Land Subsidence within the Mojave Water Agency Service Area

Michelle Sneed and Justin BrandtCalifornia Water Science Center

U.S. Geological Survey

Land Subsidence► Land subsidence is a gradual settling or

sudden sinking of the Earth’s surface owing to surface or subsurface movement of earth material

► More than 80% of the identified 17,000 square miles of land affected by subsidence in the Nation is a consequence of our exploitation of groundwater (National Research Council, 1991).

► Most of the groundwater related subsidence is caused by the compaction of susceptible alluvial aquifer systems that typically accompanies overdraft of these systems.

Measuring SubsidenceBench Mark

Spirit Leveling

GPS

InSAR

Extensometer*

*measures part of land subsidence

Reported subsidence caused by fluid (mostly water) withdrawal

CA Basins with Subsidence History

Subsidence is an ‘undesirable result’ in the Sustainable Groundwater Management Act (SGMA) legislation

2010

2012

Protruding Well (~2 ft in 2 yrs)

Preliminary and subject to revision

2003-08

2008-10

2007-10

InSAR-Measured Subsidence in the San Joaquin Valley

Subsidence along the DMC2007-10

2003-08Loss of capacity reported at Check Station 7 in April 2014, where flow was restricted because of subsidence upstream (Checks 2-6). The short-lived opportunity to fill San Luis Reservoir was impacted.

Land Subsidence in the Mojave Desert Aquifer-System Compaction

Galloway and others, 1999; USGS Circular 1182

► Concentrated in the fine-grained deposits (aquitards)

► Inelastic (irreversible) compaction occurs when the preconsolidation stress is exceeded

► Preconsolidation stress ≈ previous lowest groundwater level

► Storage capacity is reduced

Subsidence Damages Infrastructure & Natural Resources; Worsens Flooding

► Infrastructure damage Roads, rails, bridges, pipelines, wells,

canals, etc.► Natural resources

Reduces aquifer-system storage capacity Fissuring and channeling Rerouting/migration of riparian corridors and

wetlands (and aquatic ecosystems)

► Flooding Increased depth Longer duration Unexpected/migrating locations Restricted land uses

Since the early 1990s, subsidence has been localized and near 5 dry lakebeds

Mojave

Subsidence Magnitudes

Coyote Lake

► No subsidence 1993-2004

► Shallow wells small but

persistent declines to historically low levels

► Deep wells unknown

► Clay lithology unknown

► Compaction depth unknown permanence

unknown

Troy Lake

► Shallow and deep wells persistent declines

to historically low levels

► Clay layers range 3-100 ft in

shallow and deep parts of the aquifer system

► Compaction likely permanent in

shallow and deep parts of the aquifer system

Harper Lake

► Shallow wells persistent declines

to historically low levels

► Deep wells recoveries after

reaching historical lows in the 1980s

► Clay layers range 8-89 ft in

shallow and deep parts of the aquifer system

► Compaction likely permanent in

the shallow system possible in the deep

system

El Mirage► Shallow and deep

wells small but persistent

declines historical levels

(prior to 1980s) are unknown

► Clay layers range 5-295 ft in

shallow and deep parts of the aquifer system

► Compaction likely in shallow and

deep parts of the aquifer system

permanence unknown

Lucerne► Shallow wells

persistent declines to historically low levels

► Deep wells recoveries

(historical levels unknown)

► Clay layers range 20-40 ft in

shallow part of aquifer system only

► Compaction likely permanent in

the shallow system unlikely in the deep

system

Past Subsidence: 2 ft during 1976-98

Fissuring and Resultant Damage on Hwy247, Lucerne Valley

GPS stations are not in the

subsiding areas, and indicate uplift

Uplift has been observed

throughout the western US… thought to be related to the

drought (loss of water weight)

GPS stations are not in the

subsiding areas, and indicate uplift

Uplift has been observed

throughout the western US… thought to be related to the

drought (loss of water weight)

GPS stations are not in the

subsiding areas, and indicate uplift

Uplift has been observed

throughout the western US… thought to be related to the

drought (loss of water weight)

GPS stations are not in the

subsiding areas, and indicate uplift

Uplift has been observed

throughout the western US… thought to be related to the

drought (loss of water weight)

The Short Story► Subsidence has been restricted to 5 localized areas

and is not regional► Subsidence rates less than 1 inch/year►Aquitards compacted in response to persistent

groundwater-level declines to historically low levels►At least some subsidence likely is permanent (not

recoverable if water-levels rise)► Subsidence in some areas may continue for some

time after water levels stabilize/recover because of the presence of thick (slowly equilibrating) aquitards

Where To Go From Here? Continued, Periodic Monitoring

►MWA proactive at assessing subsidence Nearly 20-year record of subsidence Next round of assessment is tentatively planned for

2018-2019

►More satellites with higher measurement frequencies make the future of InSAR monitoring promising, and gaps in data availability less likely

►Coordinate groundwater-level and subsidence measurements Improved understanding of mechanisms leads to

informed management decisions that consider subsidence