Mapping Seawater Intrusion
using Time Domain
Electromagnetic Induction
and High Resolution
Electrical Resistivity,
Los Angeles County,
California
Ted Johnson and Nancy Matsumoto:
Water Replenishment District of Southern California
and
John Jansen: Aquifer Science and Technology
Setting
8 m3/y)
Approx.
250,000
af/y
(3.1
e
Multi-layered
sandy
aquifers
2
Million
Area =4 420
mi People
(1100 km2)San Gabriel Mtns
Groundwater
Pumping
over 800 ft deep
(> 250 m)
Merced Hills
San Gabriel Valley
Santa Monica Mtns
Los
Angeles
Puente Hills
Coyote Hills
San Gabriel River
Los Angeles River
Long Beach
Palos Verdes Hills
Pacific Ocean
1900 – 1950: Extractions Double Safe Yield
Overdraft: Groundwater levels dropping 10 ft
(3 m) per year and well below sea level.
By 1950s, groundwater was below
sea level in half of the basins
Seawater Intrusion 2 miles (3.2 km) inland. v ~ 250 ft/y (75 m/y)
Seawater Intrusion Barrier Wells
West Coast
Basin Barrier Project
Residual “Saline
Plume”
Dominguez Gap
Barrier Project
Alamitos Gap
Barrier Project
Barrier Wells & Saline Plumes
Saline Plume in multiple aquifers to different extents up to 800 feet deep (250 m).
Pacific Ocean
West
East
Injection Wells
Residual saline plumes
continue to move inland
800 ft
Silverado Aquifer
Lower San Pedro
Aquifer
Pico Formation (“bedrock”)
Modified from DWR 1961, Cross Section E-E’
(250 m)
200 ft. Sand
Aquifer
El Segundo Blvd
Injection Wells
Sepulveda Blvd
Groundwater
Flow Direction
Manhattan Beach Blvd
Legend
Wells used for contouring
Chloride Concentrations
250 - 500 ppm
500 - 1,000 ppm
1,000 - 3000 ppm
Crenshaw
?
@
190th
Goldsworthy Desalter
Hawthorne
^_
2006 Saline
Plume Map*
Silverado
(middle)
Aquifer
Torrance
3,000 - 5,000 ppm
> 5,000 ppm
1990 Saline Plume
* - based on
existing well data
Pacific Ocean
Saline Plume Definition
Cl = 600 ppm
Not so bad?
Cl = 19,800
Cl = 100 ppm
Big Problem?
No Plume Here?
Seawater
Intrusion
Fresh
Water
• 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
• Short screen wells identify salt
at that discrete depth
• Many wells needed to identify
3D nature of intrusion $$$$, or…
Electrical 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 Electromagnetic
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 Well in
“Noise Free” Areas
But what about Electrical Noisy Areas,
such as Los Angeles?
2 Sites for Pilot Test
Site A
Baseball Field
and Drainage Ditch
Site B
City
Park
0
0.25
0.5
Mile
Electrical Resistivity (ER)
Time Domain Electromagnetic
Induction (TEM)
Site A Layout
• 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 Results
Electrical Resistivity Line 2
Depth (feet)
TEM2 TEM1
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, fullscale TEM survey in February 2008 in open spaces (parks,
school yards, ball fields, etc).
H’
North
D’
35 TEM
Locations
Cross
Section
Line
D
H
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.
Thank You!
tjohnson@wrd.org