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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.

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  1. 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 CaliforniaandJohn Jansen: Aquifer Science and Technology

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

  3. By 1950s, groundwater was below sea level in half of the basins 1900 – 1950: Extractions Double Safe Yield Overdraft: Groundwater levels dropping 10 ft (3 m) per year and well below sea level. Seawater Intrusion 2 miles (3.2 km) inland. v ~ 250 ft/y (75 m/y)

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

  5. Injection Wells Residual saline plumes continue to move inland Barrier Wells & Saline Plumes Saline Plume in multiple aquifers to different extents up to 800 feet deep (250 m). West East 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. 2006 Saline Plume Map*Silverado (middle) Aquifer* - based on existing well data El Segundo Blvd Injection Wells Groundwater Flow Direction Manhattan Beach Blvd Sepulveda Blvd Redondo Beach Blvd 190th Crenshaw Hawthorne Torrance Sepulveda PCH

  7. Cl = 600 ppm Not so bad? Cl = 19,800 Big Problem? Cl = 100 ppm No Plume Here? Saline Plume Definition • Intrusion Maps biased based on well locations and screen intervals • Shallow wells may miss salt Pacific Ocean • 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 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. Electrical Resistivity (ER)

  9. 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. Time Domain Electromagnetic Induction (TEM)

  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 Depth (feet) Electrical Resistivity Line 2 TEM2 TEM1

  17. 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). Pilot Test Showed:

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

  19. 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. Results:

  20. Thank You! tjohnson@wrd.org

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