The RASA Program represents a systematic effort to study a number of the Nation's most important aquifer systems, which, in aggregate, underlie much of the country and which represent an important component of the Nation's total water supply. In general, the boundaries of these studies are identified by the hydrologic extent of each system and, accordingly, transcend the political subdivisions to which investigations have often arbitrarily been limited in the past. The broad objective for each study is to assemble geologic, hydrologic, and geochemical information, to analyze and develop an understanding of the system, and to develop predictive capabilities that will contribute to the effective management of the system. The use of computer simulation is an important element of the RASA studies to develop an understanding of the natural, undisturbed hydrologic system and the changes brought about in it by human activities and to provide a means of predicting the regional effects of future pumping or other stresses. The final interpretive results of the RASA Program are presented in a series of U.S. Geological Survey Professional Papers that describe the geology, hydrology, and geochemistry of each regional aquifer system. Each study within the RASA Program is assigned a single Professional Paper number beginning with Professional Paper 1400.

Investigations in the Old Woman Mountains area provide insights into mid‐Tertiary extensional tectonism in southeastern California. The Old Woman Mountains area is a moderately extended region that lies between two highly extended terranes, the Central Mojave Extensional Complex and the Colorado River Extensional Corridor. Two normal faults and a high‐angle fault that we interpret to be a transfer structure divide the area into four distinct structural blocks. These blocks correspond to the four mountain ranges of the area: the Old Woman, Piute, Little Piute, and Ship mountains. Other major faults involved in tilting are inferred to be buried beneath alluvium in surrounding valleys. Deformation occurred in part before, but primarily after, deposition of the Peach Springs Tuff, an 18.5 Ma regional stratigraphic marker. Transport directions of hanging walls are westward on the west side of the Old Woman Mountains area and east‐southeastward on the east side. We suggest that the Old Woman Mountains area is a less extended portion of a continuous extensional terrane that stretches from the central Mojave Desert across the Colorado River to the transition zone of the Colorado Plateau. Lower strain in the Old Woman Mountains area than in most of the Colorado River Extensional Corridor and Central Mojave Extensional Complex and age relations across the terrane can be explained by a model in which the principal locus of upper crustal breakaway to an east dipping low‐angle simple shear zone shifted at about 18–20 Ma from a position far to the west to just east of the Old Woman Mountains area.

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Abstract:The karst aquifer systems in southern Kentucky can be dynamic and quick to change. Microorganisms that live in these unpredictable aquifers are constantly faced with environmental changes. Their survival depends upon adaptations to changes in water chemistry, taking advantage of positive stimuli and avoiding negative environmental conditions. The U.S. Geological Survey conducted a study in 2001 to determine the capability of bacteria to adapt in two distinct regions of water quality in a karst aquifer, an area of clean, oxygenated groundwater and an area where the groundwater was oxygen depleted and contaminated by jet fuel. Water samples containing bacteria were collected from one clean well and two jet fuel contaminated wells in a conduit-dominated karst aquifer. Bacterial concentrations, enumerated through direct count, ranged from 500,000 to 2.7 million bacteria per mL in the clean portion of the aquifer, and 200,000 to 3.2 million bacteria per mL in the contaminated portion of the aquifer over a twelve month period. Bacteria from the clean well ranged in size from 0.2 to 2.5 mm, whereas bacteria from one fuel-contaminated well were generally larger, ranging in size from 0.2 to 3.9 mm. Also, bacteria collected from the clean well had a higher density and, consequently, were more inclined to sink than bacteria collected from contaminated wells. Bacteria collected from the clean portion of the karst aquifer were predominantly (,95%) Gram-negative and more likely to have flagella present than bacteria collected from the contaminated wells, which included a substantial fraction (,30%) of Gram-positive varieties. The ability of the bacteria from the clean portion of the karst aquifer to biodegrade benzene and toluene was studied under aerobic and anaerobic conditions in laboratory microcosms. The rate of fuel biodegradation in laboratory studies was approximately 50 times faster under aerobic conditions as compared to anaerobic, sulfur-reducing conditions. The optimum pH for fuel biodegradation ranged from 6 to 7. These findings suggest that bacteria have adapted to water-saturated karst systems with a variety of active and passive transport mechanisms.

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