THE HYDROGEOLOGY OF MOAB- SPANISH VALLEY, SOUTHEASTERN UTAH, WITH EMPHASIS ON MAPS FOR WATER-RESOURCE MANAGEMENT AND LAND-USE PLANNING by Mike Lowe, Janae Wallace, and Stefan Kirby Utah Geological Survey and Charles Bishop 2007 Funded by: U.S. PA through Utah Division of Water QualityFunded by: U.S. PA through Utah Division of Water Quality 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

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

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 = 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 ft –Total Jgc ~ 1200 ft Hydrostratigraphy Scan-line Survey

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

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 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

18% PRISTINE 82% 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 fill and 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 2.1cfs 2.8 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, 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. DomainArea (acres) Flow* (cubic feet per second) Average nitrate concentration (background) (mg/L) Number of wells sampled Current number of septic tanks permitted *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. DomainArea (acres) Flow amount (cfs) Current density (acres/ system) Number of septic tanks permitted Projected number of total septic tanks Calculated lot-size recommen- mg/L (acres) Lot-size recomm- endation (acres) /15** /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/yr Estimated: Subsurface inflow-12,300 Infiltration ppt-730 Seepage-Kens Lake Total Recharge=17,330 Steady-state calibration: Subsurface inflow-12,765 Infiltration ppt-728 Seepage-Kens Lake Total Recharge=16,650 DISCHARGE-acre-ft/yr Estimated: Seepage to streams-1140 Withdrawal wells/springs Seepage to Colo. Riv Total Discharge=17,330 Steady-state calibration: Seepage to streams-1099 Withdrawal wells/springs Seepage to Colo. Riv Total Discharge=16,650