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Ground water facts

Apr 21, 2017



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    5. Groundwater and Groundwater Measurement Methods CE/ENVE 320-04 Ground water sample acquisition is a key phase of many environmental investigations in natural systems. The sample acquisition process for ground water samples has been previously discussed. In addition to ground water sample collection, an understanding of ground water flow in aquifer systems is critically important in determining ability of a specific aquifer to meet the needs of a public water supply or transport or impede the flow of contaminated ground water due to ground water contamination. This section will begin with an introduction to groundwater from a hydrological prospective. We will introduce some of the basic terms in hydrology and aquifer systems as well as the key components to ground water flow and the important parameters in determining flow rates in ground water systems. Hydrology is the science of water occurrence, movement and transport. Hydrogeology is the part of hydrology that deals with the occurrence, movement and quality of water beneath the Earth's surface. Because hydrogeology deals with water in a complex subsurface environment, it is a complex science. On the other hand, much of its basic terminology and principles can be understood readily by non-hydrogeologists.

    This section presents basic terms and principles of hydrogeology. The first section introduces many key terms and concepts in definition form. Subsequent definitions include graphics to aid in explanation. The following sections introduce principles of a ground water movement, using these terms. Graphics are included to further define terms and illustrate concepts.

    Ground water is water held within the interconnected openings of saturated rock beneath the land surface.

    The hydrologic cycle shows that when rain falls to the ground, some water flows along the land surface to streams or lakes, some water evaporates into the atmosphere, some is taken up by plants,

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    and some seeps into the ground. As water begins to seep into the ground, it enters a zone that contains both water and air, referred to as the unsaturated zone or vadose zone. The upper part of this zone, known as the root zone or soil zone, supports plant growth and is crisscrossed by living roots, holes left by decayed roots, and animal and worm burrows. Below lies an intermediate zone, followed by a saturated capillary fringe, which results from the attraction between water and rocks. As a result of this attraction, water clings as a film on the surface of rock particles.

    Water moves through the unsaturated zone into the saturated zone, where all the interconnected openings between rock particles are filled with water. It is within this saturated zone that the term "ground water" is correctly applied. Ground water is held in aquifers, which are discussed in the following sections.

    Ground water is often thought of as an underground river or lake. Only in caves or within lava flows does ground water occur this way. Instead, ground water is usually held in porous soil or rock materials, much the same way water is held in a sponge.

    Unconfined Aquifers

    In unconfined aquifers, the ground water only partially fills the aquifer and the upper surface of the ground water (the water table) is free to rise and decline. The ground water is at atmospheric pressure. The height of the water table will be the same as the water level in a well constructed in that unconfined aquifer. The water table typically mimics, in a subdued way, the topography of the land surface above, resulting in a water table with hills, valleys, or flat areas. It is

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    important to note that unconfined aquifers, especially those close to the surface, can be vulnerable to contamination from activities on the land surface.

    Confined Aquifers

    Confined aquifers may also be referred to as artesian aquifers.

    A confined aquifer is sandwiched between confining beds (layers of impermeable materials such as clay which impede the movement of water into and out of the aquifer). Because of the confining beds, ground water in these aquifers is under high pressure. Because of the high pressure, the water level in a well will rise to a level higher than the water level at the top of the aquifer. The water level in the well is referred to as the potentiometic surface or pressure surface.

    Even in a confined aquifer, water seeks its own level. Geological strata are not perfectly horizontal. At some point the lithological unit that comprises the confined aquifer is exposed to the surface. This is the aquifer'srecharge zone, and it may be miles away from where one hopes to construct a well. The "confined" aquifer is actually unconfined at the recharge zone. In order for pressure to build, the water level in the recharge zone must be at a higher elevation than the base of the confining unit. When a well is drilled through the confining unit, usually far from the recharge zone, the water in this well will rise to the level of the water at the recharge zone. In some instances this may be above the surface of the ground, in which case the well is called aflowing artesian well. This same situation, where the level of the water at the recharge zone is above the base of the confining unit, leads to the appearance of springsor seeps where the confining unit is penetrated by a hillside.

    It is important to note that confining beds not only serve to hamper the movement of water into and out of the aquifer, they also serve as a barrier to the flow of contaminants from overlying unconfined aquifers. For this same reason, however, contaminants that reach a confined aquifer through a poorly constructed well or through natural seepage, can be extremely difficult and expensive to remove.

    This section introduces the basic concepts of ground water and surface-water interactions, the meaning of water levels in piezometers and wells, and the measurement of water level and discharge.

    Surface Expressions of Ground Water Whether streams, ponds, lakes, or oceans, any surface-water bodies are likely to be surface expressions of ground water. Some, like puddles after a rainstorm, are ephemeral, or short-lived. Others, like the oceans, are perennial, or long-lasting. In either case, we may gain valuable information about subsurface conditions by examining surface-water/ground water interactions. In some cases and under some conditions, surface water is moving toward ground water. This is the case with losing streams, and it also may be true for losing ponds, lakes, wetlands, or puddles. The opposite condition might also exist: Water might be moving from ground water toward the surface "gaining" stream, pond, wetland, or other water body.

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    To determine the direction of movement, consider water levels in wells or piezometers near the surface-water body. The wells or piezometers must be open to the same body of rock or sediment that holds the surface water, and they must be hydraulically connected. A lower level in the wells indicates that the surface- water body is losing, whereas a higher level in the wells indicates that it is gaining (see figure below). Temperature may be another indicator. In temperate regions, ground water tends to be colder than surface water during the summer and warmer than surface water during the winter. Some environments in which this would not be true would be zones of hydrothermal activity (e.g., hot springs), or areas where the surface water originates as melt water from glaciers. Differences in water chemistry might indicate flow directions, as well. Any time a surface-water body appears to be losing water to the ground water, c6nsider the possibility that the surface water may be perched. In this situation, the rate at which water is added to the surface water exceeds the rate at which the underlying rock or sediment can transmit or "drain" the surface water. For example, a heavy rain or sudden snowmelt might cause a temporary pond to form on clay-rich soil. A clay lens near the surface in some glacial materials might cause the formation of a perpetual wetland at the surface, whereas the regional water table lies much farther below.

    Subsurface Expressions of Ground Water Geologists look for outcrops of rock to provide clues about subsurface geology. But where no outcrops exist, their information may come from drillhole data. Likewise, hydrogeologists can learn a great deal from "outcrops" of ground water: springs, seeps, and some other surface-water bodies. But where no hydrogeologic outcrop exists, hydrogeologists must rely on data from holes drilled to puncture the ground water's surface and allow us to examine its nature.

    Why Is This the Water Level? Ask that question any time you encounter a water level, whether surficial or sub-surficial. If surface water, is it gaining or losing? If subsurface, is the water level from an aquifer or aquitard? If an aquifer, is it confined, semiconfined, or unconfined? Is the water level the result of flow from fractures? Is it from a cave, mine, conduit, tunnel, pipeline, or drainage tile? Was the water level measured in a piezometer? A well? What is the screened or open interval in the well?

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    One fairly unusual subsurface expression of ground water is water in caves or caverns. Some of these hold subsurface springs, streams, and ponds. Of course, these features occur only where large underground openings exist, and they are particularly likely to be in karst regions. Other underground conduits might not be natural. For example, such human-made features as tunnels, underground mines, or buried conduits or pipes might be affected by ground water. In these cases, consider how permeable the walls of the opening are, and particularly with mines or pipes, consider whether