Setting
description
Transcript of Setting
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.
Thank You!Thank You!