MALIBU WATER QUALITY
Figure 4 - A-A’ Hydrogeology Cross-Section.
water flow and chemistry from various researchers and the local lifeguard station, hydrogeology data from localized on-site wastewater reports, water use by parcel from the local water agency and land use from the City of Malibu and County of Los Angeles (Stone, 2010; RMC, 2013). Additional new data was collected along the near and offshore areas in form of geophysics, monthly water levels throughout the basin, water chemistry (surface and groundwater). New monitoring wells were installed, in order to collect continuous cores from ground surface to bedrock (Earth Forensics, 2013; RMC, 2013). After the installation of these new wells a series of aquifer tests were run in order to determine the hydraulic properties of both the fine-grained low-permeable (LPZ) semi-confining layer and the underlying gravel layer (Fig. 4).
Historical work
Local work has consisted of university studies of the lagoon; geotechnical studies of all building construction, including seismic fault studies; wastewater disposal studies involving percolation testing (both seepage pits and leach lines) of the shallow soil and groundwater systems; removal of several underground storage tanks and testing of contaminated soils (Leighton, 1994; Ambrose & Orme, 2000; RMC, 2013). These studies, project reports and government filings gave a com- prehensive understanding of the upper 7.5 meters of geology within the CCAM. However, very little had been done below the LPZ (approximately 7.5 meters below ground surface (bgs)) or within the underlying gravels.
Geological Setting
There are three primary hydrostratigraphic units within the Malibu Creek Groundwater Basin (MCGB): alluvium (which is comprised of LPZ and surface soils); the underly- ing coarse-grained strata commonly referred to as the “Civic Center Gravels (CCG)” and bedrock (Leighton, 1994; ECI, 2000). Bedrock is at or near land surface in the upland areas, and beneath the unconsolidated sediments that are present
in the CCAM along Malibu Creek and beneath the associated Lagoon (Fig. 4).
A large bedrock valley lies beneath the CCAM portion of the City of Malibu. The data used to construct the bedrock surface included previous modeling efforts (Stone, 2004), additional resistivity geophysical surveys completed by the USGS (Izbicki, 2009) and resistivity surveys and borehole drilling conducted in 2009 (RMC, 2012 – concept injection plan) and 2013 (Cardno-Entrix, 2013). Unconsolidated mate- rials containing zones of permeable sand and gravel deposits overlie the bedrock (Leighton, 1994; ECI, 2000). These aquifer units provide pathways for groundwater to flow from upland recharge areas to discharge areas along Malibu Creek/Lagoon and out beneath the sea floor.
The geomorphic shape and characteristics of the bedrock layer are consistent with two ancient water courses carved by Malibu Creek leading to the ocean (Earth Forensics, 2013; RMC, 2013). The estimated location of the ancient courses dated at 60,000 and 20,000 years ago, are shown in Fig. 5.
In 2009, an offshore seismic reflection survey was conducted to map the top of bedrock and to determine whether the CCG observed in recently installed deep wells continued offshore. This survey was followed by an electrical resistivity survey in 2013 to determine if the mapped offshore formations transmit- ted groundwater offshore and at what location(s).
Water-bearing formations in the MCGB are composed of Holocene alluvium consisting of clays, silts, sands and gravels. Alluvial sediments deposited in the CCAM by Malibu Creek, and other small drainages are estimated to range in thick- ness from a feather edge near the valley walls to 53.3 meters in the central part of the main body of alluvium. The basin hydrogeology is depicted in cross-section shown in Fig. 4.
The offshore extension of the CCG was identified in a 2013 resistivity survey conducted west of Malibu Lagoon along the shoreline and immediately offshore as a higher resis- tivity zone present below a shallow zone consisting of low resistivity material (LPZ). The low resistivity material layer
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