Groundwater level and hydrochemistry in the San Jacinto basin, riverside county, California

The San Jacinto basin, structurally a sediment-filled graben (4 km by 40 km) formed by right-stepping en echelon faulting in the right-slipping San Jacinto fault zone, provides most of the water demand in the San Jacinto valley. Pumping of groundwater in the past 80 yeas had caused water level to drop at some locations by 90 m. Analysis of seasonal and long-term fluctuations of water levels suggests at least four hydrologic subregimes in the basin. Stiff and Piper diagrams indicate four major types of subsurface water. Major ion concentrations appear stable over the recorded period from 1951 to 1986 despite repeated changes of water levels in the past 48 years. Hydrologic discontinuities are recognized from contour maps of four-year mean winter water elevations and four-year mean concentrations of total dissolved solids (TDS). Some discontinuities coincide with faults while others probably signify sedimentary facies boundaries. A trough of low TDS (reflecting ancient channel deposits of high hydraulic conductivity) lies southwest of the present channel of the San Jacinto River. This implies the San Jacinto River has migrated northeast probably in response to tectonic tilting of the basin. That TDS trough also suggests that most of the subsurface water leaves the basin about 7 km south of the present river exit. Because of its mixing with high-TDS water from the northwest, the northwest moving subsurface water appears to have been displaced southward before it exists the basin. Contours of sulfate, which is almost absent in the water from the northwest, provide a better indication of exit location.