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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