1. 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

2. Multi-layered sandy aquifers
Setting
San Gabriel Valley
Coyote Hills
Palos Verdes Hills
Puente Hills
Merced Hills
Santa Monica Mtns
Long Beach
San Gabriel River
Los Angeles River
Los Angeles
San Gabriel Mtns
Pacific Ocean
Approx. 250,000 af/y (3.1 e8 m3/y)
Groundwater Pumping
Multi-layered sandy aquifers
over 800 ft deep (> 250 m)
Area = 420 mi2 (1100 km2)
4 Million People
West Coast Basin
Central Basin
Newport-Inglewood Uplift
Newport-Inglewood
Uplift

3. 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)

4. Seawater Intrusion Barrier Wells
West Coast
Basin Barrier Project
Residual “Saline Plume”
Alamitos Gap
Barrier Project
Dominguez Gap
Barrier Project

5. Barrier Wells & Saline Plumes
Saline Plume in multiple aquifers to different extents up to 800 feet deep (250 m).
West
East
Injection Wells
Residual saline plumes continue to move inland
Pacific Ocean
200 ft. Sand Aquifer
Silverado Aquifer
800 ft
(250 m)
Lower San Pedro Aquifer
Pico Formation (“bedrock”)
Modified from DWR 1961, Cross Section E-E’

6. Groundwater Flow Direction
El Segundo Blvd
2006 Saline Plume Map* Silverado (middle) Aquifer * - based on existing well data
Injection Wells
Groundwater Flow Direction
Manhattan Beach Blvd
Sepulveda Blvd
Redondo Beach Blvd
190th
Crenshaw
Hawthorne
Torrance
Sepulveda
PCH

7. Saline Plume Definition
Intrusion Maps biased based on well locations and screen intervals
Cl = 600 ppm
Not so bad?
Cl = 100 ppm
No Plume Here?
Cl = 19,800
Big Problem?
Pacific Ocean
Shallow wells may miss salt
Production wells with long screens and high Q may average/dilute salt
Fresh Water
Seawater
Intrusion
Short screen wells identify salt at that discrete depth
Many wells needed to identify 3D nature of intrusion $$$$, or…

8. 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.

9. 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.

10. Electrical Methods Work Well in “Noise Free” Areas

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

12. 2 Sites for Pilot Test

13. Electrical Resistivity (ER)

14. Time Domain Electromagnetic Induction (TEM)

15. Site A Layout
TEM:
40m x 40m single turn loop (500 ft, 150 m depth, noisy data).
20m x 20m 4-turn loop (600 ft, 180 m depth, much less noise).
ER:
42 electrodes over 1,345 linear feet.
350 ft (106 m) depth of penetration (not deep enough to reach saline plume).

16. Site A Results
Electrical Resistivity Line 2
TEM2 TEM1
Depth (feet)

17. 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).

18. D D’ H’ H
North
35 TEM Locations
Cross Section Line
Pacific Ocean

21. 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.

22. Thank You!