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

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Saltwater Intrusion in Los Angeles Area Coastal Aquifers— the Marine Connection SUPPORTING SOUND MANAGEMENT OF OUR COASTS AND SEAS Saltwater Intrusion in Los Angeles Area Coastal Aquifers— the Marine Connection O ne-third of the water supply for coastal areas of Greater Los Angeles comes from local ground-water sources. Salt- water has penetrated a part of the supply, and a signifi cant part of the remaining supply is at risk. U.S. Geological Survey (USGS) scientists, working in cooperation with local water agencies, are studying the connection between coastal aquifers and the offshore geol- This computer-generated perspective view of the Greater Los Angeles region highlights the locations of “bar- ogy to better understand the riers” (shown in yellow) to saltwater intrusion into coastal aquifers. These barriers consist of freshwater processes and pathways of injection wells. Because some saltwater continues to infi ltrate 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. saltwater intrusion. This image combines high-resolution offshore bathymetric data and onshore digital-eleva- tion data. (Image produced by J.V. Gardner and P. Dartnell, U.S. Geological Survey.) Saltwater from the Pacifi c Ocean is is more complex than previously imag- on the geometry of geologic structures and seeping into some Los Angeles Basin ined. U.S. Geological Survey (USGS) buried erosional features, such as ancient coastal aquifers and replacing freshwater. scientists, working in cooperation with the stream channels. Another system uses Without treatment, this ground water Water Replenishment District of Southern laterally directed acoustic pulses to create does not conform to drinking-water or California and the Los Angeles County detailed three-dimensional images of the agricultural standards. Department of Public Works, are expand- sea fl oor. In addition to providing accurate This problem is signifi cant because ing efforts to understand the geologic much of the water used by the nearly 10 processes and fl uid pathways that control POPULATION AND SALTWATER INTRUSION million residents of Los Angeles County saltwater intrusion. 10 comes from ground-water sources. Al- Potential pathways for saltwater intru- Barriers to intrusion constructed though not all coastal aquifers in the re- sion include hydraulic connection to aquifer 8 Hydraulic gion are at risk, the existing resources are beds exposed at the sea fl oor, fl ow along gradient 6 reversal vital and must be protected to maintain buried ancient stream channels, and fl ow Saltwater intrusion adequate supplies of potable water. through crushed rock in fault zones. The 4 at Redondo Beach In the 1950’s, construction began USGS and its cooperators are focusing their First well drilled County (in millions) 2 on the fi rst of three “barriers” in an at- efforts on (1) conducting geologic and hy- European tempt to halt saltwater intrusion. Each drologic studies on land and (2) surveying Population of Los Angeles settlement begins 1800 1850 1900 1950 2000 barrier consists of a series of injection and mapping the sea fl oor near the coast. Year wells that essentially form a subsurface To investigate the marine connection Saltwater intrusion in coastal aquifers of Los wall of freshwater designed to keep to saltwater intrusion in coastal areas of Angeles County is a result of increasing use of saltwater from penetrating further into Greater Los Angeles, scientists are using a ground-water resources accompanying popula- tion growth. This diagram shows the growth of aquifers. The barriers are only partly ef- variety of techniques to study the geology Los Angeles County’s population and signifi cant fective; saltwater continues to infi ltrate offshore. An acoustic technique, known as events in the history of ground-water usage. In in some areas. refl ection seismology, produces vertical the 1920’s, the overall fl ow of coastal ground water reversed, leading to saltwater intrusion in New studies show that the ground-wa- profi les that provide information on the some coastal aquifers. ter geohydrology of the coastal aquifers layering of strata in the subsurface and also U.S. Department of the Interior USGS Fact Sheet 030–02 U.S. Geological Survey 2002 SALTWATER INTRUSION INTO COASTAL AQUIFERS IN THE LOS ANGELES BASIN In the late 1800’s, water wells pumped by wind A pumping well power were fi rst used to tap into the ground water of potentiometric surface the Los Angeles Basin (A). This technology provided abundant freshwater for residents of the parched coastal region, allowing for expansive growth of sea water confining material both population and agriculture. Increased pumping aquifer 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 B to drop below sea level. As a result, a landward-di- cone of depression develops rected 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 saltwater drilled to inject high-quality freshwater into the wedge intrudes freshwater ground, creating hydraulic pressure ridges or “bar- riers” to saltwater intrusion (C). Ideally, these bar- riers would stem the fl ow of saltwater into coastal C injection well aquifers. However, the barriers are not completely effective. pressure gradient New studies by the U.S. Geological Survey and forms barrier its cooperators show that the geology of the Los Angeles Basin is much more complex than origi- nally conceived. A seismic profi le from the Port of Long Beach (D) shows the complex 0 D A research vessel chartered by the U.S. Geological geology of the area. The sediment layers, Survey leaves port to collect data on submarine geol- shown as different colors, provide many ogy in Los Angeles and Long Beach Harbors. These potential pathways for saltwater intrusion. By understanding this geology, scientists can data, obtained using an acoustic technique known 0 1 MILE as refl ection seismology, are helping scientists un- better determine where and how fast water moves within the various beds of sediment Approximate depth 01KILOMETER derstand the mechanisms of saltwater intrusion in 2,300 feet coastal areas of Greater Los Angeles. both onshore and offshore. Long Beach bathymetry, these images reveal features Buried ancient stream San channel and channel axis of the sea-fl oor morphology of both shal- Pedro Older drainage and low shelf areas and deeper slopes and can- San P edro channel axis yons (see perspective image on previous 6 B 8 ay Huntington page). 6 6 Beach 10 Sediment thickness (meters) 8 10 Other techniques—such as deep continu- PPalosalos V Verdeserdes 8 Fault FFaultault Z Zoneone 15 10 Fold axes in sediment layers ous coring to collect sediment and porewa- 25 10 ter samples for analyses, as well as gravity 6 8 10 modeling and fl uid-fl ow modeling—pro- 12 Newport vide additional new data for improving 5 10 8 6 Beach 6 6 understanding of the geohydrology, sedi- 4 mentology, seismic hazards, and tectonic 15 6 15 45 50 8 evolution of strata in the Los Angeles Ba- 16 35 18 sin. By integrating new data sets and ideas San Gabriel with existing conceptual models, scientists 200 Canyon are able to more accurately predict the fl uid 0 5 MILES fl ow in ground-water systems and better 300 Depth contours in meters 05KILOMETERS (1 meter = 3.281 feet) understand the marine connection to salt- water intrusion in the region. Bathymetric and high-resolution seismic data allow scientists to create accurate maps of the offshore Los The efforts of the USGS and its coopera- Angeles region. This map shows the San Gabriel Canyon, faults, folds, and sediment-fi lled ancient stream tors in studying saltwater intrusion in coastal channels that cross the offshore shelf areas in the region. The ancient stream channels and faults are potential pathways for the infi ltration of saltwater into coastal aquifers. New studies focus on mapping areas of the Los Angeles Basin are leading the seaward extent of coastal aquifers below the sea fl oor, locating underwater rock outcrops, and under- to a better understanding of how the ground- standing the physical properties and porewater chemistry of sediments. water supplies crucial to populous coastal regions can be managed and sustained. Such Edited by research is only part of the USGS efforts James W. Hendley II and Peter H. Stauffer For more information contact: U.S. Geological Survey to protect people’s lives and property from Graphic design by Kevin R. Evans, Rudolph R. Contreras, 345 Middlefi eld Road, MS-999 geologic and environmental hazards in the Sara Boore, and Susan Mayfi eld Menlo Park, CA 94025 (650) 329-5042 coastal zones of the United States. http://walrus.wr.usgs.gov COOPERATING ORGANIZATIONS Water Replenishment District of Southern California This fact sheet and any updates to it are available online Brian D. Edwards and Kevin R. Evans Los Angeles County Department of Public Works at: http://geopubs.wr.usgs.gov/fact-sheet/fs030-02/ Printed on recycled paper.
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