An Introduction to Ground Water Hydrology

Section 2 - General ground water principles

Ground Water Flow

Flow Potential and Hydraulic Gradient

Precipitation falling on the land at any elevation above sea level is drawn by the force of gravity to lower and lower elevations, and finally back to the sea. Water carried aloft and precipitated at higher elevations acquires potential energy, or flow potential, that is given up as it journeys downward. Whereas surface water loses its potential very rapidly in time, ground water may retain the potential to flow for long periods of time. A stream flows at several feet per second, while ground water in the adjacent soil and rock flows only several feet per year. Were it not for the slow rate of water movement through the porous materials of the earth's crust, ground water would not occur at high elevations and within only a few feet of the ground surface; it would pass back to the oceans as rapidly as streamflow. Because ground water is maintained at higher elevations under hills, the water table tends to retain the general form of the surrounding topography, and is often referred to as a "subdued replica of the land surface."

The elevation of the water table in Figure 8 indicates the relative potential for ground water flow. Flow always follows the ground water slope, or hydraulic gradient, from areas of high potential to areas of low potential. This hydraulic gradient is maintained because of the slow downward movement of ground water. During prolonged drought, however, water tables lower and hydraulic gradients become flatter and flatter until there is much less potential for water movement. Note that Figure 8 and the following figures are highly exaggerated in the vertical dimension and that in reality hydraulic gradients are less, or flatter, than shown.

Recharge and Discharge

High potentials occur where water is added (recharged) to a ground water system, and low potentials occur where ground water is removed (discharged). This relationship is illustrated in Figure 8 where the water table is shown as being higher under the hills and sloping downward to where it intersects the surface of the water in the stream and swamp. Lines representing typical flow paths (also called flow lines) show that ground water is moving down away from the hilltops and up towards the stream and swamp into which ground water discharges. The stream and swamp are discharge areas, and the hilltops are recharge areas. There are a great variety of geologic situations that determine whether an area is one of recharge or discharge. Man-made situations, such as pumping a well, also affect recharge and discharge areas. Some of these complexities are discussed later in this handbook.

Flow Divides

Topography has much to do with the movement of both surface and ground water. Just as streams have drainage basins that are defined by divides that follow ridge lines, ground water flow systems are defined by ground water flow divides. The simplest type of ground water divide occurs beneath a hill where the gradient of the water table indicates that ground water flows essentially in opposing directions, similar to rain that falls on the hilltop (Figure 8). Usually in Maine, surface water divides and ground water divides coincide along ridge lines, but the situation illustrated in Figure 9 is also possible.

Ground water flow divides also occur where two directions of flow converge in a region of discharge, such as beneath the stream and swamp in Figure 8. The shallow ground water flow does not cross under, but discharges into the stream and swamp. The analogy with overland runoff can be applied here, as with the ridge line described above. Streams are not recognized as surface water divides, but it is obvious that overland runoff cannot cross from one side of a stream to another, and that, in fact, two directions of overland runoff converge at a common region of flow into a larger surface water body.

Shallow and Deep Flow Systems

Ground water divides are imaginary surfaces across which flow does not take place. In Figure 8, shallow ground water flow does not take place from the right to the left side of the central hill, nor from the right side of the stream to the left side. Deeper ground water flow, however, does pass under the hill and the stream and swamp. This deeper, or intermediate flow system bypasses local, shallower points of recharge and discharge. This ground water is derived from some recharge area outside of the figure; it will ultimately discharge at some point, probably the river to which the small stream shown in the figure drains. Still deeper, regional flow systems may occur in the underlying materials, whether loose sediments or fractured bedrock. All of these flow systems may occur within a ground water drainage basin. Overall ground water flow within the basin is from the higher elevations to the mouth of the stream that drains the basin. Many local ground water divides and directions of ground water flow occur within this larger drainage basin.

Most ground water flow patterns in Maine could not be classified as being "ideal"; rarely are the unconsolidated deposits texturally similar throughout (homogeneous) and equally permeable in all directions (isotropic), and bedrock is probably never so. The change in permeability between a sand or gravel overburden and the underlying bedrock can be very large. In some cases, the bedrock may be more permeable than an overlying unit, but for the most part, the poor permeabllity of the bedrock surface has a strong influence on the directions of ground water movement in overlying sediments that are about 20 feet or less thick (typical of Maine). Water will follow the path that offers the least resistance to its movement; thus, it will flow more freely in the more permeable overburden. Figure 10 indicates how contrasts in permeablility affect general directions of ground water movement.

Within the overburden, intermediate flow systems are usually controlled by the bedrock surface topography, as suggested by Figure 8. Ground water flow within the fractured bedrock is influenced by the overall topography of the land surface, the topography of the bedrock surface, and the geometry of the fracture systems. Faults have definable directions, and joints occur in what are called "sets," that have similar map directions and attitudes in space (dips). The fracture sets and their interconnnections give bedrock preferred directions of permeability. On a local scale, fractured rock is strongly anisotropic, that is, permeability parallel to an open fracture is very high, but is essentially zero perpendicular to that fracture (Figure 11). On a regional scale, however, fracture geometry has less influence on overall ground water movement than has the configuration of the land surface.

Flow Near Streams and Lakes

Ground water movement in the vicinity of a stream flowing through an area blanketed by unconsolidated material is shown in Figure 12. The water table intersects the stream at its surface and rises away from the stream. Ground water discharges across a seepage face from the stream banks and directly into the stream itself. Thus ground water becomes surface water. This condition is called a gaining stream and is the normal situation in Maine, whether the stream crosses consolidated or unconsolidated materials. Under drought conditions surface water seeps downward through the stream bottom to become ground water. This condition is called a losing stream.

Lakes, like streams, tend to receive ground water inflow through at least some of their shore and bottom. As shown in Figure 13, ground water inflow is in addition to surface water inflow. Water loss occurs as normal outflow and evaporation. Figure 13 shows a typical lake in a bedrock depression where unconsolidated sediments cover most of the shore and lake bottom. Ground water flow occurs both in these unconsolidated sediments and in the adjacent bedrock.

A kettle pond is quite a different type of lake, as it usually occurs entirely within a permeable sand and gravel deposit (Figure 14). Such a pond typically has a very limited watershed area from which surface water flows into it. Its water level is representative of the local water table elevation, and most water flowing into the pond is ground water. A kettle pond may lose water by ground water outflow, as indicated in Figure 14. Other kinds of lakes also may contribute to ground water flow over some or all of their shore and bottom.

Flow Rates and Quantities

Speed of ground water flow through a porous substance is related to the hydraulic conductivity of the material through which it passes, the hydraulic gradient, and the effective porosity of the material as in the following relationship:

v = K i / n_e

where: v = average seepage velocity (length/time)
             K = hydraulic conductivity or permeablility (length/time)
              i = hydraulic gradient (length/length), and
            n_e = effective porosity (dimensionless)

In general, the higher the permeablility and gradient, the more rapid the rate of flow. At the same hydraulic gradient, flow through a coarse clean sand is more rapid than through a less permeable clayey fine sand. Similarly, flow along a zone of fracturing is more rapid than through poorly fractured becdrock that is much less permeable. At a given permeability, flow is most rapid where the gradient is steepest such as near points of ground water discharge that include streams, springs, and pumping wells. Hydraulic gradient, however, is not only indicative of flow velocity, but is also indicative of the potential expended in moving ground water from one point to another. Clay is so poorly permeable that a steep gradient must develop before water flows through it. A coarse gravel deposit commonly has a very flat gradient, indicating that little potential is expended in moving water from one place to another. Of the two, permeability and gradient, permeability is usually the more important factor for determining the speed of ground water movement. Typical ground water flow rates in Maine within marine clay deposits are 0.02 to 0.8 feet per year, within glacial till deposits 5 to 11 feet per year, and within sand and gravel deposits 40 to 100 feet per year.

Quantity of ground water flow through a porous substance is related to the hydraulic conductivity, hydraulic gradient, and cross-sectional area of the material as in the following relationship:

Q = K i A

where: Q = quantity of flow (length³/time)
            K = hydraulic conductivity (length/time)
             i = hydraulic gradient (length/length), and
            A = cross-sectional area (length²)

In terms of ground flow through porous materials, their saturated thickness and breadth are just as important as the permeability and gradient. Under the same conditions of permeability and gradient, a broad sand and gravel deposit conducts a greater volume of ground water than does a narrow bedrock fracture zone.

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Last updated on March 25, 2009

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