Abstract: Stratigraphic and Sedimentologic Controls on the Hydrogeology of the Alameda Creek Fan (Niles Cone), Alameda County, California

Christine E. Koltermann

Quantitative depositional modeling, petrophysical relations, and groundwater flow simulation were combined to relate the hydrologic structure of the Quaternary Alameda Creek fan aquifers to the geologic controls on aquifer development. This approach predicts grain size and hydraulic conductivity distributions throughout the fan. Results of the depositional model show that tectonics, paleoclimate change, and sea level change strongly influenced the large scale trends in fan sediment texture. The sequence of stacked, offset aquifers formed during colder, wetter periods, while the aquitards formed during warmer, drier periods in the geologic history of the fan. Due to right lateral strike-slip motion on the Hayward Fault, the deeper aquifers are predominantly located on the orthwest side of the fan, while the upper aquifers are located closer to the modern source area. The lack of hydraulic connection across the bay of the deeper aquifers has been caused by the barrier to stream flow and sediment transport presented by the Coyote Hills during fan development. Depositional modeling also predicted the thick, regional Irvington aquitard as a consequence of the previous sea level high stand. Petrophysical modeling results show that predicted hydraulic conductivity values are consistent with observed values over a seven order of magnitude range. Groundwater flow model simulations reproduce observed hydrogeologic features under both natural and pumping conditions. These features include hydraulic gradients, hydraulic connections between aquifers, locations and

discharge of flowing wells and springs, the effects of the Hayward fault, and the alteration of the flow system which led to salt water intrusion.

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