James Kingsbury

National Water-Quality Assessment Program Water Quality of the Mississippi embayment-Texas coastal uplands aquifer system and Mississippi River Valley alluvial aquifer By James A. Kingsbury, Jeannie R.B. Barlow, and Brian G. Katz 99° 94°30' 90°

About 8 million people rely on groundwater from the Mississippi embayment- KANSAS ! MISSOURI !!! KENTUCKY ! ! ! ! ! ! ! TENNESSEE ! ! !! ! ! Texas coastal uplands aquifer system for drinking water. The Mississippi River ! ! ! ! ! ! !! ! ! ! !

ARKANSAS ! 36° ! ! ! TENNESSEE !!! !! !! ! ! !! !!!!!! ! ! !!!!! ! ! ! Shallow groundwater is most vulnerable to contamination !! ! ! ARKANSAS ! ! !! ! See Inset Valley alluvial aquifer also provides drinking water for domestic use in rural areas, ! ! ! !!! ! ! ! !! !!!!!!!! ! !! ! ! !!!!!!!!!! ! ! ! ! ! !!!!!!! !!!! ! ! ! !!! ! ! OKLAHOMA ! ! ! ! One or more !! ! ! where groundwater residence times are short-- ! ! ! ! ! but is of primary importance to the region as a source of water for irrigation. ! ! ! ! ! MISSISSIPPI !! ! ! ! ! ! ! ! ! ! ! ! ! ! pesticides or volatile organic compounds were detected in samples ! Monitoring wells and public- ! supply wells sampled in the ! Irrigation withdrawals from this aquifer are among the largest in the !! Groundwater is a potential source of phosphorus to streams in the lower ! Memphis, Tennessee area ! ! !! ! from 42 percent of wells that sampled shallow groundwater in the ! ! MISSISSIPPI Mississippi River watershed-The median concentration of dissolved ! ! ! Nation and play a key role in the economy of the area, where annual crop ! ! ! ! ! !!! ! ! ! ! !! ! ! Mississippi embayment-Texas coastal uplands ! !!! ! ! ! ! ! ! !!! ! ! ! ! ! ! phosphorus (0.6 mg/L) in the Mississippi River Valley alluvial aquifer was more ! !! ! sales total over 3 billion dollars. The reliance of the region on both aquifers for ! ! !! ! ! TEXAS ! ! ! ! ! !!! ! aquifer system and Mississippi River Valley alluvial aquifer. 31°30' ! ! than 10 times greater than is typical for groundwater. Groundwater discharge ! drinking water and irrigation highlights the importance of long-term ! ! !! These contaminants were detected most commonly and at ! contributes a signifcant portion of base flow to streams. An association !! LOUISIANA ! management to sustain the availability and quality of these resources. ! !! ! the highest concentrations where groundwater residence times ! ! between elevated concentrations of phosphorus and iron suggest that ! ! ! ! ! ! !! ! ! ! ! were short (groundwater less than 20 years old) and the phosphorus transport is facilitated by the reducing conditions in the aquifer. ! ! ! Mississippi River Valley alluvial aquifer groundwater was oxic (containing dissolved oxygen). Mississippi embayment-Texas coastal uplands aquifer system !( Primarily domestic and public-supply wells in the METX aquifer system 27° !( Primarily irrigation wells in the MRVA aquifer !( Shallow monitoring well in Memphis, Tennessee area <20 years old >20 years old Mississippi River Valley alluvial aquifer 10 Base from U.S. Geological Survey 1.5 digital data 1:2,500,000 Water table Albers Equal-area projection North American Datum of 1983 1 Water table Terrace deposits— 0.1 Public unconfined aquifer 1 Mississippi embayment-Texas supply well 0.01 coastal uplands aquifers— unconfined 0.001

Public liter per micrograms in 0.5 supply Monitoring No pesticides detected

Total pesticide concentration, concentration, pesticide Total 0.0001 wells wells 0 5 10 15 20 25 30 USEPA recommended criterion

in milligrams per liter as P Domesitc well for total phosphorus in Ecoregion X Irrigation well Estimated groundwater age, in years 0

Dissolved phosphorus concentration, 0 5,000 10,000 15,000 20,000 Dissolved iron concentration, in micrograms per liter Vulnerability of deep aquifers may increase in areas where withdrawals are greatest-Large water-level declines in the

Mississippi embayment-Texas coastal uplands aquifer system

have increased the potential for contamination from shallow groundwater. 7,000 Estimated nitrogen An example of this is the middle Claiborne aquifer underlying Memphis, 6,000 inputs at land surface Tennessee, in areas where the overlying confining units are thin or absent. 5,000 Con ning layers In some other areas, increases in chloride and dissolved solids have 4,000 Confining units Mississippi embayment-Texas coastal occurred where saltwater has been drawn from lower parts of the aquifer 3,000 upland aquifers—confined into areas of freshwater.

in kilograms Nitrogen input, 2,000 Mississippi River Valley Thick con ning unit-low vulnerability Con ning unit absent-increased vulnerability 1,000 alluvial aquifer 20 Shallow aquifer water level Shalllow aquifer water level 30 Well depth 59 feet 30 Well depth 91 feet 10 Nitrate in groundwater 35

40 land surface surface land

Photo courtesy of USDA NRCS Water produced for domestic and public supply is generally surface land 40 50

1 below feet in level, Water Water level, in feet below feet in level, Water 45 of high quality- 60 1950 1960 1970 1980 1990 2000 2010 Nitrate concentrations in shallow groundwater 1950 1960 1970 1980 1990 2000 2010 1955 1965 1975 1985 1995 2005 Few contaminants were detected at concentrations greater 1955 1965 1975 1985 1995 2005 were low despite widespread fertilizer use- 0.1 than human-health benchmarks. Of those that were, Although agriculture is the predominant land Laboratory reporting level contaminants derived from natural sources such as radon and

per liter as Nitrogen use near the wells in the Mississippi River Valley GX12ML00CRK2200 arsenic were more common and at higher concentrations 0.01 alluvial aquifer, nitrate was detected infrequently.

Concentration, in milligrams Subunits of the Missisippi Shallow parts Pleistocene than those derived from anthropogenic River Valley alluvial aquifer of the Miss emb- terrace When nitrate was detected, concentrations were low Texas coastal deposits sources such as pesticides. uplands aquifer (Memphis system area, TN) (usually less than 1 mg/L) in large part because of the 120 140 anoxic conditions throughout most of the aquifer Specific conductance Specific conductance EXPLANATION Middle Claiborne aquifer Middle Claiborne aquifer 100 120 Oxic which result in the removal of nitrate through Mixed denitrification. 80 100

Anoxic Redox conditions

CENTIMETER

IN MICROSIEMENS PER PER MICROSIEMENS IN

SPECIFIC CONDUCTANCE SPECIFIC

CENTIMETER IN MICROSIEMENS PER PER MICROSIEMENS IN 60 CONDUCTANCE SPECIFIC 80 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 7,000 Estimated nitrogen 6,000 inputs at land surface 5,000

4,000

3,000

in kilograms

Nitrogen input, 2,000

1,000

10 Nitrate in groundwater

Photo courtesy of USDA NRCS 1 Nitrate concentrations in shallow groundwater were low despite widespread fertilizer use- 0.1 Although agriculture is the predominant land Laboratory reporting level

per liter as Nitrogen GX12ML00CRK2200 use near the wells in the Mississippi River Valley 0.01 alluvial aquifer, nitrate was detected infrequently.

Concentration, in milligrams Subunits of the Missisippi Shallow parts Pleistocene River Valley alluvial aquifer of the Miss emb- terrace When nitrate was detected, concentrations were low Texas coastal deposits uplands aquifer (Memphis system area, TN) (usually less than 1 mg/L) in large part because of the

EXPLANATION anoxic conditions throughout most of the aquifer Oxic which result in the removal of nitrate through Mixed denitrification. Anoxic Redox conditions