This may cause severe disruption of utilities such as sewer and water lines and damage to structures and roads. Subsidence can also cause flooding, particularly in coastal areas. Between 1906 and 1987, land in the Houston/Baytown region of Texas subsided by between 1 and 10 feet, resulting in pronounced flooding of valuable land adjacent to Galveston Bay. When policy-makers recognized the value of remaining ground water in preventing subsidence and concomitant flooding, they formulated a plan to conserve ground water in situ by developing sufficient surface supplies to accommodate 80 percent of the projected demand for Houston by the year 2010 (Schoek, 1995). The most dramatic example of subsidence is found in the San Joaquin Valley of California, where land surfaces have fallen up to 40 feet in some areas.

A unique problem associated with subsidence caused by prolonged overdrafting has been the development of sinkholes in some areas of Florida where natural flow patterns in limestone aquifers have been perturbed. Land subsidence generally occurs when aquifer pressure levels are significantly lowered in basins where the substrate is primarily fine-grained material such as clays and silts, which are more compressible than more rigid coarse grains such as sand or limestone and sandstone formations. Subsidence caused by the consolidation of fine-grained material cannot be reversed by artificially injecting additional water into the formation. Subsidence is reversible only in aquifers usually dominated by sands, gravels, or sandstone, which can accept the additional fluids.

Saline ground water is found in aquifers throughout the United States. Ground water depletion may cause intrusion of poorer-quality water into high-quality water supplies. In some coastal regions, particularly in California and Florida, there are serious sea water intrusion problems caused by the attenuation of fresh ground water flows toward the ocean. The in situ value of ground water in these cases derives from providing a barrier to salt water intrusion. Overdrafting can depressurize confined aquifers, leading to the intrusion of salt water into portions of the aquifer that formerly contained high-quality water (see Figure 2.4). Salt water intrusion problems are not limited to coastal areas. Problems with saline ground water have been documented in 41 states (Atkinson et al., 1986). A number of methods are available to combat salt water intrusions, including artificial recharge, reductions in extractions, establishment of a pumping trough along the coast, formation of pressure ridges through artificial water injection, and installation of subsurface barriers.

Discharges from unconfined aquifers are the source of about 30 percent of the nation's stream flow (Frederick, 1995). This source of surface water is especially important in sustaining stream flow during dry periods, the so-called base flow. Ground water levels have a direct impact on lake levels and on the amount of freshwater flowing through estuaries to the oceans. Reductions in surface water flows can have adverse impacts on the aesthetic values, recreational potential, and use of surface waterways for transportation.

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