GROUND WATER non-water bearing and form the base of the Santa Clara–Calleguas Basin. Although these rocks are not an The Santa Clara–Calleguas drainage basin is part important source of ground water, the erosion and of the tectonically active Transverse Ranges subsequent deposition of these rocks are the source of physiographic province. The mountains are composed the unconsolidated deposits that form the Santa Clara– of a variety of consolidated marine and terrestrial Calleguas ground-water basin. The sedimentary rocks sedimentary and volcanic rocks of Late Cretaceous of Cretaceous age are exposed in the Topatopa through Quaternary age. The subbasins of the Santa Mountains north of the ground-water basin and in the Clara–Calleguas Basin are filled with a mixture of Simi Hills and Santa Susana Mountains south of the consolidated and unconsolidated marine and terrestrial basin (California State Water Resources Board, 1956, coastal deposits of Tertiary and Quaternary age. These pl. 10). These rocks are generally non-water bearing basin-fill sediments and consolidated rocks form a except within the poorly cemented and fractured complex set of aquifer systems that have been the sandstones in the hills near Simi Valley (Turner, 1975, primary source of water supplies since the early 1900s. p. 3). Agriculture has been the main user of ground water, The consolidated Tertiary sedimentary rocks and in recent years public supply and industry have underlie most of the ground-water basin and compose become significant users of ground water. The the surrounding mountains and hills. These rocks are geohydrology of the basin is discussed in detail in predominantly marine in origin and are nearly reports by California Department of Public Works impermeable except for the slightly permeable (1934), California Department of Water Resources sandstones and within fracture zones. Some of these (1954, 1958, 1974a,b, and 1975), California State Miocene formations contain oil and tar sand beds, Water Resources Board (1956), Mann and Associates natural gas, and related methane and brines. The Pico (1959), and Turner (1975). The reader is referred to Sandstone of Pliocene and Pleistocene epochs these reports for a more complete description of the underlies the unconsolidated deposits throughout most geohydrology of the Santa Clara–Calleguas Basin. of the ground-water basin and crops out in the mountains on the north side of the Santa Clara River Valley (California State Water Resources Board, 1956, Geologic Framework pl. 10). These rocks are also considered to be of low permeability and non-water bearing. For this report, the lithologic units mapped by Volcanic rocks and related intrusive rocks of Webber and others (1976), Dibblee (1988, 1990a,b, Miocene age underlie parts of the southern Oxnard 1991, 1992a,b,c,d), and Dibblee and Ehrenspeck Plain, South Pleasant Valley, and Santa Rosa Valley (1990) in the Santa Clara–Calleguas Basin and subbasins (figs. 7 and 8D,E). Although these rocks are surrounding area were grouped into two general considered non-water bearing, they have been categories: (1) upper Cretaceous and Tertiary bedrock, developed for water supply where alluvial deposits are and (2) Quaternary unconsolidated deposits. The absent, such as in the Santa Rosa Valley subbasin. outcrop pattern of these combined units is shown in These volcanic and intrusive rocks also crop out in the figure 7A and their stratigraphic relations are shown in Santa Monica Mountains along the southern and figure 7B. southeastern boundaries of the ground-water basin (California State Water Resources Board, 1956, pl. 10) Consolidated Rocks and in the offshore submarine canyons along the The upper Cretaceous and Tertiary consolidated southwestern boundary of the basin (Kennedy and rocks include sedimentary, volcanic, igneous, and others, 1987, pl. 2A). metamorphic rocks. These rocks are virtually

28 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Unconsolidated Deposits than 2,000 ft in the Santa Clara River Valley near The Quaternary unconsolidated deposits consist Ventura (Dibblee, 1992a,b,c,d) and consist of a series of the Santa Barbara Formation (Weber and others, of relatively uniform fine-grained sand layers 100 to 1976), the Las Posas Sand (Dibblee, 1988, 1990a,b, 300 ft thick separated by silt and clay layers 10 to 20 ft 1991, 1992a,b,c,d; Dibblee and Ehrenspeck, 1990), the thick. The upper part of San Pedro Formation consists San Pedro Formation (Weber and others, 1976), and the of lenticular layers of sand, gravel, silt, and clay of Saugus Formation (Weber and others, 1976; Dibblee, marine and continental origin. The continental fluvial 1988, 1990a,b, 1991, 1992 a,b,c,d), all of the silt, sand, and gravel deposits within the upper part of Pleistocene epoch, and unconsolidated alluvial and the San Pedro Formation are referred to as the Saugus fluvial deposits of the Pleistocene to Holocene epoch. Formation by Dibblee (1988, 1990a,b, 1991, In the Santa Clara–Calleguas Basin, the unconsolidated 1992a,b,c,d) and Dibblee and Ehrenspeck (1990). deposits are grouped together into the upper-aquifer These deposits reach a maximum thickness of more system and the lower-aquifer system (fig. 7B). than 5,000 ft in the Piru subbasin in the Santa Clara The Santa Barbara Formation, mapped by River Valley (Dibblee, 1991). The sand and gravel Weber and others (1976), overlies consolidated Tertiary layers range from 10 to 100 ft thick and are separated rocks in most of the ground-water basin and consists of by silt and clay layers that generally are 10 to 20 ft marine sandstone, siltstone, mudstone, and shale. The thick. The Santa Barbara and San Pedro Formations are thickness and lithology of the formation varies absent in the Santa Rosa Valley subbasin east of the considerably throughout the basin, but the formation is San Pedro Fault and in the South Pleasant Valley thickest, more than 5,000 ft, in the Ventura area (Yerkes subbasin southeast of the Bailey Fault. In the eastern and others, 1987). The formation is of low permeability part of the Santa Rosa Valley subbasin and in the and generally contains water of poor quality eastern part of the South Pleasant Valley subbasin, throughout most of the basin (Turner, 1975) and, recent alluvial and terrace deposits were deposited therefore, is not considered an important source of unconformably on the marine shale and sandstone beds ground water. In the East Las Posas Valley subbasin, of the Santa Margarita Formation (Late Miocene) or the Santa Barbara Formation contains layers of sands rest unconformably on the Conejo Volcanics (Middle and gravels that are an important source of water to Miocene). For this study, the Santa Margarita wells in areas where younger unconsolidated deposits Formation in the Santa Rosa Valley subbasin is are absent or are unsaturated. The coarse-grained grouped with the unconsolidated sediments of the section of the Santa Barbara Formation in the East Las lower system. During the Pleistocene epoch, major Posas Valley subbasin is commonly referred to as the changes in sea level resulted in cycles of erosion and “Grimes Canyon” member (California Department of deposition (Dahlen, 1992). The sequence of deposits Water Resources, 1956). above the erosional unconformities typically starts with The Santa Barbara Formation and the lower part a basal conglomerate that is laterally extensive, of the San Pedro Formation mapped by Weber and relatively more permeable than the underlying others (1976) consist of shallow marine sand and deposits, and a potential major source of water to wells gravel beds that were indicated as a separate formation, perforated in these deposits. These coarse-grained the Las Posas Sand, by Dibblee (1988, 1990a,b, 1991, layers of fluvial and beach deposits are interbedded 1992a,b,c,d) and Dibblee and Ehrenspeck (1990). with extensive fine-grained layers. These deposits reach a maximum thickness of more

Ground Water 29 4 2 T T

N

N A' 45' ° ILES 118 0M R18W Piru Lake Oaks , stratigraphic column and Thousand B KILOMETERS 0 .

Fillmore B' D' ter flow model, and 01 01

° Creek from Weber and others, 1976) 119

o

l

Calleguas ?

E'

maril

Clara

aula C'

Ca

Santa P

D Santa

lRio

E

ard

Oxn R22W 15' C °

119 E

Ventura ? Fault McGrath

n

ore

i

ard

h

a B

Pl

Oxn

Offs

? A 30' ? ° Shoreline ? 119 not Area Fault Pitas Point ? mapped Pacific Ocean Generalized surficial geology of the Santa Clara–Calleguas ground-water basin and extents layers in numerical ground-wa A,

° ° 07' 30" 22' 30" 34 34 A Figure 7. related aquifer designations of geologic units by source and aquifer-system model layers, Ventura County, California. (Modified

30 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California The Late Pleistocene and Holocene deposits are EXPLANATION unnamed, consist of relatively flat-lying marine and

UNCONSOLIDATED DEPOSITS continental unconsolidated deposits, and are regionally grouped into the upper system of water-bearing Upper-Aquifer System – deposits (fig. 7B). These deposits, which were derived Alluvium (Shallow aquifer) from local sources and from the Santa Clara River and Submarine shelf deposits Calleguas Creek, were deposited unconformably on the Holocene (Oxnard aquifer) older unconsolidated deposits and contain basal Upper Pleistocene deposits conglomerates that are laterally extensive and produce (Mugu aquifer) substantial ground-water supplies. In the Mound and Lower-Aquifer System – Oxnard Plain subbasins, the basal zones are overlain San Pedro Formation with fine-grained deposits of low permeability. Alluvial

Quaternary (Upper and Lower Hueneme and Fox Canyon aquifers) Undifferentiated and fluvial sand and gravel deposits with interbedded Submarine slope outside Santa Clara- fine-grained deposits of the Holocene epoch

Cenozoic Pleistocene Calleguas Basin Undifferentiated unconformably overlie the Late Pleistocene deposits. Santa Barbara Formation The basal deposits of the Holocene epoch consist of (Grimes Canyon) gravel and sand, which are overlain by fine-grained BEDROCK (Consolidated or deposits throughout most of the Santa Clara River non-water bearing) Valley and Oxnard Plain subbasins. These basal Volcanic rocks (Miocene) deposits are relatively more permeable than underlying

Tertiary deposits, and are potential major sources of water to Sedimentary, igneous, and wells completed in the saturated parts of these deposits. metamorphic rocks (Pliocene-Cretaceous) Interbedded sand layers occur within the fine-grained deposits throughout most of the Oxnard Plain. With the Mesozoic exception of recent coarse-grained channel deposits Faults – Identified during this study Faults – Weber and others (1976); along the Santa Clara River and Calleguas Creek, the Green and others (1978) thin layer of Holocene deposits that are not coincident ? Approximate boundary of with minor tributaries are relatively fine grained and undifferentiated submarine slope relatively low in permeability. D D' Line of section – (see figure 8) Boundary of Santa Clara-Calleguas Hydrologic Unit Ground-water subbasin boundary – Extent of active flow region in upper and lower layers (Subbasins are named in fig. 1) Model-layer extents – Onshore upper and lower

Submarine extent of upper

Submarine extent of lower USGS multiple-well monitoring site with geophysical logs Other wells with geophysical logs used for cross sections Other wells with geophysical logs

Figure 7—Continued.

Ground Water 31 w w o o fl fl , layer 1 4 -aquifer -aquifer -aquifer gional gional model layers system included in re model system, layer 2 included in re model Aquifer system wer wer ormation not ormation not Upper Lo system, layer 2 F Lo F y 3 alle V RASA yon arita sandstones g yon w model w model w o o fl fl included in northeastern Santa Rosa wer Hueneme ox Can

Shallo Upper Hueneme Lo F Grimes Can Santa Mar gional gional Aquifers 2 -water and surface-water flow model of the Santa 9,10 yon urner (1975) T yon 5 2 Green and others (1978) semiperched ox Can ormation not included in re ormation not included in re Recent alluvial and Oxnard Mugu Hueneme F Grimes Can F Not Included F an ormation, wsley o e sands) v 1 an deposits) gressi ll, and alluvial f Dibblee fi w re cial fi ormation e deposits) ormations an, terrace, and marine e, felsic-andesites to basalts) v Saugus F Las Posas Sand (Marine shallo aqueros Sandstones, Modelo F ormation ne-grained lak fi e and intrusi v opanga-V ood plain deposits, arti ood plain deposits and alluvial f ood plain, alluvial f fl fl fl 8 e sands) Lithologic units and F ater Sandstone, Cozy Dell Shale, Matilija v xtrusi 8 er and er and er and e sands and siltstones) v v v v ormation, Monterey Shale, Rincon Mudstone, T 7 gressi ormation, T ormation 11 w re uvial sediments) uvial sandstones and uvial claystones and sandstones) ormation ormation ormation, Santa Susana F fl fl fl ormation eber and others (1976) ormation ormation olcanics ormation, Coldw opanga F W ormation ormation Alluvium V Alluvium ormation Alluvium edro F er T ormation uvial sediments) uncal F deposits) fl F Sisquoc F J w errestrial errestrial conglomerates, sandstones, and shales) errestrial errestrial and marine e errestrial errestrial conglomerate, sandstones, and marine shales) Recent (Lagoonal, beach, ri Recent (Lagoonal, beach, ri Older (Lagoonal, beach, ri terrace deposits) Saugus F (T San P (Marine clays and sands terrestrial Santa Barbara F (Marine shallo Pico F (Marine siltstones, sandstones, and conglomerates) Repetto f (T Santa Margarita F (T Conejo (T Lo (Marine transgressi Sespe F (T Llajas F (Marine sandstones, mudstones, and claystones) Martinez F (T Chico F (Sandstones with shales) 6 6 6 wer) series (epoch) Geologic (Upper) Pleistocene (Lo Pleistocene

aleocene

Holocene Late Early Pliocene Miocene Oligocene Eocene P

nary Quater

ertiary T Stratigraphic column and related aquifer designations of geologic units by source system model layers in the ground system Cretaceous Geologic Upper

era Mesozoic Geologic Cenozoic Figure 7B. Clara–Calleguas Basin, Ventura County, California—Continued

32 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California

dro and Santa Barbara Formations (Weber and (Weber dro and Santa Barbara Formations der of Oxnard Plain. -water and surface-water flow model of the Santa g.11.2). fi . gional Aquifer-System Analysis Program of the U.S. Geological Survey. Aquifer-System gional orebay by Turner (1975). Turner orebay by alley subbasins only. alley V entura County subbasins. entura County subbasins of Santa Paula, Fillmore, Piru, and Las Posas Valley and may be time equivalent to parts of the San Pe and may be time equivalent Valley Fillmore, Piru, and Las Posas entura County subbasins of Santa Paula, V V ormation everywhere except in Pleasant Valley where the Santa Barbara Formation was assigned to the Grimes Aquifer. assigned to the Grimes was where the Santa Barbara Formation Valley in Pleasant except ormation everywhere g. 3). ed on the basis of ash-deposit age dates (Yerkes and others, 1987, ed on the basis of ash-deposit age dates (Yerkes fi w aquifer included in the Oxnard Plain Forebay and inland subbasins. Semiperched part of Shallow aquifer not included in remain and inland subbasins. Semiperched part of Shallow w aquifer included in the Oxnard Plain Forebay Stratigraphic column and related aquifer designations of geologic units by source system model layers in the ground fi ormations from Dibblee (1988; 1990a,b; 1991; 1992a,b,c,d) and Ehrenspeck (1990). Las Posas and Pleasant Includes Mud Pit and Claystone Members. F Perched aquifer designated in parts of the Oxnard Plain only From the current study as part of Southern California Re Shallo Restricted to the Oxnard Plain and F Modi Mapped in eastern Mapped in western San Pedro F 1 2 3 4 5 6 7 8 9 10 11 Figure 7B. Clara–Calleguas Basin, Ventura County, California—Continued others, 1976,

Ground Water 33 3N/21W-11Qa 800 600 400 200 200 400 600 800 FEET Sea level 1,000 1,200 1,400 1,600 1,800 2,000 2,200 2,400 Hueneme. , A' ′

C–C (SP2) 3N/21W-16H5-8

? 4N/18W-28Ea , Section

C

3N/21W-20J3 4N/18W-20La ? subbasin

Santa Paula ? (RP1)

, Los Posas. ′ 4N/18W-31D3-7 B Land – Surface

B 3N/22W-36J1 Piru subbasin , Section B

Land

Surface

2N/22W-2Ca

4N/19W-33F1

2N/22W-3Ka Country Club Fault 4N/19W-32G1 Santa Clara River.

′

2N/22W-9Ma A 4N/19W-31Ca –

A 2N/22W-8L1

, Section

4N/20W-35Ka A Mound

subbasin 2N/23W-13Ka

Fillmore subbasin

3N/20W-3D7

Section E-E'

2N/23W-14N1

Coastline

3N/20W-5C4 3N/20W-6Aa Oak Ridge Fault for location of sections.) A Coastal. (See figure 7 , ′ E – E , Section E acific Ocean P Shallow aquifer Oxnard aquifer Mugu aquifer Upper Hueneme aquifer Lower Hueneme aquifer Fox Canyon aquifer Undifferentiated or non-water- bearing rocks (includes Grimes Canyon aquifer) Queried where doubtful Dashed where inferred. inferred. Throw indicated by arrows Dashed where location and inclination amaica Fault J EXPLANATION , Pleasant Valley. 10,000 20,000 30,000 FEET Fault - Contact - ′ Hydrogeology of the Santa Clara–Calleguas ground-water basin, Ventura County, California. Number in parentheses is localsite monitoringname. well Sections bend at well locations

D

Indicates truncated depth of well Well location and State well number

? –

(RP1)

D 4N/18W-31D3-7 A Vertical exaggeration 15:1 0 600 400 200 200 400 600 800 system system aquifer aquifer Upper- Lower- 1,000 1,200 1,400 1,600 1,800 2,000 2,200 2,400 FEET , Section A Sea level Figure 8. D

34 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 800 600 400 200 200 400 600 800 Sea level FEET 1,400 1,200 1,000 1,000 1,200 1,400 1,600 1,800 2,000

B' 3N/19W-19K2 irview Fault

Fa

(P7)

3N/20W-35R2 2N/21W-3B1 Land Central Las Posas Fault Surface

alley subbasin

East Las Posas V

2N/21W-12H1

(LP1) 2N/21W-11J3 Land Surface subbasin

Oxnard Plain

(SG-1) est Las Posas alley subbasin 2N/21W-15J1

V W rooHonda Arroyo 2N/22W-23B3-7

subbasin

Oxnard Forebay 2N/22W-27Ca

2N/21W-17F5 2N/22W-31Ha subbasin

Oxnard Plain A' 45' °

ILES

118 Section E-E'

Piru 0M Lake N2W12(CM-3) 1N/23W-1C2

KILOMETERS

Coastline 0

B' D'

Creek ver

°

01 01 Ri

119

leguas l

a

C

Clara E'

C'

a Point Mugu t

an S

D 15' C ° E 119 Port

Hueneme B Pacific Ocean 10,000 20,000 30,000 FEET Oxnard Slope 1 Fault

Location of Section Pacific Ocean

30' A ° B ° ° Vertical exaggeration 15:1 0 119 07' 30" 22' 30" 34 34 800 600 400 200 200 400 600 800 FEET 1,000 1,200 1,400 1,600 1,800 2,000 —Continued. Sea level B Figure 8

Ground Water 35 A' 45' ° 118 MILES 0 Piru Lake KILOMETERS

0

B' D'

k

ee Cr

°

01 01

iver

R

119

Calleguas

Clara E'

C'

a Point Mugu

t

n

a S

D 15' C ° E 119 Port

C' Hueneme

B

2N/21W-32E1

ic Ocean Slough Revolon Location of Section Pacif 2N/21W-31Pa

30' A ° ° °

119 07' 30" 22' 30" 34 34 1N/22W-1M3S Camarillo Fault Southern Springville Fault subbasin

Land Oxnard Plain

Surface 1N/22W-16Ra Canyon

Old Hueneme Clay deposits,

1N/22W-20N2

Section

E-E' N2W2D (CM-2) 1N/22W-29D1 Coastline (Undifferentiated) ertiary Marine Sediments T Hueneme Canyon Fault acific Ocean P C 200 200 400 600 800 FEET 1,000 1,200 1,400 1,600 Sea level 10,000 20,000 30,000 FEET —Continued. Vertical exaggeration 15:1 0 Figure 8 C

36 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California A' 45' ° 118 MILES 0 Piru Lake KILOMETERS

0 B' D'

D'

Creek r

°

01 iary 01 ive

and and

lcanics R lcanics) Santa 119 Conejo ormation

Tert (includes

Vo

Vo Sediments Margarita

F leguas

Cal Land

Clara E'

Surface

C' Point Mugu

2N/20W-24E1

anta

S

D 2N/20W-23H2

subbasin

Santa Rosa 15' C °

E San Pedro Fault

119 ertiary Rocks

Port T 2N/20W-21L1

(Undifferentiated) Hueneme B alley

South V

Pleasant 2N/20W-29B1 subbasin alga Creek Calleguas

Location of Section Pacific Ocean

30' A ° ° °

119 07' 30" 22' 30" 34 34 2N/20W-30M1 alley olcanics

North V

V Camarillo Fault Pleasant subbasin 1N/21W-1B4 alley

South V

Pleasant subbasin

1N/21W-15D2

1N/21W-21K4

eoo Slough Revolon

1N/21W-28N2 S2W3Q (Q-2) 1S/21W-32Q2 subbasin

Oxnard Plain

Section E-E' S2W83(CM-1) 1S/21W-8L3 ertiary

Marine T Sycamore Canyon Fault Coastline Sediments olcanics V acific Ocean P ault F D 0,000 20,000 30,000 FEET 400 200 200 400 600 800 1,000 1,200 1,400 FEET Mugu Slope 1 Sea level —Continued. Vertical exaggeration 15:1 01 D Figure 8

Ground Water 37 A' 45' ° 118 MILES 0 Piru Lake KILOMETERS

0

B' D'

k

ee Cr ° 01 01

E'

River

119

a

Calleguas

lar

C E' C' Point Mugu

Bailey Fault

Santa

Section Mugu

D-D' Canyon D

S2W83(CM-1) 1S/21W-8L3 15' C °

E ertiary Marine T 119 1S/21W-6F1 Sediments Port and Volcanics

North Mugu Lagoon Fault

(Undifferentiated) S2W11(CM-6) 1S/22W-1H1

Hueneme

B Location of Section Pacific Ocean (CM-5) 1N/22W-35E1

30' A ° ° ° 119 07' 30" 22' 30" 34 34 Canyon Old Hueneme Clay deposits, Land

Surface

Old Hueneme Canyon Fault

N2W2G (CM-4) 1N/22W-28G1

Section C-C' N2W2D (CM-2) 1N/22W-29D1 subbasin Oxnard Plain ault

F

? 1N/22W-18L2 ertiary Rocks T (Undifferentiated) ?

Hueneme Canyon

1N/22W-7Ea

Section

B-B' ault

El Rio F N2W12(CM-3) 1N/23W-1C2 2N/23W-2600a

McGrath Fault

at lr River Clara Santa

Section A-A'

ault 2N/23W-14N1 F

Mound

subbasin 2N/23W-10R1 Oak Ridge Santa Clara-Calleguas Hydrologic Unit Boundary E ault 400 200 200 400 600 800 F 1,000 1,200 1,400 1,600 1,800 entura oothills

V F Sea level FEET IN ELEVATION, 10,000 20,000 30,000 FEET —Continued. Vertical exaggeration 15:1 0 E Figure 8

38 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Aquifer Systems Upper-Aquifer System Shallow Aquifer—The Shallow aquifer extends The water-bearing deposits were previously from land surface to a depth of 60 to 80 ft along the divided into six aquifers in the Santa Clara–Calleguas Santa Clara and the Arroyo Las Posas flood plains and Basin within the two regional aquifer systems (Turner, throughout most of the Oxnard Plain and Pleasant 1975). Using geophysical and geochemical data Valley subbasins (figs. 7 and 8). Along the flood plain collected as a part of the USGS RASA Program, the of the Santa Clara River, the shallow aquifer consists of aquifer designations were realigned into seven major predominantly sand and gravel and is an important aquifers. The unconsolidated deposits of the late source of ground water. During prolonged droughts, Pleistocene and Holocene epochs are grouped into the the Shallow aquifer becomes dewatered in the upper regional upper-aquifer system, which includes the reaches of the Santa Clara River and Arroyo Las Posas. Shallow, Oxnard, and Mugu aquifers (fig. 7B). The Beneath the Oxnard Plain and Pleasant Valley lower-aquifer system is composed of complexly faulted subbasins, the Shallow aquifer consists of fine-to- and folded unconsolidated deposits of the Pliocene and medium sand with interbedded clay layers and is Pleistocene epochs and include the upper and lower referred to as the “semiperched aquifer”; the clay Hueneme, Fox Canyon, and Grimes Canyon aquifers layers separate the Shallow aquifer from the underlying (fig. 7B). The lower aquifer extends to about 1,600 ft Oxnard aquifer. The Shallow aquifer occasionally below sea level in the Oxnard Plain subbasin to more becomes perched locally because of pumping from the than 2,000 ft below sea level in the Mound subbasin Oxnard aquifer. Water quality is poor throughout most (fig. 8 A,E). All these aquifers extend offshore within of the Oxnard Plain and Pleasant Valley subbasins and the continental shelf (fig. 8); however, the thickness, consequently few wells are perforated opposite this structure, and extent of the submarine outcrops vary aquifer. across the basin for the upper- and lower-aquifer Oxnard Aquifer—The Oxnard aquifer lies at the systems (figs. 7 and 8). base of the Holocene deposits and consists of sand and The onshore part of the Oxnard Plain is gravel deposited by the ancestral Santa Clara River and subdivided into a confined region and an unconfined the Calleguas Creek and by their major tributaries. The region. The unconfined region includes the Oxnard coarse-grained basal deposits of the Holocene epoch Plain Forebay and the northeastern part of the Oxnard are referred to as the “Oxnard aquifer” (Turner, 1975). Plain. The confined region was subdivided into The base of the aquifer ranges from about 150 to 250 ft Northwest and South Oxnard Plain model subareas for below land surface throughout most of the Oxnard the water-management analysis in this study (fig.1). Plain subbasin (fig. 8). The basal deposits range in The submarine shelf extends (fig. 7A) southwestward thickness from less than 10 to 200 ft and are a major from the coastline and is subdivided along the McGrath source of water to wells in the Piru, Fillmore, Santa Fault as an extension of the onshore separation between Paula, Oxnard Plain Forebay, and Oxnard Plain the Mound subbasin and the Oxnard Plain (figs. 1 subbasins. Hydraulic conductivity in the Oxnard and 7); these subbasins are hereinafter referred to as the aquifer is about 190 ft/d near Port Hueneme (Neuman “offshore Mound” and “offshore Oxnard Plain” and Witherspoon, 1972). The Oxnard aquifer is subbasins. For the water-management analysis in this relatively fine grained in the Mound, Pleasant Valley, study, the offshore Oxnard Plain was subdivided into Santa Rosa Valley, and Las Posas Valley subbasins; this northern and southern regions separated by the aquifer is not considered an important source of ground Hueneme submarine canyon. water in these subbasins. Throughout most of East and West Las Posas Valley subbasins, the Oxnard aquifer is unsaturated.

Ground Water 39 In the Piru and Fillmore subbasins, there are few Clara River was the primary source of sediment for if any clay layers separating the Shallow and Oxnard both aquifers. The Mugu aquifer is differentiated from aquifers; therefore, ground water can move freely the lower-aquifer system because it is less indurated between the two. In the Santa Paula subbasin, the Santa and relatively undisturbed. However, because of the Clara River has migrated south of the ancestral river similarities between these deposits, many investigators that deposited the sediments of the Oxnard aquifer and include the upper Pleistocene deposits in the mostly overlies non-water-bearing rocks of Tertiary lower-aquifer system. In the Pleasant Valley, Santa age. As a result, the Santa Clara River does not overlie Rosa Valley, East Las Posas Valley, and West Las Posas the Oxnard aquifer throughout most of the Santa Paula Valley subbasins, the Mugu aquifer sediments were subbasin. derived from South Mountain and the surrounding hills In the Oxnard Plain Forebay subbasin, there are and are finer grained than sediments derived from the relatively few clay layers separating the Shallow and Santa Clara River. Oxnard aquifers. Alluvial fans derived from the Throughout most of the ground-water basin, the mountains north of the Mound subbasin pushed the Mugu aquifer extends from about 200 to 400 ft below Santa Clara River south toward South Mountain. In the land surface (fig. 8) and consists of sand and gravel Oxnard Plain Forebay subbasin, clay layers were interbedded with silt and clay. The silt and clay layers eroded by the Santa Clara River, and sand and gravel retard the vertical movement of water through the were deposited in their place; owing to the absence of Mugu aquifer and confine or partly confine the aquifer. clay, this subbasin is artificially recharged by surface Over most of the ground-water basin, the top of the spreading of water diverted from the Santa Clara River. aquifer is relatively flat; however, the base of the The Oxnard aquifer is considered to be unconfined in aquifer has a more irregular surface (Turner, 1975) the Oxnard Plain Forebay subbasin. owing to a regional uncomformity. This uncomformity, Throughout the Oxnard Plain and Pleasant which is most pronounced in the Mound and the East Valley subbasins, the Shallow and Oxnard aquifers are Las Posas Valley subbasins (fig. 8A,B,E), is due to separated by clay layers. These clay layers confine or deformation during deposition of older alluvium that partly confine the Oxnard aquifer throughout most of contains the Mugu aquifer. the Oxnard Plain and Pleasant Valley subbasins. Few production wells are perforated solely in the Previous investigators (California Department of Water Mugu aquifer; most are also perforated in the overlying Resources, 1956; Turner, 1975) reported that the clay Oxnard aquifer or in the underlying lower-aquifer layers separating the Shallow and Oxnard aquifers in system. In general, wells that are perforated opposite the Point Mugu area are thin or absent, allowing free both the Oxnard and Mugu aquifers, which are similar interchange of water in this part of the subbasin. in thickness, obtain most of their water from the However, data, collected from several multiple-well Oxnard aquifer because it is significantly more monitoring sites constructed in the Point Mugu area as permeable. Hydraulic conductivities estimated from a part of this study (Densmore, 1996), indicate that slug tests at the multiple-well monitoring sites relatively thick clay layers separate the Shallow and constructed for this study range from less than 1 to Oxnard aquifers. 98 ft/d; most, however, are less than 25 ft/d (E.G. Mugu aquifer—The Mugu aquifer (Turner, Reichard, U.S. Geological Survey, written commun., 1975) is composed of the basal part of the unnamed 1995). When individual wells at the same multiple-well upper Pleistocene deposits. In the Piru, Fillmore, Santa monitoring site were tested, the estimated hydraulic Paula, Mound, Oxnard Plain Forebay, and Oxnard conductivity of the Oxnard aquifer was almost always Plain subbasins, these deposits are similar to those of higher than that estimated for the Mugu aquifer. the underlying lower-aquifer system because the Santa

40 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California In subbasins in which the Mugu aquifer is Santa Clara–Calleguas Basin (fig. 7B). The lower part predominantly coarse-grained (the Piru, Fillmore, and of the San Pedro Formation as defined by Weber and Santa Paula subbasins), wells perforated in both the others (1976) and the upper part of the Las Posas Sand Mugu aquifer and the underlying lower-aquifer system as defined by Dibblee are referred to as the “Fox obtain most of their water from the Mugu aquifer. This Canyon aquifer” in the Las Posas, Pleasant Valley, and is shown by a wellbore flowmeter test completed on Oxnard Plain subbasins (Turner, 1975). The coarse- well 3N/21W–11J5 in the Santa Paula subbasin (see grained layers of the Santa Barbara Formation as figure A5.1 in Appendix 5). Although this well is defined by Weber and others (1976) are commonly perforated predominantly in the lower-aquifer system, referred to as the “Grimes Canyon aquifer” in the East almost all the water yielded by the well is derived from Las Posas Valley subbasin and parts of the Pleasant the Mugu aquifer. As stated previously, the Mugu Valley subbasins (Turner, 1975). In most of the other aquifer is less indurated than the lower-aquifer system, subbasins, the Santa Barbara Formation is of low which would account for its greater water-yielding permeability, yields poor-quality water, and is not capacity. In the subbasins where the Mugu aquifer is considered an important source of water. Regional fault predominantly fine grained, wells yield significant systems (figs. 7 and 8) segregate the lower-aquifer quantities of water from the aquifer only if they are system into many parts and affect the flow of water perforated opposite the basal coarse-grained zone. This between and within the subbasins. laterally extensive basal zone, which, as noted earlier, Upper and Lower Hueneme Aquifers—The is due to a regional unconformity, yields water readily Hueneme aquifers constitute the upper part of the San to wells. Many wells are not perforated opposite this Pedro Formation beneath the Oxnard Plain mapped by zone, however, because its thickness is 20 ft or less Weber and others (1976), and the Saugus Formation throughout many of the subbasins. Results of the beneath the Santa Clara River Valley subbasins mapped wellbore flowmeter test for well 1N/21W–15D2 by Dibblee (1988, 1990a,b, 1991, 1992a,b,c,d) and (figure A5.1 in Appendix 5) in the Pleasant Valley Dibblee and Ehrenspeck (1990). These aquifers consist subbasin show that the basal zone of the Mugu aquifer of lenticular layers of sand, gravel, silt, and clay. The yields significantly more water per foot of aquifer sediments constituting the aquifers have been subjected penetrated than does the underlying lower-aquifer to considerable folding, faulting, and erosion since system. deposition. These deposits were divided into upper and lower aquifers based on data from electric logs which Lower-Aquifer System show a decrease in electrical resistivity at the contact between the aquifers. The decrease is attributed to the The lower-aquifer system consists of the folded presence of more fine-grained deposits in the Lower and faulted Pleistocene continental and marine deposits Hueneme aquifer than in the Upper Hueneme. The of the Saugus, San Pedro, and Santa Barbara Upper Hueneme aquifer reaches a maximum thickness Formations as defined by Weber and others (1976) and of more than 700 ft (fig. 8A) and the Lower Hueneme the Saugus Formation and the Las Posas Sand as aquifer reaches a thickness of more than 2,000 ft in the defined by Dibblee (1988, 1990a,b, 1991, 1992a,b,c,d) axis of the Santa Clara syncline in the Santa Paula, and by Dibblee and Ehrenspeck (1990). For this study, Fillmore, and Piru subbasins. In areas of the basin that the unconsolidated deposits of the Saugus and the have been uplifted since deposition (fig. 8A,D,E), upper part of the San Pedro Formations as defined by much of the sediments constituting Hueneme aquifers Weber and others (1976) and the Saugus as defined by have been removed by erosion. Dibblee were split into the “Upper Hueneme” and “Lower Hueneme” aquifers, respectively, for the entire

Ground Water 41 In the Oxnard Plain subbasin, the Upper thicknesses of 100 to more than 300 ft. In contrast, the Hueneme aquifer is predominantly fine grained in two overlying Lower Hueneme aquifer is characterized as a areas along the coast line between Port Hueneme and series of relatively high resistivity zones 10 to 100 ft in Point Mugu (Old Hueneme Canyon on figure 8C,E). thickness separated by relatively low resistivity zones These fine-grained deposits are more than 200 ft thick 10 to 20 ft in thickness. Most of the electric logs near the coast, and they extend about 3.5 mi inland. inspected show there was a significant shift in the Turner (1975) attributed these deposits to a lagoonal or spontaneous potential curve opposite the Fox Canyon embayment depositional environment throughout most aquifer, indicating a change in the aquifer mineralogy of the San Pedro Formation deposition. Inspection of and (or) a change in the water quality of the aquifer. lithologic and electrical logs collected during the Historically, very few wells tapped the Fox drilling of the multiple-well monitoring sites Canyon aquifer of the ground-water basin, except in constructed for this study indicates that these the East and West Las Posas Valley and the Pleasant fine-grained deposits are ancestral submarine canyons Valley subbasins. Because yield is significantly less in (fig. 8C,E) that were backfilled during a rise in sea this aquifer than in the upper aquifer system, few water level. The submarine canyons were carved into the San wells were perforated solely in the Fox Canyon aquifer. Pedro Formation sometime prior to the deposition of This limited testing of the hydraulic properties of the the deposits of the upper Pleistocene. These backfilled aquifer. For previous investigations, electric logs from ancestral submarine canyons are important hydrologic petroleum wells were used to define the character and features because they are low permeable barriers to extent of the aquifer. High-resistivity zones on those ground-water flow and may contribute to coastal logs, which indicate possible coarse-grained zones of subsidence (fig. 9). The hydraulic conductivity of the good quality water, led to the belief that the Fox fine-grained deposits in the ancestral submarine Canyon aquifer would be a major source of water to canyon, estimated from a slug test at the CM-5 wells. multiple-well monitoring site (fig. 8E), was 0.1 ft/d To help determine the hydraulic properties of the (E.G. Reichard, U.S. Geological Survey, written Fox Canyon aquifer, at least one piezometer at 13 of commun, 1995). the 23 multiple-well monitoring sites constructed for Fox Canyon Aquifer—The Fox Canyon aquifer this study was perforated opposite the aquifer. The constitutes the basal part of the San Pedro Formation lithologic and electric logs for these sites indicate mapped by Weber and others (1976). The aquifer relatively low permeability; the Fox Canyon aquifer consists of weakly indurated very fine- to medium- consists of predominantly fine- to very fine-grained grained fossiliferous sand with occasional gravel and sand that is indurated to slightly indurated (Densmore, clay layers of shallow marine origin. As stated 1996); this is coincident with the high-resistivity zones previously, Dibblee (1992a,b,c,d) mapped these on the electric logs and reflects the low dissolved-solids deposits as a separate formation, which he designated concentration of water in the aquifer and the induration as the Las Posas Sand. The marine deposition of the of the aquifer sediments. The low permeabilities were sediments of the Fox Canyon aquifer resulted in a confirmed by slug tests that indicate hydraulic relatively uniform series of layers, which can be conductivities ranging from 1 to 9 ft/d (E.G. Reichard, correlated by the electric logs, over large areas of the U.S. Geological Survey, written commun., 1995). ground-water basin (Turner, 1975). The Fox Canyon These hydraulic conductivities are considerably lower aquifer is identified on electric logs by zones of than those of the overlying aquifers. relatively high resistivity that are almost identical for

42 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 119°15' 119° 118°45' Lake

Cr r Piru Sespe C Cr

Pole u ir Cr per P r C San Hop T in a t 4 ount a r M Fillmore N ulphu S Paul

ber a 34° Tim iver 22' Cr R 30" dge Ellsworth Santa Oak Ri Paula

ain hMount Big Clara Sout Mtn

C r Santa

Ventura o F da Arroyo S Saticoy reeman Diversion roy

Ar Arroyo imi undellnca Hon alley Ar a Saticoy osas Simi V Spreading Las P Barr Wash Hills Grounds Las Posas Hills reek T rillo C El Rio ey 2 Cama Spreading Cr Grounds Camarillo N Oxnard PlElRio Beardsl a ejo in Con Revol

Oxnard on

Pacif TIDAL E S 3 584 lough Thousand Oa ks ic Calleguas

34° Z 901 07' 30" Ocean 0 10 MILES

0 10 KILOMETERS Z 583

R22W R18W

EXPLANATION Santa Clara-Calleguas ground-water basin Shallow alluvium and unconsolidated deposits Oil field Hydrologic unit boundary Outside Santa Clara-Calleguas ground-water basin Fault Ground-water subbasin Shallow alluvium Syncline boundary A Consolidated and unconsolidated deposits Anticline Z 583 Bench mark and designation

TIDAL E Z 3 584 583 0 0 Z901 Z583 TIDAL 3 1939-60 ?

? 1 Bend in 1 traverse ? ? ?

? 2 2 ? 1939-78

TOTAL SUBSIDENCE, IN FEET E 584 TOTAL RELATIVE SUBSIDENCE, IN FEET 3 3 024681012 1920 1940 1960 1980 2000 B DISTANCE ALONG TRAVERSE, IN MILES C YEAR

Figure 9. Subsidence in Oxnard Plain and Pleasant Valley, Santa Clara–Calleguas ground-water basin, Ventura County, California. A, Geographic features. B, Subsidence profile. C, Subsidence of bench marks through time.

Ground Water 43 To determine the relative contribution of water The Grimes Canyon aquifer is also present in from the Fox Canyon aquifer to wells perforated in the the southeastern part of the Oxnard Plain subbasin and Fox Canyon and overlying aquifers, available throughout most of the Pleasant Valley subbasins flowmeter logs were inspected and additional logs were (Turner, 1975); many production wells tap this aquifer collected (see table 5 in the “Ground-Water Discharge” throughout the Pleasant Valley subbasin. Lithologic section). The flowmeter log collected at well 2N/22W- and electric logs collected from multiple-well 13N2 in the Oxnard Plain Forebay subbasin (in monitoring sites constructed for the RASA study Appendix 5) shows that little, if any, water enters the indicate that much of the deposits that contain the wells from the Fox Canyon aquifer, and almost all the Grimes Canyon aquifer are relatively fine grained and water pumped comes from the basal zone of the water is relatively high in dissolved-solids (Densmore, overlying Lower Hueneme aquifer. Flowmeter logs 1996). Although deposits similar to those of the Grimes collected from wells in the Oxnard Plain and the Canyon aquifer are present beneath the western part of Pleasant Valley subbasins indicate that, in most of the the Oxnard Plain subbasin, few production wells tap wells tested, the flow contribution from the Fox these deposits owing to their greater depth in that part Canyon aquifer is less than the flow contribution from of the subbasin. the overlying aquifers. Data from the flowmeter logs from the Pleasant Valley and the Oxnard Plain subbasins indicate that the Fox Canyon aquifer is a Ground-Water Recharge major source of water to wells perforated throughout the lower-aquifer system only if the overlying Lower Sources of recharge to the aquifer systems Hueneme aquifer is absent or is predominantly fine include streamflow infiltration, direct infiltration of grained. Based on well construction data, this may be precipitation on the valley floors of the subbasins and the case throughout most of the East and West Las on bedrock outcrops in adjacent mountain fronts, Posas Valley, Oxnard Plain, and Pleasant Valley artificial recharge of diverted streamflow and imported subbasins. surface water, percolation of treated sewage effluent, Grimes Canyon Aquifer—The Santa Barbara and infiltration of excess irrigation water (irrigation Formation (Weber and others, 1976), which consists of return flow) in some agricultural areas. For previous non-water-bearing marine sandstone, siltstone, studies, recharge was estimated using consumption and mudstone, and shale, underlies the Fox Canyon aquifer water-balance methods based on precipitation and throughout most of the ground-water basin and is streamflow data for various historical periods considered the base of the ground-water system (Grunsky, 1925; California Department of Public throughout most of the basin. However, in parts of the Works, 1934; California State Water Resources Board, ground-water basin, the upper part of the Santa Barbara 1956; Mann and Associates, 1959; California Formation contains water-bearing deposits referred to Department of Water Resources, 1975). as the “Grimes Canyon aquifer” (Turner, 1975). In the East Las Posas Valley subbasin, the Streamflow Infiltration Grimes Canyon aquifer predominantly consists of Streamflow infiltration is the largest component layers of well-indurated sandstones and conglomerate of ground-water recharge in the Santa Clara–Calleguas with high resistivity as indicated by the electric logs, basin and includes gaged and ungaged streamflow. The characteristic of well-indurated sandstone and Santa Clara River and the Calleguas Creek have been conglomerate layers. Because the sediments are well altered substantially by regulated flow; the construction indurated, the hydraulic conductivity of the aquifer is of the Santa Felicia Dam (Lake Piru) transformed flow relatively low. However, the Grimes Canyon aquifer is in the Santa Clara River system from predominantly an important source of water in the East Las Posas winter and spring floodflows to significant summer and Valley subbasin where the overlying aquifers are absent fall low flows. or are unsaturated.

44 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Gaged Streamflow of the flow in the Santa Clara River is lost to ground- Previous estimates of annual subbasin water recharge (fig. 10B). Streamflow loss is low for streamflow-infiltration rates are summarized in table 3. many of the wettest years, such as 1969 and 1984 These reported estimates were aggregated into (fig. 10A), which may indicate a significant contribution of ungaged inflow prior to or during averages for the wet and dry periods used in this study 3 (fig. 2A). The total estimated gaged streamflow periods with relatively low flow (less than 200 ft /s). infiltration reported by the California Department of The annual range of gaged streamflow loss in the Santa Water Resources (1975) for 1937–67 ranged from 0 to Clara River for 1956–93 varied from about 2,700 to 297,700 acre-ft annually (table 3). These estimates 97,800 acre-ft/yr (table 3). On a climatic basis, total yield average wet-year and dry-year infiltration rates infiltration for the Santa Clara River was about 34,000 that are 67 and 57 percent of estimated runoff, (22 percent of flow) and 25,100 (37 percent of flow) respectively. The ratios of wet-year to dry-year acre-ft/yr for wet- and dry-year periods during infiltration for the Santa Clara River and for the total 1956–93, respectively; for the Calleguas Creek above basin during the period were 2.0 and 2.7, respectively Highway 101, it ranged from 0 to 6,100 acre-ft/yr for (table 3). For streamflows less than 250 ft3/s (about 500 the period of record (1973–93) (table 3). The wide acre-ft/d), the rates of infiltration on the Santa Clara range of streamflow loss also was subject to the effects of additional inflow from treated municipal sewage River were about 14 percent, and for several dry years 3 (such as 1952 and 1958) the rates ranged from 50 to 70 between gaging stations of about 12 ft /s (8,700 acre- percent (California Department of Water Resources, ft/yr) and irrigation return flow. 1975, fig. 15). Streamflow infiltration along the Santa Clara Streamflow loss for the Santa Clara River for wet River, estimated as part of a sediment-transport study, and dry seasonal flows less than 250 ft3/s (about is 23 percent of flow per mile for flows less than 100 ft3/s, 20 percent of flow per mile for flows from 100 to 500 acre-ft/d) was determined by subtracting 3 downstream gaged streamflow (gaging station 500 ft /s, 6 percent of flow per mile for flows from 500 to 1,000 ft3/s, and less than 2 percent of flow per mile 11114000) from the sum of upstream gaged inflows 3 (gaging stations 11108500, 11110000, 11110500, 713, for floodflows greater than 1,000 ft /s (Brownlie and 11113000, 11113500) (fig. 4). Similarly, the Taylor, 1981). streamflow loss for Calleguas Creek was estimated as Densmore and others (1992) estimated the difference between downstream streamflow streamflow infiltration for a summer drought under (11106550) and gaged inflows (11106850 and conditions of controlled releases from Lake Piru. The 11106400) for flows less than 10 ft3/s (20 acre-ft/d) controlled releases result in an increase in infiltration rate with increased channel width in Piru Creek when (fig. 4). Seasonal streamflow losses in the Santa Clara 3 River and the Calleguas Creek varied widely but releases exceed 200 ft /s (Steve Bachman, United generally show several patterns (fig. 10). Regression of Water Conservation District, oral commun., 1996). seasonal streamflow loss in relation to total gaged These various infiltration estimates collectively streamflow indicates an overall loss of 35 percent for suggest that infiltration is dependent on antecedent wet-year seasons (fig. 10A) and 52 percent for dry-year conditions, which include antecedent ground-water seasons (fig. 10B) for the Santa Clara River. Loss from levels; magnitude of the streamflow and related the Calleguas Creek during low-flow conditions is properties, such as channel width; and current and generally either 0 percent during winter and fall antecedent regulated flows. seasons or 100 percent during spring and fall seasons (fig. 10C). During dry-year summers, 70 to 100 percent

Ground Water 45 (62,500; infiltration Basin total oor runoff. 1 99,800) (40,300; 108,600) fl 12,400–143,300 — — 0–297,700 —

Creek ow. fl — — — — 0–6,100 Calleguas otal infiltration T River Santa Clara ltration values during dry and wet periods, respectively. — during dry and wet periods, respectively. ltration values 12,300–138,900 (51,300; 65,000) — — 0–185,800 (26,400; 51,800 ) 2,700–97,800 ( 25,100; 34,000) ura County, California fi 2 includes estimates of runoff from ungaged tributaries. from ungaged includes estimates of runoff are average in are average ow fl includes estimates of runoff from ungaged tributaries and valley- tributaries from ungaged includes estimates of runoff ow fl Ungaged Recharge ltration from treated waste water or irrigation return or irrigation water ltration from treated waste Mountain-Front fi 1,400–56,200 ( 11,500; 30,200) — — 3,600–190,000 (22,400; 80,300) 3,800–78,500 (13,200; 34,200) in ow fl Montalvo Forebay of (23,600; 74,600) (12,100; 22,100) Oxnard Plain — 1,000–145,000 1,000–39,300 — — (14,900; 15,700) Santa Paula — 0–16,200 (4,100; 9,300) 4,200–24,400 — — Streamflow infiltration orks (1934, table 59). For Santa Clara River, the net stream Santa Clara River, orks (1934, table 59). For W Gaged ater Resources (1975, tables 23 and 24). For basin total the net stream ater Resources (1975, tables 23 and 24). For W er Valley. v Fillmore ater Resources Board (1956, tables 12–15). (20,500; 26,800) leakage for Calleguas Creek does not include additional stream leakage for Calleguas W — 0–33,600 (0; 0) 1,800–49,100 — — ow fl . Stream Piru the California State

— 6,800–68,600 (18,500; 38,400) 6,400–68,300 (19,100; 36,100) — — Summary of estimated ranges and averages gaged ungaged streamflow infiltration in the Santa Clara-Calleguas Basin, Vent Estimates from the California Department of Public Estimates include only the Santa Clara Ri Estimates from Estimates from Mann (1959), Plates 28–31 Estimates from the California Department of Estimates from this study 1 2 3 4 5 6 1993 1932 1951 1957 1967 6 1 3 4 5 1893 to 1937 to 1937 to 1937 to 1956 to ime period ime periods are reported in water years except from this study. All estimates are in acre-feet per year. Numbers in parentheses All estimates are in acre-feet per year. from this study. years except ime periods are reported in water of estimate T Table 3. [T , no estimate was made] , no estimate was

46 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 500 TOTAL STREAMFLOW IN RELATION TO A STREAMFLOW LOSS FOR WET-YEAR SEASONS 1958 LINE OF STREAMFLOW LOSS EQUAL TO TOTAL STREAMFLOW 1994 400 LINEAR REGRESSION LINE FOR WET-YEAR SEASON LOSS 1967 1967 SEASON ENDING FOR YEAR SHOWNWINTER 1966 SPRING 1995 SUMMER 1993 300 FALL

200 1966 1985 1966 1982 1979 1981 1969 1978 1979 1992 1986 1965 1984 100 1959 1957 1956 1984 1957 1984 1985 1969 1968 1987 1958 1984 1986 1980 1956 1957 1957 1967 1965 1968 1956 1965 1985 1982 1981 1982 1968 1970 0 0 100 200 300 400 500 600 700 800 900 1,000

500 TOTAL STREAMFLOW IN RELATION TO STREAMFLOW LOSS FOR DRY-YEAR B SEASONS LINE OF STREAMFLOW LOSS EQUAL TO TOTAL STREAMFLOW 400 LINEAR REGRESSION LINE FOR

STREAMFLOW LOSS, IN ACRE-FEET PER DAY DRY-YEAR SEASON LOSS 1972 SEASON ENDING FOR YEAR 1962 SHOWN WINTER SPRING SUMMER 300 FALL 1992 1962 1972 1973

1974 1971 200 1972 1988 1987 1991 1970 1989 19751987 1959 1970 1964 1988 100 1959 1971 1978 19771990 1991 1989 1972 1988 1990 1990 1963 19591990196119761988 1960 1962 1960 1963 1989 1965 1973 1963 1961 1961 1991 1977 1974 1964 1976196419601964 1973 1961 196319771976 1987 1975 0 1960 1976 0 100 200 300 400 500 600 700 800 900 1,000 TOTAL GAGED INFLOW, IN ACRE-FEET PER DAY

Figure 10. Estimated seasonal streamflow losses for gaged inflows in the Santa Clara River and Calleguas Creek and tributaries, Ventura County, California. A, Santa Clara River streamflow in wet-years seasons. B, Santa Clara River streamflow in dry-year seasons. C, Calleguas Creek streamflow in wet-and dry-year seasons.

Ground Water 47 C 20 STREAMFLOW IN RELATION TO STREAMFLOW LOSS FOR WET-YEAR SEASONS STREAMFLOW IN RELATION TO STREAMFLOW LOSS FOR DRY-YEAR SEASONS LINE OF STREAMFLOW LOSS EQUAL TO TOTAL STREAMFLOW SEASON ENDING FOR YEAR SHOWN WINTER SPRING SUMMER FALL 1993 1994 1989 1992 1994 1993

1992 1994 1990 10 1990 1991

1991 1989 1991 1989 1989 1990 STREAMFLOW LOSS, IN ACRE-FEET PER DAY

8 3 5 3 5 2 78 80 76 75 77 985 97 97 98 97 9 0 19741977 197619 19 19 1983 19821 1983 1 1 19 1986 1 198 1 198 1 1984 01020 TOTAL GAGED INFLOW, IN ACRE-FEET PER DAY

Figure 10—Continued.

Ungaged Streamflow alluvium and becomes ground-water recharge. This Infiltration of streamflow in ungaged drainage assumption may result in an overestimate of recharge basins at the boundary of an alluvial aquifer system is during floodflows. referred to as “mountain-front recharge.” Mountain- Previous estimates of mountain-front recharge front recharge occurs along the arroyos and the small range from 1,400 to 190,000 acre-ft/yr for 1893–1967 tributary stream channels of the 64 ungaged tributary (table 3). In some wet years such as 1969, 1978, 1979, drainage basins that drain into the ground-water 1980, 1983, 1986, and 1993, measured outflow at the subbasins from the surrounding mountain fronts of the downstream gaging station at Montalvo (11114000) Santa Clara–Calleguas Basin. This component of (fig. 4) on the Santa Clara River was greater than gaged streamflow constitutes a small but significant inflow from the major tributaries. This difference can contribution to streamflow and ground-water recharge, be attributed to the contribution of ungaged streamflow. especially during wet years. For this study, it was Based on gaging-station data, this ungaged streamflow assumed that the streamflow percolates into the may have ranged from 39,800 to 479,800 acre-ft/yr for the Santa Clara River for 1956–93 and from 300 to 7,800 acre-ft/yr for Calleguas Creek for 1973–93 (the period of record).

48 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California For this study, mountain-front recharge was Previous estimates of direct infiltration for water estimated by means of a modified rational method years 1894 through 1957 (California Department of using gaged streamflow data from two small Public Works, 1934; Mann and Associates, 1959; subdrainage basins, Hopper and Pole Creeks (fig. 4), California Department of Water Resources, 1975) are referred to as “index” basins. The fraction of summarized in table 4. The total estimated infiltration precipitation assumed to be mountain-front recharge for the Santa Clara River Valley subbasins ranges from was estimated as the ratio of total seasonal streamflow 0 to 90,800 acre-ft/yr (table 4) and averages 30,400 and for each ungaged subdrainage basin to the average total 5,300 acre-ft/yr for wet-year and dry-year periods, seasonal precipitation for an index basin. To estimate respectively (Mann and Associates, 1959). mountain-front recharge, estimates of seasonal Direct infiltration was estimated as a percentage precipitation were required for each of the subdrainage of precipitation and ranged from no infiltration in the basins for each wet year and dry year (fig. 3). It was confined parts of the Mound, Oxnard Plain, and North assumed that most of the runoff from the ungaged Pleasant Valley subbasins to as much as 6,238 acre- drainage basins infiltrates near the mountain fronts and ft/yr in the unconfined Fillmore subbasin. The does not contribute significantly to mainstem percentage of precipitation was based on the modified streamflow. rational method in which the amount of potential The amount of recharge was estimated as the recharge is the fraction of runoff from the index index-basin streamflow fraction of precipitation subdrainage basin multiplied by the total volume of multiplied by the average total volume of seasonal precipitation for each ground-water subbasin. This precipitation (drainage area multiplied by kriged method may overestimate potential recharge during seasonal precipitation) for each of the 64 ungaged periods of sustained rainfall when soil moisture is tributary subdrainage basins. Seasonal (winter, spring, exceeded and overland runoff to stream channels summer, and fall) estimates for wet and dry years were occurs. Total estimated recharge as direct (valley-floor) made for all 64 subdrainage subbasins. The average infiltration ranges from 18,300 to 32,700 acre-ft/yr percentages of precipitation that became mountain- (fig. 11A, table 4) during dry- and wet-year periods, front recharge during the period of record for the two respectively; this estimate included an additional index subdrainage basins, Pole and Hopper Creeks, 2,200 acre-ft/yr of direct bedrock infiltration along the were 4 and 7.5 percent, respectively. Estimates of basin margins, which is described in a later section in mountain-front recharge ranged from about 3,800 to the context of developing estimates of inflow for the 78,500 acre-ft/yr for 1956–93 (table 3) and averaged subareas of the ground-water model. 34,200 and 13,200 acre-ft/yr for wet- and dry-year periods, respectively. The estimates of seasonal Artificial Recharge mountain-front recharge ranged from zero for most of Artificial recharge is a major contributor to the Oxnard Plain to as much as 12,000 acre-ft per ground-water recharge in the Oxnard Plain Forebay season for the Santa Clara River Valley subbasins and the Piru subbasins (fig. 11A). Artificial recharge (figs. 1 and 11A). was started in 1929 adjacent to Piru and Santa Paula Creeks and the Santa Clara River near Saticoy. The use Direct Infiltration of streamflows for recharge, as well as for agriculture, Recharge also occurs as direct infiltration of supplemented the growing use of the ground-water precipitation on the valley floors (hereinafter referred resources. Additional surface-water storage was to as “valley-floor recharge”) and as direct infiltration provided by construction of Santa Felicia Dam on Piru of precipitation on bedrock outcrops (hereinafter Creek in the early 1950s. Major diversions along the referred to as “bedrock recharge”). These components Piru and Santa Paula Creeks and along the Santa Clara of recharge constitute a small but significant River at Saticoy and Freeman have been used for contribution to streamflow and ground-water recharge, artificial recharge of the upper-aquifer system. especially during wet years.

Ground Water 49 A 50 10 Oxnard Plain Forebay subarea South Oxnard Plain subarea

40 8

30 6

20 4

10 2

0 0 10 10 Northwest Oxnard Plain subarea Northeast Oxnard Plain subarea

8 8

6 6

4 4

2 2

0 0 20 20 18 Piru subbasin18 Fillmore subbasin 16 16 14 14 12 12 10 10 8 8

INFLOW, IN THOUSANDS OF ACRE-FEET PER SEASON 6 6 4 4 2 2 0 0 10 10 Santa Paula subbasin Mound subbasin

8 8

6 6

4 4

2 2

0 0 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 YEAR

Figure 11. A, Estimates of seasonal ground-water inflows to the subbasins and to the Oxnard Plain subareas of the Santa Clara–Calleguas ground- water basin, Ventura County, California, 1891–1993, and B, Annual estimated and reported ground-water pumpage in the Santa Clara–Calleguas ground- water basin, Ventura County, California, 1891–1993.

50 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California A 10 10 North Pleasant Valley subbasin South Pleasant Valley subbasin

8 8

6 6

4 4

2 2

0 0 10 10 West Las Posas Valley subbasin South Las Posas Valley subbasin

8 8

6 6

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0 0 10 10

INFLOW, IN THOUSANDS ACRE-FEET PER SEASON East Las Posas Valley subbasin Santa Rosa Valley subbasin

8 8

6 6

4 4

2 2

0 0 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 YEAR

EXPLANATION

Estimated mountain-front recharge Estimated irrigation return flow recharge Estimated treated wastewater recharge Reported artificial recharge Estimated valley-floor recharge

Figure 11—Continued.

Ground Water 51 B 50 Oxnard Plain Forebay subarea South Oxnard Plain subarea

40

30 TOTAL REPORTED 20 PUMPED-BACK RECHARGE

10

0 50 Northwest Oxnard Plain subarea Northeast Oxnard Plain subarea

40

30

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0 50 Piru subbasin Fillmore subbasin

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20 PUMPAGE, IN THOUSANDS OF ACRE-FEET PER YEAR

10

0 50 Santa Paula subbasin Mound subbasin

40

30

20

10

0 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 YEAR

Figure 11—Continued.

52 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California B 50 North Pleasant Valley subbasin South Pleasant Valley subbasin

40

30

20

10

0 50 West Las Posas Valley subbasin South Las Posas Valley subbasin

40

30

20

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0 50 East Las Posas Valley subbasin Santa Rosa Valley subbasin

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30

PUMPAGE, IN THOUSANDS ACRE-FEET PER YEAR 20

10

0 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 320 YEAR Santa Clara-Calleguas ground-water basin 280

240 EXPLANATION ADJUSTED 200 TOTAL Estimated agricultural pumpage 160 BASIN Reported municipal pumpage PUMPAGE Total reported pumpage 120 USED FOR Total reported agricultural pumpage MODEL Total reported non-agricultural pumpage 80

40

0 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 YEAR

Figure11—Continued.

Ground Water 53 otal direct recharge infiltration T (12,900; 34,800) 400–62,200 2 — — 4,000–306,400 (20,200; 108,300) 18,300–32,700

otal T — — — — 2,200 oor runoff. bedrock recharge fl alley alley/ Rosa Santa V V — — — — 640–2,400 Pleasant est W Basin, Ventura County, California (1,000; 3,000)

Las Posas 0–5,400 — — — 350–2,100 lleguas ltration of precipitation includes potential runoff. fi — — — — 800–4,700 (570; 3,400) Las Posas East/South — — — — 3,600 Plain Oxnard includes estimates of runoff from ungaged tributaries and valley- tributaries from ungaged includes estimates of runoff ow fl (1,600; 4,500) (500; 3,200) Forebay 100–12,200 Montalvo 0–8,000 0–9,500 (0; 0) 4 — 1,700 of Oxnard Plain ltration during dry and wet periods, respectively. —, no estimate made] ltration during dry and wet periods, respectively. Mound fi 0–4,500 (750; 2,200) — — — 0 (2,300; 6,600) 0–25,600 0–11,800 0–26,200 (0; 9,600) 3 (900; 8,000) — 4,500 Santa Paula oor recharge. Period of record is in calendar years. oor recharge. fl Fillmore alley- (2,800; 13,600) orks (1934, table 59). For the Santa Clara River, the net stream the Santa Clara River, orks (1934, table 59). For er Valley and West Las Posas subbasins. West and Valley er 500–37,900 W v 0– 21,200 (4,400; 11,800) 0–29,800 ( 0; 11,700) 2 — 3,100–6,200 ater Resources (1975, tables 3, 19, and 20). For basin total, the estimated recharge from direct in basin total, the estimated recharge ater Resources (1975, tables 3, 19, and 20). For . Numbers in parentheses indicate the average in . Numbers in parentheses indicate the average W ater Resources Board (1956, tables 12–15). Piru W 200–15,100 0–11,300 (2,700; 6,600) 0–11,300 (0; 4,800) 1 (1,100; 5,600) — 950–1,900 Associates (1959, plates 28–31). Summary of selected estimates ranges and averages direct infiltration precipitation in subbasins the Santa Clara–Ca California Department of Public Includes only the Santa Clara Ri California State Mann and California Department of Final estimates from this study for v 1 2 3 4 5 6 years Period of 1894 to 1932 1937 to 1951 1937 to 1957 1937 to 1967 1891 to 1993 record in water Table 4. [All estimates in acre-feet per year 1 3 4 5 6

54 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Artificial recharge began with offstream and controlled releases of combined imported water spreading-works to help provide an adequate and and natural streamflow from Lake Piru during summer dependable water supply for agriculture. Spreading- and fall are diverted at the Freeman Diversion. About works were operated by the Santa Clara Conservation 2,500,000 acre-ft was artificially recharged along the District: Santa Clara River streamflow was diverted Santa Clara River Valley of which 378,054 acre-ft was near Saticoy beginning in 1928–29; Piru Creek at Piru (October 1931–December 1993), 1,228,615 streamflow was diverted near Piru beginning in acre-ft at Saticoy (October 1928–December 1993), 1930–31; and Santa Paula Creek streamflow was 868,408 acre-ft at El Rio (December 1955–December diverted near Santa Paula beginning in water year 1931 1993), and 26,968 acre-ft at Santa Paula. Some of the (Freeman, 1968). The initial capacities of the surface water diverted at Saticoy and later at the diversions for the Saticoy, Piru, and Santa Paula Freeman Diversion was directly delivered by pipelines spreading grounds (fig. 4) were 120, 60, and 25 ft3/s, for irrigation. About 239,966 acre-ft of the diversions respectively (Freeman, 1968). These sites represent was delivered directly through the Pleasant Valley some of the earliest efforts to divert and artificially pipeline (September 1958–December 1993) from recharge shallow ground-water aquifers. The Saticoy surface-water diversions, and an additional 4,161 acre- and Piru spreading grounds have been in continuous ft was delivered to John Lloyd Butler farms (March operation since their construction more than 70 years 1970–May 1991) for irrigation (Greg Middleton, ago. The Santa Paula diversion was operated for about United Water Conservation District, written commun., 11 years (1930–41) (fig. 11A), recharging a total of 1994). 26,968 acre-ft. Since the 1960s, most artificial recharge at El The UWCD added additional spreading grounds Rio has been pumped back for nearby irrigation or at El Rio (fig. 4) in 1955 for diversions at Saticoy and delivery by pipeline to adjacent subbasins. During added the Pleasant Valley pipeline and reservoir in October 1955–December 1977, about 389,600 acre-ft 1957 for additional storage capacity. Earthen dikes was recharged at El Rio and about 170,974 acre-ft was were used to divert as much as 375 ft3/s of streamflow pumped back. Net recharge during this period was from the Santa Clara River at Saticoy. However, about 218,600 acre-ft and the pump-back rate was between 1955 and 1983, there were 81 dike failures at 44 percent. During July 1979–December 1993, about the diversion (United Water Conservation District, 411,300 acre-ft was recharged and about 231,400 acre- 1983). The earthen dike and diversion was relocated ft (44 percent) was pumped back at El Rio. The ratio of about 1 mi upstream after the riverbed degraded during pumpage to recharge for the El Rio artificial storage the large flood of 1969, but the dike remained prone to and recovery system (ASR) for 1978–93 ranged from failures at streamflows greater than 1,600 ft3/s. A 0.38 in wet years to 1.5 in dry years. concrete dike and diversion structure, called the Freeman Diversion, was constructed in 1991. It is more durable and provides a larger diversion capacity of 460 ft3/s. Natural streamflow during winter and spring

Ground Water 55 Irrigation Return Flow Ground-Water Discharge Deep percolation of excess applied irrigation Discharge of water from the aquifer systems water (hereinafter referred to as “irrigation return includes ground-water discharge as pumpage from flow”) is an additional source of artificial recharge to wells, evapotranspiration along the river flood plains, the ground-water system. However, areally extensive and offshore flow along submarine outcrops. Some confining units retard the recharge of irrigation return additional intermittent baseflow to rivers occurs at the flow to the upper-aquifer system throughout most of subbasin boundaries, but the baseflow generally the Oxnard Plain and Mound subbasins. Irrigation infiltrates again in the downstream subbasin and thus is return flow is redirected by drains throughout most of not considered a loss to the ground-water flow system. the southern part of the Oxnard Plain subbasin to During the wet periods, however, ground water streamflow that discharges to the Pacific Ocean through discharges as stream baseflow to the Pacific Ocean; this Revolon Slough (fig. 4). Increases in nitrate base-flow component of discharge to the ocean was concentrations in ground water from wells in the larger prior to the 1930s (Freeman, 1968). upper-aquifer system (Izbicki and others, 1995; Izbicki and Martin, 1997) and related increases in ground- water levels may indicate that some irrigation return Pumpage flows are infiltrating back to the upper-aquifer system The first wells were drilled on the Oxnard Plain in the Santa Clara River Valley and Las Posas Valley in 1870 following the severe drought of 1853–64 and subbasins and in the Oxnard Plain Forebay and Santa during a sustained dry climatic period (1840–83) Rosa Valley subbasins. The deep percolation of (fig. 2). Although pumping occurred during the late irrigation return flow within these subbasins consists of 1800s and early 1900s, pumpage was minimal and varying amounts of surface water and ground water. therefore was not estimated for this report. These first The amount of return flow was estimated based on a artesian flowing wells typically were drilled to depths 70-percent irrigation efficiency of applied water of 90 to 143 ft, and discharges were about 500 to (Blaney and Criddle, 1950, 1962) for the areas of 1,000 gal/min (Freeman, 1968). Many wells were irrigated agriculture estimated from five land-use maps. completed during 1870–71 for irrigation of field crops. Estimates by Koczot (1996) were based on areas and During the early development of the ground-water crop types delineated from land-use maps for 1912 resources, the drilling of wells diminished the flow of (Adams, 1913), 1927 (Grunsky, 1925; Koczot, 1996), the springs and the artesian wells. By 1912, as many as 1932 (California Department of Public Works, 1934), 42 pumping plants were operating north of the Santa 1950 (California Department of Public Works, 1950), Clara River, providing water for irrigation and and 1969 (California Department of Water Resources, domestic use (Freeman, 1968). 1970). The resulting annual estimates were about By 1920, a progressive lowering of water levels 17,900 acre-ft for 1912; 46,100 acre-ft for 1927; throughout the Santa Clara River Valley and the 45,700 acre-ft for 1932; 52,600 acre-ft for 1950; and Oxnard Plain subbasins required the replacement of 67,900 acre-ft for 1969. When the estimates for the many centrifugal pumps with deep turbine pumps. By Oxnard Plain and Mound subbasins are excluded, the 1924, many of the previously undeveloped areas of the annual estimates of irrigation return flow are reduced to Santa Clara–Calleguas Basin were being used for about 11,800 acre-ft for 1912; 26,900 acre-ft for 1927; agriculture (Grunsky, 1925). On the basis of a 1912 22,400 acre-ft for 1932; 27,700 acre-ft for 1950; and land-use map, estimated agricultural pumpage yields a 37,900 acre-ft for 1969 (Koczot, 1996). basinwide average rate of withdrawal of about 33,500 acre-ft/yr, which results in a potential total withdrawal of about 267,700 acre-ft for the years 1919–26 of the dry-year period 1919–36 (fig. 2).

56 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Ground water initially was developed total withdrawal of about 513,500 acre-ft for 1927–30. predominantly for agricultural use. Agricultural Calculated pumpage estimated from the 1932 land-use ground-water pumpage was estimated indirectly from map is at about 174,000 acre-ft/yr, yielding an land-use maps for periods prior to the metering of estimated total withdrawal of about 2,610,000 acre-ft pumpage; Koczot (1996) estimated pumpage using for 1931–45. Estimates of agricultural pumpage, based selected land-use maps and consumptive-use estimates on the 1950 land-use map, yield a basinwide average for 1912, 1927, 1932, 1950, and 1969. Land-use maps rate of pumpage of 180,000 acre-ft/yr and a total were used instead of electrical power records because withdrawal of about 2,880,000 acre-ft for 1946–61. of the labor required to construct pumpage records for By 1967, about 800 wells equipped with deep- large timespans and because many wells were not well turbine pumps provided more than 90 percent of powered by electricity. These land-use maps were used the water demand in the basin (Freeman, 1968). On the to delineate agricultural consumptive use which was basis of 1969 land use, estimates of agricultural used to estimate pumpage for periods prior to metering pumpage yield a basinwide average rate of withdrawal not represented by land-use maps. The 1912 land-use of about 201,700 acre-ft/yr, yielding an estimated total map was used for 1919–26; the 1927 map was used for pumpage of 3,227,200 acre-ft for 1962–77. 1927–30; the 1932 map was used for 1931–45; the Reported pumpage was compiled from the 1950 map was used for 1946–61; and the 1969 map technical files of the FGMA and UWCD for July 1979– was used for 1962–77. These land-use time periods December 1993. These data generally were semiannual were based on a combination of factors including land totals of user-reported agricultural, nonagricultural, use, climate, water levels, and historical events. The and total pumpage. Early pumpage data were land-use pumpage estimates were used as initial incomplete for the Las Posas Valley, Pleasant Valley, agricultural pumpage for the simulation of ground- and Santa Rosa Valley subbasins. For these areas, 1984 water flow but were adjusted for some periods during FGMA reported pumpage was used to represent model calibration (fig. 11B). Municipal pumpage for pumpage for 1978 through 1983. Estimated and the cities of Ventura, Camarillo, and Oxnard and for the reported total annual pumpage were combined for the Channel Islands Beach Community Services District entire Santa Clara–Calleguas Basin and range from 760 (near Port Hueneme); pumpage for a fish hatchery in acre-ft for 1912 to as much as 301,400 acre-ft for 1990, the southern end of the Piru subbasin; and pumpage of which was during the last sustained drought. artificial recharge in the Oxnard Plain Forebay Reporting of metered pumpage began in the subbasin were estimated independently and combined 1980s; the total reported basinwide pumpage was with the agricultural pumpage for the total estimated 2,468,610 acre-ft during the 10-year period 1984–93 pumpage prior to 1983. (Greg Middleton, United Water Conservation District, Ground-water development continued to spread written commun., 1994). Of this reported total in the ground-water basin during the severe drought pumpage, 37 percent was from the Oxnard Plain period of 1923–36, tapping deeper aquifers for subbasin, 37 percent from the upper Santa Clara River agricultural supplies (fig. 2). As the surface-water Valley subbasins, 13 percent from the Las Posas Valley resources became fully developed in the early 1930s, subbasin, 9 percent from Pleasant Valley subbasin, new ground-water development began to provide a 3 percent from the Mound subbasin, and 1 percent significant proportion of the water resources. In the from the Santa Rosa Valley subbasin. 1930s, the first deep wells were drilled in the Pleasant Valley and Las Posas Valley subbasins. Calculated agricultural pumpage, estimated from the 1927 land- use map, yields a basinwide average rate of withdrawal of about 128,400 acre-ft/yr for 1927 and an estimated

Ground Water 57 Evapotranspiration the submarine shelf farther offshore. Because of the Evapotranspiration (ET) from the regional alternating layers of coarse- and fine-grained ground-water flow system is restricted to the river flood sedimentary deposits in these coastal aquifer systems, plains, where ground water and streamflow infiltration submarine leakage through the tops of the upper- and are within the depths of the root zones of riparian lower-aquifer systems that crop out along the vegetation. ET was not calculated for parts of the submarine shelf probably is small. Outside of some coastal areas of the Oxnard Plain subbasin where the folklore, there are no estimates or evidence, such as Shallow aquifer is “semiperched.” cold seeps, of submarine discharge in the Ventura area. Previous estimates of annual ET for the Santa However, the possibility of seawater intrusion along the Clara River Valley subbasins range from 11,700 acre- coastal Oxnard Plain subbasin has long been ft/yr for 1892–1932 (California Department of Public recognized (Grunsky, 1925; California Department of Works, 1934) to 13,724 acre-ft/yr for 1958–59 (Mann Public Works, 1934; Freeman, 1968); geochemical and Associates, 1959). The estimated average ET for evidence of seawater intrusion in the upper- and lower- the entire Santa Rosa Valley subbasin for 1972–83 is aquifer systems (Izbicki, 1991, 1992, 1996a) indicates about 4,300 acre-ft/yr (Johnson and Yoon, 1987). a hydraulic connection to the submarine outcrops of the Previous estimates of the ET rate vary widely, ranging aquifer systems (figs. 7 and 8). Coastal flow was from 1.1 ft/yr (California Department of Water estimated using the ground-water flow model Resources, 1974a,b) to 2.4 ft/yr (California developed for this study and is described later in the Department of Public Works, 1934) to as much as 5.2 report (see Simulation of Ground-Water Flow). ft/yr (Mann and Associates, 1959). Borehole electromagnetic-induction (EM) logs The total area classified as land with riparian of monitoring wells installed as part of the RASA vegetation or as a flood plain was estimated from the Program indicate that seawater intrusion occurs along five land-use maps (1912, 1927, 1932, 1950, 1969) multiple coarse-grained beds that are commonly, but compiled for the RASA study (Koczot, 1996; not exclusively, the basal units of the seven major Predmore and others, 1997). A combination of riparian aquifers that compose the upper- and lower-aquifer land distributions from the 1912, 1927, 1932, and 1950 systems (figure A5.2 in Appendix 5). These basal units maps of the Conejo Creek area yields an estimated total commonly occur above regional unconformities that of 14,945 acres of riparian vegetation along the stream are related to the major sea-level changes during the channels for predevelopment conditions in the basin. Pleistocene epoch. Natural gamma and EM The 1932 land-use map for the entire basin indicates a geophysical logs collectively indicate that the flow of total riparian area of 11,237 acres. The most detailed seawater from the ocean occurs laterally through the set of land-use maps (1950) for the entire basin yielded submarine outcrops and remains confined to the most a reduction to 6,539 acres of riparian land by 1950. By transmissive coarse-grained beds that are bounded by 1969, the total was only 2,265 acres. The model, fine-grained layers (figure A5.2 in Appendix 5). A developed for this phase of the RASA study, was used cross-sectional solute transport model developed for to simulate the evapotranspiration along the flood plain the Port Hueneme area supports the conceptual of the Santa Clara River, Calleguas Creek, and its framework of lateral intrusion, with vertical intrusion major tributaries. impeded by shallow fine-grained confining units (Nishikawa, 1997). Seawater intrusion forms a relatively sharp interface with fresh ground water as it Coastal Flow along Submarine Outcrops enters the basal coarse-grained beds of the aquifer Discharge from the regional ground-water flow systems laterally and remains stratified in the layered systems probably occurs as lateral flow to the Pacific coastal alluvial-aquifer systems of the Santa Clara– Ocean through outcrops that are exposed along the Calleguas Basin. steep walls of the submarine canyons and that truncate

58 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Ground-Water Levels, Movement, and artesian-flowing conditions of the late 1800s to below Occurrence or near land surface in most wells completed in the upper-aquifer system in the Oxnard Plain subbasin The largest source of discharge from the ground- (fig. 13). The effects of ground-water development and water flow system in the Santa Clara–Calleguas Basin overdraft first appeared in 1931 when water levels in is pumpage. Pumpage has caused water-levels to wells in parts of the Oxnard Plain declined below sea decline below sea level (fig. 12) which has resulted in level (Freeman, 1968). In the 1930s, the first deep wells seawater intrusion and changes in ground-water were drilled in the Pleasant Valley and Las Posas Valley quality, altered ground-water vertical-hydraulic subbasins. Before development, water levels in the gradients, reduced streamflow, reduced in ET, and lower-aquifer system probably were higher, but the caused land subsidence. Long-term hydrographs of water-level patterns probably were similar to the water levels in production wells (figs. 13 and 14) and in patterns shown in figure 12A for 1931.Well owners in the multiple-zone observation wells (fig. 15) show coastal areas began to recognize the connection fluctuations driven by multiple-year to decadal changes between the ground-water reservoirs and the ocean in recharge and seasonal to multiple-year changes in when they observed that water-level changes in wells pumpage. corresponded with the rising and falling phases of the ocean tides (Freeman, 1968). The Santa Clara Water Upper- and Lower-Aquifer-System Water Levels Conservation District officially recognized the linkage between overdraft and seawater intrusion in their Little information exists on predevelopment annual report of 1931 (Freeman, 1968). water levels in the upper- or lower-aquifer system Ground-water pumpage increased during the during the periods of early ground-water development. 1940s with the widespread use of the deep turbine In the 1870s, wells near the coast on the Oxnard Plain pump. The effects of permanent overdraft were subbasin were reported to deliver water to the second exemplified by the lack of recovery of water levels to floor of homes under the natural artesian pressures of historical levels after the spring of 1944, which marked the Oxnard aquifer (Freeman, 1968). Several early the end of the wettest climatic period in the 103 years ground-water-level maps were constructed for parts of of historical rainfall record at Port Hueneme (fig. 2A). the basin (Adams, 1913; Grunsky, 1925), but the first The effects of overdraft also were recognized landward map of the entire basin was completed for fall 1931 in the Santa Clara River Valley when ground-water (California Department of Public Works, 1934), which levels declined about 20 ft in the Fillmore subbasin was during a period of agricultural development and a (fig. 14). Water levels in the southern Oxnard Plain and severe drought (1923–36, fig. 2). Pleasant Valley were below sea level by 1946 As the surface-water resources became fully (Freeman, 1968). In 1949, water-level altitudes were used in the early 1930s, ground-water development 30 ft below sea level in parts of the Oxnard Plain began to provide a significant part of the water subbasin, and one of the first wells intruded by resources. If the conditions in 1931 represent, in part, seawater was identified along the coast in the Silver conditions prior to major ground-water development, Strand well field (north of Port Hueneme) (Freeman, then ground water in all the aquifers initially moved 1968). The direction of subsurface flow within the from the landward recharge areas toward the west or upper aquifers near the coast has been landward since southwest to the discharge areas along the submarine approximately 1947 (California Department of Water outcrops offshore in the Pacific Ocean (fig. 12A). By Resources, 1958). the 1930s, water levels had declined as a result of the 1927–1936 drought (figs. 12A and 13), changing from

Ground Water 59 T 2 N T 4 N Model-grid boundary of flow region and subareas 800 Modeled faults Wells 0 0 7700 45' 10 MILES

° 700 600 118 R18W 660000 Piru Lake

500 10 KILOMETERS 0 0 7700 700

0 66000 600 Shows altitude of 00 550000

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Shows simulated altitude offor water-level year indicated. Contour intervals vary water level for year indicated.where Dashed approximately located. Datum is sea level. Contour intervals vary

0 0

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0 2

, Fall 1932—Composite measured in upper- and lower-aquifer systems River 3300 A 0 200 10 0 0 ° 33000 0

220 80 700 60 119 11000

0 at Clara Santa 4 30 0 600 0 2 300 440 500 3300 400 0

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0 0 0 303

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0 808

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200 2 0 1100 0 Subbasin names are given in figure 1 440 , Spring 1993—upper-aquifer system. 0 C Hydrologic Unit boundary River and selected streams 330 Ground-water subbasin boundary– 5 100 10 80 2200 R22W 60 40 5 1100

15' 0

°

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0 5 119 5

n

a

e 5 5 c , Spring 1993—lower-aquifer system. O B

fic ci Fall 1931 - Composite ofLower Upper aquifer and systems - Measured Measured contours modified from California Department of Public Works (1934). 1932 - Upper Aquifer System - Simulated Pa A Ground-water level altitudes in the Santa Clara–Calleguas ground-water basin, Ventura County, California. Consolidated and unconsolidated deposits Shallow alluvium and unconsolidated deposits Shallow alluvium ° ° 34 22' 30" 34 07' 30" Santa Clara-Calleguas ground-water basin Outside Santa Clara-Calleguas ground-water basin Figure 12. simulated upper-aquifer system.

60 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California T 2 N T 4 N 45' 10 MILES ° 118 Piru R18W Lake 10 KILOMETERS

300

330000 0 0

0

0 300 3

0

0

0 0 200 2

200 2

200 River 0 0 110000 1100 0 ° 00 880 3300 8080 6600

119 6600 4400

0

440 -60 0

at Clara Santa

0 0 220

4 2 20

- -40

-40 0 20

0

0 -2- 6 6

0 2 -20 -

40

0 -

60 2 k

0

e

-20- 0

-20

4 e

6

-40 - r

-60 0 -

00 0

2200 1100 0 C

880 0 s

a

0 0 u 4

- -4

g

0 e

440 l

l

a 0 0

C -60

40 -80 20

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0 0

60 2

-20 -

0 0

-20

0 2 - 0 -2 0 0 0 1100 808

6060 0

4400 -20

R22W

0 0

2200 2

0 0 15' °

119 0 n Model fault in lower-aquifer system a ce O

Spring 1993 - Lower-Aquifer system c fi ci Pa —Continued. B ° ° 34 22' 30" 34 07' 30" Figure 12

Ground Water 61 T 2 N T 4 N 45' 10 MILES ° 700 118

660000 Piru R18W Lake 10 KILOMETERS

0 600 0 7700 0 66000 500500

500 500500 500

440000 300

0 0

0

0

0

0

2 200

4 400 0 200 0 4400 0

0

River 3300

0 0 3 400 300 600

° 200 100 500 119

400

0

0 0

0

8 8

100

00 1 100100

at Clara Santa 2200 0

20 0 300 2 6600

30 4400 0 0

0 0 0

1 10

0 0 10

2200 1001 k

0 e

0 e

3300 r

C

00 s 10

a

0 2 20

2 u

g

e l

l

a

C 0

1 10 0 100 0

200 0

4 40

0 0 0 2002 660

0 0 1001 0 100 880 R22W 80 2200 0

60 40 4 40

20 0

6 60 15'

° 10

119 0 880

1100

n a e c O

c fi ci Spring 1993 - Upper-Aquifer system a P —Continued. C ° ° 34 22' 30" 34 07' 30" Figure 12

62 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California OXNARD PLAIN FOREBAY SUBBASIN OXNARD PLAIN FOREBAY SUBBASIN PIRU SUBBASIN 150 150 600

50 50 500

-50 -50 400

-150 -150 300 2N/21W-7P2 4N/19W-33D3 2N/21W-11J1 [ ] -250 2N/22W-13N2 [ ] 2N/21W-15M3 [ ] 2N/22W-23K4 [ ] 400 2N/22W-26B3 119° 118°45' MOUND SUBBASIN 300 200 Lake Piru 200 100 T 4 N 100 0 34° River 22' 30" 0 -100 Santa Clara 2N/23W-24G1 [ ] 2N/23W-14K1 [ ] -100

-200 T WEST LAS POSAS VALLEY 2 N SUBBASIN 3N/19W-29K4 850 R18W ek re P C 750 a s c a if u i g c e l l 05MILES 650 34°7'30" a O C 05KILOMETERS ce R 22W an 550

450 1N/22W-36B2 [ ] 2N/20W-23K1 1N/21W-31L1 [ ] 300 100 350 200 0 250 100 -100 150 0 EAST LAS POSAS VALLEY SUBBASIN -200 SANTA ROSA VALLEY SUBBASIN 2N/20W-10H1 [ ] OXNARD PLAIN SUBBASIN 2N/20W-9F1 [ ] 2N/21W-24F1 [ ] 500 2N/21W-34D1 200 1N/22W-21B3 [ ] 400 1N/22W-20E2 [ ] 1N/22W-4F4 100 100 100 300 0 0 0 200 -100 -100 -100 100 -200 -200 0 -200 OXNARD PLAIN SUBBASIN NORTH PLEASANT VALLEY EAST LAS POSAS VALLEY SUBBASIN OXNARD PLAIN SUBBASIN SUBBASIN

Figure 13. Measured and simulated water-level altitudes in wells completed in the lower-aquifer system of the Santa Clara–Calleguas ground-water basin, Ventura County, California.

Ground Water 63 EXPLANATION Santa Clara-Calleguas ground-water basin Shallow alluvium and unconsolidated deposits Outside Santa Clara-Calleguas ground-water basin Shallow alluvium Consolidated and unconsolidated deposits Hydrologic Unit boundary Ground-water subbasin boundary– Subbasin names are given in figure 1 River and selected streams Landward model-grid boundary of flow region and subregions Wells with water-level data Existing well (Unpublished data from California Department of Water Resources and County of Ventura Public Works Agency, written commun., 1993)

2N/20W-23K1 300

200

100 SEA LEVEL

IN FEET ABOVE 0 WATER-LEVEL ALTITUDE, 1920 1935 1950 1965 1980 1995 YEAR Water-level hydrograph: colored curves show water levels for the well with identification number of the same color. The water-level curves are dashed when the time between measurements exceeds one year. The dotted line in the corresponding color represents the land-surface altitude at the well site. Green curves are simulated water levels in lower-aquifer system. See figure 1 for locations of subbasins.

Figure 13—Continued.

When ground-water pumpage approached wells in the West and South Las Posas Valley subbasins recorded maximum levels in 1951, which was at the indicate a water-level recovery in the upper-aquifer end of a drought, water-level declines reached a new system beginning in the 1950s (fig. 14) related to historical low in the upper-aquifer system (fig. 14) and increased irrigation return flow along Arroyo Simi and levels began to decline significantly in the lower- Beardsley Wash, importation of water which reduced aquifer system in the Oxnard Plain subbasin (fig. 13). local pumpage, discharge of pumped ground water into By 1950, water levels had declined below sea level in Arroyo Simi to control shallow ground-water levels, the lower-aquifer system as far inland as the Pleasant and discharge of treated municipal sewage into Arroyo Valley subbasin (fig. 13). Through 1950, water levels in Las Posas. most wells completed in the lower-aquifer system remained near land surface (fig. 13). Water levels in

64 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California SANTA PAULA SUBBASIN OXNARD PLAIN FOREBAY SUBBASIN FILLMORE SUBBASIN FILLMORE SUBBASIN 300 200 500 500

200 100 400 400

100 0 300 300

0 -100 200 200 3N/21W-16K2 [ ] 2N/22W-12R1 4N/20W-26A2 4N/19W-30R1 3N/22W-36K2 [ ] 119° 118°45' SANTA PAULA SUBBASIN 300 Lake Piru T4N 200

100 ver Ri 34°22'30" Clara Santa 0 2N/22W-2C1 [ ] 2N/22W-11A1,2 [ ]

T2N

2N/22W-9J1 [ ] 4N/19W-25K2 2N/22W-8P1 700 2N/22W-8N1 [ ] 300 R18W 600 ek re C s 200 a u 500 g e l l a 34°7'30" 05MILES 100 C Pacific 400 R22W 05KILOMETERS PIRU SUBBASIN 0 Ocean 3N/19W-29E2 [ ] 1N/22W-16E1 1N/21W-7H1 100 100 3N/19W-29E3 -100 900 MOUND SUBBASIN 0 0 2N/22W-22R1 800 200 -100 -100 700 100 -200 -200 600 OXNARD PLAIN SUBBASIN OXNARD PLAIN SUBBASIN 0 1N/21W-14A1 1N/22W-3F4 100 500 100 -100 EAST LOS POSAS VALLEY OXNARD PLAIN FOREBAY 0 SUBBASIN SUBBASIN 0 2N/19W-3A1 [ ] 2N/21W-16J1 [ ] -100 2N/19W-4K1 [ ] 2N/21W-12F1 [ ] -100 2N/20W-12G2 [ ] 450 600 -200 -200 SOUTH PLEASANT VALLEY 350 OXNARD PLAIN SUBBASIN SUBBASIN 500 1N/22W-36L1 2N/20W-23R1 250 100 300 400

150 0 200 300

50 -100 100 200

-50 -200 0 100 WEST LOS POSAS VALLEY OXNARD PLAIN SUBBASIN SANTA ROSA VALLEY SUBBASIN SOUTH LOS POSAS VALLEY SUBBASIN SUBBASIN

Figure 14. Measured and simulated water-level altitudes in wells completed in the upper-aquifer system of the Santa Clara–Calleguas ground-water basin, Ventura County, California.

Ground Water 65 EXPLANATION Santa Clara-Calleguas ground-water basin Shallow alluvium and unconsolidated deposits Outside Santa Clara-Calleguas ground-water basin Shallow alluvium Consolidated and unconsolidated deposits Hydrologic Unit boundary Ground-water subbasin boundary– Subbasin names are given in figure 1 River and selected streams Landward model-grid boundary of flow region and subregions Wells with water-level data Existing well (Unpublished data from California Department of Water Resources and County of Ventura Public Works Agency, written commun., 1993)

1N/21W-7H1 100

0

-100 FEET ABOVE SEA LEVEL N -200 WATER-LEVEL ALTITUDE, 1920 1935 1950 1965 1980 1995 YEAR Water-level hydrograph: colored curves show water levels for the well with identification number of the same color. The water-level curves are dashed when the time between measurements exceeds one year. The dotted line in the corresponding color represents the land-surface altitude at the well site. Green curves are simulated water levels in upper-aquifer system. See figure 1 for locations of subbasins.

Figure 14—Continued,

The lowering of water levels continued in the treated municipal sewage into Conejo Creek (figs. 13 upper- and lower-aquifer systems in the Oxnard Plain and 14). The hydrographs of wells in the Mound subbasin through the next dry period, 1959–64, subbasin and wells near the Hueneme submarine furthering seawater intrusion (figs. 13 and 14). Water- canyon (figs. 13 and 14) show little to no additional level hydrographs (fig. 13) for many wells in the lower- decline during these decades. By the late 1960s, aquifer system in the North Pleasant Valley and the Las thousands of acres of aquifer had been intruded by Posas Valley subbasins indicate a monotonic decline seawater in the Port Hueneme and Point Mugu areas, through the 1950s and 1960s. Water levels started to and coastal farmland had been lowered by land recover in the Santa Rosa Valley subbasin beginning subsidence (see “Land Subsidence Effects”) owing to around 1965 because of decreased pumpage in the several decades of sustained overdraft. upper- and lower-aquifer systems and discharge of

66 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 2N/22W-23B (3[FC],4[LH],5[UH],6[M],7[O]) 2N/21W-7L 150 3N/20W-35R (3[FC],4[LH],5[M],6[O]) (2[FC],3[FC],4[M]) 1N/22W-26J (3[M],4[O],5[S]) 150 25 100 350 100 -25 50 300 LSE = 590 feet 50 -75 0 250 0 -125 -50 200 OXNARD PLAIN SUBBASIN -50 150 1N/22W-27C (2[M],3[O],4[S]) -100 25 OXNARD PLAIN FOREBAY EAST LAS POSAS VALLEY -150 SUBBASIN SUBBASIN -25 OXNARD PLAIN FOREBAY SUBBASIN -75 119° 118°45' OXNARD PLAIN SUBBASIN Lake Piru 1N/22W-20J (4[LH],5[UH],6[M],7[M],8[O]) T4N 25 NS -25 34°22'30" River -75 OXNARD PLAIN SUBBASIN Santa Clara NS NS 1N/23W-1C (2[FC],3[LH],4[UH],5[O]) 25 -25 T2N -75 OXNARD PLAIN SUBBASIN 1N/22W-20M (1[FC],2[LH], 3[UH],4[M],5[O],6[S]) R18W 25 2N/20W-16A (2[M],3[O],4[S]) ek 300 re -25 C s a 250 u -75 g e 05MILES P l OXNARD PLAIN SUBBASIN a 34°07'30" l c a 200 i C 05KILOMETERS 1N/22W-29D (1[LH], fic R22W 150 2[UH],3[UH],4[O]) O EAST LAS POSAS VALLEY 25 ce 1N/21W-19L (10[M],11[M], an 12[O],13[O],14[S]) SUBBASIN -25 100 2N/21W-11J (3[FC],4[LH],5[M]) -75 50 200 OXNARD PLAIN SUBBASIN LSE = 378 feet 1N/22W-28G (1[FC], 0 150 2[LH],3[UH],4[M],5[O]) 25 -50 100 -25 -100 50 -75 -150 0 1N/22W-36K OXNARD PLAIN SUBBASIN -125 (5[FC],6[LH],7[UH],8[M],9[O]) 1N/21W-32Q (2[GC],3[FC], -50 OXNARD PLAIN SUBBASIN 25 4[UH],5[M],6[M],7[O]) -100 1N/22W-27R (3[M],4[O],5[S]) 25 25 -25 WEST LAS POSAS VALLEY -25 SUBBASIN -25 -75 2N/21W-34G -75 (2[FC],3[LH],4[M],5[O]) -75 -125 50 OXNARD PLAIN SUBBASIN OXNARD PLAIN SUBBASIN -125 OXNARD PLAIN SUBBASIN 0 1N/22W-35E (1[FC], 1S/22W-1H 2[LH],3[UH],4[M],5[O]) (1[UH],2[M],3[M],4[O]) 1S/21W-8L (3[UH],4[O],5[O]) -50 25 25 25 -100 -25 -25 -25 -150 -75 -75 -75 -200 -125 -125 -125 NORTH PLEASANT VALLEY OXNARD PLAIN SUBBASIN OXNARD PLAIN SUBBASIN OXNARD PLAIN SUBBASIN SUBBASIN

Figure 15. Measured and simulated water-level altitudes at sites with multiple wells of different depths completed in the Santa Clara–Call eguas ground-water basin, Ventura County, California.

Ground Water 67 EXPLANATION Santa Clara-Calleguas ground-water basin Shallow alluvium and unconsolidated deposits Outside Santa Clara-Calleguas ground-water basin Shallow alluvium Consolidated and unconsolidated deposits Hydrologic Unit boundary Ground-water subbasin boundary– Subbasin names are given in figure 1 River and selected streams Landward model-grid boundary of flow region and subregions Well and data - Multiple-well site sampled as a part of this study. (Densmore, l996) NS, indicates that data is not shown. 2N/21W-34G 50 (2[FC],3[LH],4[M],5[O]) Aquifer Designation 0 [S] = Shallow Upper [O] = Oxnard Aquifer -50 [M] = Mugu -100 [UH] = Upper Hueneme [LH] = Lower Hueneme

SEA LEVEL Lower -150

IN FEET ABOVE Aquifer [FC] = Fox Canyon -200 [GC] = Grimes Canyon WATER-LEVEL ALTITUDE, 1989 1990 1991 1992 1993 1994 YEAR Water-level hydrograph and well number with colored curves showing water levels for the well with sequence number of the same color. The water-level curves are dashed when the time between measurements exceeds one year. Letters in brackets are aquifer designations. The dotted line, when shown, represents the land-surface altitude (LSA) at the multiple-well site. Dashed orange curve is simulated water levels in upper-aquifer system (layer 1) and dashed black curve is simulated water levels in lower-aquifer system (layer 2).

Figure 15—Continued.

Water levels in both aquifer systems in the that water-levels began to recover in the late 1970s. Oxnard Plain subbasin partially recovered in the late This recovery was related to importation of water that 1960s owing to increased artificial recharge in the reduced local pumpage, discharge of pumped ground Oxnard Plain Forebay subbasin and natural recharge water into Arroyo Simi to control shallow ground- owing to a wetter climate. The water levels from wells water levels, and discharge of sewage effluent into in the upper-aquifer system in the Santa Clara River Arroyo Las Posas. Similar water-level recoveries in the Valley subbasins also showed recovery during the late Santa Rosa Valley subbasin began in about 1965 1960s and early 1970s. The absence of wells completed (figs. 13 and 14) owing to decreased pumpage and in the lower-aquifer system in the upper Santa Clara discharge of sewage effluent into Conejo Creek and River Valley subbasins precluded an assessment of the some water-level recovery near stream channels in history or distribution of water levels there. Data from shallower wells. wells in the East Las Posas Valley subbasin indicate

68 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California By the end of the most recent drought (1987– subbasins had recharged owing to increased streamflow 91), water levels were below sea level throughout the during wet periods or increased discharge of treated Oxnard Plain, Mound, and Pleasant Valley subbasins in sewage effluent. both aquifer systems and below sea level in the lower- aquifer system throughout the West Las Posas Valley Water-Level Differences Between Aquifers subbasin. In the inland subbasins, such as the South Differences in water levels occur between the Pleasant Valley and West Las Posas Valley subbasins, different aquifers (fig. 15) in the Santa Clara–Calleguas water levels in many of the wells were near the Basin. The water levels in the coastal Oxnard aquifer historical lows in 1991 (figs. 13 and 14). are lower than the water levels in the Shallow aquifer Beginning in 1992, which is the start of the most during dry-year periods and become higher than the recent wet period, there was an increase in recharge water levels in the Shallow aquifer during recoveries owing to, in part, the increased capacity for artificial (fig. 15) in wet-year periods. Large water-level recharge at the Freeman Diversion and to a temporary differences occur between the Shallow and the reduction of pumpage from the coastal subbasins underlying aquifers during the irrigation season, owing to increased surface-water supplies through especially within the South Oxnard Plain subarea. pipeline deliveries, conservation practices, and new These differences are primarily due to thick deposits of irrigation technology that increased irrigation silt and clay in the Shallow aquifer that retard the efficiency. Pumpage was reduced because of a drilling movement of ground water between the Shallow and moratorium established by the FGMA in 1983 on new the Oxnard aquifers. Water levels for the RASA wells completed in the upper-aquifer system in the monitoring wells completed in the Shallow aquifer Oxnard Plain. A comparison of the water-level maps show little seasonal change owing to ground-water for 1931 and 1993 indicates that by 1993 water levels pumping or precipitation (fig. 15). Other shallow wells had recovered in the upper-aquifer system and were in the northern part of the Oxnard Plain subbasin show greater than levels in 1931 (fig. 12A,C). Water levels in rises that are related to precipitation and declines that 1993 were about 5 ft higher near the coast, more than may be related to leakage (Neuman and Gardner, 1989, 20 ft higher in the Oxnard Plain Forebay than the 1931 figs. 2 and 3). Previous investigators estimated that levels, and above sea level throughout most of the vertical leakage from the shallow semiperched system Oxnard Plain. The water-level map for the lower- to the Oxnard aquifer ranges from 6,000 acre-ft/yr aquifer system shows that water levels were below sea (California Department of Water Resources, 1971) to level in the South Oxnard Plain subarea and Pleasant 20,000 acre-ft/yr (Mann and Associates, 1959). Valley subbasins (fig. 12B). Water-level data were not Similarly, wells with depths of less than 50 ft available for other inland subbasins for 1931; however, completed in the Santa Clara River Valley subbasins the hydrographs of long-term water levels indicate also have higher water levels than those of nearby wells subdued fluctuations, or decline and recovery cycles completed deeper in the upper-aquifer system. These (fig. 14), that may indicate that the shallower parts of elevated water levels may indicate some degree of the upper-aquifer system in these ground-water hydraulic separation between the Shallow (recent alluvium) aquifer and the underlying aquifer along the Santa Clara River.

Ground Water 69 Except for those wells tapping the Shallow Paula subbasins were 10 to 25 ft lower for water levels aquifer, water levels in wells in the coastal subareas in the lower-aquifer system than for levels in the upper- and Santa Clara Valley subbasins indicate spring and aquifer system. summer declines followed by recovery during late fall and winter of each year. The seasonal fluctuations in Inter-Aquifer Flow wells in the upper-aquifer system are comparable with Flow between aquifers can be an important the changes in the wells in the lower-aquifer system consideration in the management of water resources. north of the Hueneme submarine canyon. In the Vertical water-level differences (figs. 13–15) indicate Oxnard Plain subbasin south of the Hueneme the potential for upward and downward flow between submarine canyon and in the Pleasant Valley subbasin, aquifers and aquifer systems. However, these seasonal fluctuations in water levels are greater in the differences can result in appreciable leakage only if a lower-aquifer system than in the upper-aquifer system. conductive pathway is present. Vertical flow between The smaller water-level differences and seasonal aquifers can occur as leakage through coarse-grained fluctuations near Port Hueneme are partly due to the sedimentary layers, through and around fine-grained source of water (seawater intrusion) along the near- layers, and as vertical flow in and around well bores. shore submarine canyon outcrops, which tends to Vertical flow between the semiperched and the subdue the water-level fluctuations and changes in upper-aquifer systems also can occur through failed water levels between aquifers. In contrast, the larger and abandoned wells (Stamos and others, 1992). water-level differences near Point Mugu are, in part, Estimates of the number of abandoned and potentially due to offshore faulting, which creates a barrier to failed wells range from 167 (Predmore, 1993) to 238 ocean inflow for the lower-aquifer system. However, (Ventura County Resource Management Agency, wells completed in the Mugu aquifer have water-level Environmental Health Department, 1980) in the fluctuations that are similar to those of the lower- Oxnard Plain and as many as 1,215 wells throughout aquifer system. The similarity in seasonal fluctuations Ventura County (Predmore, 1993). Wellbore heat-pulse in the Mugu aquifer and the lower-aquifer system, in flowmeter tests in selected wells in the Oxnard Plain part, may be due to well-construction practices; well subbasin indicate that intraborehole flow rates of 3 to screens typically span the Mugu aquifer and parts of 11 gal/min may occur in some failed wells. This the lower-system aquifers. Flowmeter logs of wells suggests a total maximum leakage of about 800 to screened opposite both the Mugu aquifer and the 4,220 acre-ft/yr for periods when the hydraulic lower-aquifer system indicate a significant contribution gradients are downward. The hydrographs for the from the Mugu aquifer (table 5). Water levels in the multiple-observation well sites show that the heads in Pleasant Valley subbasin are about 50 ft lower in the producing aquifers can vary seasonally and Mugu-equivalent aquifer than water levels in the climatically (fig. 15). Thus, during wet-year periods or Oxnard-equivalent aquifer. This sustained water-level during periods of reduced pumpage, heads in the difference, along with water-level responses measured aquifer system can result in intraborehole discharge during short-term aquifer tests (Hanson and Nishikawa, from the ground-water flow system to the overlying 1996) and geophysical data (Densmore, 1996; semiperched systems. Conversely, during dry-year Appendix 6), indicates that these aquifers are separated periods or in areas of increased pumpage, heads in the by fine-grained confining beds. The difference in water semiperched system could be greater than heads in the levels between the Oxnard aquifer and the lower- underlying aquifers and could result in leakage as aquifer system increases during periods of pumping recharge to the ground-water system. For example, and decreases during seasonal periods of recovery. wellbore leakage of as much as 11 gal/min was Water levels in the lower-aquifer system were measured with a heat-pulse flowmeter in failed consistently more than 100 ft lower than water levels in monitoring well 1N/22W-27R2. However, detailed the upper-aquifer system in the inland subbasins of chemical sampling at nearby multiple-completion Pleasant Valley, West Las Posas Valley, and East Las monitoring wells 1N/22W-27R3–5 (Izbicki, 1996a) Posas Valley. For the inland Santa Clara River Valley indicates that the effects of this wellbore leakage were subbasins, water-level differences in the Piru and Santa not areally extensive.

70 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Vertical flow also can occur from the underlying Source, Movement, and Age of Ground Water marine sedimentary rocks or from brines related to oil The source, movement, and age of ground water deposits. Methane is reported to discharge from some in the Santa Clara–Calleguas Basin can be inferred production wells that are completed to depths just from the isotopic content of ground-water and surface- above the oil fields just west of Pleasant Valley in the water samples. Based on deuterium isotope samples, Oxnard Plain subbasin (fig. 9). Geochemical data most of the water in the upper- and lower-aquifer indicate that the amounts of leakage from deeper and systems is derived from streamflow infiltration of older formations in the southern part of the Oxnard high-altitude precipitation along the Santa Clara River Plain and South Pleasant Valley subbasins probably are that originated largely as runoff of precipitation falling small (Izbicki, 1991, 1996a, figs. 3 and 5). at the higher altitudes of the surrounding mountains (Izbicki, 1996b, fig. 3). Isotopic data also suggest a Source of Water to Wells local contribution of mountain-front recharge and The relative contribution of water to wells direct infiltration of locally derived precipitation in the completed in multiple aquifer systems is dependent on Las Posas and Pleasant Valleys and along the margins the local stratigraphy and on well construction. The of the Santa Clara River Valley (Izbicki, 1996b). vertical distribution of ground-water withdrawals from Although a large component of irrigation return flow wells was estimated from flowmeter logs of 17 wells may contribute to infiltration, no large areas of the completed as part of the RASA Program and other Oxnard aquifer in the Oxnard Plain had an isotopic studies (table 5, fig. 17B presented later in the “Model signature similar to that of evaporated waters. Analysis Boundaries” section, figure A5.1 in Appendix 5). of ground-water samples for the hydrogen isotope Where wells are perforated across younger aquifers tritium indicates that recent recharge (since 1952) has and older aquifers, most of the water is produced from occurred largely in the Santa Clara River Valley the more transmissive younger aquifers [table 5, figure subbasin, the Oxnard Plain Forebay subbasin, the A5.1 in Appendix 5]. Combined with the stratigraphy, northwestern part of the Oxnard Plain subbasin, and flowmeter logs indicate that the most productive and the South Las Posas Valley subbasin (Izbicki, 1996b, areally extensive water-bearing zones commonly occur fig. 5). Tritium data also indicate that the artificial as basal coarse-grained layers that overlie major recharge from the Oxnard Plain Forebay subbasin has regional unconformities. However, the relative largely infiltrated the upper-aquifer system. Ages contribution to any particular well from less productive determined by carbon-14 analysis of ground-water aquifers may increase with increased pumping rates samples indicate that water in the upper-aquifer system and decreased water levels in the more productive directly beneath the Saticoy spreading grounds is aquifers (table 5). relatively young (less than 500 years old), but water in The most important aspects of well construction the lower-aquifer system beneath the El Rio spreading are the vertical extent of the well screen and the depth grounds ranges from 700 to more than 13,000 years old and location of the pump intake relative to the well (Izbicki, 1996b, fig. 6). Samples from the lower-aquifer screen. Wells that are screened across the basal layer of system near the coast range from about 7,000 to 23,000 the upper-aquifer system can derive as much as years old (Izbicki, 1996b, fig. 6). Samples from wells 70 percent of the wellbore inflow from this relatively in the Las Posas Valley and Pleasant Valley subbasins thin layer. Wells that are completed only in the lower- yielded ages of about 700 to 6,000 years old (Izbicki, aquifer system can derive 100 percent of the wellbore 1996b, fig. 7). Collectively, these data indicate that the inflow from the basal coarse-grained layer in the upper-aquifer system is recharged by streamflow Hueneme aquifer (table 5). Flowmeter logs are not yet infiltration and mountain-front recharge; the lower- available for wells throughout most of the Oxnard Plain aquifer system has received little recent water; and and Las Posas Valley subbasins; for wells in all the ground water moved relatively slowly under the Piru, Fillmore, and Santa Rosa Valley subbasins; and hydraulic gradients present prior to water development. for wells screened only in the upper-aquifer system.

Ground Water 71 system Percent from Grimes Canyon eme/Fox Canyon/ Oxnard/Mugu/Huen —/17/3/17/63 —/30/21/11/38 —/9/91/0/— —/54/46/0/— —/—/5/36/59 —/—/19/26/55 —/4/5/60/31 —/—/0/28/72 —/—/0/21/79 —/—/34/53/0 —/10/27/37/26 —/17/20/58/5 —/40/30/23/7 —/—/49/51/— —/—/25/53/22 —/—/19/37/44 —/—/58/42/— —/—/100/0/— —/12/88/—/— —/—/71/29/— Inflow 9/91 4/96 17/83 30/70 54/46 17/83 40/60 12/88 aquifer systems —/100 —/100 —/100 —/100 —/100 —/100 —/100 —/100 —/100 —/100 —/100 —/100 upper/lower Percent from rate ell-test 1500 1,000 4,000 1,414 2,000 1,168 2,000 2,500 1,128 3,150 2,000 4,000 1,121 2,500 4,000 1,440 2,000 1,100 2,000 (gal/min) W 407 discharge ta Clara–Calleguas Basin, Ventura County, California 1 1 1 1 1 1 1 1 1 1 2 log ear of 1991 1991 1991 1991 1992 1991 1991 1991 1991 Y 1980 1980 1980 1980 1980 1979 1980 1980 1980 1980 1984 flowmeter (feet) 863 923 700 screen 1,433 1,013 1,321 1,220 1,363 1,000 1,083 1,003 1,106 Depth to bottom of well 403 443 571 400 603 420 383 503 443 463 350 526 (feet) well screen Depth to top of ell 18 12 18 18 18 18 18 18 18 18 9 18 W casing (inches) diameter (feet) 883 960 702 depth 1,453 1,033 1,341 1,240 1,383 1,020 1,103 1,023 1,126 otal drilled T Subbasin alley alley alley alley alley alley alley xt. Total depth of well and depth to top and bottom of well screen in feet below land surface. —, no data] land surface. depth of well and to top bottom screen in feet below Total xt. V V V V V V V South Pleasant South Pleasant Oxnard Plain South Pleasant Oxnard Plain South Pleasant South Pleasant South Pleasant South Pleasant Oxnard Plain Oxnard Plain Oxnard Plain ARD#20 N Local well name WELL#04 WELL#01 WELL#03 WELL#05 WELL#07 WELL#06 WELL#08 WELL#10 WELL#09 WELL#11 PVCWD- PVCWD- PVCWD- PVCWD- PVCWD- PVCWD- PVCWD- PVCWD- PVCWD- PVCWD- PTMUGU#03 OX Summary of well-construction data and discharge rates inflows from flowmeter logs wells in selected subbasins the San -3K1 -3R1 -4D4 -4K1 -8R1 -10G1 -15D2 -21H2 -22C1 -28D1 -31L1 State well No. Table 5. [State well No.: See well-numbering diagram in te 1N/21W 1N/21W 1N/21W 1N/21W 1N/21W 1N/21W 1N/21W 1N/21W 1N/21W 1N/21W 1N/21W 1N/22W-3F5

72 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California system Percent from Grimes Canyon eme/Fox Canyon/ Oxnard/Mugu/Huen ow —/—/58/42/— —/—/58/42/— —/—/53/47/— —/—/25/75/0 —/—/25/67/8 —/—/18/68/14 —/50/50/—/— —/44/56/—/— —/33/67/—/— —/—/100/—/— —/100/0/—/— —/73/27/—/— —/72/28/—/— Infl 50/50 44/56 33/67 100/0 73/27 72/28 aquifer systems —/100 —/100 —/100 —/100 —/100 —/100 —/100 upper/lower Percent from rate ell-test 500 1,000 1,500 4,000 2,065 2,065 1,906 2,485 4,015 1,960 1,000 1,500 2,500 (gal/min) W discharge ta Clara–Calleguas Basin, Ventura County, California— 3 3 3 1 4 5 log ear of 1992 1993 1994 1994 1991 1991 1991 Y owmeter 1992 1992 1992 1980 1994 1983 fl (feet) 700 700 screen 1,463 1,180 1,092 Depth to bottom of well 480 403 580 752 260 (feet) well screen Depth to top of ell 18 18 18 18 16 W casing (inches) diameter (feet) 700 700 depth 1,483 1,190 1,112 otal drilled T Subbasin alley alley ed Power, written commun, 1992). ed Power, V V North Pleasant South Pleasant Mound Oxnard Forebay Santa Paula vid Gardner, written commun., 1993). vid Gardner, ULA#12 alley Water Conservation District (Lee Miller, written commun.,1991). District (Lee Miller, Conservation Water alley Local well name ater Conservation District (Jim Gross, written commun., 1991). ater Conservation WELL#02 WELL#2 PA V W SAINT JOHNS#6 PVCWD- VICTORIA- ELRIO#12 SANTA Summary of well-construction data and discharge rates inflows from flowmeter logs wells in selected subbasins the San -20M5 -34G1 -8F1 -13N2 -11J5 Data from Pleasant Data from Geotechnical Consultants, Inc. (T Data from Fugro-McClelland, Inc. (Da Data from Fugro-McClelland, Inc. (Curtiss Hopkins, written commun., 1994). Data from United 1 2 3 4 5 State well No. Table 5. Continued 2N/20W 2N/21W 2N/22W 2N/22W 3N/21W

Ground Water 73 Land-Subsidence Effects More than 7,900 acre-ft of brines, 8,000 acre-ft Ground-water withdrawals, oil and gas of oil, and 72 million cubic feet of natural gas were production, and tectonic movement are three potential withdrawn from oilfields in the Oxnard Plain subbasin causes of land subsidence in the Oxnard Plain and (fig. 9) between 1943 and 1991 (Steven Fields, adjacent subbasins (fig. 9) (Hanson, 1995). Operations Engineer, California Department of Ground-water levels in the Oxnard Plain subbasin have Conservation, Division of Oil and Gas, written declined steadily since the first wells were completed commun., 1992). Pressure declines equivalent to more in the 1870s. Ground water, however, has remained a than 1,100 ft of water-level decline have occurred in primary source of water since the early 1900s. Oil and the Oxnard oilfields since the onset of oil and gas gas has been produced in the Santa Clara–Calleguas production. These declines alone could potentially Basin since the 1920s and in the Oxnard Plain subbasin account for local subsidence of 1.5 to 2.0 ft (California since the 1940s. The basin is a part of the tectonically Division of Oil and Gas, 1977). active Transverse Ranges physiographic province. Tectonic activity in the form of plate Ventura County has delineated a probable subsidence- convergence and north-south crustal shortening has hazard zone that includes parts of the Piru, Fillmore, resulted in an average regional horizontal movement in Santa Paula, Mound, Oxnard Plain Forebay, Oxnard the subbasins north of the Oxnard Plain of about Plain, and Pleasant Valley subbasins (Ventura County 0.007 ft/yr over the past 200,000 years (Yeats, 1983). Board of Supervisors, 1988). Vertical movement, as uplift north of the Oxnard Plain Since the early 1900s, water-level declines in the subbasin and as subsidence in the Oxnard Plain upper- and lower-aquifer systems in the Oxnard Plain subbasin, has been caused by plate convergence and subbasin have ranged from about 50 to 100 ft. Water related earthquakes throughout the basin. For the levels in wells at the multiple-well monitoring sites are southern edge of the Oxnard Plain subbasin (fig. 9A), lower in the lower-aquifer system than in the upper- elevation data from bench marks (BM) on bedrock (for aquifer system—by 20 ft near the Hueneme submarine example, BM Z 583) indicate that the 0.17 ft of canyon along the central coast and by about 80 ft near subsidence that occurred during 1939–78 (at a rate of the Mugu submarine canyon along the southern coast about 0.004 ft/yr) may be related to tectonic activity. of the Oxnard Plain subbasin. Because early pumpage Data from a coastal leveling traverse near the data are unavailable for the Oxnard Plain subbasin, the southeastern edge of the Oxnard Plain (fig. 9A,B) total quantity of water withdrawn is unknown. indicate that as much as 1.6 ft of subsidence occurred However, reported pumpage data indicate that during during 1939–60 at BM E 584 (0.07 ft/yr) and an 1979–91 about 822,000 acre-ft of ground water was additional 1 ft occurred during 1960–78 (0.06 ft/yr). withdrawn from the Oxnard Plain subbasin at a During 1960–92, 0.5 ft of subsidence (0.02 ft/yr) was relatively constant rate. This pumpage has resulted in measured at BM Z 901, which is southwest of BM water-level declines that, in turn, have increased the E 584 and at the edge of the coastal Oxnard Plain. effective stress on the aquifer-system sediments. An Bench-mark trajectories (fig. 9C) indicate that increase in the effective stress on aquifer sediments subsidence continues and may be driven by extreme beyond their preconsolidation stress results in water-level declines that occur during drought periods. compaction and reduction of pore space and Farther inland, where water-level and oilfield pressure mechanically squeezes water from sediments. declines are largest, greater subsidence might be expected.

74 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California