0

20

40

60

80

100

0

20

40

60

80 100 10

0 80

60

40

20

0

100 80

60

40

20

0

PERCENTCALCIUM (Ca)

MAG

NESIU

M (M

g)

SODIUM (Na) PLUS POTASSIUM

(K)

CHLORIDE (Cl) PLUS FLUORIDE (F)

BICA

RBON

ATE

(HCO

3) P

LUS

CARB

ONAT

E (C

O 3)

SULFATE (SO4 )

CHLO

RIDE

(Cl)

PLUS

FLUO

RIDE

(F) P

LUS

SULF

ATE

(SO 4)

CALCIUM (Ca) PLUS M

AGNESIUM (M

g)

PERC

ENT PERCENT

100

80

60

40

20 0

20

40

60

80

100 0

20

40

60

80

100

0

0 20

40

60

80

100

100

80

60

40

20

0 100

80

60

40

20

0

0

20

40

60

80

100

0

20

40

60

80 100 10

0 80

60

40

20

0

100 80

60

40

20

0

PERCENTCALCIUM (Ca)

MAG

NESIU

M (M

g)

SODIUM (Na) PLUS POTASSIUM

(K)

CHLORIDE (Cl) PLUS FLUORIDE (F)

BICA

RBON

ATE

(HCO

3) P

LUS

CARB

ONAT

E (C

O 3)

SULFATE (SO4 )

CHLO

RIDE

(Cl)

PLUS

FLUO

RIDE

(F) P

LUS

SULF

ATE

(SO 4)

CALCIUM (Ca) PLUS M

AGNESIUM (M

g)

PERC

ENT PERCENT

100

80

60

40

20 0

20

40

60

80

100 0

20

40

60

80

100

0

0 20

40

60

80

100

100

80

60

40

20

0 100

80

60

40

20

0

15

805

15

8

Otay RiverDrainage Basin

San Dieguito River Drainage Basin

San Diego River Drainage Basin

San Vicente Reservoir

Otay River

El Capitan Reservoir

LakeHodges

LakeSutherland

Sweetwater River

San Diego River

San Dieguito River

Tijuana River Drainage Basin

Nacion

La

Fault

0 10 MILES5

0 10 KILOMETERS5

Multi-levelmonitoring wells

MexicoUnited States

SDCCSDMC

SDORSDOT

SDNB SDMC

SDOTSDOR

SDCC

SDNB

Sweetwater RiverDrainage Basin

5

15

8

117 116116 30’

33

3230’

San Dieguito River Drainage Basin

San Diego River Drainage Basin

Sweetwater River Drainage Basin

Tijiana River Drainage Basin

MexicoUnited States

Area of map

CALIFORNIA

Otay RiverDrainage Basin

For more information see—http://ca.water.usgs.gov/sandiego

Changes in EM and Chloride ConcentrationBackground

Major-ion Chemistry

Stable Isotopes of H and O

Geophysical Logs

Tidal Effects on Salinity

Stable Isotopes of H and O in SDNB

Delineating Zones of Seawater Intrusion in a Coastal Southern California AquiferDanny A. Cronquist1, 2, Stephen Banister1, 3, and Robert Anders1

1-USGS San Diego WSC, 4165 Spruance Rd., Ste. 200, San Diego, CA 921012-San Diego State University, 5500 Campanile Drive, San Diego, CA 921823-Naval Facilities Engineering Command, SW, 1220 Pacific Hwy, San Diego, CA 92132

The regional assessment of groundwater resources in the San Diego area was designed as an integrated set of five drainage-basin investigations, to evaluate the suitability of the San Diego Formation and overlying unconsoli-dated Quaternary alluvium for use as a drinking water supply. The San Diego Formation is composed of thinly bedded sandstone and conglomerate, which originated as marine and non-marine sediment during the late Pliocene and early Pleistocene, ranges in thickness from about 100 feet to more than 800 feet, and is overlain by about 100 feet of unconsolidated Quaternary alluvium from the Linda Vista and Bay Point Formations. An integral part of the investigation is the installa-tion of as many as 6 monitoring wells at 10 sites to depths depths of as much as 2,000 feet. Data includes lithologic information, geophysical logs, and water-quality samples analyzed for a broad range of constituents including major and minor dissolved ions, trace metals, volatile organics, pesticides, wastewater indica-tors, and stable and radiogenic isotopes. In addition, the multiple-well monitor-ing sites are equipped with real-time, water-level recording equipment and the data is available via the project website shown below.

1. Groundwater in the Sweetwater and Otay River drainage basins is characterized as mixed cation-Cl to Na-Cl type. The percentages of major ions indicate also that the chemical composition of several groundwater water samples collected along the San Diego Bay resemble the major-ion composition of seawater. 2. Chloride concentrations greater than seawater likely result from seawater altered by evaporation.

3. The stable isotopes of hydrogen and oxygen indicate the groundwater in the Sweet-water and Otay River drainage basins can be separated into at least three distinct sources of recharge.

4. The major-ion chemistry and stable isotopic values for groundwater samples col-lected from SDNB2 has changed since 2006.

5. EM logs, in combination with the water-quality data, can be used to distinguish be-tween high-salinity zones that are caused by seawater intrusion from those that are caused by “freshening” of a previously-saline aquifer by recently recharged water.

Gamma and electromagnetic induction (EM) logs were obtained prior to the installation of the monitoring wells, and during subsequent site visits. A natural gamma tool is designed to measure the total intensity of gamma-ray emissions from the formation. EM logs are sensitive to changes in lithol-ogy and water quality; because the lithology remains constant repeated EM logs can be used to show changes in water-quality due to natural recharge processes, seawater intrusion, or other processes.

Major-ion Chemistry in SDNB

The stable isotopes of hydrogen and oxygen can be used to identify the differ-ent sources of recharge in the Sweetwater and Otay River drainage basins. These dif-ferent groups are distinguishable by: (1) iso-topic values comprised of a mixture of groundwater and seawater; (2) lighter (more negative) ground-water isotopic values that are characteristic of recharge which origi-nates in the mountains to the east of the Sweetwater and Otay River drainage basins; and (3) intermediate isotopic values which are characteristic of local precipitation as the source of recharge to the multiple-well monitoring sites.

The stable isotopes of hydrogen and oxygen of 12 groundwater samples collected from the SDNB multiple-depth monitoring site since 2006 indicate the groundwater is comprised of a mixture of groundwater and seawater. Also shown is a simple two-member mixing line be-tween native freshwater and seawater. The stable isotopic values from the coastal monitoring wells plot along the mixing line and suggest that water from these wells are as much as 70-percent seawa-ter, and the percentage of seawater in groundwa-ter samples collected from SDNB has changed since 2006.

Major-ion composition of 55 groundwater samples collected from three of the multiple-well monitor-ing sites located along the San Diego Bay and from two multiple-well monitoring sites lo-cated on a plateau away from the coastal area is presented using a trilinear diagram. Per-centages of major ions on a charge-equivalent basis indicate the chemical composition of the groundwater in the Sweetwater and Otay River drainage basins can be charac-terized as mixed cation-Cl to Na-Cl type. The percentages of major ions indicate also that the chemical composition of several groundwater samples col-lected along the San Diego Bay re-semble the major-ion composi-tion of seawater.

Major-ion composition of 9 groundwater samples collected from the San Diego Navy Base (SDNB) multiple-depth monitoring site since 2006 is presented using a trilinear diagram. The chemical composition of groundwater samples collected from SDNB1 and SDNB3 resemble the major-ion composition of seawater. There chemistry has remained relatively unchanged since 2006. While SDNB2 major-ion samples plot as more bicarbonate and carbonate rich water that has changed since 2006. These differences imply SDNB has multiple zones of water recharge some of which are seawater intrusion.

A comprehensive investigation is being conducted by the U.S. Geological Survey in the San Diego, California area to evaluate the suitability of the San Diego Formation and overlying alluvial deposits for use as a drinking water supply. As many as 6 monitoring wells were installed at 10 sites to depths of as much as 2,000 feet for this investigation. Litho-logic information was compiled from descriptions of drill cuttings collected at each borehole site and from observations recorded during drilling. Electromagnetic induction (EM) logs were obtained prior to the installation of the monitoring wells, and during subsequent site visits. EM logs are sensitive to changes in lithology and water quality; because the lithology remains constant repeated EM logs can be used to show changes in water-quality due to natural recharge processes, seawater intrusion, or other processes. Monitoring wells were sampled for major-ions, selected minor-ions and trace elements, the stable isotopes of oxygen and hydrogen, and age-dating constituents, including tritium and carbon-14, to identify sources of high-chloride groundwater to the monitoring wells in the San Diego area.

This paper presents lithologic, geophysical, and water-quality data collected from three of the multiple-well monitoring sites located along the San Diego Bay and from two multiple-well monitoring sites located on a plateau away from the coastal area. These monitoring sites were chosen based on their location near the coast and on their close proximity to each other. Lithologic data indicate the hydrogeology of the San Diego area is characterized by alternating layers of marine and non-marine sediment lacking continuity over long distances. Chloride concentrations in sampled wells ranged from 136 milligrams per liter (mg/L) to 40,000 mg/L, the high chloride concentrations suggest the presence of sea-water intrusion in several near-coastal monitoring wells. Bromide-to-chloride ratios indicate that chloride concentrations greater than seawater likely result from seawater altered by evaporation. A relation established between EM logs, lithology, and water quality allowed for an estimate of the quality of water in intervals not sampled by wells. Finally, the EM logs, in combination with the water-quality data, is used to distinguish between high-salinity zones that are caused by seawater intrusion from those that are caused by “freshening” of a previously-saline aquifer by recently recharged water.

Changes in chloride concentrations and electromagnetic conductivity logs for SDNB and SDMC are presented below. Conductivity has increased to a depth of about 580 feet below land surface (fbls), with a corresponding in-crease in chloride concentrations. The presence of the high-salinity groundwater indicates seawater intrusion is occurring within this zone. Conductivity has increased below a depth of 820 fbls and suggests that seawater intru-sion is occurring in this zone as well, although to a much lesser extent. In contrast, chloride concentrations and electrical conductivity have decreased from 560 fbls to 820 fbls and at 1,460 fbls. The absence of high-salinity groundwater with in this zone indicates “freshening” of a previously-saline aquifer by recently recharged water.

SDNBSDMCSDORSDOT SDCC Seawater

SDNB

Seawater

0 200 0 1,4007000

100

200

300

400

500

600

700

DEPT

H, IN

FEE

T BE

LOW

LAN

D SU

RFAC

E

800

900

1,000

1.100

1,200

1,300

1,400

1,500

1,6001,620

SDNB

0 200 0 4000

100

200

300

400

500

600

700

800

900

1,000

1.100

1,200

1,300

1,400

1,500

1,6001,620

SDMC

0 200 0 400

SDOT

100

0

200

300

400

500

600

700

800

900

1,000

1.100

1,200

1,300

1,400

1,500

1,6001,620

Gamma,in counts

per second0 200 0 400

Electromagneticconductivity, in

millimhos-meter

Gamma,in counts

per second

Electromagneticconductivity, in

millimhos-meter

Gamma,in counts

per second

Electromagneticconductivity, in

millimhos-meter

Gamma,in counts

per second

Electromagneticconductivity, in

millimhos-meter0

100

200

300

400

500

600

700

800

900

1,000

1.100

1,200

1,300

1,400

1,500

1,6001,620

SDOR Gamma,in counts

per second0 200

Electromagneticconductivity, in

millimhos-meter0 400

SDCC

100

0

200

300

400

500

600

700

800

900

1,000

1.100

1,200

1,300

1,400

1,500

1,6001,620

–50

–45

–40

–35

–30

–25

–20

–15

–10

–8 –7 –6 –5 –4 –3 –2 –1 0

δDEU

TERI

UM, I

N P

ER M

IL

δOXYGEN-18, IN PERMIL

Global Meteoric Water Line

SDNB

SDMC

SDOT

SDOR

SDCC

Group 3

Group 2

Group 1

SDNB1 2006SDNB1 2008SDNB1 2011SDNB2 2006SDNB2 2008SDNB2 2011SDNB3 2006SDNB3 2008SDNB3 2011

–50

–60

–40

–30

–20

–10

0

–8 –7 –6 –5 –4 –3 –2 –1 0

δD, I

N P

ER M

IL

δ18O-, IN PERMIL

Global Meteoric Water Line

SDNB

2006

2008

2011

Seawater mixing line

10%

20%

30%

40%

50%

60%

70%

80%

90%

Resemblingseawater

-5,000 0 5,000 10,000 20,00015,000

SDNB

-500 1,5001,0005000

SDMC320

700

500

−500

−600

−128

0 500 1,000 1,500 2,000 2,500 3,000

0

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

−200 0 200 400 600

Chloride

TDS

DEPT

H, IN

FEE

T BE

LOW

LA

ND

SURF

ACE

CHANGE IN CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

CHANGE IN TDS CONCENTRATION, IN MILLIGRAMS PER LITER

CHANGE IN ELECTROMAGNETIC CONDUCTIVITY, IN MILLIMHOS PER METER

800 -4,200

991

-238

2,800

60

86

−200 −100

−40

−100

−130

0 100 200 300 400 500

0

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

−50 0 50 100 150

Chloride TDS

DEPT

H, IN

FEE

T BE

LOW

LA

ND

SURF

ACE

240

329

190

760

590

43,000

43,200

43,400

43,600

43,800

44,00011.6

11.7

11.8

11.9 4/24/12 12:00 4/29/12 12:00 5/4/12 12:00 5/9/12 12:00 5/14/12 12:00 5/19/12 12:00

FBS TDS Dashed line is incomplete data

DATE AND TIME

DEPT

H TO

WAT

ER, I

N F

EET

BELO

W L

AND

SURF

ACE

TOTA

L DI

SSOL

VED

SOLI

DS,

IN M

ILLI

GRAM

S PE

R LI

TER

From the April 24, 2012, through May 21, 2012, an in situ salinity transducer was installed at a depth of 200 feet below land surface to measure total dissolved solids (TDS) concentrations values every 15 minutes. This data was compared against the mixed semi-diurnal tidal influenced water levels of SDNB3. The TDS concentrations reached a maximum of 43,844 mg/L on May 1st at 0745 and reached its minimum value of 43,259 mg/L on May 9 at 1730.