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

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)

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)

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

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 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

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)

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

Stable O vs Stable H

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

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

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