Study Area and Regional Geology

Determining the source of saline groundwater from the Mississippi River Valley Alluvial aquifer

in southeast Arkansas

Justin Paul and Dr. Daniel LarsenDepartment of Earth Sciences; University of Memphis

South-Central GSA Meeting March 17, 2014

Study Area

and

Regional Geology

Modified from Wikipedia

Modified from Cox et al. (2013)

Alluvial Aquifer

• Quaternary sands and gravels (Ackerman, 1996)

• Capped by silt and clay confining unit (Ackerman, 1996)Modified from Ackerman

(1996)

Occurrence of saline

groundwater

• Chloride condition could be due to evaporative processes in near surface (Kresse and Clark, 2008).

• Cannot discount vertical migration of saline fluids along faults (Kresse and Clark, 2008).Modified from Kresse and Clark

(2008)

Soils• Mostly clay-rich

varieties derived from backswamp deposits (Saucier, 1994)

• On the whole, elevated Cl concentrations in backswamps (Kresse and Clark, 2008)

Modified from Kresse and Clark (2008)

Sand-blows• Cox et al.

(2004&2007)

• Tell us multiple things:▫ Paleoseismicity▫ Elevated pore

pressures

Modified from Cox et al. (2007)

Local Faults• Arkansas & Saline

River Fault Zones

• Area I has same orientation as regional structural grain

• Area II is distinctly linear

Area I

Area II

Liquefaction Fields

Brines at Depth• Jurassic age formations

• Evaporative and shallow marine deposits associated with opening of Gulf of Mexico (Harry and Londono, 2004)

• Basinal brines with unusual chemistry (Hanor and McIntosh, 2007)

Modified from AR Geological Survey

Geothermal Anomaly at Depth

Modified from SMU Geothermal Laboratory Google Earth Application

Hypotheses• Chloride condition due to…

1. Evapotranspiration processes whereby clay-rich soils restrict recharge and concentrate chloride in infiltrating

surface water.

2. Injection of chloride-rich fluids from depth into the aquifer through faults during previous earthquakes and still

migrating today.

3. Regional rivers recharging relatively chloride-rich water into the alluvial aquifer when river levels are higher than

the water-table.

Methods

Geochemical and statistical techniques to solve this hydrogeologic problem:

•Principle Component Analysis

•Spatial Statistical Analysis

•Hydrologic Tracer Analysis

Principle Component Analysis

• n= 177

• EV-1=91% of variance▫ Heavy negative weights on Ca, Mg,

Na, Cl, SO4

▫ Dilute end-member

• EV-2=4% of variance▫ Heavy positive weight on Cl▫ Heavy negative weights on Ca and

SO4

Alluvial Aquifer

Sparta Aquifer

-300 -250 -200 -150 -100 -50 0 50 100

-80

-60

-40

-20

0

20

40

60

80

Kresse DataLarsen-Paul Data

Eigenvector-1

Eige

nvec

tor-

2

-1600 -1400 -1200 -1000 -800 -600 -400 -200 0 200

-150

-100

-50

0

50

100

150

Kresse DataLarsen-Paul Data

Eigenvector-1

Eige

nvec

tor-

2

Desha; Rel. Dilute

Desha; Salty

Desha; Dilute

Chicot; Rel. Salty

Chicot; Salty

Chicot; Very Salty

• n= 57

• EV-1=84% of variance▫ Heavy negative weights on Na

and Cl

• EV-2=9% of variance▫ Heavy positive weight on Ca

and Cl▫ Heavy negative weight on Na

Both Desha; Dilute

Spatial Analysis

• Seeking statistical relationship between location and density of sand-blows to Cl content in groundwaterModified from Kresse and Clark (2008) and Cox et al.

(2007)

Hydrologic Tracer Analysis• Modern water• <60 years

Tritium (3H)

• Geologically old water• Atmospheric, crustal, or mantle

source

4He, 3He/4He

• Intermediate age water• Assess carbon sources

14C, 13C/12C• Sensitive to evaporation• Water-rock interactions

2H/1H, 18O/16O• Recharge temperatures• Recharge contributions and

sources

Noble Gases

• Water-rock interactionsTrace Elements

Interpretations

-7 -6 -5 -4 -3 -2 -1 0

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

f(x) = 4.41045850121939 x − 10.7973951006412

f(x) = NaN x + NaNf(x) = NaN x + NaN

GMWL

Linear (GMWL)

ArkMWL

Linear (ArkMWL)

Alluvial Waters

Linear (Alluvial Waters)

Linear (Alluvial Waters)

VSMOW

Deep Waters

Miss. River Water

δ 18O

δ2H

Stable O vs Stable H

Cl content vs 14C age in alluvial groundwaters

0 200 400 600 800 1000 1200 14000.0

200.0

400.0

600.0

800.0

1000.0

1200.0

1400.0

1600.0

1800.0

Cl (mg/L)

14C

Age

Desha ; Dilute ;

Backswamp Desha ;

Salty ;Backswam

p

Chicot ; Very

Salty ;Backswam

p

Conclusions• Using geochemistry and statistics to solve a

hydrogeological problem

• Methods will test vastly different hypotheses1. Near-surface evaporative concentration of chloride

in recharging groundwater

2. Injection of chloride-rich waters from depth through faults

3. Regional rivers recharging relatively chloride-rich water into the alluvial aquifer

• Evidence suggests evap. evolved, pre-modern crustal waters mixing with fresher, younger meteoric waters

Special Thanks

• Tim Kresse- data

• Geological Society of America- funding

• U. of Memphis Dept. of Earth Sciences- support

• U. of Arkansas Stable Isotope Lab- support

• U. of Miss. Geology & Geo. Engineering Dept.-support

• South-Central GSA- travel considerations