MORGAN VALLEY HYDROGEOLOGY STUDY

By Janae Wallace, Mike Lowe, Jon King,

Walid Sabbah, and Kevin Thomas

Utah Geological Survey

View to northwest of Weber River and Morgan Valley

OBJECTIVES• CHARACTERIZE RELATIONSHIP OF GEOLOGY TO GROUND-

WATER CONDITIONS– Compile geologic map and cross sections– Develop hydrostratigraphy– Determine thickness of valley-fill aquifer (use gravity survey to

make isopach map)– Define hydrogeologic setting (map recharge/discharge areas)– Compile water-yielding characteristics, including fractured-rock

aquifers• DEVELOP WATER BUDGET FOR DRAINAGE BASIN• DETERMINE WATER QUALITY

– Mostly valley-fill data (52 wells) from previous UGS study, added 10 bedrock wells/springs

• CLASSIFY GROUND WATER IN VALLEY-FILL AQUIFER• DETERMINE HIGH NITRATE SOURCE THROUGH STABLE

ISOTOPE DATA• ENVIRONMENTAL TRACER DATA FOR 10 ALLUVIAL; 10

BEDROCK WELLS/SPRINGS (AGE DATING)

METHODS• Geologic map – simplified geologic map compiled from several sources with most

of the younger surficial deposits stripped off (King)

• Three cross sections produced to attempt to define locations and offsets on faults, thicknesses of Tertiary cover, and likely underlying pre-Tertiary units – geology below Morgan Valley is likely more complicated than shown (King)

• Aquifer characteristics estimated included specific capacity, transmissivity, hydraulic conductivity and storativity – from specific capacity using TGUESS algorithm or aquifer test by others (Sabbah)

• Gravity survey included 350 data points throughout valley (Thomas)

• Defined hydrogeologic setting on presence or absence of thick clay layers and direction of vertical hydraulic gradient (Lowe)

• Developed water budget from PRISM annual precipitation data for last 10 years, annual evapotranspiration data derived from current landuse and natural vegetation data, and other available data (Sabbah)

• Standard water-quality sampling procedures, 2004 and 2009: general chemistry, nutrients, stable isotopes/environmental tracers, and age dating (Wallace)

HYDROGEOLOGIC SETTING - 1• Geologic units range from Proterozoic to Holocene age (lithologic

columns for Morgan Valley area and Willard thrust sheet)

• Area is structurally complex, mostly related to three major episodes of mountain building (Early Proterozoic deformation, high-grade metamorphism, and igneous intrusion; Cretaceous compression and development of Sevier fold and thrust belt; and late Cenozoic extension, resulting in basin-and-range-type features, including normal faults and trough shared by Ogden and Morgan Valleys

• Potential fractured-rock aquifers are identified on lithologic columns (mostly carbonates, with some sandstones/quartzites)

• In many areas, potential fracture-rock aquifers covered by thick sequences of Tertiary units (such as Wasatch Formation and Norwood Tuff)

Blue indicates formation has potential water-bearing properties (potential aquifer)

Blue indicates formation has potential water-bearing properties (potential aquifer)

Geologic Map

By Jon King

Tertiary units

HYDROGEOLOGIC SETTING - 2

• Cross sections indicate that thickness of Tertiary units may make fractured-rock aquifers prohibitively deep (thousands of feet) for water wells in many areas and structural complexity makes targeting specific rock units risky where covered by these Tertiary units (also, not all geologic units exist in subsurface at all locations)

• Ground-water flow in fractured rock aquifers largely controlled by structure, especially dip, and water-yield to wells largely controlled by amount of fracturing encountered by well

• Compiled water-yielding characteristics for fractured-rock aquifers highly variable, even for aquifers with more than one set of data, and data from aquifer tests conducted by others may be of variable quality (reported transmissivities from 3.5-8250 square feet/day with highest reported for Wasatch Formation [valley fill?])

HYDROGEOLOGIC SETTING - 3• Valley-fill aquifer has historically been the most important aquifer

in Morgan Valley. Many wells screened in both Quaternary and Tertiary. Many wells shallow. Generally unconfined with flow from valley margins to valley center and then downstream toward head of Weber Canyon

• Thickness of valley fill is 0 at valley margins, is less than 200 feet in much of the valley, but may be greater than 600 feet thick near towns of Morgan and Enterprise

• Valley-fill is predominantly coarse grained (gravel and sand) and is a primary recharge area (vulnerable to contamination from activities on the land surface)

• Valley fill is productive aquifer with transmissivities ranging from 6.75-8815 square feet/day based on our data (U.S.G.S. report estimated 40,000-50,000 for a Morgan City well). Areas of high transmissivity correspond to areas with great aquifer thickness)

CottonwoodCreek Weber River East Canyon

Creek

Schematic block diagram showing groundwater conditions

Location of gravity stations And cross section locations

Isopach map (showingrelatively valley-fill thickness)

Different Types of Recharge/Discharge Scenarios

Recharge-area mapAll Primary Recharge

Sc = Q/S

Q – pumping rate

S -drawdown

T = bk

b = aquifer thickness

k = hydraulic conductivity

TGUESS algorithm using Sc data –Implements Cooper-Jacob approximation of Theis equation

k = T/b

T – transmissivity

B – aquifer thickness

S = Sy + (Ss x b)

Sy – specific yield

Ss – Specific storage

b – aquifer thickness

Integrated land use and natural vegetation map

used for estimating ET

Estimated ET rates for dominant vegetation and land use

Summary of Integrated

Water Budget for Morgan

Valley

Water-Quality Data

-Overall quality-Classification-Nitrate-Environmental Tracers

WATER-QUALITY RESULTS

• TDS RANGE: 91 – 1,018 mg/L; (437 av) BEDROCK: 256-772 mg/L; (526 av)

• NITRATE RANGE: <0.01 – 12.8 mg/L; AVERAGE NITRATE 2.6 mg/L

• BEDROCK NO3 :<0.01 – 28 mg/L (1.4; 4.6)

• 4 WELLS EXCEEDED MCL FOR NO3 (including 1 bedrock well) ; 2 FOR ARSENIC

TOTAL-DISSOLVED-SOLIDSMAP

Kilometers0 2 4 6 8 10 12 14 16

G r o u n dW a t e r C l a s s T D S B e n e f i c i a lU s eC l a s s I A & I BC l a s s I IC l a s s I I IC l a s s I V

0 t o 5 0 0 m g / L5 0 0 t o3 , 0 0 0 m g / L3 , 0 0 0 t o1 0 , 0 0 0 m g / LG r e a t e r t h a n1 0 , 0 0 0 m g / L

P r i s t i n e a n di r r e p l a c e a b l eD r i n k i n gw a t e rL i m i t e d u s eS a l i n e

GROUND WATER CLASS

TDS BENEFICAL USE

CLASS IA/IB 0 to 500 mg/L PRISTINE/

IRREPLACABLE

CLASS II 500 to 3,000 mg/L

DRINKING WATER

CLASS III 3,000 to 10,000 mg/L

LIMITED USE

CLASS IV >10,000 mg/L SALINE

CLASSIFICATION MAP

TDS and Location of Bedrock samples

NITRATE CONCENTRATION

Location of public supply well (formerly Wilkinson Dairy) this pictureWas taken during the development phase of the housing project. The Water from this well has been sampled numerous times and relativelyHigh nitrate concentrations persist (latest sampled yielded ~9 mg/L)

View upgradient of Hardscrabble Canyon (no apparent land use to contribute nitrate, but some wells in the area have persistent, but sporadic nitrate concentrations)

0

10

20

30

40

50

60

70

80

90

1st Qtr 2nd Qtr 3rd Qtr 4th Qtr

EastWestNorth

Fields taken from USGS http://www.rcamnl.wr.usgs.gov/isoig/isopubs/itchch16.html#16.4.2

N

wellNitrate28 mg/L

wellNitrate28 mg/L

N

WELLS SAMPLED FOR ENVIRONMENTAL

TRACERS

‘0 2 4 6 8 10

Miles

Kilometers0 2 4 6 8 10 12 14 16

Environmental Tracer/Age Data

CC

C*

-Tritium

C carbon -14 Modern

*C carbon -14 Modern Tritium age

Overall younger water for alluvial samples--longer residence time for older water in bedrock?

CONCLUSIONS• OVERALL GROUND-WATER QUALITY:

98% PRISTINE - 2% DRINKING-WATER

• OVERALL LOW NO3 CONCENTRATION- 4 wells > 10 mg/L

• OXYGEN-NITROGEN ISOTOPES: Manure/Septic and Soil N (mixed?)

• Modern-Age and Mixed-Age Water (all 3 Carbon-14 wells Modern; 2 Tritium are pre-bomb water, 6 Tritium are Modern; 12 are Mixed) -overall historical recharge age

• Nitrate likely human related-localized (not NPS)

CONCLUSIONS• Geology is complex with potential fractured-rock aquifers covered

by thick Tertiary deposits in many areas

• Penetrating targeted fractured-rock aquifers in these covered areas may be hit or miss and well yield will be determined by amount of fractures intercepted by well

• Valley-fill aquifer safer target and generally productive - may make more sense to target valley-fill aquifer and pump water to where it is needed

• Valley fill is greatest (>600 feet) in center of valley near towns of Morgan and Enterprise

• Coarse-grained valley fill vulnerable to surface sources of pollution

• Valley fill contains mostly high quality ground water.

• Nitrate an issue in some areas.

• Widespread use of septic tanks combined with shallow depth of some wells may be a future concern

• Water budget indicates inflows exceed outflows – water deficit