The San Fernando Aquifer composed of active alluvial fans and floodplains. Seasonal streams emanating from Pacoima and Big Tujunga Canyons drain the complex western San Gabriel Mountains and deposit coarse, highly permeable alluvium (Tetra Tech 2021). The more shallow western part derives mainly from Tertiary and pre-Tertiary sedimentary rocks, and is underlain by less permeable, fine-grained deposits containing persistent shallow ground water and poorer water quality (Figure 5). Large earthquakes such as the 1994 Northridge Earthquake created fractures across all layers (EPA 2023). Late Pleistocene deposits have been cut by active faults and warped over growing folds. Holocene alluvial fans are locally ponded behind active uplifts. The resulting complex pattern of deposits has a major effect on liquefaction hazards. Young sandy sediments generally are highly susceptible to liquefaction where they are saturated, but the distribution of young deposits, their grain size characteristics, and the level of ground water all are complexly dependent on the tectonics of the valley. The basement rocks here include high-grade metamorphic rocks of Precambrian age (Morton & Yerkes 1987). 

Naturally occurring trace elements and minor elements, such as chromium and fluoride, are present in the SFB groundwater at high concentrations in some areas. Additionally, radioactivity from the decay of naturally occurring uranium and thorium isotopes in the sediments is a natural component of the groundwater (USGS 2012). Dropping levels of groundwater across the basin are consistent, the unsaturated zone is growing larger (Tetra Tech 2021).