Water resources and hydrochemistry of the Alluvial and Sparta aquifers of the Grand Prairie region

Water resources and hydrochemistry of the Alluvial and Sparta aquifers of the Grand Prairie region

Chad D. Cooper, Ralph K. Davis, Kenneth F. SteeleUniversity of Arkansas, Department of GeosciencesArkansas Water Resources Center

Multiple ground-water uses Ground-water level declines Critical Ground-Water Area as of 1998 Salinization may be related to ground-

water level decline Water-quality variations resulting from

lower water levels can impact all uses

The Problem

Objectives Delineate spatial distribution and

magnitude of varying water quality conditions as related to areas of ground-water level decline

Determine areas of ground-water salinization (high in chloride, SpC, TDS)

Examine varying water-quality conditions spatially over the study area

Provide baseline water-quality data to water users of the region

Physiography of Study Area

Approximately 8,900 square kilometers

Little to no reliefDominated by

agricultural production

1999 Land Use/Land Cover(summer)

Water level, Alluvial aquifer, spring 1998

Declines in Alluvial:

1915-1933 – up to 60 feet, Arkansas Co.

1938-1953 – from 10-20 feet

1955-1961 – from 3-10 feet

1969-1996 - 32 feet, Lonoke Co.

Wells completed in the Alluvial aquiferAbout 7,000 wells!

Water level, Sparta aquifer, spring 1998 Declines in Sparta:

1949 – 50 feet near Pine Bluff

1958-1965 – 160 feet at Pine Bluff

1969-1999 – 70 feet+ in Arkansas Co.

1988-1999 – almost 20 feet in Lonoke Co.

Wells completed in the Sparta aquifer

About 325 wells (and rising)!

Sample Sites

July/Aug 2000

78 Alluvial 39 Sparta 8 QA/QC Analyses

at WQL of AWRC

Ground-water quality, Alluvial aquifer

Good to acceptableCalcium-

magnesium-bicarbonate type

No MCL’s exceededSMCL’s exceeded:

Mn (0.05 mg/L)Fe (0.3 mg/L)

Chloride: 3-172 mg/L

Ground-water quality, Sparta aquifer

BetterRange from Ca-Na-

HCO3 type to Na-HCO3 type

No MCL’s exceededSMCL’s exceeded:

Mn (0.05 mg/L)Fe (0.3 mg/L)

Chloride: 2-56 mg/L

Salinization related to declines?

Cl, Alluvial Aquifer Cl, Sparta Aquifer

Salinization related to declines?

TDS, Alluvial Aquifer TDS, Sparta Aquifer

Spatial trends, Sparta aquifer

GW

FLOW

Lonoke

Jefferson

Arkansas

Prairie


Ionic Strength Calcium Saturation


0

10

20

30

40

50

60

70

80

90

0 20 40 60 80 100

Distance from northwest end (km)

Cal

ciu

m a

nd

Sod

ium

(m

g/L)

0

10

20

30

40

50

60

70

80

90

Sod

ium

/Cal

ciu

m (

meq

/L)

Ca

Na

Na/Ca

LSA76

LSA46

PSA67

LSA92725

ASA74

PSA61

PSA76

ASA23

ASA72

ASA43 ASA7

ASA15

ASA85

AMI5

ASA67

Transect Location

Results

GW

FLOW

Spatial trends, Alluvial aquifer

Conductivity vs. Well Depth


Ionic StrengthCalcium Saturation

37 of 78 samples saturated


pH vs. Ca Saturation

pH~6.85


pH vs. Iron

pH~6.85

Results and Conclusions

Salinization not widespreadWater quality for both aquifers:

Alluvial: Acceptable, with high Cl and Fe Sparta: Better, lower Cl and Fe

Varying hydrochemistry not result of ground-water level declinesVarying hydrochemistry is instead result of geochemical evolution along flowpath

Results and ConclusionsDominant processes:

Alluvial: Ca saturation, precipitation of calcium- and iron-rich cementsSparta: Ca saturation, precipitation of calcium-rich cement; cation exchange

Future work:Combine and compare with other datasets from the regionStatistical analysis of spatial dataExamine relationships with riversIron in the Alluvial aquifer?

AcknowledgementsArkansas Water Resources Center

(AWRC)Water Quality Lab, AWRCArkansas Soil and Water Conservation

CommissionUnited States Geological Survey, Little

RockArkansas Department of

Environmental QualityCenter for Advanced Spatial

Technologies, University of Arkansas

Questions?

Water resources and hydrochemistry of the Alluvial and Sparta aquifers of the Grand Prairie region

Technology

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