##### U.S. Geological Survey   Fact Sheet 030–02

**Saltwater Intrusion in Los Angeles Area Coastal Aquifers—the Marine Connection**

| **ABSTRACT**

     One-third         of the water supply for coastal areas of Greater Los Angeles comes from         local ground-water sources. Saltwater has penetrated a part of the supply,         and a significant part of the remaining supply is at risk. U.S. Geological         Survey scientists, working in cooperation with local water agencies, are         studying the connection between coastal aquifers and the offshore geology         to better understand the processes and pathways of saltwater intrusion. |
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| This computer-generated perspective view of the Greater Los Angeles region highlights the locations of "barriers" (shown in yellow) to saltwater intrusion into coastal aquifers. These barriers consist of freshwater injection wells. Because some saltwater continues to infiltrate into Los Angeles Basin aquifers, U.S. Geological Survey scientists are investigating the geology both onshore and offshore to better understand the pathways of saltwater intrusion. This image combines high-resolution offshore bathymetric data and onshore digital-elevation data. (Image produced by J.V. Gardner and P. Dartnell, U.S. Geological Survey.) |

Saltwater from the Pacific   Ocean is seeping into some Los Angeles Basin coastal aquifers and replacing   freshwater. Without treatment, this ground water does not conform to drinking-water   or agricultural standards.

This problem is significant   because much of the water used by the nearly 10 million residents of Los Angeles   County comes from ground-water sources. Although not all coastal aquifers in   the region are at risk, the existing resources are vital and must be protected   to maintain adequate supplies of potable water.

| **POPULATION         AND SALTWATER INTRUSION**                                                                                                                                                                                                                                                                                                                                                                                                                    |
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| Saltwater       intrusion in coastal aquifers of Los Angeles County is a result of increasing       use of ground-water resources accompanying population growth. This diagram       shows the growth of Los Angeles County’s population and significant       events in the history of ground-water usage. In the 1920’s, the overall       flow of coastal ground water reversed, leading to saltwater intrusion in       some coastal aquifers. |

In the 1950’s,   construction began on the first of three “barriers” in an attempt   to halt saltwater intrusion. Each barrier consists of a series of injection   wells that essentially form a subsurface wall of freshwater designed to keep   saltwater from penetrating further into aquifers. The barriers are only partly   effective; saltwater continues to infiltrate in some areas.

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| A research       vessel chartered by the U.S. Geological Survey leaves port to collect data       on submarine geology in Los Angeles and Long Beach Harbors. These data,       obtained using an acoustic technique known as reflection seismology, are       helping scientists understand the mechanisms of saltwater intrusion in coastal       areas of Greater Los Angeles. |

New studies show that   the ground-water geohydrology of the coastal aquifers is more complex than previously   imagined. U.S. Geological Survey (USGS) scientists, working in cooperation with   the Water Replenishment District of Southern California and the Los Angeles   County Department of Public Works, are expanding efforts to understand the geologic   processes and fluid pathways that control saltwater intrusion.

Potential pathways for saltwater   intrusion include hydraulic connection to aquifer beds exposed at the sea floor,   flow along buried ancient stream channels, and flow through crushed rock in   fault zones. The USGS and its cooperators are focusing their efforts on (1)   conducting geologic and hydrologic studies on land and (2) surveying andmapping   the sea floor near the coast.

  To investigate the marine connection to saltwater intrusion in coastal areas   of Greater Los Angeles, scientists are using a variety of techniques to study   the geology offshore. An acoustic technique, known as reflection seismology,   produces vertical profiles that provide information on the layering of strata   in the subsurface and also on the geometry of geologic structures and buried   erosional features, such as ancient stream channels. Another system uses laterally   directed acoustic pulses to create detailed three-dimensional images of the   sea floor. In addition to providing accurate bathymetry, these images reveal   features of the sea-floor morphology of both shallow shelf areas and deeper   slopes and canyons (see perspective image above).

Other techniques—such   as deep continuous coring to collect sediment and porewater samples for analyses,   as well as gravity modeling and fluid-flow modeling—provide additional   new data for improving understanding of the geohydrology, sedimentology, seismic   hazards, and tectonic evolution of strata in the Los Angeles Basin. By integrating   new data sets and ideas with existing conceptual models, scientists are able   to more accurately predict the fluid flow in ground-water systems and better   understand the marine connection to salt-water intrusion in the region.

The efforts of the USGS   and its cooperators in studying saltwater intrusion in coastal areas of the   Los Angeles Basin are leading to a better understanding of how the ground-water   supplies crucial to populous coastal regions can be managed and sustained. Such   research is only part of the USGS efforts to protect people’s lives and   property from geologic and environmental hazards in the coastal zones of the   United States.

| **SALTWATER         INTRUSION INTO COASTAL AQUIFERS IN THE LOS ANGELES BASIN**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
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| In       the late 1800’s, water wells pumped by wind power were first used to       tap into the ground water of the Los Angeles Basin (A). This technology       provided abundant freshwater for residents of the parched coastal region,       allowing for expansive growth of both population and agriculture. Increased       pumping through the early 1900’s caused potentiometric levels (the       levels to which pressure in the aquifer would make water rise in cased wells)       along the coastline to drop below sea level. As a result, a landward-directed       pressure gradient caused saltwater to begin invading coastal aquifers as       early as the 1920’s (B).      In the 1950’s, sets of closely spaced wells were drilled to inject       high-quality freshwater into the ground, creating hydraulic pressure ridges       or “barriers” to saltwater intrusion (C). Ideally, these barriers       would stem the flow of saltwater into coastal aquifers. However, the barriers       are not completely effective.      New studies by the U.S. Geological Survey and its cooperators show that       the geology of the Los Angeles Basin is much more complex than originally       conceived. A seismic profile from the Port of Long Beach (D) shows the complex       geology of the area. The sediment layers, shown as different colors, provide       many potential pathways for saltwater intrusion. By understanding this geology,       scientists can better determine where and how fast water moves within the       various beds of sediment both onshore and offshore. |  |
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| Bathymetric       and high-resolution seismic data allow scientists to create accurate maps       of the offshore Los Angeles region. This map shows the San Gabriel Canyon,       faults, folds, and sediment-filled ancient stream channels that cross the       offshore shelf areas in the region. The ancient stream channels and faults       are potential pathways for the infiltration of saltwater into coastal aquifers.       New studies focus on mapping the seaward extent of coastal aquifers below       the sea floor, locating underwater rock outcrops, and understanding the       physical properties and porewater chemistry of sediments.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |

*By Brian D.   Edwards and Kevin R. Evans*

*Edited by James W. Hendley   II and Peter H. Stauffer*

*Graphic design by Kevin   R. Evans, Rudolph R. Contreras, Sara Boore, and Susan Mayfield; Web layout by Carolyn Donlin*

**COOPERATING ORGANIZATIONS**  Water Replenishment District of Southern California  Los Angeles County Department of Public Works

For more information contact:  U.S. Geological Survey  345 Middlefield Road, MS-999  Menlo Park, CA 94025  (650) 329-5042

Visit the Coastal   and Marine Geology website

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For questions about the   content of this report, contact Brian   Edwards

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| U.S. Department of the Interior | U.S. Geological Survey | Geologic Division | Coastal and Marine Geology |

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Michael Diggles