Occurrence of natural and anthropogenic hexavalent chromium (Cr VI) in groundwater near a mapped plume, Hinkley, CA

Task 5

Evaluation of historic and present-day groundwater movement

An existing MODFLOW-based groundwater flow model originally developed by Andrews and Neville (2003), and updated for estimation of management alternatives within the area (Arcadis/CH2M-Hill, 2011), will be used to evaluate historic and present-day groundwater movement in the study area. This task will be done by PG&E consultants in collaboration with U.S. Geological Survey hydrologists familiar with groundwater flow models, and with their use within the Mojave Desert of southern California. The modeling work will be divided into four steps: 1) review of the suitability of the groundwater flow model for the purposes of this study, 2) update and recalibration of the model, 3) use of the model to estimate historical groundwater levels, and 4) use of the model with particle tracking to aid in the visualization of groundwater movement and interpretation of groundwater age data.

The U.S. Geological Survey in collaboration with PG&E and their consultants will review the existing groundwater flow-model to determine its suitability simulate groundwater flow in Hinkley Valley with respect to the model domain, boundary conditions, hydraulic properties (including the influence of aquifer heterogeneity on transport of Cr VI), and spatial and temporal discretization. On the basis of the initial evaluation the model will be updated, and the updated model will be recalibrated to available data. Depending on the results of the model evaluation, it may be necessary extend the model domain to appropriate hydrologic boundaries. In addition, the model may need to be refined to improve: 1) simulation of groundwater flow between the bedrock aquifer and the alluvial deposits, 2) simulation of drying and rewetting of alluvial deposits through time, 3) simulation of the effect of the Lockhart and Mount General Faults on groundwater flow, and 4) simulation of the effect of rapid, intermittent infiltration and subsequent recharge of large quantities of streamflow from the Mojave River. Other enhancements not specifically listed also may be required. Historic water-level and streamflow data (available digitally from U.S. Geological Survey, Mojave Water Agency, PG&E consultants, and Project Navigator data bases), test-drilling data collected by PG&E consultants during well installation (including texture data available digitally from Project Navigator), pumping data (available from the U.S. Geological Survey and the Mojave Watermaster), groundwater flow data collected during tracer tests done within the IRZ, geologic and hydrologic data from the western, northern and eastern subareas collected as part of this study, and other available data (including areal imagery) will be used to update aquifer property and other hydraulic data within the groundwater flow model prior to calibration.

The updated and calibrated groundwater flow model will be used to estimate historical groundwater levels and movement within the study during hydrologically important periods including: 1) prior to agricultural pumping (1931 pre-development conditions within the USGS Regional groundwater flow model), 2) after the onset of agricultural pumping during the time when discharges of Cr VI containing wastewater occurred from the compressor station, 3) as agricultural pumping declined but prior to pumping for land treatment of Cr VI by PG&E, and 4) present-day conditions, including the effect of ongoing management activities (such as pumping for land treatment, injection to control plume migration, and movement of groundwater within the IRZ) intended to control or remediate the plume.

A particle-tracker will be added to the model to aid in the visualization of water movement through the valley, and the distribution of groundwater age, and increase understanding of Cr VI movement from the compressor station. Reactive transport of Cr VI in groundwater will not be simulated. Special attention will be given to the influence of rapid, intermittent infiltration and subsequent recharge of large quantities of streamflow from the Mojave River on groundwater flow and movement. Particle-tracking results will be compared with tracer data collected as part of this study to evaluate the source, movement, and age of groundwater in a manner similar to a regional analysis of groundwater flow and movement in the Mojave River groundwater basin by Izbicki and others (2004). Model properties may need to be adjusted to improve calibration and simulation results where indicated on the basis of this comparison.

Cooperating Agency: Lahontan Regional Water Quality Control Board
Project Chief: John A. Izbicki
Phone: 619-225-6131
Email: jaizbick@usgs.gov

California Water Conditions

Real-Time California Streamflow Conditions