Stable isotopes of delta(18)O, delta(2)H, and (13)C, radiogenic isotopes of (14)C and (3)H, and ground water chemical compositions were used to distinguish ground water, recharge areas, and possible recharge processes in an arid zone, fault-bounded alluvial aquifer. Recharge mainly occurs through exposed stream channel beds as opposed to subsurface inflow along mountain fronts. This recharge distribution pattern may also occur in other fault-bounded aquifers, with important implications for conceptualization of ground water flow systems, development of ground water models, and ground water resource management. Ground water along the mountain front near the basin margins contains low delta(18)O, (14)C (percent modern carbon [pmC]), and (3)H (tritium units [TU]), suggesting older recharge. In addition, water levels lie at greater depths, and basin-bounding faults that locally act as a flow barrier may further reduce subsurface inflow into the aquifer along the mountain front. Chemical differences in ground water composition, attributed to varying aquifer mineralogy and recharge processes, further discriminate the basin-margin and the basin-center water. Direct recharge through the indurated sandstones and mudstones in the basin center is minimal. Modern recharge in the aquifer is mainly through the broad, exposed stream channel beds containing coarse sand and gravel where ground water contains higher delta(18)O, (14)C (pmC), and (3)H (TU). Spatial differences in delta(18)O, (14)C (pmC), and (3)H (TU) and occurrences of extensive mudstones in the basin center suggest sluggish ground water movement, including local compartmentalization of the flow system.

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The vertical component of specific discharge was estimated across several depth zones in the aquifer system at Cañutillo, Texas. The specific discharge was estimated using temperature and thermal conductivity data from four observation wells bottoming near the base of the aquifer system. Specific discharge was first calculated (using a steady‐state model) from the slopes of conductive heat flow versus temperature plots. The heat flow plots for all four wells suggested a zone of downward ground‐water flow from ∼ 70 to ∼ 220 feet depth and one or two zones of upward flow from ∼ 800 to ∼ 220 feet depth. Head data from observation wells did support the conclusion of an upper zone of downflow as indicated by the temperature data; however, the head data suggested that ground water should presently be moving downward from ∼ 220 to ∼ 800 feet. A computer model was developed to determine if the temperatures may be remanent and therefore reflect prepumping ground‐water flow, particularly in the deep flow zone. Computer model estimates were made of during‐pumping downflow in the upper flow zone. For the deep flow zone computer model estimates were made of prepumping upflow. The computer model estimates and steady‐state vertical specific discharge estimates were similar for the upper zone having downward flow. In the deep zone prepumping upflow estimates modeled on the computer were the same order of magnitude as steady‐state heat flow specific discharge estimates. Hence, present temperature data may demonstrate a significant remanent groundwater flow component and therefore might be used to estimate past vertical ground‐water flow.

The projected growth of Hernando County, located along the coast of west‐central Florida, has motivated the county and the Southwest Florida Water Management District to assess ground water resources in the area. As part of the Hernando County Water Resources Assessment Project, a computationally efficient modeling approach was developed to simulate regional ground water flow and salt water intrusion in two adjoining ground water basins in which Hernando County is located. In this modeling approach, the regional ground water flow simulation was divided into two stages: fresh water and sharp‐interface simulations. The first stage was performed using a calibrated fresh water model. Following the fresh water simulation, a calibrated sharp‐interface model was used to evaluate the potential extent of migration of the fresh water/salt water interface as a result of implementing ground water management scenarios. Predictive modeling results indicate that projected future ground water withdrawals will cause less than 0.6 m of additional drawdown within Hernando County. Results also suggest that by the year 2050 the fresh water/salt water interface will migrate landward less than 1.6 km from its 1994 position.

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