USFS/USGS HDROGEOLOGICAL FAULT ZONE MODELING IN THE SAN BERNARDINO NATIONAL FOREST

Tunneling associated with the Arrowhead water conveyance project beneath the San Bernardino National Forest could affect both surface and subsurface forest resources. Sensitive ecosystem components supported by springs discharging from faults could be affected by hydraulic disruptions from tunneling. A discrete feature network (DFN) model was constructed to evaluate this potential impact.

Surface geologic data combined with subsurface fracture data provide the basis for the DFN model. Limited pressure head data and Lugeon interval testing from boreholes indicate the possibility of fault bounded flow compartments. These data constrain the hydrologic properties. The presence of faulted bedrock requires the translation of poorly constrained fault structures into a numerical model honoring these limited data.

Specifically within the fault zones, flow can occur both along and across the faults. Spatially, there are fault core zones and associated fault damage zones that commonly have different hydrologic properties. Using special features within the DFN, this spatial heterogeneity is modeled. Resultant pressure head drops in the model are similar to field observations, a correlation that increases confidence in the DFN model.