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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
Approx. 250,000 af/y (3.1 e 8 4 Million People m 3 /y)
Santa Monica Mtns Merced Hills
Los Angeles
San Gabriel Valley
Los Angeles River
Puente Hills Coyote Hills
San Gabriel 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).
West East Injection Wells
Residual saline plumes continue to move inland
200 ft. Sand Aquifer Silverado Aquifer Lower San Pedro Aquifer Modified from DWR 1961, Cross Section E E’ Pico Formation (“bedrock”)
Injection Wells
El Segundo Blvd Groundwater Flow Direction Manhattan Beach Blvd
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 190th Torrance
2006 Saline Plume Map* Silverado (middle) Aquifer * - based on existing well data
Saline Plume Definition
Cl = 100 ppm Cl = 600 ppm Not so bad?
No Plume Here?
Cl = 19,800 Big Problem?
• Intrusion Maps biased based on well locations and screen intervals • Shallow wells may miss salt
Seawater Intrusion Fresh Water
• 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?
0
2 Sites for Pilot Test
Site A Baseball Field and Drainage Ditch Site B City Park
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 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, full scale TEM survey in February 2008 in open spaces (parks, school yards, ball fields, etc).
D H’
North
D’ 35 TEM Locations Cross Section Line 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.