The document discusses land subsidence, which is the sinking or settling of land caused by underground water withdrawal. It notes that groundwater overdraft is the principal cause of subsidence, and it has impacted over 1,200 square miles in Arizona, with up to 20 feet of subsidence measured. The document outlines how subsidence occurs, where it is located nationally and in Arizona, its impacts such as damage to infrastructure and earth fissures, monitoring techniques, and potential mitigation strategies.

1. AWE Luncheon
March 6, 2015
Leslie T. Katz
Land Subsidence:
What is it and why do we care?
AWE Luncheon
March 6, 2015
Leslie T. Katz

2. What is land subsidence and why does it occur?
Where is land subsidence a problem – nationally
and in Arizona?
How does land subsidence effect us?
What mitigation strategies are available to avoid
or minimize land subsidence?

3. What is Land Subsidence?
Deformation of land surface due to compaction,
consolidation, or collapse of the subsurface
Groundwater overdraft is principal cause – 43% of
Arizona’s water supply from groundwater

4. What is Land Subsidence?
 Pumping reduces fluid pressure in pore spaces
between sediment grains in unconsolidated
materials
 Results in reduction in support for “skeleton” of
aquifer system
 Weight of over-burden causes collapse, particularly
in fine-grained layers

5. Generalized Basin & Range
Schematic

6. What is Actually Occurring in the Aquifer?

7. What is Actually Occurring in the Aquifer?

8. What Do We Need to Know?
• Results in largely permanent loss in aquifer
storage
• Can continue after pumping stops due to delayed
drainage from fine-grained layers
• Is exacerbated by increasing over-burden

9. What Do We Need to Know?
 Recharge basins
 Wastewater discharge ponds
Not all basins are created equal
 Same water level decline does not result in same
subsidence
 Thickness of fine-grained units is critical!

10. Where is Subsidence Occurring in US?

11. Occurs across southern AZ
First documented in 1940s
when pumping increased
25 distinct subsidence areas
identified
> 1,200 sq miles of land
impacted
Up to 20 feet of subsidence
measured
Where is Subsidence
Occurring in AZ ?

12. Occurs across southern AZ
First documented in 1940s
when pumping increased
25 distinct subsidence areas
identified
> 1,200 sq miles of land
impacted
Up to 20 feet of subsidence
measured
Where is Subsidence
Occurring in AZ ?

13. Occurs across southern
AZ
First documented in
1940s when pumping
increased
25 distinct subsidence
areas identified
> 1,200 sq miles of land
impacted
Up to 20 feet of
subsidence measured
Where is Subsidence Occurring in AZ ?

14. How is Subsidence Monitored?
InSAR = Interferometric Synthetic Aperture Radar
Used by ADWR since 2002

15. West Phoenix
Subsidence
2010-2014

16. Hawk Rock
Subsidence
2010-2014

17. What about Earth Fissures?
• Caused by differential settling
• Can show up suddenly and be
widened/deepen during rain storms
• Can be miles long, >10 feet wide, and 100s of
feet deep

18. What about Earth Fissures?

19. Where do Earth Fissures Occur?
• Most occur along basin margins
• Picacho Basin has about 2/3 of mapped
fissures
• Other areas include Luke, Wilcox, Apache
Junction, and Chandler Heights

21. What are the Impacts of Subsidence and
Earth Fissures?
Largely permanent loss in aquifer storage
Damage to surface and subsurface infrastructure
 Surface – buildings, roads, bridges, pipelines,
canals, recharge & wastewater basins
 Subsurface – pipelines, foundations, underground
tanks

22. Problems with gravity conveyance systems
 Sewer systems, CAP canal, irrigation ditches
Fissures can provide avenues for contaminants to
enter aquifers
What are the Impacts of Subsidence and
Earth Fissures?

23. What Can be Done?
• Characterize aquifer, focusing on identifying
fine-grained layers
• Analyze and project maximum anticipated
drawdown
• Design well fields and pumping strategies to
minimize well interference and drawdown
• Identify signs of existing subsidence and/or
fissures

24. What Can be Done?
Monitor changes over time
 Track pumping and water level data
 Geologic reconnaissance
 InSAR, if available
 Fissure mapping, if available
 Aerial photography
 Geophysics

25. Final Thoughts
Case Studies
 Aquifer recharge facility siting and design
 Underground gas storage project design
 Wastewater discharge pond design
 Recovery planning

26. Final Thoughts
Future Projections
 Several factors conspiring to increase subsidence
into the future
 Drought
 Over-allocation of CAP
 Climate change
 Will need technical but also legal strategies in AZ

27. Questions?