3D HYDROGEOLOGIC FRAMEWORK MODEL OF THE YUCAIPA GROUNDWATER BASIN

A comprehensive hydrogeologic evaluation of the Yucaipa Basin is essential to help local water districts to sustainably manage groundwater resources. Previous hydrologic and geologic investigations of the area provide information on hydrogeologic conditions, but there is no comprehensive study that describes the hydrogeology of the subsurface throughout the entire basin. The Yucaipa Basin is located in a geologically complex region between the northwest-southeast trending San Andreas fault zone and the ancestral Banning fault. Several northeast-southwest oriented dip-slip faults run through the Yucaipa Basin, providing structural relief between the sediment-filled Yucaipa graben and uplifted Mesozoic crystalline rocks that form the Crafton Hills. Several of these faults have been identified as potential barriers to groundwater flow within the sedimentary units of the graben. Quaternary alluvial and wash deposits comprise a relatively thin upper layer of sediments in the Yucapia Basin, which are underlain by the upper member of the San Timoteo Formation (Pleistocene and Pliocene) and older sedimentary formations. In this study, available geologic and geophysical information are compiled to construct a 3D hydrogeologic framework model of the Yucaipa Basin. The distribution of subsurface geology is constrained by regional depth-to-basement gravity studies and lithologic information from about 400 borehole driller’s logs. The lithologic types identified in the driller’s logs are simplified into textural categories (e.g., fines, sands, and gravels), and the extent of these lithologic facies throughout the basin are being modeled. In general, preliminary results show an expected grain-size distribution, with gravels deposited proximal to high topographic relief areas, sands deposited in wash areas, and fines furthest downstream. This observation is consistent with current topographic and tectonic conditions. These textural distributions are also being analyzed in the context of the geologic history of the area and possible hydrogeologic implications.