Geologic Setting, Geohydrology, and Ground-Water Quality near the Helendale Fault in the Mojave River Basin, San Bernardino County, California

The proximity of the Moja ve River groundwater basin to the highly urbanized Los Angeles region has resulted in rapid population growth and, consequently, an increase in the demand for water. The Mojave River, the primary source of surface
water for the region, normally is dry—except for periods of flow after intense storms; therefore, the region relies almost entirely on ground water to meet its agricultural and municipal needs. The area where the Helendale Fault intersects the Mojave River is of particular hydrogeologic interest because of its importance as a boundary between two water-management subareas of the Mojave Water Agency. The fault is the boundary between the upper Mojave River Basin (Oeste, Alto, and
Este subareas) and the lower Mojave River Basin (Centro and Baja subareas); specifically, the fault is the boundary between the Alto and the Centro subareas. To obtain the information necessary to help better understand the hydrogeology of the area near the fault, multiple-well monitoring sites were installed, the surface geology was mapped in detail, and water-level and water-quality data were collected from wells in the study area.