THE HYDROGEOLOGY OF MOAB-SPANISH VALLEY, SOUTHEASTERN UTAH, WITH EMPHASIS ON MAPS

FOR WATER-RESOURCE MANAGEMENT AND LAND-USE

PLANNINGby

Mike Lowe, Janae Wallace, and Stefan Kirby

Utah Geological Survey

and

Charles Bishop2007

Funding provided by:

•Utah Division of Water Rights

•U.S. Environmental Protection Agency

•Grand County

•Utah School and Institutional Trust Lands Administration

•Utah Geological Survey http://geology.utah.gov/online/ss/ss-120/ss-120.pdf

14 X 1.25, 18 square miles

• Moab-Spanish Valley is a semi-rural area in Grand and San Juan Counties that is experiencing an increase in residential development

• Development in Grand County is on a community sewer system, but development in San Juan County uses septic-tank systems

• Nitrate contamination in the valley-fill aquifer is a potential threat

• Water-resource managers need a better understanding of ground-water conditions to better appropriate and manage water rights

• Local government officials desire land-use management tools to help preserve water quality

BACKGROUND

PURPOSE• Characterize the relationship of geology to

ground-water conditions in the Glen Canyon and unconsolidated valley-fill aquifers

• Map ground-water recharge and discharge areas for the valley-fill aquifer – vulnerability to surface sources of pollution

• Classify the ground-water quality of the Glen Canyon (east of valley only) and valley-fill aquifers – baseline water quality/beneficial uses

• Using a ground-water flow model to apply a mass-balance approach, provide septic-tank density/lot-size recommendations to protect water quality

RELATION OF GEOLOGY TO GROUND_WATER CONDITIONS

After Doelling (1988)

• Salt Tectonics• Major faults and folds• Uplift and erosion• Dissolution and

collapse• Valley fill deposition

Spanish Valley

NE SW

• Glen Canyon geometry and extent

• Fault and fracture characteristics of Glen Canyon aquifer relating to ground-water occurrence and movement

• Valley-fill aquifer geometry and sediment types

• Geologic framework data to be used to construct new ground-water flow model, which includes Glen Canyon aquifer, and assist with resource management

Purpose

NavajoSandstone

KayentaFormation

WingateSandstone

• Glen Canyon Group (Lower Jurassic)-dual permeablity aquifer • Wingate Sandstone, Kayenta Formation, and Navajo Sandstone

– Mostly eolian sandstones, some fluvial (Kayenta)• Navajo rock properties

– Unfractured K = 0.004 to 5 ft/day (Hood and Patterson, 1984; Freethey and Cordy, 1991)

– Fractured K up to 88 ft/day (Freethey and Cordy, 1991)• Thickness (Doelling, 2001, 2004)

– Jn up to 800 ft– Jk 100–300 ft– Jw 250-450 ft– Total Jgc ~ 1200 ft

Hydrostratigraphy

Scan-lineSurvey

• Outcrop extent

• Recharge from the La Sals (Sumsion, 1971; Blanchard, 1990)

• Major local recharge from Mill Creek and Pack Creek (Sumsion, 1971; Blanchard, 1990)

• This type of map helps to map lineaments

Glen Canyon Aquifer

GEOLOGY•Valley margin faults, due to gouge, may act as barriers to flow perpendicular to valley

•There are two structural ground-water compartments due to offset along the Moab fault

• The Glen Canyon Group ranges in thickness from about 330 feet south and southeast of Moab to about 1300 feet beneath southeastern Moab-Spanish Valley

• Glen Canyon Group is absent in the subsurface near Moab

Structure Contour

109°37'30"W 109°35'0"W 109°32'30"W

109°30'0"W

109°27'30"W 109°25'0"W 109°17'30"W

38°37'30"N

38°35'0"N

38°30'0"N

38°25'0"N

109°22'30"W 109°20'0"W

38°32'30"N

38°27'30"N

109°30'0"W

.

0.004 - 0.007

0.007 - 0.010

0.010 - 0.0140.014 - 0.018

0.018 - 0.021

0.021 - 0.0250.025 - 0.028

0.028 - 0.032

0.032 - 0.036

explanation

domain a

Tl median = 47 m (154ft)

Ld max = 0.035m/m2 ( 0.011ft/ft2)

Or = 127 (1)N = 1337

domain b

Tl median = 45 m (148ft)

Ld max = 0.031 m/m2 ( 0.009ft/ft2)

Or = 141 (3)N = 566

domain c

Tl median = 31 m (102ft)

Ld max = 0.032 m/m2 (0.009ft/ft2)

Or = 142 (2)N = 2498

domain d

Tl median = 71 m ( 233ft)

Ld max = 0.026 m/m2 ( 0.008ft/ft2)

Or = 128 (4)N = 306

domain e

Tl median = 46 m (151ft)

Ld max = 0.011 m/m2 ( 0.003ft/ft2)

Or = 154 (5)N = 318

domain f

Tl median = 74 m (243ft)

Ld max = 0.026 m/m2 ( 0.008ft/ft2)

Or = 35 (10)N = 135

lineament density (m/m2)

GlenCanyonGroupoutcrop

extent ofcolor orthophoto

N = number of lineaments

Or = mean orientation (standard deviation)

Ld max = maximum lineament density

Tl median = median trace length

lineament parameter description

extent of lineament domain

0 210.5

0 1 2

Lineament Domains

• Based on color-Orthophotos

• Unimodal orientation

• NW trending except in SE

• Density variation

Outcrop Joint Data

• Examined at outcrop along NE margin of valley 25 sites, ~1300 joints measured

• Principal (NW-striking) and secondary (NE-striking) joint sets

• Variety of trace lengths, terminations

• Open or partially infilled -calcite most common

• Fracture density varies

The valley fill of Moab-Spanish Valley consists mainly of stream, alluvial-fan, mass-movement, and wind-blown deposits, and generally lacks extensive fine-grained layers.

The valley fill is more than 400 feet thick near the Colorado River northwest of Moab. The valley fill thins to about 100 feet over a concealed bedrock high southeast of Moab and then thickens to more than 300 feet beneath southeastern Moab-Spanish Valley.

Moab

Based on 165 drillers’ logs of water wells

• 72 WELLS SAMPLED 1968-2004• TDS RANGE: 140 – 1818 mg/L; AVERAGE TDS 687 mg/L• NITRATE RANGE: 0.06 – 7.37 mg/L; AVERAGE

NITRATE 2.2 mg/L• MOST WELLS NO3 <1 mg/L ()• 3 WELLS EXCEEDED PRIMARY WATER QUALITY

STANDARD FOR RADIONUCLIDES; 1 SECONDARY FOR IRON; 15 FOR SULFATE

No GCG

G roundW ater C lass

TD SBenefic ia l

U se

C lass IA & IB

C lass II

C lass III

C lass IV

0 to 500 m g/L

500 to3,000 m g/L

3,000 to10 ,000 m g/L

G reater than10,000 m g/L

Pris tine andirrep laceable

D rinkingw ater

L im ited use

Sa line

18% PRISTINE82% DRINKING-WATER QUALITY

GROUND-WATER QUALITY CLASSIFICATION

Sole Source Aquifer designation, 2002

MASS-BALANCE APPROACH(Key parameters)

• Domain Acreage

• Background nitrate concentration

• Current number of septic tanks

• Ground water available for mixing

• Nitrogen loading from septic tanks

SITE SPECIFIC SEPTIC-TANK DENSITY VS. WATER QUALITY DEGRADATION STUDIES

(CALCULATIONS)

Q=TLI

(VOLUME of Discharge= Transmissivity x Length of flow x Hydraulic Gradient)

Background N mass + N mass from new septic tanks

Volume of discharge through aquifer + Waste-water volume

•Two layers (valley filland bedrock)•216 rows, 82 columns• Cell size ranges from 140 x 160 feet to 500 x 500 feet

Downs and Kovacs, 2000

1.08 cfs

2.1cfs2.8 cfs

Nitrogen Loading

Septic-tank discharge of 242 gallons/day

percapita indoor usage – 70 gallons/day – Utah Division of Water Resources, 2001

San Juan County’s average 3.46 person household – U.S. Census Bureau, 2002

Estimated nitrogen loading of 54.4 mg/L

average nitrogen loading – 17 g N per capita per day (Kaplan, 1988)

San Juan County’s average 3.46 person household – U.S. Census Bureau, 2002

15 percent retainment of nitrogen in tank (later removed during pumping) (Andreoli and others, 1979)

Parameters used to perform a mass-balance analysis for different ground-water flow domains in Moab-Spanish Valley, Grand and San Juan Counties, Utah.

Domain Area(acres)

Flow* (cubic feet per second)

Average nitrate concentration (background)

(mg/L)

Number of wells sampled

Current number of septic tankspermitted+

1 1396 1.08 2.68 16 74

2 3397 2.06 3.50 12 59

3 6749 2.82 0.78 18 77

*data were derived using ground-water flow computer model . +septic systems were estimated by the Southeast Utah Health Department (Jim Adamson, 2002, written communication; Lance Christie 2003, verbal communication).

Projected septic-tank density versus nitrate concentration for domain 1 in Moab-Spanish Valley, Grand and San Juan Counties, Utah, based on 74 existing septic tanks

Results of the mass-balance analysis using the best-estimate nitrogen loading of 54 mg N/L* for different ground-water flow domains in Moab-Spanish Valley, Grand and San Juan Counties, Utah.

Domain Area (acres)

Flow amount

(cfs)

Current density (acres/system)

Number of septic tanks permitted

Projected number of

total septic tanks

Calculated lot-size

recommen-dation

@1 mg/L (acres)

Lot-size recomm-endation(acres)

1 1396 1.08 19 74 132 10.5 10

2 3397 2.06 58 59 171 20/15** 20

3 6749 2.82 88 77 222 30/16** 20

*best-estimate calculation is based on a nitrogen load of 17 g N per capita per day (from Kaplan, 1988) for a 3.46-person household and 242 gallons per capita as the amount of water generated per household based on the 2001 Utah State Water Plan (Utah Division of Water Resources, 2001a). **second number after/ corresponds to the calculated lot-size recommendation based on an allowable degradation of overall nitrate concentration to be 5 and 3 mg/L, respectively, for domains 2 and 3.

Conclusions• Movement of water in the Glen Canyon aquifer controlled by fracture

characteristics• Glen Canyon aquifer lies directly beneath valley-fill along much of Spanish

Valley, but is absent in subsurface near Moab• Glen Canyon aquifer lineaments (usually joint zones) increase regional

permeability parallel to valley axis• Outcrop scale joints more complex, generally increase permeability parallel to

valley axis • Valley margin normal faults (where present) may decrease permeability

perpendicular to valley axis• The valley-fill generally lacks confining beds, and is mostly primary recharge

area; it is considered vulnerable to surface sources of pollution• Overall ground-water quality is good: 18% Pristine; 82% Drinking-water

quality• Poorest quality water in valley-fill aquifer is where Glen Canyon Group is

absent in subsurface• Based on our ground-water modeling using the mass-balance there are three

ground-water flow domains in the valley-fill aquifer; to be protective of ground-water quality lot sizes for development using septic-tank systems should be no smaller than 10 or 20 acres, depending on domain

Ground-Water Budget

RECHARGE-acre-ft/yrEstimated:

Subsurface inflow-12,300

Infiltration ppt-730

Seepage-Kens Lake- 3300

Total Recharge=17,330

Steady-state calibration:

Subsurface inflow-12,765

Infiltration ppt-728

Seepage-Kens Lake- 3157

Total Recharge=16,650

DISCHARGE-acre-ft/yrEstimated:

Seepage to streams-1140

Withdrawal wells/springs -6400

Seepage to Colo. Riv. - 9530

Total Discharge=17,330

Steady-state calibration:

Seepage to streams-1099

Withdrawal wells/springs - 6398

Seepage to Colo. Riv. - 9153

Total Discharge=16,650