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Description
Land development, whether urban or rural, is dependent on several factors, most importantly the water supply. Areas which are not self-sufficient in this vital resource must carefully evaluate their groundwater supplies before land development is undertaken. One such area of interest is the Mount Laguna area. The study of this problem was begun in the summer of 1974 with the aid of a National Science Foundation Grant # GY 11160. The data collected included lithologic and structural features, soil types, chemical properties and rates of spring flow, and water use from wells. One particular geomorphologic feature, the meadow, is thought to play an integral part in the groundwater system of the Mount Laguna area. Four of these meadow areas, Agua Dulce, Filaree Flat, Ames Valley, and Laguna Meadow were chosen for a geophysical evaluation of their subsurface geometries and the stratification within them. Two geophysical methods were employed in this study: 1) seismic refraction, and 2) electrical resistivity. Seismic refraction was chosen as the principal investigative method of this study while the electrical resistivity data was used to supplement and confirm the seismic refraction data. Four stratigraphic units were defined by the seismic refraction method: 1) a residual soil displaying a velocity of about 1100 ft/sec, 2) a weathered bedrock unit showing an average velocity of 2300 ft/sec, 3) a wet weathered bedrock unit with velocities ranging from 4700 to 6400 ft/sec, and 4) a bedrock unit which displayed velocities greater than 8500 ft/sec. The overwhelming majority of the electrical resistivity curves were of the H-type which have the following resistivity relationship between layers: ρ1 > ρ2 < ρ3 . The resistivity value of the second layer varied from approximately 10 to 100 ohm-meter which is indicative of the presence of moisture in varying degrees. A generally systematic disparity was observed between the seismic refraction and electrical resistivity data. The electrical resistivity usually showed a much thinner first zone and much greater depth to bedrock than the seismic refraction data. This disparity could be attributed to capillary action and water-filled bedrock fractures respectively. Interpretation of the data points to a relatively planar and nearly horizontal bedrock which is overlain by three stratigraphic units, residual soil, weathered bedrock, and wet weathered bedrock. The central portions of the meadows exhibit shallower depths to bedrock which is overlain solely by the residual soil unit. The other two units are encountered as the margins of the meadows are approached and the full stratigraphic spectrum is developed underneath the slopes. This stratigraphic distribution is observed only in the granitic terrain. Metamorphic terrain exhibits greater depth to bedrock which is overlain solely by the residual soil unit. Groundwater recharge, storage, and movement seems to be restricted to the forested slopes cradling the meadow areas where the full stratigraphic spectrum is developed. Capillary action does, however, effectively moisten the residual soil unit to within a few feet of the surface.
