Abstract: Influence of Alluvial Fan Deposits on Ground Water Flow Paths - A Case Study

Jessica B. Donovan, Julie A. Hayes, Kimberly S. Jolitz

Results of an extensive site investigation conducted in Palo Alto, California illustrate the relationship between the depositional orientation of alluvial fan deposits and ground water flow paths in this portion of the south San Francisco Bay Region. The site is situated near the northwestern margin of the Santa Clara Valley at a location approximately one-third of the distance between the foothills of the Santa Cruz Mountains and San Francisco Bay. Current ground surface topography indicates that the site is situated along the transition zone between the San Francisquito Creek alluvial fan to the west and the Matadero Creek fan to the east. The regional ground water gradient is north to northeast in this area.

Two primary water-bearing aquifers were identified within the alluvial fan deposits. They are termed the A Aquifer, which extends between approximate depths of 13-55 feet, and the B Aquifer, encountered between depths of 60-120 feet. The configuration of the coarse-grained alluvial deposits comprising the A Aquifer has been delineated using an extensive body of data consisting of 220 cone penetration test (CPT) soundings and 150 monitoring wells.

Due to the comparatively large size of its drainage basin, San Francisquito Creek transports a greater volume of water and sediment than the smaller Matadero Creek. As a result, the predominant depositional orientation in the A Aquifer is easterly, becoming northerly in the vicinity of Matadero Creek. As indicated by the distribution patterns of chemicals in ground water, variations in the configurations of coarse and fine-grained deposits cause localized refraction of ground water flow away from the regional gradient.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California