GROUNDWATER HYDROLOGY IN MALIBU CREEK’S LOWER FLOOD PLAIN
IT’S IMPLICATIONS FOR WASTEWATER DISCHARGE
By Matt Horns
August 3, 2012
Ambrose and Orme produced a document that is posted on the Ecomalibu web site’s “studies” page dated May 2000 and titled “Lower Malibu Creak and Lagoon Resource Enhancement & Management.”
For this article, “Malibu Creek’s Lower Flood Plain” includes the area designated by Ambrose and Orme that is underlain by substrate described as “ fluvial and estuarine” on their map on page 1-5. This includes flat-lying terrain upstream and within Serra Retreat, all the shopping centers, the Colony, the Courthouse/Library, and of course the creek and the lagoon. Malibu Civic Center is located on a rise at slightly higher elevation, so it could be considered just inside, just outside, or at the margin of the flood plain.
The flood plain is underlain by unconsolidated alluvium (sediments including sand, silt, gravel, cobbles, and boulders) deposited by Malibu Creek, with some lenses of sand deposited in a beach or near-shore environment. This sediment is around 200 feet deep at a maximum toward the center of the flood plane and becomes thinner toward the margins. The depth of the alluvium is a result of the lower canyon filling with sediment as sea level has risen several hundred feet in the last 10,000 years or so.
Figure 1-7 on page 1-13 of the Ambrose/Orme document presents a north-south vertical cross-section through this sedimentary basin. This illustration is based on theoretical models.
Figure 1-8 on page 1-16 shows more detail. This illustration is based on extensive field data.
Now, to groundwater hydrology:
The velocity at which groundwater flows though alluvium is determined by 3 factors. One is the gravitational force on groundwater at whatever elevation on whatever celestial body the site is located at. In this case we are talking about Planet Earth’s mean ocean tide levels at Malibu, California, North America. Another is the alluvium’s permeability, which is rather high in this case. The third factor is known as “hydraulic head.” This is the difference in elevation between the water table at various locations. Typically groundwater flows away from locations with a higher water table toward areas with a lower water table. The exception is when groundwater is overlain by an impervious layer and pressure builds, sometimes forcing groundwater upward though the substrate.
Most of the flood plain’s surface lies at an elevation of only 10-20 feet (less in some places, more in others). Most of the sedimentary layers have fairly high permeability rates (the ability of groundwater to flow through them). When elevated above the ocean and allowed to drain efficiently, groundwater typically flows rapidly down-gradient and away though this type of alluvium, allowing for well-drained conditions. Here, the water table along the beach is maintained at a shallow depth by the adjacent ocean. This slows subsurface drainage out of the alluvium and creates very shallow depths to groundwater.
IMPLICATIONS FOR SEPTIC SYSTEMS
When a large amount of rainfall infiltrates into the ground, the water table is elevated at somewhat uniform levels. When water is injected into the ground, for example by a septic leach field, a leaky water pipe, or an injection well, a “mound” is created in the water table. Due to hydraulic gradients formed by this mound, shallow groundwater flows away from the mound in all directions. This is how some of the septic system effluent along Colony Road could possibly flow away from the beach and end up in storm drains and in the lagoon.
IMPLICATIONS FOR SEWAGE TREATMENT SYSTEMS
Waste water injection might not work:
Injecting treated waste water into the ground beneath the flood plain at a rate in which it all flows down-gradient through the alluvium into the ocean is a lot like having the bath tub faucet turned on continually at a rate that will allow it to all flow through the drain and not overflow the tub. Consultants have supposedly determined that the designed discharge rate from the proposed sewage treatment plant will not overflow (in this case emerge from the ground and become surface runoff). The water table at the proposed injection well is already quite shallow and is certain to rise when pumping begins. I see a very thin margin of error in this case. I am not convinced that subsurface flow through the “buried stream channels” that they have found will convey water rapidly enough away from the well to prevent “day-lighting” of a substantial portion of that effluent.
“Waste Water” is a thing of the past:
Southern California’s domestic water supplies are diminishing while demands for water rise. Many established water treatment systems are being retrofitted (at huge financial cost) to recycle their treated water and use it for irrigation. Including a recycling system in the original design is much more cost-effective. Tapia recycles most of its treated water. Pepperdine recycles almost all it theirs. If a sewage treatment plant is built for Malibu, it is unthinkable to me that recycling would not be a part of the design.