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

Eig

envecto

r-2

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

-150

-100

-50

0

50

100

150

Kresse DataLarsen-Paul Data

Eigenvector-1

Eig

envecto

r-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)

14

C A

ge

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