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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