. Feinstein and J .T. Krohelski, U.S. Geological Survey)

The size of the drop relative to the average condition before there was pumping is called drawdown. Just as water levels can be mapped for different elevations in the ground-water system, so can drawdown be mapped at the water table and for deeper horizons.

The pattern of drawdown around pumping centers is called the "cone of depression". The map below shows the size of two large regional cones of depression along Lake Michigan in 1980. The drawdown corresponds to conditions in a deep aquifer system heavily used for water supply.

Thumbnail map of 1980 drawdown contours in northeastern Illinois and southeastern Wisconsin for the deep sandstone aquifer (LARGE FILE) (source: U.S. Geological Survey Circular 1186)

The cone of depression around Chicago reached its deepest level in the early 1980s before Lake Michigan replaced some of the pumped ground water as a source of drinking water. At its center, the drawdown exceeded 800 ft in 1980, a very large drop. It has rebounded somewhat since then.

In the area around Milwaukee-Waukesha, the maximum drawdown at the center of the cone was almost 400 ft in 1980. In the subsequent years, this cone has deepened and expanded.

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