A More Accurate and Powerful Tool for Managing Groundwater Resources and Predicting Land Subsidence: an application to Las Vegas Valley

Zhang, Meijing

Dept. of Geosciences, Virginia Tech

Advisor: T.J. Burbey

Figure from Http://www.environment.scotland.gov.uk/our_environment/water/groundwater.aspx

Relationship between land subsidence and hydraulic head

Surface Water

Groundwater

Aquifer System

Relationship between land subsidence and hydraulic head

Total stress σ

Water pressure

Effective stress σ’

' p

Relationship between land subsidence and hydraulic head

Pumping well

Total stress σ

Water pressure

Effective stress σ’

hSb kgdhdpd '

Δb is the land subsidence. Sk is the skeletal storage coefficient , and Δh is the change in hydraulic head

According toTerzaghi's one-dimensional consolidation theory, deformation occurs only in vertical direction

Generalized surficial geologic map of Las Vegas Valley

Geologic cross-section (A-A’) illustrates the stratigraphic and fault relations interpreted from well log data. (From Bell, 2008)

BedrockSand and gravel

Silt and clay interbed

A A’Bedrock

Sand and gravel

Silt and clay interbed

Fault

A

A’

Groundwater has been pumped since 1905; More Than 1.5 m of subsidence has been observed since 1935

Bedrock

Fault

Pumping well Recharge

well

To help mitigate the ongoing occurrence of land subsidence, an artificial recharge program was initiated in 1989

Pumping and Recharging wells

0

20

40

60

210

235

310

285

260

1996 1998 2000 2002 2004 2006

Water DepthSubsidence

Seasonal and long-term subsidence and water level patterns at the Lorenzi site, Las Vegas, Nevada

A significant percentage of the subsidence is delayed relative to the water-level decline

What causes subsidence and delayed drainage?

A significant percentage of the subsidence is delayed from the water-level decline

Subsidence map for Las Vegas Valley from 1992 to 1997 (From Bell, 2002)

Subsidence bowls are offset from the major pumping center. Over time, the valley has yielded a very complex subsidence pattern, much more so than the water-level distribution

To better manage groundwater resources and predict future subsidence we have updated and developed a more accurate groundwater management model for Las Vegas Valley

Layer2

Deep-zone Aquifer Layer4

Developed-zone Aquifer

Near-surface Aquifer

Layer3

Layer1

The vertical conceptual model layer distribution (From Yan, 2007)

Faults

50m-Cell

The model incorporates MODFLOW with the SUB (subsidence) and HFB (horizontal flow barrier) packages

Extended simulation period from 1912-2010

1.7 million cells

Groundwater flow equation

z

hSW

z

hK

zy

hK

yx

hK

x szzyyxx

)()()(

K is the component of the hydraulic conductivityW is the volumetric flux per unit volume of sources or sinks of waterSs is the specific storageS’s is the specific storage of the interbedKv’ is the vertical hydraulic conductivity of the interbed

The unequilibrated heads within the interbeds can be described by the one-dimensional diffusion equation

t

h

K

S

z

h

v

s

2

2

Sources of observation data

Groundwater level data can be obtained from the USGS

Groundwater monitoring network

Pumping and Recharging wells

Las Vegas Valley Water District and State Engineer’s Office will provide needed pumping and artificial recharge data for the extended period of record

Subsidence map for the period 1963-1980 (from Bell, 2008) (left)

GPS

Land subsidence data

InSAR and PS-InSAR

Benchmarks established in 1935 and 1963

Currently only one continuous GPS station has been monitored for more than a few years

Provides surface deformations from interferometric synthetic aperture radar (data available from 1992-2010)

Permanent scatterer velocity maps (2002-2010) showing target velocities in mm/yr for the Las Vegas basin

(provided by Youquan, Zhang)

mm

/year

BLUE= UpliftRED= Subsidence

??

?

Limitation of the traditional inverse method

How to specify the number of zones ???

Where each zone is for each parameter ???

The objective of this investigation Observed land

subsidenceObserved drawdown

APE (Adjoint Parameter Estimation) algorithm and

UCODE

Inversely CalibrateHydrologic Parameters

MODFLOW

Automatically identify suitable

parameter zonations

Objective function

|hsimulated-hobserved|

|subsimulated-subobserved|

Minimize

+

h is the groundwater level sub is land subsidence

Estimated Transmissivity Zones after 3 Iterations

True Synthetic Transmissivity Zones

To verify the validity of the algorithm, a MODFLOW 2000 hypothetical model is developed, and the APE algorithm is executed to create approximate spatial zonations of T, Sske and Sskv

Note that the colors in each frame only indicate different zones and the colors (number of zones) change after each iteration

Estimated Specific Storage Zones after 3 Iterations

True Synthetic Specific Storage Zones

The estimated zonations approach the true parameter zonations

Observed vs. simulated (a) final drawdown, and (b) final subsidence.

Where do we go from here?

Our next goal is to apply the APE algorithm to Las Vegas Valley to build a complete management model for water purveyors

If necessary, global methods will be employed

A parallel method will be incorporated

Conclusions

An updated groundwater management model for Las Vegas Valley model is being developed.

We have outlined an automated parameter estimation process that can greatly aid the calibration of ground water flow models like those of LVV.

Accurate parameterization will provide a far more accurate and precise groundwater model that can be used to more accurately predict future trends on the basis of future pumping patterns.

Thanks!