Mapping Seawater Intrusionusing Time Domain

Electromagnetic Induction and High Resolution Electrical

Resistivity, Los Angeles County, California

Ted Johnson and Nancy Matsumoto:Water Replenishment District of Southern California

andJohn Jansen: Aquifer Science and Technology

SettingSetting

San Gabriel ValleySan Gabriel Valley

Coyote Hills

Palos Verdes Hills

Puente Hills

Merced Hills

Santa Monica Mtns

Long BeachLong Beach

San Gabriel RiverLos Angeles River

Los Los AngelesAngeles

San Gabriel MtnsSan Gabriel Mtns

Pacific Ocean

Newport-Inglewood

Uplift

Newport-Inglewood Uplift

4 Million PeopleArea = 420 mi2 (1100 km2)Approx. 250,000 af/y (3.1 e8 m3/y)Groundwater PumpingMulti-layered sandy aquifersover 800 ft deep (> 250 m)

By 1950s, groundwater was belowsea level in half of the basins

Seawater Intrusion 2 miles (3.2 km) inland. v ~ 250 ft/y (75 m/y)

1900 – 1950: Extractions Double Safe Yield

Overdraft: Groundwater levels dropping 10 ft (3 m) per year and well below sea level.

West CoastBasin Barrier Project

Dominguez GapBarrier Project

Alamitos GapBarrier Project

Seawater Intrusion Barrier Wells

Residual “Saline Plume”

Silverado Aquifer

Lower San Pedro Aquifer

Pico Formation (“bedrock”)

200 ft. Sand Aquifer

Modified from DWR 1961, Cross Section E-E’

West East

Injection Wells

Paci

fic

Oce

anBarrier Wells & Saline Barrier Wells & Saline PlumesPlumes

Residual saline plumes continue to move inland

800 f

t

(250 m

)

Saline Plume in multiple aquifers to different extents up to 800 feet deep (250 m).

2006 2006 Saline Saline Plume Plume Map*Map*

Silverado Silverado (middle) (middle) AquiferAquifer

* - based on * - based on existing well existing well

datadata

Groundwater Flow Direction

Redondo Beach Blvd

Cre

nsha

w

Haw

thor

ne

Sepulveda

Torrance

190th

Manhattan Beach Blvd

El Segundo Blvd

Sep

ulve

da B

lvd

PCH

Injection Wells

Legend

_̂ Goldsworthy Desalter

@? Wells used for contouring

Chloride Concentrations

250 - 500 ppm

500 - 1,000 ppm

1,000 - 3000 ppm

3,000 - 5,000 ppm

> 5,000 ppm

1990 Saline Plume

Paci

fic

Oce

an

Saline Plume DefinitionSaline Plume Definition

SeawaterIntrusion

• Intrusion Maps biased based on well locations and screen intervals• Shallow wells may miss salt• Production wells with long screens and high Q may average/dilute salt

Fresh Water

• Short screen wells identify salt at that discrete depth• Many wells needed to identify 3D nature of intrusion $$$$, or…

Cl = 100 ppmNo Plume Here?

Cl = 600 ppmNot so bad?

Cl = 19,800Big Problem?

Electrical Electrical Resistivity (ER)Resistivity (ER) Electrical Current is induced into

the ground through two current electrodes.

Voltage drop is measured across potential electrodes.

Electrode array is expanded to increase depth of penetration.

Resistivity of formation/fluids measured in ohm-meters (Ω-m).

Modern systems use 42 to 56 (or more) electrodes with automated switching.

Time Domain Time Domain Electromagnetic Electromagnetic Induction (TEM)Induction (TEM) Square transmitter loop laid out on

surface 20 m to 100 m per side. Current of several amps cut off

nearly instantaneously. Creates broad frequency EM pulse. Nearly vertical propagation of pulse

induces eddy currents in conductive units (such as saline water).

Receiver measures magnetic field from eddy currents over time.

Data is modeled into layered system. To reduce background noise and

increase depth, multiple wire turns can be used.

Electrical Methods Work Electrical Methods Work Well in “Noise Free” AreasWell in “Noise Free” Areas

But what about Electrical But what about Electrical Noisy Areas, such as Los Noisy Areas, such as Los Angeles? Angeles?

2 Sites for Pilot 2 Sites for Pilot TestTest

S ite A

B aseball F ie ld and Drainage Ditc h

C ityP ark

S ite B

0 0.25 0.5M ile

Electrical Electrical Resistivity (ER)Resistivity (ER)

Time Domain Time Domain Electromagnetic Electromagnetic Induction (TEM)Induction (TEM)

• TEM: o 40m x 40m single turn loop (500 ft, 150 m depth, noisy data).o 20m x 20m 4-turn loop (600 ft, 180 m depth, much less noise).

• ER:o 42 electrodes over 1,345 linear feet.o 350 ft (106 m) depth of penetration (not deep enough to reach saline plume).

Site A LayoutSite A Layout

Dep

th (

feet)

Site A ResultsSite A ResultsElectrical Resistivity Line 2

TEM2 TEM1

Pilot Test Showed:Pilot Test Showed:• ER and TEM can work in urbanized environment with care.

• Mapped bulk resistivity (formation + fluids). Interpreted freshwater (> 20 Ω-m), brackish water (3 – 20 Ω-m), and saline water (<3 Ω-m).

•ER higher resolution but could not reach deep enough (L = d * 3.5 or 2,800 ft line length (850 m) to reach 800 feet deep (250 m). Not possible in study area for this much open space.

• Multi-turn TEM proved best for this study. Therefore, full-scale TEM survey in February 2008 in open spaces (parks, school yards, ball fields, etc).

35 TEM Locations

Pacifi

c Ocean

Cross Section Line

North

D

D’

H’

H

Results:Results:• Multi-turn TEM coils successfully mapped resistivity in an urban environment to 800 feet deep (250 meters).

• 3D subsurface resistivity identified at 35 sites in 2 weeks for $50,000 (U.S.)

• Saline plume found mostly to be discontinuous and varying shape/size across basin likely due to heterogeneity.

•Drilling 2 deep multi-level monitoring wells in summer to confirm/refine results. Maps will be redrawn after completion. Ultimately leading to RO plants for treatment.

tjohnson@wrd.org

Thank You!Thank You!