INFLUENCE OF THE LOCKHART FAULT ZONE ON GROUNDWATER FLOW IN HINKLEY, CALIFORNIA

Groundwater flow across the Lockhart Fault zone near Hinkley Valley, California was evaluated using geophysical and groundwater age data to understand the movement of a hexavalent chromium plume originating from the Hinkley compressor station. Recent mapping in the study area shows the fault zone consists of multiple strands striking west-northwest through unconsolidated Mojave River deposits. Differences in water levels on either side of the fault zone show the fault zone is an impediment to groundwater flow. Horizontal point velocity probe (PVP) data collected in monitoring wells within the fault zone showed groundwater flow vectors deflect north-northwest on the upgradient side of fault strands. Groundwater flow vectors measured using the PVP had a greater westward deflection compared to flow directions estimated from groundwater level data. Groundwater velocities measured using the PVP were lowest within the Lockhart Fault zone with a median velocity of 3.2 feet per day (ft/day) compared to median velocities of 11.3 ft/day upgradient and 5.4 ft/day downgradient of the fault zone. Younger groundwater was present on the upgradient and downgradient sides of the overall fault zone, and older groundwater with unadjusted carbon-14 ages as old as 5,650 years before present was present in water from wells within parts of the fault zone. The groundwater flow and age data are consistent with limited groundwater movement across the fault zone with the majority of groundwater flow recharging Hinkley Valley from the Mojave River on the eastern side of the fault zone. At least some of the groundwater recharged from the Mojave River would have flowed beneath the compressor station—the source of anthropogenic hexavalent chromium releases to Hinkley Valley—and to the northwest along the downgradient side of the Lockhart Fault zone.