Ground Water Modeling of a Coastal Aquifer
Malibu, California
Otto Figueroa, M.Sc. Candidate, California State University Fullerton
W. Richard Laton, Ph.D., Assistant Professor, California State University Fullerton
Abstract
The city of Malibu, California is one of the few localities in this part of the
country that does not operate a city-wide sewer system. For this reason, all new
developments must assess the impact of the proposed developments to ground water
elevations onsite and offsite. These investigations are further complicated by the
geomorphic expressions of eustatic changes and tectonic uplift during the late Quaternary.
Flights of marine terraces have been mapped along the Southern California coast. A
marine terrace is characterized by a gently ocean-ward dipping abrasion platform on
bedrock units that is overlain by a three to seven meter marine sand layer that is in turn
overlain by non-marine terrace deposits.
A ground water model was developed for a fourteen-acre site on a marine terrace
in Trancas Canyon, Malibu. A Geographic Information System (GIS) (MapInfo©) was
used in tandem with the Visual ModFlow© 3-dimensional ground water modeling
software to determine both the flow direction and the affects on ground water levels due
to waste water disposal onsite. The GIS was used to develop a model of the subsurface
geology of the site and immediate vicinity. The GIS database was also useful in
assigning hydrogeologic parameters including hydraulic conductivity and porosity of each
geologic unit to the model. By importing the geologic model directly into the ground
water modeling environment from the GIS, only minor adjustments were required prior to
running the model.
Ground water elevations observed in late January 2003 and estimated ground
water recharge rates from surrounding sites were used to calibrate the model in a steadystate mode. Constant Head boundary conditions were set just north of the site boundary
to simulate recharge from existing single family homes in the surrounding hills.
Recharge wells were set along Broad Beach Road to simulate recharge from the septic
systems of homes along the beach. A General-Head boundary condition was used to
simulate the Pacific Ocean along Broad Beach. After calibration, a recharge wells were
added east of the site to simulate possible future recharge from proposed condominiums
and recharge points were added onsite to simulate recharge from the future single-family
homes onsite. Results showed that most of the flow is directed away from Broad Beach
and that the coastal water table was affected slightly less than the local tidal influence.