Geohydrologic data were collected from Irwin Basin at Fort Irwin National Training Center in the Mojave Desert of southern California by the U.S. Geological Survey during 1992-96 to determine the quantity and quality of ground water available in this basin. In addition to data collected from existing wells and test holes, 17 monitoring sites were constructed in Irwin Basin to provide data on subsurface geology, ground-water levels, and ground-water quality. Eleven of these sites were multiple-well monitoring sites that were constructed to provide depth-dependent geohydrologic data in the aquifer system. The aquifer system of Irwin Basin, defined on the basis of hydrologic data collected from wells in Irwin Basin, consists of an upper and a lower aquifer. A 1994 water-table contour map shows that a cone of depression beneath Irwin Basin well field has developed as a result of ground-water development. Water-quality samples collected from Irwin Basin wells to determine potential sources of ground-water degradation indicate that water in three areas in the basin contains high nitrate and dissolved-solids concentrations. The stable isotopes of oxygen and hydrogen indicate that present-day precipitation is not a major source of recharge in this basin. Tritium and carbon-14 data indicate that most of the basin was recharged before 1953 and that this water may be more than 14,000 years old. Purpose and ScopeThe objectives of this study were to describe the geohydrologic and geochemical framework of the ground-water basins that supply water to the base, to develop ground-water-flow models to help refine the understanding of the geohydrology of the basins, and to Previous StudiesPrevious studies by the U.S. Geological Survey include a hydrogeologic reconnaissance by Kunkel and Riley (1959) that identified unconsolidated waterbearing and consolidated nonwater-bearing deposits and established that Irwin, Bicycle, and Langford Basins were favorable for development of ground water. Yount and others (1994) did detailed geologic mapping in Irwin Basin and part of Langford Basin and determined the location of several faults on the base.Several consulting and engineering investigations have been completed for the base (C.F. Hostrup and Associates, 1955; James M. Montgomery and Associates, 1981; Wilson F. So and Associates, 1989) to determine the long-term availability of ground water for the Fort Irwin NTC. The Montgomery study in 1981 concluded that ground water was being mined from Irwin and Bicycle Basins. Wilson F. So and Associates (1989) determined that if projected Irwin Basin water demands were accurate, water levels would be "critically deep" at some time between the years 2011 and 2020.

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Twenty-three multiple-well (nested) monitoring sites were constructed in the western part of the Santa Clara-Calleguas Hydrologic Unit and groundwater basin between October 1989 and May 1994. This report is a compilation of the data collected at these sites from October 1989 through April 1995 and includes location and design of the monitoring sites, lithologic data, geophysical logs, groundwater level measurements, and water-quality analyses. Grainsize distribution and mineralogical analyses also are included. Ninety-nine piezometers were installed at the 23 sites to enable collection of depthdependent data. Lithologic logs were compiled from descriptions of drill cuttings for each site and from observations recorded during logging. Generalized stratigraphic columns were compiled by grouping similar lithologic units. Grain-size and mineralogical analyses were used to identify and characterize the various lithologic units and to distinguish changes among them. Geophysical logs provided information on the character of the lithologic units and on the presence and chemical characteristics of ground water. Water levels were measured periodically during 1989-94. Water-quality data were collected periodically during 1989-95. INTRODUCTION Purpose and Scope The U.S. Geological Survey (USGS) constructed 23 multiple-well monitoring sites in the western part of the Santa Clara-Calleguas Hydrologic Unit between October 1989 and May 1994. The purpose of the monitoring sites was to provide data on geology, water levels, groundwater quality, and aquifer properties at multiple depths in the aquifer system. Most existing data were obtained from wells perforated solely in the upper part of the aquifer system or perforated through several aquifers. The monitoring sites were constructed as part of two concurrent U.S. Geological Survey studies of the area: (1) a Regional Aquifer-System Analysis (RASA) study of groundwater conditions in the Santa Clara-Calleguas Hydrologic Unit; and (2) a cooperative study with United Water Conservation Introduction 1 District (UWCD) to investigate seawater intrusion in the Oxnard Plain, the coastal part of the Santa Clara-Calleguas Hydrologic Unit. The objectives of the RASA study are (1) to determine the geohydrologic framework of the coastal and desert basins in southern California, and (2) identify and analyze major issues and problems affecting the use of ground water in these basins (Martin, 1986). The Santa Clara-Calleguas basin was selected for intensive study, as a representative coastal basin, under the RASA program. The objective of the cooperative study with UWCD is to develop and apply solute-transport and simulation-optimization modeling techniques for evaluating management options to control seawater intrusion in the Oxnard Plain. Data collected from the monitoring sites will be essential in completing both studies. The purpose of this report is to present a compilation of the data collected at the monitoring sites from October 1989 (when the first site was constructed) through April 19...

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