Gravity survey of the Lucerne Valley groundwater basin: Implications for basin structure and geometry
Lucerne Valley is located at a structurally complex part of the Eastern California shear zone but the geometry of the groundwater basin located there is poorly constrained. To constrain basin geometry for groundwater assessment and location of potential faults, 433 gravity measurements were collected in the central portion of the basin. Absolute gravity values range from 979,332 to 979,409 meals and isostatic anomalies range from -26.8 to -16.7 meals. Analysis of these new data, coupled with existing gravity data, suggest the thickest part of the Lucerne Valley groundwater basin exists near the San Bernardino mountains and that the basin shoals northward. The gravity data indicate a shallow basin dominated by undulating basement topography as well as previously unrecognized structures across the basin, such as the extension of the Silver Reef fault. A gravity high in the center of the Lucerne Valley groundwater basin suggests high-density basement complex exists in the subsurface. Two-dimensional forward and three-dimensional inverse modeling of isostatic anomalies show an approximate basement depth of 300 m (980 ft) in the Lucerne Valley groundwater basin, and 550 m (1,800 ft) in the Fifteenmile groundwater basin. Potential areas of groundwater recharge are more likely to exist in the southern portion of the basin where basin sediments are thicker and possibly coarser grained. Storage potential estimates range from 1.9 x 106 to 6.7 x 106 acre-feet in the Lucerne Valley groundwater basin of water and 4.9 x 106 to 8.7 x 106 acre-feet of water in the Fifteenmile Valley groundwater basin.