SEQUENCE STRATIGRAPHIC CONTROLS ON SURFACE TO GROUNDWATER RECHARGE IN THE CHAIN-OF-LAKES AREA, WEST SIDE OF THE MAIN BASIN, LIVERMORE – AMADOR GROUNDWATER BASIN, CALIFORNIA

In order to better understand groundwater flow and recharge in the Livermore – Amador Groundwater Basin, the Zone 7 Water Agency initiated a sequence stratigraphic pilot study on the west side of the Main Basin. The study area included the western part of the chain-of-lakes area. Of specific interest was the Lake I area, considered a significant basin recharge area. The objective of the investigation was to evaluate the hydrostratigraphy and aquifer architecture in the Lake I area to assess the effectiveness of the surface to groundwater recharge into deeper groundwater aquifers that are used for municipal supply by Zone 7. By evaluating subsurface data using sequence stratigraphy, the geometry and distribution of aquifer and aquitard sediments are linked to the original depositional processes that formed the sediments. The sequence stratigraphic study proved to be valuable in assessing groundwater flow and the effectiveness of groundwater recharge in the Lake I area. Four stratigraphic sequences were identified based on the analysis of (1) outcrop data at Lake I, (2) approximately 40 well logs (including both geophysics and lithologic descriptions), and (3) groundwater elevation data. Four generalized lithofacies were identified in the study area based primarily on geophysical log character tied to lithologic descriptions of cuttings. Each sequence is marked by a sequence boundary at the base of a series of fining upward braided fluvial channels containing sand and gravel with interstratified flood plain and lacustrine clays and silts. Each sequence is capped by a laterally extensive lacustrine interval. The laterally extensive lacustrine intervals in concert with the subtle angular discordances between the sequences appear to be the controlling features to groundwater flow in this area of the basin.