Agriculture in southeastern Arkansas

1. POTENTIAL SOURCES OF SALINE GROUNDWATER IN THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER - SOUTHEASTERN ARKANSASCHRIS 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) AREAII (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 ModelHuff 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 ModelMason, 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. 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 • 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 Cooper, C.D., 2002

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

50. Kresse, 1999. “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.