1. POTENTIAL SOURCES OF SALINE GROUNDWATER IN THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER - SOUTHEASTERN ARKANSAS CHRIS KING, P.G.

2. Agriculture in southeastern Arkansas
Irrigation of row crops (rice, soybeans, cotton) and aquaculture (catfish) results in intensive use of alluvial aquifer
Mississippi River Valley alluvial aquifer is the source of 97% of groundwater used
Isolated areas in Ashley, Desha, and Chicot Counties have >100 mg/L Cl

3. Crowley’s Ridge
From: Cooper, C.D., 2002

4. Many areas with high Cl and TDS concentrations are less than 5 miles in diameter
Large area of the alluvial aquifer in Chicot County contains TDS concentrations exceeding 1,000 mg/L
Numerous studies have been done, however no clear explanation as to origin

5. FIGURE 2 - HIGH TDS AREAS OF SOUTHEAST ARKANSAS – NORTHEAST LOUISIANA
HUFF AND BONCK, 1993
AREA I
(Kresse, 2008)
AREA II
(Kresse, 2008)

6. Cl concentrations >70 mg/L are unsuitable for rice plants
Elevated Na levels are associated with higher salinity groundwater
High Na in irrigation water results in deterioration of soil structure (flocculation) and decreased permeability

7. From: Cooper, C.D., 2002

8. IRRIGATION PROJECT AREAS
Bayou
Meto N
Grand
Prairie
Boeuff-Tensas
High Demand
Area
Saline Water
Areas

9. Structural Features of the Lower Mississippi Valley
Mississippi Embayment is dominant structural feature in this area
Represents northern arm of the Gulf Coastal Plain sediment wedge of Tertiary and Quaternary deposits
The wedge of sediments thickens tremendously south of a line of marginal faults in southern Arkansas

10. FIGURE 3 - MISSISSIPPI EMBAYMENT
From: Armstrong, O.P., 2005

11. The study area is located between the South Arkansas and Pickens-Gilbertown fault zones
Between these fault zones is a large gap where marginal faults haven’t been mapped
Two structural features in this area are the Monroe Uplift and Desha Basin

12. FIGURE 4 - MAJOR STRUCTURAL FEATURES IN SOUTHEASTERN ARKANSAS
From: Armstrong, O.P., 2005

13. FORMATION OF THE MISSISSIPPI EMBAYMENT
Late in the Precambrian Era (>600 mya), rifting occurs along eastern Arkansas as continents drift apart
Guccione, M.J., 1993

14. Formation of the Mississippi Embayment
Late Cretaceous rifting and structural downwarping of fractured Paleozoic rocks produced a southward plunging syncline
The axis of the syncline approximates the present course of the Mississippi River

16. EARLY TERTIARY PERIOD
3 major marine transgressions (sea level rises) in Mississippi Embayment:
Sediments deposited in streams, swamps, and shallow coastal environments
Deposition of sand and clay, with lignite coal accumulation in swamps
Source: National Geographic

17. LATE TERTIARY PERIOD - Sea level much lower
- Ancestral Mississippi River forms in a well-defined but narrow and shallow valley
Source: National Geographic

18. Quaternary Period (<1.6 MYA)
Several major glacial periods provide massive amounts of coarse sediment to the lower Mississippi Valley:
Mississippi River is a braided stream flowing in a wide, shallow valley
Sand and gravel deposited in a series of channels that migrate across the valley
Periodic wind storms blow fine sand and silt out of channels (deposited in dunes and ridges)

19. QUATERNARY <1.6 MYA
Guccione, M.J., 1993

20. Mississippi River erodes Tertiary sediments on west of Crowley’s Ridge, Ohio River on the east
Eventually the Mississippi River cuts through Crowley’s Ridge near Cairo, Illinois
Approximately 10,000 years ago glaciation ends, Mississippi switches from a braided to meandering stream

21. Guccione, M.J., 1993

22. MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER
Quaternary alluvial deposits adjacent to Mississippi River:
- Clay and silt serves as a semi-confining unit above
- Sand and gravel in middle to lower part of alluvium
Sands and gravels function as the Mississippi Alluvial Valley Aquifer

23. Beneath gravel are Tertiary age Jackson and Claiborne Group:
Top of formation represents an unconformity (erosion surface)
Deposited in coastal environment
Low-permeability clays, silts, and highly permeably sands
Deposits occur as discontinuous lenses with variable thicknesses

24. FIGURE 1 – CROSS-SECTION THROUGH THE ALLUVIAL AQUIFER
IN ASHLEY AND CHICOT COUNTIES
UPPER CONFINING UNIT
ALLUVIAL AQUIFER
TERTIARY DEPOSITS
From: Gonthier and Mahon, 1992

25. CROSS-SECTION OF ALLUVIAL AQUIFER
(WEST-EAST THROUGH CENTRAL ASHLEY & CHICOT COUNTIES)
MRVA CONFINING UNIT
MRVA AQUIFER
BASAL GRAVEL
TERTIARY AGE DEPOSITS
Gonthier, G.J. and Mahon, G.L., 1992

26. The Tertiary Age Jackson Group and underlying Cockfield and Cook Mountian Formations are sometimes referred to as the “upper Claiborne and Jackson Group undifferentiated”

27. FIGURE 5 - GEOLOGIC CROSS SECTION THROUGH SOUTHEASTERN ARKANSAS
Difficult to distinguish w/
Discontinuous sand beds
Approx. 2800’ - Cretaceous
From Fitzpatrick, D.J., 1985

28. Recharge
The alluvial aquifer is confined from above where the overlying clay and silt is thick and continuous
Most recharge occurs from infiltration in point bar deposits and along major streams during high stages.
An estimated net recharge of 0.5 in/yr occurs by upward flow through the base of the aquifer from Tertiary and Cretaceous age aquifers

29. Groundwater Flow in Alluvial Aquifer
Flow within the alluvial aquifer is in the direction of hydraulic gradient, south-southeast
Intense pumping for irrigation creates local hydraulic gradients, capable of causing plume migration

30. FIGURE 9 – POTENIOMETRIC SURFACE OF THE MISSISSIPPI RIVER VALLEY
ALLUVIAL AQUIFER, SPRING 2004
From: Schrader, T.P., 2006

32. Lower Confining Unit
Thinning of the Cockfield surface through channel erosion has occurred in southern Chicot County
Upward flow from below is possible in areas where the Cockfield is thin or absent
Saltwater may flow from southern Chicot County into Louisiana via a fluvial channel eroded into the surface of the Cockfield Fm.

33. FIGURE 8 - SURFACE OF TERTIARY JACKSON FORMATION WITH
TDS CONCENTRATIONS FROM ALLUVIAL AQUIFER PLOTTED
From: Huff and Bonck, 1993

34. Regional Flow Model Huff and Bonck, 1993
Precipitation falling on the outcrop areas west of the Mississippi Alluvial Plain flows eastward and downgradient through the Sparta and deeper Carrizo-Wilcox aquifers
Deeply circulating groundwater comes into contact with waters with TDS concentrations of 3,000 to >10,000 mg/L
Upward flow occurs further east and may enter the alluvial aquifer from below where the lower confining unit has been eroded

35. Generalized section showing regional geology and hydrology in the Mississippi River Alluvial Plain
From: Huff and Bonck, 1993

36. Regional Flow Model Mason, P.G., 2001
The linear NW to SE trend of the Desha Basin may have at first been a preferential pathway for flow into the Cockfield, but where the syncline closed to the southeast, flow may have been restricted, trapping Cl-rich waters
The Midway group has a dip of approximately 25 feet per mile toward the axis of the Desha Basin
Very little deep-aquifer data is available for southeastern Arkansas

37. Bedinger and Reed, 1961. Water Resources Circular No
Bedinger and Reed, Water Resources Circular No. 6, Geology and Ground-water Resources of Desha and Lincoln Counties, Arkansas
“The bottom of the Sparta sand marks the maximum depth at which fresh water can be obtained.”

38. U.S. GEOLOGICAL SURVEY HYDROLOGIC INVESTIGATIONS ATLAS HA-309
“Geohydrology of the Coastal Plain Aquifers of Arkansas”
Sea Level
500’

39. Chemistry of Alluvial Aquifer Groundwater
Chemical constituents in groundwater vary widely across the Mississippi alluvial plain

40. From: Green, B.G. (USDA-ARS)

41. Chemistry of Alluvial Aquifer Groundwater (cont.)
In Arkansas rainwater, virtually all of the Na, Cl, and Mg originate from seawater
Evapotranspiration alone can theoretically produce TDS near 100 mg/L
Cation exchange occurs in clays of the unsaturated zone
Kresse and Fazio, 2002

42. FIGURE 10 – NORTH-SOUTH TRENDING BAND OF ELEVATED
CHLORIDE CONCENTRATIONS IN CHICOT COUNTY
4 MILES
From Fitzpatrick, D.J., 1985

44. Mixing from another source can produce waters with TDS >350 mg/L
Poor quality water sources cited include:
De-watering of clay lenses/confining units storing ancestral water
Upward recharge from Tertiary and older aquifers along faults or updip
Irrigation returns flows along with high evapotranspiration and ion exchange in the soil zone
- Recharge from rivers

45. Cooper, C.D., 2002
Areas further within the alluvial aquifer flow system have higher TDS concentrations due to increased residence time
As recharge enters the flow path near the fall line, it picks up calcium in the soil zone
More calcium, along with other dissolved constituents, are picked up along the flow path during rock-water interaction

46. SPECIFIC CONDUCTANCE IN THE ALLUVIAL AQUIFER VERSUS WATER LEVELS
Cooper, C.D., Wilson, A.D., Davis, R.K., and Steele, K.F., 2001

47. Hypotheses that may explain spatial variations in concentrations of specific chemical parameters
Cooper, C.D., 2002
Movement of water from deeper Tertiary and older units either along faults or updip
De-watering of clay lenses/confining units that may be storing connate paleo-waters of differing chemistry through increased pumping

48. GROUNDWATER WITHDRAWALS
Increased pumping of groundwater for irrigation is the most significant development in southern Arkansas
When the rate of groundwater withdrawal is sufficient to strongly influence flow, areas of higher salinity may increase in extent or migrate

49. Cooper, C.D., 2002
Spatial distributions of hydrochemical variations do not suggest that increased pumping has modified the geochemistry of the alluvial and Sparta aquifers

50. Kresse, “Ground-water Resources and Water Quality in the Vicinity of the Pine Bluff Municipal Area – Jefferson County, Arkansas.”
Water Quality in the Sparta, Cockfield, and alluvial aquifers of Jefferson County do not show evidence of increases in major cations and anions when compared to historical data.

51. Median TDS values for the Sparta aquifer were within 1 mg/L from the early 1950’s to 1999, despite a large cone of depression which developed resulting in water levels dropping approximately 250 feet

52. Chicot County
An area of high TDS concentrations is located near a major magnetic anomaly
Suggests upward transport of saline fluids via fault(s) from a deeper aquifer

53. Figure 6 - Aerial Magnetic Survey U.S. DOE, 1980

54. FIGURE 7 – Intersection of deep faults in Chicot County
From: Huff and Bonck, 1993

55. Areas of localized faulting
Mason, P.G., 2001
High Cl concentrations are found in an area 5 miles north of Greenville, Mississippi in the Cockfield aquifer
This area is also known for anomalously high hydraulic gradients and pH, which suggests the presence of faulting

56. Deep wells in southern Arkansas
Very few wells draw from aquifers below the Sparta
Below the Sparta and Wilcox Formation is the thick Midway shale, a major barrier to fluid flow
Below the Tertiary section is a thick sequence of carbonates
Near the center of the Monroe Uplift these carbonates form productive Cretaceous and Jurassic petroleum reservoirs

57. FIGURE 3 - STRUCTURAL FEATURES OF THE LOWER MISSISSIPPI RIVER VALLEY
DESHA BASIN
Kresse and Fazio, 2002

58. OIL & GAS TEST WELLS IN SOUTHEASTERN ARKANSAS
As of 2008, AOGC database lists 116 wells having been drilled in Ashley County; 39 in Chicot County
Many are at least 5,000 feet deep
Arkansas Geological Survey

59. Pathways for intrusion of deeper saline groundwater into alluvial aquifer
Direct or indirect migration along a deep fault
Upward flow from below where the Cockfield/Jackson confining unit is thin
Dewatering of clay lenses or confining units
Upward movement from underlying Sparta due to high pumping rates in alluvial aquifer (Cl concentrations in parts of Sparta aquifer are not as high as those in overlying Cockfield aquifer)
Movement through abandoned oil and gas test holes
MOST ACCEPTED
SCENARIOS

60. Areas for further research
Geochemical modeling of existing data
Collection of site-specific data from areas with elevated Cl concentrations:
- Coring and analysis of saturated and unsaturated zone
- Sample wells over a growing season for trends
- Monitor water levels in the same wells over time
- Examine local and regional soil types

61. REFERENCES
Armstrong, O.P., 2005, “An Overview of Mississippi Embayment Petroleum Potential of NE Arkansas,” unpublished online report.
Bedinger, M.S. and Reed, J.E., 1961, “Geology and Ground-Water Resources of Desha and Lincoln Counties, Arkansas,” Water Resources Circular No. 6,
Arkansas Geological and Conservation Commission.
Fitzpatrick, D.J., 1985, “Occurrence of Saltwater in the Alluvial Aquifer in the Boeuf-Tensas Basin, Arkansas,” U.S. Geological Survey Water-Resources Investigation
Report
Cooper, C.D., 2002, “Spatial Characterization of Hydrochemistry for the Alluvial and Sparta Aquifers of the Grand Prairie Region, Eastern Arkansas,” University of Arkansas
M.S. Thesis.
Cooper, C.D., Wilson, A.D., Davis, R.K., and Steele, K.F., 2001, “Hydrochemical Characterization for the Alluvial and Sparta Aquifers of Eastern and South-Central Arkansas:
Final Data Report,” University of Arkansas and Arkansas Water Resources Center.
Cox, R.T., Larsen, D., Forman, S.L., Woods, J., Morat, J., and Galluzzi, J., 2004, “Preliminary Assessment of Sand Blows in the Southern Mississippi Embayment,” Bulletin of
the Seismological Society of America, Vol. 94, No. 3, pp
Greene, B.G., “Shrimp Culture in Low-Salinity Water in Arkansas,” USDA-ARS unpublished technical presentation, date unknown.
Gonthier, G.J. and Mahon, G.L., 1992, “Thickness of the Mississippi River Valley Confining Unit, Eastern Arkansas,” U.S. Geological Survey Water-Resources Investigations
Report
Guccione, M.J., 1993, “Geologic History of Arkansas Through Time and Space,” University of Arkansas with funding by the National Science Foundation, 63 pp.
Hosman, R.L., 1969, “Geohydrology of the Coastal Plain Aquifers of Arkansas,” U.S. Geological Survey Hydrologic Investigations Atlas HA-309.
Hosman, R.L., 1996, “Regional Stratigraphy and Subsurface Geology of Cenozoic Deposits, Gulf Coastal Plain, South-Central United States,” U.S. Geological Survey
Professional Paper1416-G.
Huff, G.F., and Bonck, J.P., 1993, “Saltwater in shallow aquifers in east central and northeastern Louisiana and southeastern Arkansas.” U.S. Geological Survey Open File
Report
Kresse, T.M., 2008, “Occurrence, Distribution, and Source of Elevated Chlorides in the Mississippi River Valley Alluvial Aquifer in Southeastern Arkansas,” 2008
South-Central Geological Society of America Annual Meeting Abstracts with Programs.
Kresse, T.M. and Fazio, J.A., 2002, “Pesticides, Water Quality and Geochemical Evolution of Ground Water in the Alluvial Aquifer, Bayou Bartholomew Watershed, Arkansas,”
Arkansas Department of Environmental Quality Water Quality Report WQ
Mason, P.G., 2001, “Monitored Salinity and Water Levels in the Cockfield Aquifer, Washington County, Mississippi,” Mississippi Department of Environmental Quality,
Hydrologic Investigations Report
Reed, T.B., 2004, “Status of Water Levels and Selected Water-Quality Conditions in the Mississippi River Valley Alluvial Aquifer in Eastern Arkansas, 2002,”
U.S. Geological Survey Scientific Investigations Report
Schrader, T.P., 2006, “Status of Water Levels and Selected Water-Quality Conditions in the Mississippi River Valley Alluvial Aquifer in Eastern Arkansas, 2004,”
U.S. Geological Survey Scientific Investigations Report
U.S. Geological Survey Fact Sheet FS , 2002, “The Mississippi River Alluvial Aquifer in Arkansas: A Sustainable Water Resource?”
U.S. Geological Survey Fact Sheet , 2005, “Ground-Water Models of the Alluvial and Sparta Aquifers: Management Tools for a Sustainable Resource.”
Zachry, D.L, Steele, K.F., Wood, L.J., and Johnston, D.H., “Stratigraphy and Hydrology of Upper Cretaceous and Tertiary Strata, Columbia and Union Counties, Arkansas,”
University of Arkansas, 1986.