MALIBU WATER QUALITY
Figure 5 - Ancient Course of Malibu Creek and Bedrock Contours
is thought to consist of clay-rich unconsolidated estuarine material (Cardno-Entrix, 2013), similar to materials identified in onshore borings, and correlates with the LPZ previously identified. The resistivity of the CCGs was higher on the west side of the groundwater basin than on the east, suggesting that the aquifer contains fresher water and is more perme- able on the west side of the basin, correlating with one of the identified ancient Malibu Creek channels. The resistivity of the CCG zone was lower by an order of magnitude on the east side, suggesting that the groundwater in this area is brackish, or the aquifer contains more silt and clay, or both. Based on the resistivity survey results, the fresh to brackish water zone appears to rise towards the sea floor offshore and south of the beach on the western side of the groundwater basin. This suggests that the CCGs continue offshore and discharge fresh groundwater to the seafloor (Cardno-Entrix, 2013).
Hydrology/hydrogeology
The MCGB is located in a Mediterranean climate, char- acterized by cool wet winters and warm dry summers. The majority of precipitation occurs between November and April. This area lies in the semi-permanent high-pressure zone of the eastern pacific (Porse et al., 2018). As a result, the climate is mild, tempered by cool sea breezes occasionally interrupted by infrequent periods of extremely hot weather, winter storms or Santa Ana winds. Average annual rainfall is 31.2 centimeters (Jones & Stokes, 2009).
18 TPG •
Apr.May.Jun 2021
The MCGB is a small alluvial basin, approximately 2.45 square kilometers in size, located along the Los Angeles County coastline (DWR, 2016). The basin is bounded by the Pacific Ocean on the south, and by the Santa Monica Mountains, composed of non-water-bearing Tertiary age rocks, on all remaining sides. The valley is typified by steep canyons that generally run north to south and is drained by Malibu Creek to the Pacific Ocean (DWR, 2003, 2014 and 2016).
Malibu Creek is the primary surface water feature within the MCGB. Historically, Malibu Creek has formed a freshwater lagoon near the ocean outfall (Fig. 2). This is confined by a sand berm which is naturally eroded during times of high stream flow. During low stream flow periods, a sand spit closes the lagoon to tidal flushing. This damming of the surface water has a dramatic effect on shallow groundwater flow in the vicinity of the creek.
Groundwater flow within the basin is controlled by both Malibu Creek and the LPZ that divides the basin into an upper unconfined aquifer and a lower semi-confined aquifer (the CCGs). This separation in hydrogeology lessens towards Malibu Creek where the aquifer system acts as a single uncon- fined aquifer with little to no fines from surface to bedrock. This is confirmed via several monitoring wells along the creek (Leighton, 1994; Earth Forensics, 2013; RMC, 2013).
In order to verify the geophysical findings and to further calibrate the water level surveys, a series of wells was installed (Fig. 3) (Earth Forensics, 2013). Three of these wells penetrated bedrock and several others were to the top of the LPZ. In each
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