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on the other hand, may yield as much as 2,000 gallons of water per minute because vast amounts of water I Ie between the sand and gra i ns and pebb I es. Crysta II i ne rocks, I I ke granite, appear to have no available water when an individual rock is examined. Yet, moderate amounts of water can be contained in fractures interlacing an extensive rock mass. Because of great hydrologic variation In rocks and soi 15, and the differences in the rates at which aquifers recharge, there is a great range I n the amount of water ava i lab 1 e from location to location, season to season, and year to year. Ground water aquifers may provide a sustained yield of less than a gallon to several thousand gallons of water per minute. Aggravating Circumstances &Consequences If water is pumped out of an aquifer faster than It Is recharged, the water level In the aquifer--the ground water table--Is said to drop. Seasonal fluctuations from irrigation or changes in recharge are quite common. If the excessive withdrawal of water continues over an extended period of time and the natural recharge does not offset the total amount of water extracted, It is said the ground water Is being Ilmined.11 There Is ev I dence that once vast amounts of water are withdrawn from deep aquifers at rates exceeding recharge, the aquifers never again can be recharged with the original volume of water. Consol idation and subsidence processes can fill in the space once occup I ad by water. Ef forts to artificially recharge underground aquifers have met with I imited success. Human activities greatly affect the avallabil ity of ground water in some areas. Widespread and excess I ve pump i ng of underground waters can lower the ground water tab I e under many square mi les. This Is the situation In some irrigated farming areas in eastern Colorado. In many areas wells have had to be deepened to obtain sufficient water as a ground water table drops. The lower the ground water table, the more it costs to dri II a well and to pump the water to the surface. It eventually may cost more to pump water than the water Is worth, especially for agricultural and Industrial applications using large volumes of water. Paving large areas, altering or removing vegetation, grading, sand and gravel mining, diverting storm runoff away from established channels and other manipulation of the land and surface hydrology can change recharge rates in the Immediate vicinity, thus altering ground water levels. Mitigation An adequate understanding of the ground water system is the best measure of protection from property damage and devaluation from a reduction or loss of water supply. ay using such knowledge, construction and property development can be accomplished without encountering unexpected ground water situations. Hydrogeologic investigations can reveal not on I y sp'ec If i c character i st i cs, but the i nterra I at i onsh I ps between and among human endeavors and natural factors. Know I edge of the amount of water withdrawn per year, the recharge rates and other facts may reQu 1 re the amount of water withdrawn annually from wells to be restricted and the number of wells limited to keep the walls from drying up. Restrictions on land development may be needed to preserve recharge from natural sources and thus maintain the ground water supp I y and protect I and investments. Land Use In semi-arid Colorado ground water considerations are Increasingly considered in maKing land usa decisions. Normally land uses are not restricted, although they may entail construction modifications to protect or enhance ground water supply and related property Values. Too much or too I I tt I e ground water, year around or seasona II y, can be compensated for ord I nar i I Y through geo I og i c Invest 1 gat I on and responsive planning. Developments which rely on ground water should be permitted only after it Is establ ished that sufficient water exists and that the necessary amounts can be withdrawn indefinitely without jeopardizing the supply. 21