1. Determining the source of saline groundwater from the Mississippi River Valley Alluvial aquifer in southeast Arkansas Justin Paul and Dr. Daniel Larsen Department of Earth Sciences; University of Memphis South-Central GSA Meeting March 17, 2014

2. Study Area and Regional Geology
Modified from Wikipedia
Modified from Cox et al. (2013)

3. Alluvial Aquifer Quaternary sands and gravels (Ackerman, 1996)
Capped by silt and clay confining unit (Ackerman, 1996)
Modified from Ackerman (1996)

4. 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)

5. 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)

6. Modified from Cox et al. (2007)
Area I
Area II
Liquefaction Fields
Sand-blows
Cox et al. (2004&2007)
Tell us multiple things:
Paleoseismicity
Elevated pore pressures
Local Faults
Arkansas & Saline River Fault Zones
Area I has same orientation as regional structural grain
Area II is distinctly linear
Modified from Cox et al. (2007)

7. Modified from AR Geological Survey
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

8. Geothermal Anomaly at Depth
Modified from SMU Geothermal Laboratory Google Earth Application

9. 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.

10. Methods
Geochemical and statistical techniques to solve this hydrogeologic problem:
Principle Component Analysis
Spatial Statistical Analysis
Hydrologic Tracer Analysis

11. Principle Component Analysis
Chicot; Very Salty
Chicot; Salty
Alluvial Aquifer
Desha; Dilute
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
Chicot;
Rel. Salty
Desha; Salty
Sparta Aquifer
Higher TDS to left, higher Cl to north
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

12. Spatial Analysis
Seeking statistical relationship between location and density of sand- blows to Cl content in groundwater
Modified from Kresse and Clark (2008) and Cox et al. (2007)

13. Hydrologic Tracer Analysis
Interpretations
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
Trace Elements

14. Stable O vs Stable H

15. Cl content vs 14C age in alluvial groundwaters
Chicot ; Very Salty ;
Backswamp
Desha ; Dilute ;
Backswamp
Desha ; Salty ;
Backswamp

16. 2. Injection of chloride-rich waters from depth through faults
Conclusions
Using geochemistry and statistics to solve a hydrogeological problem
Methods will test vastly different hypotheses
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
Sub-bullets of diff hypoths

17. 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