WHY GEOLOGY IS CRITICALLY IMPORTANT EVEN WHEN YOUR GEOPHYSICS IS GREAT

In recent years the California Department of Water Resources greatly improved subsurface characterizations of groundwater systems through abundant airborne electromagnetic (AEM) survey lines. To the casual observer, it may appear that the new AEM data together with existing borehole logs are all that is needed to support the necessary modeling and management of groundwater. Another crucial step involving geologic interpretation, however, is needed. Examples of why this is so illustrate how geologic reconstruction of the origin of the sediments is necessary to adequately interpret the subsurface data and produce the 3D subsurface hydrostratigraphic maps needed in groundwater model building. Examples from the Central Valley aquifer system include the incised valley-fill deposits (‘paleovalleys’ in the popular press) on eastside fans, sequence-stratigraphic influences on aquifer-aquitard structure and vertical flow, the expected but uncharacterized differences in depositional environments between fan and inter-fan areas, and the mid-valley transitions from fan to through-going fluvial deposits. None of the above can be adequately anticipated or characterized without the application of geologic principles and interpretation. All examples will be presented in both geologic and hydrogeologic contexts, highlighting potential pathways for improved modeling and management of groundwater as well as improved understanding of the Quaternary geology.