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Ground-Water Quantity and Quality: Natural Factors
Ground water fluctuates considerably, both in regard to water level and water quality. Many of these changes are natural and occur continuously. Most of the changes are either small or temporary, although some events, for example a significant climatic change, can have a considerable effect on ground water availability and movement.
The most obvious change in ground water levels is related to day-to-day and seasonal variations in precipitation. Levels are usually highest in spring and lowest in summer, but both the level and the timing of ground water highs and lows can vary from year to year as shown in Figure 30. Over several decades, minor changes in climate can cause noticeable variations in typical ground water levels. There are droughts of several years' duration, and longer periods of more or less, precipitation.
Water wells respond to rainfall by showing a higher static water level. The degree to which a given rainstorm affects a particular well depends on numerous variables, especially those related to infiltration rate. However, the basic nature of the well, gravel or rock, water-table or artesian, also causes significant variation in the degree and rate of response of a well.
Most wells constructed in unconsolidated overburden respond to rainfall within hours of the onset of the storm, assuming the storm is large enough to satisfy local soil moisture deficits. These wells are typically water-table wells in which the downward-moving recharge water has direct access to the water-bearing zone. Shortly after a rainstorm, the water table rises, and dug wells show a positive response (Figure 31). Artesian wells constructed in unsonsolidated sediments tend to respond more slowly to rainfall, possibly several day or weeks later, because of the poor permeability of the confining layer.
Rock wells usually respond to rainfall within several days or weeks, similar to a confined aquifer in unconsolidated materials. The exception occurs where rock wells penetrate an extensively fractured material in an area of thin overburden. The numerous fractures are more likely to be open to the surface and to carry recharge water directly down to the saturated zone. Even though drilled into bedrock, such wells are of the water-table type and respond to rainfall much as do water-table wells in unconsolidated sediments (Figure 32).
Ground water quality varies seasonally, and often with each rainstorm. During the periods of significant recharge, ground water tends to be less mineralized; that is it is diluted by the fresher recharge. Soluble constituents in overburden could, however, cause recharge to be highly mineralized. Recharge raises water levels and increases flow rates, thereby reducing travel time within a porous material. Late summer, when water tables (and potentiometric surfaces) are at their lowest seasonal level, is the period when ground water is most mineralized. Coastal wells are also likely to be highest in chlorides during this period because the water table is at its lowest level, and the salt-water interface has migrated significantly landward. Increased recharge following a rainstorm often freshens a ground water supply. For example, not only does a spring increase in discharge after a rainfall, but its water quality responds in a variety of ways to the recharge, as indicated in Figure 33.
Aquifer Composition and Length of Flow Path
Water stored in and moving through earth materials dissolves many substances from the surfaces of the interstices and fractures. The length of time a given volume of water is in contact with certain soil or rock types, the degree of consolidation of the water-bearing material, and the mineral makeup of the porous materials affect the quality of water obtainable from the subsurface.
Shallow and deep ground water flow systems have significantly different flow-path lengths that affect water quality. Shallow flow passes into and out of the subsurface in a much shorter distance that deeper flow. Even assuming rates of flow in a shallow and deep system are similar, ground water in the deeper system is in contact with the rock strata for much longer periods than water in the shallower system. In fact, ground water in the deeper system is likely to flow more slowly than that in the shallower system. As a general rule, ground water derived from greater depths tends to be more highly mineralized than water derived from near-surface sources. Water recovered from a recharge zone (where ground water is moving downward) is usually less mineralized than water recovered from a discharge zone (where ground water is returning to the surface). Residence time refers to the period of time that a given droplet of ground water is in contact with rock and soil from which it dissolves various chemical constituents. The longer the residence time, whether caused by distance traversed, slow rate of movement, or both, the greater the degree of mineralization of ground water obtained from a particular rock or soil type.
The amount of consolidation of water-bearing materials plays an imortant part in ground water quality. The more surface area to which ground water is exposed, the greater the opportunity for chemical and physical reactions to take place. In unconsolidated material this surface area is porportional to the porosity of the material; thus, a highly porous clay material exposes more surface area to ground water held within it than does a more permeable, but less porous, gravel. Crystalline rocks have very low porosity; thus, little surface area is exposed to ground water contained in rock fractures.
Ground water chemistry often varies according to chemical composition of the soil and rock through which it flows. Limey soils and rocks release calcium ions to ground water. Materials bearing iron sulfide release iron, and possibly manganese and sulfur compounds, to ground water. Granites may contribute constituents, including fluoride and radon gas, to ground water.
Hardness refers to the ability of water to form suds with a soap. Hard water causes difficulty in making suds, leaves a ring in the bathtub, forms soap curds in clothing, and builds up scale in boilers and tea kettles. It is caused mostly by calcium and magnesium carbonates dissolved from soil and rocks, most especially those composed of, or containing, limestone.
The ability of water to conduct an electric current is called specific conductance. The more mineralized a water, the lower is its resistance to the flow of an electric current, and thus, the higher its specific conductance. The parameter is measured very quickly and easily with an electronic instrument that is used in a laboratory or lowered down a well.
The hydrogen ion concentration in water is indicated by its pH. Seven is neutral pH, greater than seven is alkaline, and lower than seven is acidic. Most ground water tends to be acidic, some to the extent that it attacks copper plumbing, as well as the rocks containing it.
Alkalinity is a measure of the ability to neutralize acid. It is related to the presence in the water of certain salts and carbonate and bicarbonate ions.
Excess chlorides in ground water cause it to taste "salty." Sea spray, salt water intrusion, de-icing salts for highways, and animal wastes contribute chlorides to ground water.
Iron and manganese are dissolved from soils and rocks containing various minerals, commonly iron and manganese sulfides. Such rock types are common in Maine, causing much of our ground water to be high in these two substances. They are chemically similar, thus generally occur together. Iron and manganese are not toxic, but impart objectionable taste to water and may leave brown stains on porcelain and in clothing.
Nitrogenous compounds are usually derived from animal and plant materials, but are contributed also by fertilizers in agricultural and urban areas. Nitrate is the most common form of these compounds to occur in ground water. Human and other animal wastes are the cause of serious nitrate pollution in various parts of the world.
Many rocks in Maine contain uranium. As uraniuim decays, one of the products is radon, a radioactive gas. This gas can diffuse into ground water and is found in many bedrock wells, sometimes in very high concentrations. Radon gas in a domestic water supply will, in turn, diffuse into air of a home. High radon levels in homes is considered to be a cause of cancers of the respiratory system.
Air Pressure and Tides
In addition to rainfall, changes in atmospheric pressure and variations in ocean tides in the coastal region affect artesian wells. Some respond even to minute changes in the gravitational attraction of the sun and moon.
Ground water in an artesian aquifer is in direct contact with the atmosphere only where a well is drilled. A change in air pressure acts only on the water surface exposed in the well. A rise in air pressure (more weight of air) causes the water level in the well to go down. A drop in air pressure causes a corresponding rise in the water level in the well (Figure 34). The ratio of air-pressure change to change in well water level is called the barometric efficiency of the artesian aquifer. It is most often given as a percent. If air pressure changes an equivalent of six feet of water, and the well's water level changes three feet, the barometric efficiency of the aquifer is 50%.
Barometric efficiency is a measure of the rigidity of the confining layer. The more this confining layer is able to resist change due to variation in air pressure, the greater the effect of the air pressure on the water in a well penetrating the aquifer. Thirty feet of shale overlying an artesian aquifer would be more rigid than three feet of clay, and a well in an aquifer underlying the shale would have a higher barometric efficiency.
Artesian wells within several miles of the coast respond to the changing tides because of the load of seawater placed, and then removed, on the confining strata, and because of the decrease in the ground water discharge to the ocean basin at high tide. Tidal efficiency is the ratio of feet of tidal change to feet of change in well-water level, and is usually given as a percent. If a nine foot tide causes a three foot change in water level, the aquifer tapped by that particular well has a tidal efficiency of 33%. The weight of the tide acts on the aquifer, rather than on the level of water in the well, just the opposite of the atmospheric pressure (Figure 35). Tidal efficiency is a measure of the flexibility of the confining layer. In theory, the barometric and tidal efficiencies of a particular aquifer should together equal 100%, or 1.
These two kinds of aquifer efficiencies are useful for measuring the degree of confinement of a particular aquifer. The reaction of three coastal bedrock wells to the same tidal forces (the three wells are approximately equidistant from the coast), shown in Figure 36, suggests large differences in the nature of the water-bearing fractures intersected by each well. Comparison of water-level changes in these three wells to rainstorms indicates that the rock fractures must be more open to the land surface and recharge in the two wells showing the least tidal efficiency.
Proximity to the Ocean
Fresh water occurs beneath the continent and moves slowly in the direction of the ocean, where it comes into contact with salt water that saturates the intergranular spaces and rock fractures of the ocean basin. The line of contact is called the fresh-water/salt-water interface. Its location is determined by two factors: (1) the difference in density between fresh and salt water and (2) the flow potential of the fresh ground water.
Fresh water is less dense than salt water. Where both occur in a saturated material, the fresh water tends to float as a lens on top of the salt water. The situation is analogous to an iceberg that floats in the sea with about 75% of its bulk below ocean level. Figure 37 is a simplified illustration of an oceanic island where ground water derived from and continiuosly replenished by rainfall saturates the subsurface. The water table is typically higher under the center of the island and slopes downward to where it intersects the sea. The height of the water-table mound above sea level determines the thickness of the fresh water lens under the island. The higher the water table, the deeper the lens of fresh water. Thus, the greater the elevation of the land above sea level, and the greater the amount of precipitation falling on the land, the larger the volume of fresh water available on the island.
The relationship know as the Ghyben-Herzberg Ratio is a calculation of the static (non-flowing) relationship between the column of fresh water and the column of salt water pictured in Figure 37. These two columns are equal in weight. Their difference in height is caused by the difference in density between salt water and fresh.
If "t" is the height of the water table above sea level (in Figure 37), and "h" is the thickness of the fresh water lens below sea level, the following relationship can be derived where the density of sea water is 1.025 and fresh water is 1.000:
(h + t) x 1.000 = (h) x 1.025
A more detailed view of the fresh-water/salt-water relationship is illustrated in Figure 38. The interface is on the seaward side of the coastline. Under static conditions, the interface would be at the shore. However, in reality, ground water is not static, but flows towards and continually discharges into the sea, as indicated by the arrows in the figure. It is this flow that causes the seaward displacement of the fresh-water/salt-water interface and permits the drilling of fresh water wells in Maine within a few feet of the coastline.
Because the interface position is very dependent upon the height of the water table above sea level, seasonal changes in water-table level and daily changes in sea level cause the interface to change position. With each tidal cycle, the interface migrates inward and outward. A nine foot tidal change should cause a 40 x 9 (h=40t), or 360 foot variation in the interface position. Similarly, a seasonal water-table change of five feet should cause a seasonal interface change of 200 feet. Thus the lens of fresh water that exists below sea level shrinks slightly with each high tide, and shrinks significantly in late summer when the water table is lowest.
Temperature of ground water is ordinarily within a few degrees of the mean annual air temperature prevailing in a given locale. Ground water temperatures in Maine tend to be a relatively constant 40o to 50oF, a property useful to some commercial operations, for example the raising of fish in hatcheries. In summer, ground water is cooler than the air and feels cool (Figure 39). Streams receiving much ground water are cool, and typically good sources for trout and other cold-water fishes. Buildings and industrial boilers are often cooled using the relatively low temperature of ground water. In winter, however, ground water is warmer than the air, so that it seems warm (Figure 40). Streams receiving much ground water are warmer and tend not to freeze over so rapidly. The same buildings that are cooled in summer using ground water can be heated in winter using heat pumps to concentrate the caloric content of ground water.
Last updated on March 25, 2009
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