Evolution of the conceptual hydrogeologic and ground-water flow

model for Las Vegas Valley, Clark County, Nevada

David J. Donovan

Southern Nevada Water Authority

Geological Society of America Annual Meeting

November 14, 2000

Outline

• Model development

• Improvement in geologic understanding

Conceptual Model DevelopmentHydrologic Parameters

• Location, timing and value of:– Water levels– Ground water production / injection– Spring, creek flow– Major and minor wash flow– Water usage by bare-soil and pheatophytes– Natural recharge– Secondary recharge– Interbasin flow

Conceptual Model DevelopmentHydrogeologic Parameters

• Location, and value of:– Transmissivity - Horizontal permeability– Leakance - Vertical permeability– Storativity - Ability to store water

• Spatial distribution determined by a combination of geology and hydrology

• Assigned value primarily determined by hydrologic analysis (aquifer tests / modeling)

1981 MSS Image (2,3,1)

> 5,000 meters of fill

Me

ters

ab

ove

me

an

se

a l

eve

l

Topography of Las Vegas Valley

Major structures and lithologies in Las Vegas Valley

Hydrologic flow system and Hydrogeologic setting

-1000

-4000

-3000

-2000

-5000

2000

3000

0

1000LowerPaleozoicCarbonate Rocks

Carbonates ?PaleozoicLower

UpperPaleozoicTransitionalMesozic

SliciclasiticRocks

&Rocks Rocks

CratonalCambrian

Mesozoic to

?

pre-Cambrian

Basement

SPRING MOUNTAINS

Frenchman Mtn.Las Vegas

WashSpringsLas VegasMountain

La Madre

Alluvial FanRed Rock

Las Vegas Boulevard (Strip)

West EastEdge of Las Vegas Valley Hydrographic Basin

Ground water flow directions

0

-5000Hydrogeologic cross-section. Vertical exaggeration: 4 to 1

025005,0007,500

10,00012,500

-2,500-5,000-7,500

-10,000-12,500-15,000

??

UTM meters east (NAD 27)

Hydrologic flow system. Vertical exaggeration: 1 to 1

ToColorado

River

ToPahrump

High permeability area inLas Vegas Valley

High permeability area

Maxey and Jameson cross-section

East

INTERBEDDED SANDY GRAVELS

SILTY CLAYS

CALICHE WITH SAND AND GRAVEL

DISTRICTMAIN WELL FIELD

NEW WELLS

Generalized geologic cross-section of Las Vegas Valley

EXPLANATION

Modified from Maxey and Jameson (1948)

Vertical Exageration ~ 17.5U D

U D U D

500

250

(m)

West

0 5

KILOMETERS

CARBONATE BEDROCK

500

250

750

1000(m)

"Near-surface water""Shallow aquifer""Middle aquifer""Deep aquifer"

Concept of allostratigraphic units

Allostratigraphic units cross-section

"middle

0

250

500

Fault

CLASTIC ROCKS

UD Inferred

U

part""lower

part"

CARBONATE

750

West

1000

METERS

B

?

Surface Location of D TO D' cross-section

Vertical Exaggeration = 10X

B'

Land

Sand Sandy c-g deposits

KILOMETERS

Gravelly c-g depositsInterbeddedSandy silt

2.50

ROCKS

Unknown

East

Hydrostratigraphic units cross-section

24A

0

250

500

Fault

5

6 6B?

UD Inferred

U

4

4B

3B

750

West

1000METERS

B

(aquifer)

(aquifer)(aquitard)

3A1B

2

1A

1X

East

Surface Location of D TO D' cross-section

Vertical Exaggeration = 10X

B'

Land

Very high permeability

Low permeability Moderate permeability

KILOMETERS

Unknown

2.50

EXPLANATION

CarbonateRocks

Clastic Rocks

Hydrostratigraphic unit fence Diagram

1X1A

1B

1B

1A

1X

1X

4

4

4B

4B3A

2

5

66

5

3B 2

2

3A

2A

2B

4

6B?

3B

0 1 2 3 4 5

N

W

S

E

0

0

100

100

200

200

300

300

400

400

500

500

600

600

700

0

250

500

750

750

1000

Meters above

6?

Area OfInvestigationBoundary

D

D'

B

B'

Low Permeability

ModeratePermeability

HighPermeability

CarbonateBedrockClasticBedrock

PotentiometricSurface

Wells usedto create fence diagram

PotentiometricSurface

1A

1B

1X

2

2A

3A

3B

4

4A

4B

5

6

6B

2B

mean sea level

Elevation

Duck CreekAquifer

Las VegasSprings

Las VegasWash

Aquifer

Aquitard

Relationship between allostratigraphic and hydrostratigraphic units

Increase in sorting

Increasing distance from source

Locally cemented

Poorly sorted, cementedcoarse-graineddeposits

Well sortedlightly cemented

moderately coarse-graineddeposits

fine-grained deposits

Hydrostratigraphic Units

shallow geochemical zone

aquitard

aquitard

aquitard

aquitard

primary aquifer fornon-municipal wells

primary aquifer formunicipal wells

aquifer ?

aquifer ?

Las VegasWash

Aquitard

Las VegasSpringsAquifer

DuckCreekAquifer

(confiningunit)

Very Simplified diagram ofthe ground-water systemin the central part ofLas Vegas Valley

0

25

50

75

100

125

150

175

200

225

250

275

300

325

350

375

400

425

450

Model Layers - Gross Simplification

Layer 1 - Las Vegas Wash Aquitard

Leakance Boundary - Las Vegas Creek Aquifer Plus Twin Lakes Aquitard

Layer 2 - La Madre Mountain Aquifer

Leakance Boundary - Unnamed Aquitard at the Top of the Duck Creek Aquifer

Layer 3 - Duck Creek Aquifer

1

2

3

Transmissivity map Layer 1

8 Transmissivity Zones

Range of Values:

100 to 5,000 ft2/d

Upper Leakance layer

8 Leakance Zones

Range of Values:

1.0E-8 to 1.6E-4 ft2/d

Transmissivity map Layer 2

11 Transmissivity Zones

Range of Values:

50 to 30,000 ft2/d

Lower Leakance layer

6 Leakance Zones

Range of Values:

1.0E-8 to 1.0E-3 ft2/d

Transmissivity map Layer 3

6 Transmissivity Zones

Range of Values:

50 to 1,000 ft2/d

Table - TransmissivityTransmissivity

code Type Layer 1 Layer 2 Layer 30 Inactive 0 0 01 Carbonate Rock 101 551 1,0002 Clastic Rock 102 252 1003 Igneous/Metamorphic Rock 103 53 504 Cemented Alluvial Fan gravel 1,500 1,0045 U/F U/C Alluvial Fan Gravel/Sand 2,500 8,0056 Faulted Sands/Silts/Gravel 1,001 6,0067 Unfaulted/Undiff. Silts/Sands/Clays 1,000 307 5008 Local moderately permeable areas 5,000 12,6009 Very permeable areas 50,000 30,009

10 N/A11 Faulted Carbonate Rock 50112 Faulted Clastic Rock 5213 Faulted Igneous/Metamorphic Rock14 Local odd unit 1,214

Table - Leakance

Leakance L 1-2 L 2-3code Type

0 Inactive 0 01 Carbonate Rock 1.1E-06 1.0E-062 Clastic Rock 2.0E-08 2.0E-083 Igneous/Metamorphic Rock 1.0E-08 1.0E-084 Cemented Alluvial Fan gravel 1.5E-065 Alluvial Fan Gravel/Sand 1.6E-04 1.0E-046 Faulted Sands/Silts/Gravel 1.3E-05 1.0E-037 Unfaulted/Undiff. Silts/Sands/Clays 1.0E-07 1.0E-078 Local moderately permeable areas 1.5E-069 Very permeable areas

10 N/A11 Faulted Carbonate Rock12 Faulted Clastic Rock13 Faulted Igneous/Metamorphic Rock14 Local odd unit

Conclusions

• Hydrologeologic models primarily test geologic concepts

• Geologic understanding has increased in the last two decades

• Hydrologic data is the compilation of causes and effects on the ground-water system

• Most observed ground-water system changes are a result of multiple causes

Artesian wells in Las Vegas Valley

1912 1998

Table - StorativityStorage Layer 1 Layer 2 Layer 3

code Type0 Inactive1 Carbonate Rock 1.0E-01 1.001E-02 1.1E-032 Clastic Rock 1.0E-01 1.002E-02 1.2E-033 Igneous/Metamorphic Rock 1.0E-01 1.003E-02 1.3E-034 Cemented Alluvial Fan gravel 2.5E-01 1.004E-025 Alluvial Fan Gravel/Sand 8.050E-036 Faulted Sands/Silts/Gravel 1.006E-027 Unfaulted/Undiff. Silts/Sands/Clays 1.5E-01 1.070E-03 1.7E-038 Local moderately permeable areas 1.001E-019 Very permeable areas 5.009E-02

10 N/A11 Faulted Carbonate Rock12 Faulted Clastic Rock13 Faulted Igneous/Metamorphic Rock14 Local odd unit 1.101E-0115 Local odd unit 2 4.015E-02

Location of Las Vegas Valley

Allostratigraphic units

shallow geochemical zone

Tule SpringsAlloformation

Lone MountainAllogroup

ParadiseValley

Alloformation

silt

Very Simplified diagramof the basin-fill inthe central part ofLas Vegas Valley

0

25

50

75

100

125

150

175

200

225

250

275

300

325

350

375

400

425

450

gravel

silt

gravel with

silt

gravel

silt

gravel

caliche

(Las Vegas Fm.)

Major lithic types: Las Vegas Valley