SMALL-SCALE SPATIAL SMALL-SCALE SPATIAL VARIATION OF WATER QUALITY VARIATION OF WATER QUALITY WITHIN THE MISSISSIPPI RIVER WITHIN THE MISSISSIPPI RIVER

VALLEY ALLUVIAL AQUIFER VALLEY ALLUVIAL AQUIFER ARKANSAS, USAARKANSAS, USA

SMALL-SCALE SPATIAL SMALL-SCALE SPATIAL VARIATION OF WATER QUALITY VARIATION OF WATER QUALITY WITHIN THE MISSISSIPPI RIVER WITHIN THE MISSISSIPPI RIVER

VALLEY ALLUVIAL AQUIFER VALLEY ALLUVIAL AQUIFER ARKANSAS, USAARKANSAS, USA

Ken SteeleKen Steele11, Tim Kresse, Tim Kresse22 and Ralph Davis and Ralph Davis11

11Department of Geosciences, University of Department of Geosciences, University of Arkansas, Fayetteville Arkansas, Fayetteville 22Water Division, Arkansas Water Division, Arkansas

Dep.t of Environmental Quality, Little RockDep.t of Environmental Quality, Little Rock

OBJECTIVEOBJECTIVE

To ascertain spatial variability of major ion concentrations on small scale within the alluvial

aquifer in Arkansas

To ascertain spatial variability of major ion concentrations on small scale within the alluvial

aquifer in Arkansas

Pulaski County

ARKANSAS

CountyWoodruff

CountyMonroe

Alluvial

Aquifer

U S A

Alluvial Aquifer

Composition

Sand, silt, clay grades downward to sand and gravel (~ 45 m thick)

Discontinuous lenses of clay, silt or sandy silt

Overlying unit (6 m thick) composed of fine silt and clay

Mineralogical heterogeneity

Rainfall1270 mm annually but little recharge during summer

Yield 3,785 to 11,355 liters per minute

Site Descriptions

Site Site CropsCropsSoil Mapping Soil Mapping

UnitUnitReliefRelief

mm

Well DepthsWell Depths

mm

Water DepthWater Depth

mm

PulaskiPulaski

CountyCounty

Soybean

Vegetables

Keo & Rilla –well drained

Perry – clayey ~ 0.45

Shallow 9 - 10

Deep 24 – 30

Shallow 7-9

Deep ~ 8

MonroeMonroe

CountyCountySoybean

RiceFoley & Dundee-

poorly drained

~ 0.3 Shallow 8 - 9

Deep 30 - 34

Shallow 4 - 5

Deep ~ 5

WoodruffWoodruff

CountyCountySoybean

Dubbs –permeable

Amagon-more clay

~ 0.3Shallow 9 -15

Deep 24 - 30

Shallow ~ 6

Depth ft P-X P-Y P-Z

Depth ft M-Y M-Z M-R

0-5 silt silt sand 0-2 sand

5-10silt

claysilt

claysand

2-4clay silt sand

10-15

clay clay sand

clay clay4-6

clay clay sand

15-20 sand silt silt 6-8 sand sand sand20-22 sand sand sand 8-10 sand silt sand22-14 sand sand sand 10-12 sand sand sand

12-14 sand sand sand14-16 sand sand16-18 sand18-20 sand

Pulaski County Site Monroe County Site

Monroe County 3 shallow monitoring wells

3 deep irrigation wells Pulaski County

3 shallow monitoring wells 1 domestic well

3 deep irrigation wellsMonitoring wells 20 - 212 m apart

Irrigation wells 1.3 km apart

Irrigation wells 0.8 km from monitoring wells

Woodruff County 4 monitoring wells

1 domestic well

Sample Collection

SiteShallow

monitoring and domestic wells

Deep irrigation

wells

Monroe

Pulaski

counties

June 24-25 2002

August 11-12, 2002August 11-12

2002

Woodruff

County June 18

1996None

Monroe County

Shallow Monitoring wells ~ 30 feet deep

Deep Wells ~ 100 feet

SmallSmall

scalescale

00 500500

FeetFeet

SepticSeptic tanktank

PulaskiPulaski

CountyCounty

MonroeMonroe

CountyCounty

Monitoring Monitoring WellWell

Monroe County Monroe County Pig penPig pen

14.45

Monroe County

High SOHigh SO44, ,

Cl, and Cl, and NONO33

0

5

10

15

20

25

30

35

40

0 10 20 30 40

Recharge Cl mg/L

No

n-r

ech

arg

e C

l m

g/L

TemporalTemporal

Spatial Spatial VariationVariation

Plots of recharge versus non-recharge periods ion concentrations allow simultaneous comparison of spatial and temporal variations

Recharge Conditions

Wells assigned recharge or non-recharge conditions based on:

• Typical recharge period (October – June)

Septic tank effluent and Irrigation return flow recharge not likely

• Best scientific judgment based on:

Rainfall Ground water levels

0

10

20

30

40

50

60

70

0 20 40 60 80

Recharge SO4

No

n-r

ech

arg

e S

O4

Monroe-SPulaski-SMonroe-DPulaski-DWoodruff-S

0

5

10

15

20

25

30

35

40

0 10 20 30 40

Recharge Cl mg/L

No

n-r

ec

ha

rge

C

l m

g/L

Monroe-SPulaski-SMonroe-DPulaski-DWoodruff-S

0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300 350 400 450

Recharge HCO3 mg/L

No

n-r

ech

arg

e H

CO

3 m

g/L

Monroe-S

Pulaski-S

Monroe-D

Pulaski-D

Woodruff-S

max/minmax/min

MM P P WW

Shallow Shallow 1.9 1.6 4.11.9 1.6 4.1

Deep Deep 1.91.9 1.7 1.7

max/minmax/min

MM P P WW

Shallow Shallow 3.7 4.9 4.43.7 4.9 4.4

Deep Deep 1.41.4 4.7 4.7

max/minmax/min

M M P W P W

Shallow Shallow 3.7 4.9 4.43.7 4.9 4.4

Deep Deep 1.41.4 4.7 4.7

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80

Recharge Ca mg/L

Non

-rec

harg

e C

a m

g/L

Monroe-S

Pulaski-S

Monroe-D

Pulaski-D

Woodruff-S

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

Recharge Na mg/L

Non

-rec

harg

e N

a m

g/L

Monroe-SPulaski-SMonroe-DPulaski-DWoodruff-S

0

5

10

15

20

25

30

0 10 20 30

Recharge Mg mg/L

No

n-r

ech

arg

e M

g m

g/L

Monroe-S

Pulaski-SMonroe-D

Pulaski-DWoodruff-S

0

1

2

3

4

5

0 1 2 3 4 5

Recharge K mg/L

Non

-rec

harg

e K

mg/

L

Monroe-SPulaski-SMonroe-DPulaski-DWoodruff-S

max/min max/min

M P WM P W

Shallow Shallow 5.3 1.8 3.55.3 1.8 3.5

Deep Deep 2.02.0 -- --

max/minmax/min

MM P P W W

Shallow Shallow 2.9 1.6 4.52.9 1.6 4.5

Deep Deep 2.02.0 -- --

max/minmax/min

MM P P W W

Shallow Shallow 2.9 1.7 >6.72.9 1.7 >6.7

Deep Deep 1.51.5 1.3 1.3

max/minmax/min

M P WM P W

Shallow Shallow ---- 5.4 5.4 -- --

Deep Deep -- -- -- -- -- --

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2(Ca+Mg)/HCO3 meq

(Na+

K)/

Cl

meq

Monroe-NPulaski-NM Wells NP Wells NMonroe - RPulaski - RWoodruff-S

All points to the left are shifted to the right within the acceptable

ratio zone when Na+K are added to Ca+Mg

These two points are moved to the left within the acceptable ratio zone

when SO4 is substracted

from Ca+Mg.

Calcite-richCalcite-rich

unitunit

Clay-richClay-rich units

Clay-richClay-rich

unitunit

Non-rechargeNon-recharge

period waterperiod water

tabletable

RechargeRecharge

period waterperiod water

tabletable

RechargeRecharge

period waterperiod water

tabletable

Sandy siltSandy silt

Sandy siltSandy silt

WellsWells

CONCLUSIONSCONCLUSIONS

• There is prevalent and extensive small-There is prevalent and extensive small-scale spatial variation of ion scale spatial variation of ion concentrationsconcentrations

• There is temporal variation for Ca, Mg, There is temporal variation for Ca, Mg, Na, SONa, SO44, HCO, HCO33

• Significant cation exchange occurs for Significant cation exchange occurs for Ca and Na (and some for Mg and K)Ca and Na (and some for Mg and K)

• Deep wells may or may not have ion Deep wells may or may not have ion concentrations greater than shallow concentrations greater than shallow wellswells

CONCLUSIONS (continued)CONCLUSIONS (continued)

• Spatial variation of water quality is attributed Spatial variation of water quality is attributed to:to:–discontinuous sand, silt and clay lenses discontinuous sand, silt and clay lenses

with heterogeneous mineralogywith heterogeneous mineralogy–differences in residence timedifferences in residence time

–lower water tables can allow fine-grained lower water tables can allow fine-grained sediment units to de-water thus increasing sediment units to de-water thus increasing ion concentrationsion concentrations

• Temporal variation is caused by:Temporal variation is caused by:

–fluctuating water tables that can fluctuating water tables that can change flow pathschange flow paths

SUMMARYSUMMARY

Complex physical and mineralogical Complex physical and mineralogical heterogeneity of the alluvial aquifer heterogeneity of the alluvial aquifer sediments and varying recharge sediments and varying recharge cause significant variability of the cause significant variability of the ground-water chemistryground-water chemistry

0

5

10

15

20

25

30

35

40

0 10 20 30 40

Recharge Cl mg/L

No

n-r

ech

arg

e C

l m

g/L

Monroe-SPulaski-SMonroe-D

Pulaski-DWoodruff-S

max/minmax/min

MM P W P W

Shallow Shallow 6.8 3.6 4.36.8 3.6 4.3

Deep Deep 1.31.3 -- --

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

Recharge SO4

No

n-r

ech

arg

e S

O4

Monroe-SPulaski-SMonroe-DPulaski-DWoodruff-S

max/minmax/min

MM P P WW

Shallow Shallow 3.7 4.9 4.43.7 4.9 4.4

Deep Deep 1.41.4 4.7 4.7

0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300 350 400 450

Recharge HCO3 mg/L

No

n-r

ec

ha

rge

H

CO

3 m

g/L

Monroe-S

Pulaski-S

Monroe-D

Pulaski-D

Woodruff-S

max/minmax/min

MM P P WW

Shallow Shallow 1.9 1.6 4.11.9 1.6 4.1

Deep Deep 1.91.9 1.7 1.7

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80

Recharge Ca mg/L

No

n-r

ech

arg

e C

a m

g/L

Monroe-SPulaski-SMonroe-DPulaski-DWoodruff-S

max/minmax/min

MM P W P W

Shallow Shallow 2.9 1.6 4.52.9 1.6 4.5

Deep Deep 2.02.0 -- --

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

Recharge Na mg/L

No

n-r

ech

arg

e N

a m

g/L

Monroe-S

Pulaski-S

Monroe-D

Pulaski-D

Woodruff-S

max/minmax/min

MM P P WW

Shallow Shallow 5.3 1.8 3.55.3 1.8 3.5

Deep Deep 2.02.0 -- --

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Recharge Mg mg/L

No

n-r

ech

arg

e M

g m

g/L

Monroe-SPulaski-SMonroe-DPulaski-DWoodruff-S

max/minmax/min

MM P P WW

Shallow Shallow 2.9 1.7 >6.72.9 1.7 >6.7

Deep Deep 1.51.5 1.3 1.3

0

1

2

3

4

5

0 1 2 3 4 5

Recharge K mg/L

No

n-r

ech

arg

e K

mg

/LMonroe-SPulaski-SMonroe-DPulaski-DWoodruff-S

max/minmax/min

MM P P WW

Shallow Shallow ---- 5.4 5.4 ----

Deep Deep -- -- -- -- ----