U.S. Department of the Interior 

U.S. Geological Survey Seawater Intrusion in a 

Coastal California Aquifer u

G

S

S

This report is a summary of recent work on 

seawater intrusion in aquifers underlying the 

Oxnard Plain, Ventura County, California. It 

is part of a series of reports describing the 

results of the U.S. Geological Survey's Southern California Regional Aquifer-System Analysis (RASA) study of a southern California 

coastal ground-water basin. The geologic setting and hydrologic processes that affect seawater intrusion in aquifers underlying the 

Oxnard Plain are similar to those in other 

coastal basins in southern California.

Introduction

Seawater intrusion in aquifers underlying the Oxnard Plain, Ventura County, California, was first observed in the early 1930's and 

became a serious problem in the mid-1950's 

(California Department of Water Resources, 

1965) (fig. 1). Historically, local agencies 

responsible for the management of ground 

water used a criterion of 100 milligrams per 

liter (mg/L) chloride to define the leading edge 

of the seawater front. It was assumed that all 

high-chloride water from wells behind the 

front originated from seawater that entered 

aquifers through outcrop areas in submarine 

canyons. Recent work (Izbicki, 1991; Stamos 

and others, 1992) showed that other sources of 

high-chloride water to wells are present and 

that the areal extent of seawater intrusion in 

the upper aquifer system is smaller than previously believed.

Hydrogeology

The Oxnard Plain, 60 miles northwest of 

Los Angeles, has an area of 120-square miles 

(mi2) and is underlain by a complex system of 

aquifers more than 1,400 feet thick. These 

aquifers (like many similar coastal aquifers in 

southern California) can be divided into an 

upper and a lower aquifer system (fig. 2).

The upper aquifer system consists of relatively flat-lying alluvial deposits about 400 

feet thick and contains two aquifers that have 

been developed for water supply the Oxnard 

and Mugu aquifers. The Oxnard aquifer, about 

180 feet below land surface, is the primary 

water-yielding zone. The Oxnard aquifer is 

underlain by the Mugu aquifer and overlain by 

a thick, areally extensive clay deposit. This 

clay deposit separates the Oxnard aquifer from 

a shallow unconfined aquifer that previous 

researchers have referred to as the 'perched 

aquifer.' (Use of this name in this report does 

not imply that perched conditions exist in the 

Oxnard Plain.) The Oxnard and Mugu aquifers 

crop out in Hueneme and Mugu submarine

R22W R21W119°05'

T1S 

34°05'

5 KILOMETERS

EXPLANATION

UNCONSOLIDATED 

DEPOSITS

CONSOLIDATED 

ROCKS

BATHYMETRYDepth of water, in feet 

below sea level. Interval 

50 feet 

 50 

CHLORIDE

CONCENTRATIONS

(mg/L)-

100-250

250-500

500-1,000

Map note:

@ Indicates Hueneme Canyon

@ Indicates Mugu Canyon

Figure 1. Chloride concentrations in water from wells in the upper aquifer system in 

the Oxnard Plain, 1955-89. (Data from California Department of Water Resources 

and County of Ventura Public Works Agency.)
canyons less than one-quarter mile offshore 

(fig. 2). The perched aquifer crops out immediately offshore all along the coast in the study 

area.

Native water in the Oxnard and Mugu 

aquifers is generally fresh and has chloride 

concentrations about 40 mg/L. However, in 

some areas (especially near Mugu submarine 

canyon) interbedded fine-grained deposits in 

the Oxnard and Mugu aquifers contain saline 

water. Prior to the onset of seawater intrusion, 

the Oxnard and Mugu aquifers were extensively pumped for water supply. The perched 

aquifer contains fresh and saline water, but it 

is not used as a source of water supply. Saline 

water in the perched aquifer results from a 

combination of (1) seawater that recharged the 

aquifer through offshore outcrops or infiltrated 

into the aquifer through coastal wetlands or 

during coastal flooding [in some cases this 

water was partly evaporated prior to recharge], 

(2) concentration of dissolved minerals resulting from the evaporative discharge of ground 

water, (3) infiltration of irrigation return water.

The lower aquifer system consists of 

alternating layers of alluvial sand and clay 

about 5 to 50 feet thick. These deposits grade 

to marine near the coast and overlie finegrained marine sands that are more than 100 

feet thick and are separated by marine silt and 

clay interbeds as much as 50 feet thick. The 

deposits of the lower aquifer system have been

MARINE AND"

VERTICAL SCALE GREAi L , c .« CRATED 

0 5 MILES

5 KILOMETERS

Figure 2. Generalized geologic sections 

through Hueneme (A in figure 1) and Mugu 

submarine canyons (B in figure 1).

folded and faulted. Marine seismic-reflection 

data (Greene and others, 1978) and testdrilling data show that the lower aquifer system crops out in Hueneme submarine canyon, 

but it does not crop out in Mugu submarine 

canyon because of offshore faults and uplift of 

partly consolidated marine and volcanic rocks 

(fig. 2).

Native water in the lower aquifer system 

generally is fresh and has chloride concentrations that range from 40 to 100 mg/L. The 

lower aquifer system is surrounded and underlain by partly consolidated marine and volcanic rocks that contain saline water. Chloride 

concentrations are as high as 3,400 mg/L in 

the partly consolidated marine rocks and can 

exceed 1,000 mg/L in the volcanic rocks.

Ground-water pumping has caused 

water levels in parts of the Oxnard and Mugu 

aquifers to decline below sea level and below 

the water level in the perched aquifer. Seawater entered aquifers through outcrop areas in 

Hueneme and Mugu submarine canyons in the 

mid-1950's and advanced inland in response 

to changes in the amount and distribution of 

pumping (fig. 1). By 1989, about 23 mi2 of the 

upper aquifer system was believed to be 

intruded by seawater. Because of increasing 

chloride concentrations, pumping was shifted 

from the upper system to the lower system; 

subsequently, water levels in the lower aquifer 

system declined to below sea level. Increasing 

chloride concentrations were observed in the 

lower aquifer system near Mugu submarine 

canyon as early as 1985, and high-chloride 

water in the lower aquifer system near 

Hueneme submarine canyon was discovered 

as part of this study in 1989 (Izbicki, 1991).

After 1993, a combination of groundwater management strategies and increased 

availability of water from the Santa Clara 

River for ground-water recharge caused water 

levels in wells near the coast to rise above sea 

level and above water levels in the perched 

aquifer. Water levels in parts of the lower aquifer system near Hueneme submarine canyon 

also rose above sea level at that time, but water 

levels in the lower aquifer system near Mugu 

submarine canyon were still below sea level as 

late as 1996.

Seawater Intrusion in the Upper Aquifer System

Prior to 1989, most of the wells used to 

monitor chloride concentrations in the upper 

aquifer system were abandoned (unused) agricultural supply wells, and many of these wells 

were screened in more than one aquifer. As 

part of this study, 32 monitoring wells were 

installed in the upper aquifer system at 14 

multiple-well sites within the area believed to 

be intruded by seawater (Densmore, 1996). 

These wells are screened in individual aquifers 

(the Oxnard, Mugu, or perched aquifer) to

enable collection of aquifer-specific waterlevel and water-quality data.

Chloride concentrations in water from 

most wells completed as part of this study 

were less than 250 mg/L (fig. 3) and in water 

from many wells, chloride concentrations 

were not much higher than that of native 

(fresh) ground water. However, chloride concentrations in water from some wells near 

Hueneme and Mugu submarine canyons were

R21W 119°05'

34°05'

5 KILOMETERS

EXPLANATION

(See fig. 1 for base-map explanation) 

SOURCES OF HIGH-CHLORIDE 

WATER TO WELLS- Seawater 

intrusion; chloride concentration, 

in mg/L

500-5,000 

>5,000

Water from fine-grained deposits;

chloride concentration>500 mg/L 

Water from partly consolidated marine

and volcanic rocks; chloride concentration varies with depth

MULTIPLE-WELL SITES-Numbered 

sites are referred to in text; chloride 

concentration, in mg/L 

<250

250-2,500 

>2,500

Figure 3. Areal extent of seawater 

intrusion and high-chloride water from 

other sources in aquifers underlying the 

Oxnard Plain, 1993.
as high as 17,000 mg/L in the Oxnard aquifer, 

and as high as 6,300 mg/L in the Mugu aquifer 

(Izbicki and others, 1995). Chloride concentrations in the perched aquifer near the coast 

were as high as 23,000 mg/L. For comparisons, seawater has a chloride concentration of 

19,000 mg/L; the EPA Maximum Contaminant Level (MCL) for chloride in a public 

water supply is 250 mg/L; and salt-sensitive 

crops common in the study area, such as 

strawberries, are adversely affected by chloride concentrations as low as 100 mg/L.

Results of sample analyses showed that 

water from abandoned agricultural wells was 

not representative of ground water in the 

Oxnard or Mugu aquifers. This is because 

these wells have (1) large diameter casings and 

are difficult to purge of stagnant water prior to 

sample collection, (2) screened intervals that 

may be open to more than one aquifer (as a 

result, chloride concentrations in individual 

aquifers cannot be defined), and (3) steel casings that are subject to corrosion and failure 

especially in areas where water in the perched 

aquifer is saline.

Corrosion and failure of abandoned 

wells may result in leakage of water from the 

perched aquifer into underlying aquifers. For 

example, chloride concentrations in water 

from well 27R1, located in what was believed 

to be a highly intruded part of the upper aquifer system, increased from less than 70 mg/L 

in the early 1960's to 1,900 mg/L in 1990 (fig. 

4). The increase in chloride concentrations 

was used as evidence of increasing seawater 

intrusion in the Oxnard aquifer. However, less 

than 150 feet away, water from well 27R4 

(screened in the Oxnard aquifer) had a chloride concentration in 1990 of 180 mg/L. At the 

same time, water in well 27R5, screened in the 

perched aquifer, had a chloride concentration 

of 23,000 mg/L. The water level in well 27R5 

was higher than the water level in wells 27R1 

or 27R4 (fig. 4), suggesting that water could 

move from the overlying perched aquifer into 

the Oxnard aquifer through the failed casing of 

well 27R1. Leakage of water at a rate of 9 

gal/min was confirmed on the basis of a heatpulse velocity log (R.T. Hanson, U.S. Geological Survey, written commun., 1994). Similar 

results were obtained at two additional locations where multiple-well sites were located 

near abandoned wells. Predmore (1993) identified more than 160 abandoned, and potentially failed, wells in the area believed to be 

intruded by seawater.

Results of a direct-current surfaceresistivity survey in the area believed to be 

intruded by seawater (Zohdy and others, 1993) 

agreed with water-quality data from multiplewell sites and showed that the areal extent of 

seawater intrusion in the upper aquifer system 

was smaller than previously believed. Interpretation of geologic, water-quality, and 

surface-resistivity data by Stamos and others 

(1992) suggested that poor-quality water from

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1,900

-

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23,000

Screened

interval

I

E

1t10 ^

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1

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'PERCHED

AQUIFER1

OXNARD

AQUIFER

MUGU 

AQUIFER

Figure 4. Water levels and chloride 

concentrations in wells 1N/22W-27R 1,3-5 

(Number is chloride concentration, in 

mg/L.)

fine-grained deposits may be an additional 

source of high-chloride water to wells. The 

presence of poor-quality water in the finegrained, lagoonal deposits near Point Mugu 

was confirmed using test-drilling data and 

electromagnetic logs (R.T. Hanson, U.S. Geological Survey, written commun., 1994).

Major-ion data (Izbicki, 1991) and oxygen and deuterium (8 18O and 8D) isotopic 

data (Izbicki, 1996) were used to identify the 

source of high-chloride water to wells and to 

redefine the areal extent of seawater intrusion 

in the Oxnard and Mugu aquifers. Using this 

approach, seawater cannot be positively identified from other sources of high-chloride 

water at chloride concentrations less than 500 

mg/L. The redefined areal extent of seawater 

intrusion in 1993 was 5.5 mi2 in the Oxnard 

aquifer and 3.3 mi2 in the Mugu aquifer (fig. 

3) considerably less than the 23 mi2 (of the 

upper aquifer system) mapped in previous 

studies. Much of the apparent reduction in the 

areal extent of seawater intrusion results from 

the identification of additional sources of highchloride water to wells in the Oxnard and 

Mugu aquifers. For example, near Mugu submarine canyon the seawater front is preceded 

by water from fine-grained lagoonal deposits 

that has chloride concentrations greater than 

500 mg/L in both the Oxnard (4.5 mi2 of highchloride water) and Mugu (3.1 mi2) aquifers.

On the basis of the redefined areal extent 

of seawater intrusion, seawater in the Oxnard 

and Mugu aquifers advanced only 2.7 and 1.9 

miles, respectively, from aquifer outcrops in 

the submarine canyons between 1955 and 

1992 (fig. 3). As a consequence, the seawater 

must be moving slower than previously 

believed. This is consistent with estimates of 

the rate of movement of the seawater front 

made on the basis of tritium and chlorofluorocarbon data (Michel and others, in press).

Seawater Intrusion in the Lower Aquifer System

The lower aquifer system also contains 

multiple sources of high-chloride water to 

wells. These sources include seawater intrusion and movement of high-chloride water 

from partly consolidated marine and volcanic 

rocks that surround and underlie the basin. 

Although not discussed specifically in this 

report, leakage of water through the corroded 

and failed casings of abandoned wells also is a 

source of high-chloride water to wells in the 

lower aquifer system. As part of this study, 25 

monitoring wells (some as deep as 1,500 feet) 

were installed in the lower aquifer system and 

underlying marine rocks at 10 sites near Hueneme and Mugu submarine canyons (Densmore, 1996).

Chloride concentrations in water from 

wells installed in the lower aquifer system as 

part of this study were as high as 2,500 mg/L 

near Mugu submarine canyon and 7,100 mg/L 

near Hueneme submarine canyon. Chloride 

concentrations in water from wells installed in 

the surrounding and underlying marine rocks 

were as high as 3,400 mg/L (Izbicki and others, 1995).

High-chloride water from wells in the 

lower aquifer system near Mugu submarine 

canyon is enriched in iodide relative to a mixture of native water with seawater (fig. 5). 

These data suggest that high-chloride water in 

the lower aquifer system near Mugu submarine canyon is not the result of seawater intrusion. This interpretation was confirmed by 

518O and 8D data, which show that highchloride water near Mugu submarine canyon 

is meteoric, rather than oceanic, in origin 

(Izbicki, 1991). The chemical and isotopic 

composition of high-chloride water from wells 

near Mugu submarine canyon is similar to that 

of high-chloride water from wells as far as 15 

miles inland. These inland wells have been 

invaded by high-chloride water from the 

marine and volcanic rocks that surround and 

underlie the lower aquifer system. Analyses of 

water-quality data suggest that the source of

1,000,000

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uj o

§ | 10,000

3i 5i

1,000

. High-chloride ground water fromy 

the lower system near 

Mugu submarine canyon 

Seawater

_o All other water

1 10 100 1,000 

CHLORIDE, IN MILLIMOLE PER LITER

Figure 5. Chloride-to-iodide ratio as a 

function of chloride in water from wells in 

the Oxnard Plain.
most of the high-chloride water to wells in the 

lower aquifer system near Mugu submarine 

canyon is the partly consolidated marine 

rocks, although the volcanic rocks are a locally 

important source.

The 8 18O and 8D data confirm that highchloride water near Hueneme submarine canyon is a mixture of native water and seawater. 

Seawater intrusion in the lower aquifer system 

was first identified in November 1989 in water 

from a well near Hueneme submarine canyon 

(fig. 3). This well is screened between 720 and 

760 feet below land surface. By May 1991, 

water from a well at the same site, but 

screened between 500 and 520 feet, also was 

intruded by seawater. In August 1993, no 

increase in chloride concentration and no evidence of seawater intrusion was found in 

water from a well at the site screened at a 

depth of 830 to 870 feet or in water from other 

multiple-well monitoring sites inland from 

Port Hueneme.

Electromagnetic logs show that seawater 

intrudes the lower aquifer system through relatively thin layers within the aquifer (R.T. Hanson, U.S. Geological Survey, written 

commun., 1994). Intruded layers are presumably areally extensive and more permeable 

than the unintruded layers. Clay deposits having low permeability restrict the vertical 

movement of seawater between more permeable layers. The layered nature of seawater 

intrusion within the lower aquifer system was 

simulated using a solute-transport model 

developed by the U.S. Geological Survey 

(Tracy Nishikawa, written commun., 1996).

Strategies for Controlling Seawater 

Intrusion

In previous studies, the area! extent of 

seawater intrusion in aquifers underlying the 

Oxnard Plain was overestimated because other 

sources of high-chloride water to wells were 

not recognized. Control of these other sources 

may require new solutions, such as the 

destruction of abandoned (unused) wells (Ventura County Public Works Agency, 1995). 

More accurate understanding of the areal 

extent of seawater intrusion also may allow 

increased pumping from the upper aquifer system, or the use of management solutions such 

as injection barriers, which had previously 

been rejected as impractical for control of the 

widespread seawater intrusion believed to 

exist in the upper aquifer system underlying 

the Oxnard Plain (Venture County Public 

Works Agency, 1995).

Results of a recently completed U.S. Geological Survey simulation-optimization study 

of alternative water-resource management 

strategies (Reichard, 1995) showed that a significant reduction in pumping, particularly in 

the lower aquifer system, and a large quantity 

of additional water (either recharged to aquifers or used in place of ground-water pumping) would be required to control seawater 

intrusion. Numerous projects to reduce water 

demand and develop supplemental water 

sources through the expansion of existing 

ground-water recharge facilities, expansion of 

existing water-distribution facilities, use of 

reclaimed water, and injection of imported or 

reclaimed water into aquifers underlying the 

Oxnard Plain are planned by local water agencies and municipalities (Venture County Public Works Agency, 1995). Information 

obtained from the U.S. Geological Survey 

RASA study of southern California coastal 

basins will aid in the development of those 

projects and aid in the effective management 

of the ground-water resources of the basin.

References Cited

California Department of Water Resources, 

1965, Seawater intrusion: Oxnard Plain, 

Venture County, Bulletin No. 63-1, 59 p.

Densmore, J.N., 1996, Lithologic and groundwater data for monitoring wells in the 

Santa Clara-Calleguas ground-water 

basin, Venture County, California, 1989- 

95, U.S. Geological Survey Open-File 

Report 96-120,179 p.

Greene, G.H., Wolf, S.C., and Blom, K.G., 

1978, The marine geology of the eastern 

Santa Barbara Channel with particular 

emphasis on the ground water basins offshore from the Oxnard Plain, Southern 

California, U.S. Geological Survey 

Open-file report 78-305, 104p.

Izbicki, J.,A., 1991, Chloride sources in a California coastal aquifer, in Peters, Helen, 

ed., Ground water in the Pacific Rim 

Countries: American Society of Civil 

Engineers, IR Div/ASCE, Proceedings, 

p. 71-77.

Izbicki, J.A., 1996, Use of 818O and 6D to 

define seawater intrusion, in Bathala, 

C.T., ed., North American Water and 

Environmental Congress: American 

Society of Civil Engineers, IR Div/ 

ASCE.

Izbicki, J.A., Martin, Peter, Densmore, J.N., 

and Clark, D.A., 1995, Water-quality 

data for the Santa Clara-Calleguas 

Hydrologic Unit, Venture County, California, October 1989 through December 

1993, U.S. Geological Survey Open-File 

Report 95-315, 124 p.

Michel, R.L., Busenberg, E., Plummer, L.N., 

Izbicki, J.A., Martin, Peter, and Densmore, J.N., in press, Use of tritium and 

chlorofluorocarbons to determine the 

rate of seawater intrusion in a coastal 

aquifer: Proceedings of the 16th Seawater Intrusion Meeting, Caligari, Italy.

Predmore, S.K., 1993, Use of a geographic 

information system to identify abandoned wells: U.S. Geological Survey 

Open-File Report 93-147, 1 map.

Reichard, E.G., 1995, Groundwater-surface 

water management with stochastic surface water supplies: A simulation optimization approach: Water Resources 

Research, v. 31, no. 11, p. 2845-2865.

Stamos, C. L., Predmore, S.K., and Zohdy, 

A.R., 1992, Use of D-C Resistivity to 

map saline ground water, in Engman, 

Ted, ed., Saving a threatened resource-- 

In search of solutions: American Society 

of Civil Engineers, IR Div/ASCE, p 80- 

85.

Ventura County Public Works Agency, 1995, 

Annual Report-Fox Canyon Groundwater Management Agency, Ventura, CA., 

32p.

Zohdy, A.R., Martin, Peter, Bisdorf, R.J., 

1993, A study of seawater intrusion 

using direct-current soundings in the 

southeastern part of the Oxnard Plain, 

California: U.S. Geological Survey 

Open-File Report 93-524, 139 p.

For additional information on this 

report contact the author:

John A. Izbicki

5735 Kearny Villa Road, Suite O

San Diego, C A 92123

(619)637-6831

e-mail: jaizbick@usgs.gov

For information on the southern 

California RASA study contact the 

project chief:

Peter Martin

5735 Kearny Villa Road, Suite O

San Diego, C A 92123

(619)637-6827

e-mail: pmmartin@usgs.gov

For information on other RASA 

studies contact the program director:

Hayes Grubb

8011 CameronRoad

Austin, TX 78754

(512)873-3000

e-mail: hfgrubb@usgs.gov

July Fact Sheet 125 -96

GPO 786-658/59149