Page 868 - anf1995elsmere_deis

The local occurrence and accumulation of petroleum has also impacted the Towsley
Formation and the local groundwater regime. Oil generated within the deeper portions of

the basin has migrated vertically and horizontally within permeable sedimentary units and
has either reached the surface in the Elsmere Canyon area or has accumulated in the
subsurface within Pico deposits and lower Towsley beds. Because the coarse-grained lower
member of the Towsley Formation is typically oil-bearing in the subsurface and tar-saturated
in surface exposures, its ability to efficiently transmit groundwater has been further limited.

9.4.3 1 un er Flow stem and H drauli R lati 11 hi 8

Infiltration and recharge to the local aquifers is limited because of steep topographic
gradients and-thin soils developed over well-indurated bedrock with limited permeability.
During moderate and heavy periods of precipitation, a substantial portion of the
precipitation received within the Elsmere Canyon watershed leaves the property as surface

flow. Precipitation contributing to local recharge includes infiltration at topographically
higher areas that act as groundwater drainage divides. East Firebreak Road ridge is the
primary drainage divide at the project property, with topographically lower drainage divides
also coinciding with elevated areas flanking the main Elsmere Canyon drainage on the

north, south and west. This configuration of drainage divides, in combination with relatively
impermeable Towsley strata, produces an inward-directed, mainly northwest-sloping
groundwater gradient that closely follows the Elsmere Canyon surface drainage pattern in
the northwest area of the project property.

Within the local groundwater system, recharge and discharge within San Gabriel Formation
rocks occurs through complex fracture pathways under flow rates controlled by fracture size,

density and interconnectivity. Because of steep hydraulic gradients, the fracture network
developed in these rocks, characterized by limited storage capacity, exhibits fairly responsive
recharge and discharge to moderate and heavy precipitation.

Groundwater recharge to Eocene rocks occurs by direct infiltration of precipitation and by

subsurface flow from the saturated portion of the San Gabriel Formation in hydraulic
communication with these strata. Like the San Gabriel Formation, discharge from Eocene
water-bearing strata occurs from springs and seeps in outcrops located within Elsmere
Canyon, and flows into shallow alluvial deposits in the canyon drainage.

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