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Ground Water Quantity and Quality: Human Effects
Man, too, affects water level and quality, although it is impossible to clearly separate human factors from natural ones. Pumping wells, for example, can steepen or reverse local flow gradients and draw in contaminants. Pumping wells also affect other pumping wells, but are in turn affected by such natural factors as aquifer boundaries and the presence of surface-water bodies.
Well Interference and Aquifer Boundaries
If two or more wells obtain ground water from the same aquifer, their cones of depression (or pressure relief) will interact if the wells are close enough to one another or are pumped at a great enough rate of discharge. Where cones of depression contact each other, the wells cause mutual interference, as pictured in Figure 41. The drawdown in one well causes some drawdown in the adjacent well and vice versa, so that the pumping levels in both wells are lower than they otherwise would be. Well interference is most typical in gravel aquifers in Maine, but occurs occasionally in domestic bedrock wells. It is very possible that well interference is more common in rock wells than expected, but goes unnoticed. Where two rock wells interfere, it shows that the water-bearing fractures supplying both wells are hydraulically connected. If the interconnection is direct, the interference effect can be very significant, as shown in Figure 42. This figure is drawn from the records of a well on High Head Peninsula, Harpswell, Maine. Of 27 observation wells monitored, only four show certain interference from a nearby pumping well. Three of the four are in the same area of the peninsula where the wells tend to yield more than the local average yield. The interference suggests that the higher yields are related to more numerous interconnections among the water-bearing fractures.
Significant problems with well interference in Maine's bedrock aquifers have not been documented, however, the number of high yielding municipal and other bedrock wells with significant cones of depression is still small.
As has been discussed, pumping wells affect the surrounding water level in the form of a cone of depression (or cone of pressure relief). The shape of the cone of depression is altered by aquifer boundaries, beyond which a significantly different amount of ground water (more water or less water) is available to the pumping well. Less water is available to a pumping well where an aquifer is bounded by poorly permeable material. Examples of this type of boundary interference would be places where a gravel aquifer is surrounded on its sides by clay and underlain by till. Bedrock may also be a poorly permeable boundary to a gravel aquifer, as depicted in Figure 43. For crystalline bedrock aquifers, such boundaries are fracture walls, or distinctly less fractured rock in the case of a zone of highly sheared material.
The opposite type of boundary condition occurs where a well is in close proximity to a surface-water body such as a lake or stream. The lake or stream bottom, in theory, is a boundary of infinitely high permeability. (Fine sediments on the bottom of the lake or stream, however, may make this boundary much less permeable.) Additional water will be drawn from this source if the cone of depression extends to the surface-water body, and if the lake or stream bottom is relatively permeable. Figure 44 illustrates this condition, known as induced infiltration, in which the normal discharge of ground water to a stream has been reversed, and the stream itself loses water to the discharging well. The cone of depression in this case extends farther in the direction opposite the stream because of the greater volume of aquifer in that direction required to supply water to the pumping well.
In some cases induced infiltration contaminates well water. The technique of placing wells where they will benefit from induced infiltration, however, is often used to increase their long-term yield.
All of the various hydraulic parameters such as yield, drawdown, interference, and boundary conditions can be predicted through detailed geologic observation and analysis of pumping tests. For example, the area affected by a given well, pumping at a particular rate for a certain time, can be calculated with sufficient accuracy so that potential well-interference problems and contamination problems can be avoided before they occur.
Water level changes in wells being pumped in Maine typically are of short, or at most seasonal, duration because the long-term withdrawal rate is not greater than the long-term recharge rate. Long-term downward trends in water levels caused by pumping are not known in Maine, although some may be occurring. In other parts of the country, a notable example being the High Plains district of Texas, ground water withdrawals for agricultural purposes have far surpassed natural recharge for many decades. The ground water has been mined (ground water mining), and local water levels have dropped several hundred feet. Ultimately, the local water resource will be depleted by virtue of being too expensive to be pumped hundreds of feet to the surface, and the agricultural activities will change. Over many centuries, the High Plains water table could rise again if most pumping ended.
In Maine, lowered water levels could become bothersome in coastal regions subject to salt-water intrusion, in some of the larger gravel aquifers utilized by several municipalities and commercial firms, and in agricultural areas where there is a large-scale shift to irrigation of field crops. Such situations can be prevented by regulating the annual withdrawal to match the annual recharge.
Ground Water Contamination
Man has an effect on water quality as well as water quantity. Contaminants released to air, water, or land, for example, can find their way into ground water supplies, as suggested by Figure 45. Raindrops form on airborne dust particles and bring them to the ground surface. Compounds are leached out of the particulate matter into the droplets before they reach the surface. Gases are also absorbed by water droplets, and are carried to the ground by rain. Chloride-enriched precipitation is common in the coastal areas, where salt-water spray is blown aloft. Some rain downwind of industrial areas has a low pH, and is referred to as acid rain. A number of lakes in Maine and the Northeast are reported to be decreasing in pH because of acid rain (Davis and others, 1977). The source of some of this contamination may be as distant as the Midwest region. Actual, or potential, effects of acid rain on ground water in Maine are not well known.
Water from polluted streams is sometimes drawn into the ground water system by reversed flow gradients caused by pumping a well adjacent to the polluted stream. At other times, flooding streams carry contaminants onto the floodplains, where they are leached downward into the ground water after the flood has passed.
Leachate from solid and liquid wastes placed on or under the ground surface migrates downward into ground water. Spilled substances such as petroleum products commonly pollute subsurface water.
The opportunity for ground water pollution is present nearly everywhere, not just in the vicinity of populated places. Yet most ground water remains of good quality because of natural cleansing properties of the unsaturated porous substances which may overlie the water table. The presence of air with oxygen to react with biologic and organic pollutants is one of the most significant of natural cleansing factors. Ion-exchange capacity of clay soils and sediments is also very improtant in the removal of such things as metals from recharge water. Absorption of contaminants by soil particles, and adsorption (physical attraction) onto the particles remove many contaminants as well. Microbes are involved with the breakdown and retention of organic pollutants moving through the soil. All in all, porous materials are superior water-reclamation media and generally maintain ground water in a potable state. Man, however, often overwhelms the natural processes available at a given site and contaminates ground water.
Once in the ground water system, contaminants travel the various paths followed by ground water and are sometimes able to migrate considerable distances. Different contaminants travel at different rates and to different distances from the source of introduction to the system. A single large discharge of a particular contaminant may react differently from a small, but continuous discharge of the same waste, and the distance a certain pollutant can migrate through a particular geologic deposit is subject to a variety of chemical, physical, and geological factors.
A critical factor in contaminant travel is the biological and chemical nature of the pollutant. Most pathogenic organisms, for example, are attenuated within 100 feet of their source, but viruses apparently can migrate much farther. Some chemicals, such as nitrate-nitrogen, chlorides, and numerous household and industrial organic chemical compounds (generally, hazardous wastes) are not broken down by physical, biological, or chemical processes that naturally occur in geologic formations and aquifers. Their concentrations are decreased almost entirely by dilution. Thus these materials can migrate great distances in a ground water system at ever decreasing, but still undesirable, concentrations. Other substances, such as lead and various metals adhere to soil particles and generally travel only short distances.
In a general way, less permeable substances, especially those containing clays with available ion-exchange sites (a water softener, for example, exchanges sodium ions for calcium and magnesium ions), are better suited to waste attenuation, while gravel soils are least suited. Furthermore, the thickness of the unsaturated material through which recharge water must pass before becoming part of the ground water body is an important factor. Aquifers underlying a thick, unsaturated soil cover are better protected than those with little or no overburden.
Last updated on March 25, 2009
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