CHARACTERIZATION OF THE SHALLOW AND DEEP AQUIFERS IN THE EAST NEWPORT MESA AREA, ORANGE COUNTY, CA

A conceptual hydrogeologic model of the East Newport Mesa area will be developed to characterize lateral and vertical flow patterns between the shallow-most semi-perched, semi-confined aquifer and the underlying regional, potable, confined aquifer. The deep zone, although relatively shallow in the study area, is thought to be hydraulically connected to the principal groundwater production zone of the Orange County Groundwater Basin, which yields more than half of the water demand to over 2.3 million people in north and central Orange County. It is therefore important that the relationship between these two zones be better understood to determine the degree of hydraulic connection and possibility of contaminant migration from the shallow to the deep zone. The Newport Mesa is made up of Lower Miocene to Upper Pleistocene semi-consolidated marine sediments, and is one in a series of uplifted highlands trending northwest along the Newport-Inglewood fault zone. Groundwater flow in the East Newport Mesa has not been extensively studied due to the lack of sufficiently deep production or monitoring wells in the area. Model development will be based primarily on correlation of lithologic logs, geochemical analysis and groundwater level trends of monitoring wells, and known regional geologic structure from basin-wide reports. Hydraulic heads in the area show that groundwater flow in the shallow zone variably trends to the south with levels above sea level, while the deep zone has heads below sea level with groundwater flow to the northwest, towards a pumping depression induced by municipal water supply wells. A primary feature of the conceptual model may be the presence of an aquifer mergence zone, i.e., an angular or erosional unconformity in which the intervening aquitard between the two zones is eroded away, thus hydraulically joining the two aquifers. This would potentially provide a pathway for shallow groundwater to migrate vertically into the deep zone, a draining effect primarily driven by the significant amount of pumping from the deep zone northwest of the study area. This conceptual model will potentially serve as the foundation for a numerical flow model, and related contaminant transport model, with implications for managing and protecting drinking water production wells in the Orange County Groundwater Basin.